Effect of orientation on the steady-state performance of vapor compression cycles

The number of vapor compression cycles in microgravity is still small, especially relative to the increasing rate of developments supporting space exploration. Ground-based inclination testing helps to understand the effect of gravity on two-phase cycles and can increase confidence into their operability in space. An investigation was conducted by operating a vapor compression cycle at different orientations, always for a long enough time to achieve steady-state operation. Experiments were conducted with R134a and R1234ze(E) across three different mass fluxes. In general, both refrigerants reacted similarly to inclination changes. Significant mass flow rate oscillations were observed in the suction line due to inclination changes in a transient study. These had larger amplitudes at lower flow rates. The steady-state conditions plotted as a function of the inclination angle for one set of control parameters resulted in sinusoidal behaviors with varing “amplitudes”. A semi-mechanistic heat exchanger model was leveraged to track the hydrostatic pressure drop of all coil-segments as the test rig was rotated. Based on comparing experimental and model results, it is hypothesized that changes in orientation led to an accumulation of refrigerant in the evaporator causing a higher pressure drop not captured by the model.