6. HEAT PIPE DESIGNThese are intended to provide information on the selection of the pipe working fluids, wick structures, container materials, and to present a systematic design theory procedure for a given set of data. 6.1 Fluid, Wick Structure, and Materials for Heat Pipes6.1.1 Fluid Selection
That is, the selected fluid for the heat pipe must have a melting point temperature belowand a critical point temperature abovethe pipe operating temperature. Several fluids canoften be used for a given operating temperature. The relative merits of different fluid canbest be observed by examining heat pipe theory as follows:For comparison of the relative merits of various fluids for heat pipes, an approximatetheory is developed for a simple cylindrical heat pipe .The following simplifyingassumptions are made:1. The pipe is capillary limited.2. Vapor pressure losses are negligible.3. Wick thickness twis much smaller than the vapor core radius.4. Heat flux density is uniform at the evaporator or condenser surface.5. Thermal conductivity of the liquid –saturated wick is proportional to that of the liquid. The heat transport factor (QL)c, max, underthese assumptions, can then be calculated bythe following equation;(QL)c, max=2σrc−ΔP−ρlgLtsinϕFl+Fvor------------------------------------6.1(QL)c, max=2(σρlλμl)(Krc)(2πrctw)…………………………………6.2Here, the terms in parentheses represent, form left to right, the liquid property, the wickproperty, and the wick cross-sectional area, respectively. The above equation indicatesthat for a pipe of fixed wick structure and dimensions its heat transport factor (QL)c, maxisdirectly proportional to the liquid transport factor, which is defined as(σρlλμl).For a minimum temperature gradient of a heat pipe, the temperature drop across theliquid-saturated wick must be minimum. And from the above examination, it can seenthat for heat pipes with a large heat transport capability but a small temperature gradient ,one may select fluids that have a large liquid transport factor and conductance factor , astheir working fluids. In addition, toxicity and flammability of the fluid may also have tobe considered in certain applications.6.1.2 Wick SelectionThe purpose of a heat pipe wick is three fold:i.To provide the necessary flow passages for the return of liquid from thecondenser to the evaporator. ii.To provide surface pores at the liquid-vapor interface for the development ofcapillary pumping pressure.iii.To provide a heat flow path from the wall of the container to the liquid-vaporinterface.
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