Characterization of pool boiling heat transfer from porous-coating-enhanced surfaces
Development of techniques for enhancement and optimization of thermal management technologies has been a highly active area of research in recent decades in response to the rapid emergence of compact, high-power electronic systems. Immersion cooling by boiling is one of the preferred methods for high power density applications, due to its passive nature and high heat transfer coefficients obtained. Pool boiling heat transfer has been extensively studied in recent decades to understand the inherent mechanisms yielding the high heat transfer rates, as well as to further enhance the heat transfer by simple modifications or additions to existing approaches. This thesis aims to provide detailed fundamental analysis of heat transfer enhancement by surface-coating-based boiling enhancement methods and to quantitatively analyze the dependence of heat transfer performance on coating properties. For passive systems, which cannot afford active cooling due to either form factor or other application constraints, two-phase pool boiling heat transfer provides highly effective immersion cooling. Surface enhancement techniques, such as surface coatings, may further augment the cooling efficiency of such systems. The experimental study presented in this thesis analyzes the effects of variation of particle size on the pool boiling performance of FC-72 obtained by free-particle and sintered-coating enhancement techniques. In the free-particle technique, loose copper particles are placed on a heated copper surface, whereas in the sintered-coating technique, copper particles are sintered to the copper surface. The particle coatings provide additional vapor nucleation sites in the cavities formed at particle-surface and particle-particle contact points, thereby enhancing boiling performance over a polished surface. The boiling performance enhancement is studied for particle sizes varying from 45–1000 µm at a constant coating layer thickness-to-particle diameter ratio (δ/d) of approximately 4 for both techniques. High-speed flow visualizations are performed to understand the boiling patterns and bubble departure parameters that provide explanations for the trends observed in the boiling curves. The measured wall superheat is observed to be significantly lower with a sintered coating compared to the free-particle layer for any given particle size and heat flux. Performance trends with respect to particle size, however, are remarkably similar for both enhancement techniques, and an optimum particle size of ~100 µm is identified for both free particles and sintered coatings. The free-particle technique is shown to offer a straightforward method to screen the boiling enhancement trends expected from different particulate layer compositions that are intended to be subsequently fabricated by sintering. From the experimental investigation of pool boiling from coated surfaces, it was observed that for a given surface coating, several additional parameters might affect the heat transfer performance of the surface, and the coating porosity and particle morphology did not vary independent of each other. Hence, to further understand the effects of coating properties, pool boiling heat transfer of FC-72 is studied from coatings with independently varying coating porosities with two different particle morphologies. Surfaces are fabricated with same size particles (90–106 µm) having different morphologies, viz., spherical and irregular, at a constant coating layer thickness-to-particle diameter ratio (d/d) of approximately 4, with porosities varying over a wide range (∼40%–80%). The morphology and size of the particles affect the pore geometry, porosity, permeability, thermal conductivity, and other characteristics of the sintered coating. In turn, these characteristics impact the heat transfer coefficient and critical heat flux (CHF) during boiling. The porous structure formed by sintering is quantitatively characterized using image analysis and numerical simulation based on micro-computed tomography (µ-CT) scans to study the geometric and effective thermophysical properties of the coatings. Critical coating properties affecting the boiling performance metrics are identified, and regression analysis is employed to observe the dependence of these metrics on the coating properties. Coatings with irregular particles or lower porosity are observed to yield higher heat transfer coefficients than those with spherical particles or higher porosities. The relative strength of dependence of the heat transfer coefficient and CHF on the coating porosity, pore diameter distribution, particle diameter distribution, particle sphericity distribution, necking and interfacial areas, permeability, and thermal conductivity of the coatings are determined. The importance of high-fidelity coating characterization to understand the heat transfer behavior of coatings is demonstrated. Plans for future work are outlined based on the current findings. Proposed additional studies include investigation of the single bubble dynamics to further the understanding of bubble behavior that influences the heat transfer performance for free-particle and sintered-coating techniques. The quantitative regression analysis from µ-CT scans may be further extended to include a more rigorous model that employs codependence of critical inputs, to determine a predictive correlation for the boiling heat transfer performance based on coating properties.
Garimella, Purdue University.
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