Experimental Investigations on Pool Boiling Heat Transfer over External Micro - Finned Cylindrical Surfaces

Abstract

The pool boiling heat transfer is preferred over the single-phase heat transfer when a large amount of energy transmission is needed in a small space. Pool boiling results in high heat transfer due to involvement of liquid to vapour phase change. The pool boiling process is widely used for high heat removal in various engineering applications such as in HVAC (Heating, Ventilation and Air Conditioning) industry, steam generators, nuclear power reactors in power plants, electronic components cooling, etc. Based on the literature review, it was concluded that further investigations related to pool boiling over plain and micro-finned cylindrical surfaces at different pressures and bubble departure diameter needs to be undertaken. Also, it was concluded that though numerous studies related to nucleate boiling over enhanced surfaces were carried out in past and correlations were suggested for the prediction of BHTC, the available correlations were applicable for limited surface characteristics and heat flux range for the micro-finned cylindrical surfaces. In the present research work, a passive technique was utilised and pool boiling set-up was designed and developed to perform pool boiling experiments over plain and micro-finned cylindrical surfaces. The set-up was validated by comparing the pool boiling experimental results over plain cylindrical surface with the values predicted by the correlations of Ribatski and Jabardo, Jung et al., and Gorenflo and Kenning. The experiments were conducted with R141b and R123 as the working fluids, at four different pressures (100 kPa, 125 kPa, 150 kPa and 175 kPa). Based on the experimentation, it was observed that for a plain cylindrical surface, the average increase in the boiling heat transfer coefficient with pressure increase from 100 kPa to 175 kPa was 7.42% to 20% for R141b as the working fluid and 9% to 27.4% for R123 as the working fluid. The average increase in the boiling heat transfer coefficient with pressure increase from 100 kPa to 175 kPa for the micro-finned cylindrical surfaces MCS-1, MCS-2, MCS-3 and MCS-4 using R141b was in the range of 9% to 28.72%, 12.2% to 55%, 7.4% to 25.3% and 8.7% to 30.9% respectively. The same using R123 was in the range of 9.5% to 38.6%, 11.7% to 35.5%, 11.4% to 33.3% and 14% to 38%, respectively. The variation in heat transfer enhancement for micro-finned cylindrical surfaces compared to that for plain cylindrical surfaces was due to the difference in surface geometry and wettability. The static contact angle was used to characterise the wettability of the surface. It was found from the static contact angle measurement that the micro-finned cylindrical surfaces have higher static contact angles than plain cylindrical surfaces. Higher contact angle led to an increase in the active nucleation site density. Hence, it may be inferred that the increase in boiling heat transfer coefficient over micro-finned cylindrical surfaces was due to decreased surface wettability. Using R141b as the working fluid, the average increase in the boiling heat transfer coefficient for the micro-finned cylindrical surfaces MCS-1, MCS-2, MCS-3, and MCS-4 in comparison to the plain cylindrical surface at 100 kPa, 125 kPa, 150 kPa and 175 kPa pressures was in the range of 108% to 119%, 7% to 35%, 11% to 21% and 62% to 79% respectively. Using R123, the average increase in the boiling heat transfer coefficient for the micro-finned cylindrical surfaces MCS-1, MCS-2, MCS-3, and MCS-4 in comparison to the plain cylindrical surface at 100 kPa, 125 kPa, 150 kPa and 175 kPa pressures was in the range of 69% to 84%, 3% to 10%, 16% to 23%, and 29% to 40%, respectively. The distinction of pool boiling characteristics over plain and micro-finned cylindrical surfaces at different pressures using R141b and R123 could be attributed to different fluid properties of the working fluids. Pool boiling over the plain surface with R123 resulted in a higher boiling heat transfer coefficient than R141b at all tested pressures, whereas the pool boiling characteristics over micro-finned cylindrical surfaces were greatly affected by micro-finned surface geometry, surface wettability and fluid properties. A correlation of boiling heat transfer coefficient was developed using dimensional analysis. For the derivation of the new correlation model, the effect of geometrical parameters, operating pressure and thermo-physical properties of fluids were considered. The whole data set's absolute average deviation was 13.43 %, with a root mean square deviation of 0.0273. All the predicted values were within ± 15 % of the experimental values of the boiling heat transfer coefficient. As a part of the study, the bubble departure diameters were also measured at the same heat flux condition over plain and micro-finned cylindrical surfaces at different pressures. The experimental values of bubble departure diameters for the plain surface were compared with the widely used correlations and found to be within ±20%. For predicting the bubble departure diameter over the micro-finned cylindrical surface, a new correlation was proposed which showed a good agreement with the measured bubble departure diameter data. The total MAE (mean absolute error) of the new model bubble departure diameter was about 6.79% for the whole range of data points. The present parametric study reveals that the surface with lower surface wettability and lower fin depth and fin pitch gives better performance. Also, with the increase in pressure, boiling performance improves due to a rise in bubble departure frequency with a smaller bubble departure diameter. The correlation derived for BHTC can be used to predict the boiling heat transfer coefficient over different geometry micro-finned cylindrical surfaces with reasonable accuracy