Local heat transfer coefficient for pool boiling of R-134a and R-123 on smooth and enhanced tubes

The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m² for the TBIIHP tube and 3.1–73.2 kW/m² for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m². °C and 5331–25950 W/m². °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m² and 7.5–60.7 kW/m² for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m². °C and 535.4–3181.8 W/m². °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.     

Condensation of R-134a on horizontal integral-fin titanium tubes

Experimental research was conducted to evaluate the condensation of R-134a on horizontal smooth and integral-fin (32 fpi) titanium tubes of 19.05 mm outer diameter. Experiments were carried out at saturation temperatures of 30, 40 and 50 °C and wall subcoolings from 0.5 to 9 °C. The results show that the condensation heat transfer coefficients (HTCs) on the smooth tubes are well predicted by the Nusselt theory with an average error of +2.38% and within a deviation between +0.13% and +5.42%. The enhancement factors provided by the integral-fin tubes on the overall condensation HTCs range between 3.09–3.94, 3.27–4 and 3.54–4.1 for the condensation temperatures of 30, 40 and 50 °C, respectively. The enhancement factors increase by increasing the wall subcooling and with the rise of the condensing temperature. The condensate flooded fraction of the integral-fin tubes perimeter varies from 25% to 20% at saturation temperatures of 30 °C and 50 °C, respectively. The correlation reported by Kang et al. (2007) [1] predicted the experimental data with a mean deviation of ?5.5%.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Experimental study of forced convective boiling for non-azeotropic refrigerant mixtures R-22/R-124 in horizontal smooth tube

In this article, the experiment is carried out with four stages of test section and new experimental data of non-azeotropic refrigerant mixtures R-22/R-124 are presented. Existing correlations are analysed and new ones are proposed for pure refrigerants. The nucleate boiling correlation of the refrigerant mixtures is modified so as to obtain the evaporation heat transfer coefficient of the refrigerant mixtures in a smooth tube. The result shows that the majority of the predictions fall within ±30% of the experiments.     

Prediction of Local Bundle Boiling Heat Transfer Coefficients: Pure Refrigerant Boiling on Plain,Low Fin,and Turbo-BII HP Tube Bundles

New correlations predicting the local boiling heat transfer coefficients for vertical flow over a tube bundle have been developed. These correlations were developed for three different classes of tubes: smooth, low-finned, and Turbo-BII HP. These correlations are based on experiments with refrigerants R-134a, R-507A, and R-410A, at a saturation temperature of 4.4°C (40°F). Each one accounts for the local flow structure and/or local geometry particular to each tube. Each one also uses a void fraction model designed specifically for vertical two-phase flow in a tube bundle as an integral part of the correlation. The correlations have been compared to recent local bundle boiling data and show good agreement to within ± 20%.     

Enhancement of boiling heat transfer in restricted spaces in compact horizontal tube bundles

ln desalinization devices and some heat exchangers making use of low‐quality heat energy, both wall temperatures and heat fluxes of heated tubes are quite low and generally cannot cause boiling in flooded‐type tube bundle evaporators with a large tube spacing. But when the tube spacing is very small, boiling in restricted spaces can occur and induce a higher heat transfer than that of a falling film or pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes, and heating status of tubes as well as surface status (smooth and roll‐worked) on boiling in restricted spaces in compact horizontal tube bundle evaporators under atmospheric pressure. The experimental results provide a restricted space boiling database for water in smooth and enhanced surface tube bundles. Of particular importance is information concerning the influence of tube spacing of flooded‐type tube bundle evaporators, especially for the case of zero pitch, when the neighboring tubes are contacting each other. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 394–401, 2001     

Effect of pore size on the nucleate pool boiling of structured enhanced tubes

In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20 mm, 0.23 mm and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 mm pore size. For R-11 and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a ‘cross-over’ characteristic, which existing pored tubes do. The connecting gaps of the present tube are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m² and saturation temperature 4.4° C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube     

Nucleate pool boiling characteristics from coated tube bundles in saturated R-134a

Nucleate pool boiling heat transfer from plasma coated copper tube bundles with porous copper (Cu) immersed in saturated R-134a was experimentally studied. The bundle is composed of 15 tubes (of which the number of heated/instrumented tubes was varied) arranged in four different configurations with a pitch-to-diameter ratio of 1.5. The influences of various parameters, for instance, bundle arrangements and heat flux were clarified. Tests were conducted with both increasing and decreasing the heat flux. The data presented indicated that at low heat fluxes, the vertical-in-line tube bundles have the highest bundle factor. A configuration factor was proposed which can be used to characterize the geometric arrangements of the bundles.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Film Condensation of R-134a and R-236fa,Part 2: Experimental Results and Predictive Correlation for Bundle Condensation on Enhanced Tubes

Local test results for two enhanced condensing tubes (next-generation versions of the Wieland Gewa and Wolverine Turbo enhanced condensing tubes) are obtained for refrigerants R-134a and R-236fa on the center row of a three row-wide tube bundle. The “bundle effect” on the heat transfer performance of the test section is observed and described. New predictive methods for falling film condensation on bundles are proposed, based on a modification of the previous vertical single-row method with condensate slinging. The modifications performed to the experimental setup to allow for bundle tests are described. For two types of enhanced tubes and two refrigerants, the local heat flux is correlated as a function of condensation temperature difference, the film Reynolds number, the tube spacing, and liquid slinging effect.     

Generalized neural network correlation for flow boiling heat transfer of R22 and its alternative refrigerants inside horizontal smooth tubes

The correct prediction of refrigerant boiling heat transfer performance is important for the design of evaporators. A generalized neural network correlation for boiling heat transfer coefficient of R22 and its alternative refrigerants R134a, R407C and R410A inside horizontal smooth tubes has been developed in this paper. Four kinds of dimensionless parameter groups from existing generalized correlations are selected as the input of neural network, while the Nusselt number is used as the output. Three-layer perceptron is employed as the universal approximator to build the relationship between the input and output parameters. The neuron number of hidden layer is determined by the performance of model accuracy and the standard sensitivity analysis. The experimental data of the four refrigerants in open literatures are used for correlation. The results show that the input parameter group based on the Gungor–Winterton correlation is better than the other three groups. Compared with the experimental data, the average, mean and root-mean-square deviations of the trained neural network are 2.5%, 13.0% and 20.3%, respectively, and approximately 74% of the deviations are within ±20%, which is much better than that of the existing generalized correlations.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.     

Evaporative characteristics of R-134a and R-600a in horizontal tubes with perforated strip-type inserts

Experimental heat transfer coefficients for R-134a and R-600a in horizontal tubes with vertically positioned perforated strip-type inserts are reported in this paper. Tests were conducted using a single-tube evaporator test facility. The test section used was 2000 mm long, 10.6 mm inside diameter, horizontal, smooth copper tube with perforated strip-type inserts made from the same material (copper). Test parameters were varied as follows: heat flux 9.1-31.2 kW/m²; mass velocity 82.3-603.3 kg/m² s; quality 0-0.85, and a saturation temperature of 6 °C. The flow pattern were identified for different test tubes and flow conditions. The heat transfer coefficients for R-600a were higher than those for R-134a. The heat transfer performance and pressure drop can be improved up to 2.5 and 1.5, respectively for a 96 perforated holes enhanced tube. All comparisons were based on the same nominal mass flow rate. Finally, an empirical correlation was developed.     

In-tube passive heat transfer enhancement in the process industry

Enhanced heat transfer surfaces are used in heat exchangers to improve performance and to decrease system volume and cost. In-tube heat transfer enhancement usually takes the form of either micro-fin tubes (of the helical micro-fin or herringbone varieties), or of helical wire inserts. Despite a substantial increase in heat transfer, these devices also cause non-negligible pressure drops.By making use of well-proven flow pattern maps for smooth tubes and the new ones for smooth and enhanced tubes, it is shown from the refrigerant condensation data that flow patterns have a strong influence on heat transfer and pressure drop. This is done for data obtained from in-tube condensation experiments for mass fluxes ranging from 300 to 800 kg/m² s at a saturation temperature of 40 °C, for refrigerants R-22, R-134a, and R-407C. The flow regimes, pressure drops, heat transfer coefficients, and the overall performance of three different tubes, namely a smooth-, 18° helical micro-fin-, and a herringbone micro-fin tube (each having a nominal diameter of 9.51 mm), are presented and compared to the performance of smooth tubes with helical wire inserts (with pitches of 5 mm, 7.77 mm and 11 mm corresponding to helical angles of 78.2°, 72°, and 65.3°, respectively).     

LDV assisted bubble dynamic parameter measurements from two enhanced tubes boiling in saturated R-134a

LDV measurements and heat transfer experiments for nucleate pool boiling from two horizontal enhanced tubes with porous copper on copper surfaces immersed in saturated R-134a were conducted. The influence of tube position (alignment) and tube pitch on bubble dynamics and boiling characteristics were studied. Photographs indicate that the average number of bubbles increases with heat fluxes, which is the same as those in previous studies of single tube. However, the bubble departure diameters of the upper tube show an opposite trend with an increase as compared to previous single-tube studies. LDV measurements show that the present tube arrangement has significant influence on local velocity in both magnitude and trend. The heat transfer mechanism and modeling for the upper tube were studied and developed.     

Boiling heat transfer to R-22/DMF mixtures

The boiling heat transfer rates to R-22/DMF solutions under pool boiling conditions on horizontal tubes have been investigated. The tests were conducted for a range of values of heat flux q, pressure p, and mole fraction x.     

Experimental investigation on two-phase flow boiling heat transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner diameters

An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C₃H₈ and CO₂ in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5–40 kW m^?2, mass flux range of 50–600 kg m^?2 s^?1, saturation temperature range of 0–15 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on Wang et al. (1997) [5] and Wojtan et al. (2005) [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and ?0.48% average deviation.     

Nucleate boiling of two and three-component mixtures

For three pure fluids and their two- and three-component mixtures, heat transfer coefficients were measured in nucleate pool boiling on the upward facing copper surface of 40 mm diameter. The more-, moderate- and less-volatile components in mixtures are refrigerants R-134a, R-142b and R-123, respectively. Heat transfer coefficients of mixtures were less than the interpolated heat transfer coefficients between pure components, with more reduction at higher heat flux. Two correlations originally developed for two-component mixtures by Thome and Shakir and by Fujita and Tsutsui reproduced well the measured heat transfer coefficients of three- as well as two-component mixtures. This result implies that the boiling range included in the correlations accounts for heat transfer reduction in mixture boiling.     

Thermodynamic property formulations and heat transfer aspects for replacement refrigerants: R-123 and R-134a

This paper reports analytical relations for the thermodynamic properties enthalpy, entropy, heat capacities at constant pressure and temperature of the replacement refrigerants R-123 and R-134a. These refrigerants are considered promising as substitutes for the fluids R-11 and R-12, respectively, which are two of the most widely used CFC refrigerants. In addition to the properties, the three real gas isentropic exponents k_p,v,k_v,T, k_p,T are calculated, which may be used instead of the classical exponent k=c_p/c_v in the ideal gas isentropic change equations to describe with good accuracy the real gas behaviour. A systematic study to research the influence of various parameters on heat transfer during condensation of R-123 and R-134a on horizontal integral-fin tubes is also carried out. The results are useful in refrigeration applications to improve the basic design, as a significant concern about new refrigerants to replace the CFCs has increased very rapidly due to the destruction of stratospheric ozone and global warming. © 1997 John Wiley & Sons, Ltd.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.