Steady state model of a loop heat pipe (LHP) with coherent porous silicon (CPS) wick in the evaporator

Abstract

A theoretical study is conducted to explore the effect of different parameters on the performance of a Loop Heat Pipe (LHP). These parameters are evaporator temperature, condenser temperature, total mass charge, the tube size of piping system, and pumping distance between evaporator and condenser. This paper presents a steady state model that describes the thermodynamics, heat transfer, and fluid mechanics inside an LHP. A LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference that developed between the evaporator and condenser to circulate a working fluid through a closed loop. The loop heat pipe efficiently transports the heat generated by a highly localized concentrated heat source and discharges this heat to a convenient smk. The steady state LHP model is described by the conservation equations, thermodynamic relations, and capillary and nucleate boiling limits. The loop heat pipe cycle is presented on a temperahre-entropy diagram. A relationship is developed to predict the ratio of the heat of evaporation to the heat leaked to the compensation chamber. This work predicts the sue of a LHP, the pumping distance, and the maximum power that can he dissipated for a fixed source temperature. Nomenclature A CPL CPS h CP hce e k L LHP m M P Q R T r V Wick cross sectional area, m2

Parametric analysis of loop heat pipe operation: a literature review

“…For example, in Teflon wicks, the convective effects are more important than the conductive effects due to the small wick thermal conductivity. According to Hamdan et al [31], a conductive-convective model of the evaporator wick is necessary for thick wicks with a low thermal conductivity. The axial heat leak across the evaporator metallic envelope is indirectly affected by the porous medium characteristics, as it depends on the evaporator wall temperature, which is linked to the heat transfer coefficient between the evaporator wall and the evaporation interface in the porous wick.…”

“…The effect of the wick characteristics on the LHP operating limits has been presented by Hamdan et al [21,31]. Calculation results have been obtained for a coherent porous silicon wick, whose characteristics were imposed by the fabrication technology.…”

“…The constrained space of such applications requires to design specific LHPs. Various models have been developed for the LHP characterization [7,23,[29][30][31][32]. The steady state models are useful to size new-designed LHPs and to predict LHP performance for various fixed external conditions.…”

Analytical Model for Characterization of Loop Heat Pipes

“…The originality of our model compared with Kaya et al's [12,13] and Hamdan et al's [14] model is the possibility to simulate various LHP evaporator geometries of cylindrical or flat disk shapes and to take into account the conductive heat transfer through the wall between the heat source and the reservoir. This heat leak can be dominant 1) in miniature LHPs for which the reservoir is located close to the evaporator, 2) in the case of high evaporator wall thermal conductivity having a great thickness compared with the other dimensions, and 3) in the case of poor wick thermal conductivity.…”

“…To predict the LHP characteristic curve or performance at steady state, various models have been developed [11][12][13][14]. Most of these models are based on the steady-state energy-conservation equations written for each component of the LHP.…”