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

Plate heat exchangers;Local heat transfer;Visualization;R245fa

Jin, Shenghan

关键词: Plate heat exchangers;    Local heat transfer;    Visualization;    R245fa;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/97353/JIN-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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^{-2}s^{-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 flow regime is non-uniform across the plate, while the introduction of heat flux further complicates the distribution. Upon heating, nucleation sites start to form in the liquid pool region. The motion of the bubbles indicate that the liquid pool is rather stagnant and buoyancy is the primary driving force. Both the number and activeness of nucleation sites increase with heat flux. Regular bubbly flow, irregular bubbly flow, film flow, partial film flow and dry-out zones are identified in the diabatic flow visualization. Nucleation sites are primarily found along the bottom gasket edge and near the contact points. Even near the dry-out region, nucleation site is found in the liquid retained near the contact points.To better understand the local heat flux and heat transfer coefficient in the complex flow regimes of PHEs, a new experimental approach is developed. It measures the local heat transfer and simultaneously visualizes the flow regime, while preserving the real geometry and operating conditions. The test section is of the original shape of a FPHE heat exchanger. A heat flux plate is built by deploying thermocouples on the inner surfaces of two original chevron plates and filling the space in between with clay as the conductive material. With its thermal resistance calibrated, the instrumented plate functions as a heat flux sensor. It is installed to measure fluid heat transfer on one side, while leaving visual access on the other side through transparent plates. The transparent plates are fabricated using the original plates as the mold to fully preserve the geometry. Results of R245fa flow boiling are presented, with the effect of heat flux, inlet vapor quality and flow arrangements investigated. With simultaneous heat transfer measurement and flow visualization, the new approach provides a viable solution for better understanding the interaction between heat transfer and flow regime.