De-risking thermal management challenges in automotive vehicles: oil circulation and thermal contact resistance

Haider, Syed Angkan

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https://hdl.handle.net/2142/129729 Copy

Description

Title

De-risking thermal management challenges in automotive vehicles: oil circulation and thermal contact resistance

Author(s)

Haider, Syed Angkan

Issue Date

2025-04-30

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

Miljkovic, Nenad

Doctoral Committee Chair(s)

Miljkovic, Nenad

Committee Member(s)

• Bradshaw, Craig
• Wang, Sophie
• Elbel, Stefan

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 circulation ratio (OCR)
• refrigerant-oil mixture
• oil separator
• thermal contact resistance
• thermal interface material (TIM)

Abstract

Thermal management of automotive vehicles involves several different challenges, especially with vehicles becoming more electrified and compact. The challenges root from the requirement for thermal management across multiple different systems and maintaining a high energy efficiency in a wide range of operating conditions. The key focus for this study has been on two major thermal management challenges in automotive vehicles: (1) oil circulation in the air conditioning unit and (2) thermal contact resistance in power electronics. The first part of the work emphasizes oil circulation in the air conditioning system of automotive vehicles. These systems work by employing the vapor compression cycle which has four major components, namely, compressor, condenser, expansion device and evaporator. In such systems, the oil originates from the compressor where it is used as a lubricant to reduce the wear and tear between moving parts. But this oil, mixed with the refrigerant, can leave the compressor and travel around the system, in a phenomenon known as oil circulation. While oil is beneficial to the compressor, in the rest of the system, it can have detrimental effects and has been known to cause reduction in cooling capacity and coefficient of performance (COP). This research focuses specifically on the measurement of oil circulation ratio (OCR), defined as the ratio of the mass flow rate of oil to that of the total mass flow rate of the refrigerant-oil mixture which travels around the system. The standard method for measuring OCR is prescribed in ASHRAE Standard 41.4 (2015) which describes the method of sampling of refrigerant-oil mixture using an evacuated type sampling cylinder to draw in a sample from the liquid line of the system. But the approach described in the standard has several limitations and fails to account for different parameters which can affect results. This study investigates sampling using two different configurations: flow-through cylinder and evacuated cylinder. Using experimental results of measured OCR, thermodynamic modeling, and flow visualizations with a high-speed camera, the effects of parameters such as valve opening speed, sampling cylinder orientation, sampling cylinder size and degree of subcooling were observed. The study was concluded by pointing out the limitations of the ASHRAE standard and providing recommendations on how to accurately and repeatably measure OCR measurements via these sampling techniques. Even though the standard method of OCR measurement involves sampling, it can be tedious, alters the steady operation of a system, and can have several limitations. As an extension of the refrigerant-oil research, this study also looked into novel methods of measuring OCR online so that sampling methods could be avoided. Online oil concentration sensors are already available in the market. They typically work by using the change in refrigerant-oil mixture properties due to the addition of oil to predict the liquid line OCR. However, these sensors can be very expensive and often require regular calibration. This study focuses on a novel method of OCR measurement using an oil separator installed at the discharge of the compressor. The oil separator separates the oil flow from the pure refrigerant flow. The separated pure refrigerant flows around the system and the separated oil returns to the compressor suction. By measuring the refrigerant and oil flow rates, and by performing mass flow corrections to account for refrigerant dissolved in returning oil and oil entrained by refrigerant vapor, the system OCR was predicted. The results of OCR from this oil separator-based approach were compared with liquid line sampling. The OCR from the separator-based approach showed good agreement with liquid line sampling. In the second part of the work, the focus was shifted towards another important thermal challenge, that of thermal contact resistance, in electronic components of automotive vehicles. In electronic components, often the most common mechanism of heat transfer is conduction. Conduction heat transfer between two solids suffers from thermal resistance resulting from the microscopic surface gaps existing at the interface of the solids due to surface roughness of the solids. The microscopic gaps occupied by air results in high thermal resistance and poor heat dissipation. This can cause overheating and eventual damage to electronic components. To avoid this, thermal interface materials (TIMs) are inserted at the interface. TIMs are typically made of highly conductive material which can close the existing air gaps and result in improved heat transfer. This study investigated different commercially available TIMs to first identify the state-of-the-art and to create a benchmark for the study. A calorimeter bar setup based on ASTM D5470 Standard was used for the measurement. Different types of TIMs ranging from thermal pastes, PCMs, graphene, carbon nanotubes, and several others were tested to find the best in the market. The study also focused on the development of a novel composite TIM. The novel TIM was made by applying Galinstan, a liquid metal alloy, to the surface of an etched copper foil as copper is one of the most thermally conductive metals. The surface of the copper was etched to ensure that the liquid metal adhered to it to prevent pump out. The liquid metal has high thermal conductivity compared to other pastes and liquid and also provides good contact with the solid surfaces. The novel composite TIM showed thermal resistance values much lower than those obtained with the high-end commercially available TIMs. This study addresses two critical challenges in automotive thermal management: oil circulation ratio in air conditioning systems and thermal contact resistance in electronics. The research contributes to the field by (1) offering recommendations for measuring OCR accurately and repeatably, (2) introducing a novel oil separator-based method for OCR measurement, and (3) developing an innovative TIM to reduce thermal contact resistance. These efforts will help to advance the field by improving thermal performance and ensuring better efficiency.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129729

Copyright and License Information

Copyright 2025 Syed Angkan Haider

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