Dynamics of Bubble Behavior in Nucleate Boiling of R134a in an Enhanced Horizontal Tube

Abstract

The design of tubes in heat exchangers has a significant influence on heat transfer coefficients (HTCs), affecting nucleate boiling at heat exchanger surfaces. Many researchers have explained heat transfer enhancement techniques applied to tubes by identifying empirical correlations, both dimensional and non-dimensional, for HTCs in terms of surface roughness factor, vapor density, and liquid density based on their experimental results. However, there has been a lack of research analyzing the mechanisms that affect the HTCs in depth. For example, the influence of bubble density, surface roughness factor, and the dynamics of bubble behavior have rarely been explained. In this regard, the present study focuses on mechanism analysis in terms of the value of HTC, specifically revealing the dynamics of bubble behavior. Additionally, this study reports the results of an experimental investigation of pool boiling heat transfer of R134a in a plain tube, a low-fin tube, and an enhanced (Thermoexcel-E) tube as well as results of examining the dynamics of bubble behavior observed on the surface of an enhanced tube. Ultimately, all these investigations aim to determine the heat transfer characteristics of the enhanced tube. In this study, however, circulated water flowing inside an evaporator is used as a heat source instead of a cartridge heater. The heat transfer enhancement ratio of the Thermoexcel-E tube is 2.69-4.13 in the range of heat fluxes tested when the values of HTC from the Thermoexcel-E tube are compared to counterparts from a plain tube. Meanwhile, analysis of the dynamics of bubble behavior observed on the surface of the enhanced tube allows determination of the enhanced tube's heat transfer characteristics. The findings of this study can provide the refrigeration industry with a basic framework for better design using high-efficiency evaporators.