| Popis: |
Icing, or the formation of ice from water via freezing or water vapor via desublimation,is a phenomenon that commonly occurs within air cycle-based refrigeration systems andrequires thermal control that limits system performance. In aircraft applications icing frequentlyoccurs in the heat exchangers and turbine(s) that are part of the air cycle machine,the refrigeration unit of the environmental control system. Traditionally, water vapor isremoved from an air cycle machine via condensing in a heat exchanger and subsequenthigh-pressure water separation. This approach is not capable of removing all of the vaporpresent at low altitude conditions, corresponding to a high risk of icing. To mitigate icingunder these conditions, a membrane dehumidifier is considered to separate the water vaporthat remains after condensing and liquid water separation.Three distinct investigations are conducted as part of this work. The first is aimed atmodeling approaches for desublimation frosting, or frost growth on sufficiently cold flatsurfaces. This results in a novel, analytical, and non-restrictive solution well-suited for representingfrost growth and densification in moist air heat exchangers. The second investigationconcerns membrane dehumidification and module design. A custom component modelis developed and verified under aircraft conditions, then the Pareto frontier of volumetricallyefficient membrane modules is characterized via a multi-objective optimization study.The final investigation evaluates three two-wheel air cycle subsystem architectures withdiffering dehumidification approaches: (1) condenser-based, (2) membrane dehumidifier-based,and (3) combined. Steady-state simulations are run for each of these over a range offlow rates and altitudes. The results demonstrate that incorporating a membrane dehumidifierreduces the turbine inlet saturation temperature, which mitigates icing in the turbineand reduces the required bypass flow, thus increasing the cooling capacity. |
