Evaporation heat transfer and friction characteristics of R-134a flowing downward in a vertical corrugated tube

Differently from most previous studies, the heat transfer and friction characteristics of the pure refrigerant HFC-134a during evaporation inside a vertical corrugated tube are experimentally investigated. The double tube test sections are 0.5 m long with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tubes are one smooth tube and two corrugated tubes, which are constructed from smooth copper tube of 8.7 mm inner diameter. The test runs are performed at evaporating temperatures of 10, 15, and 20 °C, heat fluxes of 20, 25, and 30 kW/m², and mass fluxes of 200, 300, and 400 kg/m² s. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is measured directly by a differential pressure transducer. The effects of heat flux, mass flux, and evaporation temperature on the heat transfer coefficient and two-phase friction factor are also discussed. It is found that the percentage increases of the heat transfer coefficient and the two-phase friction factor of the corrugated tubes compared with those of the smooth tube are approximately 0-10% and 70-140%, respectively.     

Evaporation heat transfer and flow characteristics of R-134a flowing through internally grooved tubes

This article describes experimental investigations of the heat transfer coefficient and pressure drop of R-134a flowing inside internally grooved tubes. The test tubes are one smooth tube and four grooved tubes. All test tubes are made from type 304 stainless steel, have an inner diameter of 7.1 mm, are 2,000 mm long and are installed horizontally. The test section is uniformly heated by a DC power supply to create evaporation conditions. The groove depth of all grooved tubes is fixed at 0.2 mm. The experimental conditions are conducted at saturation temperatures of 20, 25 and 30°C, heat fluxes of 5, 10 and 15 kW/m², and mass fluxes of 300, 500 and 700 kg/m² s. The effects of groove pitch, mass flux, heat flux, and saturation temperature on heat transfer coefficient and frictional pressure drop are discussed. The results illustrate that the grooved tubes have a significant effect on the heat transfer coefficient and frictional pressure drop augmentations.     

Evaporation flow pattern and heat transfer of R-22 and R-134a in small diameter tubes

The flow patterns and heat transfer coefficients of R-22 and R-134a during evaporation in small diameter tubes were investigated experimentally. The evaporation flow patterns of R-22 and R-134a were observed in Pyrex sight glass tubes with 2 and 8 mm diameter tube, and heat transfer coefficients were measured in smooth and horizontal copper tubes with 1.77, 3.36 and 5.35 mm diameter tube, respectively. In the flow patterns during evaporation process, the annular flows in 2 mm glass tube occurred at a relatively lower vapor quality compared to 8 mm glass tube. The flow patterns in 2 mm glass tube did not agree with the Mandhane’s flow pattern maps. The evaporation heat transfer coefficients in the small diameter tubes (d _i  < 6 mm) were observed to be strongly affected by tube diameters, and to differ from those in the large diameter tubes. The heat transfer coefficients of 1.77 mm tube were higher than those of 3.36 mm and 5.35 mm tube. Most of the existing correlations failed to predict the evaporation heat transfer coefficient in small diameter tubes. Therefore, based on the experimental data, the new correlation is proposed to predict the evaporation heat transfer coefficients of R-22 and R-134a in small diameter tubes.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.     

Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations

An experimental investigation has been carried out to study the heat transfer characteristics during evaporation of R-134a inside a single helical microfin tube. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and four mass velocities of 53, 80, 107 and 136 kg/m² s for each tube inclination angle during evaporation of R-134a. The results demonstrate that the tube inclination angle, α, affects the boiling heat-transfer coefficient in a significant manner. For all refrigerant mass velocities, the best performing tube is that having inclination angle of α = +90°. The effect of tube inclination angle, α, on heat-transfer coefficient, h, is more prominent at low vapor quality and mass velocity. An empirical correlation has also been developed to predict the heat-transfer coefficient during flow boiling inside a microfin tube with different tube inclinations.     

Condensation heat transfer and pressure drop characteristics of R-290, R-600a, R-134a and R-22 in horizontal tubes

This paper presents the experimental results of condensation heat transfer coefficients of hydrocarbon (HC) refrigerants R-290 and R-600a, hydrochlorofluorocarbon (HCFC) refrigerant R-22, and hydrofluorocarbon (HFC) refrigerant R-134a in a horizontal double-pipe heat exchanger having pipe inner diameters of 10.07, 7.73, 6.54, and 5.80 mm. The condensation process experiments were conducted at mass flux of 35.5–210.4 kg/m² s and condensation temperature of 40°C. The main results were summarized as follows: The average condensation heat transfer coefficients of R-290 and R-600a were higher than those of R-22 and R-134a. The pressure drops of the four refrigerants were in the order of R-600a > R-290 > R-134a > R-22. The pressure drops of R-600a, R-290, R-134a, and R-22 were approximately 6–15, 9.8–12.5, 4.3–6.7, and 2.1–4.6% higher, respectively, in the 10.7 mm diameter tubes compared to the 5.80 mm diameter tubes. Comparing the condensation heat transfer coefficients of our experimental results with those of other correlations, our experimental data in all the test tubes coincided best with that of Haraguchi et al.     

Augmentation of heat transfer by twisted tape inserts during condensation of R-134a inside a horizontal tube

An experimental investigation has been carried out to find the heat transfer coefficient during condensation of R-134a vapor inside a horizontal tube. Experiments were conducted for the condensation of R-134a inside a plain tube and tubes with different twisted tape inserts. Twisted tapes with different twisted ratios of 6, 9, 12 and 15 were inserted in the refrigerant side, one by one, in the full length of test-condenser. For each inserted tube and the plain tube, test runs were carried out for the mass velocities of 92, 110, 128 and 147 kg/s-m². An empirical correlation has also been developed to predict the enhanced heat transfer coefficient.     

Experimental study of R134A condensation heat transfer inside the horizontal micro-fin tubes

The Investigation of the two-phase flow patterns and their transitions during the condensation has gained increasing interest and importance from the well-known phenomenon that the heat transfer characteristics are strongly dependent on the flow patterns. Therefore, it is very important to study on which heat transfer enhancement approach is suitable for an individual flow pattern inside a condenser, so that an accurate heat transfer mechanism can be understood that is consistent with the flow patterns. The condensation heat transfer for R134a in the two kinds of in-tube three-dimensional (3-D) micro-fin tubes with different geometries is experimentally investigated. Based on the flow pattern observations, the flow patterns in the Soliman flow regime map are divided into two-flow regimes; one with the vapor-shear-dominant annular regime and the other with the gravitational-force-dominant stratified-wavy regime. The flow regime transition criterion between the annular regime and the stratified-wavy regime is at Fr equal to 2. In the annular regime, the heat transfer coefficients h of the two kinds of in-tube 3-D micro-fin tubes decreases as the vapor quality x decreases. The regressed condensation heat transfer correlation from the experimental data of the annular flow region is obtained. The dispersibility of the experimental data is inside the limits of ±25%. In the stratified-wavy regime, the average heat transfer coefficient h of the two kinds of in-tube 3-D micro-fin tubes increases as the mass flux increases and the number of micro fins in the 3-D micro-fin tube is not the controlling factor for the performance of a condensation heat transfer. The regressed condensation heat transfer correlation of the stratified-wavy flow regime is experimentally obtained. The dispersibility of the experimental data is inside the limits of ±22%. Combined with the criteria of flow pattern transitions, the correlations can be used for the design of a condenser with 3-D micro-fin tubes.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in a single circular microtube

The condensation heat transfer coefficients of R-22, R-134a and R-410A in a single circular microtube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The test section is a smooth, horizontal copper tube of 1.77 mm inner diameter. The experiments were conducted at mass flux of 450-1050 kg/m² s, saturation temperature of 40 °C. The test results showed that in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski correlation. In case of two-phase flow, the condensation heat transfer coefficient of R-410A was higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensing heat transfer. And also, recently proposed correlation in the single circular microtube is considered not adequate for small diameter tube. Therefore, it is necessary to develop accurate and reliable correlation to predict heat transfer characteristics in the single circular microtube.     

Flow boiling heat transfer study of R-134a/R-290/R-600a mixture in 9.52 and 12.7 mm smooth horizontal tubes: Experimental investigation

A detailed experimental investigation is carried out to study the flow boiling heat transfer behavior of R-134a/R-290/R-600a (91%/4.068%/4.932% by mass) refrigerant mixture in smooth horizontal tubes of diameter 9.52 and 12.7 mm. The heat transfer coefficients of the mixture are experimentally measured under varied heat flux conditions for stratified flow patterns using a coaxial counter-current heat exchanger test section. The tests are conducted for refrigerant inlet temperatures between ?9 and 5 °C and mass flow rates ranging from 3 to 5 g s^?1. Kattan–Thome–Favrat maps are used to confirm the flow patterns for the tested conditions. The magnitude of the heat transfer coefficient with respect to flow patterns and different mechanisms of boiling are discussed. The heat transfer coefficient of the refrigerant mixture is also compared with that of R-134a for selected working conditions. The significance of nucleate boiling in the overall heat transfer process under these testing conditions is highlighted.     

Two-phase frictional pressure drop multipliers for SUVA R-134a flowing in a rectangular duct

The adiabatic two-phase frictional multipliers for SUVA, R-134a flowing in a rectangular duct (with D_H = 4.8 mm) have been measured for three nominal system pressures (0.9 MPa, T_sat = 35.5 °C; 1.38 MPa, T_sat = 51.8 °C; and 2.41 MPa, T_sat = 75.9 °C) and three nominal mass fluxes (510, 1020 and 2040 kg/m²/s). The data is compared with several classical correlations to assess their predictive capabilities. The Lockhart–Martinelli model gives reasonable results at the lowest pressure and mass flux, near the operating range of most refrigeration systems, but gives increasingly poor comparisons as the pressure and mass flux are increased. The Chisholm B-coefficient model is found to best predict the data over the entire range of test conditions; however, there is significant disagreement at the highest pressure tested (with the model over predicting the data upwards of 100% for some cases). The data shows an increased tendency toward homogeneous flow as the pressure and flow rate are increased, and in fact the homogeneous model best predicts the bulk of the data at the highest pressure tested.     

Prediction of film boiling heat transfer coefficients for binary mixtures

Film boiling of binary liquid mixtures may be significantly different from that of single-component liquids due to the mass diffusion effect. A theoretical analysis is performed to outline the effects of mass diffusion phenomena on film boiling heat transfer process from a horizontal cylinder heating surface to the binary liquid mixtures of ethylene oxide/water and ethanol/benzene over whole range of compositions. These two binary systems are chosen for illustrating the strong and weak mass diffusion effects, respectively, on film boiling. Furthermore, a simple correlation for predicting heat transfer coefficient is proposed to demonstrate the idea that the dimensionless F factor can satisfactorily account for the mass diffusion effect on film boiling heat transfer of binary mixtures.     

Roughness and surface material effects on nucleate boiling heat transfer from cylindrical surfaces to refrigerants R-134a and R-123

This paper presents results of an experimental investigation carried out to determine the effects of the surface roughness of different materials on nucleate boiling heat transfer of refrigerants R-134a and R-123. Experiments have been performed over cylindrical surfaces of copper, brass and stainless steel. Surfaces have been treated by different methods in order to obtain an average roughness, Ra, varying from 0.03 μm to 10.5 μm. Boiling curves at different reduced pressures have been raised as part of the investigation. The obtained results have shown significant effects of the surface material, with brass being the best performing and stainless steel the worst. Polished surfaces seem to present slightly better performance than the sand paper roughened. Boiling on very rough surfaces presents a peculiar behavior characterized by good thermal performance at low heat fluxes, the performance deteriorating at high heat fluxes with respect to smoother surfaces.     

Determination of heat transfer coefficients on the surfaces of plastic tubes

 The paper discusses the statistical steady heat and momentum transfer problem in the inlet section of the plastic tubes. The modified two equation k–ɛ turbulent model utilizing variability of turbulent Prandtl number, Pr_t, was used for the analysis. Considering the thermophysical anisotropy of the tube material, a balance of local temperatures and local heat fluxes on the boundary between the fluid and the tube wall was assumed (conjugate heat transfer problem). The thermal boundary condition on the external surface of the tube (temperature) measured in the experiment was taken into account. The boundary problem described was solved by the control volume method. The values of the parameters of Pr and Re obtained from the experiments were included in the numerical calculations. Based on the results obtained, profiles of mean fluid temperatures, local Nusselt numbers on the internal and external surface of the tube, and profiles of temperatures on the internal surface of the tube and inside of the tube wall were determined. The analysis shows that changes in Pr_tand turbulence intensity, Tu, influence the local values of Nusselt numbers, and it also shows that the results for the local Nusselt numbers inside the tube obtained from numerical calculations are of great accuracy in comparison with results published in the available literature. Received on 11 June 2001     

Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.