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Advanced Two-Phase Heat Exchangers for Environmental Control


OBJECTIVE: Develop compact heat transfer technologies to improve the performance of two-phase heat exchangers. DESCRIPTION: Tube-fin heat exchangers are used to acquire or reject heat in a wide range of military and commercial thermal control systems, including ground and sea-based environmental control units, refrigeration systems, and waste heat regeneration systems. As power-density of tactical systems continue to increase, the use of two-phase thermal control systems is becoming more widespread due to improved thermal capacity when cooling or heating through the latent heat of a working fluid. Improving the effectiveness of two-phase heat exchangers used to transfer heat between the working fluids and air streams will lead to reductions in size, weight, and power consumption of military environmental control systems. Improved heat exchanger designs utilizing microchannels, plate-fin configurations, and surface enhancements offer performance advantages over traditional tube-fin heat exchangers, but have not yet been fully utilized in many military and commercial systems. While developers have incorporated these improvements in single-phase systems, they have yet to be utilized reliably with two-phase systems. This is largely due to two-phase flow maldistribution and condensation build-up in the heat exchanger, which results in poor heat transfer performance, as well as high manufacturing costs. Technologies are sought to improve the volumetric heat transfer performance over tube-fin heat exchangers while maintaining flow rates, temperatures, and differential pressures characteristic of heat exchangers used for environmental control. Methods to ensure proper distribution of a two-phase mixture at the entrance to multiple fluid passages and through the device must be proven. PHASE I: Develop concepts to provide improved heat transfer performance over conventional heat exchangers and provide a uniform distribution of a two-phase working fluid at the inlets to multiple fluid passages. Validate design performance through analytical modeling or subscale demonstration of components as appropriate. PHASE II: Demonstrate a working prototype of an evaporator sized to a 5-ton (60,000 BTU/hr) Environmental Control Unit. Experimentally validate the unit"s performance over a variety of refrigerant flow rates and inlet qualities, assuming an air side 80/67 degrees F dry bulb/wet bulb temperature and 2000 cfm. Complete a cost analysis of concepts established to ensure the selected technology is competitive with tube-fin manufacturing processes. PHASE III: Optimize the concept design for manufacturability and heat transfer performance using the knowledge gained during Phases I and II. This heat exchanger must meet military unique requirements such as corrosion resistance, shock and vibration. Perform detailed design and fabrication of evaporator for transition into advanced environmental control systems demonstrations. Conduct extended testing to verify performance. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Increasing the effectiveness of two-phase heat exchangers provides an enabling technology to reduce size and weight, as well as improve the energy efficiency, of commercial HVAC systems.
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