Milan Visaria

Application of Two-Phase Spray Cooling for Thermal Management of Electronic Devices

Recent studies provide ample evidence of the effectiveness of two-phase spray cooling at dissipating large heat fluxes from electronic devices. However, those same studies point to the difficulty predicting spray performance, given the large number of parameters that influence spray behavior. This paper provides a complete set of models/correlations that are required for designing an optimum spray cooling system. Several coolants (water, FC-72, FC-77, FC-87 and PF-5052) are used to generate a comprehensive spray-cooling database for different nozzles, flow rates, subcoolings, and orientations. High-speed video motion analysis is used to enhance the understanding of droplet formation and impact on the devices surface, especially near the critical heat flux (CHF) point. A previous CHF correlation for normal sprays is modified for both inclination and subcooling effects. A new user-friendly CHF correlation is recommended which shows excellent predictive capability for the entire database. Also discussed in this paper is a new theoretical scheme for assessing the influence of spray overlap on cooling performance.

A Systematic Approach to Predicting Critical Heat Flux for Inclined Sprays

This study provides a new systematic approach to predicting the effects of spray inclination on critical heat flux (CHF). Experiments were performed with three pressure spray nozzles over a broad range of inclination angles at five flow rates and subcoolings of 15°C and 25°C. These experiments also included high-speed video analysis of spray formation, impact, and recoil for a 1.0 X 1.0 cm 2 test surface. Inclined sprays produced elliptical impact areas, distorted by lateral liquid flow that provided partial resistance to dryout along the downstream edge of the impact ellipse. These observations are used to determine the locations of CHF commencement along the test surface. A new theoretical model shows that increasing inclination angle away from normal decreases both the spray impact area and the volumetric flux. These trends explain the observed trend of decreasing CHF with increasing inclination angle. Combining the new model with a previous point-based CHF correlation shows great success in predicting the effects of spray inclination on CHF.

Application of two-phase spray cooling for thermal management of electronic devices

Recent studies provide ample evidence of the effectiveness of two-phase spray cooling at dissipating large heat fluxes from electronic devices. However, those same studies point to the difficulty predicting spray performance, given the large number of parameters that influence spray behavior. This paper provides a complete set of models/correlations that are required for designing an optimum spray cooling system. Several coolants (water, FC-72, FC-77, FC-87 and PF-5052) are used to generate a comprehensive spray-cooling database for different nozzles, flow rates, subcoolings, and orientations. High-speed video motion analysis is used to enhance the understanding of droplet formation and impact on the devices surface, especially near the critical heat flux (CHF) point. A previous CHF correlation for normal sprays is modified for both inclination and subcooling effects. A new user-friendly CHF correlation is recommended which shows excellent predictive capability for the entire database. Also discussed in this paper is a new theoretical scheme for assessing the influence of spray overlap on cooling performance.

Performance of thermal enhancement materials in high pressure metal hydride storage systems

Over the past two years, key issues associated with the development of realistic metal hydride storage systems have been identified and studied at Purdue University’s Hydrogen Systems Laboratory, part of the Energy Center at Discovery Park. Ongoing research projects are aimed at the demonstration of a prototype large-scale metal hydride tank that achieves fill and release rates compatible with current automotive use. The large-scale storage system is a prototype with multiple pressure vessels compatible with 350 bar operation. Tests are conducted at the Hydrogen Systems Lab in a 1000 ft2 laboratory space comprised of two test cells and a control room that has been upgraded for hydrogen service compatibility. The infrastructure and associated data acquisition and control systems allow for remote testing with several kilograms of high-pressure reversible metal hydride powder. Managing the large amount of heat generated during hydrogen loading directly affects the refueling time. However, the thermal management of hydride systems is problematic because of the low thermal conductivity of the metal hydrides (∼ 1 W/m-K). Current efforts are aimed at optimizing the filling-dependent thermal performance of the metal hydride storage system to minimize the refueling time of a practical system. Combined heat conduction within the metal hydride and the enhancing material particles, across the contacts of particles and within the hydrogen gas between non-contacted particles plays a critical role in dissipating heat to sustain high reaction rates during refueling. Methods to increase the effective thermal conductivity of metal hydride powders include using additives with substantially higher thermal conductivity such as aluminum, graphite, metal foams and carbon nanotubes. This paper presents the results of experimental studies in which various thermal enhancement materials are added to the metal hydride powder in an effort to maximize the effective thermal conductivity of the test bed. The size, aspect ratio, and intrinsic thermal conductivity of the enhancement materials are taken into account to adapt heat conduction models through composite nanoporous media. Thermal conductivity and density of the composite materials are measured and enhancement metrics are calculated to rate performance of composites. Experimental results of the hydriding process of thermally enhanced metal hydride powder are compared to un-enhanced metal hydride powder and to model predictions. The development of the Hydrogen Systems Laboratory is also discussed in light of the lessons learned in managing large quantities of metal hydride and high pressure hydrogen gas.Copyright