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This paper presents heat transfer coefficient and pressure gradient of R1234ze(E) in a tube of 0.643 mm. In addition, visualization sections are added for evaluation of flow patterns. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at exit of data points are presented in the same plot ( Fig. 8 ). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. The single-phase friction factor shows fRe = 64 in laminar and underestimation of Churchill prediction in transition (1700

This paper presents the discussion of regimes of two-phase flow of R1234ze(E) in a 24-port microchannel tube with average hydraulic diameter of 0.643 mm, obtained by visualization. The experiment is conducted in the same facility in the previous work for R32 (Li and Hrnjak, 2019). The conditions cover mass flux from 50 to 225 kg m−2 s−1. The inlet saturation temperature is 30 °C. The two-phase flow is generated by adding heat in several steps to refrigerant initially subcooled at the entrance of test line. As vapor quality increases at a fixed mass flux, the flow is firstly plug/slug, then transitional, and finally is annular flow regime at high quality. When mass flux is 50 kg m−2 s−1, no annular flow observed in the tube. The vapor quality of boundary between two flow patterns decreases as mass flux increases. The annular flow starts at x = 0.85 (G = 100 kg m−2 s−1) and x = 0.63 (225 kg m−2 s−1). The transitional flow starts at x = 0.8 (G = 50 kg m−2 s−1) and x = 0.3 (225 kg m−2 s−1). Comparing to R32 and R134a, flow patterns of R1234ze(E) transition at lower quality due to the lower vapor density and thus higher vapor velocity. Three flow pattern maps in literature are compared to the results and they have limited agreement with our observations. The plug/slug flow behaves as homogeneous. The velocity at interface between liquid slug and vapor plug is close to the homogeneous velocity. The vapor plug length fraction also agrees to the homogeneous void fraction. The length of vapor plug increases dramatically as vapor quality increases at fixed mass flux. The length of liquid slug first increases and then unchanged as quality increases. The video supports that there are liquid droplets formed from the liquid slug and liquid ring collision.

This paper presents visualization of the two-phase flow of R134a in a 24-port microchannel tube with the average hydraulic diameter of 0.643 mm. The experiment is conducted on the same facility in the previous work for R32 (Li and Hrnjak, 2019). Mass flux covers the range from 50 to 250 kg-m−2s−1. The two-phase flow is generated by adding heat in several steps to originally subcooled refrigerant at the very entrance. When mass flux is 50 kg-m−2s−1, no annular flow is observed in the tube. The annular flow starts at x = 0.9 when mass flux is 100 kg-m−2s−1 and x = 0.6 when 250 kg-m−2s−1. The transitional flow starts at x = 0.25 (G = 50 kg-m−2s−1) and x = 0.8 (250 kg-m−2s−1). Revellin and Thome (2007), Ong and Thome (2011), and Zhao and Hu (2000) correlations are plotted against the measurements. None of them fit our data since the experiment is in lower mass flux than the database of their correlations’. The interface velocity and vapor plug length fraction (close to void fraction) are measured from the high speed videos. The velocity and vapor fraction agree to results based on the homogeneous assumption. In plug/slug flow, both the vapor plug and liquid slug lengths are uneven. The length distribution of plug and slug seems to follow Beta distribution. When the quality is low, the quantity of short vapor plugs is larger than long plugs. At fixed mass flux, when quality increases, length of vapor plugs increases and length of liquid slugs decreases.

This paper presents the heat transfer coefficient, pressure gradient, and flow pattern of R1234yf in a microchannel tube. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at the exit of data points are presented in the same plot. The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg m−2s−1, heat flux from 0 to 6 kW m−2, vapor quality from 0 to 1, and inlet saturation temperature from 10 to 30 °C. Comparing the correlations to the HTC measurements at very low quality (about 0.1), Gorenflo, D., and Kenning, D. (2010, Pool Boiling, in: VDI Heat Atlas, 2nd ed, Springer, pp. 757–788) agree with the results. As vapor quality increases, pressure gradient increases. The adiabatic pressure gradient is a strong function of mass flux and saturation pressure (temperature). Flow patterns of R1234yf are also affected by mass flux and saturation pressure. The heat transfer coefficient is a strong function of mass flux and heat flux. The saturation temperature has a smaller effect on HTC in the condition range (10 – 30 °C). Under the test range, the accelerating pressure drop is insignificant compared to friction. Comparing to the results, Mishima, K., and Hibiki, T. (1996, “Some Characteristics of Air-Water Two-Phase Flow in Small Diameter Vertical Tubes,” Int. J. Multiph. Flow, 22(4), pp. 703–712) and Muller-Steinhagen, H., and Heck, K. (1986, “A Simple Friction Pressure Drop Correlation for Two-Phase Flow in Pipes,” Accessed March 1, 2018)., 20, pp. 297–308.) have small mean absolute error (MAE) to predict local pressure gradient. For the heat transfer coefficient, Sun, L., and Mishima, K. (2009, “An Evaluation of Prediction Methods for Saturated Flow Boiling Heat Transfer in Mini-Channels,” Int. J. Heat Mass Transf, 52(23–24), pp. 5323–5329) and Gungor, K. E., and Winterton, R. H. S. (1986, “A General Correlation for Flow Boiling in Tubes and Annuli,” Int. J. Heat Mass Transf, 29(3), pp. 351–358) have an MAE less than 30%.

This paper presents heat transfer coefficient and pressure drop of R32 measured in the same facility as introduced in Li and Hrnjak (2017). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. The pressure drop of R32 is lower than R134a, and the heat transfer coefficient is higher at the same condition. The pressure drop increases with quality and mass flux but decreases with reduction of saturation pressure. R32 heat transfer coefficient increases when heat or mass flux rises. Heat transfer coefficient of R32 first increases when quality increases due to convective effects and then drops at moderate quality due to the absence of nucleate boiling and dry-out. A comparison of adiabatic pressure drop and heat transfer coefficient to correlations has been made. Based on the Blasius equation and homogeneous density, the viscosity model from Cicchitti et al. (1960) has the best fit to predict two-phase pressure drop. Kim and Mudawar (2012) has the smallest MAE to predict pressure drop. For heat transfer coefficient, Sun and Mishima (2009) is the best fit. Further research will be on expanding the database and understanding hydraulic and boiling behavior.

A new facility has been built for experimental study on microchannel and introduced in this paper. Six heat transfer coefficient and six pressure drop measurements can be conducted simultaneously in one pass from vapor quality of 0 to 1 on the facility. Two water-jacket-design heat exchangers are placed top and bottom on each test section for heating the refrigerant. In each test section, the top and bottom sides are heated and controlled separately to ensure uniform conditions. The environmental heat loss and wall temperature measurements are corrected to increases confidence of results. Port geometry is measured with an optical microscope. Heat transfer coefficient and pressure drop of R134a are measured in a 24-port microchannel tube. The single-phase pressure drop follows the rule of round tube and f Re is 64. The single-phase Nusselt number is more sensitive to Reynolds number, compared to large tubes. As vapor quality increases, two-phase pressure drop increases until quality of 0.9, and then stops increasing or even drops; two-phase heat transfer coefficient increases until quality of 0.5 to 0.6 and then drops. Higher mass flux or lower saturation temperature will increase the pressure drop. Heat flux and mass flux have strong impact on heat transfer coefficient, and the effect of saturation temperature is not clear.