Heat and Mass Transport in Heat Pipe Wick Structures

A novel experimental approach is developed for characterizing the performance of heat pipe wick structures. This approach simulates the actual operation of wick structures in a heat pipe. Open, partially submerged, sintered copper wicks of varying pore size are studied under the partially saturated conditions found in normal heat pipe operation. A vertical wick orientation, where the capillary lift is in opposition to gravity, is selected to test the wicks under the most demanding conditions. Mass transport measurements of the working fluid, in addition to the temperature field, are obtained for the porous wicks under the action of a discrete heat source (evaporator) mounted on one end. The working fluid, supplied from a condenser pool, evaporates from the wick surface primarily in the evaporator region, and is condensed and collected into a container separate from the pool, to yield mass flow rates. Thus the liquid-pumping capability of the wick, coupled with flow impedance, is measured as a function of applied heat flux. Repeatable results with low uncertainty are obtained. A careful analysis of the transport paths for heat and mass transfer in the wick structure confirms that mass transfer due to vaporization of the working fluid is the largest contributor to heat dissipation from the wick. The expected and measured values of evaporation rate are in good agreement. Results are also presented in terms of overall effective conductance based on measured temperatures. Nomenclature A = Area (m 2) h fg = Enthalpy of vaporization (kJ/kg) k = Thermal conductivity (W/m·K) L = Length (m) m = Mass flow rate (kg/s) L Nu = Nusselt number (/ hL k) P = Pressure (kPa) Pr = Prandtl number q = Heat transfer rate (W) q " = Heat flux (W/m 2) R = Thermal resistance (°C/W) L Ra = Rayleigh number (  3 / eff g Ts Tb L   ) t = Time T = Temperature (°C) x = Axial location relative to x wick1 (cm) y = Liquid level height (m) * To whom correspondence should be addressed: 765-494-5621; [email protected] 2 Subscripts ann = Annulus app = Applied atm = Atmospheric bath = Outer bath cp = Copper pad e = Epoxy/wick interface eff = Effective ep = Epoxy es = Evaporator front surface evap = Evaporation g = Gage (relative to atmospheric) h = Heater-average value in = Input ins = Heater insulation losses l = Liquid …