Flow boiling in horizontal pipes: integral measurements, optical diagnostics, and numerical simulations

Abstract

Flow boiling in horizontal pipes is the heat transfer mechanism central to a wide variety of applications including refrigeration systems, high-temperature heat pumps, organic Rankine cycle systems and concentrating solar power plants operating in the direct steam generation mode. Refrigerants such as R245fa often serve as the working fluid in these applications, but few studies have been published investigating flow boiling of R245fa in pipes of sizes above the micro-scale. There is a lack of detailed data available with high spatial and temporal resolution for boiling flows, which could enhance fundamental understanding of the hydrodynamic and thermal behaviour and aid effective design and operation of heat transfer systems. Numerical simulation of boiling flows can complement detailed experimental data, but it presents a complex computational challenge, particularly in macro-scale geometries. This work presents experimental and numerical investigations towards a full description of flow boiling. A bespoke experimental facility was constructed for flow boiling of R245fa in a horizontal 12.6 mm pipe, and an experimental database has been populated with measurements of pressure drops, heat transfer coefficients and flow patterns. Laser-induced fluorescence and particle image velocimetry techniques were also developed and successfully applied in this facility, providing detailed spatio-temporally resolved interfacial property, phase distribution and liquid-phase velocity-fi eld data. Predictive methods in the literature are compared to experimental data and improvements suggested based on the optical measurements of interface location. Numerical simulations of elongated bubble flow provide new insights into the effects of buoyancy and inertia on bubble shape and dynamics. A new theoretical model for the fi lm thickness around the bubble is presented and compared to experimental data. A phase change model is validated in OpenFOAM for growth of a small bubble, and is then used to simulate stratifi ed flow based on a set of experimental conditions from this work.