Mitigation of Orion Ammonia Boiler Outlet Coolant Thermal Stratification

Part of NASA’s Orion Multi-Purpose Crew Vehicle (MPCV) active thermal control system is the Ammonia Boiler Heat Exchanger (ABHX). The ABHX is used as a topper when the Orion radiators cannot provide sufficient cooling and is the sole method of cooling the spacecraft after the command module/service module separation prior to reentry. Ammonia and propylene glycol water (PGW) flow through the heat exchanger in a counter-flow fashion, allowing the evaporating ammonia to cool the PGW. After exiting the heat exchanger , the PGW travels through a 0.5-inch diameter tube with three bends and enters temperature sensor block containing two thermistors. Development testing showed that the floe at the sensor block was not well mixed - the two PGW temperature sensors registered temperature differences of up to 5°C.

A gravity fed water test stand was constructed of 1 inch clear PVC pipe to investigate the stratification. The test stand contained a long lead in section to allow the flow to fully develop. The bend area was geometrically scaled and the water flow rate was set to match the PGW Reynolds number of approximately 1000. Dye injection was used to visually assess the flow and mixing.

Baseline testing clearly showed that the flow was poorly mixed at the thermistor location. Turbulators of different types and lengths were added to mix the flow. In the end, a turbulator configuration was chosen that resulted in well mixed flow while adding minimal pressure drop.

In the present work, the Orion stratification issue is described, the test stand configuration and scaling is detailed, and the test is discussed. The chosen turbulator configuration is explained and its effect on the performance of the Orion ABHX is described.