Study of transient oil migration at compressor and improved oil return

Wang, Xin

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https://hdl.handle.net/2142/129603 Copy

Description

Title

Study of transient oil migration at compressor and improved oil return

Author(s)

Wang, Xin

Issue Date

2025-05-01

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

Miljkovic, Nenad

Doctoral Committee Chair(s)

Miljkovic, Nenad

Committee Member(s)

• Wang, Xiaofei
• Tang, Ke
• Zhang, Yuanhui

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Oil migration
• HVAC&R
• Refrigeration
• Two-phase flow
• Compressor
• Lubricant
• Refrigerant-oil properties
• Air-conditioning

Abstract

Oil is essential in the vapor compression system to ensure lubrication inside the compressor. The oil inside the compressor can reduce friction and minimize wear, providing sealing to achieve high volumetric efficiency. However, oil migrating to other system components negatively impacts the heat transfer efficiency, pressure drop, and overall system performance. Refrigeration and air conditioning systems experience significant transient behaviors during startup and shutdown phases, leading to dynamic changes in temperature, pressure, and fluid distribution. For HVAC&R systems that require frequent on-off cycling, oil and refrigerant migration during transient startup and shutdown has great impact on the system performance and reliability. These transients notably influence the migration patterns of both refrigerant and lubricating oil, impacting system efficiency, compressor reliability, and overall operational stability. The first section explores the transient effects in oil migration and property changes at the compressor suction and the discharge. Oil flow behaviors and oil migration are quantified and analyzed by the high-speed camera recording and video analyzing during compressor cold and warm startup. The oil-refrigerant mixture flow transitions from two-phase flow to vapor refrigerant flow and oil annular mist flow. Severe oil foaming happens inside the compressor and at the discharge. Plug flow, foam flow, bubbly flow and annular mist flow are observed within three minutes after cold startup before steady state is reached. In contrast, for restarting after a short idle time, the oil flow develops to steady-state annular mist flow within one minute after warm startup without foam and phase transition happening. The difference between oil flow behaviors is explained by mixture property variations such as oil-refrigerant solubility, viscosity, and refrigerant distribution. The characterization helps to reduce oil migration and further improves the oil management strategies of the system. After shutdown, refrigerant migrates from the high-pressure side to the low-pressure side instantaneously. The retained oil absorbs the vapor refrigerant and mixes with the liquid refrigerant. The mixture flow at the compressor suction and discharge is observed. At the suction, vapor refrigerant solubility rapidly increases due to the pressure increase. Mixture film breaks from the top of the tube wall and flows down, then accumulates at the bottom of the tube within seconds. At the discharge, the refrigerant solubility fluctuates within a small range. Viscosity increases because of temperature decrease, then decreases because of refrigerant mass fraction increase. Oil in contact with the inner wall of the discharge stops immediately, while oil droplets in the vapor core flow slow down gradually. The vapor refrigerant, liquid refrigerant, and oil relationship is investigated and estimated after shutdown to better understand oil and refrigerant migration. In the following section, strategies to clean existing oil retention and enhance oil return are investigated. Oil retention and pressure drop of refrigerant flow through the horizontal tube with oil are quantified. Pure refrigerant flow flushes the test tube with pre-added oil, the remaining oil is weighed after flushing for different times. Flow visualization shows flow regime change during flushing. Different refrigerant mass flux, test pressure, and temperature are tested for the potential capacity to improve oil removal. The result shows that oil retention is significantly reduced with higher refrigerant mass flux. Understanding oil migration and management in vapor compression systems is critical for improving system reliability and efficiency. By characterizing transient behaviors during startup and shutdown, as well as refining oil return strategies, this study provides valuable insights into optimizing HVAC&R system performance. Implementing these findings can enhance compressor durability, reduce energy losses, and improve overall system stability applications.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129603

Copyright and License Information

Copyright 2025 Xin Wang

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