| Integrated light emitting diode (LED) with large power density has been projectinga brilliant lighting market owing to much higher luminous efficiency than conventionallights. Accompanying with the significant increase in integration and power density,however, the heat dissipation becomes one of the key factors affecting the LEDperformance, limiting its further development. Conventional cooling techniques areunsatisfactory to meet the requirement of the high heat flux cooling inhigh-power-density integrated LED. As a result, towards the heat dissipation problem inhigh-power-density integrated LED, a new type of the fin-type gravity heat pipe radiatorhas been proposed in this thesis. In this design, series of circle-shape fins are verticallyoriented to the outer surface of the heat pipe, by which heat is dissipated by naturalconvection. In addition to the features of the simple structure, no capillary wick andeasy manufacturing, such design not only facilitates the natural convention heat transferbut also effectively solve the fouling problem encountered in the fin type heat radiator.More importantly, the performance of the proposed fin type gravity heat pipe radiator isable to fulfill the heat dissipation demand of high-power-density integrated LED.Following this new design, we designed and fabricated a lab-scale fin type gravity heatpipe radiator and experimentally investigated its heat transfer performance. The effectsof the working fluids, heating power and charge of working fluid on the heat transferperformance were also examined. Main conclusions are presented as follows.①When the heating power was150W and the heat flux was37.5W/cm~2, theoverall thermal resistance of the fin type gravity heat pipe radiator was in arange of0.14K/W-0.25K/W. In the meantime, the surface temperature of theevaporative sector was lower than65℃while the temperature difference inthe surface temperature of the condensing sector was lower than3℃,indicating that the new radiator exhibits good temperature uniformity.Moreover, it is also found that this radiator could reach stable within3minuteswhen the operating conditions vary.②The radiator proposed in this thesis showed a good start-up performance withwater as working fluid. The start-up time decreased with increasing the heatingpower but increased with increasing the charge of working fluid. Under thesteady state, the thermal resistance decreased and increased with increasing the heating power and charge of working fluid, respectively. In this experiment, itis found that due to the T-shape outlet design of the evaporative sector, theworking temperature of the radiator fluctuated but remained in5℃. For givenoperating conditions, the radiator with water as working fluid showed theoverall thermal resistance of0.08K/W~0.33K/W and heat transfer coefficientof3200W/m~2·K~5700W/m~2·K.③When ethanol was chosen as working fluid, the radiator showed good start-upperformance for most given heating power. In the case of20%charge ofworking fluid and200-W power density, the bottom temperature of theevaporative sector increased to100.5℃and the performance of the radiatorbecame worse. In this testing, it is found that when ethanol was used, the effectof ethanol vapor’s shear force and carrying to condensed ethanol wassignificant. As a result of the shear force, the condensed ethanol would stay atthe insulated sector, blocking the ethanol vapor flow and thus leading to theincrease of the pressure in the evaporation chamber. As such, the surfacetemperature of the evaporator was rapidly increased, lowering the coolingperformance. When the charge of working fluid was30%, the start-upperformance of the radiator was favorable. Besides, the overall thermalresistance of0.07K/W~0.34K/W and the overall heat transfer coefficient of3500W/(m~2·K)~6000W/(m~2·K) were achieved under given conditions.④With respect to acetone as working fluid, this radiator showed good start-upperformance under most cases. Rapid start-up and response to the variation inthe heating power were achieved. When the charge of working fluid was30%and the heating power was200W, the bottom temperature of the evaporativesector firstly increased to95℃and then decreased to maintain at78℃. Theoverall thermal resistance decreased with increasing the heating power. It isalso found that the overall thermal resistances of20%and30%charge ofworking fluid crossed each other due to the blockage in the insulating sectors.For given operating conditions, the overall thermal resistances ranged from0.07K/W to0.37K/W and the overall heat transfer coefficients are in therange of3000W/m~2·K~5700W/m~2·K.⑤Experimental results showed that the optimal charge of working fluid for water,ethanol and acetone was all30%. Due to the T-shape outlet of the evaporativesector, the working temperature fluctuated in the case of water as working fluid while the fluctuation became large in the case of ethanol and acetone asworking fluids as a result of the shear force and carrying effect. Such largefluctuation caused a rapid increase in the surface temperature of the radiator,making it unsatisfied for the LED heat dissipation requirement. |
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