N23A-T024   TITLE: Compact Condensers Enabled by Additive Manufacturing

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): General Warfighting Requirements (GWR)

OBJECTIVE: Develop novel, additively manufactured refrigerant to water condensers for use in electronic cooling systems.

DESCRIPTION: Cooling of high power electronics via refrigerant evaporation presents a significant opportunity to reduce the size, weight, and power (SWaP) consumed by thermal management systems. Recent advances have led to demonstration of compact microchannel evaporators with heat transfer coefficients in excess of 100 kW/m2^K and low pumping powers. Condensers are often the largest component in a two-phase cooling system. Commercial condensers are typically shell and tube designs with heat transfer coefficients less than 5000 W/m^2K. Military systems often use a secondary coolant loop to transport heat from condensers. For example, naval platforms use a freshwater cooling system, while air platforms have a polyalphaolefin (PAO) coolant loop. Recent progress in metal additive manufacturing has enabled the fabrication of air heat sinks and cold plates with complex flow geometries. Additive manufacturing also allows for thinner walls, reducing weight and thermal resistance. Most metal additive manufacturing technologies are based on powder processing, with a limited number of alloys commercially available. Process parameters can greatly affect the microstructure and mechanical properties of the resulting metallic heat exchanger, leading to concerns about long term durability. Heat transfer enhancement using internal structures embedded inside flow passages is commonly used to increase local convection heat transfer, but has been limited to simple structures by conventional manufacturing techniques. Additive manufacturing can enable more complex structures, but demonstrations to date have been largely limited to intuitive designs. Topology optimization has recently been applied to the design of additively manufactured cold plates, but the designs are limited to two-dimensional optimization due to computational complexity. New tools are needed to enable the full potential of additive manufacturing to realize optimal three-dimensional designs. This STTR topic seeks innovative condenser designs enabled by metal additive manufacturing for efficient cooling of electronics.

PHASE I: Design a compact, additively manufactured, metal heat exchanger for condensing a refrigerant with saturation temperatures below 35 �C (R134a preferred) using freshwater at 25 �C for heat rejection. Pressure drop on both the refrigerant and water side should be minimized to optimize efficiency. Verify feasibility using modeling and/or component demonstration. Perform rough manufacturing cost analysis. Develop a Phase II plan.

PHASE II: Demonstrate a prototype R134a condenser using the concept developed in Phase I. The prototype should be able to reject 50 kW of heat to a fresh water system at 25 �C. Evaluate the efficiency of the prototype under various electrical and cooling loads and temperatures. Performance data, including heat transfer and pressure drop, shall be collected at a variety of flow rates (both refrigerant and water), temperatures, and entrance qualities. Validate analytic models developed in Phase I and evaluate scalability of design.

PHASE III DUAL USE APPLICATIONS: Design and develop the next generation of compact, high efficiency condensers using the knowledge gained during Phases I and II. These heat exchangers must meet military unique requirements, e.g., shock and vibration. Advanced condensers developed here would be suitable for use in a variety of commercial applications, from hybrid vehicles to building air conditioning.

REFERENCES:

1.       X Zhang, R Tiwari, A.H. Shooshtari, and M.M. Ohadi, "An additively manufactured metallic manifold-microchannel heat exchanger for high temperature applications," Applied Thermal Engineering, vol. 143, pp. 899-908 (2018). https://doi.org/10.1016/j.applthermaleng.2018.08.032

2.       S. Sun, P. Liebersbach, and X. Qian, "3D topology optimization of heat sinks for liquid cooling," Applied Thermal Engineering, vol. 178, 115540 (2020). https://doi.org/10.1016/j.applthermaleng.2020.115540

3.       H. Moon, D. J. McGregor, N. Miljkovic, and W. P. King, "Ultra-power-dense heat exchanger development through genetic algorithm design and additive manufacturing." Joule vol. 5, pp 3045-3056 (2021). https://doi.org/10.1016/j.joule.2021.08.004

 

KEYWORDS: thermal management; two-phase cooling; condenser; additive manufacturing; heat exchanger

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