Computational analysis for sublimation enhanced heat transfer of CO2 jet impingement cooling

Abstract

Impinging jet has been applied to various industry fields, such as heating, cooling, drying or cleaning surfaces. Because of the benefits of high local heat transfer coefficients of impinging jet, this technique is widely employed for spot cooling. Liquid and gas phase fluids can be commonly used as coolants in this technique. Gas jet cooling needs simple equipment for cooling but the gas jet has lower cooling efficiency than liquid jet cooling. In case of liquid jet cooling, closed-loop system to recirculate coolant is needed. For the electronics cooling or the case requiring simple cooling system, the gas jet with high cooling efficiency is requested. This study suggests the novel idea of dry-ice assisted jet impingement cooling and investigates its performance prediction computationally. When the carbon-dioxide (CO2) passes through the tiny orifice gap or jet nozzle, it experiences the rapid temperature drop as well as pressure decrease by the Joule-Thomson effect. This temperature drop causes the formation of small CO2 dry-ice particles. Besides the enhanced cooling performance by lowered bulk-jet temperature, the significantly improved heat transfer can be expected by the additional sublimation effect between the dry-ice particles and cooling target. Computational analysis of solid-gas two-phase jet impingement flow was aided by using a baseline two-phase solver of commercial CFD package. Additional in-house code accounting for the sublimation effects is embedded into the solver. The effect of sublimation on heat transfer performance was investigated by the variance of flow parameter like Reynolds number, solid phase concentration and jet geometries. As a result, gas-solid jet with sublimation has higher heat transfer coefficient and sublimation maintains the temperature of jet flow low. As the amount of sublimation increases with Rew increasing, heat transfer enhancement is observed more clearly with higher Rew. Irregular distribution of Local heat transfer coefficient near stagnation region is caused by turbulence effect of jet core development and sublimation makes more irregular distribution of local heat transfer coefficient. The jet core development which affects the activity of sublimation near the stagnation region is changed by inlet velocity, nozzle-spacing distance and nozzle width. The more developed jet core is impinged on surface, the more amount of sublimation occurs. Therefore, heat transfer is enhanced with higher inlet velocity, longer nozzle-spacing distance and narrow nozzle width.