Flow Condensation Heat Transfer Coefficient Research Articles

Flow condensation heat transfer correlation development of R32-oil mixture inside a micro-fin tube

The air-conditioners employ R32 as refrigerant because it has low GWP. The heat transfer performance of R32 in the heat exchangers would be influenced by the lubricating oil cycled with R32. In order to obtain the quantitative effect of oil on the flow condensation heat transfer characteristics, flow condensation heat transfer correlation of R32-oil mixture is needed. In this study, an experimental rig for testing the flow condensation heat transfer coefficients of R32-oil mixture is established. The test conditions covering the actual working conditions of air-conditioners are designed, including the condensation temperature ranging from 40 to 50 °C, mass flux ranging from 100 to 400 kg m − 2 s − 1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0 % to 5 %. The results show that the flow condensation heat transfer coefficients decrease with the increasing of oil concentration because of the high viscosity of the oil. A new condensation heat transfer correlation is developed based on the enhanced structure of the micro-fin tube and physical properties of the working fluid, and it agrees with 82 % of the experimental results within a deviation of ± 20 %.

Experimental investigation and correlation development of flow condensation with R123/MWCNTs nanorefrigerant inside a horizontal tube

The current experimental study is focused on examining the behaviour of heat transfer of pure R123 and R123/multi-walled carbon nanotubes nanofluid condensing in a smooth horizontal pipe at a range of different working conditions, like the mass velocity of about 200–500 kg/m2s, multi-walled carbon nanotubes volume fraction of approximately 0.01, 0.05, and 0.1 %, vapour quality of approximately 0.05–0.9, and saturation temperature of about 35.3 and 40.9 °C. The heat transfer coefficient of pure R123 and nanorefrigerant is compared in this investigation. According to the results, employing multi-walled carbon nanotubes improves the heat transfer coefficient for all selected volume fractions. It is observed that the heat transfer coefficient for both pure and nanorefrigerant falls as the saturation temperature rises from 35.3 to 40.9 °C. For a mass velocity of about 400 kg/m2s, 0.1 % fraction of multi-walled carbon nanotubes results in the highest heat transfer coefficient increase of 29 % compared to the base refrigerant. The mean absolute error analysis is evaluated to forecast the accuracy of the experiments by comparing the experimentally captured condensation heat transfer data with available correlations, which shows good agreement. A new correlation for flow condensation heat transfer coefficient is suggested, with an acceptable mean absolute percentage error of about 9.6 %.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Flow condensation heat transfer of propane refrigerant inside a horizontal micro-fin tube

Due to the phase-out of CFCs and HCFC refrigerants, propane is a nature refrigerant occurring substance produced by natural gas production and oil refining. As a result, propane has a higher latent heat and lower density than conventional refrigerants while maintaining a comparable saturation pressure and thermal conductivity. So, propane refrigerant is already widely used in domestic fridges and freezers for many years. However, propane’s operating pressures and temperatures are well suited for air conditioning equipment, including chillers. This study investigates the contributions of different heat transfer mechanisms in two-phase flow condensation heat transfer coefficients for propane inside a 6.3-mm ID micro-fin copper tube. Data were collected through an experiment with a two-phase flow condensation. Measurements were taken at different refrigerant mass fluxes from 100 to 300 kg/m2s and heat fluxes from 3 to 9 kW/m2. In addition, the experiments investigated effect of vapor quality, mass flux, and heat flux on the heat transfer coefficients. Finally, a new heat transfer coefficient correlation was developed based on the experimental data with good agreement.

Flow condensation heat transfer and pressure drop of R134a alternative refrigerants R513A and R450A in 0.95-mm diameter minichannels

Due to the Kigali amendment to the Montreal Protocol, refrigerants such as R134a are targeted for phasedown or phaseout due to their global warming potential (GWP) and lower GWP refrigerants are being considered to replace R134a. Since the condenser is typically responsible for 50% of the charge of a refrigeration system, it is vital to have a fundamental understanding of the flow condensation heat transfer performance of low GWP refrigerants such as R513A and R450A. Experimental flow condensation heat transfer coefficient data are reported for R134a, R513A, and R450A in seven parallel 0.95 mm diameter mini-channels for a range of mass fluxes (i.e., 200 – 500 kg/m2s) and qualities (i.e., 0.2 – 0.8) at a saturation temperature of 40 °C. The heat transfer coefficient uncertainties for all experiments were ± 6.3 – 21.2%, with an average of ± 9.8%. Condensation heat transfer coefficients for R134a, R513A, and R450A all increased with increasing mass flux and quality. R513A condensation heat transfer coefficients were 2.6 – 25.6% lower than R134a heat transfer coefficients and pressure drops were 4.5 – 14.0% lower than R134a pressure drops. R450A heat transfer coefficients were 2.4% higher than R134a at higher mass fluxes and qualities and up to 11.7% lower than R134a at lower mass fluxes than R134a heat transfer coefficients; R450A pressure drop were comparable to R134a pressure drop (i.e., 5.0% higher to 9.5% lower). The data were compared to three condensation correlations – Shah (2016), Kim and Mudawar (2013), and Cavallini et al. (2011); the data were predicted with mean average errors of 23.4%, 15.9%, and 23.4%, respectively.

Experimental study of condensation heat transfer of R134a inside the micro-fin tubes at high mass flux

The flow condensation heat transfer coefficient of R134a inside two helical micro-fin tubes with outer diameter of 6.35 mm and helical angles of 18o and 28o respectively were experimentally studied at a high mass flux ranging from 500 to 1100 kg m−2 s−1. The condensation temperatures are 35°C, 40°C and 45°C, and the vapor qualities ranges from 0.95 to 1.00 at the inlet section and from 0.00 to 0.05 at the outlet section. Experimental results show that there are annular and intermittent flow regimes inside the tube under experimental conditions, the ratio of annular flow region to the total heat transfer region is about 70%. The liquid film flow changes from rotational flow to coexistence of rotational and horizontal flows when the vapor quality is 0.475±0.08, 0.486±0.06 and 0.521±0.05, respectively and the corresponding saturation temperature is 35°C, 40°C and 45°C. Based on the experimental data, the heat transfer coefficient increases with increasing mass flux and fin helical angle, and with decreasing saturation temperature and Reynolds number of cooling water. In addition, the heat transfer coefficient of annular flow is greater than that of intermittent flow, in other words, the heat transfer coefficient increases with increasing vapor quality. The heat transfer coefficient obtained at high mass flux was compared with the calculated data of some published correlations used at low mass flux. It was found that most correlations underestimate the experimental heat transfer coefficient inside the micro-fin tube, and these prediction deviation increases with increasing mass flux and fin helical angle. By introducing some dimensionless numbers as well as considering the flow mechanism of liquid film inside the tube, two heat transfer coefficient correlations were proposed based on the experimental data. The ratio of liquid film thickness to fin height was used to characterize whether the liquid film flow is rotational flow or coexistence of rotational and horizontal flows in the proposed correlation, which shows a better prediction accuracy with a mean deviation of less than 7.3%.

Experimental investigation on convective condensation heat transfer in horizontal 4mm diameter coated tube

Convective flow condensation heat transfer coefficient and pressure drop characteristics inside a coated tube and an equivalent smooth tube with an inner diameter of 4 mm were conducted by using refrigerant R410A. A microporous surface coating layer of approximately 0.2-mm thickness was sintered in the inner surface of the test tube. All condensation runs in the two tested tubes were conducted at saturation temperature 40 ± 0.2 °C, a mass flux range from 307.3 kg/(m2 s) to 998.5 kg/(m2 s), and a vapor quality range from 0.2 to 0.9. The experimental results show that the micro surface coating layer produces 32.97% to 44.78% higher heat transfer coefficient than the smooth tube when vapor quality ranged from 0.9 to 0.2 along the test tube, and yielded a 21.3% overall thermal benefit compared with the smooth tube. For mass flux of 450 kg/(m2 s), 650 kg/(m2 s), and 875 kg/(m2 s), the surface-coated tube produces 37.70% to 54.57%, 28.29% to 41.05%, and 24.9% to 51.39% higher benefits than that of the equivalent smooth tube, respectively. Experimental data indicate that the surface-coated tube presented better thermal performances at high mass flux and high vapor quality region. Ten available condensation heat transfer coefficient correlations for annular or stratified/annular flow were examined, and the prediction results show that correlations over predict condensation heat transfer coefficient of smooth tube and present a better prediction for the surface-coated tube.

General equation for flow condensation heat transfer coefficient in different orientations of helical coils of smooth tubes using genetic programming

There are several experimental studies on heat transfer during condensation in coiled tubes. But there is no well-verified method for calculating of heat transfer coefficient. In this study, a general non-linear correlation for estimation of heat transfer coefficient during flow condensation in different orientations of smooth coiled tubes is proposed. The correlation has been obtained by correlating the Nusselt number with two-phase Reynolds number, reduced pressure, Froude number, tube to coil diameter ratio, and inclination angle of coil axis to horizontal using Genetic programming (GP). This model has been validated with 503 experimental data points from 9 sources, which include different tube diameters, coil diameters, inclination angles, orientations, working fluids, mass fluxes and saturation temperatures. The new correlation predicts experimental data points with an excellent value of average absolute relative deviation (AARD) of 9.20%. The same database is also compared to 9 available correlations for straight and coiled tubes. Their deviations are significantly higher than the present correlation. In addition, impact of each input parameter on heat transfer coefficient in coiled tubes has been discussed.

Machine learning algorithms to predict flow condensation heat transfer coefficient in mini/micro-channel utilizing universal data

Miniature condensers utilizing mini/micro-channel has been recognized as one effective technique for designing a compact heat rejection device. However, because of the complex behaviors in phase-change systems like flow condensation, accurately predicting heat transfer coefficients can be a challenging task. In this study, a large database is utilized to develop machine-learning based models for predicting condensation heat transfer coefficients in mini/micro-channels. A consolidated database of 4,882 data points for flow condensation heat transfer in mini/micro-channels is amassed from 37 sources that includes 17 working fluid, reduced pressures of 0.039 – 0.916, hydraulic diameters of 0.424 mm – 6.52 mm, mass velocities of 50 < G < 1403 kg/m2s, liquid-only Reynolds numbers of 285 – 89,797, superficial vapor Reynolds number of 44 – 389,298, and flow qualities of 0 – 1. This consolidated database is utilized to develop four machine learning based models viz., Artificial Neural Netoworks (ANN), Random Forest, AdaBoost and Extreme Gradient Boosting (XGBoost). A parametric optimization is conducted and ANN and XGBoost showed the best predicting accuracy. The models with dimensionless input parameters: Bd, Co, Frf, Frfo, Frg, Frgo, Ga, Ka, Prf, Prg, Ref, Refo, Reg, Rego, Suf, Sug, Sufo, Sugo, Wef, Wefo, Weg, and Wego predicted the test data for ANN and XGBoost models with MAEs of 6.8% and 9.1%, respectively. The optimal machine-learning models performed better than a highly reliable generalized flow condensation correlation. Models were also able to predict excluded datasheets with reasonable accuracy when data points including the specific working fluid were part of the training dataset of the remaining datasheets. The work shows that machine learning algorithms can become a robust new predicting tool for condensation heat transfer coefficients in mini/micro channels.

Experimental study of heat transfer coefficient, pressure drop and flow pattern of R1234yf condensing flow in inclined plain tubes

The purpose of the present study is to investigate the condensation heat transfer coefficient, pressure drop and flow pattern characteristics of R1234yf refrigerant in an inclined plain tube with an internal diameter of 8.3 mm. Experiments are conducted at four mass fluxes of 80, 160, 240 and 320 kg/m2s, mean vapor qualities in the range of 0.12 to 0.75, saturation temperature of 25 °C, and for tubes with inclination angles in the range of -90° (vertically downwards) to +90° (vertically upwards). The effects of different parameters such as mass flux, mean vapor quality, and the inclination angle of the tube on flow pattern, condensation heat transfer coefficient, and pressure drop are investigated. According to the results, it is found that inclination angle significantly influenced the flow pattern. Furthermore, it is observed that generally, increasing vapor quality and mass flux, enhances the heat transfer coefficient for all the inclination angles. At low mass fluxes or low vapor qualities, the highest heat transfer coefficient occurs in the tube with the inclination angle of -30°, while at higher mass fluxes or higher vapor qualities, the effect of inclination angle on heat transfer is negligible. With increasing the vapor quality, total pressure drop increases for tubes with the negative or zero angles, while it generally decreases, for the tubes with positive angles. In addition, total pressure drop increases with increasing mass flux at constant vapor quality for all the inclination angles. Experimental results of this study are also compared with the existing correlations in literature that predict the amount of heat transfer coefficient and frictional pressure drop. Finally, based on the present data, a new correlation is developed and suggested to predict the flow condensation heat transfer coefficient in inclined plain tubes.

Experiment and simulation on downward flow condensation of nitrogen in vertical tubes

The heat transfer characteristics of nitrogen downward flow condensation are measured in 2 mm and 1 mm vertical tubes. The saturation temperature of nitrogen ranges from 100.4 to 108.0 K. The mass flux varies from 32.7 to 78.6 kg/m2 s for 2 mm tube and from 52.4 to 314.4 kg/m2 s for 1 mm tube. Experimental results of the flow condensation heat transfer coefficient are proposed and analyzed for different vapor qualities, mass fluxes, tube diameters and saturation temperatures. An annular condensation model with the consideration of the interfacial shear stress and the gravity is proposed based on the assumption of uniform film thickness around the tube’s circumference, and the model is validated by the experimental data. Based on the model, the flow and heat transfer characteristics, including the thickness of the liquid film and the condensation heat transfer coefficient are analyzed. Finally, the model and several previous correlations of flow condensation heat transfer coefficient are evaluated by the comparison with the experimental data.

Experimental investigation on flow condensation heat transfer and pressure drop of nitrogen in horizontal tubes

The characteristics of flow condensation heat transfer and pressure drop of nitrogen are measured in smooth horizontal tubes with an inner diameter of 2 mm and 1 mm. The tests are conducted at saturation temperatures from 101.0 to 106.6 K, with mass fluxes from 13.1 to 78.6 kg/m2s for 2 mm tube, and from 65.5 to 262.0 kg/m2s for 1 mm tube. The impacts of mass flux, vapor quality, tube diameter and saturation temperature on the heat transfer coefficient and pressure drop are analyzed. Several previous correlations of flow condensation heat transfer coefficient and frictional pressure drop are compared with the experimental data. Since the thermal properties of nitrogen are different from that of non-cryogenic fluids, such as density, viscosity, surface tension and contact angle, the flow condensation heat transfer characteristic of nitrogen is different from that of non-cryogenic fluids. Considering the influence of thermal properties, a revised heat transfer coefficient correlation based on the Shah correlation is proposed for flow condensation of nitrogen in smooth horizontal tubes, with the mean absolute error of 11.2%.

Experimental Investigation of Condensation Heat Transfer Characteristics of R-134a Vapor in Horizontal Heat Exchanger

An experimental investigation of refrigerant R-134a two-phase flow condensation heat transfer coefficient and pressure drop in condenser tube section of refrigeration system under different operating conditions is presented. The experimental and theoretical investigations are based on test conditions in range of 10 -17 kW/m2 for heat flux, 42-63 kg/m2s for mass flux, vapor quality 1-0.03 and saturation temperature 44 to 49˚C. The experimental tests are conducted on test rig supplied with a test section to simulate the water cooled double pipe heat exchanger, which is designed and constructed in the present work. “The experimental results have revealed that, the heat flux and mass flux have significant impacts on the heat transfer coefficient. “The heat transfer coefficient was increased with increase in heat flux and mass flux at prescribed test conditions, where the enhancement in heat transfer coefficient was about 47% and 14% for relatively higher heat flux and mass flux, respectively. “The enhancement in the heat transfer coefficient was about 51% for relatively lower saturation temperature 45.97˚C and 43% for higher vapor quality 0.88 compared to other values at constant test conditions. “The pressure drop was higher in the range of 12% and 49% for relatively higher mass flux and heat flux respectively. “The present work results have validated by comparison with predictive models and with similar research work results and the comparison has revealed an acceptable agreement.

Simple and general correlation for heat transfer during flow condensation inside plain pipes

This work proposes a new general and simple model to determine the local flow condensation heat transfer coefficient inside plain pipes. The model considers two regimes corresponding to high mass fluxes and/or high thermodynamic qualities and low mass fluxes and/or low thermodynamic qualities. For each region, a new model is suggested which resembles the single-phase heat transfer coefficient model but defining an equivalent Reynolds number in terms of the sum of the superficial liquid and vapour Reynolds numbers. The models consider that the superficial vapour Reynolds number plays a major role in controlling the heat transfer coefficient. The model is able to predict the heat transfer coefficient from channels with a hydraulic diameter of 67 μm up to pipes with a hydraulic diameter of 20 mm for several fluids. No noticeable effect of the diameter of the channel, shape or fluid properties on the heat transfer coefficient has been observed for the studied cases.

Assessment of body force effects in flow condensation, part II: Criteria for negating influence of gravity

This study concerns the development of a set of mechanistic criteria capable of predicting the flow conditions for which gravity independent flow condensation heat transfer can be achieved. Using FC-72 as working fluid, a control-volume based annular flow model is solved numerically to provide information regarding the magnitude of different forces acting on the liquid film and identify which forces are dominant for different flow conditions. Separating the influence of body force into two components, one parallel to flow direction and one perpendicular, conclusions drawn from the force term comparison are used to model limiting cases, which are interpreted as transition points for gravity independence. Experimental results for vertical upflow, vertical downflow, and horizontal flow condensation heat transfer coefficients are presented, and show that, for the given test section, mass velocities above 425kg/m2s ensure gravity independent heat transfer. Parametric evaluation of the criteria using different assumed values of mass velocity, orientation, local acceleration, and exit quality show that the criteria obey physically verifiable trends in line with those exhibited by the experimental results. As an extension, the separated flow model is utilized to provide a more sophisticated approach to determining whether a given configuration will perform independent of gravity. Results from the model show good qualitative agreement with experimental results. Additionally, analysis of trends indicate use of the separated flow model captures physics missed by simpler approaches, demonstrating that use of the separated flow model with the gravity independence criteria constitute a powerful predictive tool for engineers concerned with ensuring gravity independent flow condensation heat transfer performance.

Assessment of body force effects in flow condensation, Part I: Experimental investigation of liquid film behavior for different orientations

Body force effects in flow condensation vary depending on channel orientation and fluid mass velocity, making the design of systems intended to operate in multiple orientations more complicated than those at a fixed orientation. This study examines the effects of body force on liquid film development for flow condensation of FC-72 in horizontal, vertical upflow, and vertical downflow orientations. Two test sections are utilized, one capable of providing high-speed imaging of liquid film development, and the other designed to allow detailed measurements of flow condensation heat transfer coefficient. High speed imaging shows that for low FC-72 mass velocities, flow regimes differ significantly among the three orientations, with vertical upflow exhibiting falling film behavior, horizontal flow showing stratification, and vertical downflow displaying annular co-current flow. For the case of low mass velocity horizontal flow, interfacial disturbances in the form of a sinusoidal wave are clearly visible with wavelengths on the order of 1–10mm. As mass velocity is increased, the liquid film is seen to exhibit similar behavior for all three orientations due to interfacial shear stress negating body force effects. Heat transfer measurements reinforce these trends, with circumferential variations in heat transfer coefficient present for horizontal flow at low mass velocities, and differences in the axial variations in heat transfer coefficient seen when comparing vertical upflow to vertical downflow. As mass velocity is increased, differences in heat transfer coefficient are reduced, with the highest mass velocities exhibiting almost no variation with orientation. This convergence of values indicates the ability of interfacial shear stress to mitigate body force effects at sufficiently high mass velocities.

Stratified Flow Condensation of CO&lt;sub&gt;2&lt;/sub&gt; in a Tube at Low Temperatures

This study presents an experimental investigation of CO2 flowing condensation at the saturation temperature of-10°C, mass flux in the range from 40 to 60kgm-2s-1 and vapour quality ranging from 0.2 to 0.8, in a 6.52mm inside diameter horizontal tube. Previous research on refrigerant condensation has shown that under these conditions, CO2 two phases are expected to develop as a stratified flow. The significant feature of the stratified flow heat transfer is vapour film condensation in the upper region which dominates the overall heat transfer process. Test series in this study confirm that the saturation-to-tube wall temperature difference has a significant influence on the condensing heat transfer coefficient when the temperature difference is within 3K. Comparisons between the experimental results and the predictions by the Dobson, Cavallini and Thome models show that CO2 stratified flow condensation heat transfer coefficients are over-predicted by these models with mean deviations of 104%, 81% and 127%, respectively.

A Common Method for Calculation of Flow Boiling and Flow Condensation Heat Transfer Coefficients in Minichannels With Account of Nonadiabatic Effects

Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena; however, their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. A similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such a case the heat transfer coefficient is primarily dependent on the convective mechanism. In this article a method developed earlier by the authors is applied to calculations of the heat transfer coefficient for flow condensation. The modifications of interface shear stresses between flow boiling and flow condensation are considered through incorporation of the so-called blowing parameter, which differentiates between these two modes of heat transfer. Satisfactory consistency with well-established correlations for condensation has been found, as well as with selected experimental data.

PURE NH3 AND CO2 HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SMOOTH TUBES FOR NH3 TWO-STAGE AND NH3/CO2 CASCADE REFRIGERATION SYSTEMS: A REVIEW

This paper presents a review of differences and similarities of in-tube heat transfer and pressure drop between ammonia (NH3) and carbon dioxide (CO2) from the perspective of the design of heat exchangers for NH3 two-stage and CO2/NH3 cascade refrigeration systems. The focus is on differences in thermophysical properties and thus different characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO2/NH3 cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for flow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict flow condensation heat transfer coefficients was developed based on experimental results for CO2 at -15°C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate flow boiling heat transfer coefficients for NH3 at -20°C, if a flow pattern can be appropriately determined for a flow condition. Also, it is presented that existing correlations can predict well the heat transfer coefficients for CO2 flow boiling at -15 and -30°C. It is shown that some correlations can predict pressure drop relatively well for NH3 and CO2 two-phase flow. The NH3 and CO2 flow evaporation heat transfer and pressure drop characteristics at -40°C are compared with predicted values.

CO 2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO 2 flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m −2 s −1, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO 2 two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.