Numerical simulation for the effects of nozzle geometry and engine thrust on vacuum plume radiation characteristics

Chemical engines are commonly used for spacecraft attitude and orbit control, where high-temperature and high-pressure exhaust expands into the vacuum, generating a plume-like flow field, known as the vacuum plume. The vacuum plume contains abundant gases with radiation capability, such as H₂O and CO₂. The radiation characteristics of the vacuum plume depend on critical engine design parameters, such as combustion chamber thermal properties, nozzle expansion ratio, and outlet expansion angle. This study investigates the effects of nozzle geometry and engine thrust on plume radiation characteristics, using the coupled Computational Fluid Dynamics and Direct Simulation Monte Carlo (CFD-DSMC) method for flow field simulation and the Backward Monte Carlo Method (BMCM) for infrared radiation analysis. The results demonstrate that reducing the expansion ratio increases both pressure and temperature at the nozzle outlet, thereby increasing the infrared radiation intensity of the vacuum plume. In contrast, expanding the nozzle outlet angle enhances shock wave dispersion near the lip, ultimately decreasing radiation intensity. For the truncated nozzle, a reduction in outlet diameter leads to a lower expansion ratio but a larger outlet expansion angle. Therefore, due to the complex interaction between expansion ratio and angle, the infrared radiation intensity initially decreases and then increases as the truncated nozzle outlet diameter decreases. Moreover, the results indicated that higher thrust leads to an increase in radiation intensity, as expected. Finally, our finding suggests that increasing engine pressure and reducing nozzle throat size can minimize the infrared radiation characteristics of the vacuum plume for a given thrust.