Energy Rejection Systems for Very High Altitude Aircraft

High altitude aircraft are now being used for atmospheric sampling and other purposes, but significant advances in performance must be achieved for sustained flight at altitudes up to 30 km. It is proposed to replace the standard metal finned circulating glycol/water heat exchanger with a droplet heat exchanger developed in this program. A circulating liquid sprayed into a ducted airstream creates a dense droplet field that directly transfers heat from the liquid to the gas and is then captured at the end of the duct by inertial separation from a sharply turning flow. A droplet heat exchanger efficiently rejects engine heat under widely varying conditions while greatly reducing heat exchanger weight and lowering aircraft drag. Heat transfer can be increased for high altitude flight, lower weight increases altitude capability and together with decreased drag increases range. Heat transfer and droplet collection can be maintained during speed and attitude changes, and the system tolerates rain, snow, and dust. DHXs have already been demonstrated experimentally; a Phase 1 experimental program is proposed to demonstrate the feasibility of a droplet heat exchanger for a UAV.