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Additive Manufactured Non-Metallic Heat Spreading Device for Enhanced Integrated Cooling and Electrical Performance in T-Type Inverter System
Keywords: thermal management, power density, integrated cooling
As energy demands and power electronics density scale concurrently, reliability of such devices is being challenged. Inadequate thermal management can cause system-wide failures due to thermal run-away, thermal expansion induced stresses, interconnect fractures and many more. Conventional techniques used to cool devices consist of heavy, metallic systems such as cold plates and large heat sinks, which can significantly reduce the overall system power density. Moreover, the manufacturing of such components is expensive and often requires custom-made cold plates for improved integration with the electronic system. Although used as a standard practice, these metallic thermal management systems have the potential to intensify electro-magnetic interference (EMI) when coupling with high frequency switching power electronics, and the material density increases the weight of the system, which is detrimental in mobile applications. Lastly, cold plates and heat sinks can create non-uniform cooling profiles in the electronics due to the insufficient management of hot-spots. To combat these drawbacks, a new heat spreader design has been proposed which reduces weight and EMI effects while eliminating hot-spots through localized fluid impingement. Using an additive manufacturing-based pathway, the creation of intricate internal geometries gives the ability to direct fluid exactly where needed for uniform and controllable cooling profiles. Through this manufacturing process, non-metallic materials can made into a light-weight, mobile-conscious thermal management package. The working principle behind the performance lies in the direct fluid impingement onto hot-spots which enhances the local heat transfer performance. Upon doing so, maximum device temperatures are shown to decrease, and more uniform temperatures are able to be maintained throughout the module. Strategically placed outlet paths convectively spread heat away from the module in excess of high conductivity metals, such that optimal cooling profiles can be produced when properly designed. Even without the benefit of conduction, while localized cooling has the potential to increase average module temperatures, the overall maximum can be reduced, consequently providing improved reliability through reduced thermal gradients. Using computational fluid dynamics, a parametric study of the heat spreader design can be performed with respect to two variables: heat transfer coefficient and pressure drop. Design considerations such as nozzle geometry and placement, impingement angles, cross-flow interactions and manifold layouts are chosen to reduce the pressure drop across the device while maintaining the appropriate heat transfer coefficient levels. Once suitable nozzle features are chosen, a custom-designed manifold system is designed to provide the ideal flow rate to each nozzle for optimal cooling profiles. Lastly, outlet paths are placed such that the heat removed through impingement is spread to create a uniform temperature profile in the electronics. When combined together, low pressure drop manifolds and high heat transfer nozzle features form a coupled solution that can be custom-tailored for an assortment of hot-spot arrangements. The manufacturability of such designs is an issue for conventional techniques, but not for additive manufacturing. Using AM, these complex designs are printed to create exact models in numerous light-weight, non-metallic materials. Furthermore, AM not only gives design flexibility but can accommodate different manufacturing techniques, such as investment casting. Through 3D printing a negative-image model of the heat spreader internal geometry, investment casting allows for the desired material to be implemented in each appropriate setting. However, the surfaces of such models have the potential to contain high surface roughness and small print defects. Therefore, post-processing is used to create an ideal situation where intricate features are maintained and surface friction factors are reduced. In addition to printing intricate details, the use of non-metallic materials creates a mobile-conscious system that also limits the development of intensified EMI. An experimental flow loop is used to test the performance of the device with regards to pressure drop and fluid temperatures. Moreover, a resistive heater assembly is used to simulate power losses from electrical devices, mimicking the integration with a 150kW T-type inverter topology developed for electrified aircraft propulsion. Temperature measurements are also taken in the assembly, in addition to the monitoring of heater power consumption. Initial experimental results show at a flow rate of 2 GPM and an input wattage of 200 W, the heat spreader gives a pressure drop of 9 PSI and a heat transfer coefficient of 3800 W/(m2*K). Furthermore, a performance metric has been developed by taking the pressure drop multiplied by the thermal resistance and cooling system mass. When taking the area of under curve of this metric plotted versus flow rate, the new heat spreader shows a 190% performance increase compared to an off-the-shelf cold plate selected for devices with power losses in the range of 200 W. These performance values are with the first-generation heat spreader design that was developed as a proof of concept. Using CFD, this design will be further optimized with the goal to minimize pressure drop and increase the heat transfer coefficient. Additionally, manufacturing methods with the emphasis on minimizing pressure losses are being developed. Using a mix of investment casting and chemical vapor smoothing, there is an ample amount of opportunity to further improve performance while creating an efficient manufacturing method.
David Huitink, Assistant Professor
University of Arkansas
Fayetteville, AR

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