EFFECTS OF COOLING SCHEMES ON THE PERFORMANCE OF SPACE CLOSED BRAYTON CYCLE

Abstract

Closed Brayton cycle can efficiently convert the fission energy of the reactor into electricity, which is an ideal energy conversion system for long life, high-power and compact space reactor. In order to ensure the safe and reliable operation of turbines, compressors and generators, the space closed Brayton cycle intends to cool bearings and generators by working fluid, such as He-Xe mixture. So the cooling will inevitably affect the cycle efficiency. In this paper, three different cooling schemes for bearings and generators were proposed for the space closed Brayton cycle. They are: (1) bypassing some gas from the compressor outlet to cool bearings and generators and joining the mainstream before the reactor inlet, (2) bypassing some gas from the compressor outlet to cool bearings and generators and joining the mainstream after the reactor outlet, and (3) setting an independent cooling loop. The influence of bleeding gas flow rate on the key components was analyzed, and the three schemes were compared. The results show that the temperature difference at hot end of recuperator, molar specific power of reactor, turbine inlet temperature and molar specific work of schemes (1) and (2) decrease with increase of bleeding ratio. The scheme (3) is not effected by bleeding ratio. For a given bleeding ratio, the increasing of compression ratio will promote the space Brayton cycle efficiency of the scheme (1) and (2) at first, then the efficiency reaches its maximum value, with further increasing of compression ratio, the efficiency will reduce. But further increasing of bleeding ratio will reduce the temperature difference at hot end of recuperator, the cycle efficiency and compression ratio can not reach the maximum when the bleeding ratio increases. Therefore, there are optimal bleeding ratio, maximum cycle efficiency and corresponding compression ratio. And the maximum cycle efficiency of scheme (1) and (2) increases first and then decreases with the increase of bleeding ratio. The scheme (1) has the highest efficiency of power generation, 21.63%, because the bypass can reduce the core heat absorption and the turbine inlet temperature is the highest. The scheme (3) has the lowest efficiency which is 19.78% with the highest molar specific power which is 0.8832kJ∙mol-1 , and this scheme can reduce the core inlet temperature and the power of recuperator. The analysis provides a reference for the cooling design of the space closed Brayton cycle in the future.