Statistical Modeling of Plume Exhausted from Herschel Small Nozzle with Baffle

摘要

Constantly released Helium on board of the Herschel spacecraft is used to cool three scientific instruments down to 0.3 K. The Helium is released by small nozzles creating a counter-torque. This compensates the torque caused by the solar pressure acting on the spacecraft surfaces. An optimization of the nozzle shape did not allow avoiding severe plume impingement on the spacecraft surfaces and consequently the application of baffles has been considered to reduce plume impingement effects. Two baffle shapes, cylindrical and conical, with different radii and lengths have been analyzed numerically. The analysis has been performed with a kinetic approach, namely, the direct simulation Monte Carlo (DSMC) method to cope with changing flow regime from continuum in a subsonic part of the nozzle through transitional to free-molecular flow inside the baffle. A direct application of DSMC-based software would require large computer resources to model the nozzle and plume flows simultaneously. Therefore, the computation was split in two successive computations for the nozzle and nozzle/plume flow. Computations of plume flow with and without baffles were performed to study effects of baffle size and shape on the plume divergence, and plume impingement on the Herschel spacecraft. It has been shown that small baffles even widen the plume. An increase of radius/length of the baffle decreases the plume divergence, however even the largest baffle could not meet the requirements.