Cavitation is a destructive phenomenon in control valves.In order to delay cavitation,a multi-series of perforated cylindrical plates,called trims,are used.Previously,the effects of orifice diameter and different type...Cavitation is a destructive phenomenon in control valves.In order to delay cavitation,a multi-series of perforated cylindrical plates,called trims,are used.Previously,the effects of orifice diameter and different types of trims have been investigated.In this study,by numerical analysis,a globe control valve was investigated by employing four different cases(without trim,with one trim,with two and three trims)and the impact of the number of these trims on the intensity,formation region and the initiation point of cavitation was analyzed.It was found that the addition of one stage or two stages of trims reduces the intensity and delays the onset of cavitation,relative to the valve without trim.However,no significant differences in terms of intensity and initiation point of cavitation were observed in the cases where two or three trims were used.Therefore,due to the high cost of producing the trims,and the severe drop in flow coefficient,it is not economically and technically justified to increase the number of trims to more than three.展开更多
In this paper,the performance of a solar thermal system with a focus on space heating was investigated.A 70 m^(2) detached house was considered in the weather conditions of the city of Tehran,Iran.A thermosyphon solar...In this paper,the performance of a solar thermal system with a focus on space heating was investigated.A 70 m^(2) detached house was considered in the weather conditions of the city of Tehran,Iran.A thermosyphon solar water heater with a flat plate collector combined with an auxiliary electrical heater supplies the heating demand of the house.The proposed system was modeled and analyzed using TRNSYS software.In this regard,the TRNBuild module was employed for the building load calculation.The model has been simulated for one year of operation.The effects of the solar collector’s surface area and storage volume were assessed.The results show that for a solar collector with a 15 m^(2) surface area,the solar fraction is 0.29 in January,during which the solar radiation is the lowest.Using solar collectors of10 m^(2) and 5 m^(2) surface areas,the solar fraction falls to 0.23 and 0.14,respectively in January.Besides,two cases of 150 L and 300 L storage tanks are taken into account.Eventually,it is found that using a 15 m^(2) solar collector and a 150 L storage tank can appropriately provide the building’ s heating demand taking the thermal performance and economic aspects into consideration.展开更多
Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern th...Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern the location of cavitation erosion before collapse.Thus,this study introduces a new numerical approach based on the improved delayed detached eddy simulation(IDDES)turbulence modeling for predicting cavitating flows.Then,the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows.The method for cavitation erosion prediction,a modified version taken from previous studies,was applied as a post-processing tool.The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study.The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well,including shedding processes and the length,shape,and collapse of cavitating structures.Using the numerical analysis,three main stages were detected for the present cavitating flow,and the vorticity-cavitation interactions were investigated by the vorticity transport equation.The streak-like and tube-like cavitating(STLIC and TULIC)structures were introduced in the second stage,initiated by flow instability,and entirely governed by corresponding turbulent flow structures.The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls.The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle.Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.展开更多
文摘Cavitation is a destructive phenomenon in control valves.In order to delay cavitation,a multi-series of perforated cylindrical plates,called trims,are used.Previously,the effects of orifice diameter and different types of trims have been investigated.In this study,by numerical analysis,a globe control valve was investigated by employing four different cases(without trim,with one trim,with two and three trims)and the impact of the number of these trims on the intensity,formation region and the initiation point of cavitation was analyzed.It was found that the addition of one stage or two stages of trims reduces the intensity and delays the onset of cavitation,relative to the valve without trim.However,no significant differences in terms of intensity and initiation point of cavitation were observed in the cases where two or three trims were used.Therefore,due to the high cost of producing the trims,and the severe drop in flow coefficient,it is not economically and technically justified to increase the number of trims to more than three.
文摘In this paper,the performance of a solar thermal system with a focus on space heating was investigated.A 70 m^(2) detached house was considered in the weather conditions of the city of Tehran,Iran.A thermosyphon solar water heater with a flat plate collector combined with an auxiliary electrical heater supplies the heating demand of the house.The proposed system was modeled and analyzed using TRNSYS software.In this regard,the TRNBuild module was employed for the building load calculation.The model has been simulated for one year of operation.The effects of the solar collector’s surface area and storage volume were assessed.The results show that for a solar collector with a 15 m^(2) surface area,the solar fraction is 0.29 in January,during which the solar radiation is the lowest.Using solar collectors of10 m^(2) and 5 m^(2) surface areas,the solar fraction falls to 0.23 and 0.14,respectively in January.Besides,two cases of 150 L and 300 L storage tanks are taken into account.Eventually,it is found that using a 15 m^(2) solar collector and a 150 L storage tank can appropriately provide the building’ s heating demand taking the thermal performance and economic aspects into consideration.
文摘Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries.Detached vortical flows normally generate heaps of cavitating flow structures,which,in turn,govern the location of cavitation erosion before collapse.Thus,this study introduces a new numerical approach based on the improved delayed detached eddy simulation(IDDES)turbulence modeling for predicting cavitating flows.Then,the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows.The method for cavitation erosion prediction,a modified version taken from previous studies,was applied as a post-processing tool.The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study.The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well,including shedding processes and the length,shape,and collapse of cavitating structures.Using the numerical analysis,three main stages were detected for the present cavitating flow,and the vorticity-cavitation interactions were investigated by the vorticity transport equation.The streak-like and tube-like cavitating(STLIC and TULIC)structures were introduced in the second stage,initiated by flow instability,and entirely governed by corresponding turbulent flow structures.The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls.The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle.Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.
