Experimental Studies on the Effect of Ultrasonic Field Application in Nucleate Pool Boiling Regime under Various Operating Pressures with R-141b as Working Medium

Abstract

Pool boiling has several industrial applications and is a research topic of great interest over the decades due to its inherent capabilities of large heat transfer rates with small temperature differences. The requirement of further higher rates of heat transfer in a wide range of energy intensive applications has made the augmentation of boiling heat transfer a dynamic domain of research for the past several decades and, in line, many active and passive methods have been developed. However, there have been a very few studies available related to usage of ultrasonic field in boiling under different pressure conditions. The present study discusses the experimental investigation of saturated pool boiling including the effect of operating pressure and ultrasonic field to pool boiling. In the preliminary part of the study, a novel approach to evolve the fluid surface interaction parameter Csf and index m of the well-known Rohsenow correlation was conducted using the approach of genetic algorithm. A C++ computer program was prepared based on the binary coded genetic algorithm (BCGA) technique. For the testing of the program, the pool boiling experiments were conducted using R141b as the working fluid and a plain Cu disc as a heating surface under six different pressure conditions ranging from 80.5 kPa to 130.5 kPa. The experimental results were analysed using regression analysis and observed a value of 0.0045 for Csf and 0.699 for index m for the surface- fluid combination. For the given range of various parameters and non-dimensional numbers, the BCGA program was tested with initial population of 100 and finally converged to 0.0048 and 0.6817 for Csf and index m respectively. The results indicate the efficacy of the approach and the inherent generalisation makes it suitable for other surface-fluid combinations and characterized surfaces as well. The experimental investigations of the effect of operating pressure on pool boiling of R141b over plain Cu as well as Si-coated surface were conducted and are discussed as a parametric study. The Si-coated surface was prepared using direct current (DC) sputtering technique. The working fluid R141b was boiled in saturated pool condition under pressure ranging from 80.5 kPa to 130.5 kPa and the acquired experimental data and trends were compared with the existing correlations and theories. Within the pressure range considered, the surface superheat variation was found to be insignificant at lower heat fluxes, however at higher heat fluxes, the maximum reduction in surface superheat was found to be 9.5ºC and 14.8⁰C for iv the plain Cu surface and Si-coated surface respectively and corresponded to 80.5 kPa pressure. In comparison with the results of saturated pool boiling under atmospheric conditions, at the pressure of 130.5 kPa, a corresponding increase in the heat transfer coefficient of 12.1% for the plain Cu surface and of 17.8% for Si-coated surface was observed at a heat flux of 225 kW/m2 and 272 kW/m2 respectively. Also, the accompanying augmentation in the critical heat flux was observed as 13.3% for the plain Cu and 21.2% for the Si-coated surfaces. Based on the experimental data, a correlation was developed for predicting heat transfer coefficients within the given pressure range. Experimental investigations were conducted to study the effect of operating pressures (in the range of 80.5 kPa to 130.5 kPa) in the presence of ultrasonic field (31 kHz and 40 kHz frequencies) on saturated pool boiling of R141b over plain Cu surface. An ultrasonic assembly was designed, fabricated and installed in the boiling vessel for delivering of the ultrasonic field during the boiling process. It was found that ultrasonic field is more effective to enhance boiling at higher operating pressures. The surface superheat was found to be reduced by a maximum value of 2.6°C with 31 kHz of ultrasonic field and 130.5 kPa operating pressure at lower heat flux of 113 kW/m2. This also led to further decrease of bubble departure diameter and a corresponding increase in bubble departure frequency. The maximum augmentation in heat transfer coefficient was calculated as 37.1% and 11.4% for frequency of 31 kHz and 40 kHz respectively at the lower heat flux of 113 kW/m2. The maximum improvement in Nusselt number (Nu) was found to be 25.3% at 31 kHz frequency of ultrasonic field and corresponding to 130.5 kPa operating pressure conditions. The study suggests the usage of higher operating pressures for saturated pool boiling in the presence of ultrasonic field for superior augmentation in heat transfer performance. Also, the investigation suggests the use of ultrasonic field with lower frequency for more improvement in pool boiling heat transfer performance.