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With the growing concern of environment problems, there are urgent demands for environment-friendly refrigerants in the refrigeration industry. As a natural refrigerant, R600a not only has zero ozone depletion potential and quite low global warming potential, but also has better cooling performance than other refrigerants. Due to its good cooling performance and environment-friendly characteristics, R600a has been used as an alternative refrigerant in heat transfer applications such as refrigerators, freezers, and heat pumps. It is known that accurate prediction of two phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube play an important role in design and optimization of refrigerators, freezers, and heat pumps. According to this issue, a detailed experimental study was carried out to investigate the two-phase flow pattern transition and frictional pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation temperature from 282.4 to 304 K, mass fluxes from 67 to 194 kg/(m2 s). Based on a high speed camera, four main flow regimes can be observed: plug flow, stratified-wavy flow, slug flow and annular flow. The flow map of R600a has been drawn and comparisons between the experimental data of flow patterns and transition lines in the literature have been made. The results showed that with the increase of mass flux, the inception of the annular flow regime occurs earlier. The intermittent flow regime takes place over a narrow range of vapor quality and annular flow regime occupies a wide range of vapor quality. In addition, the trend of Barbieri and Ong and Thome I / A transitions lines are consistent with the experimental data. However, both of them overestimate the inception of the annular flow regime. After taking the influence of vapor inertia, liquid viscous and surface tension forces into account, a modified Weber number We * has been introduced to the new flow pattern transition model. Based on the relationship of X tt and We *, a new correlation has been proposed and it can well fit with the experimental data of flow pattern. Furthermore, a detailed discuss of frictional pressure drop of annular flow and non-annular flow has been made. The results indicated that frictional pressure drop increases with the increases of vapor quality. The tendency of frictional pressure drop presents different the changing laws in different flow regime. The frictional pressure drop increases slowly in the non-annular flow regime while it increases rapidly when the flow pattern turns into the annular flow regime. The reason is that the vapor-phase pressure drop often dominates the total two-phase frictional pressure drop. The annular flow regime owns higher void fraction than the non-annular flow regime. Nine classic prediction correlations of pressure drop were evaluated and the results showed that the correlation of Gronnerud and Muller-Steinhagen and Heck give the best fit to the experimental frictional pressure drop of annular flow and non-annular flow with a mean absolute relative deviation of 38.5% and 19.7% respectively.

Boiling heat transfer exists in many industrial heat exchangers such as the evaporator of the refrigerator and the air conditioner, the reboiler of the chemical equipment and steam generator of the nuclear reactor. In the process of boiling, bubble movement can speed up the heat transfer between liquid and solid, leading to the improvement in heat transfer efficiency. Traditional refrigerants used in the industrial facilities have serious environmental problems, therefore, new substitutes are needed. Hydrocarbons are good alternatives. Ethane was suggested as one component of a mixed- refrigerant for its zero ozone depletion potential, low global warming potential and high thermodynamic properties. Therefore, it is important to investigate the bubble behavior of ethane in saturated nucleate pool boiling to design and optimize the industrial equipment like evaporator, improve natural gas liquefaction technology, and research characteristics of heat transfer and multiphase flow as well. Although there are a lot of researches on the heat transfer characteristics of pool boiling, the research on the bubble behavior in ethane is still lacking. In this paper, visualization experiments on the pool boiling heat transfer characteristics of ethane were carried out at 0.2 MPa. The heat fluxes varied from 14.65 kW m−2 to 80.79 kW m−2. Boiling occurred on the upward facing side of a smooth vertical copper cylinder with 20 mm diameter, connected to a DC power supply to control the heat flux on the surface. The measured roughness of the surface was 68.1 nm (root mean square average of the height deviations) and 50.7 nm (the arithmetic average of the absolute values of the surface height deviations). Acquisition of the temperature and pressure was accomplished by using a Keithley 2700 and six thermocouples. The experimental apparatus has a steel ruler inside the pool as the reference length to measure the bubble sizes and the maximum uncertainty for length measurement is 0.060 mm. Some conclusions can be drawn. (1) According to the change of diameter with time, the growth process was divided into three stages. The bubble growth process has obvious segmentation characteristics, and piecewise prediction models are recommended. (2) The bubbles in boiling pool exhibit the characteristics of axial symmetry and uneven distribution; the shape of separation of ethane bubbles has ellipsoidal and irregular shapes relative to normal temperature liquids. (3) There were trail changes and additional movements of large bubbles. Compared to normal temperature liquids, the trail changes were relatively simple; and the bubbles are bubbly and slug flow of two flow regimes. (4) System pressure, heating wall properties like surface roughness, liquid properties like contact angle and surface tension have an important influence on bubble behavior. This paper suggests that the heat transfer prediction equation should consider them.

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%.

In recent years, due to the environmental problems of traditional chlorinated refrigerants, it has become an urgent task to search for suitable substitutes. Hydrocarbons, with zero ozone depleting potential, low global warming potential and high thermodynamic performance, are good alternative refrigerants in refrigerators and heat pump systems. In addition, hydrocarbons also the important parts of the natural gas. As one kind of hydrocarbons, ethane and its mixtures have been widely applied in the mixed refrigerant throttling refrigeration systems and recommended to be used in the cascade refrigeration systems to replace R13 and R503. As the heat transfer characteristics of ethane play important parts in the design of the heat exchangers in refrigeration cycles, the heat transfer coefficients during condensation should be well examined and analyzed. Although there are massive experimental studies of heat transfer on flow condensation of hydrocarbons in horizontal tubes, the published experimental investigations for ethane are rare. Therefore, it’s desirable to conduct an experimental investigation on condensation heat transfer of ethane in a horizontal tube and find suitable correlations. In this paper, condensation heat transfer of ethane was studied experimentally in a horizontal tube with inner diameter of 4 mm. The heat transfer test section was a copper plate (100 mm× 20 mm× 200 mm) embedded twelve four-wire PT100 platinum resistance thermometers. Using the assumption of two-dimensional heat conduction for the copper plate, both the wall temperature and heat flux could be obtained. The tests were carried out at saturation pressures from 1.01 MPa to 2.56 MPa for mass fluxes from 100 kg/(m2 s) to 257 kg/(m2 s) and vapor qualities from 0 to 0.8. The uncertainties for the heat transfer coefficient with a 95% confidence interval were less than 3.33% under the employed operation conditions. The effects of mass flux, vapor quality and saturation pressure on condensation heat transfer coefficient were examined and analyzed. Some conclusions can be drawn that (1) in relative high mass fluxes and vapor qualities, the condensation heat transfer coefficients increased with the increasing mass flux and vapor quality while decreased with the increasing saturation pressure; (2) in relative low mass fluxes and vapor qualities, the vapor quality, mass flux and saturation pressure had a little influence on condensation heat transfer coefficients. In addition, the experimental data were compared with nine well-known correlations of condensation heat transfer coefficient. The comparison results showed that Chato (1962) correlation and Wang et al. (2002) correlation relatively agreed well with the experimental data with the mean absolute relative deviations less than 35%. The other correlations showed larger predicted results than the experimental data with the mean absolute relative deviations more than 55%. And the Chato correlation and Wang et al. correlation were both in good agreement with the relative low heat transfer coefficients rather than the relative high heat transfer coefficients.