Heat Transfer Characteristics Of Self-wetting Nanofluids Pulsating Heat Pipes For LED Automotive Headlights

Nowadays,the LED lighting has become the mainstream of automotive lighting.However,traditional heat dissipation methods have difficulties in meeting the increasing heat flux density and heat dissipation needs of LED headlights.This has become one of the bottlenecks that restrict the development of LED lights.In this paper,self-rewetting nanofluids pulsating heat pipes（PHPs）technology was investigated for the thermal management of LED headlights to address the heat dissipation challenges of LED headlights.The main research contents and conclusions are as follows.（1）Visualization experimental platform was built.The different flow states and operation mechanisms of MWCNTs/H₂O nanofluids in a PHP with an inner diameter of 2mm and a charge ratio of 40%were investigated.The start-up pattern and temperature variation characteristics of PHP under different conditions were discussed.The results showed that the main flow pattern of the PHPs was the gas plug flow,accompanied by bubbly,mixed and annular flow patterns.With the increase of heating power,the heat transfer performance of the PHPs was gradually enhanced.At an inclination of 0°,the PHPs had the fastest start-up speed and the best heat transfer effect.The heat transfer performance of the PHPs was best when the evaporation and condensation sections had the same length.（2）Experimental platform for heat transfer characteristics was built.Heat transfer performances of ultrapure water,n-butanol self-rewetting fluid,MWCNTs/H₂O nanofluid and mixed solution of n-butanol self-rewetting fluid with MWCNTs dispersion were investigated in a PHP.The highlight of this study was to analyze their thermal performances under varying operating conditions,including different heating powers,inclination angles and ambient temperatures.Meanwhile,thermophysical properties of working fluids were also explored in order to better understand the working mechanisms of PHP.The experimental results indicated that the mixed solution,i.e.,self-rewetting nanofluids showed excellent enhancement of the thermal transport performance at a low heating power range.The enhancement ratio of the self-rewetting nanofluid was about7-17%.The results also indicate that the inclination angle also had a non-negligible effect on thermal performance of the working fluids,especially,their heat transfer limits in the PHP.In addition,a lower ambient temperature could generally improve the heat transfer performance of investigated working fluids in the PHPs.（3）A 2-D PHP model was established.The effect of charge ratio on a single-loop PHP is studied using simulation.The results of the research show that for single-loop PHPs,with a charge ratio of 50%,starts faster and has the best heat transfer.In the simulation of the multi-loop PHP,the growth of small bubbles was recorded in the time period of2.0-2.5s.Multi-loop PHPs start-up is faster and requires less time to reach stable operation compared to single-loop PHPs.Finally,the self-rewetting nanofluids PHPs technology was used for thermal management of LED headlights.PHPs heat sink was designed and manufactured,and its performance was tested.The test results showed that the PHP heat sink had excellent heat dissipation performance with thermal resistance no higher than 2.108°C/W.The research would establish the foundation for the engineering application of PHPs cooling technology in LED automotive headlights.