where am is the average film coefficient, at is the liquid film coefficient and a, is the vapor-liquid interface coefficient. Required heat transfer area and condenser length are determined from the mass flow rate, heat transfer coefficient and allowable temperature drop through the condenser. Although this was a point calculation it was judged sufficiently representative of an optimized system to be used in estimating total systems mass. Electric Power Converter: Thermionic System. In this design, the thermionic converters are attached to heat pipes coming out of the core to avoid the inevitable failure of in- core converters due to fuel swelling. Converter performance can be obtained by analyzing the electrode surface, sheath, and plasma simultaneously [14]. This calculation was done using SIMCON [15]. Thermionic converter performance depends on emitter temperature, collector temperature, cesium vapor temperature and gap distance. One mm thick tungsten emitters and molybdenum collectors were used with cesium vapor in the gap. Gap distance was set at 0.254 mm and cesium vapor temperature was held at 600 K. The converter cells are arranged along the core heat pipe condenser sections and wired in series to obtain the desired voltage. Each cell produces about 0.6 V. Power conditioning is not discussed here. Converter efficiency is shown in Fig. 9 versus current density for various collector temperatures from 900 to 1300 K with a constant emitter temperature of 1900 K. The converter efficiency curves have peaks at about 7.5 A/cm2. Power density curves for the same collector temperature range have peaks at about 15 A/cm2. High currents lead to substantial energy losses through joule heating, so converter efficiency is The condenser is cooled by radiator heat pipes. The heat transfer coefficient was calculated assuming laminar flow film condensation using the following equation:
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