Experimental Heat Transfer Coefficients Research Articles

Investigation on the performance of modified solar distillation system using different composite materials: An experimental and theoretical study

The present study explores enhancing the productivity of solar still through the utilization of different enhancement configurations, such as Iron Hemispherical Pieces (IHPs) and Steel Wool Pads (SWPs). Two solar stills were used in this study, one of them is a conventional solar still (CSS) as a comparison reference and the other is a modified solar still (MSS) with different enhancement configurations. Different parameters were continuously monitored to evaluate their influence on the efficiency of the solar stills. Mathematical model was performed to calculate the experimental heat transfer coefficients and the productivity for the stills under different amounts of basin water. Energy Efficiency (EE) and Exergy Efficiency (EXE) were employed to analyze their impact for the different configurations. The findings indicated that increasing the number of IHP to 120 IHPs leads to increase the productivity by 35 % compared to 30 IHPs. Using 5 L of water yielded an accumulated productivity of 3800 ml, whereas employing 15 L decreased productivity by 26 %. Moreover, the addition of Steel Wool Pads (SWPs) significantly enhanced productivity by 28 %. Additionally, the MSS + 120IHP configuration exhibited maximum EE and EXE values of 47.53 % and 3.2 %, respectively, with daily average values of 24.53 % and 1.55 % at 5 L of water. Similarly, the MSS + 120IHP + 60SWP configuration attained maximum EE and EXE values of 55.24 % and 3.5 %, respectively, with daily average values of 30.00 % and 1.90 %. Subsequently, the experimental measurements of hourly production were validated against the corresponding theoretical values, demonstrating good agreement between them. Finally the cost evaluation was performed and the MSS has the minimum cost per liter (CPL) compared with the conventional one.

Experimental study of R600a boiling in microfin tube

This paper presents experiments to investigate flow boiling and pressure gradients of R600a. The study employed microfin and smooth copper tubes simulating conditions in a domestic refrigerator, using low mass fluxes (MFs) and saturation temperatures (Tsat). The test sections featured a tube length of (400 mm), an inner diameter of (4.3 mm), and an outer diameter of (5 mm). Test conditions varied, including MFs of (25, 35, and 45 kg m−2 s−1), vapor quality from (0.18–1), Tsat of (−15, −9, and −5 °C), and HFs of (10, 14, and 18 kW m−2). Both tubes shared identical dimensions, clearly comparing microfin impact on flow boiling characteristics. The study evaluated enhancement factor (EF), penalty factor (PF), and overall efficiency (η) by comparing heat transfer coefficients (HTC) and pressure gradients in the microfin tube versus the smooth tube. Results indicate a significantly boosted HTC in the microfin tube compared to the smooth tube, with a reasonable increase in pressure drop. Under specific conditions (HF of 10 kW m−2, Tsat of −9 °C, and MFs of 25 kg m−2 s−1), the average HTC is enhanced by 325%, while the pressure drop is increased by 150%. Experimental HTC and frictional pressure gradient comparisons aligned well with values predicted by existing literature correlations.

Experimental Heat Transfer Coefficients for Zeotropic Mixture Condensation of Hexamethyldisiloxane/Octamethyltrisiloxane and Ethanol/Hexamethyldisiloxane

AbstractLegal requirements for refrigerants necessitate a continuous development of these fluids, also focusing on mixtures. This article contributes three aspects to the ongoing research: (1) Experimental mean heat transfer coefficients were determined for the condensation of two novel zeotropic mixtures, hexamethyldisiloxane (MM)/octamethyltrisiloxane and ethanol/MM, gathered for filmwise condensation on a vertical tube. (2) Both mixtures were investigated with several compositions to provide a comprehensive understanding of the mixture effects. (3) The results were taken to develop a novel superposition approach to predict mixture condensation.

Experimental Evaluation of Condensation of R32 and R134a over a Plain Tube

The rise in demand for universal energy, combined with environmental concerns, prompted a search for a new technology that was compatible with the existing scenario. Nowadays, most conventional refrigerants are being phased out due to environmental restrictions, which has led to the development of alternative refrigerants. Such newly developed alternative refrigerants need further experimental assessment regarding their thermophysical properties. The current work has experimentally looked at the heat transfer phenomena that occur when the refrigerants R32 and R134a condense over a single horizontal plain tube at saturation temperatures of 40–50 °C, wall sub-cooling temperatures ranging from 3 °C to 11 °C, and heat fluxes of 8–31 kW/m2. The results showed that at a saturation temperature of 40 °C, R32 and R134a have a coefficient of condensation heat transfer about 4.0–9.0% higher than that at saturation temperatures of 45 °C and 50 °C. At the same wall sub-cooling temperature, R32 has a 50–55% higher condensing-side heat transfer coefficient than R134a. The validation of the experimental condensing-side heat transfer coefficient with Nusselt’s model revealed a deviation of ±10%. Further, a modified correlation based on Nusselt’s model predicted the experimental condensing-side heat transfer coefficient with an error band of ±5%.

Investigation of convective heat transfer in a cold flow pebble bed nuclear reactor using advanced pebble probe heat transfer sensor

The aim of this study was to investigate the local heat transfer coefficients between the pebbles and the coolant gas in a PBR using an advanced measurement technique that consists of a heated pebble probe, a micro-foil heat flux sensor mounted flush on the surface of the heated pebble probe, and a thermocouple in the center of the bed void in front the sensor. In this work, the local heat transfer coefficients were derived at various axial levels, radial and angular locations and at different superficial inlet gas velocities that cover both transitional and turbulent flow conditions. The local heat transfer coefficients were found to be 22-32%, 34-39%, and 18-20% higher near the wall at superficial inlet gas velocities of 0.3,1.2, and 2m/s, respectively. This is due to higher volumetric flow at the wall where larger void in the pebble bed exists compared to the center region of the bed where the flow of the gas in the packed bed follows the least resistance path. Large deviations were obtained between the experimental overall heat transfer coefficients in the bed and the predictions of seven correlations that were selected from the literature. Furthermore, these correlations cannot predict the local heat transfer coefficients inside the PBR. This necessitates the development of new correlations for the prediction of the local heat transfer coefficients using the data obtained in this work. A pseudo-3D correlation was developed and was found to provide predictions that are in good agreement with the experimental values for the conditions used, with an averaged absolute relative error (AARE) of 3.33% at high Reynolds numbers for our operating and design conditions.

Flow pattern-based heat transfer analysis of microchannel flow boiling: An experimental investigation

The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

Effect of Horizontal Tube Position on Wall-to-Bed Heat Transfer in Air-Solid Fluidized Bed of Large Particles

Fluidized beds are extensively used in a variety of industrial applications, such as heat recovery systems, coal combustion, and solid particle drying. The effect of the position of the heat transfer tube on heat transfer from a horizontal bare tube in a 150 mm square cross-section fluidized bed containing large particles, ragi, and mustard is investigated. The influence of varying fluidizing velocities on heat transport is also examined for a fixed bed height of the large particles. When the position of the tube from the distributor plate was increased in 0.028m increments from 0.52 m to 0.080 m and then to 0.108 m, the heat transfer coefficient was reduced. Due to enhanced fluidization, all bed heights yielded almost comparable heat transfer coefficient values at velocities exceeding 1 m/s, 170 and 160 W/m2K for ragi and mustard, respectively. The dependency of the heat transfer coefficient on fluidizing velocity is qualitatively like that of small particles. An error deviation of around 8.8% was found when comparing experimental heat transfer coefficient values to those predicted from empirical correlations in the published literature.

A comprehensive study with high-speed camera assisted visualizations of HFO-1234ze(E) condensation inside an enhanced tube

Flow condensation of HFO-1234ze(E) inside a helical micro-finned tube of 4.28 mm ID underwent experimentation. Flow patterns were visualized with a high-speed camera while varying vapor qualities from 0.01 to 0.95 and mass fluxes from 100 to 400 kg m−2 s1, at saturation temperature of 30 °C. Intermittent, stratified-wavy, transitional, and annular flow pattern types were identified. The flow pattern map of Doretti et al. (validated by Cavallini et al.) accurately predicted the flow patterns. Heat transfer coefficients (HTCs) and frictional pressure drops (FPDs) were also measured for mass fluxes ranging from 100 to 600 kg m−2 s−1 at saturation temperatures of 30 °C and 40 °C. Both HTC and FPD elevated with higher mass fluxes and vapor qualities which stemmed from the presence of observed shear stress-driven annular flow patterns. HTC and FPD are generally higher at lower saturation temperatures. Experimental HTC and FPD have been compared against values estimated with empirical models. Furthermore, the average enhancement factor of the micro-finned tube was 2.94 with an area enhancement factor of 2.39. Plotting enhancement factor versus non-dimensional gas velocity provided detailed insight into the augmentation effect of micro-finned tubes under different operating conditions.

Heat transfer enhancement of supercritical carbon dioxide in eccentrical helical tubes

A type of eccentrical helical tube (EHT) was proposed for more efficient and compact supercritical carbon dioxide (SCO2) heat exchangers in refrigeration systems. The forced convective heat transfer of SCO2 flowing in three designed eccentrical helical tubes with pitches of 25, 50 mm and eccentricities of 0.9, 1.5 mm was simulated by using the three-dimensional steady Reynolds-averaged Navier-Stokes equations, shear stress transport turbulence model and energy equation in ANSYS CFX 2019 R2 at mass fluxes of 200, 400 kg/m2s, inlet pressures of 8, 9 MPa, outwards wall heat fluxes of 12, 24 kW/m2. The flow and heat transfer models were validated in the plain tube with experimental mean heat transfer coefficient and empirical Darcy friction factor. Influences of pitch, eccentricity and operational conditions on heat transfer enhancement were identified. Flow pattern, heat transfer mechanism and vortex kinematics were clarified. Heat transfer enhancement of SCO2 in the EHT was compared with water and air in the EHT, twisted elliptical tube and conical tube, respectively. A pitch of 25 mm and eccentricity of 1.5 mm at 6 mm tube diameter can achieve a better thermal-hydraulic performance. A higher SCO2 mass flux raises Nusselt number and reduces friction factor, a higher inlet pressure and a larger wall heat flux reduces Nusselt number but increases friction factor. The ratio f/f0 is ranged in 2.52–1.68 and drops off with increasing inlet pressure and nominal Reynolds number, the ratios Nu/Nu0, ψ and η curves show a concave shape near the pseudocritical point ranged in 1.68–1.06 and 1.06–1.55, 1.23–0.83 and 0.83–1.30, 0.67–0.50 and 0.50–0.92 at the pitch of 25 mm and eccentricity of 1.5 mm, inlet pressure of 8 MPa, mass flux of 400 kg/m2s, and wall heat flux of 12 kW/m2. A helical flow pattern with a core flow occurs in the EHT, the local heat transfer coefficient can be correlated to absolute helicity. The high-wall shear stress on the ridge of the EHT is responsible for the heat transfer enhancement. The f/f0 value of the EHT is in between the twisted elliptical tube and the conical tube, but the Nu/Nu0 and ψ values are comparable.

A Comparative Analysis of Two-Phase Flow Boiling Heat Transfer Coefficient and Correlations for Hydrocarbons and Ethanol

This study will present a comprehensive review of the two-phase flow boiling heat transfer coefficient of hydrocarbons such as propane (R-290), butane (R-600), iso-butane (R-600a), and ethanol at various experimental conditions. Studying the multiphase flow heat transfer coefficient is crucial for many types of heat transfer equipment to achieve higher efficiency for more compact design and cost reduction. One reason we chose hydrocarbons as refrigerants in this study is that they are of an ozone depletion potential equal to zero (ODP = 0) and a deficient level of direct global warming potential (GWP = 3). Moreover, hydrocarbons’ thermodynamic and thermophysical characteristics qualify them to be a strong candidate for more heat transfer applications, initially, by constructing a database for the working fluids using multiple existing experimental work. The current data that this study have collected for the flow boiling spans a wide range of parameters, such as mass flux, heat flux, operating pressure, saturation temperature, etc. Furthermore, by comparing the experimental multiphase heat transfer coefficient database with the anticipated values of each correlation, the prediction performance of 26 correlations found in the literature was assessed. This study allows the best prediction method to be selected based on the minimum deviation of predicted results from the experimental database provided based on the mean absolute error (MAE) calculated from the assessed correlations. The conclusions of such a study can also be helpful for developing more accurate correlation methods for these fluids and improving the prediction of their flow boiling characteristics.

Experimental Firing Test Campaign and Nozzle Heat Transfer Reconstruction in a 200 N Hybrid Rocket Engine with Different Paraffin-Based Fuel Grain Lengths

Firing test campaigns were carried out on a 200 N thrust-class hybrid rocket engine, using gaseous oxygen as an oxidizer and a paraffin-wax-based fuel. Different fuel grain lengths were adopted to extend the fuel characterization under different operating conditions, and to evaluate rocket performances and internal ballistics in the different configurations. In addition to data collected under a 220 mm propellant grain length, two further test campaigns were carried out considering 130 mm and 70 mm grain lengths. Two different injector types were adopted in the 130 mm configuration; in particular, a showerhead injection system was used with the aim to contain high-amplitude pressure oscillations observed during some firing tests in this engine configuration. Parameters such as the chamber pressure and temperature inside the graphite nozzle, space-averaged fuel regression rate and nozzle throat diameter were measured. The results allowed for the investigation of different issues related to hybrid rockets (e.g., fuel regression rate, engine performance, nozzle ablation under different conditions). The focus was mainly directed to the nozzle heat transfer, through the reconstruction of the convective heat transfer coefficient for different tests in the 70 mm grain length engine configuration. The reconstruction took advantage of the experimental data provided by the nozzle embedded thermocouple. Then, the experimental convective heat transfer coefficient was used to validate the results from some empirical correlations. The results showed significant differences between the experimental convective heat transfer coefficients when considering tests with different oxidizer mass flow rates. Furthermore, the predictions from the empirical correlations proved to be more reliable only in cases characterized by oxidizer-rich conditions.

Condensation heat and mass transfer characteristics of zeotropic refrigerant mixture R1234yf/R32 inside small-scale tube: Flow patterns observation and non-equilibrium film model calculation

Minichannel heat exchanger has been more and more widely used in refrigeration industry and other fields due to its high heat transfer efficiency, lightweight and low carbon emissions and other advantages. And, the zeotropic mixture R1234yf/R32 with low global warming potential (GWP) is a promising substitute for R410A in commercial refrigeration. In this study, the experiment investigation of condensation heat and mass transfer characteristics of R1234yf/R32 in minichannel test tube (inner diameter (ID) 2 mm) were conducted. The influencing factors including mass flow rate, vapor quality, mass fraction and tube diameter on the mass transfer and heat transfer coefficient (HTC) were discussed in detailed. The flow pattern was taken during condensation experiment. Meanwhile, based on the discussion of flow pattern, thermophysical properties, tube diameter and composition variation in the liquid film and at the vapor-liquid interface, the effects on heat transfer and mass transfer during condensation were analyzed. The calculation results using the non-equilibrium film model considering the mass transfer on both the liquid and vapor sides were compared and agreed well with the experimental HTC with an average deviation of 14.9%. In addition, the heat transfer degradation coefficient (HTDC), vapor-liquid interface temperature and diffusion flux of R32 were reasonably evaluated by the calculation model, and the comparison analysis were conducted with that of ID 4 mm in the previous study.

Two mathematical models of flow boiling and flow instability in rectangular expanding microchannel heat exchangers and structure optimization

Flow boiling instability in microchannels was widely studied to improve the reliability of microchannel heat exchangers. Two rectangular expanding microchannel heat exchangers have been experimentally investigated, one channel with cross-cutting and the other without cross-cutting. This study aims to optimize the heat transfer capabilities of heat exchangers. Two mathematical models containing heat transfer correlation and instability parameters have been developed. The experimental heat transfer coefficient (HTC) was used to validate them. Based on the models, the structure of the expanding microchannel was optimized, and the effect of expanding microchannel on flow instability was analyzed. As analyzed, in the models for two heat exchangers with and without cross-cutting, the mean deviations are 10.03% and 5.95%, respectively. The effect of flow instability suppression increases with the increase of the radiation angle. Moreover, the cross-cutting in expanding microchannels could further steady flow when adding fins in downstream channels. When adjusting the number of cross-cutting in cold plates from 1 to 4, the maximum increment of HTC is 58.17 kW/(m2·K). The influences of adjusting the number of fins in microchannels on the HTC are obvious for cold plates without cross-cutting. When the position of added fins is moved downstream in channels, the HTC of the heat exchanger could be increased by 26.58%. The results of this study provide some insights for inhibiting flow boiling instability and further optimizing the expanding microchannel structure to enhance heat transfer capability.

Experimental and numerical investigation of the use of new generating refrigerant R513A in the gravity-assisted heat pipe

In this study, theoretical–experimental and life cycle climate performance analyses of the gravity-assisted heat pipe filled with R513A were carried out. The heat carried by the heat pipe and the internal heat transfer coefficients of the evaporator and condenser were investigated. As the temperature set value (30, 35, 40, 45, 50 °C) increased, the heat transferred by the heat pipe increased and the averaged experimental data were obtained as 27.65, 49.30, 74.95, 99.40, 114.90 W, respectively. For the given temperature set values, the experimental internal heat transfer coefficient of the evaporator was obtained as 946, 1125, 1360, 1652, and 1215 Wm−2 °C−1. Experimental condenser internal heat transfer coefficients were obtained as 1385, 1460, 1527, 1563, and 1417 Wm−2 °C−1. According to the life cycle climate performance analysis of the given system, it has been determined that 82.22 % of the total emissions are caused by direct emissions and 17.78 % by indirect emissions. Direct emissions are related to leaks of the working fluid. Choosing a low global warming potential working fluid will significantly reduce emission and thus reduce the overall emission of the gravity-assisted heat pipe.

Experimental Investigation of InTube Condensation of HFO-1234yf

An increase in global warming potential and other environmental concerns are demanding new environmentally friendly refrigerants. For effective performance of refrigeration and air conditioning system condenser design play a vital role. Experiments were conducted to study the condensation of R1234yf refrigerant inside a copper tube of 8.4 mm in diameter and 750 mm in length. The heat transfer coefficients of refrigerant were calculated against mass flux varying from 150–300 kg/m2 s, quality of refrigerant 0.3 to 0.8, and saturated temperature 30 and 500 C. The experimental heat transfer coefficients were compared to the heat transfer coefficients by the recent MM Sha correlation. The experimental results are in good agreement with an absolute mean deviation of nearly 20% with the MM Sha heat transfer coefficient.

Experimental investigation on high-temperature flow boiling heat transfer characteristics of R245fa in a horizontal circular tube

Working fluid is the lifeblood of thermodynamic cycle for its heat transfer behaviour is critical to the design of heat exchangers in a thermodynamic cycle. Though being widely applied in organic Rankine cycles and high-temperature heat pump cycles, R245fa has witnessed little research on its high-temperature flow boiling heat transfer characteristics by far. In the present study, an experimental investigation on the flow boiling heat transfer characteristics of R245fa with a mass flux of 248–460 kg·m−2∙s−1, a heat flux of 5.5–17.3 kW·m−2 and an evaporating temperature of 80–115 °C in an inner diameter of 10 mm horizontal smooth circular tube is conducted. The influences of vapor quality, mass flux and evaporating temperature on the flow boiling heat transfer coefficient are also analyzed. The comparison of experimental heat transfer coefficients and the predicted heat transfer coefficients based on five heat transfer coefficient correlations is conducted and it is discovered that Gungor and Winterton's correlation with a mean deviation of 20.4% gives the best fit to the experimental data. Thus, based on Gungor and Winterton's work, an improved heat transfer coefficient correlation with a mean deviation of 12.8% is developed, in which 95% of the data points fall within the range of ±30%.

EXPERIMENTAL INVESTIGATION ON TWO-PHASE HEAT AND FLUID FLOW IN POROUS HEAT SINKS

The current paper presents a novel experimental study on flow boiling in heat sinks with rectangular cross-section parallel microchannels fabricated by conventional powder metallurgy process. The dimensions of microchannel heat sinks are 20 mm × 20 mm. Porosities of heat sinks produced by sintering vary between 13.9% and 21.3%. A pure (nonporous) copper heat sink was also produced to compare the experimental data with each other. Channel sizes and numbers of all heat sinks used in the tests are identical. Flow boiling tests were carried out with deionized water at a flow rate of 238.2 kg/m<sup>2</sup> s and 430.32 kg/m<sup>2</sup> s. The fluid enters the heat sink at 80°C, completely liquid. The tests were carried out under constant heat flux conditions and the heat fluxes are 46.5 W/cm<sup>2</sup> and 61.5 W/cm<sup>2</sup>. Experimental studies have shown that the heat transfer coefficients in the heat sink with the highest porosity are from 28% to 33.3% higher than in the pure copper heat sink. Experimental heat transfer coefficients increase with increasing flow rate and decrease with increasing heat flux and vapor quality. On the other hand, the pressure drop values of the heat sink sample with the highest porosity were measured to be one and half times more than that of the pure copper heat sink. The imaging technique was used during the experiments. With the imaging technique, it was determined that the fluctuations in the pressure values were caused by the reverse steam flow. Additionally, the heat transfer coefficient values obtained experimentally were compared with the existing correlations in the literature. Experimental results showed that pure heat sinks are more compatible with the results given in the literature than porous heat sinks.

Transient heat transfer characteristics of submerged heat source under seismic excitation

In this experimental based study, experiments are carried out to investigate the transient heat transfer behavior from the horizontal heat source submerged in the partial filled water pool including the effect of submergence depth of heat source. Further, experiments are conducted and analyses have been made to investigate the heat transfer behavior under the seismic environment along with the consideration of seismic excitation parameters: natural frequency of liquid and amplitude of displacement.The transient heat transfer coefficient for natural convection and boiling region are evaluated with and without excitation cases. Experimental heat transfer coefficients were compared with the available correlations and found inadequate. Therefore, new correlations have been proposed for transient heat transfer coefficient for each region. For the seismic condition, separate correlation has been proposed to predict the vibrational heat transfer coefficient in case when strong sloshing occurs with wave breaking at the free surface. Proposed correlations are predicted quite satisfactory results.

Development of flow boiling mechanism model and its application to rectangular narrow channel under atmosphere pressure

Bubble sliding is an important phenomenon in subcooled flow boiling that contributes significantly to overall heat transfer, especially in geometrically constrained geometries. While the widely used wall boiling model which embedded in the CFD commercial software for subcooled boiling simulation, assumes that the bubble jumps directly into subcooled mainstream when its diameter grows to a critical value. While in narrow channel, almost all the bubbles slide along the heating surface before lift-off, the size increases continuously during the sliding process, which is greatly different from the assumption in the original wall boiling model. In this paper, heat transfer induced by bubble sliding is introduced to the original heat partitioning strategy. A force balance mechanism model validated by visualization experiments in narrow rectangular channel is applied to determine whether bubbles slide on the heating surface and when to detach from the nucleation site. The nucleate site density and bubble sliding velocity obtained from our previous experiment are also utilized in the new model. The applicability of the proposed model is demonstrated by comparing the calculated heat transfer coefficients with the experimental heat transfer coefficients. The wall temperature predictions of the new model are closer to experimental data than the original model, typically are close to the boiling region. The heat partitioning components show that the heat transfer due to bubble sliding is the highest fraction of surface heat dissipation, which is consistent with experiment observations. Finally, the wall heat flux partition, bubble sliding distance, bubble departure diameter and lift-off diameter are analyzed. The proposed model has been successfully applied to the analysis of flow and heat transfer characteristic of plate type fuel element in marine nuclear power system.

Experimental investigation on the flow condensation of R450A, R515B, and R1234ze(E) in a 7.0 mm OD micro-fin tube

The flow condensation of R1234ze(E), R450A, and R515B was conducted in a 6.14 mm inner diameter micro-fin tube. The fin height is 0.18 mm and the fin number is 50. The helix angle is 18° and the area enhancement factor is 1.63. During the experimental tests, the saturation temperature is 30 and 40 °C. The mass fluxes range from 50 to 400 kg m-2 s-1. The vapor qualities are between 0.1 and 0.96. The heat transfer coefficients and frictional pressure gradients of the three refrigerants were compared with each other. The experimental heat transfer coefficient increases with vapor quality and mass velocity, except at high vapor quality, where a higher mass velocity does not imply a higher heat transfer coefficient, probably due to the liquid film thickness distribution. Generally speaking, R450A presents slightly higher heat transfer coefficients at high mass fluxes and smaller pressure gradients, whereas R515B shows lower heat transfer coefficients and pressure drops very close to those of R1234ze(E). Moreover, some models of the heat transfer coefficient and pressure gradient specifically developed for micro-fin tubes were chosen to explore their feasibility for the new refrigerants and tube. Cavallini et al.’s model shows the best agreement for the heat transfer coefficient, and Diani et al.’s model shows the best agreement for the frictional pressure gradient.