Investigation of heat transfer mechanism of low environmental pressure large-space spray cooling for near-space flight systems

There is an imperative need to employ the on-board electro-driven equipment since the concept of all-electric and more-electric technology has been applied in the design of near-space flight systems. This application will inevitably bring a dramatic increase in the waste heat because of incomplete energy conversion, which will easily cause an overheating in the electrical device. What's worse, the near-space flight system will encounter extremely tough thermal environment where the atmosphere is rarefied due to the high altitude and the temperature of the introduced air into the plane is awfully high owing to the aerodynamic heat, both of which will pose a great challenge for the near-space thermal control system. Aiming to design an efficient thermal protection strategy of the on-board permanent magnet synchronous motor for the steering of major flight control which operates constantly in the near-space cruise stage, this paper proposes a novel low environmental pressure large-space spray cooling system. Revolving the scheme, a small-scaled experimental prototype was designed and established, upon which thermal tests were conducted to estimate influence of several parameters such as environmental pressure, nozzle inlet temperature and spray volumetric flow rate upon the cooling performance. Both flash boiling and subcooled spray phenomena were discussed, results of which suggests the former is more desirable due to the temperature uniformity and high coolant utilization. In addition, capable of providing performance predictions and guidances for future full-scaled tests, an empirical experimental correlation with the relative error of only ±8% was acquired on the basis of the data in the flash boiling region.