An analytical model has been developed to describe the operation of simple and concentric gas‐loaded heat pipes and to assess the extent of deviations from ideal behavior due to diffusion, viscosity, and sonic flow. The model predicts, with reasonable accuracy, the start‐up power, the maximum heat transport, and the thermal regulation (=Δ density/Δ input power). Experimental results will be presented for Li/He heat pipes. This approximate analytical model should enable spectroscopic and kinetic heat pipe users to design and operate heat pipes optimally without extensive and costly computer solutions of the full Navier‐Stokes equations. The model is based on approximate solutions of diffusion/convection equations, in which the convective velocity distribution of a nearly ideal hat pipe is assumed to be identical to that of an ideal heat pipe. The vapor is treated as a one‐dimensional compressible fluid. Among the more important results are (1) The start‐up power QSU, defined as the power required to bring the metal vapor density to 95% of n0, the stagnation density, at the exit of the heated zone (or adiabatic zone if present) is virtually independent of n0. (2) For a heat pipe whose thermal losses are dominated by radiation, the start‐up power varies as T9/40, where T0, the stagnation temperature, is defined by n(T0)=n0. (3) The sonic flow limit QCF may be approximated as QCF= 1/2 hfg An0c(T0), where hfg is the heat of vaporization per atom, A is the cross‐sectional area, and c(T0) is the speed of sound at T0. (4) The thermal regulation properties of a concentric heat pipe may be approximated from the thermal regulation properties of simple heat pipes.
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