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Vapor compression cycles would have many applications in the space industry if it was not for the uncertainty imposed by microgravity environments on two-phase systems. A first step towards Zero-G for technologies involving fluid dynamics can be terrestrial testing at different orientations. For vapor compression cycles, there is very little litera...
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... interpretation of some inclination test results requires knowledge of the relative component positioning. This is shown in Figure 1 and Figure 3. In the horizontal orientation, the compressor discharge enters the condenser at the highest point of the system from where it flows down in a helical coil condenser. ...
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... versa, if gravity pulls the liquid refrigerant away from the expansion valve or towards the compressor, the angle is defined as negative. This is exemplified in Figure 3. Subsequent experiments showed that the prediction of a positive angle is not always as trivial, but the definition will be kept for consistency. ...
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... The inclination angle was changed to í µí¼ = +30°, which lowers the evaporator with respect to the compressor. The suction line is then an inclined tube with upward flow, as can be seen from Figure 3. After 45 seconds, the test stand was turned back to the horizontal position (í µí¼ = 0°). ...
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Citations
... Brendel et al. [11] installed a VCC on an inclinable test stand which allowed to change the inclination angle during continuous operation of the cycle. An anomaly, later labeled as unstable steady-state, occurred during inclination testing with R134a which was not readily explainable: The test stand was operated in the horizontal orientation at steady-state with a mass flow rate of 1.82 g/s. ...
... Previous publications of experimental data from this test stand focused on anomalies due to orientation changes [11] as well as dynamic changes of the inclination angle every 2 min or every 20-40 s [15,16]. For the research presented in this paper, steady-state operating conditions were achieved at each step of a series of inclination angles. ...
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.
... One of the studies found the VCC to be almost orientation independent while the others found strong effects including a loss of subcooling and liquid flooding of the compressor. The three studies are summarized in the introduction of Brendel et al. (2021). ...
Vapor compression cycles are rarely used in microgravity environments. Although the energy savings would be appreciable, the uncertainty of the cycle behavior in microgravity is too large relative to more established cooling technologies. One approach to increase confidence is testing the cycle at different orientations on the ground to change the gravity vector experienced by the cycle. A previous study found a small scale, liquid-to-liquid vapor compression cycle was able to operate continuously through 360 degrees. Increased stability of the cooling capacity was found for higher mass fluxes. The instability was quantified by the relative deviation of a measurement due to inclination changes from its horizontal steady-state value. This study continues the research with the same test rig but using air-to-refrigerant, fin-and-tube heat exchangers instead of the previously used liquid-to-refrigerant, tube-in-tube heat exchangers. The smaller diameter of the fin-and-tube evaporator led to higher mass fluxes which further increased the stability of the cycle compared to the previous, tube-in-tube heat exchanger study. Combining data from both test stand configurations, the instabilities decreased smoothly from 24% to 3% over a wide range of mass fluxes from 3 kg m-2 s-1 to 60 kg m-2 s-1 for the investigated test stand. The influence of the different heat exchanger geometries and an effect called "unstable steady-states" on the instability quantifier are discussed.
... This reduces the heat exchanger footprint compared to an air to refrigerant heat exchanger. A tube-in-tube heat exchanger is chosen over a flat-plate heat exchanger due to an expected higher stability against changes in orientation and the gravity vector (Brendel et al., 2021) 7 . While the operation in microgravity does not require gravity independent components, testing prior to launch at varying orientations can increase the confidence in the technology if the system is stable. ...
Thermoelectric cooling, reversed Brayton cycles and Stirling cycle devices are common cooling technologies employed on spacecraft where temperatures below the radiator temperature are needed. Although used less often in microgravity, vapor compression refrigeration was proposed for microgravity applications as early as the 1970s. Researchers proposed the vapor compression cycle over other cooling technologies for its superior energy efficiency in the typical refrigerator/freezer and air-conditioning temperature range. The higher energy efficiency should alleviate the mass penalty of the thermal system, conceivably both for satellites and manned spacecraft. Despite numerous propositions, the literature lacks a discussion of the absolute equivalent mass benefit achievable and the value of it relative to the total mass of the thermal system of the spacecraft or the spacecraft itself. This paper presents the equivalent mass benefits of employing a vapor compression system over other cooling technologies using a mass penalty factor for power consumption. Additionally, a prototype vapor compression cooler sized to fit into an ISS locker is presented.
Long duration manned space travel is projected to bring a need for large cooling capacities in microgravity for typical freezing and refrigeration temperatures. Among possible cooling technologies, the vapor compression cycle has the highest coefficient of performance but the confidence for microgravity applications is very low. This research effort experimentally investigated the effects of hyper and microgravity on a vapor compression cycle during parabolic flights. A total of 122 parabolas were flown over four days with a repeating pattern of 5 sequential parabolas. Transient data of the evaporation temperature and cooling capacity for selected parabolas are presented as the test stand experienced alternating hyper and microgravity levels. Most measurements and performance indicators showed mild effects on the cycle operation during the varying g-forces. For example, the refrigerant side cooling capacity fluctuated on average within a band of 15 % through all sets of parabolas. A loss of superheat due to gravity changes was observed for one set of parabolas, in which the operating conditions of the cycle showed only 3 K superheat at the onset of gravity changes. For all other sets of parabolas, superheat and subcooling were maintained. Flight results were compared with inclination testing in a laboratory using the same test stand. Inclination changes from -90° to + 90° impacted both the liquid and suction line mass flow rate while varying the gravity level between 0 and 1.8 g affected the suction line mass flow rate but not the liquid line mass flow rate.
