Conical spiral tube bundle are universally used in heat transfer enhancement in heat exchangers.The heat transfer and resistance of the tube bundle are affected greatly by the conical structure,so the analysis of it i...Conical spiral tube bundle are universally used in heat transfer enhancement in heat exchangers.The heat transfer and resistance of the tube bundle are affected greatly by the conical structure,so the analysis of it is necessary.In order to a further evaluation,the heat transfer and resistance characteristics of conical spiral tube bundle are investigated with regression analysis based on numerical simulation data.The correlations of heat transfer and pressure drop of conical spiral tube bundle are proposed both in laminar and turbulent fluid flow.On the based of the field synergy principle,the synergy of four vectors,the velocity,the velocity gradient,the temperature gradient and the pressure gradient,are calculated and discussed via the user defined function（UDF） program.The synergy angles β and θ,which respectively denote the performance of heat transfer enhancement and pressure drop of the conical spiral tube bundle,are analyzed.Finally,the comprehensive performance of the conical spiral tube is evaluated by the synergy angle γ and all of the three synergy angles of conical spiral tube bundle are compared to both bare tube and thin cylinder-interpolated tube.The analysis of the synergy angles shows that the heat transfer enhancement and pressure drop of conical spiral tube bundle are smaller than that of the thin cylinder-interpolated tube,while the comprehensive performance of conical spiral tube bundle is greater.The analysis of the heat transfer and pressure drop of conical spiral tube is valuable and instructional on the design and optimum of conical spiral tube bundle heat exchangers.展开更多
The single-phase heat transfer characteristics in a PWR fuel assembly are important. Many investigations attempt to obtain the heat transfer characteristics by studying the flow features in a 5 x 5 rod bundle with a s...The single-phase heat transfer characteristics in a PWR fuel assembly are important. Many investigations attempt to obtain the heat transfer characteristics by studying the flow features in a 5 x 5 rod bundle with a spacer grid. The field synergy principle is used to discuss the mechanism of heat transfer enhancement using mixing vanes according to computational fluid dynamics results, including a spacer grid without mixing vanes, one with a split mixing vane, and one with a separate mixing vane. The results show that the field synergy principle is feasible to explain the mechanism of heat transfer enhancement in a fuel assembly. The enhancement in subchannels is more effective than on the rod＇s surface. If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. Increasing the blending angle of the split mixing vane improves heat transfer enhancement, the maximum of which is 7.1%. Increasing the blending angle of the separate mixing vane did not significantly enhance heat transfer in the rod btmdle, and even prevented heat transfer at a blending angle of 50%. This fmding testifies to the feasibility of predicting heat transfer in a rod bundle with a spacer grid by field synergy, and upon comparison with analyzed flow features only, the field synergy method may provide more accurate guidance for optimizing the use of mixing vanes.展开更多
In order to reveal the mechanics of composite regeneration by coupling cerium-based additive and microwave for a diesel particulate filter, a composite regeneration model by coupling cerium-based additive and microwav...In order to reveal the mechanics of composite regeneration by coupling cerium-based additive and microwave for a diesel particulate filter, a composite regeneration model by coupling cerium-based additive and microwave for a diesel particulate filter was established based on field synergy theory. Performance evaluation on field synergy and composite regeneration of the diesel particulate filter was conducted by using the vortex crushing combustion and field synergy mathematical models. The results show that the peak temperature of the particulate filter body reaches 1180-1190 K when the regeneration time is 175 s, and there are optimal coordination degree between the velocity vector and temperature gradient of the filter body and the maximum ratio0.56-0.60 of the best burning regeneration region is obtained. Accordingly, the largest regeneration combustion rate inside the particulate filter body and the highest regeneration efficiency at the moment are achieved.展开更多
The microenvironment,which involves pollutant dispersion of the urban street canyon,is critical to the health of pedestrians and residents.The objectives of this work are twofold:(i)to effectively assess the pollutant...The microenvironment,which involves pollutant dispersion of the urban street canyon,is critical to the health of pedestrians and residents.The objectives of this work are twofold:(i)to effectively assess the pollutant dispersion process based on a theory and(ii)to adopt an appropriate stratigy,i.e.,wind catcher,to alleviate the pollution in the street canyons.Pollutant dispersion in street canyons is essentially a convective mass transfer process.Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature,we apply the field synergy theory to the study of pollutant dispersion in street canyons.In this paper,a computational fluid dynamics(CFD)simulation is conducted to investigate the effects of wind catcher,wind speed and the geometry of the street canyons on pollutant dispersion.According to the field synergy theory,Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons.The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%.Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.展开更多
To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio(D/dp<10) have now been considered in many areas. Fluid-to-wall heat transfer...To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio(D/dp<10) have now been considered in many areas. Fluid-to-wall heat transfer coefficient is an important factor determining the performance of this type of beds. In this work, local fluid-to-wall heat transfer characteristic in packed beds was studied by Computational Fluid Dynamics(CFD) at different Reynolds number for D/dp=1.5, 3.0 and 5.6. The results show that the fluid-to-wall heat transfer coefficient is oscillating along the bed with small tube-to-particle diameter ratio. Moreover, this phenomenon was explained by field synergy principle in detail. Two arrangement structures of particles in packed beds were recommended based on the synergy characteristic between flow and temperature fields. This study provides a new local understanding of fluid-to-wall heat transfer in packed beds with small tube-to-particle diameter ratio.展开更多
The experimental study of natural convection in allglass evacuated tube solar collectors is performed through the experimental platform of the solar-assisted fuel cell system.The experimental facility includes solar c...The experimental study of natural convection in allglass evacuated tube solar collectors is performed through the experimental platform of the solar-assisted fuel cell system.The experimental facility includes solar collectors with different length and diameter tubes, different coating materials, and with / without guide plates, respectively. Threedimensional mathematical models on natural and forced convections in the solar collectors are established and the experimental data is validated by field synergy and entransy principles. The results of natural convection show that the water temperature increases and thermal efficiency decreases gradually with the evacuated tube length. The thermal efficiency increases when absorption rates increase from 0. 95 to 1. 0 and emission rates decrease from 0. 16 to 0. 06. The thermal efficiency of solar collectors is increased after being equipped with the guide plate, which is attributed to the disappearance of the mixed flowand the enhancement of the heat transfer at the bottom of the evacuated tube. The results of forced convertion indicate that the Reynolds, Nusselt and entransy increments of the horizontal double collectors are higher than those of the vertical single collector while the entransy dissipation is lower than that of the vertical single collector. It is concluded that the solar collectors with guide plates are suitable for natural convection while the double horizontal collectors are suitable for forced convection in the thermal field of solar-assisted fuel cell systems with lowand medium temperatures.展开更多
Shape and quantity of helical baffles have great impact on the shell-side performance of helical baffled heat exchangers （HBHE）. In this work, three physical models of HBHE with baffles of different shape （trisecti...Shape and quantity of helical baffles have great impact on the shell-side performance of helical baffled heat exchangers （HBHE）. In this work, three physical models of HBHE with baffles of different shape （trisection, quadrant and sextant sector） were investigated. Numerical simulations were performed on HBHE at three helix an- gles （10°, 25° and 40°） by the software ANSYS CFX. Analyses of numerical results indicate that the sextant HBHE shows relatively better fluid flow performance because the leakage flow in the triangle area is evidently reduced and the fluid streamline appears much closer to an ideal spiral flow, while the trisection and quadrant HBHE show more scattered and disordered streamline distributions. The convective heat transfer coefficient and pressure drop in three types of HBHE were presented. Further investigations on the shell side performance with different helical baf- fles were implemented by the field synergy theory. Both theoretical and numerical analyses gave support on the re- lations between helical baffle shape and shell-side performance. This paper may provide useful reference for the selection of baffle shade and auantitv in HBHE.展开更多
Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic fo...Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic force and fluid velocity,was derived from the basic equation describing the motion of magnetic beads in microchannels. It is proposed that one of the most important approaches for separation efficiency enhancement is to improve the coordination of magnetic force field and fluid flow field. A T-shaped microchannel magnetophoretic separator was designed based on the angle. And then a two-dimensional dynamic model of magnetic beads moving in microchannels was established to study the separation efficiency of T-shaped microseparator by combined use of finite element method and Runge-Kutta method. The results show that the capture effi-ciency of T-shaped microseparator is much higher than that of the straight microseparator at the same conditions. For small magnetic beads at high fluid velocities,the designed T-shaped microseparator could still keep high separation efficiency whereas the conventional straight microseparator fails to separate the magnetic beads. Further analysis shows that the mechanism of separation efficiency enhancement lies in the synergy of magnetic force field and flow field,which directly leads to large deflected velocity of the magnetic beads from the main stream,and thus increasing the separation efficiency. It is anticipated that the results in this paper are theoretically helpful for the optimum design of highly efficient magnetophoretic separators.展开更多
This study combines the three-dimensional model of the high-temperature proton exchange membrane fuel cell(HT-PEMFC)with theoretical analysis,by optimizing the structure of the fuel cell,adding a semicircular baffle i...This study combines the three-dimensional model of the high-temperature proton exchange membrane fuel cell(HT-PEMFC)with theoretical analysis,by optimizing the structure of the fuel cell,adding a semicircular baffle in the gas channel and implementing novelly arranged obstacles to improve the PEMFC performance. The effects of velocity distribution,interface reactant concentration and pressure drop on performance are studied. The results show that adding obstacles in the gas channel will produce vertical velocity and can improve output performance,especially in the case of high current density and higher baffle radius. The superiority of the optimized structure in mass transfer capacity is proved,and a mechanism explanation is given for the improvement of performance.展开更多
The experiment was conducted to investigate the heat transfer performance of supercritical CO_2 in a casing heat exchanger by comparing their heat transfer,entropy production unit number,non-dimensional entropy produc...The experiment was conducted to investigate the heat transfer performance of supercritical CO_2 in a casing heat exchanger by comparing their heat transfer,entropy production unit number,non-dimensional entropy production rate and field synergy factor.The results show that both heat transfer and entropy production unit number in four tubes decrease with water temperature increasing.Heat transfer and entropy production unit number in multiple tubes( i. e.,triple straight tube and double helix tube) is higher than their single counterparts; the non-dimensional entropy production rate increases with water temperature. Non-dimensional entropy production rate of triple straight tube and double helix tube is far below the single tube. Field synergy factor of double helix tube is much higher than that of the triple straight tube under the same condition. Further experiment was carried out in double helix tube,under various CO_2 pressure and inlet water temperature,the results are analyzed and reported in this paper.展开更多
Flow and heat transfer characteristic of supercritical carbon dioxide(SCO_(2))are numerically investigated in the horizontal and vertical tubes.TWL turbulent Prandtl number model could well describe the behavior of SC...Flow and heat transfer characteristic of supercritical carbon dioxide(SCO_(2))are numerically investigated in the horizontal and vertical tubes.TWL turbulent Prandtl number model could well describe the behavior of SCO_(2) affected by the buoyancy.Under the cooling condition,the heat transfer performance of SCO_(2) along the upward direction is best and that along the downward direction is worst when bulk fluid temperatures are below the pseudocritical temperature.Reducing the ratio of heat flux to mass flux could decrease the difference of convective heat transfer coefficient in three flow directions.Under the heating condition,heat transfer deterioration only occurs in vertical upward and horizontal flow directions.Heat transfer deterioration of SCO_(2) could be delayed by increasing the mass flux and the deterioration degree is weakened in the second half of tube along the vertical upward flow direction.Compared with the straight tube,the corrugated tube shows better comprehensive thermal performance.展开更多
Printed circuit heat exchanger(PCHE)has been widely used in supercritical carbon dioxide(S-CO_(2))power systems because of its high heat transfer efficiency and good compactness.However,due to the large variety of PCH...Printed circuit heat exchanger(PCHE)has been widely used in supercritical carbon dioxide(S-CO_(2))power systems because of its high heat transfer efficiency and good compactness.However,due to the large variety of PCHE configurations,channel selection in practical applications lacks a basis.Therefore,this paper discussed the heat transfer and friction characteristics and the synergy of three fields in the channel under the guidance of the field synergy principle for four typical PCHE channels.Additionally,the comprehensive performance of four channels was compared.Finally,the heat transfer and friction factor correlations for S-CO_(2)in four channels were established.The findings demonstrate that the synergy of velocity and pressure fields of the straight channel PCHE is better(β≈180°),so its resistance loss is relatively small.The zigzag and sinusoidal wavy channels and the airfoil fins can reduce the angle a between the temperature gradient and velocity,thus enhancing the heat transfer.The sinusoidal wavy channel can reduce flow resistance compared to the zigzag channel due to the rounded corners.The streamlined airfoil structure can guide the flow and reduce backflow,thus reducing resistance losses.In the range of Re studied in this paper,the maximum error of the proposed heat transfer and friction factor correlations of PCHE is 7.0%,which shows good fitting accuracy.The research in this paper can provide a reference for the selection and design of PCHE with different channel configurations.展开更多
Cooling water is an important part in a Spallation Neutron Source target cooling system, but the unstable vortexes at the exits of the slits between every two tungsten target slices have a negative impact on the stabl...Cooling water is an important part in a Spallation Neutron Source target cooling system, but the unstable vortexes at the exits of the slits between every two tungsten target slices have a negative impact on the stable running of the target system. We apply the field synergy principle for fluid flow to obtain the optimal flow field, which has a uniform velocity distribution without eddy, and then, optimize the geometrical structure of the cooling water flow channel based on the optimal flow field. The results show that when the cooling water flows in the optimized channel, the eddy sizes decrease, the time fluctuations of velocity and pressure almost vanish, and the volume flow rates of the cooling water in each parallel slit are uniform. Therefore, it effectively improves the running stability of the target system with the premise of satisfying the target heat load.展开更多
To deal with the aerodynamic heating on the aircraft surface,a potential solution is to utilize liquid cooling via the channels in part of the fuselage.This is a typical problem of flow and heat transfer in channels w...To deal with the aerodynamic heating on the aircraft surface,a potential solution is to utilize liquid cooling via the channels in part of the fuselage.This is a typical problem of flow and heat transfer in channels with unilaterally-heated surfaces.The enhancement of heat transfer in the channel is significant due to the high heating flux.The optimal velocity and temperature fields are obtained first based on the field synergy optimization method.Four rib configurations are proposed to produce the longitudinal vortices suggested by the optimal velocity field.The flow and heat transfer characteristics of different rib configurations are obtained by numerical simulation.The numerical simulations show that the heat transfer enhancement of the rib configurations are quite different,but the pressure drop increases similarly in the laminar flow range of Re = 500–1500.The mechanism of heat transfer enhancement with the single/double-inclined ribs for the unilaterally-heated channel is analysed.The best enhancement of geometric parameter among the investigated parameters such as the angle,length,radius and the spacing of the ribs is obtained.展开更多
This review paper summarizes constructal design progress performed by the authors for eight types of heat sinks with ten performance indexes being taken as the optimization objectives,respectively,by combining the met...This review paper summarizes constructal design progress performed by the authors for eight types of heat sinks with ten performance indexes being taken as the optimization objectives,respectively,by combining the methods of theoretical analysis and numerical calculation.The eight types of heat sinks are uniform height rectangular fin heat sink,non-uniform height rectangular fin heat sink,inline cylindrical pin-fin heat sink(ICPHS),plate single-row pin fin heat sink(PSRPHS),plate inline pin fin heat sink(PIPHS),plate staggered pin fin heat sink(PSPHS),single-layered microchannel heat sink(SLMCHS)with rectangular cross sections and double-layered microchannel heat sink(DLMCHS)with rectangular cross sections,respectively.And the ten performance indexes are heat transfer rate maximization,maximum thermal resistance minimization,minimization of equivalent thermal resistance which is defined based on the entransy dissipation rate(equivalent thermal resistance for short),field synergy number maximization,entropy generation rate minimization,operation cost minimization,thermo-economic function value minimization,pressure drop minimization,enhanced heat transfer factor maximization and efficiency evaluation criterion number maximization,respectively.The optimal constructs of the eight types of heat sinks with different constraints and based on the different optimization objectives are compared with each other.The results indicated that the optimal constructs mostly are different based on different optimization objectives under the same boundary condition.The optimization objective should be suitable chosen based on the focus when the constructal design for one heat sink is performed.The results obtained herein have some important theoretical significances and application values,and can provide scientific bases and theoretical guidelines for the thermal design of real heat sinks and their applications.展开更多
Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored b...Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.展开更多
Tangential leakage loss reduction has great significance on improving the performance of scroll compressors.In this study,the flow field of a scroll compressor working with CO_(2) was numerically investigated.The deve...Tangential leakage loss reduction has great significance on improving the performance of scroll compressors.In this study,the flow field of a scroll compressor working with CO_(2) was numerically investigated.The development characteristics of the tangential leakage flow in different working chambers were carried out,which was obtained by analyzing the field quantities distributions.The impacts of the radial clearances and sidewall roughness on the tangential leakage were also taken into consideration,in order to explore the feasible method of the flow control for the tangential leakage in scroll compressors.Results showed that the tangential leakage flow had various characteristics in the suction and compression chambers due to the different interactions between the tangential leakage flow and mainstream.Owing to little reverse pressure gradient,the tangential leakage flow maintained the typical jet form in the suction chambers.By contrast,the mixing of the tangential leakage flow and mainstream induced the passage vortex and secondary flows in the compression chamber.The secondary flow was the primary factor that results in the occurrence of localized high temperature region rather than the tangential leakage.With the increase of the radial clearance,the volumetric efficiency declined and the discharge temperature increased rapidly.In terms of flow control of the tangential leakage,the increase of sidewall roughness by 2μm could achieve the same effect as the decrease of the radial clearance by 4μm,while the volumetric efficiency increased by almost 5%and average discharge temperature decreased by 5K.展开更多
Carbon nanotube nanofluids have wide application prospects due to their unique structure and excellent properties.In this study,the thermal conductivity properties of carbon nanotube nanofluids and SiO2/water nanoflui...Carbon nanotube nanofluids have wide application prospects due to their unique structure and excellent properties.In this study,the thermal conductivity properties of carbon nanotube nanofluids and SiO2/water nanofluids were compared and analyzed experimentally using different preparation methods.The physical properties of nanofluids were tested using a Malvern Zetasizer Nano Instrument and a Hot Disk Thermal Constant Analyzer.Combined with field synergy theory analysis of the heat transfer performance of nanofluids,results show that the thermal conductivity of carbon nanotube nanofluids is higher than that of SiO2/water nanofluids,and the thermal conductivity of nanofluid rises with the increase of mass fraction and temperature.Moreover,the synergistic performance of carbon nanotube nanofluids is also superior to that of SiO2/water nanofluids.When the mass fraction of the carbon nanotube nanofluids is 10%and the SiO2/water nanofluids is 8%,their field synergy numbers and heat transfer enhancement factors both reach maximum.From the perspective of the preparation method,the thermal conductivity of nanofluids dispersed by high shear microfluidizer is higher than that by ultrasonic dispersion.This result provides some reference for the selection and use of working substance in a microchannel cooling concentrated photovoltaic and thermal(CPV/T)system.展开更多
A cooling system consisting of several heat exchange modules is a necessary part of an automobile, and its performance has a direct effect on a vehicle's energy consumption. Heat exchangers, such as a charged air ...A cooling system consisting of several heat exchange modules is a necessary part of an automobile, and its performance has a direct effect on a vehicle's energy consumption. Heat exchangers, such as a charged air cooler (CAC), radiator, oil cooler, or condenser have different structures and can be arranged in various orders, and each combination may produce different effects because of interactions among them. In this study, we aimed to explore the principles governing interactions among adjacent heat exchangers in a cooling system, using numerical simulation and experimental technology. 3D models with different combinations were developed, compared, and analyzed comprehensively. A wind tunnel test platform was constructed to validate the computational results. We found that the heat dissipation of the modules was affected slightly by their relative position (the rules basically comply with the field synergy principle), but was independent of the modules' spacing within a certain distance range. The heat dissipation of one module could be effectively improved by restructuring, but with a penalty of higher resistance. However, the negative effect on the downstream module was much less than expected. The results indicated that the intensity of heat transfer depends not only on the average temperature difference between cold and hot mediums, but also on the temperature distribution.展开更多
Periodic whole cross-section computation models are established for segmental baffle heat exchanger, shutter baffle heat exchanger, and trapezoid-like tilted baffle heat exchanger. The reliability of models is verifie...Periodic whole cross-section computation models are established for segmental baffle heat exchanger, shutter baffle heat exchanger, and trapezoid-like tilted baffle heat exchanger. The reliability of models is verified by comparing the simulated results to the results obtained from the Bell-Delaware method. Due to the orthogonal assembly of the baffles, the shell side fluid shows the twisty flow of trapezoid-like tilted baffle heat exchanger. The essential mechanism on disturbing flow and heat transfer enhancement is revealed by defining the non-dimensional factor η of the shell side fluid flow direction of heat exchanger and the field synergy principle. The results show that at the same Reynolds number, the shell side fluid convection heat transfer coefficient of trapezoid-like tilted baffle heat exchanger is 12.43%-24.33% and 6.71%-11.51% higher than those of segmental baffle heat exchanger and shutter baffle heat exchanger, respectively. The shell side fluid flow velocity field and the pressure gradient field of trapezoid-like tilted baffle heat exchanger and shutter baffle heat exchanger decreases compared with that of segmental baffle heat exchanger, so the shell side fluid flow resistance and pressure drop is increased; the shell side comprehensive performance of trapezoid-like tilted baffle heat exchanger is 5.85%-9.06% higher than that of segmental baffle heat exchanger, and 15.27%-23.28% higher than that of shutter baffle heat exchanger. In this study, a baffle structure with higher efficiency of the energy utilization for the heat exchanger is provided.展开更多
基金supported by National Basic Research Program of China(973 Program,Grant No. 2007CB206900)Scholarship Award for Doctoral by Ministry of Education of China(Grant No. 10000071183646)
文摘Conical spiral tube bundle are universally used in heat transfer enhancement in heat exchangers.The heat transfer and resistance of the tube bundle are affected greatly by the conical structure,so the analysis of it is necessary.In order to a further evaluation,the heat transfer and resistance characteristics of conical spiral tube bundle are investigated with regression analysis based on numerical simulation data.The correlations of heat transfer and pressure drop of conical spiral tube bundle are proposed both in laminar and turbulent fluid flow.On the based of the field synergy principle,the synergy of four vectors,the velocity,the velocity gradient,the temperature gradient and the pressure gradient,are calculated and discussed via the user defined function（UDF） program.The synergy angles β and θ,which respectively denote the performance of heat transfer enhancement and pressure drop of the conical spiral tube bundle,are analyzed.Finally,the comprehensive performance of the conical spiral tube is evaluated by the synergy angle γ and all of the three synergy angles of conical spiral tube bundle are compared to both bare tube and thin cylinder-interpolated tube.The analysis of the synergy angles shows that the heat transfer enhancement and pressure drop of conical spiral tube bundle are smaller than that of the thin cylinder-interpolated tube,while the comprehensive performance of conical spiral tube bundle is greater.The analysis of the heat transfer and pressure drop of conical spiral tube is valuable and instructional on the design and optimum of conical spiral tube bundle heat exchangers.
基金Supported by National Natural Science Foundation of China(Grant No.51376022)
文摘The single-phase heat transfer characteristics in a PWR fuel assembly are important. Many investigations attempt to obtain the heat transfer characteristics by studying the flow features in a 5 x 5 rod bundle with a spacer grid. The field synergy principle is used to discuss the mechanism of heat transfer enhancement using mixing vanes according to computational fluid dynamics results, including a spacer grid without mixing vanes, one with a split mixing vane, and one with a separate mixing vane. The results show that the field synergy principle is feasible to explain the mechanism of heat transfer enhancement in a fuel assembly. The enhancement in subchannels is more effective than on the rod＇s surface. If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. Increasing the blending angle of the split mixing vane improves heat transfer enhancement, the maximum of which is 7.1%. Increasing the blending angle of the separate mixing vane did not significantly enhance heat transfer in the rod btmdle, and even prevented heat transfer at a blending angle of 50%. This fmding testifies to the feasibility of predicting heat transfer in a rod bundle with a spacer grid by field synergy, and upon comparison with analyzed flow features only, the field synergy method may provide more accurate guidance for optimizing the use of mixing vanes.
基金Projects(51176045,51276056)supported by the National Natural Science Foundation of ChinaProject(531105050037)supported by the Changjiang Scholars and Innovative Research Team in University,ChinaProjects(201208430262,201306130031)supported by the National Studying Abroad Foundation Project of China
文摘In order to reveal the mechanics of composite regeneration by coupling cerium-based additive and microwave for a diesel particulate filter, a composite regeneration model by coupling cerium-based additive and microwave for a diesel particulate filter was established based on field synergy theory. Performance evaluation on field synergy and composite regeneration of the diesel particulate filter was conducted by using the vortex crushing combustion and field synergy mathematical models. The results show that the peak temperature of the particulate filter body reaches 1180-1190 K when the regeneration time is 175 s, and there are optimal coordination degree between the velocity vector and temperature gradient of the filter body and the maximum ratio0.56-0.60 of the best burning regeneration region is obtained. Accordingly, the largest regeneration combustion rate inside the particulate filter body and the highest regeneration efficiency at the moment are achieved.
基金This research was supported by the National Natural Science Foundation of China(Grant No.51778511)the European Commission H2020 Marie S Curie Research and Innovation Staff Exchange(RISE)award(Grant No.871998)+2 种基金Hubei Provincial Natural Science Foundation of China(Grant No.2018CFA029)Key Project of ESI Discipline Development of Wuhan University of Technology(Grant No.2017001)the Fundamental Research Funds for the Central Universities(Grant No.2019IVB082).
文摘The microenvironment,which involves pollutant dispersion of the urban street canyon,is critical to the health of pedestrians and residents.The objectives of this work are twofold:(i)to effectively assess the pollutant dispersion process based on a theory and(ii)to adopt an appropriate stratigy,i.e.,wind catcher,to alleviate the pollution in the street canyons.Pollutant dispersion in street canyons is essentially a convective mass transfer process.Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature,we apply the field synergy theory to the study of pollutant dispersion in street canyons.In this paper,a computational fluid dynamics(CFD)simulation is conducted to investigate the effects of wind catcher,wind speed and the geometry of the street canyons on pollutant dispersion.According to the field synergy theory,Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons.The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%.Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.
基金supported by the National Natural Science Foundation of China(5127618151476173)the National Basic Research Program of China(2011CB 710705)
文摘To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio(D/dp<10) have now been considered in many areas. Fluid-to-wall heat transfer coefficient is an important factor determining the performance of this type of beds. In this work, local fluid-to-wall heat transfer characteristic in packed beds was studied by Computational Fluid Dynamics(CFD) at different Reynolds number for D/dp=1.5, 3.0 and 5.6. The results show that the fluid-to-wall heat transfer coefficient is oscillating along the bed with small tube-to-particle diameter ratio. Moreover, this phenomenon was explained by field synergy principle in detail. Two arrangement structures of particles in packed beds were recommended based on the synergy characteristic between flow and temperature fields. This study provides a new local understanding of fluid-to-wall heat transfer in packed beds with small tube-to-particle diameter ratio.
基金The National Natural Science Foundation of China(No.51376110,51541604)the Major International(Regional) Joint Research Project of the National Natural Science Foundation of China(No.61320106011)
文摘The experimental study of natural convection in allglass evacuated tube solar collectors is performed through the experimental platform of the solar-assisted fuel cell system.The experimental facility includes solar collectors with different length and diameter tubes, different coating materials, and with / without guide plates, respectively. Threedimensional mathematical models on natural and forced convections in the solar collectors are established and the experimental data is validated by field synergy and entransy principles. The results of natural convection show that the water temperature increases and thermal efficiency decreases gradually with the evacuated tube length. The thermal efficiency increases when absorption rates increase from 0. 95 to 1. 0 and emission rates decrease from 0. 16 to 0. 06. The thermal efficiency of solar collectors is increased after being equipped with the guide plate, which is attributed to the disappearance of the mixed flowand the enhancement of the heat transfer at the bottom of the evacuated tube. The results of forced convertion indicate that the Reynolds, Nusselt and entransy increments of the horizontal double collectors are higher than those of the vertical single collector while the entransy dissipation is lower than that of the vertical single collector. It is concluded that the solar collectors with guide plates are suitable for natural convection while the double horizontal collectors are suitable for forced convection in the thermal field of solar-assisted fuel cell systems with lowand medium temperatures.
基金Supported by the National Natural Science Foundation of China(51106090)the National Key Basic Research Program of China(2013CB228305)the Independent Innovation Foundation of Shandong University(2012TS190)
文摘Shape and quantity of helical baffles have great impact on the shell-side performance of helical baffled heat exchangers （HBHE）. In this work, three physical models of HBHE with baffles of different shape （trisection, quadrant and sextant sector） were investigated. Numerical simulations were performed on HBHE at three helix an- gles （10°, 25° and 40°） by the software ANSYS CFX. Analyses of numerical results indicate that the sextant HBHE shows relatively better fluid flow performance because the leakage flow in the triangle area is evidently reduced and the fluid streamline appears much closer to an ideal spiral flow, while the trisection and quadrant HBHE show more scattered and disordered streamline distributions. The convective heat transfer coefficient and pressure drop in three types of HBHE were presented. Further investigations on the shell side performance with different helical baf- fles were implemented by the field synergy theory. Both theoretical and numerical analyses gave support on the re- lations between helical baffle shape and shell-side performance. This paper may provide useful reference for the selection of baffle shade and auantitv in HBHE.
基金supported by the National Natural Science Foundation of China (Grant No.50925624)the National Basic Research Program of China ("973" Project) (Grant No.2012CB720404)the Science and Technology Commission of Shanghai Municipality (Grant No.11XD1403100)
文摘Magnetophoresis is one of the most important separation methods in biological and chemical engineering. In this paper,a novel impact parameter on separation efficiency,i.e.,the angle between the vectors of magnetic force and fluid velocity,was derived from the basic equation describing the motion of magnetic beads in microchannels. It is proposed that one of the most important approaches for separation efficiency enhancement is to improve the coordination of magnetic force field and fluid flow field. A T-shaped microchannel magnetophoretic separator was designed based on the angle. And then a two-dimensional dynamic model of magnetic beads moving in microchannels was established to study the separation efficiency of T-shaped microseparator by combined use of finite element method and Runge-Kutta method. The results show that the capture effi-ciency of T-shaped microseparator is much higher than that of the straight microseparator at the same conditions. For small magnetic beads at high fluid velocities,the designed T-shaped microseparator could still keep high separation efficiency whereas the conventional straight microseparator fails to separate the magnetic beads. Further analysis shows that the mechanism of separation efficiency enhancement lies in the synergy of magnetic force field and flow field,which directly leads to large deflected velocity of the magnetic beads from the main stream,and thus increasing the separation efficiency. It is anticipated that the results in this paper are theoretically helpful for the optimum design of highly efficient magnetophoretic separators.
基金supported by the De-fense Industrial Technology Development Program (No. JCKY2018605B006)the Aviation Science Fund (No. 201928052002)
文摘This study combines the three-dimensional model of the high-temperature proton exchange membrane fuel cell(HT-PEMFC)with theoretical analysis,by optimizing the structure of the fuel cell,adding a semicircular baffle in the gas channel and implementing novelly arranged obstacles to improve the PEMFC performance. The effects of velocity distribution,interface reactant concentration and pressure drop on performance are studied. The results show that adding obstacles in the gas channel will produce vertical velocity and can improve output performance,especially in the case of high current density and higher baffle radius. The superiority of the optimized structure in mass transfer capacity is proved,and a mechanism explanation is given for the improvement of performance.
基金Hujiang Foundation of China(No.D14003)Yangtze River Delta Technology Joint Research,China(No.10195811000)
文摘The experiment was conducted to investigate the heat transfer performance of supercritical CO_2 in a casing heat exchanger by comparing their heat transfer,entropy production unit number,non-dimensional entropy production rate and field synergy factor.The results show that both heat transfer and entropy production unit number in four tubes decrease with water temperature increasing.Heat transfer and entropy production unit number in multiple tubes( i. e.,triple straight tube and double helix tube) is higher than their single counterparts; the non-dimensional entropy production rate increases with water temperature. Non-dimensional entropy production rate of triple straight tube and double helix tube is far below the single tube. Field synergy factor of double helix tube is much higher than that of the triple straight tube under the same condition. Further experiment was carried out in double helix tube,under various CO_2 pressure and inlet water temperature,the results are analyzed and reported in this paper.
文摘Flow and heat transfer characteristic of supercritical carbon dioxide(SCO_(2))are numerically investigated in the horizontal and vertical tubes.TWL turbulent Prandtl number model could well describe the behavior of SCO_(2) affected by the buoyancy.Under the cooling condition,the heat transfer performance of SCO_(2) along the upward direction is best and that along the downward direction is worst when bulk fluid temperatures are below the pseudocritical temperature.Reducing the ratio of heat flux to mass flux could decrease the difference of convective heat transfer coefficient in three flow directions.Under the heating condition,heat transfer deterioration only occurs in vertical upward and horizontal flow directions.Heat transfer deterioration of SCO_(2) could be delayed by increasing the mass flux and the deterioration degree is weakened in the second half of tube along the vertical upward flow direction.Compared with the straight tube,the corrugated tube shows better comprehensive thermal performance.
基金supported by the National Key R&D Program of China(No.2022YFB3304001)the Key R&D Special Program of Shaanxi province(No.2022GXLH-01-04)。
文摘Printed circuit heat exchanger(PCHE)has been widely used in supercritical carbon dioxide(S-CO_(2))power systems because of its high heat transfer efficiency and good compactness.However,due to the large variety of PCHE configurations,channel selection in practical applications lacks a basis.Therefore,this paper discussed the heat transfer and friction characteristics and the synergy of three fields in the channel under the guidance of the field synergy principle for four typical PCHE channels.Additionally,the comprehensive performance of four channels was compared.Finally,the heat transfer and friction factor correlations for S-CO_(2)in four channels were established.The findings demonstrate that the synergy of velocity and pressure fields of the straight channel PCHE is better(β≈180°),so its resistance loss is relatively small.The zigzag and sinusoidal wavy channels and the airfoil fins can reduce the angle a between the temperature gradient and velocity,thus enhancing the heat transfer.The sinusoidal wavy channel can reduce flow resistance compared to the zigzag channel due to the rounded corners.The streamlined airfoil structure can guide the flow and reduce backflow,thus reducing resistance losses.In the range of Re studied in this paper,the maximum error of the proposed heat transfer and friction factor correlations of PCHE is 7.0%,which shows good fitting accuracy.The research in this paper can provide a reference for the selection and design of PCHE with different channel configurations.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51006060, 51036003)the Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant No.201150)
文摘Cooling water is an important part in a Spallation Neutron Source target cooling system, but the unstable vortexes at the exits of the slits between every two tungsten target slices have a negative impact on the stable running of the target system. We apply the field synergy principle for fluid flow to obtain the optimal flow field, which has a uniform velocity distribution without eddy, and then, optimize the geometrical structure of the cooling water flow channel based on the optimal flow field. The results show that when the cooling water flows in the optimized channel, the eddy sizes decrease, the time fluctuations of velocity and pressure almost vanish, and the volume flow rates of the cooling water in each parallel slit are uniform. Therefore, it effectively improves the running stability of the target system with the premise of satisfying the target heat load.
基金supported by discipline construction fund of Tsinghua University。
文摘To deal with the aerodynamic heating on the aircraft surface,a potential solution is to utilize liquid cooling via the channels in part of the fuselage.This is a typical problem of flow and heat transfer in channels with unilaterally-heated surfaces.The enhancement of heat transfer in the channel is significant due to the high heating flux.The optimal velocity and temperature fields are obtained first based on the field synergy optimization method.Four rib configurations are proposed to produce the longitudinal vortices suggested by the optimal velocity field.The flow and heat transfer characteristics of different rib configurations are obtained by numerical simulation.The numerical simulations show that the heat transfer enhancement of the rib configurations are quite different,but the pressure drop increases similarly in the laminar flow range of Re = 500–1500.The mechanism of heat transfer enhancement with the single/double-inclined ribs for the unilaterally-heated channel is analysed.The best enhancement of geometric parameter among the investigated parameters such as the angle,length,radius and the spacing of the ribs is obtained.
基金supported by the National Natural Science Foundation of China(Grant Nos.51779262,51506220 and 51579244)。
文摘This review paper summarizes constructal design progress performed by the authors for eight types of heat sinks with ten performance indexes being taken as the optimization objectives,respectively,by combining the methods of theoretical analysis and numerical calculation.The eight types of heat sinks are uniform height rectangular fin heat sink,non-uniform height rectangular fin heat sink,inline cylindrical pin-fin heat sink(ICPHS),plate single-row pin fin heat sink(PSRPHS),plate inline pin fin heat sink(PIPHS),plate staggered pin fin heat sink(PSPHS),single-layered microchannel heat sink(SLMCHS)with rectangular cross sections and double-layered microchannel heat sink(DLMCHS)with rectangular cross sections,respectively.And the ten performance indexes are heat transfer rate maximization,maximum thermal resistance minimization,minimization of equivalent thermal resistance which is defined based on the entransy dissipation rate(equivalent thermal resistance for short),field synergy number maximization,entropy generation rate minimization,operation cost minimization,thermo-economic function value minimization,pressure drop minimization,enhanced heat transfer factor maximization and efficiency evaluation criterion number maximization,respectively.The optimal constructs of the eight types of heat sinks with different constraints and based on the different optimization objectives are compared with each other.The results indicated that the optimal constructs mostly are different based on different optimization objectives under the same boundary condition.The optimization objective should be suitable chosen based on the focus when the constructal design for one heat sink is performed.The results obtained herein have some important theoretical significances and application values,and can provide scientific bases and theoretical guidelines for the thermal design of real heat sinks and their applications.
基金the National Basic Research Program of China (973 Program, Grant No. 2007CB206903)the New Century Excellent Talents in University (Grant No. NCET-05-0583).
文摘Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.
基金supported by the National Natural Science Foundation of China(Grant Nos.52006010 and 52006009)。
文摘Tangential leakage loss reduction has great significance on improving the performance of scroll compressors.In this study,the flow field of a scroll compressor working with CO_(2) was numerically investigated.The development characteristics of the tangential leakage flow in different working chambers were carried out,which was obtained by analyzing the field quantities distributions.The impacts of the radial clearances and sidewall roughness on the tangential leakage were also taken into consideration,in order to explore the feasible method of the flow control for the tangential leakage in scroll compressors.Results showed that the tangential leakage flow had various characteristics in the suction and compression chambers due to the different interactions between the tangential leakage flow and mainstream.Owing to little reverse pressure gradient,the tangential leakage flow maintained the typical jet form in the suction chambers.By contrast,the mixing of the tangential leakage flow and mainstream induced the passage vortex and secondary flows in the compression chamber.The secondary flow was the primary factor that results in the occurrence of localized high temperature region rather than the tangential leakage.With the increase of the radial clearance,the volumetric efficiency declined and the discharge temperature increased rapidly.In terms of flow control of the tangential leakage,the increase of sidewall roughness by 2μm could achieve the same effect as the decrease of the radial clearance by 4μm,while the volumetric efficiency increased by almost 5%and average discharge temperature decreased by 5K.
基金supported by the National Natural Science Foundation of China(NO.51766012)Inner Mongolia Financial Innovation Funding Project in 2017+1 种基金Inner Mongolia Natural Science Foundation of China(NO.2019MS05025)the Inner Mongolia Science and Technology Major Project of China(NO.201905)。
文摘Carbon nanotube nanofluids have wide application prospects due to their unique structure and excellent properties.In this study,the thermal conductivity properties of carbon nanotube nanofluids and SiO2/water nanofluids were compared and analyzed experimentally using different preparation methods.The physical properties of nanofluids were tested using a Malvern Zetasizer Nano Instrument and a Hot Disk Thermal Constant Analyzer.Combined with field synergy theory analysis of the heat transfer performance of nanofluids,results show that the thermal conductivity of carbon nanotube nanofluids is higher than that of SiO2/water nanofluids,and the thermal conductivity of nanofluid rises with the increase of mass fraction and temperature.Moreover,the synergistic performance of carbon nanotube nanofluids is also superior to that of SiO2/water nanofluids.When the mass fraction of the carbon nanotube nanofluids is 10%and the SiO2/water nanofluids is 8%,their field synergy numbers and heat transfer enhancement factors both reach maximum.From the perspective of the preparation method,the thermal conductivity of nanofluids dispersed by high shear microfluidizer is higher than that by ultrasonic dispersion.This result provides some reference for the selection and use of working substance in a microchannel cooling concentrated photovoltaic and thermal(CPV/T)system.
基金Project (No. 51206141) supported by the National Natural Science Foundation of China
文摘A cooling system consisting of several heat exchange modules is a necessary part of an automobile, and its performance has a direct effect on a vehicle's energy consumption. Heat exchangers, such as a charged air cooler (CAC), radiator, oil cooler, or condenser have different structures and can be arranged in various orders, and each combination may produce different effects because of interactions among them. In this study, we aimed to explore the principles governing interactions among adjacent heat exchangers in a cooling system, using numerical simulation and experimental technology. 3D models with different combinations were developed, compared, and analyzed comprehensively. A wind tunnel test platform was constructed to validate the computational results. We found that the heat dissipation of the modules was affected slightly by their relative position (the rules basically comply with the field synergy principle), but was independent of the modules' spacing within a certain distance range. The heat dissipation of one module could be effectively improved by restructuring, but with a penalty of higher resistance. However, the negative effect on the downstream module was much less than expected. The results indicated that the intensity of heat transfer depends not only on the average temperature difference between cold and hot mediums, but also on the temperature distribution.
基金financially supported by the National Natural Science Foundation of China (Grant No. 21776263, No. 51006092, No. 51776190, No. 51476147)the Henan Province Science and Technology Breakthrough Plan of China (Grant No. 182102310022)the Applied Research Plan of Key Scientific Research Projects of Henan Province Higher Education of China (Grant No. 18A470001, No. 17A530006)
文摘Periodic whole cross-section computation models are established for segmental baffle heat exchanger, shutter baffle heat exchanger, and trapezoid-like tilted baffle heat exchanger. The reliability of models is verified by comparing the simulated results to the results obtained from the Bell-Delaware method. Due to the orthogonal assembly of the baffles, the shell side fluid shows the twisty flow of trapezoid-like tilted baffle heat exchanger. The essential mechanism on disturbing flow and heat transfer enhancement is revealed by defining the non-dimensional factor η of the shell side fluid flow direction of heat exchanger and the field synergy principle. The results show that at the same Reynolds number, the shell side fluid convection heat transfer coefficient of trapezoid-like tilted baffle heat exchanger is 12.43%-24.33% and 6.71%-11.51% higher than those of segmental baffle heat exchanger and shutter baffle heat exchanger, respectively. The shell side fluid flow velocity field and the pressure gradient field of trapezoid-like tilted baffle heat exchanger and shutter baffle heat exchanger decreases compared with that of segmental baffle heat exchanger, so the shell side fluid flow resistance and pressure drop is increased; the shell side comprehensive performance of trapezoid-like tilted baffle heat exchanger is 5.85%-9.06% higher than that of segmental baffle heat exchanger, and 15.27%-23.28% higher than that of shutter baffle heat exchanger. In this study, a baffle structure with higher efficiency of the energy utilization for the heat exchanger is provided.
