Oscillating heat pipes (OHPs) are very promising cooling devices. Their heat transfer performance is af- fected by many factors, and the form of the relationship between the performance and the factors is complex and ...Oscillating heat pipes (OHPs) are very promising cooling devices. Their heat transfer performance is af- fected by many factors, and the form of the relationship between the performance and the factors is complex and non-linear. In this paper, the effects of charging ratio, inclination angle, and heat input and their interaction effects on heat transfer performance of a looped copper-water OHP are analyzed. First, suppose that the relationship between the response and the variables approximates a second-order model. And use the central composite design to arrange the ex- periment. Then, the method of least squares is used to estimate the parameters in the second-order model. Finally, multi- variate variance analysis is used to analyze the model. The results show that the assumption is right, that is to say, the re- lationship is well modeled by a second-order function. Among the three main effect variables, the effect of inclination angle is the most significant, but their interaction effects are not significant. In the range of the considered factors, both the optimum charging ratio and the optimum inclination angle increase as the heating water flow rate increases.展开更多
Heat pipe is always bent in the typical application of electronic heat dissipation at high heat flux,which greatly affects its heat transfer performance. The capillary limit of heat transport in the bent micro-grooved...Heat pipe is always bent in the typical application of electronic heat dissipation at high heat flux,which greatly affects its heat transfer performance. The capillary limit of heat transport in the bent micro-grooved heat pipes was analyzed in the vapor pressure drop,the liquid pressure drop and the interaction of the vapor with wick fluid. The bent heat pipes were fabricated and tested from the bending angle,the bending position and the bending radius. The results show that temperature difference and thermal resistance increase while the heat transfer capacity of the heat pipe decreases,with the increase of the bending angles and the bending position closer to the vapor section. However,the effects of bending radius can be ignored. The result agrees well with the predicted equations.展开更多
A mathematical model was developed to predict the maximum heat transfer capacity of high temperature heat pipe with triangular grooved wick. The effects of the inclination angle and geometry structure were considered ...A mathematical model was developed to predict the maximum heat transfer capacity of high temperature heat pipe with triangular grooved wick. The effects of the inclination angle and geometry structure were considered in the proposed model.Maximum heat transfer capacity was also investigated experimentally. The model was validated by comparing with the experimental results. The maximum heat transfer capacity increases with the vapor core radius increasing. Compared with the inclination angle of0°, the maximum heat transfer capacity increases at the larger inclination angle, and the change with temperature is larger. The performance of heat pipe with triangular grooved wick is greatly influenced by gravity, so it is not recommended to be applied to the dish solar heat pipe receiver.展开更多
This paper focuses on the heat transfer performance of semi-open heat pipe which is a new type of heat pipe. After analyzing its condensation heat transfer mechanisms theoretically, several semi-open heat pipes in dif...This paper focuses on the heat transfer performance of semi-open heat pipe which is a new type of heat pipe. After analyzing its condensation heat transfer mechanisms theoretically, several semi-open heat pipes in different length ratios and upper hole diameters are studied experimentally and compared with the same dimensions closed heat pipes. Experimental results show that the heat transfer performance of semi-open heat pipe becomes better by increasing heat transfer rate. At the first transitional point, the heat transfer performance of semi-open heat pipe approaches the level of the closed heat pipe. It is suitable to choose upper small hole about 1 mm in diameter and length ratio larger than 0.6 for the semi-open heat pipe.展开更多
As for the factors affecting the heat transfer performance of complex and nonlinear oscillating heat pipe (OHP),grey relational analysis (GRA) was used to deal with the relationship between heat transfer rate of a loo...As for the factors affecting the heat transfer performance of complex and nonlinear oscillating heat pipe (OHP),grey relational analysis (GRA) was used to deal with the relationship between heat transfer rate of a looped copper-water OHP and charging ratio,inner diameter,inclination angel,heat input,number of turns,and the main influencing factors were defined.Then,forecasting model was obtained by using main influencing factors (such as charging ratio,interior diameter,and inclination angel) as the inputs of function chain neural network.The results show that the relative average error between the predicted and actual value is 4%,which illustrates that the function chain neural network can be applied to predict the performance of OHP accurately.展开更多
This work used experimental methods to study heat transfer behavior inside a heat pipe and found that heat transfer behavior inside the heat pipe was changed due to its integration with cooling plates. This change cau...This work used experimental methods to study heat transfer behavior inside a heat pipe and found that heat transfer behavior inside the heat pipe was changed due to its integration with cooling plates. This change caused the heat pipe to have copper-like heat transfer behavior. Experimental performances first built a CPU simulator with maximum heat power 300 W in accordance with the ASTM standard as heat source and measured temperature distribution by using infrared thermography and thermocouple thermometer. Observation of heat transfer behavior inside heat pipe influenced by its integration with cooling plates used color schlieren technique. A commercial CPU heat pipe cooler was also used as reference object in this work. Integration of the heat pipe with cooling plates causes the heat pipe to have the copper-like heat transfer behavior. The results indicate that rebuilding the bare heat pipe’s heat transfer behavior is the best solution for improving cooling efficiency of the heat pipe cooler.展开更多
This paper performs a simulation study of the heat transfer phenomena in a tubular U-loop pipe. We have investigated the enhancement of heat transfer with mass flow in a pipe without insert, with full length and short...This paper performs a simulation study of the heat transfer phenomena in a tubular U-loop pipe. We have investigated the enhancement of heat transfer with mass flow in a pipe without insert, with full length and short length twisted tape inserts. The length of the pipe is approximately 2436.80 mm long with 29 mm inner and 33 mm outer diameter respectively. A constant heat flux is taken which generated the boundary layer of the pipe close to the flowing fluid around the boundary. The simulations are considered for the stationary and the time dependent module for 35 seconds with different length of inserts. The comparisons are made among the results. We observed that the transfer of heat is enhanced significantly with the increase of the length of inserts inside the computational domain. We also found that, full length twisted tape inserts are more effective than comparing with the short length inserts and without insert.展开更多
A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance.The effect of heat load on the axial di...A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance.The effect of heat load on the axial distribution of capillary radius,and the effect of working temperature and wick structure on the maximum heat transfer capability,as well as the effect of the heat load and working temperature on the total thermal resistance are all investigated and discussed.It is indicated that the meniscus radius increases non-linearly and slowly at the evaporator and adiabatic section along the axial direction,while increasing drastically at the beginning of the condenser section.The pressure difference in the vapor phase along the axial direction is much smaller than that in the liquid phase.In addition,the heat transfer capacity is deeply affected by the working temperature and the size of the wick.A groove wick structure with a wider groove base width and higher groove depth can enhance the heat transfer capability.The effect of the working temperature on the total thermal resistance is insignificant;however,the total thermal resistance shows dependence upon the heat load.In addition,the accuracy of the model is also verified by the experiment in this paper.展开更多
In order to develop further the application of high temperature heat pipe in hypersonic vehicles thermal protection,the principles and characteristics of high temperature heat pipe used in hypersonic vehicles thermal ...In order to develop further the application of high temperature heat pipe in hypersonic vehicles thermal protection,the principles and characteristics of high temperature heat pipe used in hypersonic vehicles thermal protection were introduced.The methods of numerical simulation,theory analysis and experiment research were utilized to analyze the frozen start-up and steady state characteristic of the heat pipe as well as the machining improvement for fabricating irregularly shaped heat pipe which is suitable for leading edge of hypersonic vehicles.The results indicate that the frozen start-up time of heat pipe is long(10 min) and there exists large temperature difference along the heat pipe(47 °C/cm),but the heat pipe can reduce the temperature in stagnation area of hypersonic vehicles from 1 926 to 982 °C and work normally during 1 000-1 200°C.How to improve the maximum heat transfer capability and reduce the time needed for start-up from frozen state of the heat pipe by optimizing thermostructure such as designing of a novel wick with high performance is the key point in hypersonic vehicles thermal protection of heat pipe.展开更多
Some novel grooved-sintered composite wick heat pipes(GSHP) were developed for the electronic device cooling.The grooved-sintered wicks of GSHP were fabricated by the processes of oil-filled high-speed spin forming an...Some novel grooved-sintered composite wick heat pipes(GSHP) were developed for the electronic device cooling.The grooved-sintered wicks of GSHP were fabricated by the processes of oil-filled high-speed spin forming and solid state sintering.The wick could be divided into two parts for liquid capillary pumping flow:groove sintered zone and uniform sintered zone.Both of the thermal resistance network model and the maximum heat transfer capability model of GSHP were built.Compared with the theoretical values,the heat transfer limit and thermal resistance of GSHP were measured from three aspects:copper powder size,wick thickness and number of micro grooves.The results show that the wick thickness has the greatest effect on the thermal resistance of GSHP while the copper powder size has the most important influence on the heat transfer limit.Given certain copper powder size and wick thickness,the thermal resistance of GSHP can be the lowest when micro-groove number is about 55.展开更多
This paper presents an experimental study on a flat plate closed loop pulsating heat pipes. It consisted of total 40 channels with square cross section(2×2 mm2, 165 mm long) machined directly on an aluminum plate...This paper presents an experimental study on a flat plate closed loop pulsating heat pipes. It consisted of total 40 channels with square cross section(2×2 mm2, 165 mm long) machined directly on an aluminum plate(180×120×3 mm3), which was covered by a transparent plate. The working fluid employed was ethanol. As the results, the influence parameters of thermal performance were investigated, such as filling ratio, heat load and operational orientations etc. Filling ratio was found to be a critical parameter, and its effect was rather complicated. According to its values the PHP plate could have four distinct working zones with different operational characteristics and heat transfer performance. The effect of heat load on thermal performance was found to be positive, and in general, increasing the heat load would improve heat transfer performance. In order to analyze the effect of gravity on thermal performance, three different heat modes and total seven tilt angles were tested and compared. Successful operation at all orientations with respect to gravity was also achieved.展开更多
Four kinds of micro heat pipe of trapezoidal groove wick structure with different numbers of grooves or aspect ratios were studied and compared about thermal transfer performances in order to optimize the manufacture ...Four kinds of micro heat pipe of trapezoidal groove wick structure with different numbers of grooves or aspect ratios were studied and compared about thermal transfer performances in order to optimize the manufacture of micro heat pipe with groove wick structure. The results show that these micro heat pipes have excellent performance in heat transfer; the equivalent thermal conductivity coefficient is two orders of magnitude compared with that of copper; the number and aspect ratio of grooves have a prominent effect on the performance of such thermal transfer. The optimum number of grooves is lower than 60 and the best aspect ratio is near to 1.5. The temperature and thermal transport rate are almost directly proportional relationship, but this relationship will be broken up suddenly when the critical heat flux is reached.展开更多
Using ethanol or acetone as the working fluid, visualization of oscillations in steady state was observed visually by high-speed cameras, and temperature oscillating and heat transfer characteristics of closed-loop pl...Using ethanol or acetone as the working fluid, visualization of oscillations in steady state was observed visually by high-speed cameras, and temperature oscillating and heat transfer characteristics of closed-loop plate oscillating heat pipe with parallel channels(POHP-PC) were experimentally investigated by varying liquid filled ratios(50%, 70%, 85%), section scales(1 mm×1 mm and 1 mm×1.5 mm), inclination angles, working fluids and heating inputs. It was found that during operating there was mixed flow consisting of plug flow and annular flow in channels of oscillating heat pipe at steady-state. There was an equilibrium position for working fluid of condenser during oscillating, and periodic oscillations occurred up and down in the vicinity of equilibrium position. With heat input increasing, equilibrium position rose slowly as a result of vapor pressure of evaporation.Evaporation temperature oscillating amplitude possessed a trend of small-large-small and frequency trend was of small-large during steady-state. It may be generally concluded that temperature, whether evaporator or condenser, fluctuated sharply or rose continuously when oscillating heat pipe coming to dry burning state. Simultaneously, it was found that temperature difference of cooling water possibly dropped with heat input rising during dry burning state. Thermal resistance of No. 2 with acetone was lower than that of No. 1 during experiments, but No. 2 achieving heat transfer limit was earlier than No. 1. However, with ethanol, thermal resistance of No. 1 and No. 2 were similar with the heating input less than 110-120 W and filling ratios of 50% and 70%. And with filling ratio of 85%, heating transfer performance of No. 2 was better compared to No. 1 during all the experiments.展开更多
The effect of working fluid on the start-up and thermal performance in terms of thermal resistance and heat transfer coefficient of a pulsating heat pipe have been studied in the present paper. Methanol and de-ionized...The effect of working fluid on the start-up and thermal performance in terms of thermal resistance and heat transfer coefficient of a pulsating heat pipe have been studied in the present paper. Methanol and de-ionized water has been selected as the working fluid. The minimum startup power for DI water was obtained at 50% filling ratio and for methanol at 40%. The optimum filling ratio in terms of minimum startup power and minimum thermal resistance was 50% for DI water and 40% for methanol. The minimum thermal resistances for DI water and methanol were observed at vertical orientation. The evaporator side heat transfer coefficient for water was slightly more, while the condenser side heat transfer coefficient was appreciably more than that of methanol. Studies were also conducted for start-up time and temperature at different orientations and it was found that the PHP charged with methanol worked efficiently at all orientations.展开更多
Heat transfer of a capillary evaporator in a loop heat pipe was analyzed through 3D numerical simulations to study the effects of the thermal conductivity of the wick, the contact area between the casing and the wick,...Heat transfer of a capillary evaporator in a loop heat pipe was analyzed through 3D numerical simulations to study the effects of the thermal conductivity of the wick, the contact area between the casing and the wick, and the subcooling in the compensation chamber (CC) on the thermal performance of the evaporator. A pore network model with a distribution of pore radii was used to simulate liquid flow in the porous structure of the wick. To obtain high accuracy, fine meshes were used at the boundaries among the casing, the wick, and the grooves. Distributions of temperature, pressure, and mass flow rate were compared for polytetra-fluoroethylene (PTFE) and stainless steel wicks. The thermal conductivity of the wick and the contact area between the casing and the wick significantly impacted thermal performance of the evaporator heat-transfer coefficient and the heat leak to the CC. The 3D analysis provided highly accurate values for the heat leak;in some cases, the heat leaks of PTFE and stainless steel wicks showed little differences. In general, the heat flux is concentrated at the boundaries between the casing, the wick, and the grooves;therefore, thermal performance can be optimized by increasing the length of the boundary.展开更多
Multiple loop heat pipes which have two evaporators and two condensers in one loop are a kind of active heat transfer device. Since they have two evaporators and two condensers, the operating mode also becomes multipl...Multiple loop heat pipes which have two evaporators and two condensers in one loop are a kind of active heat transfer device. Since they have two evaporators and two condensers, the operating mode also becomes multiple. This work discusses the cases that multiple loop heat pipes were operated with one condenser at high temperature and the other at low temperature. To avoid the high temperature returning liquid and keep the multiple loop heat pipes work properly, the flow regulator which was made of polyethylene was designed, fabricated and applied in this test. The effect of flow regulator was confirmed and analyzed. In the test that large temperature difference existed between two sinks, it can be found according to the result that the flow regulator worked effectively and prevented the high temperature vapor to enter the inlet of common liquid line, which can keep the evaporators and returning liquid to operate at low temperature. With the increment of heat loads and the temperature difference between two sinks, the pressure difference between two condensers became larger and larger. When the pressure difference was larger than the flow regulator’s capillary force, the flow regulator could not work properly because the high temperature vapor began to flow through the flow regulator. According to the test data, the flow regulator can work properly within the sinks’ temperature 0°C/60°C and the two evaporators’ heat load 30/30 W.展开更多
基金Supported by the Natural Science Foundation of Ministry of Education of Jiangsu Province (02KJB470001).
文摘Oscillating heat pipes (OHPs) are very promising cooling devices. Their heat transfer performance is af- fected by many factors, and the form of the relationship between the performance and the factors is complex and non-linear. In this paper, the effects of charging ratio, inclination angle, and heat input and their interaction effects on heat transfer performance of a looped copper-water OHP are analyzed. First, suppose that the relationship between the response and the variables approximates a second-order model. And use the central composite design to arrange the ex- periment. Then, the method of least squares is used to estimate the parameters in the second-order model. Finally, multi- variate variance analysis is used to analyze the model. The results show that the assumption is right, that is to say, the re- lationship is well modeled by a second-order function. Among the three main effect variables, the effect of inclination angle is the most significant, but their interaction effects are not significant. In the range of the considered factors, both the optimum charging ratio and the optimum inclination angle increase as the heating water flow rate increases.
基金Project(U0834002) supported by the Joint Funds of the National Nature Science Foundation of China and Guangdong ProvinceProject (2009ZM0134) supported by the Foundational Research Funds for the Central Universities in China
文摘Heat pipe is always bent in the typical application of electronic heat dissipation at high heat flux,which greatly affects its heat transfer performance. The capillary limit of heat transport in the bent micro-grooved heat pipes was analyzed in the vapor pressure drop,the liquid pressure drop and the interaction of the vapor with wick fluid. The bent heat pipes were fabricated and tested from the bending angle,the bending position and the bending radius. The results show that temperature difference and thermal resistance increase while the heat transfer capacity of the heat pipe decreases,with the increase of the bending angles and the bending position closer to the vapor section. However,the effects of bending radius can be ignored. The result agrees well with the predicted equations.
基金Project(51076062)supported by the National Natural Science Foundation of China
文摘A mathematical model was developed to predict the maximum heat transfer capacity of high temperature heat pipe with triangular grooved wick. The effects of the inclination angle and geometry structure were considered in the proposed model.Maximum heat transfer capacity was also investigated experimentally. The model was validated by comparing with the experimental results. The maximum heat transfer capacity increases with the vapor core radius increasing. Compared with the inclination angle of0°, the maximum heat transfer capacity increases at the larger inclination angle, and the change with temperature is larger. The performance of heat pipe with triangular grooved wick is greatly influenced by gravity, so it is not recommended to be applied to the dish solar heat pipe receiver.
文摘This paper focuses on the heat transfer performance of semi-open heat pipe which is a new type of heat pipe. After analyzing its condensation heat transfer mechanisms theoretically, several semi-open heat pipes in different length ratios and upper hole diameters are studied experimentally and compared with the same dimensions closed heat pipes. Experimental results show that the heat transfer performance of semi-open heat pipe becomes better by increasing heat transfer rate. At the first transitional point, the heat transfer performance of semi-open heat pipe approaches the level of the closed heat pipe. It is suitable to choose upper small hole about 1 mm in diameter and length ratio larger than 0.6 for the semi-open heat pipe.
基金Project(531107040300) supported by the Fundamental Research Funds for the Central Universities in ChinaProject(2006BAJ04B04) supported by the National Science and Technology Pillar Program during the Eleventh Five-year Plan Period of China
文摘As for the factors affecting the heat transfer performance of complex and nonlinear oscillating heat pipe (OHP),grey relational analysis (GRA) was used to deal with the relationship between heat transfer rate of a looped copper-water OHP and charging ratio,inner diameter,inclination angel,heat input,number of turns,and the main influencing factors were defined.Then,forecasting model was obtained by using main influencing factors (such as charging ratio,interior diameter,and inclination angel) as the inputs of function chain neural network.The results show that the relative average error between the predicted and actual value is 4%,which illustrates that the function chain neural network can be applied to predict the performance of OHP accurately.
文摘This work used experimental methods to study heat transfer behavior inside a heat pipe and found that heat transfer behavior inside the heat pipe was changed due to its integration with cooling plates. This change caused the heat pipe to have copper-like heat transfer behavior. Experimental performances first built a CPU simulator with maximum heat power 300 W in accordance with the ASTM standard as heat source and measured temperature distribution by using infrared thermography and thermocouple thermometer. Observation of heat transfer behavior inside heat pipe influenced by its integration with cooling plates used color schlieren technique. A commercial CPU heat pipe cooler was also used as reference object in this work. Integration of the heat pipe with cooling plates causes the heat pipe to have the copper-like heat transfer behavior. The results indicate that rebuilding the bare heat pipe’s heat transfer behavior is the best solution for improving cooling efficiency of the heat pipe cooler.
文摘This paper performs a simulation study of the heat transfer phenomena in a tubular U-loop pipe. We have investigated the enhancement of heat transfer with mass flow in a pipe without insert, with full length and short length twisted tape inserts. The length of the pipe is approximately 2436.80 mm long with 29 mm inner and 33 mm outer diameter respectively. A constant heat flux is taken which generated the boundary layer of the pipe close to the flowing fluid around the boundary. The simulations are considered for the stationary and the time dependent module for 35 seconds with different length of inserts. The comparisons are made among the results. We observed that the transfer of heat is enhanced significantly with the increase of the length of inserts inside the computational domain. We also found that, full length twisted tape inserts are more effective than comparing with the short length inserts and without insert.
基金Supported by the 11th Five Year National Science and Technology Support Key Project of China(2008BAJ12B02)
文摘A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance.The effect of heat load on the axial distribution of capillary radius,and the effect of working temperature and wick structure on the maximum heat transfer capability,as well as the effect of the heat load and working temperature on the total thermal resistance are all investigated and discussed.It is indicated that the meniscus radius increases non-linearly and slowly at the evaporator and adiabatic section along the axial direction,while increasing drastically at the beginning of the condenser section.The pressure difference in the vapor phase along the axial direction is much smaller than that in the liquid phase.In addition,the heat transfer capacity is deeply affected by the working temperature and the size of the wick.A groove wick structure with a wider groove base width and higher groove depth can enhance the heat transfer capability.The effect of the working temperature on the total thermal resistance is insignificant;however,the total thermal resistance shows dependence upon the heat load.In addition,the accuracy of the model is also verified by the experiment in this paper.
基金Project(51076062) supported by the National Natural Science Foundation of China
文摘In order to develop further the application of high temperature heat pipe in hypersonic vehicles thermal protection,the principles and characteristics of high temperature heat pipe used in hypersonic vehicles thermal protection were introduced.The methods of numerical simulation,theory analysis and experiment research were utilized to analyze the frozen start-up and steady state characteristic of the heat pipe as well as the machining improvement for fabricating irregularly shaped heat pipe which is suitable for leading edge of hypersonic vehicles.The results indicate that the frozen start-up time of heat pipe is long(10 min) and there exists large temperature difference along the heat pipe(47 °C/cm),but the heat pipe can reduce the temperature in stagnation area of hypersonic vehicles from 1 926 to 982 °C and work normally during 1 000-1 200°C.How to improve the maximum heat transfer capability and reduce the time needed for start-up from frozen state of the heat pipe by optimizing thermostructure such as designing of a novel wick with high performance is the key point in hypersonic vehicles thermal protection of heat pipe.
基金Project(51205423)supported by the National Natural Science Foundation of ChinaProject(2012M510205)supported by China Postdoctoral Science Foundation+1 种基金Project(S2012040007715)supported by Natural Science Foundation of Guangdong Province,ChinaProject(20120171120036)supported by New Teachers’Fund for Doctor Stations,Ministry of Education,China
文摘Some novel grooved-sintered composite wick heat pipes(GSHP) were developed for the electronic device cooling.The grooved-sintered wicks of GSHP were fabricated by the processes of oil-filled high-speed spin forming and solid state sintering.The wick could be divided into two parts for liquid capillary pumping flow:groove sintered zone and uniform sintered zone.Both of the thermal resistance network model and the maximum heat transfer capability model of GSHP were built.Compared with the theoretical values,the heat transfer limit and thermal resistance of GSHP were measured from three aspects:copper powder size,wick thickness and number of micro grooves.The results show that the wick thickness has the greatest effect on the thermal resistance of GSHP while the copper powder size has the most important influence on the heat transfer limit.Given certain copper powder size and wick thickness,the thermal resistance of GSHP can be the lowest when micro-groove number is about 55.
基金Supported by the Ger man National Science Foundation (DFG)(No. GR412/33)
文摘This paper presents an experimental study on a flat plate closed loop pulsating heat pipes. It consisted of total 40 channels with square cross section(2×2 mm2, 165 mm long) machined directly on an aluminum plate(180×120×3 mm3), which was covered by a transparent plate. The working fluid employed was ethanol. As the results, the influence parameters of thermal performance were investigated, such as filling ratio, heat load and operational orientations etc. Filling ratio was found to be a critical parameter, and its effect was rather complicated. According to its values the PHP plate could have four distinct working zones with different operational characteristics and heat transfer performance. The effect of heat load on thermal performance was found to be positive, and in general, increasing the heat load would improve heat transfer performance. In order to analyze the effect of gravity on thermal performance, three different heat modes and total seven tilt angles were tested and compared. Successful operation at all orientations with respect to gravity was also achieved.
基金Projects(50605023 50436010) supported by the National Natural Science Foundation of China
文摘Four kinds of micro heat pipe of trapezoidal groove wick structure with different numbers of grooves or aspect ratios were studied and compared about thermal transfer performances in order to optimize the manufacture of micro heat pipe with groove wick structure. The results show that these micro heat pipes have excellent performance in heat transfer; the equivalent thermal conductivity coefficient is two orders of magnitude compared with that of copper; the number and aspect ratio of grooves have a prominent effect on the performance of such thermal transfer. The optimum number of grooves is lower than 60 and the best aspect ratio is near to 1.5. The temperature and thermal transport rate are almost directly proportional relationship, but this relationship will be broken up suddenly when the critical heat flux is reached.
基金Project(51306198)supported by the National Natural Science Foundation of ChinaProject(NR2013K07)supported by Beijing Key Lab of Heating,Gas Supply,Ventilating and Air Conditioning Engineering,China+1 种基金Project(331614013)supported by Beijing University of Civil Engineering and Architecture,ChinaProject(00921915023)supported by Organization Department of Beijing,China
文摘Using ethanol or acetone as the working fluid, visualization of oscillations in steady state was observed visually by high-speed cameras, and temperature oscillating and heat transfer characteristics of closed-loop plate oscillating heat pipe with parallel channels(POHP-PC) were experimentally investigated by varying liquid filled ratios(50%, 70%, 85%), section scales(1 mm×1 mm and 1 mm×1.5 mm), inclination angles, working fluids and heating inputs. It was found that during operating there was mixed flow consisting of plug flow and annular flow in channels of oscillating heat pipe at steady-state. There was an equilibrium position for working fluid of condenser during oscillating, and periodic oscillations occurred up and down in the vicinity of equilibrium position. With heat input increasing, equilibrium position rose slowly as a result of vapor pressure of evaporation.Evaporation temperature oscillating amplitude possessed a trend of small-large-small and frequency trend was of small-large during steady-state. It may be generally concluded that temperature, whether evaporator or condenser, fluctuated sharply or rose continuously when oscillating heat pipe coming to dry burning state. Simultaneously, it was found that temperature difference of cooling water possibly dropped with heat input rising during dry burning state. Thermal resistance of No. 2 with acetone was lower than that of No. 1 during experiments, but No. 2 achieving heat transfer limit was earlier than No. 1. However, with ethanol, thermal resistance of No. 1 and No. 2 were similar with the heating input less than 110-120 W and filling ratios of 50% and 70%. And with filling ratio of 85%, heating transfer performance of No. 2 was better compared to No. 1 during all the experiments.
文摘The effect of working fluid on the start-up and thermal performance in terms of thermal resistance and heat transfer coefficient of a pulsating heat pipe have been studied in the present paper. Methanol and de-ionized water has been selected as the working fluid. The minimum startup power for DI water was obtained at 50% filling ratio and for methanol at 40%. The optimum filling ratio in terms of minimum startup power and minimum thermal resistance was 50% for DI water and 40% for methanol. The minimum thermal resistances for DI water and methanol were observed at vertical orientation. The evaporator side heat transfer coefficient for water was slightly more, while the condenser side heat transfer coefficient was appreciably more than that of methanol. Studies were also conducted for start-up time and temperature at different orientations and it was found that the PHP charged with methanol worked efficiently at all orientations.
文摘Heat transfer of a capillary evaporator in a loop heat pipe was analyzed through 3D numerical simulations to study the effects of the thermal conductivity of the wick, the contact area between the casing and the wick, and the subcooling in the compensation chamber (CC) on the thermal performance of the evaporator. A pore network model with a distribution of pore radii was used to simulate liquid flow in the porous structure of the wick. To obtain high accuracy, fine meshes were used at the boundaries among the casing, the wick, and the grooves. Distributions of temperature, pressure, and mass flow rate were compared for polytetra-fluoroethylene (PTFE) and stainless steel wicks. The thermal conductivity of the wick and the contact area between the casing and the wick significantly impacted thermal performance of the evaporator heat-transfer coefficient and the heat leak to the CC. The 3D analysis provided highly accurate values for the heat leak;in some cases, the heat leaks of PTFE and stainless steel wicks showed little differences. In general, the heat flux is concentrated at the boundaries between the casing, the wick, and the grooves;therefore, thermal performance can be optimized by increasing the length of the boundary.
文摘Multiple loop heat pipes which have two evaporators and two condensers in one loop are a kind of active heat transfer device. Since they have two evaporators and two condensers, the operating mode also becomes multiple. This work discusses the cases that multiple loop heat pipes were operated with one condenser at high temperature and the other at low temperature. To avoid the high temperature returning liquid and keep the multiple loop heat pipes work properly, the flow regulator which was made of polyethylene was designed, fabricated and applied in this test. The effect of flow regulator was confirmed and analyzed. In the test that large temperature difference existed between two sinks, it can be found according to the result that the flow regulator worked effectively and prevented the high temperature vapor to enter the inlet of common liquid line, which can keep the evaporators and returning liquid to operate at low temperature. With the increment of heat loads and the temperature difference between two sinks, the pressure difference between two condensers became larger and larger. When the pressure difference was larger than the flow regulator’s capillary force, the flow regulator could not work properly because the high temperature vapor began to flow through the flow regulator. According to the test data, the flow regulator can work properly within the sinks’ temperature 0°C/60°C and the two evaporators’ heat load 30/30 W.