Variable conductance heat pipe performance analysis - Zero-to-full load

Abstract

Test data for an aluminum/ammonia/nitrogen variable conductance heat pipe (VCHP) with a central core wick are presented and used to develop an analytical model for predicting evaporator temperature as a function of heat load and environmental conditions. These data show that VCHP performance at very low heat loads is dominated by heat losses from the so-called primary adiabatic section. The present formulation is an extension of an earlier VCHP model that was found suitable for engineering calculations over a heat-load range of about 20-100% of full load; the extended model presented here includes the operating range of 0-100% of full heat load. The extended model postulates a uniform condensation flux q0 in the nonadiabatic transport section, and a second but different uniform condensation flux ql in the active condenser. A set of nonlinear, transcendental, algebraic equations describes the performance of a simple VCHP. An incomplete definition of the two condensation fluxes requires a double-iteration procedure with respect to q0 and tc to solve the set of equations. A comparison of analytical temperature distributions and experimental data points shows that the present model is adequate for most engineering applications. Nomenclature Ac = internal cross-section area for vapor/gas transport Ar = reservoir surface area for heat transfer Aw = conduction cross section of wick plus wall c — local molar density of vapor and noncondensable gas mixture D = local diffusion coefficient of vapor/gas mixture h = local heat-loss coefficient (convection plus linearized radiation) H = local latent heat of vaporization kw = thermal conductivity of pipe wall and wick structure / = axial length tc = condensation front location Lj = nondimensional length, ^j/^3 mg = mass of noncondensabl e gas Mv = vapor molecular weight pg = gas partial pressure pv = vapor partial pressure q0 = condensation heat flux in primary transport section q = condensation heat flux in active condenser section