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Performance measurement and visualization on the refrigerant distribution in the vertical manifold of the microchannel tube heat exchanger

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Title: Performance measurement and visualization on the refrigerant distribution in the vertical manifold of the microchannel tube heat exchanger
Author(s): Zou, Yang
Advisor(s): Hrnjak, Predrag S.
Department / Program: Mechanical Sci & Engineering
Discipline: Mechanical Engineering
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Thesis
Subject(s): Distribution Microchannel Evaporator R410a Liquid Fraction Visualization Froude Number
Abstract: This project studies the liquid refrigerant distribution of an intermediate vertical manifold in a multi-pass evaporator. A new test rig is designed and built to circulate R410A through the vertical microchannel manifold. Refrigerant is circulated into the manifold through the lower set of parallel microchannels and exits through the upper set. Three experimental circular manifolds were developed, each with the microchannels inserted to half depth. The first is an aluminum manifold used in industry with 5 inlet and 5 exit microchannels; the second is a transparent replica of the first; the third is a transparent manifold with 10 inlet and 10 exit microchannels used to study the effect of increasing manifold length and microchannel number. The inlet mass flow rate (min) and quality (x) are two main parameters affecting the distribution. The inlet quality was varied from 0.2 to 0.8, and the inlet mass flow rate was varied from 0.8 to 4.5 g/h per microchannel. For all three manifolds, the best distribution is generally found at high min and low x, and the worst at high x and extreme values of min. At low qualities, 0.2 and 0.4, the distribution improves with increasing mass flow rate, and at high qualities, 0.6 and 0.8, the distribution is optimum at intermediate mass flow rates, and gets worse as min increases or decreases. Likewise, for a fixed mass flow rate, the distribution generally improves as quality decreases. A comparison of the two transparent headers shows that increasing the number of microchannels in the header generally improves the distribution. The two-phase flow regimes bubbly flow, churn flow and semi-annular flow are observed through visualization results. The flow regime is dependent on min and x. At constant min, increasing the inlet quality would transition the flow regime from bubbly, to churn, and then to semi-annular flow. At a fixed intermediate inlet quality (e.g., 0.4), the flow regime would change from churn to semi-annular as min increases. The variation of liquid distribution closely follows the transition of flow regime. Based on the visualization results and force balance analysis, the liquid distribution is mainly determined by the competition between the inertial and buoyancy forces. The force balance determines where liquid goes, and further determines the distribution. Froude number is an appropriate parameter to predict the flow regime. However, it is not good enough to predict the distribution because it does not include the important effects of top pressure.
Issue Date: 2010-08-31
URI: http://hdl.handle.net/2142/17010
Rights Information: Copyright 2010 Yang Zou
Date Available in IDEALS: 2010-08-31
2012-09-07
Date Deposited: 2010-08
 

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