Turbulent liquid-vapor flow interactions and heat transfer in confined jet impingement boiling

Matthew J Rau, Purdue University

Abstract

The central purpose of this work is to investigate the fundamental thermal transport and turbulent flow characteristics of confined two-phase jet impingement. A novel experimental facility is developed that allows for the spatial mapping of heat transfer coefficient distributions resulting from two-phase impinging jets. This facility is used to study the local and area-averaged heat transfer characteristics of three orifice designs having the same total open area: a single orifice, a 3 × 3 array of orifices, and a 5 × 5 array of orifices. The jet arrays result in higher area-averaged heat transfer than the single jet; however, they display a larger relative nonuniformity in local heat transfer coefficient. The pressure drop is found to be independent of heat flux (i.e., vapor generation) for all orifices investigated. To improve the heat dissipation capabilities of two-phase jet impingement, boiling surface enhancements are investigated. Because the pressure drop is dominated by the orifice and minimally influenced by the flow on the heated surface, adding surface structures can improve heat transfer with little penalty in pressure drop. A hybrid enhancement surface having pin fins coated with a microporous layer is designed to provide increased nucleation site density and macroscale surface area. A study with the single jet is performed to first understand the heat transfer characteristics of this surface with jet impingement. The hybrid surface and surfaces representing each constituent enhancement type are investigated, including: a baseline smooth flat surface, a flat surface coated with the microporous layer, a surface with extended pin fins, and the hybrid surface. With these characteristics of the hybrid surface understood, the heat transfer and pressure drop performance of the surface with the 5 × 5 array of jets is investigated. The array more effectively distributes liquid into the hybrid enhancement structures and results in superior vapor evacuation. This scheme drastically extends critical heat flux, more than doubling it compared to the single jet on the same hybrid surface with only a slight increase in pressure drop. Compared to the single jet impinging on the smooth flat surface, the array of jets with the hybrid enhancement results in a more than four-fold increase in critical heat flux at each flow rate investigated. This impingement scheme displays low wall superheats in nucleate boiling, high critical heat fluxes, and only small increases in pressure drop with increasing heat flux. The influence of vapor generation on the flow field within the confinement gap is studied to better understand the interaction between liquid and vapor during confined jet impingement boiling. Liquid flow measurements with simultaneous vapor visualizations are performed using time-resolved stereo particle image velocimetry (PIV) with fluorescence illumination. A single jet of subcooled water impinges onto a circular heat source and measurements are acquired across a range of heat fluxes at jet Reynolds numbers of 5,000 and 15,000. The liquid flow pattern in the confinement gap (with the height maintained at four jet diameters) is found to be governed by the vapor motion. Boiling is found to disrupt both the horizontal wall jet and the vertical impinging jet flow. Specifically, rising bubbles from the heat source interact with the impinging jet causing an increase in the jet velocity decay rate prior to impingement. Vapor is a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heat source experiencing the most intense turbulence modification. Vapor-induced turbulence is found to be significant downstream of the heat source only once saturated conditions in the flow are reached. Tomographic PIV is then implemented to further study the interaction between vapor motion and the turbulent liquid flow as the confinement gap height is varied. This first application of tomographic PIV to flow boiling is significant given the complexity of confined two-phase jet impingement. Three confinement gap heights are studied; 8, 4, and 2 jet diameters. A visual hull method is used to reconstruct the time-varying region of vapor in the flow, allowing a time-resolved estimate of the vapor void fraction. Special attention is given to the interaction of vapor with turbulence and coherent vortices in the confinement gap during boiling. The three-dimensional measurements allow asymmetries in the flow, which are found to be augmented during boiling, to be described in detail. The vapor interface motion is shown to cause the formation of coherent vortices in the flow. The primary locations of these vortices do not coincide with the regions of most-intense turbulent kinetic energy, suggesting that irrotational velocity fluctuations contribute significantly to the magnitudes of turbulent kinetic energy measured in this two-phase flow. Reduced confinement gap heights are found to confine vapor within the gap, which results in less turbulence but greater bulk liquid flow unsteadiness. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Garimella, Purdue University.

Subject Area

Mechanical engineering

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