Study On Gas-liquid Two-phase Motion Mechanism And Heat Transfer Characteristics Of Thermosyphon Heat Sink For LED Cooling

In the past few years,light emitting diodes(LED)lamps have played an important role in the illumination market due to high luminous efficiency and environmental protection.The heating power per unit area of LED light chips has been increased to obtain higher heat performance,which results in the occurrence of high local temperatures in LED light chips.If heat is not effectively released,the reliability and lifetime of the LED light will decrease.Therefore,heat dissipation is an important factor in the design of LED lights,the thermosyphon heat sink releases heat of high power LED lights due to low cost and high conductivity under the condition of mild working and limited space size.The heat performance and gas-liquid two phase motion mechansim of the thermosyphon heat sink are mainly investigated in this paper,which has an important theoretical and practical significance for understanding heat transfer mechanism and improving heat transfer performance.The effects of evaporator wettability,filling ratio,inclination angle and heating power on the heat performances of a thermosyphon were investigated under the condition of natural convection by experiments.It was found that the wall temperature difference of a thermosyphon with a hydrophilic evaporator was smaller than that with a hydrophobic evaporator.The results showed that the smallest thermal resistance was at the filling ratios of 20-30%.In addition,as the heating power increased from 10 W to 14 W,the thermal resistance decreased significantly(44.1% decrease)for a thermosyphon with a hydrophilic evaporator.The maximum heat dissipation of LED heat sink was 117 W in the limit of temperature.A model considering transient mass transfer time relaxation parameter,evaporator wettability in terms of contact angle coupled Volume of Fluid(VOF)model was developed to reveal the phase change mechanism of a thermosyphon.The simulated results were compared with experimental results to validate accuracy of the model.The results showed that the model with transient mass transfer time relaxation parameter had smaller relative errors(0.27-0.73%,3.21-4.23% and 2.45-6.78%)for the absolute temperature,thermal resistances at evaporation and condensation sections than the model without transient mass transfer time relaxation parameter(2.01-2.97%,18.31-21.74% and 15.34-28.25%),respectively.In addition,the relative errors between the simulated and experimental outer wall temperatures were 0.1-0.15% for hydrophilic and hydrophobic evaporators of a thermosyphon,thereby making the model accurate and reliable.Numerical simulation was employed to investigate the effects of filling ratio,inclination angle,evaporator wettability and heating power on the heat perfo rmance and reveal the heat transfer mechanism under the condition of natural convection.The results showed that a low filling ratio of 12% would result in drying out,but a high filling ratio of 40% would prevent large bubbles from reaching the liquid surface,thereby decreasing the heat performance.Therefore,the thermal resistance was the smallest(heat performance is best)at filling ratios of 20-30%.As the inclination angle increased from 15° to 90°,the bubbles attaching to the wall of the evaporator decreased,which decreased thermal resistance by 59.5%.The bubble emission frequency increased due to the few bubbles attaching on a hydrophilic evaporator inner wall,which improved the heat performance.This was in contrast to the bubbles attaching on hydrophobic evaporator inner wall which decreased bubble emission frequency and reduced the heat performance.As the heating power increased from 10 W to 14 W,the thermal resistance reduced more significantly(44.1% decrease)for a hydrophilic evaporator than that for a hydrophobic evaporator(20.6% decrease),due to the fact that the bubble emission frequency of a hydrophilic evaporator increased more sharply(265% increase)than that of a hydrophobic evaporator(100% increase)at an inclination angle o f 90° and a filling ratio of 25%.The methods coupled thermal resistance network with finite volume method were employed to investigate the effects of fin number,fin length and heating power on the heat transfer characteristics of thermosyphon heat sinks.The results showed that the bottom maximum temperature and thermal resistance of thermosyphon heat sink decreased first and then increased as fin number increased.The best performance of thermosyphon heat sink with fin number of 20,25,30,35 and 40 were corresponding to the fin length of 30,45,60,75 and 90 mm,respectively.As the fin lengths were in the range of 30-60 mm,the bottom maximum temperature and thermal resistance of thermosyphon heat sinks changed significantly.As the heating power increased from 50 W to 200 W,the thermal resistance reduced significantly(43.3% decrease)with fin number of 30 and fin length of 60 mm.Combining response surface method(RSM)with genetic algorithm,fin number,fin length and heating power were selected as input factors,the bottom maximum temperature,thermal resistance and mass of thermosyphon heat sink were selected as objective functions,then design of experiments,response analysis and multi-objective optimization were conducted.The results showed that fin length had the most significant effects on the bottome maximum temperature and thermal resistance for thermosyphon heat sink,and fin number had the most significant effect on the mass for thermosyphon heat sink.In addition,Pareto optimization solutions were obtained for the thermosyphon heat sink.