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A wall heat transfer model for subcooled boiling flow

Helfried Steiner
Institute of Fluid Mechanics and Heat Transfer, Graz University of Technology, Austria

Alexander Kobor
Institute of Fluid Mechanics and Heat Transfer, Technical University Graz, Inffeldgasse 25, Graz 8010, Austria

Ludwig Gebhard
Institute of Fluid Mechanics and Heat Transfer, Technical University Graz, Inffeldgasse 25, Graz 8010, Austria

Abstract

The development of modern cooling systems aims in many cases at a most compact, space and weight saving design. The consequential demand for highest possible heat transfer rates has lead to the very promising concept of providing for a controlled transition from pure single-phase convection to subcooled boiling flow in thermally highly loaded regions. The application of this approach in modern engineering design requires a realistic modeling of the complex phenomena associated with the two-phase flow heat transfer. The present work proposes for the computation of the specific wall heat transfer rate a modified superposition model, where the total heat flux is assumed to be additively composed of a forced convective and a nucleate boiling component, respectively. Since the present model requires only local input quantities, it is well suited to CFD of geometrically very complex coolant flows, where the definition of global length or velocity scales would be very impractical. The wall heat fluxes predicted by the present model were compared to experimental data for the most part obtained by in-house measurements with water being the working fluid. The overall agreement is very good, particularly, in the partially nucleate boiling regime, where the effect of the bulk flow rate is significant. Deviations are primarily observed at higher wall superheats, where a strong two-way coupling between the motion of the liquid and the motion of the bubbles as well as considerable bubble-bubble interactions typically occur.

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