Visualization Of Flat-Plate Pulsating Heat Pipe And Analysis Of Factors Affecting Its Heat Transfer Characteristics

Pulsating heat pipe is a kind of high-efficiency heat transfer element that uses working fluid to oscillate back and forth between cold and hot ends for heat transfer.Due to the high heat transfer limit,simple processing,and low cost,pulsating heat pipe is considered to be one of the most promising technologies to solve the heat dissipation problem of high heat flux electronic components in confined spaces.Owing to the unique working principle and complex influencing factors,the heat transfer characteristics of pulsating heat pipe are still unclear.In this paper,the flow directions and flow patterns of the working fluid in FP-PHP and the infrared thermal image of FP-PHP at different operation stages are visually studied,and the factors affecting the unsteady and steady heat transfer characteristics of FP-PHP are also analyzed respectively.The visualization results indicate that the flow directions of water and absolute ethanol in the FP-PHP are mainly single-tube pulsation and large direction switch circulating flow,while HFE-347 can form a stable unidirectional circulating flow.The flow patterns observed in the experiment are mainly bubble flow,plug flow,long plug flow and annular flow.When the working fluid undergoes the large direction switch flow or unidirectional circulating flow,a mixed flow with multiple flow pattern types is dominant.The temperature distribution of the FP-PHP is highly affected by the heat transfer intensity of the working fluid under different operation states,so that the state of fluid flow can be evaluated through the infrared thermal image.The analysis results with respect to unsteady heat transfer characteristics of FP-PHP show that: the effects of heating powers on the start-up behaviors of FP-PHP with different working fluids are different.Specifically,with the increase of heating power,the water FP-PHP mainly starts up with gradually temperature rising,while the starting behavior of the absolute ethanol FP-PHP will change from gradually temperature rising to temperature fluctuation rising,a sudden drop in temperature is also observed during the start-up process of the HFE-347 FP-PHP.Besides,under the inclination angle of 90 ° and pre-evacuation degree of 1 Pa,the unsteady heat transfer characteristics of FP-PHP are the best.The analysis results with respect to steady heat transfer characteristics of FP-PHP show that: the FP-PHPs with water,absolute ethanol and HFE-347 as the working fluids have the lowest thermal resistance at 50%,50%,30% filling rates and 90° inclination angle.Under the same operating conditions,the thermal resistance of FP-PHP with HFE-347 is the lowest.In addition,the thermal resistance of water FP-PHP will decrease first and then increase with the decrease of pre-evacuation degree at high heating powers.The absolute ethanol FP-PHP under different pre-evacuation degrees is also simulated in this paper.The simulation results show that the pressure difference between the evaporator and condenser of absolute ethanol FP-PHP fluctuates differently under different pre-evacuation degrees,and the larger the fluctuation amplitude,the lower the thermal resistance of the FP-PHP.