Experimental and theoretical studies of spray cooling for high power electronics
Date of Issue2014
School of Mechanical and Aerospace Engineering
Spray cooling as a promising thermal management scheme has received much attention from modern industrial and technological applications, such as power electronics, nuclear power generation, high-power lasers and high-power conversion systems. The present work focused on the fundamental investigations of the spray characteristics of spray nozzles in free spray atomisation and in spray cooling. Theoretical models were developed to study the heat transfer in the non-boiling and boiling regimes of spray cooling. A closed loop system was developed to study the applications of using a multi-nozzle array on large area high heat load electronics cooling. Furthermore, the effects of structured surfaces on the spray cooling performance were scrutinised. Two optical techniques, Phase Doppler Anemometry (PDA) and Particle Image Velocimetry (PIV) were used to characterise the spray structures of pressure swirl nozzles. In free spray atomisation, the spray characteristics were found to be highly dependent on the axial distance. The spray cone produced by the pressure swirl nozzles evolved from a hollow spray cone to a full spray cone with the increase of axial distance. In spray atomisation under spray impingement, the spray cone formation of pressure swirl nozzle is largely dependent on the temperature of the impinged surface. The spray cone of a pressure swirl nozzle expands after impinging on a surface with a relatively high temperature. As a result, the impinging droplet flux near the centre of the spray cone decreases and the spray cone changes from a full spray cone to a hollow spray cone. The heat transfer experiments show that the effects of nozzle-to-surface distance on the heat transfer performance are complex and dependent on surface temperature. The expansion of the spray cone has significant effects on the surface temperature non-uniformity and heat transfer coefficient in spray cooling. A thin film flow model was developed to estimate the thickness of the liquid film formed under spray impingement. On the basis of the thin film flow model, a heat transfer model was developed to study the heat transfer in the non-boiling regime of spray cooling. The modelling results showed that the local film thickness was sensitive to the local droplet flux density and the droplet impingement cooling was the primary heat transfer mechanism in the non-boiling regime of spray cooling. To better understand the heat transfer in the boi ling regime of spray cooling, a numerical model based on the experimental spray characteristics was proposed to investigate the dynamics of droplet impingement, bubble boiling as well as their interplay in the spray cooling process. The effects of heat flux, droplet diameter, and droplet impingement frequency on the dynamics of bubble boiling were investigated. The numerical model showed that the fluxes of the collapsed bubbles due to the limited bubble size, as well as the punctured bubbles due to the droplet impingement increased as heat flux increased. A smaller impinging droplet is favourable for bubble boiling due to the more secondary nuclei induced as well as the larger fractions of the bubbles punctured at bigger diameters. A prototype of a high power closed loop spray cooling system using Rl34a as the working fluid was constructed to study the feasibility of using a multi-nozzle array on a 6U electronic card cooling. Fifty four pressure swirl nozzles were assembled in an array of 9 x 6 to cover a 6U card surface. Simple drainage concepts were introduced in the spray chamber design. The experimental results show a promising prospect of using multi-nozzle arrays on large area power electronics cooling. Heat removal of 16 kW is achieved on the 6U card surface by maintaining the surface temperature below 26.5°C. High heat transfer coefficient (up to 2.8 W /cm2 · K) and high liquid evaporation fraction (up to 0.88) are obtained. Without affecting the surface temperature non-uniformity, the control of chamber pressure is able to maintain the same operating temperature of a device at different heat loads. Experiments were conducted to study the thermal effects of differently scaled structure surfaces in an Rl34a spray cooling system. Results show that the arrangement of macro- fins plays a more important role in the cooling performance of macro-structured surfaces rather than a simple increase in the wetted area. Microstructures improve the heat transfer performance by enhancing the capillary effects and providing more potential nucleation sites on the heated surface. Taking the smooth flat surface as a reference, the micro-structured flat surface achieves a relative heat transfer enhancement of 32% compared to the 36% of the macrostructured surfaces, while the multiscale-structured surfaces which combine the features of micro- and macro- structures gain a heat transfer enhancement of up to 65%. Besides, the macro-structured surfaces prolong the transition period before CHF occurs and shorten the duration that the heated surface remains in film boiling after the occurrence of CHF while powering off the heat source.