Condensation Of R134a Research Articles

An Experimental Investigation of Pressure Drop in Two-Phase Flow during the Condensation of R410A within Parallel Microchannels

In this study, the flow condensation of R-410A within 18 square microchannels arranged horizontally in parallel was experimentally investigated. All components of pressure drop, including expansion, contraction, deceleration, and friction, were quantified specifically for microchannels. The test conditions included saturation temperature, vapor quality, and mass flux, ranging from 18.86 to 24.22 bar, 0.09 to 0.92, and 200 to 445 kg/m2·s, respectively. The frictional pressure loss made up approximately 92.89% of the overall pressure reduction. The findings demonstrate that the pressure drop rises with higher mass flux and a lower saturation temperature. By comparing with correlations and semi-empirical models outlined in the literature across various scales, specimen types, and refrigerant media, correlations developed for two-phase adiabatic flows in multi-channel configurations can effectively predict the pressure drop in microchannel condensation processes. The model introduced by Sakamatapan and Wongwises demonstrated the highest predictive accuracy, with a mean absolute deviation of 8.4%.

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.

Thermal−hydraulic characteristics of R32 and R410A condensation in plate heat exchangers with 1 mm chevron depth

Compact brazed plate heat exchangers (BPHEs) with low chevron depths are widely used in new heat pumps and air-conditioners for reducing inventory and improving efficiency. This study analyses the complete condensation of R410A and R32 inside BPHEs with a chevron depth of 1 mm. The effects of the mass flux and chevron patterns (V-shaped and W-shaped) on the heat transfer coefficient (HTC) and frictional pressure drop are investigated. The HTC of R32 is 30–50 % higher than that of R410A, which has a 10–20 % higher friction factor at high mass fluxes and vice versa at low mass fluxes. The HTC of R410A monotonically increases with the mass flux, whereas that of R32 does not. The W-shaped pattern results in 30 % and 15 % higher pressure drops compared with the V-shaped pattern for R410A and R32, respectively. The friction factor of R32 decreases more sharply than that of R410A as the mass flux increases, particularly at low mass fluxes. The correlations developed by Longo et al. and Zhang et al. show the best agreement with the experimental data for the HTCs of R32 and R410A, respectively, with mean absolute percentage deviations (MAPDs) of 15 %. The friction factor correlation of Zhang et al. accurately predicts the pressure drop of R32 for the W-shaped and V-shaped patterns. In addition, it accurately predicts the pressure drop of R410A for the W-shaped pattern, with an MAPD of less than 10 %, but over-estimates it by 37 % for the V-shaped pattern.

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%.

Two-phase flow condensation heat transfer characteristics of R-134a inside three-dimensional cylindrical micropillar enhanced tube

This paper presents an experimental investigation for evaluation of heat transfer coefficient, frictional pressure drops and flow regimes with their transitions during the condensation of R-134a inside a smooth, two microfins (MF1 and MF2) and a newly developed micropillar (MP) tubes. The experiments are carried out for a range of mass flux at an average saturation temperature of 35°C. Micropillar tube has produced the maximum heat transfer coefficient, while the microfin (MF1) tube resulted the highest frictional pressure gradient compared to other tubes. The experimental observations of microfin and micropillar tubes are plotted on the mass flux versus vapor quality flow map and Taitel and Duckler flow map to discuss the transitions within different flow patterns achieved. Visual inspection of the flow regimes with varying mass flux and vapor quality showed stratified-wavy, intermittent, and annular flow regimes. The occurrence of annular flow is found to be more in MF1 and MP tube. A performance evaluation factor (PEF) is defined to evaluate the thermal-hydraulic performance of the tubes. Using the same, it has been observed that the newly developed MP tube has a PEF value greater than unity which suggest augmentation of heat transfer coefficient at the expense of lesser pressure drop penalty.

Development and modified implementation of Lee model for condensation simulation

Numerical oscillation is a non-physical phenomenon in which the interface temperature fluctuates around the saturation temperature. The simulation for condensation is more susceptible to the numerical oscillation caused by the phase change model. To suppress the numerical oscillation and reduce interface temperature deviation in condensation simulation, a developed model modified the Lee model is presented in this study. In the present model, the mass source terms and the energy source terms are added in the governing equations to calculate the condensate within the cell and the allocation of condensation energy between the interface cells. The mass and energy source transfer model is firstly compared with the original Lee model to simulate one-dimensional Stefan problem. Indicating that with the similar accuracy the developed model could increase computational efficiency by 40 times. Then the Nusselt condensation problem is verified with a reasonable selection of model parameters, and the average error of the film thickness was controlled below 2%, as the interfacial temperature deviation within 3%. Finally, verification is also attained on the forced convective condensation of R134a and the interfacial temperature deviation is below 0.1 K. Simulation results validate the feasibility of using this model to predict heat and mass transfer in condensation. The numerical oscillations are also significantly suppressed in the simulations.

Multi-parameter classification and quantification of R-134a condensation using machine learning

Uncorrelated flow images are used to study complex features occurring in phase change processes. More specifically, machine-learning algorithms relied on three experimental datasets to classify and quantify key flow parameters of R-134a condensing as it flows upwards in a vertical straight tube. Dataset-1 has 159 data points of vapor and liquid superficial flow velocities, which were correlated within four classical flow regimes (climbing film, flow reversal, churn, and slug). The data was used to train three machine-learning algorithms (Gaussian Naive Bayes, Support Vector Machines, and the k Nearest Neighbors) and to create continuous superficial flow velocities maps for flow conditions that were not necessarily tested experimentally showing good consistency. Dataset-2 generated a sequence of 2,453 uncorrelated flow images from 25 videos, which were pre-processed and used to train Decision Tree Classifier and Convolutional Neural Network algorithms to classify flow regimes based on distinct data splitting methods. The results showed that the splitting method used significantly impacts the algorithm’s accuracy. For instance, random splitting returned F-score values higher than 97%, while splitting by experiment presented much lower F-score values. Dataset-3 had 310,839 pre-processed flow images, which were correlated with heat transfer, quality, void fraction, and mass flow rate. These images were used to train a Convolutional Neural Network algorithm, which was able quantify the three first parameters presenting relative errors in the order 7%, 1% and 1%, respectively, and classify the mass flow rate with an accuracy of 99% considering a full window size. Finally, the Kolmogorov-Smirnov test was used to further explore the different accuracies obtained with two data splitting methods.

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.

Convective condensation of R134a and R1234ze(E) inside microfin tube

Environmental concerns are forcing the replacement of the commonly used refrigerants and finding new fluids is a top priority. The hydro-fluoro-olefin (HFO) R1234ze(E), because of its smaller global warming potential (GWP) and shorter atmospheric lifetime, replaced R134a. Accordingly, for HVAC systems design, a detailed knowledge of the thermo-fluid-dynamic characteristics of the fluids and reliable predictive models are required. To improve the understanding, R134a and R1234ze(E) were employed in convective condensation experiments (saturation temperature Tsat = 35°C, mean quality xm = 0.1~0.9, quality changes Δx = 0.05~0.6, mass flux G = 43~444 kg·m−2s−1) inside a microfin tube (outer diameter D = 9.52 mm, fin number n = 60, fin height H = 0.2 mm). The results were used for two goals: the former is the comparison of the heat transfer features of the two fluids, while the latter aims at testing the performance of prediction models available in the open literature. At the saturation temperature T = 35°C, the two fluids show small differences in the thermal properties so that, as expected, the experiments highlighted a very similar behavior in the typical operating conditions of HVAC systems. In fact, for all the operating conditions marginal differences were observed in the pressure drop, the heat transfer coefficient and the flow pattern maps. The issue of prediction reliability, however, is still open. Actually, not all the models achieving good results for R134a show the same performance for R1234ze(E), especially for the pressure drop.

Composite filament with super high effective thermal conductivity

The low thermal conductivity of the clothing filament constrains the efficiency of the thermal management garments. In this paper, a super thermal conductive composite filament (STCCF) was proposed for body thermal management, which was fabricated by hollow Teflon filaments incorporated with hollow copper filaments in the heating and cooling sections. By introducing R134a into the hollow filament, a thermally driven flow was generated due to the pressure imbalance caused by the evaporation and condensation of R134a, which enhanced the heat transport capability significantly. The apparent thermal conductivity of the STCCF increased with the increasing heat power, being up to 5042 W/(m·K) at the heating temperature of 49 °C. Furthermore, bending only had a small effect on the thermal performance of STCCF due to the flow pattern transition in the curved section. Moreover, the heat transfer capacity could be enhanced by a lower temperature difference between the hot and cool sections under constant heat power. Therefore, the proposed STCCF has great potential for human body thermal management, which could contribute to human health and thermal comfort as well as building energy saving.

Numerical study of the deep removal of R134a from non-condensable gas mixture by cryogenic condensation and de-sublimation

Nowadays, the limits on greenhouse gas emissions are becoming increasingly stringent. In present research, a two-dimensional numerical model was established to simulate the deep removal of 1,1,1,2-tetrafluoroethane (R134a) from the non-condensable gas (NCG) mixture by cryogenic condensation and de-sublimation. The wall condensation method was compiled into the Fluent software to calculate the condensation of R134a from the gas mixture. Besides, the saturated thermodynamic properties of R134a under its triple point were extrapolated by the equation of state. The simulation of the steam condensation with NCG was conducted to verify the validity of the model, the results matched well with the experimental data. Subsequently, the condensation characteristics of R134a with NCG and the thermodynamic parameters affecting condensation were studied. The results show that the section with relatively higher removal efficiency is usually near the inlet. The cold wall temperature has a great influence on the R134a removal performance, e.g., a 15 K reduction of the wall temperature brings a reduction in the outlet R134a molar fraction by 85.43%. The effect of changing mass flow rate on R134a removal is mainly reflected at the outlet, where an increase in mass flow rate of 12.6% can aggravate the outlet molar fraction to 210.3% of the original. The research can provide a valuable reference for the simulation of the deep removal of various low-concentration gas using condensation and de-sublimation methods.

CFD investigation of R134a and propane condensation in square microchannel using VOF model: Parametric study using steady state solution

CFD investigation of R134a and propane condensation in square microchannel using VOF model: Parametric study using steady state solution

Prediction of effective heat transfer coefficients for vapour condensation inside horizontal tubes in stratified phase flow

In modern condensers of air conditioning systems, heat pumps, evaporators of seawater desalination systems, and heaters of power plants, the process of vapour condensation is carried out mainly inside the horizontal tubes and channels. Heat transfer processes occurring in condensers have a significant effect on the overall energy efficiency of the mentioned systems. In this paper, the experimental investigation of heat transfer during condensation of freons R22, R406a, and R407c in the plain smooth tube with d = 17 mm were carried out with the following parameters: ts = 35–40°C, G = 10–100 kg/(m2s), x = 0.8–0.1, q = 5–50 kW/m2, ΔT = 4–14 K. The unique measurements of circumferential heat fluxes and heat transfer coefficients were carried out with the thick wall method during different condensation modes. It can be inferred that with the increase of the heat flux, at the top part of the tube the thickness of the condensate film increases, which leads to the decrease in heat transfer. At the bottom of the tube, the increase in the heat flux enhances heat transfer coefficient, that is characteristic of the turbulent liquid flow in the tube. The obtained results allowed improving the prediction of effective heat transfer coefficients for vapour condensation, which takes into account the influence of condensate flow in the lower part of the tube on the heat transfer. This method generalises with sufficient accuracy (error ± 30%) the experimental data on condensation of freons R22, R134a, R123, R125, R32, R410a, propane, isobutene, propylene, dimethyl ether, carbon dioxide, and methane under stratified flow conditions. Using this method for designing heat exchangers, which utilise such types of fluids, will increase the efficiency of thermal energy systems.

R410A and R32 condensation heat transfer and flow patterns inside horizontal micro-fin and 3-D enhanced tubes

Micro-fin tube, 3-D enhanced tube and smooth tube with an inner diameter of 9.52 mm were used as test tubes to study the condensation heat transfer performance with R410A and R32 as the working fluids at different mass flow rates (150–400 kg/m2s) and vapor qualities (0.2–0.8). For R410A and R32, the heat transfer coefficient of the micro-fin tube is 2.0–2.2 times and 1.5–2.0 times that of the smooth tube, and the heat transfer coefficient of the 3-D enhanced tube is 1.4–1.5 times and 1.5–1.6 times that of the smooth tube, respectively. The micro-fin tube is effective in thinning the condensate thickness and reducing the thermal resistance. The 3-D enhanced tube promotes the generation of turbulence and droplet entrainment, which improves heat transfer of enhanced tubes. The heat transfer coefficient of R32 is greater than that of R410A due to its higher thermal conductivity, latent heat and specific heat capacity. The frictional pressure drop increases monotonically with the mass flow rate. Considering the increment in surface area and the additional pressure drop penalty, the performance evaluation factor of the enhanced tubes ranges from 0.9 to 1.4. The study presents flow pattern maps for smooth and enhanced tubes. Enhanced tubes promote the appearance of intermittent and annular flow.

Numerical simulation of flow condensation inside smooth and structured tubes

Three-dimensional numerical simulations for the flow condensation of R-134a are performed at a saturation temperature of 40 °C inside a horizontal smooth and structured tubes with hemispherical structures on the inner surface having square pitch (2.5 mm, 4 mm, and 5 mm). The numerical model for the condensation has been validated by comparing the numerical average heat transfer coefficient with the empirical correlation for smooth tubes. Liquid-vapor interfaces at different axial and longitudinal locations for both smooth and structured tubes are presented to better understand the condensation process. The temporal and spatial averaged vapor quality and liquid film thickness for the smooth and structured tube are also shown. The results indicate that the vapor quality and liquid film thickness inside structured tube are lower than the same observed inside smooth tube. Also, the average heat transfer coefficient and pressure drop for both smooth and enhanced tubes are reported and interestingly, the tube with a 2.5 mm square pitch has shown twice the heat transfer coefficient than the smooth tube.

Film thickness, interfacial waviness and heat transfer during downflow condensation of R134a

• Condensation tests are performed with R134a in a 3.38 mm tube. • Simultaneous film thickness and heat transfer measurements are carried out. • The experimental results of R134a are compared to those of R245fa. • The interfacial waves inception is delayed for R134a due to the high vapor density. • Models for condensation heat transfer, film thickness and void fraction are assessed. Coupled measurements of liquid film thickness and heat transfer during condensation inside minichannels are rare in the literature due to the technical difficulties in performing accurate and non-intrusive measures. The availability of such experimental data is fundamental for better understanding the occurring physical mechanisms and for the proper design of heat exchangers. To cover this gap, in the present work liquid film thickness and heat transfer coefficients have been simultaneously measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been run with refrigerant R134a at mass flux between 30 and 150 kg m −2 s −1 and 30 °C saturation temperature. The instantaneous liquid film thickness and interfacial waves are detected by combining two complementary optical methods: shadowgraph technique (applied to a sequence of flow pattern images recorded by a high speed camera) and chromatic confocal imaging. In the data analysis, the experimental results of R134a have been discussed in comparison to R245fa condensation data taken during a previous experimental campaign at much lower pressure. On average, R134a displays higher liquid film thickness than R245fa and the appearance of high-amplitude disturbance waves, which promote the thinning of the liquid film and enhance the heat transfer, is detected at higher mass flux for R134a as compared to R245fa. The liquid film thickness is found to decrease with the increasing mass flux for both fluids, except from 30 to 50 kg m −2 s −1 due to the transition between shear stress-driven and gravity-driven downflow condensation. The accuracy of several well-known models for the prediction of heat transfer coefficient, film thickness and void fraction has been assessed with comparison to the experimental data of both R134a and R245fa.

High Pressure Refrigerants Condensation Modeling in Horizontal Pipe Minichannels

The study of refrigerant condensation in minichannels is a very wide and complicated issue. An important aspect due to the range of the two-phase changes is mathematical and computer modeling. It allows providing calculation of heat transfer coefficient and pressure drops for many different refrigerants under different flow conditions. A mathematical model for the calculation of heat transfer coefficient and pressure drop during condensation of refrigerants in pipe minichannels is presented in this work. The model was obtained in MATLAB software based on conservation equations. The results of the calculations were verified by the authors’ own experimental investigations data and experimental data from the literature. This article concerns the results of R404A, R410A and R407C refrigerants condensation modeling.

Frictional pressure drop correlation for condensation of refrigerants in microchannel heat exchangers

ABSTRACT Condensation of HFC refrigerants in microchannel heat exchangers has gained a lot of interest recently due to high heat transfer coefficient, small size, and reduced refrigerant inventory. Additionally, these microchannel heat exchangers can be conveniently used for extensive ranges of temperature and pressure. Even though numerous research studies have been conducted on the two-phase frictional pressure drop for condensing flows in microchannels, the efforts are continuing to get more experimental data that can be used to develop an accurate frictional pressure drop correlation. The present paper uses 384 experimental data points, which the authors have obtained during the condensation of refrigerants, R134a and R410A, inside multiport rectangular microchannels with hydraulic diameters ranging from 0.66 to 1 mm, at average saturation temperatures of 30°C to 40°C, mass fluxes of 200 to 600 kg/m2s, and average vapor qualities ranging from 0.05 to 0.83. A frictional pressure drop correlation for condensing flows through microchannels has been developed, and the resulting correlation agrees with the experimental data with an MAE of 11.58% and predicts the data almost within the 20% error band. Experimental data were compared with 10 well-known frictional pressure drop correlations from the literature, and it was found that these correlations either over-predict or under-predict the data.

Influence of Saturation Temperature on Pressure Drop during Condensation of R-134a inside a Dimpled Tube: A Numerical Study

Influence of Saturation Temperature on Pressure Drop during Condensation of R-134a inside a Dimpled Tube: A Numerical Study

Local heat transfer coefficients of R32 and R410a condensation inside a brazed plate heat exchanger (BPHE)

This paper presents the local heat transfer coefficients of R32 and R410A complete condensation inside a Brazed Plate Heat Exchanger (BPHE). A new test section, with one channel on the refrigerant-side and two channels on the water-side, was specifically designed and manufactured for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in BPHE. The test section includes two thick corrugated plates instrumented with 36 copper-constantan thermocouples for measuring the plate surface temperature, the heat flux and the heat transfer coefficient in 9 different positions along the refrigerant channel. The experimental tests were carried out at a saturation temperature around 30°C, in the refrigerant mass velocity range 10 − 39 kg m−2s−1 with an inlet vapour quality around 1.00 and an outlet vapour quality ranging from 0.04 to 0.27. A transition point between gravity controlled and forced convection condensation was found for a refrigerant mass velocity around 20 kg m−2s−1. The experimental data was compared against theoretical models: Nusselt (1916) analysis for vertical surface predicts very well the data in gravity controlled condensation, while Longo et al. (2015) model shows a fair agreement with the forced convection condensation data.