Superhydrophobic heat exchangers delay frost formation and reduce defrost energy input of aircraft environmental control systems

• Published in: International journal of heat and mass transfer 2022-06, Vol.189, p.122669, Article 122669
• Main Authors: Koszut, Joe, Boyina, Kalyan, Popovic, George, Carpenter, James, Wang, Sophie, Miljkovic, Nenad
• Format: Article
• Language: English
• Subjects: ACM; Air conditioners; Aircraft; Aircraft compartments; Aircraft control; Aluminum; Commercial aircraft; Condenser; Control systems; Defrosting; ECS; Efficiency; Environmental control; Frost resistance; Frosting; Heat exchanger; Heat exchangers; Hydrophobic surfaces; Hydrophobicity; Icing; Jumping droplet condensation; Passenger comfort; Reverse-Brayton cycle; Superhydrophobic; Thermal control; Turbines; Wettability; Wind tunnels
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2022.122669

•Air cycle machine efficiency increased by using superhydrophobic heat exchangers (SHPHX).•Heat transfer rate and pressure drop is measured and compared to an uncoated counterpart.•Droplet jumping, droplet removal, and condensation are visually analyzed.•SHPHX show increased frost cycle times and can eliminate frost formation completely.•SHPHX show reduced defrost-to-cycle time ratios and lower water retention. Commercial aircraft operate over a wide range of atmospheric conditions, with temperatures exceeding 40 °C during takeoff and dropping below -50 °C at cruising altitudes near 10 km. An aircraft environmental control system (ECS) works to keep the crew and passengers comfortable by supplying them with conditioned air. The refrigeration unit of these aircraft, known as the air cycle machine (ACM), sees extreme conditions, ranging from engine bleed air exceeding 100 °C at the ACM compressor inlet to subzero temperatures at the ACM turbine outlet. These conditions can cause the ACM's condenser to frost, reducing the efficiency of the ECS. Engine bleed air can be used to defrost the condenser at the price of further reducing efficiency and cabin comfort. A larger condenser can alleviate frosting but will increase the cost and weight of the aircraft. An alternative is to apply a superhydrophobic coating to the condenser to provide increased resistance to frost growth with a minimal impact on nominal heat exchanger efficiency. In this work, we modified the surface wettability of an aluminum heat exchanger and tested it in a wind tunnel under a wide range of hot-side temperatures to study both frosting and defrosting performance. The superhydrophobic heat exchanger showed considerable improvement in system efficiency, sometimes completely eliminating frost growth and lowering the cycle normalized defrost time by up to 50%. A superhydrophobic coating on the ACM condenser of a turbine-powered aircraft has the potential to increase overall efficiency of the ECS and improve the energy efficiency of aircraft. [Display omitted]