Later Modifications to THROHPUT

Early code runs demonstrated insufficient liquid return to the evaporator section, and the eventual dry-out of the evaporator section did not agree with experimental observations. One of the methods which was employed to counteract this problem was variation of the evaporation and condensation accommodation coefficients, but this alone was insufficient (##hall88b ##hall88b, ##hall90a ##hall90a).

Further research into two areas yielded important information that made a significant impact on the modeling effort (##hall94 ##hall94). First, research into experimental accommodation coefficient determination suggested that values closer to unity were appropriate for heat pipe conditions (##nikn81 ##nikn81). Second, research into the possibility of liquid tension, or negative liquid pressure, produced several references which validated its physical existence via experimentation. Liquid tension was originally mentioned in ##bert50 (##bert50), and later multiple references are given in ##hayw70 (##hayw70) and ##apfe72 (##apfe72). Also, negative liquid pressure in heat pipes is treated specifically in ##kemm68 (##kemm68) and ##ande93a (##ande93a), and experiments using Alkali Metal Thermal to Electric Converter (AMTEC) cells are presented in ##ande92 (##ande92) and ##citeasnoun (##citeasnoun)*ande93b. Subsequent to this research, changes were made to the capillary pressure relationship in THROHPUT to allow negative liquid pressure.

The original THROHPUT code was developed with only one working fluid (lithium), one wall material (molybdenum) and one noncondensible gas (air). Adding new materials involves locating and incorporating functional forms for thermal conductivity, specific heat, density, enthalpy, internal energy, viscosity, molecular weight and critical point variables. With later modifications, the THROHPUT code can now model several working fluids (lithium, sodium, potassium, rubidium, cesium, mercury and silver), two wall materials (molybdenum and rhenium) and one noncondensible gas (air).