Vapor-side Heat Transfer Coefficient Research Articles

Grafted Nanofilms Promote Dropwise Condensation of Low-Surface-Tension Fluids for High-Performance Heat Exchangers

Summary We have designed grafted polymer coatings using initiated chemical vapor deposition (iCVD), a solvent-free, substrate-independent, vapor-phase deposition process, that are capable of stable dropwise condensation of low-surface-tension fluids. Our iCVD coatings effectively increase the vapor-side heat transfer coefficient of the condenser, are stable under industrial environments, and can be applied in a scalable fashion. We explore how tuning the grafting of these iCVD films can minimize the contact angle hysteresis and impact droplet shedding and ultimately heat transfer performance. We measure a 4- to 8-fold enhancement in vapor-side heat transfer coefficients on a variety of substrates including industrially relevant heat exchanger metals such as steel and titanium. We demonstrate that iCVD can be applied to tubular geometries, which allows for direct translation to shell-and-tube heat exchangers, used across industries. Finally, we estimate the overall cycle efficiency improvement our coatings can provide to such processes using an example organic Rankine cycle.

Heat Transfer and Pressure Drop During Laminar Annular Flow Condensation in Micro-Channels

There is wide disagreement between experimental investigations and correlations for heat transfer during condensation in micro-channels. The major problem is the fact that the vapor-side resistance is usually appreciably smaller than that on the coolant side so that methods where only the overall resistance is measured, and the vapor-side heat transfer coefficient obtained by subtraction of resistances, are prone to large uncertainty. A few more recent correlations, based mainly on data for R134a, are in fair agreement when predictions for R134a and for the same conditions are compared. The fact that wide discrepancies are found when the correlations are used for fluids with widely different properties indicates that some or all of the correlations do not capture all of the essential mechanisms. That closely similar predictions are found when the correlations are applied to R134a indicates that the datasets used in the different studies were in essential agreement. Similar comments apply for pressure drop. The special case of annular laminar condensate flow permits wholly theoretical solution without recourse to empirical input. For this mode of condensation and for specified fluid, channel geometry, flow parameters and tube wall temperatures, local heat transfer-coefficient, and local pressure gradient can be calculated as well as local quality and void fraction. The theory is outlined in the article, and recent developments are discussed. Comparisons with the correlations for heat transfer and pressure gradient are given. For the heat transfer coefficient, the results of the annular flow theory are in surprisingly good agreement with the correlations when applied to R134a. For ammonia, the theoretical results lie between the widely spread values obtained from the correlations. Results for pressure gradient given by the annular laminar flow model are generally lower than those given by the correlations.

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.

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.

HEAT TRANSFER AND PRESSURE DROP MEASUREMENTS FOR IN-TUBE CONDENSATION OF CFC-113 USING MICROFIN TUBES AND WIRE INSERTS

Accurate, repeatable heat transfer and pressure-drop measurements have been made for condensation of CFC-113 with downflow inside enhanced microfin tubes and tubes containing twisted-wire inserts. In the latter case measurements have also been made for CFC-113/air mixtures. The heat transfer rate was calculated from the coolant flow rate and temperature rise, the latter measured using a 10-junction thermopile with careful attention paid to adequate coolant mixing and isothermal immersion of the thermopile leads. The surface temperature was found from thermocouples embedded in the tube wall. One plain tube, nine microfin tubes (with different fin heights, helix angles, and number of fins), and four twisted-wire inserts (with different wire densities) were tested. Enhancement ratios (i.e., vapor-side heat transfer coefficient for the enhanced tube divided by that for a smooth tube at the same vapor-side temperature difference and vapor inlet velocity) between 1.6 and 5.6 for the microfin tubes and between 1.2 and 1.6 for the twisted-wire inserts were found, with values depending on vapor-side temperature difference, vapor inlet velocity, and air inlet mole fraction in the case of CFC-113/air mixtures. The microfin tubes showed moderate pressure-drop penalties of around 50% compared to the plain tube, while the twisted-wire inserts showed increasing pressure-drop penalty with increasing wire density.

Effects of a wavy interface on steam-air condensation on a vertical surface

A correlation of vapor-side heat transfer coefficients is proposed for condensation of a steam-air mixture on a vertical surface. The correlation is based on the analogy between heat and mass transfer and includes the effects of the wavy interface between a condensate film and bulk vapor on the condensation. For a film Reynolds number less than 350, the vapor-side heat transfer coefficients are well correlated as a function of vapor Reynolds number, air-mass fraction, Prandtl number, and Schmidt number within ±10% error. The ratio of the enhanced heat transfer coefficients for the wavy interface to those with a film Reynolds number less than 350 is well correlated as a function of film Reynolds number and vapor Reynolds number within ±15% error. The film Reynolds numbers range from about 40 to 17,600 and the vapor Reynolds numbers from about 77,000 to 500,000. The air-mass fractions also range from 0.1 to 0.7.

Condensation of pure steam and steam-air mixture with surface waves of condensate film on a vertical wall

A series of experiments has been conducted to investigate the effect of wavy interface on film condensation with and without a noncondensable gas present on a vertical wall. Concurrently, the instantaneous film thickness was measured. Overall heat transfer coefficients across the condensate film and the diffusion layer formed by a noncondensable gas were obtained in various parameters such as air-mass fraction (W = 0.1–0.7), mixture vapor velocity (Uin = 3, 5, 7 m/s) and condensate film Reynolds number (Reff,in= 0–19,000). In order to investigate the heat transfer enhancement induced by a dynamic interaction of the surface waves on condensate and the diffusion layer, vapor-side heat transfer coefficient was estimated from the measured overall heat transfer coefficient and the correlation of film-side heat transfer coefficient, and compared with that estimated by Colburn and Chilton-Colburn analogy for a smooth flat plate. The comparison showed that the vapor-side heat transfer coefficient increased clearly with the increase of film Reynolds number. The enhancement effect was dominant for low mixture vapor velocity and decreased with the increase of mixture vapor velocity.

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.

Accurate Heat Transfer Measurements for Condensation on Horizontal, Integral-Fin Tubes

In most earlier experimental investigations of condensation on low-fin tubes, vapor-side heat transfer coefficients have been found from overall (vapor-to-coolant) measurements using either predetermined coolant-side correlations or “Wilson plot” methods. When the outside resistance dominates, or is a significant proportion of the overall resistance, these procedures can give satisfactory accuracy. However, for externally enhanced tubes, and particularly with high-conductivity fluids such as water, significant uncertainties may be present. In order to provide reliable, high-accuracy data, to assist in the development of theoretical models, tests have been conducted using specially constructed plain and finned tubes fitted with thermocouples to measure the tube wall temperature, and hence the vapor-side heat transfer coefficient, directly. The paper describes the technique for manufacturing the tubes and gives results of systematic heat transfer measurements covering the effects of fin height, thickness, and spacing, tube diameter, and vapor velocity. The tests were carried out with steam, ethylene glycol, and R-113, with vertical vapor downflow. The heat flux was measured using an accurately calibrated 10-junction thermopile and paying particular attention to coolant mixing and isothermal immersion of thermocouple junctions. Care was taken to avoid errors due to the presence in the vapor of noncondensing gas and the occurrence of dropwise condensation. Smooth, consistent, and repeatable results were obtained in all cases. The data are presented in easily accessible form and are compared with the results of previous investigations, where indirect methods were used to determine the vapor-side data, and with theory.

An Experimental Study of R-113 Film Condensation on Horizontal Integral-Fin Tubes

Heat transfer measurements were made at near-atmospheric pressure on a smooth tube, on 24 integral-fin tubes having machined, rectangular-shaped fins, and on a commercial integral-fin tube. All tubes were made of copper. The vapor flowed vertically downward with a nominal velocity of 0.4 m/s. Vapor-side heat transfer coefficients were determined with a typical uncertainty of ± 7 percent using a “modified Wilson plot” technique. The vapor-side heat transfer coefficient of the integral-fin tubes (based upon the outside surface area of the smooth tube) was enhanced considerably more than the surface area enhancement provided by the fins. Heat transfer enhancements (for the same vapor-to-wall temperature difference) up to around 7 were measured for a corresponding area enhancement of only 3.9. The optimum fin spacing was found to lie between 0.2 and 0.5 mm, depending upon fin thickness and height. The data were compared with those of other investigations and with several existing theoretical models. Visual observations of condensate drainage patterns from the finned tubes were also made.

Forced Convection Condensation on a Horizontal Tube—Experiments With Vertical Downflow of Steam

The paper reports an experimental investigation into the condensation of steam flowing vertically downward over a single horizontal tube. A limited number of experiments with steam-nitrogen mixtures are also reported. For the case of “pure” steam, data have been obtained at near atmospheric pressure for vapor approach velocities in the range 5 to 81 m/s and with superheats in the range 2 to 40 K. The results are compared with theory and earlier experimental data for steam and other fluids. The vapor-side heat transfer coefficients were found to increase with velocity. For vapor velocities in excess of 30 m/s, the rate of increase of the vapor-side coefficient was greater than predicted by laminar condensate flow theory. This behavior has also been observed by earlier workers for refrigerant-113 and ethylene glycol and may indicate onset of turbulence in the condensate film. Superheat had insignificant effect on the heat transfer coefficient for the condensate film. The results for steam-nitrogen mixtures were generally in good agreement with existing equations for the “gas-layer” resistance.

Condensation of Ethylene Glycol on Horizontal Integral-Fin Tubes

The problem of heat transfer during condensation on horizontal finned tbes is extremely complex. The difficulty arises from the essentially three-dimensiona character of the condensate flow, together with significant surface tension effects. The fact that the heat transfer enhancement can significantly exceed the surface area augmentation may be attrubutable in part to the small effective plate heights associated with the fin flanks and also to the effects of surface tension in locally thinning the condensate film. Progress has been made toward the theoretical understanding of the heat transfer in locally thinning the condensate film. Progress has been made toward the theoretical understanding of the heat transfer problem and for the related phenomenon of static liquid retention on horizontal finne tubes. Experimentally, the problem is made difficult by the requirement for high accuracy in measuring the relatively high vapor-side heat transfer coefficient and the need to avoid errors that can result from the presence of noncondensing gas or from the occurrence of dropwise condensation. The large number of variables involved also presents significant difficulty. The present note reports the continuation of a systematic experimental study. New data for ethylene glycol are given here.

On the mechanism determining the transition mode from dropwise to film condensation

Different modes of transition from dropwise to film condensation were investigated analytically and experimentally. Two types of transition modes, the continuous and jump modes, were observed. The dominant mechanism of heat transfer was discussed and the controlling factors were analyzed. A close agreement between the measurement and the analysis was obtained. It was confirmed that the modes of transition were determined by the gradient of the condensation curve on the bilogarithmic coordinates and by the ratio of the vapor-side heat transfer coefficient to the overall cooling-side conductance.

Effect of Fin Spacing on the Performance of Horizontal Integral-Fin Condenser Tubes

The dependence of heat transfer performance on fin spacing has been investigated for condensation of steam on horizontal integral-fin tubes. Thirteen tubes have been used with rectangular section fins having the same width and height (0.5 mm and 1.6 mm) and with fin pitch varying from 1.0 mm to 20.5 mm. For comparison, tests were made using a plain tube having the same inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. All tests were made at near-atmospheric pressure with vapor flowing vertically downward with velocities between 0.5 m/s and 1.1 m/s. The observed heat transfer enhancement for the finned tubes significantly exceeded that to be expected on grounds of increased area. Plots of enhancement against fin density were repeatable and showed local maxima and minima. The dependence of enhancement on fin density did not depend appreciably on vapor velocity or condensation rate for the ranges used. The maximum vapor-side enhancement (i.e., vapor-side heat transfer coefficient of finned tube/vapor-side coefficient for plain tube) was found to be around 3.6 for the tube with a fin spacing of 1.5 mm.

On the heat transfer in dropwise condensation

A population balance model is used, for the case when no experimental data or theoretical formulae are available, to derive the drop size distribution of small drops which grow mainly by direct condensation. This is combined with the well-known size distribution for large drops which grow mainly by coalescence and with the growth rate of single drops to compute the heat flux during the process of dropwise condensation. The computation can be simplified by neglecting all the relatively unimportant resistances except the resistance to heat conduction through the drop body. The vapor-side heat transfer coefficient of dropwise condensation can be estimated sufficiently accurately by the simple equation ▪ where r e is half the mean spacing between the active nucleation sites on the substrate surface and r max is the maximum drop radius. A method of computing the drop nucleation rate that is independent of the classical approach is also presented; the heat flux can be estimated from the nucleation rate.