heat pipe evaporation zone. We were able to carry out experiments under a wide range of temperatures and heat flows in various heat pipe orientations using this apparatus. We used cylindrical heat pipes with diameters from 10 to 24 mm, and lengths from 100 to 600 mm. The heat pipes were either water-filled copper, or ammonia-filled stainless steel. The wick structure was made of sintered metal fibers and porosities from 60 to 85% and a thickness of 0.5 to 1.0 mm. Heat pipe wall temperatures in the evaporating, adiabatic and condensing sections were measured by thermocouples placed along the midline. The thermocouples were read by an electronic voltmeter through a commutator. The thermal power transferred by the heat pipe was determined from the heat generated by the electric heater and that absorbed by the cooling bath. The 95% confidence interval for the true heat transfer coefficients is ± 20% of the measured heat transfer coefficients. We determined the thermal resistance and heat flux of our heat pipes under appropriate operating conditions by varying the cooling bath temperature. The working fluid most well matched to spacecraft thermal control system needs was ammonia. Experimental data on the thermal resistance of the evaporator and condenser were then used to explore heat transfer models for the capillary pore structure in order to come up with appropriate relations. We included other worker’s data on water, acetone, ethanol, ammonia and freons 11 and 113; taken with evaporators made of sintered fiber, sintered powder and wire in order to extend the scope of our formulae. Theoretical Investigations Under low heat flux, heat is conducted through the evaporator to the internal meniscus, where evaporation from the liquid surface takes place without bubble formation. In this regime the convective heat transfer coefficient is independent of the heat flux. Prior theoretical work [3] predicts heat fluxes 25 to 30% lower than those obtained in the present work. This discrepancy appears to be due to a failure to take account of the specific heat flow transformation under heat flowing from the wall to the evaporating surface through the capillary structure [2], The following relation for heat transfer can be obtained using the assumption below. Nomenclature is given before the references. Figure 1 shows that the experimental data is consistent with equation (1) for conditions corresponding to a range of Reynolds numbers from 0.4 to 60. Continuing with the assumption of specific heat flow transformation, the effective thickness of the pore coat is Increased heat flow causes nucleate boiling to begin in the coat pores. This is a complex process, and its mathematical treatment is extremely difficult. Thus we must use semiempirical relationships based on models similar to our heat pipe evaporators [4], This relationship gives good results for water, ethanol and freon, but is off by up to 100% for ammonia and nitrogen, a distinct group of thermodynamically similar liquids.
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