Advances in aircraft technology have brought about a necessity for new aircraft thermal management architectures in order to maintain reasonable cost, size, weight, and power requirements of the overall thermal system. Two-phase cooling technologies such as vapor-compression systems have demonstrated significant benefits and offer a serious option for emerging new aircraft thermal management applications. Although vapor-compression technology offers a steady state solution to many of the limitations of existing aircraft thermal management systems, industry concerns about transient behavior need to be addressed. The purpose of this research was to investigate transient effects on the vapor compression system when the majority of the onboard thermal loads are cooled directly with the vapor-compression system and how these systems operate under the rapid thermal transients that a military aircraft experiences during combat missions.
Within this overall study, an experimental investigation was performed to evaluate the transient capabilities of a vapor-compression thermal bus while also rejecting waste heat to variable environmental conditions representative of the conditions experienced during a combat mission. For this analysis, a vapor-compression thermal bus was developed to directly cool four independent, high-heat-flux thermal loads. The system was then evaluated for its ability to react to the changing conditions, maintain a uniform temperature at the cooled components, and maintain desirable fluid conditions at the inlet to the system's compressor. In addition, the effects and penalties of a recuperative heat exchanger as a method to improve reliability (through increased compressor inlet superheat) was also experimentally evaluated.
In prior publications, the authors have discussed the Heat Pump Loop Thermal Bus and the advantages of using the individual throttling valves at the inlet of each cold plate to control the exit superheat from each individual cold plate. However, industry concerns about the ingestion of transient liquid slugs of refrigerant by the compressor and the concern that possibly a liquid-vapor separator at the inlet of the compressor was required were a primary reason for these demonstration experiments. During experiments, operation without accumulation of liquid upstream of the compressor was maintained. This was done while autonomously managing the superheat at the exit of each cold plate, maintaining a near uniform temperature through the cold plates, and operating efficiently by controlling the speed of the compressor. Through this evaluation, it has been shown that vapor-compression technology can operate reliably with both transient thermal loading conditions and transient environmental conditions that can be expected during military air combat operations.