Closed-Brayton-Cycle Heat Exchangers in 100-kWe Nuclear Space Power Systems

Performance expectations of closed-Brayton-cycle heat exchangers to be used in 100-kWe nuclear space power systems were forecast. Proposed cycle state points for a system supporting a mission to three of Jupiter’s moons required effectiveness values for the heat-source exchanger, recuperator and rejection exchanger (gas cooler) of 0.98, 0.95, and 0.97, respectively. Performance parameters such as number of thermal units (Ntu), equivalent thermal conductance (UA), and entropy generation numbers (Ns) varied from 11 to 19, 23 to 39 kWK, and 0.019 to 0.023 for some standard heat exchanger configurations. Pressure-loss contributions to entropy generation were significant; the largest frictional contribution was 114% of the heattransfer irreversibility. Using conventional recuperator designs, the 0.95 effectiveness proved difficult to achieve without exceeding other performance targets; a metallic, plate-fin counterflow solution called for 15% more mass and 33% higher pressure-loss than the target values. Two types of gas-coolers showed promise. Single-pass counterflow and multipass crosscounterflow arrangements both met the 0.97 effectiveness requirement. Potential reliability-related advantages of the cross-counterflow design were noted. Cycle modifications, enhanced heat transfer techniques and incorporation of advanced materials were suggested options to reduce system development risk. Carboncarbon sheeting or foam proved an attractive option to improve overall performance. INTRODUCTION Safe nuclear energy conversion is an enabling technology for enhanced science missions to the outer planets’ where power and propulsion systems using solar power are hindered by low solar energy flux. The energy provided by a small fission reactor will allow science instruments to rapidly transmit large blocks of information to earth using data transmission rates and bandwidth unattainable using batteries, solar collectors, or arrays of solar (photovoltaic) cells. A fissionpowered electric propulsion system will allow a spacecraft to visit multiple locations in a planetary system without relying on gravity-assist trajectories that lead to only short-term “fly-bys” of a planet or its moons. Once in orbit, the energy available in a fission reactor will allow extended science observations over longer durations than those currently possible using other energy sources. The recognition recently awarded these benefits of space nuclear power has revived interest in energy conversion technologies that convert a fission reactor’s thermal energy into electrical energy useful to spacecraft science instruments and electric propulsion systems. There are numerous energy conversion options. One that shows promise in attaining safe and reliable operation, high system-level efficiencies, and flexible scalability in power level is a dynamic power conversion system using a closed-Brayton-cycle (CBC) heat engine. To reduce the engine mass, thermal energy is converted to electrical work using an alternator that is integrated with the compressor and turbine on a single rotating assembly-a turboalternator compressor (TAC).’ A generic temperature-entropy diagram of a recuperated CBC is presented in Fig. 1. The diagram shows that three heat exchangers are essential components in the recuperated CBC engine systemthe heat source heat exchanger (HSHX), the recuperator, and the gas cooler (GC). In a 100-kWeclass CBC space power conversion system (PCS), the expectations of the heat exchangers can be demanding. Quantified performance analyses are needed to assess the feasibility of different system configurations. The present work introduces an operating state point reference for a 100-kWe CBC PCS. With the system state points established, sizing and performance parameters are presented for standard heat exchanger configurations used in the three exchanger roles identified. Then, the recuperator is used as an example to compare current performance expectations to AlAA Senior Member, Research Engineer, Thenno-Mechanical Systems Branch, Power and &-Board Propulsion Technology Division NASAA’M-2003-2 12597 1