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Title:Flow distribution in brazed plate heat exchangers
Author(s):Li, Wenzhe
Director of Research:Hrnjak, Pega
Doctoral Committee Chair(s):Hrnjak, Pega
Doctoral Committee Member(s):Jacobi, Anthony; Elbel, Stefan; Kozlowski, Tomasz
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Brazed plate heat exchangers
Flow distribution
Abstract:Brazed plate heat exchangers (BPHEs) have been widely used in the heating, ventilating, air conditioning, and refrigeration (HVAC&R) industry for their compactness and high heat transfer coefficient. However, they suffer from the flow maldistribution among parallel plate channels. Especially when they are used as evaporators, the non-uniform distribution of two-phase refrigerant creates an unwanted superheated region, which has a lower heat transfer coefficient and a smaller temperature difference between the refrigerant and heat-source fluid, thus lowers the heat transfer rate. This dissertation presents an experimental and numerical study of both single-phase and two-phase flow maldistribution in BPHEs. The first focus of this work is the end-plate effect, which significantly affects the determination of heat transfer coefficient in BPHEs and is often confused with the flow distribution issue. The governing equations of the end-plate effect are derived through the energy balance analysis. Numerical and analytical models are developed to solve the governing equations. The predictions by the two models agree well with each other. The heat transfer data for single-phase water in the BPHEs with different numbers of plates are used to experimentally quantify the end-plate effect and validate the developed models. Both experiments and theoretical analyses show that the influence of the endplate is more significant in a heat exchanger with a small number of plates and a lower convective heat transfer coefficient. An iteration algorithm is developed to compensate for the end-plate effect in the development of heat transfer correlations. Then, the single-phase phase flow distribution in BPHEs is experimentally and numerically investigated. In the experiments, the flow distribution is determined by the pressure differential across the channels and the developed in-channel friction factor correlation. The results indicate that in a U-type brazed plate heat exchanger, the channel flow rate first increases for the first several channels near the heat exchanger entrance due to the sudden expansion flow in the inlet header. For the rest channels, the flow rate decreases with the distance away from the entrance/exit of the heat exchanger. Such a distribution profile is associated with the axial momentum transfer in the inlet header. The influence of the total flow rate on the distribution profile is trivial, but the maldistribution is more severe with an increased number of plates. Two distribution models are developed based on the principle of equal total pressure drop for all flow paths. Two models calculate the pressure profile in the headers by 3-D CFD modeling and 1-D mass and momentum conservation equation. Besides, the single-phase flow maldistribution seems to have an insignificant influence on the heat exchanger thermal capacity, which could be the result of similar distribution profiles on both sides. In the third part of this study, a method to quantify the two-phase refrigerant flow distribution in BPHEs from the infrared images is developed. In this method, the two-phase length of the refrigerant channels is first estimated from the IR images of the BPHE sidewall. Then, the refrigerant distribution is adjusted in a BPHE evaporator model, to have the predicted two-phase length of the channels match the identified results from the IR images. The overall performance of the BPHE is also predicted in the quantification process. The proposed quantification method is validated by the experimental data: heat exchanger capacities, pressure profiles, and surface temperature of the sidewall. This non-intrusive method can effectively quantify the two-phase refrigerant distribution with the premise that all liquid refrigerant has evaporated in the BPHE. Visualization of the two-phase flow in the inlet header of the BPHEs is then accomplished through a 3-D printed transparent window. The observed flow regimes in the inlet header are periodic and three stages are identified in one cycle: top corner vapor flow, vapor jet flow, and liquid blockage. Among them, the top corner vapor flow affects the distribution most, in which the vapor refrigerant is mainly present at the top corner of the header and branches out through the first several channels, leaving the liquid refrigerant to occupy the rest flow area of the inlet header and present a single-phase like distribution profile. When the inlet vapor quality increases, the distribution of the liquid refrigerant is improved since the vapor refrigerant can reach more downstream channels to help balance the total pressure drop. With the mass flux increases, the maldistribution caused by the header-induced pressure drop is more significant, which compromises the benefit brought by the higher vapor momentum. When the number of plates is increased, the liquid refrigerant distribution is worse due to an increased pressure drop in the headers. The effect of two-phase refrigerant distribution on the heat exchanger performance is evaluated in the fifth part of this work by comparing the uniform distribution cases with the maldistributed cases. The results show that compared with the uniform distribution cases, the maldistributed cases have lower heat exchanger capacities at the same evaporation temperature or require lower evaporation temperatures to achieve the same capacity. Reducing exit superheat can overcome the adverse effect brought by the flow maldistribution but will bring difficulties in system control. The last part of this study develops a mechanistic model to predict two-phase refrigerant distribution in BPHEs. To predict the vapor refrigerant distribution, this model considers two different flow stages in the inlet header: for the top corner vapor flow, the vapor refrigerant distribution is predicted by the balance between the suction force (radial pressure gradient) and the axial momentum of each phase; for the vapor jet flow, the vapor refrigerant is assumed to be distributed evenly among channels. The time ratio of the two flow stages is estimated by assuming the irregular large-amplitude rolling waves in the feeding tube, which lead to the vapor jet flow in the header, to have the most dangerous wavelength and their traveling time is related to the liquid slug frequency. The overall vapor refrigerant distribution is the time average of the vapor distribution of the two stages. While for the liquid refrigerant distribution, it is predicted by imposing an equal total pressure for each flow path. The proposed model is validated against the experimental results.
Issue Date:2021-04-18
Rights Information:Copyright 2021 Wenzhe Li
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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