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Recently, considerable attention has been given to new environment-friendly refrigerant mixtures for applications such as heat pumps and air conditioners. This study presents flow boiling heat transfer characteristics of environment-friendly refrigerant mixture of R1234ze(E)/R600a (0.148/0.852, 0.194/0.806, 0.371/0.629, 0.623/0.377 and 0.797/0.203 by mole fraction) in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were conducted at saturation pressures from 0.215 to 0.415 MPa, mass fluxes from 67 to 194 kg m−2 s−1 and heat fluxes from 10.6 to 74.8 kW m−2. The influences of concentration, saturation pressure, mass flux and heat flux on heat transfer were analyzed and discussed. Furthermore, the experimental data were also compared with nine well-known flow boiling heat transfer correlations. The results indicated that correlation of Liu and Winterton (1991), which is the correlation for pure fluids, gives the best fit to the experimental data with a mean relative deviation of 18.2%. Finally, a new heat transfer correlation based on power-additive model was proposed and two suppression factors Smix and Fmix were introduced into the new correlation by taking account of mass transfer resistance. The new correlation makes great progress in the prediction accuracy with a mean relative deviation of 6.4%.

An experimental investigation on condensation flow pattern and heat transfer coefficient of methane/ethane mixtures (0.27/0.73, 0.54/0.46 and 0.7/0.3 by mole) in a horizontal smooth tube with inner diameter of 4 mm was carried out. The tests were conducted at saturation pressure of 1.5–2.5 MPa with mass flux of 98–257 kg m −2 s −1 and heat flux of 15.1–44.4 kW m −2 over the entire vapor quality range. The effects of concentration, saturation pressure, heat flux, mass flux and vapor quality were analyzed and discussed. A new annular/non-annular flow pattern transition criteria on condensation for zeotropic mixtures was carried out. The new transition criteria took the influence of mass transfer resistance into consideration and had satisfactory predictive ability in present study. Moreover, the experimental data were compared with many condensation heat transfer correlations of mixtures. An improved heat transfer correlation for zeotropic mixtures based on flow patterns was proposed. The new correlation combined with the equilibrium method and introduced a new correction factor ( F m ). It achieves better predicting results with a mean absolute relative deviation of 8.02%.

As a critical facility, spiral-wound heat exchanger (SWHE) has been widely used in many industrial applications. A computational fluid dynamics (CFD) model was employed with the smallest periodic element and periodic boundary conditions to examine the characteristics of the shell side of SWHE. Numerical simulation results show that the heat transfer coefficients around the tube initially increase and subsequently decrease with radial angle because of the influence of backflow and turbulent separation. The mean absolute deviation between simulated heat transfer coefficients and measured values for methane, ethane, nitrogen and a mixture (methane/ethane) is within 5% when Reynolds number is over 30000. For the pressure drop, the simulated values are smaller than the measured values, and the mean absolute deviation is within 9%. Numerical simulation results also indicate that the pressure drop and heat transfer coefficients on the shell side of SWHE decrease as the winding angle of the tubes increases. Considering the effect of winding angle on pressure drops and heat transfer coefficients, the modified correlations of Nusselt number N u = 0.308 R e 0.64 P r 0.36 ( 1 + sin θ ) − 1.38 and friction factor f = 0.435 R e − 0.133 ( sin θ ) − 0.36 , are proposed. Comparing with the experimental data, the maximum deviations for heat transfer coefficients and pressure drops are less than 5% and 11% respectively.

In recent years, due to the environmental issues, suitable substitutes are being searched to replace the traditional chlorinated refrigerants. With zero ozone depleting potential, low global warming potential and high thermodynamic performance, the hydrocarbons are suitable selections in refrigerators and heat pump systems. As hydrocarbons, R50 and R170 are also the important parts of the natural gas and often-used components in mixture refrigerants for the low-temperature Joule-Thomson refrigerator. As the flow characteristics play important parts in the design and optimization of the heat exchangers in refrigerators and air-conditioning systems, the two-phase frictional pressure drop should be well examined and analyzed. Although there are massive experimental studies on two-phase frictional pressure drop of hydrocarbons in horizontal tubes, the published experimental investigations for zeotropic mixtures of R50/R170 are rare. Therefore, it′s desirable to conduct an experimental investigation on two-phase frictional pressure drop of zeotropic mixtures of R50/R170 in a horizontal tube and find a suitable correlation. In this paper, two-phase frictional pressure drop of R50/R170 mixtures (0.27:0.73, 0.54:0.46 and 0.7:0.3 by mole) was studied experimentally in a horizontal tube with inner diameter of 4 mm. The tests were carried out at saturation pressures from 1.5 MPa to 2.5 MPa for mass fluxes from 99 kg m−2 s−1 to 255 kg m−2 s−1, vapor qualities from 0 to 0.9. The uncertainty for the pressure drop was 40 Pa and the uncertainties for the vapor quality with a 95% confidence interval were less than 11.74% under the employed operation conditions. The effects of mass flux, saturation pressure, vapor quality and concentration on two-phase frictional pressure drop were examined and analyzed. Some conclusions can be drawn that (1) the frictional pressure drops increase with the increasing mass flux and the impact becomes more obvious when the vapor quality increases; (2) the frictional pressure drops decrease with the increasing saturation pressure and the impact enhances when the vapor quality increases; (3) the frictional pressure drops increase with the increasing vapor quality and then change to be smooth even decline at high vapor qualities. And the effect of vapor quality enhances as the mass flux increases and the saturation pressure decreases; (4) the frictional pressure drops of pure R50 are lower than that of all R50/R170 mixtures and the frictional pressure drops of the initial concentration of R50/R170 mixtures at 0.27:0.73 are also lower than that of other two initial concentrations of R50/R170 mixtures. Their differences are not obvious at low vapor qualities, while the differences enhance when the mass flux and vapor quality increase as well as the saturation pressure decreases. The influence of concentration may be mainly related to the difference of the vapor densities. In addition, the experimental data were compared with twenty well-known frictional pressure drops correlations. The comparison results showed that the Friedel correlation showed the best agreement with a mean absolute relative deviation of 19.26% and 87.45% of points in the deviation bandwidth of ±30%.