Visualization experiments were conducted to clarify the operational characteristics of a polymer pulsating heat pipe(PHP).Hydrofluoroether(HFE)-7100 was used as a working fluid,and its filling ratio was 50%of the enti...Visualization experiments were conducted to clarify the operational characteristics of a polymer pulsating heat pipe(PHP).Hydrofluoroether(HFE)-7100 was used as a working fluid,and its filling ratio was 50%of the entire PHP channel.A semi-transparent PHP was fabricated using a transparent polycarbonate sheet and a plastic 3D printer,and the movements of liquid slugs and vapor plugs of the working fluid were captured with a high-speed camera.The video images were then analyzed to obtain the flow patterns in the PHP.The heat transfer characteristics of the PHPwere discussed based on the flowpatterns and temperature distributions obtainedwith thermocouples.Before starting heating,because of high wettability,large liquid slugs positioned at the evaporator section of the PHP.After starting heating,since the occurrence of boiling divided the large liquid slugs,oscillatory flowof smaller liquid slugs and vapor plugs was found in the PHP.Clear circulation flow of liquid slugs and vapor plugs was observed when the power input to the PHP was larger than 12.0 W.The flow patterns and temperature distributions confirmed that the circulation flow enhanced the heat transfer from the evaporator section to the condenser section of the PHP.In the circulation flow mode,large growth and contraction of vapor plugs were found one after another in all even-numbered PHP channels.However,the analysis of flow patterns clarified that the phase-change heat transfer rate by large growth and contraction of vapor plugs was 19%of the total heat transfer rate of the PHP.Although the generation of large vapor plugs was found in the PHP,most of the heat was transferred by the sensible heat of the working fluid.展开更多
Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat pipes.Many studies have been conducted for ultra-thin heat pipes with a centered wick structure,but this study focused on separated ...Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat pipes.Many studies have been conducted for ultra-thin heat pipes with a centered wick structure,but this study focused on separated wick structures to increase the evaporation/condensation surface areas within the heat pipe and to reduce the concentration of heat flux within the wick structure.A mathematical heat-pipe model was made in the threedimensional coordinate system,and the model consisted of three regions:a vapor channel,liquid-wick,and container wall regions.The conservation equations for mass,momentum,and energy were solved numerically with boundary conditions by using a code developed by one of the authors.The numerical results with the separated wick structures were compared with those with the centered,which confirmed the effectiveness of the separation of the wick structure.However,the effectiveness of the separation was affected by the position of the separated wick structure.A simple equation was presented to determine the optimum position of the separated wick structures.Numerical analyses were also conducted when the width of the heat pipe was increased with the cooled section,which clarified that the increase in the cooled-section width with the addition of wick structures wasmore effective than the increase in the cooled-section length.A 44%reduction in the total temperature difference of the heat pipe was obtained under the present numerical conditions.Furthermore,a comparison wasmade between experimental results and numerical results.展开更多
Extended experiments were conducted on the oscillation characteristics of merged liquid slugs in a horizontally oriented polymer pulsating heat pipe(PHP).The PHP’s serpentine channel comprised 14 parallel channels wi...Extended experiments were conducted on the oscillation characteristics of merged liquid slugs in a horizontally oriented polymer pulsating heat pipe(PHP).The PHP’s serpentine channel comprised 14 parallel channels with a width of 1.3 mm and a height of 1.1 mm.The evaporator and condenser sections were 25 and 50 mm long,respectively,and the adiabatic section in between was 75mmlong.Using a plastic 3D printer and semi-transparent filament made from acrylonitrile butadiene styrene,the serpentine channel was printed directly onto a thin polycarbonate sheet to form the PHP.The PHP was charged with hydrofluoroether-7100.In the experiments,the evaporator section was heated,and the condenser section was cooled using high-temperature and low-temperature thermostatic baths,respectively.Flow patterns of the working fluid were obtained with temperature distributions of the PHP.A mathematical model was developed to analyze the flow patterns.Themerged liquid slugs were observed in every two channels,and their oscillation characteristics were found to be approximately the same in time and space.It was also found that the oscillations of the merged liquid slugs became slower,but the heat transfer rate of the PHP increased with a decrease in the filling ratio of the working fluid.This is because vapor condensation was enhanced in vapor plugs as the filling ratio decreased.However,the filling ratio had a lower limit,and the heat transfer rate was maximum when the filling ratio was 40.6%in the present experimental range.展开更多
Centered or striped wick structures have been used to develop ultrathin heat pipes.Differing from traditional heat pipes,the centered or striped wick structures leave noncontact container surfaces with the wick struct...Centered or striped wick structures have been used to develop ultrathin heat pipes.Differing from traditional heat pipes,the centered or striped wick structures leave noncontact container surfaces with the wick structure.In this study,experiments andnumerical analyseswere conductedtoinvestigate the influenceof thesenoncontact surfaces.In the experiments,a strip-shaped wick structure was placed vertically,the top was sandwiched between wider rods and the bottom was immersed in a working fluid.The rod width was greater than the wick width;thus,noncontact surfaces were left between the rod and the wick structure.The heat was applied from the rod to the wick structure,and the evaporation heat transfer characteristics of the working fluid from the wick structure were evaluated.Water was used as the working fluid.The experiments were conducted by varying the rod and wick widths.The experimental results were obtained when the wick structures were placed separately.In the numerical analyses,the temperature and heat flux distributions in the rod were obtained.From the experimental and numerical results,it was confirmed that the noncontact surfaces caused the heat flux in the rod near both surfaces of the wick structure to concentrate,which increased the evaporation thermal resistance of the wick structure.A reduction in the noncontact surface area by increasing the wick width was found to be effective in decreasing the evaporation thermal resistance and increasing themaximumheat transfer rate of the wick structure.The separation of the wick structure increased the evaporation surface area.However,its effectiveness was limited when the heat transfer rate was small.展开更多
An ultra-thin flattened heat pipe has been developed with a centered wick structure.This structure is essential to make the heat pipe thinner.However,the centered wick structure reduces the evaporation and condensatio...An ultra-thin flattened heat pipe has been developed with a centered wick structure.This structure is essential to make the heat pipe thinner.However,the centered wick structure reduces the evaporation and condensation surface areas of the wick structure because it is sandwiched between heat pipe walls.In this study,because detailed discussion has not been made,heat transfer experiments were conducted for the wick structure sandwiched between two solid walls.This study focused on the evaporation heat transfer characteristics from the sandwiched wick structure.The experiments were conducted with three wick structures,that is,strip-shaped sintered copper powders with thicknesses of 0.5,1.0,and 1.5 mm.Water was used as working fluid.The capillary pumping performance,that is,the liquid lifting velocities of the three wick structures were the same.The experimental results of the three wick structures were compared regarding the relation between the evaporation heat transfer rate and the superheat of the working fluid.The heat transfer experiments were also conducted when one of the solid walls was removed from the wick structure.It was confirmed that even if the wick structure was sandwiched between the solid walls,sufficient evaporation of the working fluid occurred from the thin sides of the wick structure.展开更多
基金supported by JSPS KAKENHI Grant Number 22K03947.
文摘Visualization experiments were conducted to clarify the operational characteristics of a polymer pulsating heat pipe(PHP).Hydrofluoroether(HFE)-7100 was used as a working fluid,and its filling ratio was 50%of the entire PHP channel.A semi-transparent PHP was fabricated using a transparent polycarbonate sheet and a plastic 3D printer,and the movements of liquid slugs and vapor plugs of the working fluid were captured with a high-speed camera.The video images were then analyzed to obtain the flow patterns in the PHP.The heat transfer characteristics of the PHPwere discussed based on the flowpatterns and temperature distributions obtainedwith thermocouples.Before starting heating,because of high wettability,large liquid slugs positioned at the evaporator section of the PHP.After starting heating,since the occurrence of boiling divided the large liquid slugs,oscillatory flowof smaller liquid slugs and vapor plugs was found in the PHP.Clear circulation flow of liquid slugs and vapor plugs was observed when the power input to the PHP was larger than 12.0 W.The flow patterns and temperature distributions confirmed that the circulation flow enhanced the heat transfer from the evaporator section to the condenser section of the PHP.In the circulation flow mode,large growth and contraction of vapor plugs were found one after another in all even-numbered PHP channels.However,the analysis of flow patterns clarified that the phase-change heat transfer rate by large growth and contraction of vapor plugs was 19%of the total heat transfer rate of the PHP.Although the generation of large vapor plugs was found in the PHP,most of the heat was transferred by the sensible heat of the working fluid.
文摘Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat pipes.Many studies have been conducted for ultra-thin heat pipes with a centered wick structure,but this study focused on separated wick structures to increase the evaporation/condensation surface areas within the heat pipe and to reduce the concentration of heat flux within the wick structure.A mathematical heat-pipe model was made in the threedimensional coordinate system,and the model consisted of three regions:a vapor channel,liquid-wick,and container wall regions.The conservation equations for mass,momentum,and energy were solved numerically with boundary conditions by using a code developed by one of the authors.The numerical results with the separated wick structures were compared with those with the centered,which confirmed the effectiveness of the separation of the wick structure.However,the effectiveness of the separation was affected by the position of the separated wick structure.A simple equation was presented to determine the optimum position of the separated wick structures.Numerical analyses were also conducted when the width of the heat pipe was increased with the cooled section,which clarified that the increase in the cooled-section width with the addition of wick structures wasmore effective than the increase in the cooled-section length.A 44%reduction in the total temperature difference of the heat pipe was obtained under the present numerical conditions.Furthermore,a comparison wasmade between experimental results and numerical results.
基金supported by JSPS KAKENHI Grant Number 22K03947.
文摘Extended experiments were conducted on the oscillation characteristics of merged liquid slugs in a horizontally oriented polymer pulsating heat pipe(PHP).The PHP’s serpentine channel comprised 14 parallel channels with a width of 1.3 mm and a height of 1.1 mm.The evaporator and condenser sections were 25 and 50 mm long,respectively,and the adiabatic section in between was 75mmlong.Using a plastic 3D printer and semi-transparent filament made from acrylonitrile butadiene styrene,the serpentine channel was printed directly onto a thin polycarbonate sheet to form the PHP.The PHP was charged with hydrofluoroether-7100.In the experiments,the evaporator section was heated,and the condenser section was cooled using high-temperature and low-temperature thermostatic baths,respectively.Flow patterns of the working fluid were obtained with temperature distributions of the PHP.A mathematical model was developed to analyze the flow patterns.Themerged liquid slugs were observed in every two channels,and their oscillation characteristics were found to be approximately the same in time and space.It was also found that the oscillations of the merged liquid slugs became slower,but the heat transfer rate of the PHP increased with a decrease in the filling ratio of the working fluid.This is because vapor condensation was enhanced in vapor plugs as the filling ratio decreased.However,the filling ratio had a lower limit,and the heat transfer rate was maximum when the filling ratio was 40.6%in the present experimental range.
文摘Centered or striped wick structures have been used to develop ultrathin heat pipes.Differing from traditional heat pipes,the centered or striped wick structures leave noncontact container surfaces with the wick structure.In this study,experiments andnumerical analyseswere conductedtoinvestigate the influenceof thesenoncontact surfaces.In the experiments,a strip-shaped wick structure was placed vertically,the top was sandwiched between wider rods and the bottom was immersed in a working fluid.The rod width was greater than the wick width;thus,noncontact surfaces were left between the rod and the wick structure.The heat was applied from the rod to the wick structure,and the evaporation heat transfer characteristics of the working fluid from the wick structure were evaluated.Water was used as the working fluid.The experiments were conducted by varying the rod and wick widths.The experimental results were obtained when the wick structures were placed separately.In the numerical analyses,the temperature and heat flux distributions in the rod were obtained.From the experimental and numerical results,it was confirmed that the noncontact surfaces caused the heat flux in the rod near both surfaces of the wick structure to concentrate,which increased the evaporation thermal resistance of the wick structure.A reduction in the noncontact surface area by increasing the wick width was found to be effective in decreasing the evaporation thermal resistance and increasing themaximumheat transfer rate of the wick structure.The separation of the wick structure increased the evaporation surface area.However,its effectiveness was limited when the heat transfer rate was small.
文摘An ultra-thin flattened heat pipe has been developed with a centered wick structure.This structure is essential to make the heat pipe thinner.However,the centered wick structure reduces the evaporation and condensation surface areas of the wick structure because it is sandwiched between heat pipe walls.In this study,because detailed discussion has not been made,heat transfer experiments were conducted for the wick structure sandwiched between two solid walls.This study focused on the evaporation heat transfer characteristics from the sandwiched wick structure.The experiments were conducted with three wick structures,that is,strip-shaped sintered copper powders with thicknesses of 0.5,1.0,and 1.5 mm.Water was used as working fluid.The capillary pumping performance,that is,the liquid lifting velocities of the three wick structures were the same.The experimental results of the three wick structures were compared regarding the relation between the evaporation heat transfer rate and the superheat of the working fluid.The heat transfer experiments were also conducted when one of the solid walls was removed from the wick structure.It was confirmed that even if the wick structure was sandwiched between the solid walls,sufficient evaporation of the working fluid occurred from the thin sides of the wick structure.
