Condensation Of R134a Research Articles

Condensation heat transfer and pressure drop of R-410A in a 7.0 mm O.D. microfin tube at low mass fluxes

R-410A condensation heat transfer and pressure drop data are provided for a 7.0 mm O.D. microfin tube at low mass fluxes (50–250 kg/m2 s). The heat transfer coefficient of the microfin tube shows a minimum behavior with the mass flux. At a low mass flux, where flow pattern is stratified, condensation induced by surface tension by microfins overwhelms condensation induced by shear, and the heat transfer coefficient decreases as mass flux increases. At a high mass flux, where flow pattern is annular, condensation induced by shear governs the heat transfer, and the heat transfer coefficient increases as mass flux increases. The pressure drop of the microfin tube is larger than that of the smooth tube at the annular flow regime. On the contrary, the pressure drop of the smooth tube is larger than that of the microfin tube at the stratified flow regime.

Heat Transfer During Near-Critical-Pressure Condensation of Refrigerant Blends

Heat transfer during condensation of refrigerant blends R404A and R410A flowing through horizontal tubes with 0.76 ≤ D ≤ 9.4 mm at nominal Pr = 0.8–0.9 was investigated. Local heat transfer coefficients were measured for the mass flux range 200 < G < 800 kg m−2 s−1 in small quality increments over the entire vapor–liquid region. Heat transfer coefficients increased with quality and mass flux, while the effect of reduced pressure was not very significant within this range of pressures. The heat transfer coefficients increased with a decrease in diameter. Correlations from the literature were not able to predict the condensation heat transfer coefficient for these fluids at these near-critical pressures over the wide range of tube diameters under consideration. A new flow-regime based model for heat transfer in the wavy, annular, and annular/wavy transition regimes, which predicts 91% of the data within ±25%, is proposed.

Effect of Gravity during Condensation of R134a in a Rectangular Minichannel

为研究重力对小通道冷凝过程的影响，采用VOF模型对制冷剂R134a在水平放置的边长1mm方形截面小通道内的冷凝换热过程进行数值模拟.模拟过程考虑重力、表面张力和界面剪切力的综合作用，表面张力采用CSF模型.结果表明：重力对矩形小通道冷凝换热的影响不明显，蒸气在通道横截面上呈近似圆形分布；通道较短时重力对液膜的汇聚作用不明显，达到一定长度后重力作用凸显.研究结果为天舟一号货运飞船搭载的蒸发与冷凝科学实验研究项目冷凝空间实验方案设计提供了理论依据.

Pressure drop during near-critical-pressure condensation of refrigerant blends

This paper presents the results of an experimental study on pressure drop during condensation of refrigerant blends R404A and R410A at near-critical pressures inside tubes of 0.76 ≤ D ≤ 9.4 mm. Local frictional pressure gradients were measured at reduced pressures (Pr) of 0.8 and 0.9 for the mass flux range 200 ≤ G ≤ 800 kg m−2 s−1, in small quality increments. The data were compared with adiabatic and diabatic two-phase pressure drop correlations from the literature. These correlations were not able to adequately predict the pressure drop over the range of tube sizes and mass fluxes under consideration here at high reduced pressures. A new correlation for pressure drop is introduced, which accounts for property variations and microchannel effects. The correlation predicts 85% of the data within ±25%, with an average deviation between model predictions and data of ±14%.

Empirical study of heterogeneous refrigerant condensation in pipe minichannels

This paper concerns the experimental test results of homogeneous refrigerant R134a and zeotropic solutions R404A, R407C and R410A condensation in pipe minichannels with an internal diameter d = 0.90–3.30 mm. The paper presents experimental thermal and flow characteristics of condensation in comparable ranges. The experimental results were compared to the calculated results of other authors' correlations. It was found that the discrepancy between the results of tests and calculations is large, especially for zeotropic solutions, and in excess of the range of ±50%. The authors developed their own correlations, providing compatibility within ±25% for all of the tested refrigerants. Correlations are proposed for annular and annular-stratified two-phase flow structures.

Experimental study on condensation flow patterns inside inclined U-bend tubes

Effect of inclination angle/flow direction on the patterns of a two-phase flow with low mass flux in a U-bend tube is not well documented. In this study, flow patterns and transitions for condensation of refrigerant R-134a inside U-bend tubes and their contiguous straight tubes are visually observed and analyzed. In order to study the condensation flow patterns, an experimental setup is developed. The experiments have been performed for downward and upward condensation flows with different inclination angles. Low refrigerant mass fluxes were selected to vary in the range of 52–225kg/m2s. The observations showed that, the flow regime is influenced by interfacial shear stress, surface tension, centrifugal force and gravitational force. Analysis of the collected data showed that the tube inclination angle strongly influences the flow pattern. Moreover, the variety of flow regimes for the horizontal cases, vertical downward and vertical upward U-bend tube, is more than other inclinations. Considering the present range of the experimental parameters, inclinations of central region of the U-bend tube as well as the inclination angle significantly impact the two-phase flow patterns. The observed flow patterns are correlated via Weber number for different inclination angles, flow directions, and refrigerant mass fluxes.

Pressure drop during condensation of R-134a inside parallel microchannels

In this study, we experimentally investigate the pressure losses during the convective condensation of R-134a inside eight circular (diameter D = 0.77 mm) horizontal and parallel microchannels. All pressure loss contributions, including the ones related to expansion, contraction, flow direction change, acceleration, and friction, are quantified for microchannel arrangement. The test conditions include the pressure, vapor quality, heat flux, and mass velocity, ranging from 7.3 to 9.7 bar, 0.55 to 1, 17 to 53 kW m−2, and 230 to 445 kg m−2 s−1, respectively. The frictional pressure drop roughly corresponds to 95% of the net pressure loss. The influence of temperature, heat flux, and mass velocity on the pressure drop is evaluated. The results show that the pressure drop increases with an increase in mass velocity and a decrease in saturation temperature, whereas it is not affected as much by the heat flux. The experimental results are compared with correlations and semi-empirical models described in the literature. Correlations based upon the adiabatic two-phase flows within bore pipes can reasonably predict the pressure drop for condensing microchannel flows. The model proposed by Cavallini et al. (2006) presents the best prediction performance.

Experimental investigation on heat transfer coefficient during upward flow condensation of R410A in vertical smooth tubes

This paper presents an experimental investigation on condensation of R410A upward flow in vertical tubes with the same inner diameter of 8.02mm and different lengths of 300 mm, 400 mm, 500 mm and 600mm. Condensation experiments were performed at mass fluxes of 103–490 kg m−2s−1. The saturation temperatures of experimental condition were 31°C, 38°C and 48°C, alternatively. The average vapor quality in the test section is between 0.91 and 0.98. The effects of tube length, mass flux and condensation temperature on condensation were discussed. Four correlations used for the upward flow condensation were compared with the experimental data obtained from various experimental conditions. A modified correlation was proposed within a ±15% deviation range.

Condensation and evaporation heat transfer characteristics in horizontal smooth, herringbone and enhanced surface EHT tubes

An experimental investigation was performed to evaluate convective condensation and evaporation of R22, R32 and R410A inside a smooth tube (inner diameter 11.43mm), a herringbone tube (fin root diameter 11.43mm) and a newly developed enhanced surface EHT tube (inner diameter 11.5mm) at low mass fluxes. The inner surface of the EHT tube is enhanced by dimple/protrusion and secondary petal arrays. For condensation, the heat transfer coefficient of the herringbone tube is 2.0–3.0 times larger than a smooth tube and the EHT tube is 1.3–1.95 times that of the smooth tube. The heat transfer enhancement ratios of the herringbone tube and the EHT tube are larger than their respective inner surface area ratios. Mass flux has a non-monotonic relation with the condensation heat transfer coefficient in the herringbone microfin tubes; this was especially evident for R32 and R410A. For evaporation, the EHT tube provides the best evaporation heat transfer performance for all the three refrigerants; this is mainly due to the heat transfer enhancement produced from the larger number of nucleation sites, increased interfacial turbulence, boundary layer disruption, flow separation and secondary flow generation caused by the dimple and petal arrays. The evaporation heat transfer coefficient of the herringbone tube is only slightly higher than that of the smooth tube. Overall, the EHT tube provides increased heat transfer enhancement for both condensation and evaporation.

Experimental Investigations on Condensation in the Framework of ENhanced COndensers in Microgravity (ENCOM-2) Project

We report preliminary results on experimental investigations on condensation in the framework of the European Space Agency funded programme Enhanced Condensers in Microgravity (ENCOM-2) which aims at better understanding underlying phenomena during condensation. The first experiment is a study on condensation of HFE on external curvilinear surface of 15 mm height during reduced gravity experiments. It is found that the local minimum of the film thickness exists at the conjugation area of condensed film and the meniscus at the bottom of the fin; this leads to the local maximum of the heat transfer coefficient, which we also found moves towards the fin tip. The second experiment is a study of falling films hydrodynamics inside a vertical long pipe. In particular, characteristics of wavy falling films produced employing intermittent liquid feed are examined in order to assess wave effects on film condensation. Preliminary results suggest that intermittent feed simply divides the film in two autonomous regions with the wave feature of each one depending only on its flow rate. The processing of registered film thickness data can lead to the estimation of the transverse velocity profile in the film, which is mainly responsible for heat transfer during condensation. The third experiment looks at in-tube convective condensation at low mass fluxes (typical of Loop Heat Pipes and Capillary Pumped Loops) of n-pentane inside a 0.56 mm diameter channel. The results show that the mean heat transfer in the annular zone when it is elongated may be less than the mean heat transfer when it is shorter, due to the interface deformation involved by surface tension effect. When the length of this annular zone reaches a critical value, the interface becomes unstable, and a liquid bridge forms, involving the release of a bubble. The heat transfer due to the phase-change in this isolated bubble zone appears to be very small compared to the sensible heat transfer: the bubbles evolve and collapse in a highly subcooled liquid. The last experiment concerns in-tube condensation of R134a inside a square channel of 1.23 mm hydraulic diameter at mass fluxes of 135 kg m−2 s−1 and 390 kg m−2 s−1 for three different configurations: horizontal, vertical downflow and vertical upflow. For the calculated heat transfer coefficient it is found that gravity has no effect on condensation in downflow configurations at 390 kg m−2 s−1 and in upflow conditions at both values of mass velocity. The effect of gravity on the condensation heat transfer coefficient becomes noteworthy in downflow at mass velocity G = 135 kg m−2 s−1 and vapour quality lower than 0.6.

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m2s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.

Effect of applied EHD on in-tube condensation of R-134a within an assembled experimental rig including a laboratory heat exchanger

Effects of applied EHD1Electrohydrodynamic.1 on in-tube condensation of R-134a refrigerant in a double pipe heat exchanger are experimentally investigated to present applicability of the EHD technique in vertical position. The refrigerant’s vapor was flowed inside of the inner tube and cooling water was counter currently flowed in annulus section. Tests were performed at different mass flow rates 35–65kg/m2s of refrigerant and different electrical field strengths (0–8kV), under their identified breakdown strengths for two various electrode diameter sizes (4 and 6mm), while the average saturation temperature was 43 and 33°C respectively. Experimental results demonstrate a remarkable potential in utilizing EHD to enhance condensation heat transfer. It is concluded that the enhancement is driven by the effective removal of the condensate through EHD induced liquid extraction and dispersion phenomena. For the presence of the electrodes, the experimental results indicate that the maximum heat transfer ER2Enhancement ratio.2 is around 14% and 31% respectively for electrodes with diameter of 4mm (10.7mm electrode gap) and 6mm (9.7mm electrode gap), while power expenditure was negligible.

Condensation of R134a outside single horizontal titanium, cupronickel (B10 and B30), stainless steel and copper tubes

Experiments were conducted on the condensing heat transfer of eleven horizontal tubes with refrigerant R134a at saturated temperature of 40°C (1.01MPa). The tube materials are titanium, cupronickel (B10 and B30), stainless steel and copper, among whom the former four materials are of low thermal conductivity. The enhanced tubes include integral-fin and three dimensional geometries. The results indicate that the heat transfer coefficients of the enhanced copper tube are about 1.6–2.1 times of low thermal conductivity tubes with the same enhanced geometries. Compared with plain tube the mean enhanced ratio of tubes made from titanium, B10, B30 and stainless steel are 8.48, 8.31, 8.22 and 7.52, respectively. The reason why low thermal conductivity affects the condensation heat transfer of enhanced structure is discussed in detail.

Visualization on flow patterns during condensation of R410A in a vertical rectangular channel

The visualization experiments on HFC R410A condensation in a vertical rectangular channel (14.34mm hydraulic diameter, 160mm length) were investigated. The flow patterns and heat transfer coefficients of condensation in the inlet region were presented in this paper. Better heat transfer performance can be obtained in the inlet region, and flow regime transition in other regions of the channel was also observed. Condensation experiments were carried out at different mass fluxes ( from 1.6 kg/h to 5.2 kg/h) and at saturation temperature 28°C. It was found that the flow patterns were mainly dominated by gravity at low mass fluxes. The effects of interfacial shear stress on condensate fluctuation are significant for the film condensation at higher mass flux in vertical flow, and consequently, the condensation heat transfer coefficient increases with the mass flux in the experimental conditions. The drop formation and growth process of condensation were also observed at considerably low refrigerant vapor flow rate.

Pressure drop during condensation of R134a flowing inside a multiport minichannel

This paper is a continuation of the authors’ previous work on the condensation of R134a flowing inside a multiport minichannel (Sakamatapan et al., 2013) [22]. The pressure drop’s characteristics during condensation were investigated in an experiment for R134a flowing inside a multiport minichannel. The pressure drop characteristics were observed under mass flux range of 345–685kg/m2, heat flux of 15–25kW/m2, and saturation temperature of 35–45°C. Two types of multiport minichannels having 14 channels, one with a 1.1mm hydraulic diameter and another with 8 channels with a 1.2mm hydraulic diameter, were designed as a counter flow tube in a tube heat exchanger. The results showed that the total pressure drop is dominated by the frictional pressure drop. The frictional pressure gradient increased with the augmentation of mass flux and vapor quality, but the increase of the saturation temperature and channel size play an important role in the decrease of frictional pressure gradient. On the other hand, the heat flux had an insignificant effect on the frictional pressure gradient. Moreover, the miniscale correlations were conducted to predict the frictional pressure gradient, and it was found that only the multiport minichannel correlations gave a reasonable result. A new two-phase friction factor correlation was proposed using the equivalent Reynolds number concept to predict the frictional pressure gradient during condensation.

Convective Condensation Inside Horizontal Smooth and Microfin Tubes

An experimental investigation was performed for convective condensation of R410A inside one smooth tube (3.78 mm, inner diameter) and six microfin tubes (4.54, 4.6 and 8.98 mm, fin root diameter) of different geometries for mass fluxes ranging from 99 to 603 kg m−2s−1. The experimental data were analyzed with updated flow pattern maps and evaluated with existing correlations. The heat transfer coefficient in the microfin tubes decreases at first and then increases or flattens out gradually as mass flux decreases. This obvious nonmonotonic heat transfer coefficient-mass flux relation may be explained by the complex interactions between the microfins and the fluid, mainly by surface tension effects. The heat transfer enhancement mechanism in microfin tubes is mainly due to the surface area increase at large mass fluxes, while liquid drainage by surface tension and interfacial turbulence enhance heat transfer greatly at low mass fluxes.

Condensation of R-134a inside a helically coiled tube-in-shell heat exchanger

This article presents an experimental investigation of heat transfer and pressure drop characteristics of R-134a condensing inside a horizontal helical coil tube with the cooling water flowing in the shell in counter flow direction. The test runs are performed at vapor saturation temperature 35±0.5 and 40±0.5°C for the mass flux varying from 100 to 350kgm−2s−1 and vapor quality ranging from 0.1 to 0.9. The flow regimes observed during the experiment have been plotted on Taitel and Dukler and mass flux versus vapor quality flow map. The transitions between different flow regimes have also been discussed. The effect of mass flux, vapor quality and saturation temperature on the heat transfer coefficient and pressure drop have been investigated. The experimental results of the helical coil tube are compared to straight tube. The thermodynamic advantage of helical coil over straight tube is evaluated in terms of enhancement parameter. The enhancement parameter is higher than one for mass fluxes lower than 200kgm−2s−1. The correlations have been developed to predict two-phase Nusselt number and frictional pressure drop multiplier during condensation of R-134a inside horizontal helical coil tube.

Modeling of a Two-Phase Region Length of the Condensation of R134a and R404A Refrigerants in Pipe Minichannels With Periodic Hydrodynamic Instabilities

The heat removal process of a compact condenser in a refrigeration compressor system, constructed on the basis of pipe minichannels, occurs in three regions: the removal region of superheat, the two-phase proper condensation region, and the liquid subcooling region. The intensity of heat removal in the two-phase region depends on the heat efficiency of the condenser. The length of this region is determined by the process parameters and the external influences. This paper analyzes the influence of periodically generated hydrodynamic disturbances on the length of the condensation region for R134a and R404A refrigerants. Experimental tests were conducted for six diameters of pipe minichannels: 0.64, 0.90, 1.44, 1.92, 2.30, and 3.30 mm. A range of frequencies from 0.2 to 5 Hz of periodically generated disturbances showed an explicit and unfavorable influence on the decrease of the length of the proper condensation region. This influence reduced the efficiency of the process. A computational model was developed to calculate the length of the two-phase region under disturbance conditions. The model proposed was verified by the results of the experiment, and compliance was obtained in the range of ±15%.

Condensation heat transfer in smooth inclined tubes for R134a at different saturation temperatures

This paper presents the effects of saturation temperature and inclination angle on convective heat transfer during condensation of R134a in an inclined smooth tube of inner diameter of 8.38mm. Experiments were conducted for inclination angles ranging from −90° (vertical downwards) to +90° (vertical upwards) for mass fluxes between 100kg/m2s and 400kg/m2s and mean vapour qualities between 0.1 and 0.9 for saturation temperatures ranging between 30°C and 50°C in a test matrix of 637 test data points. The results showed that saturation temperature and inclination angle strongly influenced the heat transfer coefficient. With respect to saturation temperature, an increase in saturation temperature generally led to a decrease in heat transfer coefficient irrespective of the inclination angle. The effect of inclination angle was found to be more pronounced at mass fluxes of 100kg/m2s and 200kg/m2s for the range of mean vapour qualities considered. Within the region of influence of inclination, there was an optimum angle, which was between −15° and −30° (downward flow). The inclination effect corresponded to the predominance of the effect of gravity on the flow distribution. An increase in saturation temperature increased the effect of inclination on two-phase heat transfer.

Flow regime transitions during condensation in microchannels

Heat transfer and pressure drop for two-phase flow inside tubes are closely related to the corresponding flow mechanisms. The flow patterns formed in microchannels during condensation differ from those observed in conventional tubes. Using an extensive R134a condensation flow-regime database (1 < Dh < 4.91 mm, 150 < G < 750 kg m−2 s−1), new flow regime transition criteria are proposed. The data are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes and develop dimensionless transition criteria. These criteria can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing their applicability. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the development of models for predicting condensation heat transfer and pressure drop, enabling the development of optimized microchannel heat exchangers.