Study On The Effects Of Heat Transfer Distance On The Operating Characteristics Of Pulsating Heat Pipe With Different Working Fluids

With the rapid development of the semiconductor industry,the power density and the integration level of the electronic devices are getting higher and higher,which results in the sharp increase in the heat flux of electronic devices.It is well known that semiconductor devices can be damaged by excessive temperature,and thus,efficient heat transfer devices are required to dissipate the high heat flux generated by the electronic devices.Pulsating heat pipe（PHP）is a passive radiator with good heat transfer performance,and its structure is relatively simple and compact.At present,the application fields of PHPs are very wide,ranging from radiator for a small chip to heat exchanger for a large system.In different applications,PHPs with various heat transfer distances are required according to the arrangement of heat sources and heat sinks.Therefore,the heat transfer distance（adiabatic section length）of the PHP is an important parameter.However,there is still no definite conclusion about whether there is an optimal heat transfer distance to make the heat transfer performance of the PHP reach the best or whether there is a limit heat transfer distance that makes the PHP fail to start or the heat transfer performance deteriorates sharply.In addition,the thermos-physical properties of the working fluid are also an important factor affecting the performance of the PHP,and there is no relevant research on how to select an appropriate working fluid under different heat transfer distances.Based on the above research background,water,in this work,water,R1336mzz（Z）and ethanol as selected as representative working fluids,and a large number of theoretical and experimental studies have been carried out to investigate the influence of heat transfer distance on the operating characteristics of PHP:（1）Theoretical research contents and conclusionsIn the existing theoretical models of PHPs,the thickness of the liquid film is assumed to be constant,which makes that the theoretical model unable to simulate the problem of local dry-out of the PHP.In this work,the calculation on the dynamical variation of liquid film thickness with phase change process is included,and a transient theoretical model that can simulate local dry-out is established to predict the performance of PHP.Based on this theoretical model,the start-up and heat transfer processes of PHP with water,R1336mzz（Z）and ethanol as working fluids at heat transfer distances（130 mm,500 mm,1000 mm and 1500 mm）were numerically simulated.The main conclusions are as follows:1)the heat transfer distance has similar impact on the start-up,flow and heat transfer performance of the PHP with water,R1336mzz（Z）and ethanol:a)as the heat transfer distance increased,the start-up performance of PHP was dramatically deteriorated.In particular,when the heat transfer distance was large,the liquid film in the evaporation section was easily to be dried-out before the PHP started up,which resulted in rather high peak evaporation temperature before stable operation(abbreviated as T_s,max);b)as the heat transfer distance increased,the flow characteristics of working fluids at low heat input were extremely worsened.The oscillating frequency of working fluids decreased,and the stagnation time became longer at low heat input.c)as the heat transfer distance increased,the heat transfer performance of the PHP was dramatically worsened at low heat input,while the heat transfer performance of the PHP under stable operation was quite similar under high heat input.2)Although the influences of heat transfer distance on the operating characteristics of PHPs charged with different working fluids has similar trends,the degree of influence of heat transfer distance was different.As the heat transfer distance increased,the deterioration of operating characteristics of water-PHP was most significant,while the operating characteristics of R1336mzz（Z）-PHP was relatively least affected by heat transfer distance.For example,at heat input of 20 W,as the heat transfer distance increased from 130 mm to 1500 mm,the increments of T_s,max values for water,ethanol and R1336mzz（Z）-PHP were 45.88°C,28.3°C and 7.7°C,respectively,and the relative increments of thermal resistances for water,ethanol and R1336mzz（Z）-PHP were 99.6%,34.1%and 23.3%,respectively.（2）Experimental research contents and conclusionsAn experimental apparatus with detachable adiabatic section and partial visualization was built.The heat transfer distance can be flexibly changed through the detachable design,and the flow pattern variations of working fluids can be observed and studied through the partial visualization window.The start-up,flow and heat transfer characteristics of the PHP with water,R1336mzz（Z）and ethanol with different filling ratios were experimentally tested under different heat transfer distances（130 mm,500 mm,1000 mm and 1500 mm）.1)The experimental results show that at same filling ratio,the effects of heat transfer distance on the start-up,flow and heat transfer characteristics of PHPs with different working fluids have similar laws:a)The start-up performance was deteriorated with the increase of PHP,that is,the start-up time of the PHP increases and T_s,maxincreases significantly;b)With the increase of the heat transfer distance,the flow characteristics of the working fluids of the PHP degraded.Higher heat input was needed to maintain the continuous oscillation of working fluids.c)With the increase of the heat transfer distance,the thermal resistance of the PHP increased significantly under low heat flow,while the differences between the thermal resistances of PHPs under different heat transfer distances were relatively small2)According to the experimental results,it is found that the heat transfer distance has the greatest influence on the start-up,flow and heat transfer of water-PHP,followed by ethanol-PHP,and has the least influence on the R1336mzz（Z）-PHP.a)With the increase of heat transfer distance,the increment of T_s,max values was highest for water-PHP.In particular,when the heat transfer distance was 1500 mm,T_s,max values of water-PHP could be even higher than 200°C,which resulted that PHP failed to start up normally at some working conditions.The ethanol-PHP and R1336mzz（Z）-PHP had smaller increment of T_s,max values,and they started up successfully in the experimental range;b)With the increase of heat transfer distance,the heat input required to maintain the continuous oscillation flow in the water-PHP increases most significantly,followed by the ethanol-PHP,and the R1336mzz（Z）-PHP was least affected.c)With the increase of the heat transfer distance,the thermal resistance of the water-PHP increases most significantly.When the heat transfer distance increases to 1500 mm,the heat transfer performance of the water-PHP deteriorated dramatically.（3）Comparison on theoretical simulation and experimental resultsThe theoretical simulation and experimental results of T_s,max values and the thermal resistances for the PHP with different working fluids（water,R1336mzz（Z）and ethanol）,different heat transfer distance（130～1500 mm）and different heat inputs（20～100 W）were compared(T_s,maxand thermal resistance are important parameters which represent the start-up and heat transfer performance of PHP,respectively).Through the comparison between the theoretical simulation and experimental results,within the investigation range,the relative deviation of thermal resistance between the simulation results and the experimental results was within-30%～+10%,which meant that the simulation results are in good agreement with the experimental results.The theoretical model could provide a very valuable reference for the prediction of the start-up and heat transfer performance of PHPs with different working fluids at different heat transfer distances.