Abstract
Thermodynamic cycles are currently the most wanted means of converting nuclear power into available work due to the higher conversion efficiencies provided. Rankine cycles have been greatly applied for terrestrial reactors and nuclear submarines, while a lot of space projects have been using Brayton cycles for power conversion mainly due to mitigation in power-to-radiator area ratio. Regenerative Brayton cycles can present a considerable power conversion efficiency improvement when compared to the regular ones because of a reduction on the power demand from the hot heat exchanger of a cycle to achieve the same net output power. In this work, a cross-flow heat exchanger with He-Xe (40 g/mol) working fluid and Inconel 617 structural material used as the recuperator for the closed Brayton cycle of a nuclear reactor applicable for space systems is assessed in terms of heat transfer performance. The recuperator tubes are arranged in a staggered distribution around the exchanger axis. The matrix of tubes has a fixed count of 4 rows along the exchanger axis, while the number of tubes around the axis is variable, where the samples of 5, 7, 9, 12 and 16 are tested. The characteristic curves of heat transfer rate, effectiveness, convection coefficient and Colburn factor are built for each of the studied geometries in function of the Reynolds number. The obtained values for each of these parameters range between 1892.49 and 8493.21W (heat transfer rate), 0.165 and 0.325 (effectiveness), 60.3822 and 176.9682 W/m2K (cold side convection coefficient), 30.3276 and 104.3263 W/m2K (hot side convection coefficient), 0.0071 and 0.0109 (cold side Colburn factor), 0.0523 and 0.1370 (hot side Colburn factor).
Highlights
Nuclear reactors are currently seen as the best power source for space exploration vehicles due to several design advantages in comparison to their alternatives, such as compact size, low mass, long operating lifetime, resistance to hostile environments and high reliability [1]
In high power level regimes, Rankine cycles are mainly applied for power conversion in terrestrial and naval nuclear reactors, while Brayton cycles are more suitable for space nuclear power systems since they can provide a higher power-to-radiator area ratio [5], which is critical for mission success due to the need of minimum system sizes
The recuperator, shown in green, is a shell-and-tube heat exchanger, where the tubes are oriented in a 30o deviation from the radius, which increases the heat transfer area A an orients the tube flow outlet in a lower angular deviation from the heat source inlet, shown in dark blue in Figure 1 (a), and reduces head losses induced by flow direction change
Summary
Nuclear reactors are currently seen as the best power source for space exploration vehicles due to several design advantages in comparison to their alternatives (solar, chemical and radioisotope), such as compact size, low mass, long operating lifetime, resistance to hostile environments and high reliability [1]. Nowadays several research works have focused on thermal cycles due to their capacity of providing higher first law efficiencies [2] This technology has a wide range of applications in multiple engineering fields, being used for power conversion in plants using various thermal energy inputs (solar, thermoelectric, nuclear, etc) and for propulsion and power supply in vehicles developed in the naval, auto, aeronautical and space industries. In high power level regimes, Rankine cycles are mainly applied for power conversion in terrestrial and naval nuclear reactors, while Brayton cycles are more suitable for space nuclear power systems since they can provide a higher power-to-radiator area ratio [5], which is critical for mission success due to the need of minimum system sizes
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