Experimental Investigation of Two-Phase Flow Boiling Heat Transfer in Small Diameter Tubes

Ernest Gyan Bediako

Doctoral thesis

Abstract:

In recent years, there has been growing interest in exploring two-phase flow boiling heat transfer characteristics in small diameter tubes, as they offer potential improvement in heat transfer performance and design of compact systems. Despite extensive research in this area, the heat transfer processes and the dominant mechanisms controlling the heat transfer still remain unclear. This research therefore aims to experimentally investigate two-phase flow boiling heat transfer phenomenon in small diameter tubes, with a focus on understanding the underlying mechanisms, identifying critical parameters and testing predictive models for improved thermal performance. An experimental setup from the Thermal Two-Phase Lab in Norwegian University of Science and Technology was modified to investigate the heat transfer characteristics under varying operating conditions, including but not limited to mass flux, heat flux, and saturation conditions. The study was performed in a horizontal 5 mm internal diameter and 8 mm external diameter smooth stainless-steel tube with 1,1,1,2-tetrafluoroethane (R134a) as the working fluid. The main findings reveal that at low and moderate flow boiling conditions, distinct phenomena were observed. A notable peak of heat transfer coefficient near vapor quality of zero, a local minimum and heat transfer deterioration were observed in the low vapor quality region. These distinct observations were sensitive to heat flux but mildly sensitive to mass flux. The main flow patterns recorded with high speed camera were bubble, slug and intermittent flow in the low vapor quality region, annular, dry-out and mist flow in the high vapor quality region. Similar flow patterns were predicted by well-known flow pattern maps in literature. Varying saturation pressure varies the vapor quality at which the flow pattern transitions from intermittent flow to annular flow. Generally, mass flux and vapor quality favor convective boiling heat transfer whiles heat flux and saturation conditions favor nucleate boiling heat transfer. However, their interplay is very significant for clear observation. Pressure drop is observed to decrease with increasing saturation pressure. Overall, this research contributes to the fundamental understanding of two-phase flow boiling heat transfer in small diameter tubes. The insights gained from this study provide valuable guidelines for designing and improving heat exchangers and other thermal system design.

Abstract:

In recent years, there has been growing interest in exploring two-phase flow boiling heat transfer characteristics in small diameter tubes, as they offer potential improvement in heat transfer performance and design of compact systems. Despite extensive research in this area, the heat transfer processes and the dominant mechanisms controlling the heat transfer still remain unclear. This research therefore aims to experimentally investigate two-phase flow boiling heat transfer phenomenon in small diameter tubes, with a focus on understanding the underlying mechanisms, identifying critical parameters and testing predictive models for improved thermal performance. An experimental setup from the Thermal Two-Phase Lab in Norwegian University of Science and Technology was modified to investigate the heat transfer characteristics under varying operating conditions, including but not limited to mass flux, heat flux, and saturation conditions. The study was performed in a horizontal 5 mm internal diameter and 8 mm external diameter smooth stainless-steel tube with 1,1,1,2-tetrafluoroethane (R134a) as the working fluid. The main findings reveal that at low and moderate flow boiling conditions, distinct phenomena were observed. A notable peak of heat transfer coefficient near vapor quality of zero, a local minimum and heat transfer deterioration were observed in the low vapor quality region. These distinct observations were sensitive to heat flux but mildly sensitive to mass flux. The main flow patterns recorded with high speed camera were bubble, slug and intermittent flow in the low vapor quality region, annular, dry-out and mist flow in the high vapor quality region. Similar flow patterns were predicted by well-known flow pattern maps in literature. Varying saturation pressure varies the vapor quality at which the flow pattern transitions from intermittent flow to annular flow. Generally, mass flux and vapor quality favor convective boiling heat transfer whiles heat flux and saturation conditions favor nucleate boiling heat transfer. However, their interplay is very significant for clear observation. Pressure drop is observed to decrease with increasing saturation pressure. Overall, this research contributes to the fundamental understanding of two-phase flow boiling heat transfer in small diameter tubes. The insights gained from this study provide valuable guidelines for designing and improving heat exchangers and other thermal system design.

Keywords

Two-phase flow boiling, heat transfer coefficient, pressure drop, small diameter tubes, heat transfer, refrigerants, experimental investigation, compact designs

Language used: English

Date on which the thesis was submitted / produced: 29. 9. 2023

Thesis defence

• Supervisor: prof. Ing. Tomáš Vít, Ph.D.

Technical University of Liberec

Faculty of Mechanical Engineering

Doctoral programme / field:
Machines and Equipment Design / Machines and Equipment Design