N241-016 TITLE: Frost Icephobic Coating for Subfreezing Environmental Control Systems (ECS) Components

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Sustainment

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop an icephobic coating that mitigates frost formation on downstream heat exchangers inside an Environmental Control System (ECS) environment for extended periods of time with minimal maintenance required.

DESCRIPTION: Unconstrained thermal operations for an aircraft is driving integrated thermal management designs. Condenser icing is a limitation for an aircraft air conditioning system using a high-pressure water separator to remove moisture from cooling air.

Inside the aircraft’s ECS, frost can occur downstream of the Air Cycle Machine’s (ACM) turbine outlet where the turbine expands the air and drops the temperature. Most ECSs are open loop in design, allowing moist air into the system, and that moisture can reach the turbine outlet cold area. To mitigate that moisture impact, high pressure water separators are utilized to remove the majority of the humidity. In addition, hotter air (trim air) is routed at the turbine’s outlet to the condenser heat exchanger to prevent the temperature from going below freezing (example 35 °F–40 °F [1.67 °C–4.44 °C] as the controlled temp) [Ref 2]. This trim air wastes overall bleed air and limits the overall cooling capacity of the ECS due to the temperature regulation above freezing. The reheating and cooling of the bleed air in the water extraction process is carried out in a cross-flow plate-fin or tube heat exchanger called the condenser. The condenser causes water droplets to form on the hot side fin surfaces from the process of heat transfer from a cold side air/fluid stream. Frost can form on the cold side due to less than 100% water separation. On the hot side, frost can form due to bleed air cooling below its dew point during certain flight conditions [Ref 2]. Ultimately, frost buildup leads to ice accretion shedding, increasing pressure drops affecting the ACM performance, and greater thermal resistance [Ref 2]. Newer ECS designs (subfreezing condensers) aim to eliminate more moisture and mitigate icing from forming quickly on components downstream of the turbine’s outlet. However, even lower temperatures lead to quicker frost formations on the condenser exchanger and duct interfaces.

The simplest approach to mitigate these frosting issues is to design fin surfaces that prevent frost formation in the first place. Such an approach can be accomplished by encapsulating target surfaces with an icephobic (ice resistant) coating. The difficulty in mitigating ice and frost in subfreezing temperatures using icephobic coatings/surface treatments is that fast impinging moisture freezes very quickly, penetrating the coating microstructure and locking/anchoring the ice in place. In addition, icephobic coatings are typically used to facilitate ice shedding, but this SBIR topic seeks surface treatments that can prevent condensed droplets from freezing into ice in the first place. New or alternate icephobic coatings/surface treatments for heat exchanger geometries that could potentially eliminate frost growth by preventing freezing of condensed water or direct deposition of water vapor to ice crystals on the surface is the primary goal of this topic. Metrics for performance will include water condensation rate, decreased frost onset time, and reduced frost thickness.

PHASE I: Demonstrate the feasibility of the proposed formulation for the icephobic coating/surface treatment to minimize ice formation in heat exchanger/condenser components that are inside the ECS. Identify the anticipated merits of the preferred solution related to thermal performance, manufacturing, installation, durability, and cost. Develop a plan to address any technical hurdles with the coating/surface treatment. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Fully develop and analyze the selected Phase I solution for a range of condenser environments and frost icing conditions. Develop subscale and/or full-scale hardware to demonstrate the selected approach for a representative heat exchanger geometry and establish the technology and manufacturing readiness level.

PHASE III DUAL USE APPLICATIONS: Produce a final icephobic coating/surface treatment that is ready for primary applications in advanced military fighter aircraft and possibly future commercial applications. Provide documentation for icing and ECS operational limits. Provide appropriate qualification documentation for environmental testing.

The icephobic coating technology could be used on commercial aircraft ECS. Additional applications may exist for naval refrigeration applications.

REFERENCES:

1. Lee, C., Hess, E., Beaini, S., Li, S., Bacellar, D., Nasuta, D., & Martin, C. (2021). Durability and performance evaluations of superhydrophobic and icephobic coatings for tube-fin heat exchangers. International Refrigeration and Air Conditioning Conference, https://docs.lib.purdue.edu/iracc/2255/
2. Koszut, J., Boyina, K., Popovic, G., Carpenter, J., Wang, S., & Miljkovic, N. (2022). Superhydrophobic heat exchangers delay frost formation and reduce defrost energy input of aircraft environmental control systems. International Journal of Heat and Mass Transfer, 189, 122669. https://doi.org/10.1016/j.ijheatmasstransfer.2022.122669

KEYWORDS: Icephobic; coating; heat exchanger; icing; frost; condenser

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