Performance of thermionic converters with smooth and grooved molybdenum electrodes

Experiments are conducted to investigate the performance of thermionic converters with (a) smooth electrodes, (b) a grooved emitter and a smooth collector, (c) a smooth emitter and a grooved collector, and (d) grooved emitter and collector. The electrodes are made of molybdenum (Mo) and the inter-electrode gap is 0.5 mm. Experiments are performed at T_E = 1473 K–1773 K, T_C = 773 K–1023 K, and P_Cs = 10 Pa–500 Pa.; For the smooth electrodes converter, the maximum electric power density is 2.25 to 4.24 W_e/cm2 and the maximum efficiency is 13.8 to 17.0%. Because the effective work function of Mo decreases with decreasing T_E, the performance of the converter with smooth Mo electrodes at low temperatures (≤1773 K) is shown to be superior to that reported for a TI converter with a W emitter. The measured electric power density of the former is as much as 90% higher than of the latter. At T_E = 1673 K, increasing T _C, beyond its optimum of 873 K, to 1023 K causes $PMAXD$ and ηMAX for the smooth electrodes converter to decrease from 3.74 W/cm2 and 17% to ∼1.6 W/cm 2 and ∼10%, respectively. At this high collector temperature, the grooved collector converter gives the highest $PMAXD$ (∼2.4 W_e/cm2) and η MAX (14%). Visual observations of the plasma discharge in the interelectrode gap of the converter with a grooved collector show that the plasma discharge forms first near the emitter and becomes brighter and extends toward the collector with increasing current. At high discharge current, the plasma discharge stops expanding due to electron scattering, but increases in brightness.; The Cs plasma discharge model developed in this research solves the particle, momentum, and energy conservation equations and calculates the output voltage, current, and the spatial distributions of the electron temperature, T_e, electron number density, n_e, ion number density, n₊, and the electron potential, &phis; _e. The source-sink terms in the governing equations are calculated using an ionization-recombination sub-model, which incorporates excitation, deexcitation, ionization and recombination processes for both atomic and diatomic species. Incorporating associative ionization and dissociative recombination in the plasma discharge model improved the accuracy for predicting the spatial distribution of the electron temperature. Results are validated using reported measurements by earlier investigators. The results plasma discharge model are also validated by comparing them with those of Baksht et al. (1969a) and with measurements by both Reichelt (1965) and Baksht et al. (1969a). In addition, the model predictions are compared with the measured I-V curves in this work for the smooth electrodes converter.