Developing two-phase flow after an expansion valve and the effect of flow patterns on two-phase flow distribution

Yao, Yufang

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https://hdl.handle.net/2142/115863 Copy

Description

Title

Developing two-phase flow after an expansion valve and the effect of flow patterns on two-phase flow distribution

Author(s)

Yao, Yufang

Issue Date

2022-06-06

Director of Research (if dissertation) or Advisor (if thesis)

Hrnjak, Predrag

Doctoral Committee Chair(s)

Hrnjak, Predrag

Committee Member(s)

• Jacobi, Tony
• Elbel, Stefan
• Kozlowski, Tomasz

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• developing two-phase flow
• flow regime
• visualization
• distribution

Abstract

Developing two-phase flow is commonly encountered in thermal/hydraulic systems when the flow passes through valves, tube fittings, bends, or expansion/contraction in the flow area. This study investigates the development of the adiabatic two-phase flow after an expansion valve, with consequences on the evaporator in an air conditioning system. Two types of refrigerants, R134a and R245fa, with POE 32 are used as the working fluids. The flow patterns are visualized from 15 mm until 595 mm after the expansion in a straight tube. The observed flow patterns are classified into four regions as the two-phase flow progresses along the tube: well mixed, separating, separated but developing, and fully developed. Due to the sudden expansion in TEV, the flow structure in the well-mixed region is similar to homogeneous flow, with bubbles or droplets dispersed uniformly in the continuous phase. Then, the gravity/buoyancy force becomes more dominant over inertia and surface tension because of the reduction of liquid velocity and the growth of bubbles. Consequently, bubbles migrate upwards while droplets fall downward, characterizing the separating region. After phase separation, the flow characteristics may still change for a certain distance before the flow regimes are fully developed. Based on the observed developing flow regimes at various working conditions, a 3D flow pattern map is built, adding the distance from the TEV as the third dimension besides the mass flux and quality. The effect of tube geometry on developing two-phase flow is studied by comparing the flow patterns in a 5 mm tube to that of an 8 mm tube. In both cases, R134a with POE 32 is used as the working fluid. The developing two-phase flow patterns in this study are also compared with previous research by Bowers and Hrnjak [1] to check the effect of the oil. They have explored the developing flow after a needle valve using pure R134a. To get a further understanding of the developing two-phase flow, an image analysis technique is used to quantify the videos from the experiment. This method first locates the liquid-vapor interface and then estimates the void fraction and velocity of each phase. On the other hand, a developing two-phase flow model is proposed based on the force-momentum balance. The void fraction of the developing flow predicted by the model shows a similar trend to the experiment. This model can also predict the distance to fully developed flow after the expansion valve. Fluid property is another important factor determining the two-phase flow patterns. This study uses R245fa and R134a, two refrigerants with different thermal properties, to check that effect. The flow patterns of the two refrigerants in developing and developed regions are compared to study the effect of density, viscosity, and surface tension. The developed flow patterns of both refrigerants are also compared with some existing flow pattern maps. Finally, a new method is proposed to predict the transition lines between each flow pattern. After exploring developing two-phase flow, the flow patterns at the evaporator inlet are related to the two-phase flow distribution in a distributor. Four factors influencing two-phase flow distribution and flow patterns are studied: mass flow rate, distributor inlet quality, orientation, and distance from the expansion valve. In an attempt to improve the flow distribution further and quantify the effect of maldistribution, two approaches are applied: the first is manual adjustment of the resistance of each circuit individually to achieve uniform mass flow rates, and the second is the homogenization of the flow regime before division by adding an orifice in the distributor before separation to four channels. The average deviations of capacity for each branch are 9% for the baseline case, 3.3% for manual adjusting, and only 1.6% for the homogenization approach, stressing the importance of that simple strategy.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Yufang Yao

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