Pressure drop in headers of microchannel heat exchangers

Chavoshi, Amir

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https://hdl.handle.net/2142/115772 Copy

Description

Title

Pressure drop in headers of microchannel heat exchangers

Author(s)

Chavoshi, Amir

Issue Date

2022-04-22

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

Hrnjak, Predrag

Doctoral Committee Chair(s)

Hrnjak, Predrag

Committee Member(s)

• Jacobi, Tony
• Elbel, Stefan
• Zhang, Yuanhui

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)

microchannel heat exchanger, pressure drop, distribution

Abstract

Microchannel heat exchangers (MCHXs) have been widely used in the heating, ventilating, air conditioning, and refrigeration (HVAC&R) industry for their compactness and high heat transfer coefficient. However, they often underperform because of the flow maldistribution among parallel microchannel tubes, which is caused by pressure drop, as well as uneven phase distribution in headers. This non-uniform distribution of refrigerant creates an unwanted superheated region, which has a lower heat transfer coefficient and a smaller temperature difference between the refrigerant and heat-source fluid, thus decreases the heat transfer rate. This dissertation presents an experimental and numerical study of pressure drop in inlet and outlet header for both single-phase and two-phase flow in MCHXs. The first focus of this work is the experimental investigation of the pressure drop and the development of a new set of correlations for pressure loss coefficients for single-phase flow through round headers of parallel MCHXs. Compressed air is adopted as working fluid. The tested velocity through the header ranges from 1 m/s to 20 m/s while the velocity through the microchannel tube ranges from 6 m/s to 30 m/s, based on those commonly used in MCHXs. Correlations for predicting pressure drop of inlet header and outlet header are proposed, and 98% of experimental data fall into a deviation of ±15 Pa. Then, the single-phase flow distribution in MCHXs is numerically investigated. The new generated correlations and two other methods are used in a 1-D finite volume approach to evaluate single-phase pressure drop in headers of MCHXs, to predict mass flow rate distribution in microchannel tubes, and the results are compared with a Hydraulic-CFD Linked model, in which the flow in headers are simulated in 3-D by ANSYS Fluent. The results show that the 1-D finite volume models show a difference in the prediction of flow rate distribution. The model in which the flow passage in the header is assumed to be a series of dividing and combining T-manifolds shows a satisfactory agreement with the Hydraulic-CFD Linked model in the perspectives of mass flow rate distribution. The last experimental part of this study presents an investigation of the pressure drop in two-phase flow, for which flow visualization and pressure drop measurements are conducted in a vertical upward flow in a round inlet header with R134a. Pressure profiles and flow regimes for different mass fluxes and vapor qualities are demonstrated. Results show that pressure drop in the header is affected significantly by flow morphology in the header. The influences of inlet mass flux and vapor quality on the pressure drop are also presented.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Amir Chavoshi

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