Heat Transfer of Mesh-wick Heat-pipes : Bottom Heat Mode

A heat-pipe in which the working fluid circulates by gravity need not contain a wick. this type of heat pipe can be examined as a thermo-siphon. For better moistening inside the evaporating zone and improvement of the limit of flooding or entrainment, wicks of groove type are often used. In the present study, the performance of the metal wick of the copper-ethanol heat-pipe is examined by investigating the effect of inclination angle and volume of enclosed fluid on thermal resistance. In modeling the thermal resistance of the heat-pipe, it is assumed that, in the evaporating zone, the surplus fluid beyond the volume required for a complete circulation with full depth of fluid film is stagnant and the total thickness of the wick contributes the thermal resistance and that, in the condensation zone, the equivalent wick thickness to the depth of circulating fluid film accounts for the thermal resistance. In calculating the porosity, permeability and effective thermal conductivity of the wick, the equations including the normalized clearance are used. Since heat-pipes generally contain surplus fluid beyond the porous volume of the wick, the depth calculated by the equations of the Nusselt's condensation film theory is added to the depth of fluid film in the range beyond the wick capacity. The agreement between the calculated and measured differential temperature is good. The maximum heat transfer is attained at around 20 degrees of inclination angle, as is the case with thermo-siphons. The calculated wick capacity in the upright position is close to the measured value. There is no large discrepancy from the value obtained by the Katto's equation for the critical heat transfer in the limited water canal, except that the volume of enclosed fluid becomes larger.