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An analysis is presented of axially conducting gas-controlled heat pipes leading to a predictive capability for the heat and mass transfer along the heat pipe. In addition, experimental results are presented which verify the analysis, and computational results are presented which show the relative influence of various parameters which affect the system behavior. In particular it was found that axial heat conduction is of much greater importance than axial mass diffusion in establishing the wall temperature profiles and condenser heat-transfer characteristics of gas-loaded heat pipes. However, mass diffusion and, consequently, the choice of working fluid and control gas are of considerable importance in establishing the “diffusion freezeout rate” if the potential exists for freezing of vapor which penetrates the gas-blocked portion of the condenser. It is believed that the analysis and associated computer program are useful tools for designing gas-loaded heat pipes.

Temperature measurements were made within the superheated boundary layer above and adjacent to a horizontal heating surface in saturated, nucleate, pool boiling of water. A microthermocouple probe was used to measure the average temperature profiles and the temperature fluctuations within the boundary layer at heat fluxes from 1000 to 40,000 Btu/hr-sq ft. Correlations are presented for the “extrapolated” thickness of the boundary layer (δ) as well as the temperature distribution within it. It was found that the thickness (δ) could be expressed in terms of the heat-transfer coefficient as: δ = Chd. Also, the behavior of δ with system parameters was found to agree with that predicted by Han and Griffith [3] and Hsu [4] in their theories of nucleation from surface cavities. The temperature distribution in the boundary layer from the surface to 0.57δ was essentially linear and could be expressed: (T − Tb)/(Ts − Tb) = 1 − (y/δ). Above 0.57δ the temperature profile became an inverse power function of the height above the surface: (T − Tb)/(Ts − Tb) = D(y/δ)−a.

The design and manufacture of fine wire (0.001 in.) thermocouple probes are described. The probes, using arc-welded thermocouple junctions, exhibit great strength, rapid response, and high spatial resolution.