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Title:Separation of liquid-vapor two-phase flow in headers of microchannel condensers
Author(s):Li, Jun
Advisor(s):Hrnjak, Predrag
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):microchannel condenser
two-phase flow distribution
Abstract:This work presents an experimental and numerical study of separation of liquid and vapor as a way to improve condenser efficiency and heat transfer performance of typically microchannel design. This thesis is composed of three parts in the following order. 1. Separation of vapor and liquid in condensers is evaluated via numerical study as a way to improve efficiency. 2. Effects of separation of vapor and liquid on condenser performance are experimentally investigated on a MAC system. 3. Experimental study of separation of vapor and liquid in a vertical header of MCHX with flow visualization is conducted to study the separation mechanisms. The first part of the chapters evaluates the concept of separation of vapor and liquid in condenser as a way to improve efficiency. An experimentally validated microchannel condenser model indicates that separation of vapor and liquid in condenser is beneficial for performance, by either reducing the refrigerant exit temperature (enthalpy) or increasing the condensation mass flow rate at the same air side conditions. The magnitude is function of separation efficiency and optimization of the circuiting. When the sum of tube numbers of liquid and vapor pass strictly follow each pass in the baseline, the condenser refrigerant outlet temperature of the separation condenser is lower than the baseline by 0.7°C at the same refrigerant inlet state. In addition, condenser pass circuiting with different pre-assumed separation results in the header is investigated by the model. In the second part, effects of separation of vapor and liquid on condenser performance are experimentally investigated by implementing the separation condenser into an R134a MAC system. In the heat exchanger-level test, compared to the baseline condenser with the identical geometry and operating at the same condition, the separation condenser generates round 7.4% more condensate. In the system-level test, an experimental comparison at matched capacity revealed that separation condenser provided a maximum COP improvement of 6.6%. The benefit is identified and discussed: increased refrigerant-side heat transfer coefficient induced by separation of two phases. Separation efficiency in the real application is investigated and potential of further improvement is shown if separation efficiency could be increased. The last part of this work presents the experimental study of separation of two-phase flow in a vertical header of MCHX based on quantified visualization using fast camera. A header prototype is made that has an inlet in the longitudinal center part. Two sub-passes downstream are designed, lower for liquid and upper vapor flow. The header for experiment is clear to provide visual access. R-134a is used as the fluid of interest and mass flux through the inlet microchannels is controlled between 55 kg/(m2s)-195 kg/(m2s). The experiment results indicate that ideal separation in that header can happen at low mass flux up to 70 kg/(m2s). Results are presented in function of liquid and vapor separation efficiencies (ηl, ηv). Flow patterns inside the header are identified and analyzed to study the mechanisms for liquid-vapor separation. The efficiency deteriorates dramatically when the recirculation region elevates up to the top of the header, with increasing inlet flow rate and/or quality. Potential design options to improve two-phase separation are discussed. The objective should be to avoid or at least delay the recirculation region from reaching the vapor exit by reducing the liquid upward momentum or the vapor upward velocity, and decreasing liquid and vapor force interaction.
Issue Date:2016-07-21
Rights Information:Copyright 2016 Jun Li
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08

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