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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-474-8

ISBN Online: 978-1-56700-473-1

International Heat Transfer Conference 16
August, 10-15, 2018, Beijing, China

INFLUENCES OF DYNAMIC WETTING AND DEWETTING PROCESSES ON EVAPORATIVE HEAT TRANSFER

Get access (open in a dialog) DOI: 10.1615/IHTC16.kn.000025
455 pages

Abstract

Evaporative heat transfer is an efficient method for cooling applications such as spray or drop impingement cooling, mini- or microchannel flow boiling cooling, or oscillating heat pipes. In all these configurations highly dynamic wetting and dewetting processes occur which strongly interact with the heat transport processes and vice versa. An optimization of the heat transfer efficiencies should be based upon a deep and detailed understanding of these mutual interactions.
Two generic experiments are presented: a drop impingement cooling experiment and a minichannel meniscus evaporation experiment. The design of the heater element is the same in both experiments. It allows a high spatio-temporal resolution of the temperature and heat flux profile at the solid-fluid interface using IR thermography. The dynamic wetting or dewetting process is recorded in parallel using shadowgraphy. In case of the drop impingement experiment, the wetting and dewetting velocities (or Capillary number) result from the drop impingement process itself. In case of the minichannel experiment, however, these velocities can be controlled by a specific liquid-vapor displacement unit. Therefore, the two experiments complement one another and allow deep insights into the mutual interactions of the wetting and heat transfer characteristics. These experimental insights are further complemented by modelling and numerical simulations using OpenFOAM with a modified specific VOF method.
Using these complementary methods, a variety of parameter studies were performed and analyzed: wetting versus dewetting situations at different velocities, wall superheat, system pressure from atmospheric pressure up to near critical pressure, smooth versus nanocoated wall surface, and pure fluids versus binary mixture.
The results show that sensible heat transfer dominates in the wetting regime while evaporative heat transfer dominates in the dewetting regime. Microlayer formation and evaporation can be observed at higher dewetting velocities while so called contact line evaporation can be observed at lower dewetting velocities. The apparent contact angle rises with increasing wall superheat. The characteristic local maximum in heat transfer near the moving contact line diminishes when approaching the critical pressure. The heat transfer efficiency can be increased using a nanocoated heater wall that provokes thin film formation. The heat transfer efficiency decreases with a binary mixture compared to the efficiencies using the pure components.