Tenzer, Fabian Michael (2020)
Heat transfer during transient spray cooling: An experimental and analytical study.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00011344
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Spray cooling features a very high, homogeneously distributed cooling performance. Therefore it is used in various industrial applications, like cooling of high powered electronics, for quenching during metalworking or cooling of tools during hot forging. The cooling efficiency is influenced by a large number of parameters: drop diameter and velocity, mass flux, surface temperature, spray fluid and temperature, surface material and conditions, etc. The entire process is extremely complex and to date only few physical models exist describing the heat transfer rates as a function of these parameters. Instead, the heat flux is mainly predicted using empirical correlations, which are often not suited for conditions other than those from which they were derived - universality of these correlations is lacking. The present study strives to replace these empirical correlations with theories based on physics. In this study the transient spray cooling of a hot thick target is experimentally and analytically investigated. The locally resolved temporal evolution of the heat flux and surface temperature of an initially homogeneously heated substrate is measured during continuous spray impact. This experimental quantification captures the influence of various spray features (droplet diameter, velocity and mass flux), spray impact angle, spray fluid temperature, wall thermal properties and wall surface roughness, whereby the mass flux is found to be the dominating factor in determining cooling performance. Furthermore, visual observations of spray impact at high surfaces temperatures beyond the Leidenfrost point identifies no closed liquid film separated from the surface by a vapor layer, as it is often imagined for these conditions. Instead, the spray impact is governed by a superposition of single drop impacts at a dry wall. Therefore, there is no risk of unintentionally flooding the surface, thus limiting the heat flux due to an inordinately high mass flux. Spraying a surface at an oblique angle, as well as spraying with hot fluid decreases the heat flux. Increasing the surface roughness or using substrates which have a high thermal conductivity results in higher cooling performances. Theoretical models to predict the heat flux during spray cooling are developed and validated with experimental results. A model for the film boiling regime accounts for spray and wall properties and predicts the temporal evolution of surface temperature and heat flux. It agrees well with experimental data. A theory for the nucleate boiling regime indicates that the heat flux is limited by the thermal inertia of the substrate material and is not a function of the spray properties. Furthermore, the Leidenfrost point is found to be nearly independent of the spray properties. Instead, it is strongly influenced by the material of the substrate and fluid: A high thermal effusivity leads to a low Leidenfrost temperature and vice versa. This influence is captured in a newly developed theoretical prediction.

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