Abstract
This study investigates the effects of Al2O3 nanoparticles on the performance of a sintered-powder heat pipe and the boiling characteristics (including incipient superheat, boiling mode and behavior after boiling) of a sintered-powder wick. In addition, the boiling characteristics associated with a sintered-powder wick under an evaporation/boiling and a pool-boiling mode are studied. Furthermore, pool boiling for a groove wick and a smooth pipe are also investigated. The performance of a sintered-powder heat pipe charged with 0.14wt% Al2O3/water nanofluid is compared to that charged with pure de-ionized water. Results show that the minimum thermal resistance for the nanofluid is similar but the maximum heat load is considerably lower than for de-ionized water. That is, the heat pipe performance is not improved by using the Al2O3 nanofluid. A simple novel method is proposed for characterizing the boiling behavior for a heat pipe wick. In this method, a standard heat pipe is adopted and the cooled section is placed near the heated section so that boiling may occur before the capillary limit. Thus, non-condensable gas can be eliminated after sufficient evacuation and de-gassing of the working fluid during the fabrication of the heat pipe. For sintered-powder wick at an evaporation/boiling mode, the incipient superheat is 12 ℃ at an operating temperature of 60.1 ℃, while it is 15 ℃ at an operating temperature of 83.4 ℃. The incipient superheats obtained herein are higher than those in the literature, likely due to the different extents of degassing. When boiling occurs, the evaporator temperature decreases abruptly and the temperatures at the adiabatic section and condenser increase almost simultaneously. Afterwards, all the temperatures slowly approach a steady state. The thermal resistance is significantly reduced after boiling incipience. Under pool boiling, the characteristics are similar to those of the evaporation/boiling mode, except for a significantly lower incipient heat flux. For a groove wick under pool boiling, the incipient superheat is 47-50 ℃, and the boiling behavior is distinctly different from that for the sintered-powder wick. Upon reaching the incipient superheat, strong boiling suddenly prevails over the evaporator, leading to an instant, drastic temperature drop in the evaporator and suppression of boiling as a consequence. Afterwards, the evaporator gradually heats up until next strong boiling occurs. Such a periodic process continues before next higher heating power is imposed, followed by another periodic process. For a smooth cylinder surface, the boiling behavior is similar to that of the groove wick, except the boiling intensity appears weaker, according to the smaller abrupt temperature drops, and boiling may not be fully suppressed after the major boiling with an abrupt temperature drop.