Adrian Briggs

Effect of fin efficiency on a model for condensation heat transfer on a horizontal, integral-fin tube

A semi-empirical model for condensation on horizontal, integral-fin tubes has been adapted to account for ‘fin efficiency effects’. Specimen calculations have been made to investigate the effect of tube geometry and material on the enhancement ratio for condensation of steam and CFC11. The best fin spacing was found to be only weakly dependent on the other geometric variables and fin thermal conductivity. The best fin thickness was more strongly dependent on fin thermal conductivity. For the refrigerant the optimum fin thickness was smaller than presently used in practice. The model gave satisfactory agreement with experimental data for CFC113 and steam for typical fin geometries. In the case of CFC113 the enhancement ratio was almost independent of fin thermal conductivity for conductivities exceeding around 50 W m−1 K −1.

Condensation of Steam on Pin-Fin Tubes: Effect of Circumferential Pin Thickness and Spacing

Experimental data are reported for condensation of steam at atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The main geometric parameters varied were the circumferential pin spacing and thickness, since these have been shown to have a strong effect on condensate retention, and the present study shows some evidence for an optimum circumferential fin spacing. Enhancements of the vapor-side heat-transfer coefficient of up to 4 were found, compared to a plain tube at the same vapor-side temperature difference. The measured enhancements are equal to, but do not exceed, those obtained from “optimized” two-dimensional integral-fin tubes reported in the literature—an observation that is also generally true for condensation of refrigerants. The evidence suggests, however, that three-dimensional fin profiles can produce worthwhile enhancement over those obtained from simple, two-dimensional, integral-fin tubes, but that more work is needed to understand the phenomena involved so that more efficient geometries can be developed.

HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS FOR CONDENSATION OF R113 IN A VERTICAL MICRO-FINNED TUBE WITH WIRE INSERT

We report an experimental investigation of film condensation in a vertical, internally finned tube with a wire insert. The finned tube and wire insert were chosen as the best performing from a series of tubes and inserts tested separately in previous investigations. Compared with previous results for the finned tube, it was found that only for ΔT>10 K did the addition of the insert somewhat improve the heat transfer performance. However, the pressure drop for the finned tube with insert was increased. Empirical correlations for friction factors for the three tube configurations are proposed based on the present data

ENHANCED CONDENSATION OF R-113 AND STEAM USING THREE-DIMENSIONAL PIN-FIN TUBES

Experimental data are reported for condensation of R-113 and steam on six three-dimensional pin-fin tubes. Enhancements of the vapor-side heat transfer coefficient between 3.6 and 9.9 were found for R-113 and between 2.4 and 2.9 for steam when compared to a plain tube at the same vapor-side temperature difference. For R-113, heat transfer enhancement was in all cases approximately double the increase in surface area, while for steam it was largely independent of increase in surface area. Condensate retention between the pins on the lower part of the tubes was lower than for comparable two-dimensional fin tubes but was still significant.

Effects of Vapor Velocity and Pressure on Marangoni Condensation of Steam-Ethanol Mixtures on a Horizontal Tube

Careful heat-transfer measurements have been conducted for condensation of steamethanol mixtures flowing vertically downward over a horizontal, water-cooled tube at pressures ranging from around atmospheric down to 14kPa. Care was taken to avoid error due to the presence of air in the vapor. The surface temperature was accurately measured by embedded thermocouples. The maximum vapor velocity obtainable was limited by the maximum electrical power input to the boiler. At atmospheric pressure this was 7.5m/s while at the lowest pressure a velocity of 15.0m/s could be achieved. Concentrations of ethanol by mass in the boiler when cold prior to start up were 0.025%, 0.05%, 0.1%, 0.5%, and 1.0%. Tests were conducted for a range of coolant flow rates. Enhancement of the heat-transfer coefficient over pure steam values was found by a factor up to around 5, showing that the decrease in thermal resistance of the condensate due to Marangoni condensation outweighed diffusion resistance in the vapor. The best performing compositions (in the liquid when cold) depended on vapor velocity but were in the range 0.025‐0.1% ethanol in all cases. For the atmospheric pressure tests the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 15K, increased from around 55kW/m 2 K to around 110kW/m 2 K as the vapor velocity increased from around 0.8 to 7.5m/s. For a pressure of 14kPa the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 9K, increased from around 70kW/m 2 K to around 90kW/m 2 K as the vapor velocity increased from around 5.0 to 15.0m/s. Photographs showing the appearance of Marangoni condensation on the tube surface under different conditions are included in the paper. [DOI: 10.1115/1.4007893]

Condensation on a horizontal wire-wrapped tube

Measurements for film condensation of steam, R113 and ethylene glycol on a horizontal wire-wrapped tube are reported. All measurements were made at near atmospheric vapor pressure and with coolant at around 20 °C. Care was taken to avoid error due to the presence of air in the vapor. Complete wetting (film condensation) was observed in all cases. Wire diameter and pitch of winding were systematically varied and heat-transfer measurements made for a range of coolant flow rates. Data, in the form of heat flux and vapor-to-surface temperature difference, are presented. These were used to determine enhancement ratios (ratio of heat flux or heat-transfer coefficient for a wire-wrapped tube to the corresponding value for a plain tube at the same vapor-to-surface temperature difference). Enhancement ratios exceeding 3 for R113 and 2 for steam and ethylene glycol were obtained. The results are discussed in the light of earlier measurements and theory.

CONDENSATION PERFORMANCE OF SOME COMMERCIAL INTEGRAL FIN TUBES WITH STEAM AND CFC113

Integral fin tubes are used in some shell-side condensers to enhance the vapor-side heat transfer coefficient. Advanced manufacturing techniques have meant that tubes with complex fin profiles can be produced. In the present article, experimental heat transfer coefficients are presented for 12 commercially manufactured integral fin tubes, with a variety of fin profiles and dimensions, for condensation of CFC113 and steam. Best enhancement ratios of 3·3 for steam and 7·7 for CFC113 were found, for tubes with trapezoidal-cross-section fins and 26 and 50 fins per inch, respectively. The more complex Y-and T-profile finned tubes performed less well than the best of those with conventional approximately trapezoidal-cross-section fins. The results show good agreement with a recent model.

Condensation From Pure Steam and Steam–Air Mixtures on Integral-Fin Tubes in a Bank

Data are reported for condensation of steam with and without the presence of air on three rows of integral-fin tubes situated in a bank of plain tubes. The data cover a wide range of vapor velocities and air concentrations. Unlike previously reported data for plain tubes using the same test bank and apparatus, the heat-transfer coefficients for the finned tubes were largely unaffected by vapor velocity.

Condensation of ethylene glycol on integral-fin tubes : Effect of fin geometry and vapor velocity

New experimental data are reported for forced-convection condensation of ethylene glycol on a set of nine single, copper, integral-fin tubes. The first set of five tubes had fin height and thickness of 1.6 and 0.25 mm, respectively, with fin spacings of 0.25, 0.5, 1.0, 1.5, and 2.0 mm. The second set of four tubes had fin spacing and thickness of 1.0 and 0.5 mm, respectively, and fin heights of 0.5, 0.9, 1.3, and 1.6 mm. The fins were rectangular in cross section. All tubes had a fin root diameter of 12.7 mm. A plain tube of outside diameter 12.7 mm was also tested. The tests, which were performed at a near constant pressure of ∼15 kPa, covered vapor velocities between 10 and 22 m/s and a wide range ofheat fluxes. The best performing tube was that with fin spacing, height, and thickness of 0.5, 1.6, and 0.25 mm, respectively, which had an enhancement ratio (compared to the plain tube at the same vapor-side temperature difference and vapor velocity) of 2.5 at the lowest vapor velocity tested, increasing to 2.7 at the highest. For all but two of the tubes, the effect of vapor velocity on the heat-transfer coefficient of the finned tubes was less than on the plain tube, leading to a decrease in enhancement ratio with increasing vapor velocity. For two of the tubes, however, the enhancement ratio increased with increasing vapor velocity, which is the opposite trend to that found in most earlier experimental studies. This effect was thought to be due to the slight reduction in condensate flooding between the fins of these two tubes because of vapor shear.

Condensation of steam and R113 on a bank of horizontal tubes in the presence of a noncondensing gas

Abstract Data are presented for condensation from steam-air and R113-air mixtures on a bank of horizontal tubes. The test bank consisted of 10 staggered rows of four and five tubes per row. Good agreement with single-tube theory was found for the top rows when account was taken of the variation in bulk air concentration, vapor velocity, and temperature down the bank. Further down the bank, where air concentrations were higher and vapor velocities lower, discrepancies appeared between theory and experiment that were thought to be due to bouyancy effects influencing the buildup of air on the lower rows.