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A new experimental approach of characterizing two-phase flow and heat transfer in plate heat exchangers

by Shenghan Jin

Institution: University of Illinois Urbana-Champaign
Year: 2017
Keywords: Plate heat exchangers; Local heat transfer; Visualization; R245fa
Posted: 02/01/2018
Record ID: 2154162
Full text PDF: http://hdl.handle.net/2142/97353


Abstract

Heat transfer of two-phase flow is not well characterized for plate heat exchangers (PHEs) despite their wide applications. No general correlations in the public domain are capable of accounting for all different effects of geometrical parameters, working fluids and operating conditions. The approaches described in literature have either focused on measuring the heat transfer coefficient or separately visualized the flow under simplified conditions. Since the heat transfer is flow regime dependent, a new experimental approach is developed in this work to characterize the PHEs by simultaneously combining local heat transfer measurement and flow visualization.Single-phase heat transfer is first measured in order to characterize the PHEs and investigate the effect of end plates. Steady-state heat transfer coefficient of water in both frame-and-plate heat exchangers (FPHEs) and brazed plate heat exchangers (BPHEs) with various number of plates are measured. Analysis of the experimental results indicate that the end plates, instead of being adiabatic, function as fins due to the contact with their adjacent plates. To further generalize the findings, the experimental data are used to validate a thermal conduction model in ANSYS, which indicates that the fin efficiency of end plates is a function of fluid convective heat transfer coefficient and plate conductive thermal resistance. In FPHEs, the pressing force of the frame may affect the contact thermal resistance, thus change the fin efficiency. In BPHEs, the fin efficiency is much higher due to the larger contact area and higher thermal conductivity of the brazing material. It is concluded that although the effect of end plates would be quickly diluted by the increased number of plates in real applications, it could be significant when the number of plates is small, as is often the case in laboratory settings for the development of heat transfer correlations.The two-phase flow regime is then investigated in a 2-channel FPHE under adiabatic and diabatic conditions by using R245fa as the working fluid. The effects of mass flux, heat flux, vapor quality, inlet flow regime and gravity are separately investigated. The tested mass flux ranges from 5 to 50 kg m-2s-1 and heat flux ranges from 2 to 21 kW m-2, with inlet vapor quality ranging from 0.1 to 0.8. Experimental results are compared in two inlet configurations and two flow orientations. For the upward flow under adiabatic conditions, four flow regimes are identified, including the liquid pool, irregular bubbly flow, film flow and liquid dry-out zone. In the liquid dry-out regime, liquid retention is found around the contact points. The prevailing liquid pool under various inlet flow regimes indicates that it is not a result of inlet distribution, but rather a result of liquid separation in the plate channel. The visualization of downward flow further supports this hypothesis since only film flow and dry-out zone are identified and no liquid pool is found. In the tested range of adiabatic conditions, the flowAdvisors/Committee Members: Hrnjak, Predrag S (advisor), Hrnjak, Predrag S (Committee Chair), Jacobi, Anthony M (committee member), Elbel, Stefan (committee member), Zhang, Yuanhui (committee member), Del Col, Davide (committee member).

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