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Two-phase flow during condensation in smooth horizontal round tubes of R245fa, R1233zd(E), R1234ze(E), R134a, R32 from superheated vapor is visualized and presented in this paper. Flow regimes under different mass fluxes, heat fluxes, saturation pressures, specific enthalpies and tube sizes (1, 4, 6 mm) are identified. The paper describes to the flow regime transitions according to the visualizations. The driving force behind the annular-stratified flow transition is identified to be the force balance between shear, gravity and surface tension. The mechanism that dictates the annular-intermittent flow transition is the comparison between wave-height and the tube size. The slip ratio, which generates the Kelvin-Helmholtz instability, is considered to be the reason of transition from stratified-wavy to the fully-stratified flow. The more complicated scenarios where characteristics of different flow regimes coexist are detailed and methods for simplification are provided. The results are also compared to two different flow regime maps. The flow regime map that does not consider the non-equilibrium effects does not provide information beyond bulk quality 1 and 0. Additionally, it does not capture the annular entrance during condensation either. The flow regime map with non-equilibrium taken into account addresses issues above while having its own defects. For instance, it is highly empirical and some transition lines do not properly reflect experimental observations. A more mechanistic flow regime map is recommended.

A flow regime map for condensation from superheated vapor is proposed in this paper. The flow regime map takes the non-equilibrium effects into account by following the development of liquid film from the real onset of condensation to the end. It can be applied to both conventional tubes and microchannels. The transition mechanism between annular and stratified-wavy flow is determined to be the force balance between the shear force, gravity and surface tension. The transition mechanism from annular to intermittent flow is found to be the comparison between wave heights and tube diameter. The transition mechanism for stratified-wavy and fully-stratified is kept the same as elaborated by Xiao and Hrnjak (2017). The connections between the dimensionless number We, Fr and Bo and the transition criteria are analyzed. The flow regime map is validated by the experimental data in Xiao and Hrnjak (2017) and some current visualizations, which includes diabatic visualizations of R134a, R1234ze(E), R32, R245fa and R1233zd(E) condensing at 30 and 50 °C in 1, 4 and 6 mm tubes.

A mechanistic pressure drop model is proposed in this paper for condensation in horizontal smooth round tubes in order to account for the non-equilibrium effects. The model makes use of a flow regime map and void fraction correlation as well as a mechanistic heat transfer model that are all developed for condensation of superheated vapor in a vapor-compression system. The model provide seamless transition between single-phase and two-phase regions including the superheated, condensing-superheated, two-phase, condensing-subcooled and subcooled regions. Diabatic flow visualizations are used to analyze the effects on pressure drop from the formation of waves. An enhancement factor to represent the frequency and magnitude of the waves is established using Kelvin-Helmholtz and Rayleigh-Taylor instability. The two-phase pressure drop is modeled based on the single-phase pressure drop correlations, the flow regimes, void fractions as well as the enhancement factor. Data obtained from R134a, R32, R1234ze(E), R1233zd(E) and R245fa with mass fluxes from 100 kg m−2 s−1 to 400 kg m−2 s−1 and heat fluxes from 5 kW m−2 to 15 kW m−2 inside two different tubes of 4 and 6 mm are used to validate the model.

Pressure drop during condensation from superheated vapor is reported in this paper. Three different refrigerants including R134a, R32 and R1233zd(E) are evaluated in two horizontal round copper tubes with 6.1 and 4.0 mm inner diameters. The test conditions cover a range of mass fluxes from 100 kg m−2 s−1 to 400 kg m−2 s−1 and heat fluxes from 5 kW m−2 to 15 kW m−2. The condensing pressures are selected to be those that correspond to 30 °C. The measurements are all conducted with diabatic flows which start in the superheated (SH) region to observe how the onset of condensation affects the pressure drop. The visualizations of condensation are compared with the measured pressure drop, suggesting the pressure drop behaviors in different regions during the condensation process are different due to different flow regimes. The onsets of condensation all happens in the superheated region before specific enthalpy drops to that of bulk quality 1. The pressure drop depends on the mass flux, specific enthalpy and tube diameter. The result also shows that several thermal properties of the refrigerants such as the density ratio and viscosity have an effect on the pressure drop, which can be explained by the generation of waves and the differences in vapor and liquid velocity. A peak in the pressure drop curve is consistently present between the onset and end of condensation around bulk quality 1, which could be a result of the competition between increasing liquid-wave strength and decreasing flow velocity as condensation proceeds after the onset of liquid film. Since none of the current existing correlations are sufficient in accounting for the different mechanisms in different stages of the condensation process, a pressure drop model that is specifically developed for condensation of superheated vapor by taking non-equilibrium into consideration is in need.

A heat transfer model is proposed in this paper for condensation of annular and stratified flow in horizontal smooth round tubes accounting for the non-equilibrium effects. The model is based on a non-equilibrium flow regime map and void fraction correlation that is developed from diabatic flow visualization and film thickness measurement. The model unifies single-phase and two-phase heat transfer models into one continuous function throughout the superheated, condensing-superheated, two-phase, condensing-subcooled and subcooled regions with seamless transition in between. Film heat transfer coefficient based on the interfacial temperature of the flow is used as a tool for the modeling in the presence of two-phase flow, which is later converted back to heat transfer coefficient based on the bulk temperature of the flow. The two-phase flow model is developed for the annular and stratified flow. The effects of interfacial waviness, liquid entrainment, wall subcooling, gravity, tube diameter as well as non-equilibrium are discussed in separated sections. Data obtained from different refrigerants and working conditions are used for the validation of the model.

The paper presents flow characterization of condensation based on experiments with R134a in 6.1 mm inner diameter horizontal round tube. The paper presents flow visualizations and liquid film thickness measurements with mass fluxes from 50 to 200 kg m−2 s−1 and heat fluxes from 5 to 15 kW m−2, showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. All the measurements are taken at constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 °C. The result of flow visualization reveals that the condensation always starts in the bulk superheated region, which is generally defined as the beginning of condensing superheated region, as annular flow and the flow regime is strongly affected by the mass flux, instead of the heat flux. Based on the flow visualization results and underlying physics in the condensing superheated region, a new diabatic flow regime map is proposed to better represent the physics and predict flow regimes in both the two-phase and the condensing superheated region. The range of prediction of the new flow regime map extends over quality one up to the onset of condensation and the early stages of condensation are strictly in annular flow regime. A film thickness measurement technique for round tube is described as well as results of calibration. The film thickness measurement demonstrates that void fraction drops below one in superheated region, which is not taken into account for most conventional void fraction models. In addition, both the mass flux and heat flux affect the void fraction by altering the onset of condensation, a new void fraction model is proposed to include this mechanism. Moreover, having information of film distribution provides an opportunity to more realistically model the film geometry inside of the tube.

The focus of this paper is heat transfer and pressure drop during condensation in a horizontal smooth round tube. R134a in 6.1 mm inner diameter is used as an example for heat transfer measurements operated in a range of mass fluxes from 50 kg m−2 s−1 to 200 kg m−2 s−1 and heat fluxes from 5 kW m−2 to 15 kW m−2. The paper also presents pressure drop measurements with mass flux from 100 kg m−2 s−1 to 200 kg m−2 s−1, showing the effect of mass flux and heat flux on the heat transfer coefficient (HTC) and pressure drop. All the measurements are taken at a constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 °C. By connecting heat transfer results with flow characterizations, the behavior of HTC is explained from the perspectives of both mathematics and physics. The result shows that for fixed heat flux and different mass flux, even though the liquid film for higher mass flux is much thinner in the condensing superheated (CSH) region, the HTC is not affected by mass flux. In the two-phase (TP) region, however, higher mass flux clearly yields higher HTC. Opposite behavior is found when heat flux is varied and mass flux is fixed. In the CSH region, HTC increases with heat flux, while in TP region, HTC does not change with heat flux. For both cases, a peak of HTC is presented at quality one, which seems to imply some counter-acting factors that always put heat transfer largest at quality one. The counter-acting factors, however, should not exist based on flow characterization. After mathematically explaining the behavior of HTC in CSH region, film HTC (HTCf) is proposed and ready to serve as a tool in a unified heat transfer model throughout the entire CSH and TP regions. The result of film HTC shows that the film HTC goes to infinite at the onset of condensation, which is physically correct. Also, the film HTC increases with increasing mass flux. The effect of heat flux, or wall temperature on film HTC is still under discussion. In this study, higher heat flux yields lower film HTC only at high qualities, which is due to the difference in void fraction. In addition, the result of pressure drop measurement also shows that the presence of liquid film tends to increase the pressure drop, which suggests that the wavy structure of vapor–liquid interface increases the friction of the flow. The peak of the pressure drop, however, occurs at lower enthalpy for higher mass flux. The reason could be the later onset of condensation and thinner film, which delays the peak of vapor–liquid interaction at higher mass flux.