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Title:Experimental study and modeling of liquid-vapor flow in the second header of microchannel condensers with separation circuitry
Author(s):Li, Jun
Director of Research:Hrnjak, Pega
Doctoral Committee Chair(s):Hrnjak, Pega
Doctoral Committee Member(s):Jacobi, Anthony; Vanka, Surya Pratap; Elbel, Stefan; Kozlowski, Tomasz
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):microchannel condenser
two-phase flow
separation
header
heat transfer
Abstract:For a condensation process, liquid on the wall of a condenser creates an extra thermal resistance thus is detrimental to heat transfer. Separating the condensate from vapor is one of the ways to improve heat transfer and reduce pressure drop. This work presents an experimental and numerical study of separation of liquid and vapor as a way to improve the effectiveness of microchannel (MC) condensers at little cost. Condenser prototypes have been designed to have the inlet in the middle of the height. After the 1st pass (multiple parallel microchannel tubes brazed together as one part by aluminum louver fins), liquid separates from vapor inside the vertical second header (manifold tube with a plural of parallel microchannel-tube branches) mainly based on gravity. The liquid goes down to the lower, smaller passes downstream, while the vapor goes up into the upper, larger passes. This gravity-aided design only changes the pass circuitry of conventional condensers, thus it incurs almost no additional cost on manufacturing. Experiments on an R134a mobile air conditioning (MAC) system first confirm that a condenser prototype with the separation circuitry outperforms a conventional condenser baseline. In the heat exchanger-level tests, while having the same refrigerant inlet and outlet temperatures and the same air inlet condition, the separation condenser has a maximum increase of 7.4% for the refrigerant mass flow rate compared to the conventional condenser. The increase in heating capacity for that is 5.1%. In the system-level tests, while having the same cooling capacity, the COP of the system with the separation condenser is 1.3% to 6.6% higher than the system with the conventional condenser. Then, to understand the phase separation phenomena, separation experiments are conducted in two different second headers (D=15.1 mm and D=23.7 mm) for typical MAC condensers and the internal two-phase flow is visualized by a high-speed camera. R134a is the working fluid and the mass flux through the inlet microchannels is 87– 311 kg·m-2·s. Results show the liquid separation efficiency, ηL, decreases as the vapor separation efficiency, ηV, increases. As the mass flow increases the velocities increase, especially when combined with increasing quality. These two effects mix phases and reduce ηL. Three general flow regimes are revealed in the second header for the scenario of non-perfect separation: upward co-current flow, counter-current flow, and downward co-current flow. Liquid entrainment depends on whether the local vapor velocity reaches a threshold velocity related to the interfacial drag force, which is determined based on the flow regime. Based on the experimental results, a 1-D numerical model has been built for the first time to predict the separation efficiency. The modeling results show ηL is higher with a larger diameter of the header, confirmed by experiments. The model predicts ηL is higher with more inlet tubes for the same inlet mass flow rate and inlet quality to the header. For a fixed header geometry, at the same inlet mass flux and inlet quality, three refrigerants are simulated. The order of ηL at same condensing temperature is: R32 > R134a > R245fa. 3-D CFD simulation is also conducted for the separation phenomenon. Results are validated against the experimental data. It is found from the CFD results that vortices exist between each two adjacent MC tube intrusions, making the other half of the header (as opposed to the intrusion side) the free-flow region. Reverse flow is found on the intrusion side of the exit planes. Pressure variation in the axial direction is larger than that on one cross-section, indicating the usefulness of 1-D pressure drop correlations for flow in headers. With the model for the second header, an experimentally validated, steady-state condenser model has been built for the first time. For the second header, when the flow resistance downstream of it changes, the pressure boundaries change thus altering the separation efficiency. Results show that the trade-off between high quality and high mass flux for the flow in the upper passes limits the improvement of performance. Optimization for the separation condenser using R134a is performed on pass circuitry, fin density, and air velocity distribution. Fin density and air velocity capitalization work better for the separation condensers with a higher area ratio of the upper passes to the liquid passes. The optimal separation design with variable fin density and variable air distribution has a 17.8% increase of condensate flow rate (more than twice of the original 7.4% improvement by the separation prototype) compared to the conventional condenser.
Issue Date:2020-05-05
Type:Thesis
URI:http://hdl.handle.net/2142/107961
Rights Information:Copyright 2020 Jun Li. All rights reserved
Date Available in IDEALS:2020-08-26
Date Deposited:2020-05


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