Modeling of Thermofluid Phenomena in Segmented Network Simulations of Loop Heat Pipes.

摘要

The overarching goal of the work presented in this thesis is to formulate, implement, test, and demonstrate cost-effective mathematical models and numerical solution methods for computer simulations of fluid flow and heat transfer in loop heat pipes (LHPs) operating under steady-state conditions.;A segmented network thermofluid model for simulating steady-state operation of conventional LHPs with cylindrical and flat evaporators is proposed. In this model, the vapor-transport line, condenser pipe, and liquid-transport line are divided into longitudinal segments (or control volumes). Quasi-one-dimensional formulations, incorporating semi-empirical correlations for the related single- and two-phase phenomena, are used to iteratively impose balances of mass, momentum, and energy on each of the aforementioned segments, and collectively on the whole LHP. Variations of the thermophysical properties of the working fluid with temperature are taken into account, along with change in quality, pressure drop, and heat transfer in the two-phase regions, giving the proposed model enhanced capabilities compared to those of earlier thermofluid network models of LHPs. The proposed model is used to simulate an LHP for which experimental measurements are available in the literature: The predictions of the proposed model are in very good agreement with the experimental results.;In earlier quasi-one-dimensional models of LHPs, the pressure drop for vapor flow through the grooves in the evaporator is computed using a friction-factor correlation that applies strictly only in the fully-developed region of fluid flows in straight ducts with impermeable walls. This approach becomes unacceptable when this pressure drop is a significant contributor to the overall pressure drop in the LHP. A more accurate correlation for predicting this pressure drop is needed. To fulfill this need, first, a co-located equal-order control-volume finite element method (CVFEM) for predicting three-dimensional parabolic fluid flow and heat transfer in straight ducts of uniform regular- and irregular-shaped cross-section is proposed. The methodology of the proposed CVFEM is also adapted to formulate a simpler finite volume method (FVM), and this FVM is used to investigate steady, laminar, Newtonian fluid flow and heat transfer in straight vapor grooves of rectangular cross-section, for parameter ranges representative of typical LHP operating conditions. The results are used to elaborate the features of a special fully-developed flow and heat transfer region (established at a distance located sufficiently downstream from the blocked end of the groove) and to propose novel correlations for calculating the overall pressure drop and also the bulk temperature of the vapor. These correlations are incorporated in the aforementioned quasi-one-dimensional model to obtain an enhanced segmented network thermofluid model of LHPs.;Sintered porous metals of relatively low porosity (0.30 -- 0.50) and small pore diameter (2.0 -- 70 micrometers) are the preferred materials for the wick in LHPs. The required inputs to mathematical models of LHPs include the porosity, maximum effective pore size, effective permeability, and effective thermal conductivity of the liquid-saturated porous material of the wick. The determination of these properties by means of simple and effective experiments, procedures, and correlations is demonstrated using a sample porous sintered-powder-metal plate made of stainless steel 316.;Finally, the capabilities of the aforementioned enhanced segmented network thermofluid model are demonstrated by using it to simulate a sample LHP operating under steady-state conditions with four different working fluids: ammonia, distilled water, ethanol, and isopropanol. The results are presented and comparatively discussed.