Heat Transfer Mechanisms During Spray Cooling of Electronic Devices

The everyday use of electronic equipment and the increase in computation capability and therefore increasing amount of dissipated heat demands efficient systems and methods to remove the waste energy to maintain a stable operation and durability. The efficiency of convective air cooling, e.g. in data centers, is very poor, progressively reaches technical constraints und is very energy consuming and expensive. To reduce the use of energy and cost and save natural resources more efficient systems are needed in future. Two-phase systems such as spray cooling have a very high heat transport capability due to the usage of the enthaply of evaporation at the phase-change. In comparison to single-phase cooling systems, spray cooling allows the reduction of the system filling and size due to a lower demand for working fluid. Intensive research has been done in the past three decades, focussing on the increasing heat transport capability through optimisation of surface topographies. The number of studies on the description of fundamental physical effects and mechanisms is however very limited. The essential and new approaches in this work are: (a) the determination of the ratio of the governing heat transfer mechanisms convection and evaporation, depending on geometrical and operational parameters and (b) the characterisation of the heat transfer during the evaporation of coalesced droplets with the aim to give a base for a description of the transition region of the evaporation of a single droplet and a chaotic spray. The influence of the surface topography on the heat transfer on micro-milled and micro-porous sintered structures is characterised with individual heat transfer coefficients. The necessity of a discrete regard on the heat transport mechanisms is demonstrated. Additionally, extensive parameter studies are presented to determine the influence of the mass flow, heat flux, fluid subcooling as well as the saturation condition and the thermodynamic fluid properties. Results of this work are the classification of the tested surfaces into two groups as well as a dependency of the efficiency of heat transfer on the thermodynamic fluid properties and the combination of working fluid and topography, since some effects only occur for specific combinations. The need to investigate the local heat transport processes during the evaporation of coalesced droplets is also proofed. As demonstrated in prior studies, the three-phase contact line governs the heat transport. However not yet registered necking effects that significantly influence the heat transport occur during the evaporation of coalesced droplets, which was firstly made visible in this work using high time and space resolving infrared thermography. The present work presents a data base for the determination of heat transfer coefficients for convection and evaporation. The obtained results motivate further parameter studies to get a broad data set to find and characterise the parameters needed for modelling. The succesful implementation of both approaches proves the need for more research in the field of the fundamental description of the heat transport processes during spray cooling and motivates to continue both approaches to be able to develop a model to design spray cooling systems for future applications based on physics and characteristic parameters in the near future