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Abstract
An improved conventional-to-micro/minichannel criterion was proposed by using the Bond number and the liquid Reynolds number. In micro/minichannels, bubbles tend to be confined and elongated in the channel and the conventional two-phase flow theory loses its applicability. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. In this study, heat transfer correlations for elongated bubbly flow in flow boiling micro/minichannels were developed based on a collected micro/minichannel heat transfer database. The newly developed correlations not only can present a decent overall accuracy, but also estimate the parametric trends correctly. More than 97% of the data points can be predicted by the proposed correlations within a ±50% error band for elongated bubbly flow. Also, a flow-pattern-based model can be developed by combining the developed elongated bubbly flow correlations with previous annular flow correlations for predicting flow boiling heat transfer in micro/minichannels.
NOMENCLATURE
| Bl | = | Boiling number, q/(G hlv) |
| Bo | = | Bond number, g(ρl – ρv)dh2/σ |
| dh | = | hydraulic diameter, m |
| eA | = | mean absolute error, % |
| ei | = | relative error, % |
| eR | = | mean relative error, % |
| G | = | mass flux, kg m−2 s−1 |
| g | = | gravitational acceleration, m s−2 |
| h | = | heat transfer coefficient, W m−2 K−1 |
| hlv | = | latent heat of vaporization, J kg−1 |
| k | = | thermal conductivity, W m−1 K−1 |
| Np | = | number of data points |
| P | = | pressure, bar |
| Pred | = | reduced pressure |
| q | = | heat flux, kW m−2 |
| Re | = | Reynolds number, Gdh/μl |
| Rel | = | liquid Reynolds number, G(1-x)dh/μl |
| ReLO | = | liquid-only Reynolds number, Gdh/μl |
| ReVO | = | vapor-only Reynolds number, Gdh/μv |
| T | = | temperature, K |
| WeLO | = | liquid-only Weber number, G2dh/(ρlσ) |
| x | = | vapor quality |
Greek Symbols
| δo | = | initial liquid film thickness, m |
| λ | = | percentage of data within ±30% error band, % |
| μ | = | dynamic viscosity, Pa-s |
| ρ | = | density, kg m−3 |
| σ | = | surface tension, N m−1 |
| σN | = | standard deviation, % |
| ω | = | percentage of data within ±50% error band, % |
Subscripts
| A | = | annular |
| CB | = | elongated bubbly flow |
| exp | = | experimental |
| IB | = | isolated bubbly flow |
| in | = | inlet |
| l | = | liquid |
| pre | = | predicted |
| sat | = | saturated |
| v | = | vapor |
Additional information
Notes on contributors
Zan Wu
Zan Wu is presently a research associate in the Department of Energy Sciences, Lund University, Lund, Sweden. He received his B.Sc. in energy and environmental system engineering in 2008, and Ph.D. in energy engineering, all from Zhejiang University, Hangzhou, China. His research activities include multiphase flow, phase-change heat transfer enhancement techniques, microfluidics, surface modification, nanofluids, thermophysical properties, compact heat exchangers, and proton exchange membrane fuel cells. He has coauthored about 40 papers in international journals and conferences and four book chapters.
Bengt Sundén
Bengt Sundén received his M.Sc. in mechanical engineering in 1973, Ph. D. in applied thermodynamics and fluid mechanics in 1979, and became docent in applied thermodynamics and fluid mechanics in 1980, all from Chalmers University of Technology, Gothenburg, Sweden. He was appointed a professor of heat transfer at Lund University, Lund, Sweden, in 1992 and has served as Department Head of Energy Sciences, Lund University, Lund, Sweden, since 1995. His research activities include compact heat exchangers, enhancement of heat transfer, gas turbine heat transfer, combustion-related heat transfer, computational fluid dynamics (CFD) methods for laminar and turbulent fluid flow and heat transfer, evaporation and condensation on nanostructured surfaces, liquid crystal thermography, microscale and nanofluid heat transfer, computational modeling and analysis of multiphysics, and multiscale transport phenomena for fuel cells. He has published more than 600 papers in journals, books, and proceedings. He has edited 29 books and authored three textbooks. He has supervised 180 M.Sc. theses, 44 licentiate of engineering theses, and 40 Ph.D. theses. He is a fellow of the ASME, and honorary professor of Xian Jiaotong University, China, He was a recipient of the ASME Heat Transfer Memorial Award, 2011. Since 2013 he has been a guest professor at Northwestern Polytechnical University, Xi’an, China. He received the ASME HTD 75th Anniversary Medal in 2013.