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Abstract
At this time, a widely accepted model that can predict flow boiling heat transfer in microchannels with different fluids, geometries, and operative conditions is still missing. Depending on the working fluid, a predicting correlation can lead to accurate estimation or give rise to errors up to 50% and higher. The situation is further complicated when the working fluid is a zeotropic mixture of two components, due to the additional mass transfer resistance that must be estimated. In the recent years much attention has been paid to the possible use of fluorinated propene isomers in substitution for high-global-warming-potential refrigerants. The available hydrofluoroolefins cannot cover all the air-conditioning, heat pump, and refrigeration applications when used as pure fluids because their thermodynamic properties are not suitable for all the operating conditions, and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The adoption of new mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures in microchannels. In this work, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been considered: The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter has been measured at a pressure of 14 bar, which corresponds to a bubble temperature of around 26°C. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed, and such correction is found to be necessary and proper to provide a correct sizing of the evaporator.
NOMENCLATURE
| Bo | = | Boiling number = q/(G hlg) |
| C | = | parameter in the Cooper [Citation28] correlation |
| Co | = | confinement number $= (\sigma/(g (\rho_l- \rho_g) D^2))^{1/2}$ |
| cp | = | specific heat capacity $J kg_{-1} K_{-1}$ |
| Cδ0 | = | parameter in the Thome et al. [Citation30] correlation |
| D | = | hydraulic diameter, m |
| E | = | enhancement factor in Gungor and Winterton [Citation26] correlation |
| eP | = | percentage deviation = 100 (HTCCALC – HTCEXP)/HTCEXP,% |
| eR | = | average mean deviation = (1/NP) ΣeP,% |
| F | = | enhancement factor in Bertsch et al. [Citation29] correlation |
| f | = | pair frequency, Hz |
| Fc | = | mixture correction factor |
| Fr | = | Froude number = G2/(ρl2 g D) |
| G | = | mass velocity, kg m−2 s−1 |
| g | = | gravity acceleration m s−2 |
| h | = | specific enthalpy, J kg−1 |
| HTC | = | heat transfer coefficient, W m−2 K−1 |
| L | = | length, m |
| M | = | molar mass, kg kmol−1 |
| nq | = | parameter in the Thome et al. [Citation30] correlation |
| nf | = | parameter in the Thome et al. [Citation30] correlation |
| NP | = | number of data points |
| p | = | pressure, bar |
| Pr | = | Prandtl number = μ cp / λ |
| Q | = | heat flow rate, W |
| q | = | heat flux, W m−2 |
| Ra | = | arithmetical mean deviation of the profile according to ISO 4297:1998, μm |
| Re | = | Reynolds number |
| Rel | = | liquid Reynolds number = G(1−x)D/μl |
| Rp | = | roughness in the Cooper [Citation28] correlation, μm |
| S | = | suppression factor |
| T | = | temperature, °C |
| t | = | time, s |
| We | = | Weber number = G2 D / (σ ρl) |
| x | = | thermodynamic vapor quality |
| Xtt | = | Martinelli parameter |
| z | = | position, m |
Greek Symbols
| α | = | heat transfer coefficient, W m−2 K−1 |
| αq | = | parameter in the Thome et al. [Citation30] correlation, W m−2 |
| δ | = | liquid film thickness, m |
| γ | = | generic parameter |
| Δh | = | enthalpy difference, J kg−1 |
| ΔT | = | temperature difference, K |
| λ | = | thermal conductivity, W m−1 K−1 |
| μ | = | dynamic viscosity, Pa s |
| ρ | = | density, kg m−3 |
| σN | = | standard deviation = [Σ (ep – eR)2/(NP – 1)]1/2,% |
| σ | = | surface tension N m−1 |
| τ | = | pair period, s |
| ψ | = | generic parameter |
| ω | = | generic parameter |
Subscripts
| CALC | = | calculated |
| cb | = | convective boiling, referred to the HTC in Eqs. (8) (11), (12), (13), and (23) |
| crit | = | critical |
| dry | = | dryout zone |
| EXP | = | experimental |
| f | = | film, referred to the HTC in Eqs. (18) |
| fb | = | flow boiling, referred to the HTC in EquationEq. (21) |
| film | = | liquid film between the bubble and the wall, EquationEq. (15) |
| g | = | vapor |
| GL | = | glide |
| go | = | vapor only |
| l | = | liquid |
| lo | = | liquid only |
| m | = | mixture |
| min | = | minimum |
| nb | = | nucleate boiling, referred to the HTC in Eq. (3) |
| opt | = | optimal |
| ref | = | reference |
| s | = | sensible |
| t | = | total |
Additional information
Notes on contributors
Marco Azzolin
Marco Azzolin graduated in energy engineering at the University of Padova, and he is now a Ph.D. student in the Department of Industrial Engineering. The topic of his study is condensation and vaporization of refrigerants inside minichannels. At present, he is involved in the project, “ENhanced COndenser for Microgravity (ENCOM)” of the European Space Agency, funded through the MAP Condensation program (AO-2004-096).
Stefano Bortolin
Stefano Bortolin is a postdoctoral researcher in the Department of Industrial Engineering, University of Padova, Italy. He received his Ph.D. from the University of Padova in 2010. His research interests include microscale heat transfer, new refrigerants, computational fluid dynamics, and condensation over nano-engineered surfaces. He has co-authored more than 40 papers in refereed journals and conference proceedings.
Davide Del Col
Davide Del Col is associate professor at the University of Padova, Italy, where he has been teaching energy science, refrigeration technology, and renewable energies at the School of Engineering. He is a member of Commission B1 of IIR (International Institute of Refrigeration), Paris, France; a member of the ASME K-13 Committee; a member of the Steering Committee of UIT (Italian Union of Thermal-Fluid Dynamics); and an associate of INFN (Istituto Nazionale di Fisica Nucleare) in the framework of the Program ALICE (A Large Ion Collider Experiment) for the research on sensor cooling at CERN, Geneva, Switzerland. He is responsible for the Laboratory of Two-Phase Heat Transfer and the Laboratory of Solar Energy Conversion, at the Department of Industrial Engineering of University of Padova. His main research activity regards two-phase heat transfer, both condensation and flow boiling, but he is also active in the field of new refrigerants, heat pumps, and solar energy conversion. He has authored around 200 scientific papers.