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Title:The void fraction in headers of microchannel heat exchangers
Author(s):Qian, Hongliang
Director of Research:Hrnjak, Predrag S
Doctoral Committee Chair(s):Hrnjak, Predrag S
Doctoral Committee Member(s):Jacobi, Tony; Elbel, Stefan; Zhang, Yuanhui
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Void fraction
flow regimes
microchannel heat exchangers
capacitive sensors
Abstract:In HVAC&R systems, microchannel heat exchangers (MCHEs) are commonly used with the advantages of compactness, the potential for reduction of charge, and improvement of heat transfer performance compared to other traditional heat exchangers. The void fraction in the headers affects the thermal performance, refrigerant charge, and total pressure drop of MCHEs. However, due to the complex geometry of headers, measurements of void fraction in headers are challenging. A model that predicts the local void fraction between microchannel tube protrusions is also widely needed. Hence, it is essential to obtain the local void fraction in the zones between two microchannel tubes to have a better understanding of refrigerant charge inside the headers and thus the MCHEs. This dissertation demonstrates a new method to measure and model the void fraction in headers. This study first presents flow regimes and void fraction in horizontal and vertical round smooth tubes ID 7 mm with R134a in the adiabatic conditions (saturation temperature at 33 ºC) and low mass flux (40-150 kg/m2s for horizontal tubes and 65-115 kg/m2s for vertical tubes). The study on the two-phase flow in tubes is the baseline and preliminary study of two-phase flow in headers. Flow regimes are captured by a high-speed camera, while the void fraction is measured by the quick-closing valve (QCV) method. The horizontal flow patterns are compared to Wojtan-Ursenbacher-Thome flow-regime map, and some modifications based on visualization results are proposed. Void fraction results for both horizontal and vertical flows are compared to some widely used correlations. Influences of tube orientation and mass flux on the void fraction are discussed. When the vapor quality is kept constant, the void fraction of horizontal tubes is larger than that of vertical tubes. Higher mass flux also results in a larger void fraction compared to that of lower mass flux. A new capacitive sensor is then built and used to measure the void fraction and characterize flow regimes of both horizontal and vertically upward flow in the low mass flux range in circular tubes with an inner diameter of 7 mm. Three sensors with different axial lengths (D, 2D/3, and D/2) are built and evaluated to examine the possibility of utilizing shorter sensors in applications with space limitations. Results show all three sensors have the capability to measure the void fraction and detect flow regimes. Due to the nonlinear relation between capacitive signals and void fraction, a calibration procedure based on mass measurement (QCV) is proposed. After the calibration procedure, most void fraction data measured by the sensor fall into the ±15% deviations of the experimental results by QCV for horizontal flow and ±10% for vertical upward flow. Slug/stratified-wavy, stratified-wavy and annular regimes for horizontal flow (in the range of mass fluxes 40 – 150 kg m-2 s-1), and slug, churn, and annular flow regimes for vertical upward flow (in the range 65 – 115 kg m-2 s-1) are characterized with the capacitive sensor. Flow regimes are characterized based on the time plot of normalized capacitive signals, kernel density estimation (KDE), power spectral density (PSD), and visualization results from a high-speed camera. Sensors with the same design can and will be utilized directly to measure void fraction and characterize flow regimes for similar test conditions. After the capacitive sensors are validated with the flow in circular tubes, new capacitive sensors which are based on the same principles are built and calibrated to measure the cross-sectional void fraction between tubes in vertical headers of MCHEs. Eleven individual sensors are assembled into one test header to measure local capacitance independently in real-time. A calibration procedure based on visualization, pressure drop, signal patterns, and mass measurement (QCV) is proposed and used. After the calibration, most void fraction data predicted by all eleven sensors fall into ±10% deviations of the experimental results by QCV. The last experimental part of this study presents the void fraction measurement in intermediate and inlet headers with R134a by the calibrated capacitive sensors. The void fraction profiles and visualization results are demonstrated under different test conditions. Other local parameters, such as mass flux, quality, superficial velocity, and visualization results, are obtained to develop a void fraction model. The proposed void fraction model in this study is based on the drift flux model. The local void fraction predicted by the model agree well with the experimentally measured values. Most of the predictions fall into a ±15% deviation.
Issue Date:2021-04-23
Rights Information:Copyright 2021 Hongliang Qian
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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