Condensation Pressure Drop Research Articles

Study of developing a condensation heat transfer coefficient and pressure drop model for whole reduced pressure ranges

This study involves the collection of data from 10 different articles to develop experimental-based models for predicting the condensation heat transfer and the frictional pressure drop. The dataset comprises a total of 1168 condensation heat transfer coefficients and 792 frictional pressure drop data. The applied operating range considered is within the reduced pressure of 0.1–0.95, mass flux ranging from 75 to 700 kg/m2s, and an inner diameter between 3.4 and 12.5 mm. We developed the models for the condensation heat transfer coefficient based on Akers et al.’s model, whose parameters are the Prandtl number, density ratio, vapor quality, and mass flux. The total error for the condensation heat transfer coefficient is ± 22.6%. The data is further categorized into two groups of the reduced pressure: Pr < 0.5 with an error of ± 22.9% and Pr > 0.5 with an error of ± 18.9%. The frictional pressure drop models were developed based on the three different ranges of the reduced pressure. The utilized non-dimensionless parameters were the two-phase multiplier (Φlo2), the Bond number (Bo), the Weber number (We), and the Martinelli parameter (Xtt), along with the regression coefficients. Regarding the frictional pressure drop correlation, the total error is ± 32.7%, and the data is divided into three segments: Pr < 0.2 with an error of ± 24.6%, 0.2 < Pr < 0.3 with an error of ± 35.6%, and an error of ± 27.8% for the reduced pressure larger than 0.3. These correlations were developed using the MATLAB regression analysis to enhance their practicality and utility.

Thermal-Hydraulic Characterization of a Thermosyphon Cooling System for Highly Compact Edge Microdatacenter

Abstract In the European project BRAINE, an extremely efficient passive thermosyphon cooling system was developed for a novel type of Edge MicroDataCenter (EMDC) with high heat fluxes to dissipate on individual computer cards and high heat dissipation rate per unit volume in the dense array of these cards. The primary objective was the cooling of 11 nodes (which include CPUs, FGPAs, GPUs and storage) considering one heat sink evaporator per node. The present study shows the experimental results obtained with the new environmentally friendly refrigerants R1233zd(E) and R1234ze(E) as well as a validation of the solver used to design the thermosyphon for a first demo EMDC with 4 slots (i.e 4 nodes). The maximal total dissipated power was 950 W and performance ratios of heat dissipation-to-cooling fan power up to 98-to-1 were obtained. The solver validation was performed by comparing pressure drop of the different components as well as the maximum temperature of the heaters mimicking CPUs for 232 simulated datapoints and proved extremely good accuracy without any scaling or empirical factors: 99%, 78%, 53% and 95% of the datapoints for the evaporator, riser, condenser and total pressure drop, respectively, were within 30% of the experimental results, which is accuracy comparable to the best two-phase pressure drop correlations in the literature, so a very good validation result. For the temperature validation, 97% of the datapoints were within 5°C of the experimental measurements, highlighting the robustness of the solver.

Numerical investigation on flow condensation in zigzag channel of printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) has a great potential for the application in various compact energy systems. However, the there is no existing heat transfer and pressure drop correlations on condensation in PCHE zigzag semicircular channel. In this study, the flow and heat transfer characteristics of condensation in the PCHE zigzag semicircular channel were analyzed numerically. The flow patterns of annular flow, annular wavy flow and plug flow were observed in the channel, and the flow pattern transition line correlations were developed. As the mass flux increases, the transition vapor quality from annular flow to intermittent flow decreases gradually. Both the condensation pressure drop and the heat transfer coefficient reach their peaks as the vapor quality is around 0.78, and the pressure drop decreases with the increase of saturation temperature. The heat transfer coefficient decreases as the saturation temperature increases when the vapor quality is less than 0.6, while it increases slightly as the saturation temperature increases when the vapor quality is greater than 0.6. New correlations for condensation pressure drop and heat transfer in PCHE zigzag semicircular channel are developed. The average errors of the developed condensation pressure drop and heat transfer correlations are 11.0 % and 7.9 %, respectively.

Comparative analysis of condensation heat transfer and pressure drop of low GWP near-Azeotropic mixture and R-290 in a plate heat exchanger

Downward condensation of a near-azeotropic low GWP refrigerant mixture of R290/R1270 (65%/35% wt.), named HC01, was investigated inside an offset strip-fin (OSF) flow-structured plate heat exchanger (PHE). This study tested the prototype heat exchanger with HC01 and compared the condensation heat transfer coefficient (CHTC) and pressure drop (CPD) with the pure R-290 for working conditions of refrigerant mass flux (G) = 20–50 kg/m2s, inlet vapor quality (xin) = 0.2–0.9, saturation temperature (Tsat) = 40–55 °C, and heat flux (q) = 4–13 kW/m2. It was observed that the condensation of HC01 occurred entirely within the forced convection condensation flow regime, despite a slight change in the CHTC at a lower xin as the G increased. The CHTC demonstrated an increase in response to higher values of G and q, but decreased with increasing Tsat. Similarly, the CPD exhibited an increase with higher G and a decrease with lower Tsat, whereas the q had minimal influence. Comparison results showed that the HC01 exhibited 14.2% higher average CHTC but 6.5% lower CPD than that of R-290 because of the differences in properties, such as liquid thermal conductivity, viscosity, surface tension, latent heat, and discrepancies in liquid and vapor phase densities. Correlations were generated to predict the values of the Nusselt number (Nu) and friction factor (FF) of HC01, with mean absolute errors of 12.1% and 12.7%, respectively.

Research on the R513A condensation heat transfer characteristics in horizontal microfin tubes

R513A has been proposed as alternative to R134a due to its lower GWP. So, it is necessary to investigate the condensation heat transfer characteristics and pressure drop of R513A in the horizontal tubes. The 12.70-mm-OD and 9.52-mm-OD tubes were selected for condensation tests. It was found that the R134a condensation heat transfer coefficient in the 12.70-mm-OD microfin tube was similar to the R513A condensation heat transfer coefficient, and the deviation was about −3% ∼ +9%; R513A yielded 4.44% ∼ 16.01% lower pressure drop than R134a. The R513A heat transfer coefficient of the microfin tube was about 1.42–1.74 times of that of the smooth tube, and the pressure drop was 39.11% ∼ 76.63% than that in the smooth tube. Additionally, the condensation heat transfer coefficient and pressure drop of the 9.52-mm-OD test tube were 23.88% ∼ 41.62% and 40.79% ∼ 71.86% higher than those of the 12.70-mm-OD tube, respectively. The micro-fin structure inside the microfin tube resulted in efficient heat transfer, but also increased the fluid flow power consumption. Moreover, as the tube diameter decreased, the effects of sheer stress and surface tension increased. Furthermore, experimental data were also compared with previous correlations proposed for microfin tubes. More experiments are still required to develop more applicable theoretical correlations in future.

Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review

Nowadays, the demand for obtaining high heat flux values in small volumes has increased with the development of technology. Condensing flow inside mini- and microchannels has been becoming a promising solution for refrigeration, HVAC, air-conditioning, heat pumps, heat pipes, and electronic cooling applications. In these applications, employing mini/microchannels in the condenser design results in the working fluid, generally refrigerant, undergoing a phase change inside the mini/microchannels. On the other hand, the reduction in the hydraulic diameter during condensation gives rise to different flow regimes and heat transfer mechanisms in the mini- and microchannels compared to the conventional channels. Therefore, the understanding of fluid flow and heat transfer characteristics during condensation of refrigerant inside mini- and microchannels has been gaining importance in terms of condenser design. This study presents a state-of-the-art review of condensation studies on refrigerants inside mini- and microchannels. The review includes experimental studies as well as correlation models, which are developed to predict condensation heat transfer coefficients and pressure drop. The refrigerant type, thermodynamical performance, and compatibility, as well as the environmental effects of refrigerant, play a decisive role in the design of refrigeration systems. Therefore, the environmental impacts of refrigerants and current regulations against them are also discussed in the present review.

CFD simulation data report: heat transfer coefficient and pressure drop of natural gas flow condensation in spiral tube

CFD simulation data report: heat transfer coefficient and pressure drop of natural gas flow condensation in spiral tube

New Friction Factor Correlation for Phase-Change Flow in Plate Heat Exchangers

This article reports a new two-phase friction factor correlation for condensing and evaporating flows in plate heat exchangers. Over a thousand condensation pressure drop data compiled from seventeen articles were reduced to a two-phase friction factor correlation and successfully evaluated with data not used for correlation development. Similarly, over two thousand evaporation pressure drop data were collected from twenty-four articles and were empirically modeled to yield a two-phase friction factor, which was also successfully validated with independent data. The condensation friction factor correlation fit 72% of the data within ±50% and modeled 92% of the evaluation data within ±50%. Similarly, the evaporation friction factor correlation fit 64.9% of the data within ±50 and 89.8% of the evaluation data within ±50%. Pressure drop in condensers was predominantly due to convective condensation and buoyancy forces. Inertial forces and convective boiling were mainly responsible for the pressure drop in evaporators. As the Weber number was much <1, drop condensation and nucleate boiling were insignificant. Meta-analysis strongly recommended deploying plate heat exchangers as condensers for lowering the pressure drop and was inconclusive for evaporators.

A Simple and Cost-Effective Technique to Monitor the Sublimation Flow During Primary Drying of Freeze-Drying Using Shelf Inlet/Outlet Temperature Difference or Chamber to Condenser Pressure Drop

As lyophilization continues to be a critical step in the manufacturing of sensitive biopharmaceuticals, challenges often arise during the scale up to commercial scale or the transfer from one manufacturing site to another. While data from the small-scale development of the lyophilization cycle is abundant it is typically much more difficult to extract important information from commercial scale cycles, due to the lack of process analytical technologies available on the commercial line. There is often a reluctance to include wireless temperature or pressure probes during GMP operations due to the additional contamination risk, and retrofitting equipment such as the TDLAS can be prohibitively expensive. Further, as products become more advanced, the cost of consuming the product or even the availability of material may limit the opportunities to run commercial scale trials. This paper presents two novel methods to garner critical cycle information to allow for the evaluation of cycle performance without the need for expensive analytical equipment, costly revalidation and line downtime. Critically, this can be achieved using commonly available temperature and capacitance probes on existing commercial scale equipment. The first method is a calorimetric method, based on quantifying the differences in heat transfer liquid temperature between the shelf inlet and shelf outlet. This change in temperature results from the on-going sublimation, an endo-thermic reaction occurring during lyophilization. The second method uses the differential pressure between the chamber and condenser resulting from the vapor flow from vial to condenser during primary drying. As stated by the authors both methods align well and provide valuable cycle characterization data.

Condensation heat transfer and pressure drop of low-global warming potential refrigerants in smooth aluminum tubes

In this work we report condensation heat transfer coefficient and pressure drop data for R-454B, R-32, and R-454C. These refrigerants are low Global Warming Potential (GWP) replacements for R-410A. We obtained these data in a smooth 7.2 mm outer diameter (5.94 mm inner diameter) aluminum tube. In these experiments, the key variables were condensation temperature (35 °C ≤Tcond≤50 °C) and refrigerant mass flux (151.1≤G≤347.3 kg m−2 s−1). In general, for similar nominal operating conditions, we measured the highest heat transfer coefficients for R-32 and the lowest for R-454C. Among the correlations we used to predict the heat transfer data for all three refrigerants, the one developed by Thome et al. (2003) predicted the data with the greatest accuracy. Deviations between experimental data and predictions of this correlation were 21.7% for R-454B (with SBG correction), 22.0% for R-32, and 18.8% for R-454C (with SBG correction). Experimental pressure gradient data were predicted by the Friedel correlation Friedel (1979) with deviations of 13.8% for R-454B, 9.8% for R-32, and 10.2% for R-454C. In this work, we also compare the predictions of the Cavallini et al. correlation Cavallini et al. (2006) for R-454B and R-32 in smooth copper versus smooth aluminum tubes. To improve this correlation’s predictive capability for aluminum tubes, we have proposed some modifications. After incorporating these modifications, the deviations between the correlation’s predictions and our data were 10% for R-454B (with SBG correction), 8.1% for R-32, and 5.2% for R-454C (with SBG correction). These data and findings will assist the HVAC and refrigeration industry to implement low-GWP refrigerants in condensers using smooth aluminum tubes.

Hydrocarbon condensation flow and heat transfer in wave helical pipes: Effect of structure parameters

In this study, two types of wave-structured pipes for heat exchangers that are easy to process are proposed, and the condensation pressure drop and heat transfer performances for hydrocarbon refrigerant upward flow in wave helical pipes are investigated. A numerical method based on the inhomogeneous two-fluid model is introduced and validated by experimental data in literature. The effect of wave parameters (wave height and wave length) on heat transfer, pressure drop and flow patterns are studied. At lower vapor quality conditions, it is found that as wave height increases, frictional pressure drop and heat transfer coefficient exhibit a trend of initially increasing, followed by decreasing, and then increasing again. However, under higher vapor quality conditions, frictional pressure drop and heat transfer coefficient continue to increase. Besides, both the frictional pressure drop and heat transfer coefficient increase with the decrease of wave length. The averaged enhancement on frictional pressure drop for vertical wave pipes and horizontal wave pipes are respectively 1.279–5.148 and 1.238–5.808 times, while the average augmentation in heat transfer coefficient is 1.243–3.006 and 1.316–3.579 times, respectively. The comprehensive thermal performance of wave pipes was assessed using the performance evaluation criterion. The range of averaged performance evaluation criteria (under different vapor qualities) in vertical wave pipes and horizontal wave pipes are respectively 1.150–1.737 and 1.230–2.026. The horizontal wave pipe when the wave height is 5 mm and wave length is 5.24 mm shows the best overall thermal performance, with an averaged performance evaluation criterion of 2.026. The two types of wave-structured pipes proposed in this paper can increase the pipe length of the spiral-wound heat exchanger at the same volume, while also improving the performance of the spiral-wound heat exchanger. The outcomes of the study can offer some assistance in creating simple but effective spiral wound heat exchangers.

Practical design guidelines for heat transfer enhancement of condensers

Enhanced condensation is of great practical interest to further improve and miniaturize power generation and refrigeration systems. Studies in the literature that investigate enhanced condensation focus on increasing the heat flux at high vapor qualities; however, many thermodynamic cycles such as vapor compression and the Rankine cycle require complete condensation for cycle operation, and in such cases, low-quality condensation can be the limiting factor. Performance evaluation criteria (PEC) can be used to assess the relative improvement achieved by different enhancement methods, but most PEC are only applicable to single-phase flows. The present study therefore develops local design guidelines for condensing flows to determine the regions of the condenser that most need heat transfer enhancement. This is achieved by comparing the relative benefits and penalties associated with enhancing low-quality, medium-quality, and high-quality condensation equally. The effects of varying heat transfer coefficient, frictional pressure drop, temperature reduction due to pressure drop, driving temperature difference, and coupling fluid thermal resistance are considered. Three enhancement techniques are then analyzed with these approaches to determine their relative benefits and disadvantages. Cases that consider fixed geometries, fixed flow areas, and variable flow area are considered, along with comparing the relative benefits of size reduction, increased heat duty, and reduced temperature difference. Results show that heat transfer enhancement is achieved when the increase in heat exchanger conductance exceeds the reduction in the heat exchanger temperature difference induced by condensation pressure drop. Additionally, it is found that enhancement is typically most needed in the low vapor quality region of the condenser, except in situations where temperature pinching occurs along the heat exchanger.

A mechanistic model for acoustically enhanced condensation heat transfer and pressure drop

Enhancing low-quality condensation where thermal resistance is the highest is critical for size reduction and efficiency improvements for many power generation, HVAC&R, and chemical processing applications. Recently, acoustic actuation has been shown to enhance low-quality condensation by ∼30% by introducing a sound wave that oscillates between the condenser headers and resonates within the tube, agitating the liquid-vapor interface and significantly altering the distribution of the liquid phase towards the end of condensation, thereby increasing the heat transfer rate. This paper presents a multiple flow-regime model to predict the pressure drop and heat transfer during acoustically actuated condensation based on hydrodynamic changes induced by forced oscillation. Experimental results for R134a flowing through a horizontal tube across the entire quality range for mass fluxes ranging from 120 to 235 kg m−2 s−1, actuation frequencies from 2 to 20 Hz, header volumes from 150 to 1000 mL, and wall-to-refrigerant temperature differences ranging from 6 to 12 K are used for the development of the models. Enhancement mechanisms are discussed and heat-and-momentum analogy-based correlations are developed to accurately predict actuated and unactuated condensation pressure drop and heat transfer data simultaneously within 23% and 7% on average, respectively.

Experimental study of refrigerant condensation heat transfer and pressure drop in horizontal tubes with full and partial coiled wire inserts (CWIs)

The condensation heat transfer coefficient (HTC) and pressure loss in the horizontal smooth and wire-inserted tubes were investigated experimentally. Three different coiled wire inserts (CWIs) of the full length of 1000 mm and partial lengths of 100 mm and 200 mm with 0.8 mm wire thickness and 9.5 mm pitch were studied. The main condenser was a heat exchanger of double pipe type with an 8 mm inner diameter for the refrigerant and an active length of 1 m cooled by water circulating in the annulus. The experiments were conducted for different refrigerant mass velocities within 90–400 kg/m2s and low vapor mass fraction (x < 0.3). The findings revealed that using 1000 mm (full-length), 200 mm, and 100 mm length CWIs improved the HTC by 95%, 40%, and 18%, respectively, over the smooth tube. The HTC per unit pressure drop was also evaluated to assess the performance of full and partial CWIs. The results showed that the HTC per unit pressure loss decreases for all tubes as the mass velocity increases. In addition, the performance of the tubes with CWIs approached that of the smooth tube at higher mass velocities. The ratios of HTC per unit pressure drop at higher mass fluxes for full-length, 200 mm, and 100 mm length CWIs to that of the smooth tube were about 0.4, 0.78, and 0.9, respectively. Although the pressure drop increased almost linearly with the partial CWI length, the HTC increased sharply for the small inserts and gradually for the larger insert lengths. The results obtained for the condensation HTC in smooth and full wire-inserted tubes were also compared with the estimations of well-known correlations in the literature. Furthermore, three new correlations were developed to predict condensation HTC in smooth tubes and tubes with full and partial CWIs. It was shown that the new correlations are more accurate than the available ones.

Condensation Heat Transfer and Pressure Drop of Acoustically Actuated Horizontal Two-Phase Flow

An acoustic actuation technique to enhance condensation inside horizontal tubes is developed and experimentally demonstrated. Heat exchanger headers are used as Helmholtz resonators, allowing low-frequency (∼10 Hz) acoustic waves to propagate and be reflected within the condenser, improving mixing saturated vapor and condensed liquid, thereby enhancing heat transfer. Heat transfer coefficients and pressure drops are measured to evaluate the heat transfer enhancement and pressure drop penalty as a function of vapor quality, mass flux, temperature difference, and acoustic wave amplitude and frequency. Experiments are conducted using a tube-in-tube heat exchanger with an inner diameter of 14.45 mm that partially condenses refrigerant R134a at 950 kPa across vapor qualities ranging from 5% to 80% as mass fluxes of 120 and 180 kg m−2 s−1 and temperature differences of 6 and 12 K. The actuation amplitude and frequency are varied from 0.5 to 6 mm and 2 – 20 Hz, respectively to characterize the acoustic response. Heat transfer enhancement mechanisms are hypothesized and evaluated based on the experimental results, concluding that changes in two-phase flow patterns and increased convective effects lead to significant enhancement at low qualities but negligible enhancement at high qualities when the flow is annular. Existing correlations and modifications to existing models are proposed to explain the observed enhancement, and a simple correlation based on a heat-and-momentum analogy is proposed, which predicts both baseline and enhanced heat transfer data within 2.7%.

Condensation heat transfer and pressure drop characteristics inside smooth and enhanced tubes with R410A and R32

An experimental investigation was performed to evaluate the condensation heat transfer characteristics for R32 and R410A inside four horizontal round tubes with 12.7 mm outer diameter. The saturation temperatures were 35 °C and 45 °C, the vapor qualities ranged from 0.2 to 0.8, and the mass flux ranged from 50 kg m−2 s−1 to 200 kg m−2 s−1. Different inside surface structures of the four tubes were smooth (ST), hydrophobic (HYD), herringbone (HB) and hydrophobic/herringbone (HYD/HB), respectively. Effects of mass flux, vapor quality, refrigerant and saturation temperature on condensation heat transfer in different tubes were discussed with the aid of flow pattern visualizations. Experimental results show that the heat transfer coefficient of R32 was higher than that of R410A due to different physical properties of the refrigerant. HB tube had the highest heat transfer coefficient at low vapor quality and HYD tube was the best at high vapor quality. The performance enhancement factors (PEF) of HB tube and HYD tube were better, ranging from 1.07 to 1.4 and from 1.02 to 1.31, respectively. However, as the composite of HB and HYD, the HYD/HB tube had the lowest overall performance, with the PEF ranged from 0.94 to 1.12. Additionally, the specific sensitivity factor (SSF) was used to evaluate the influence of various parameters on heat transfer and pressure drop.

Experimental study on wet gas condensation heat transfer and flow resistance in horizontal intensified tube

The heat transfer and flow resistance characteristics of in-tube vapor condensation with high content non-condensable gas is significant for designing waste heat recovery devices of gas-fired boilers. In this paper, the forced convection condensation of wet gas in the horizontal tube is experimentally investigated with a non-condensable gas mass fraction of 83–95%. The condensation heat transfer coefficient, thermal resistance and pressure drop are compared between bare tube and two intensified tubes, i.e., the corrugated and spiral tube, under various Reynolds number, wall subcooling degree and inlet humidity. The results show that the in-tube condensation heat transfer coefficient increases with increasing Reynolds number and inlet humidity, but decreases with increasing wall subcooling degree. Compared with the bare tube, the heat transfer coefficient and pressure drop increase up to 47.13% and 16.00% respectively for the spiral tube, and 116.04% and 88.48% respectively for the corrugated tube. When combing the heat transfer and flow resistance factors, the maximum comprehensive performance coefficients are 1.71 and 1.44 for corrugated tube and spiral tube respectively. The superiority of corrugated tube in the wet gas condensation is benefit from effective breaking the non-condensable gas film and promoting the vapor diffusion on the interface.

Condensation flow visualization, heat transfer, and pressure drop in printed circuit heat exchangers with straight and wavy microchannels

This study experimentally investigated the condensation flow visualization, heat transfer, and pressure drop characteristics of R1234ze(E) and R32 in printed circuit heat exchangers (PCHEs) with straight and wavy microchannel geometries. The effects of the microchannel geometries and refrigerants on the condensation flow, heat transfer, and pressure drop characteristics were investigated. Three flow patterns, namely, the nondisturbance annular, annular, and plug flows, were observed in the straight microchannels, whereas only annular and plug flows were observed in the wavy microchannel. A thin liquid film was observed to form on the inner arc of the wavy microchannel in the plug flow region. The overall heat transfer coefficient and pressure drop of the PCHE with the wavy microchannel were, respectively, 11% and 18% higher than those of the PCHE. The overall heat transfer coefficient and pressure drop of R1234ze(E) were 29% lower and 42% higher, respectively, than those of R32.

Prediction of heat transfer coefficient and pressure drop of R1234yf and R134a flow condensation in horizontal and inclined tubes using machine learning techniques

Machine learning techniques have great potential to predict two-phase flow characteristics instead of classic empirical correlations. In the present study, four different machine learning models, including multi-layer perceptron artificial neural network (ANNMLP), support vector regression (SVR), K nearest neighbors (KNN), and extreme gradient boosting (XGBoost), are employed to predict the heat transfer coefficient (HTC) and the frictional pressure drop (FPD) of R134a and R1234yf condensation flow in horizontal and inclined tubes. The dataset includes 348 points from previous works and the current research. To extend the data, an experimental study is also performed on the condensation of R134a in a horizontal tube for different mass velocities and vapor qualities. The results show that, in the best model, HTC can be estimated by ANNMLP with the mean absolute percentage error (MAPE) of 7.01%. The best prediction of FPD is achieved using XGBoost machine with MAPE of 10.87% on test data. Also, the feature importance procedure is implemented to recognize the most useful features. Based on the results, the mass velocity and the inclination angle are identified as the most influencing parameters on the prediction of HTC and FPD, respectively.

Condensation heat transfer and pressure drop of R1234yf/HFC mixtures inside small diameter channels

Hydrofluoroolefin/hydrofluorocarbon mixtures, such as R1234yf/HFC mixtures, have been proposed for the substitution of R134a in refrigeration and air-conditioning applications. Indeed, these fluids present good thermodynamic properties and low environmental impact. Here, the condensation process of two mixtures, R513A (R1234yf/R134a 56/44% by mass) and R516A (R1234yf/R152a/R134a 77.5/14/8.5% by mass), has been investigated inside two channels with inner diameters equal to 0.96 mm and 3.38 mm. Both R513A and R516A are azeotropic mixtures. Condensation heat transfer tests have been conducted at 40 °C saturation temperature and mass flux ranging between 40 kg m−2 s−1 and 600 kg m−2 s−1. In the present study, particular focus is put onto condensation at low mass flux (below 100 kg m−2 s−1). Tests at low mass fluxes are very rare in the literature but of great relevance considering that chillers and heat pumps equipped with variable speed compressors work for most of their lifetime at partial loads, thus reducing the refrigerant mass flow rate in the heat exchangers and the mass flux in each channel as compared to the nominal working conditions. In the 3.38 mm diameter channel, the two-phase flow has been recorded using a high-speed camera and a comparison with two flow pattern maps is presented. The heat transfer coefficient data have been compared with the results of three semi-empirical models and with a specifically developed Artificial Neural Network model.