This paper reports the development of an experimental small loop heat pipe with polytetrafluoroethylene (PTFE) wicks. Three kinds of PTFE porous materials were fabricated, and their wick properties were evaluated. It was clarified that the peak pore radii of the PTFE porous materials ranged from 0.8 to 2:2 m, and their porosities ranged from 27 to 50%. A small loop heat pipe with PTFE wicks was designed using these properties, and an experimental small loop heat pipe, for which the wick in an evaporator is exchangeable, was fabricated. The experimental loop was tested with 1.2 and 2:2 m PTFE wicks under atmospheric conditions. The test result with a 2:2 m PTFE wick showed a high operating temperature, possibly due to vapor leak from the evaporator grooves to the compensation chamber. The test results with the 1:2 m PTFE wick showed a lower operating temperature, lower thermal resistance, and stable operation of the loop heat pipe. Nomenclature D I Wick = inner diameter of wick, m D O Wick = outer diameter of wick, m K = permeability, m 2 k Eff = effective thermal conductivity, W m 1 K 1 k L = thermal conductivity of liquid, W m 1 K 1 k Wick = thermal conductivity of wick material, W m 1 K 1 L Wick = active length of wick, m _ m Wick = mass flow rate of fluid through wick, kg s 1 P Cap = capillary limit, Pa _ Q HL = heat leak through wick, W _ Q Load = heat load to evaporator, W R E C = thermal resistance between evaporator and condenser, K=W r Pore = pore radius of wick, m T SatCore = saturation temperature of wick core, K T SatEvap = saturation temperature of evaporator, K T Cond = average temperature of condenser, K T Evap = average temperature of evaporator, K P Cond = pressure drop of condenser, Pa P Grv = pressure drop of grooves of wick, Pa P LL = pressure drop of liquid line, Pa P Tot = total pressure drop in LHP, Pa P VL = pressure drop of vapor line, Pa P Wick = pressure drop across wick, Pa P G = gravity head, Pa " = porosity, % = contact angle, deg. L = viscosity of liquid, Pa s L = density of liquid, kg m 3 = surface tension, N m 1

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This paper describes thermal performance of a small loop heat pipe device in an atmospheric condition. A comprehensive test program including start-up, power step up, power cycle and low power tests was performed. The effects of gravity on start-up and heat transport capability were also evaluated. An analytical model for the loop was developed to predict and evaluate the steady state operating performance. The test results demonstrated the robustness of the LHP. The analytical results showed good agreement with the test results except at the low power region. The feasibility of loop temperature control through compensation chamber temperature control was also experimentally demonstrated.

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