Condensate Retention Research Articles

Experimental Investigations on the Performance Enhancement Using Minichannel Evaporator with Integrated Receiver-Dryer Condenser in an Automotive Air Conditioning System

ABSTRACTThe performance comparison of two automotive air conditioning systems was experimentally studied under two typical cabin conditions viz; 27°C dry bulb temperature / 40% relative humidity and 40°C dry bulb temperature /40% relative humidity in a bench test rig for different blower speeds of the evaporator. Two mobile air conditioning systems viz; enhanced system having minichannel evaporator with integrated receiver-dryer condenser, and baseline system having conventional serpentine evaporator with parallel flow condenser were considered for the study. The compressor was the same for both the systems. The charge quantity, compressor speed, condenser air flow rate were suitably modified to maintain similar suction / discharge pressures, suction super heat and sub-cooling at all test conditions. The percentage of condensate retention during the dehumidification for both the evaporators were also compared. The comparison indicates that the average coefficient of performance of the enhanced system is higher in both low and high cabin temperature condition by about 15% and 8% respectively. The percentage of condensate retention in the evaporator of the enhanced system is lesser in the range of about 17–31% as compared to the base line system evaporator. It is also expected that a specific design of compressor for the enhanced system can yield better performance at all conditions.

Marangoni condensation of steam-ethanol mixtures on a horizontal low-finned tube

Abstract Careful heat-transfer measurements have been conducted for condensation of steam-ethanol mixtures in vertical downflow over a horizontal, water-cooled, low-finned copper tube. Care was taken to avoid error due to the presence of air in the vapor. The tube had diameter at the fin root 12.7 mm and rectangular section fins with height 1.6 mm, thickness 0.5 mm and space between fins 1.0 mm. Tests were conducted at pressures of atmospheric, 55 kPa and 14 kPa. 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%. The highest vapor velocity at approach to the tube was 7.5 m/s at atmospheric pressure and 15.0 m/s at vapor pressure 14 kPa. Effects of ethanol concentration on both retention angle and heat transfer were measured. The retention angle was strongly dependent on the vapor velocity and ethanol concentration which affected the condensation rate, composition and temperature of the condensate at the interface and consequently the surface tension of condensate. The results are compared with data for pure steam on the same finned tube and Marangoni condensation on a smooth tube under the same conditions. Similarly, results for condensate retention of water on finned tube are compared with earlier data and theoretical model. Vapor-side, heat-transfer coefficients were obtained by subtraction of coolant side and test tube wall thermal resistances from overall measurements.

Effect of circumferential pin thickness on condensate retention as a function of vapor velocity on horizontal pin-fin tubes

This paper reports the effect of circumferential pin thickness on retention angle as a function of vapor velocity on four pin-fin tubes (varying in circumferential pin thickness from 0.5 mm to 2.0 mm). Three fluids (namely water, ethylene glycol and R-141b) with high, intermediate and low surface tension to density ratios were tested. Experimentation was performed by providing downward air (to simulate vapor) through a vertical wind tunnel with velocities from 0 to 18 m/s. By providing small holes on the upper side of tube a continuous flow of condensate was ensured along the circumference of the test tubes. At low approaching zero vapor velocity; an increase in circumferential pin thickness caused a decrease in retention angle (an angle measured from top of test tube to the point of flooded flank in circumferential direction) in case of pin-fin tubes for all fluids tested. At high vapor velocity; the role of vapor shear was less effective on the upper half of pin-fin tubes when compared to the equivalent integral-fin tube (i.e. with same longitudinal fin spacing, tooth thickness, tooth height, inner and outer diameter as that of pin-fin tubes). This less effective role of vapor shear on the upper half of pin-fin tubes, when compared with integral-fin tube is thought to be due to the presence of trapped condensate in between the horizontal cuts (circumferential pin spacing) available on pin-fin tubes, which tends to resist the downward motion of the condensate. On the lower half of test tubes (as in case of test fluid R-141b) vapor velocity showed no effect on retention angle for all pin-fin tubes while for the case of equivalent integral-fin tube, retention angle was decreased with the increase of vapor velocity.

Effect of vapour velocity on condensate retention on horizontal pin-fin tubes

New experimental data for condensate retention angle as a function of vapour velocity (0–19m/s) are reported on six horizontal pin-fin tubes and an equivalent integral-fin tube (i.e. with same longitudinal fin spacing, tooth thickness, tooth height, inner and outer diameter as that of pin-fin tubes) using water, ethylene glycol and R-141b. Only geometric parameter varied was the circumferential pin spacing. For all tubes tested, an increase in vapour velocity causes an increase in condensate retention angle for the cases when retention angle was less than 90° at low approaching zero vapour velocity. For the cases when the retention angle was greater than 90° at low approaching zero vapour velocity, vapour velocity shows negligible effect on retention angle for all pin-fin tubes, while for the case of integral-fin tube (i.e. using R-141b as a test fluid where retention angle is greater than 90° at low approaching zero vapour velocity) retention angle decreased with increasing vapour velocity.

Measurements and semi-empirical correlation for condensate retention on horizontal integral-fin tubes: Effect of vapour velocity

Present study is based on simulated condensation using three fluids (water, ethylene glycol and R-141b) on integral-fin tubes, horizontally oriented in a vertical wind tunnel. Eight tubes of different dimensions were used with fixed fin root diameter and internal diameter of tubes which were kept 12.7 mm and 8.0 mm respectively. Fluid, from a tank mounted above the tube at considerable height flows through the small drilled holes in between the fins. Each tube was tested with a constant flow rate of fluid i.e. the flow rate was adjusted to a minimum value that was just sufficient to produce a smooth film of condensate. Air was used to simulate vapour with a variable velocity from 0 to 19 m/s. A hot wire anemometer was mounted just above the tube into the wind tunnel to measure the air velocity. Observation windows were available on both sides of test section to visualize the condensate retention on integral-fin tubes. Two digital cameras were mounted on both sides of test section to capture the photographs of retention angle on tubes. Condensate retention showed identical behavior on both sides of tube over the tested range of vapour Reynolds number. Retention angle is found to be a strong function of vapour Reynolds number, as retention angle increases with increasing vapour Reynolds number for fluids which have higher value of surface tension to density ratio, however, retention angle decreases with increasing vapour Reynolds number for fluids which have lower value of surface tension to density ratio (i.e. refrigerants). A semi-empirical correlation accounting for the effect of vapour velocity on retention angle is developed that predicts most of the present and earlier experimental data to within ±15% (for water and ethylene glycol) and to within ±5% (for refrigerants).

An investigation of condensate retention on pin-fin tubes

Retention angle measurements on 15 rectangular pin-fin tubes are made under static conditions (without condensation) using water, ethylene glycol and R-113. Condensate retention angles found to be considerably larger than the equivalent integral-fin tubes (i.e. with the same fin height, root diameter and longitudinal pin thickness and spacing) in a range of 5%–60%. An expression for condensate retention angle on pin-fin tubes was proposed and found to agree with the measured retention angles to within 15%.

Topography-Based Surface Tension Gradients to Facilitate Water Droplet Movement on Laser-Etched Copper Substrates

This paper describes a method for creating a topography-based gradient on a metallic surface to help mitigate problems associated with condensate retention. The gradient was designed to promote water droplet migration toward a specified region on the surface which would serve as the primary conduit for drainage using only the roughness of the surface to facilitate the movement of the droplets. In this work, parallel microchannels having a fixed land width but variable spacing were etched into copper substrates to create a surface tension gradient along the surface of the copper. The surfaces were fabricated using a 355 nm Nd:YVO4 laser system and then characterized using spray testing techniques and water droplet (2-10 μL) injection via microsyringe. The distances that individual droplets traveled on the gradient surface were also measured using a goniometer and CCD camera and were found to be between 0.5 and 1.5 mm for surfaces in a horizontal orientation. Droplet movement was spontaneous and did not require the use of chemical coatings. The theoretical design and construction of surface tension gradients were also explored in this work by calculating the minimum gradient needed for droplet movement on a horizontal surface using Wenzel's model of wetting. The results of this study suggest that microstructural patterning could be used to help reduce condensate retention on metallic fins such as those used in heat exchangers in heating, ventilation, air-conditioning, and refrigeration (HVAC&R) applications.

CAPE in Tropical Cyclones

Abstract Convective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity. Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. Without entrainment, downshear CAPE was smallest in the core and increased outward to the 600-km radius. The large number of sondes allowed the examination of soundings at the 90th percentile of conditional instability, which reflect the conditions leading to the most vigorous updrafts. Observations of convection in tropical cyclones prescribe the correct method for calculating this conditional instability. In particular, the abundance and distribution of vigorous deep convection is most accurately reflected by calculating CAPE with condensate retention and a fractional entrainment rate in the range of 5%–10% km−1.

Condensation of R-113 on Pin-Fin Tubes: Effect of Circumferential Pin Thickness and Spacing

New experimental data are reported for condensation of R-113 at near atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The only geometric parameters varied were circumferential spacing and thickness, since these have been shown to have a strong effect on condensate retention on pin-fin tubes. Heat transfer enhancement was found to be strongly dependent on the active-area enhancement, i.e., on the parts of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapor-side heat transfer enhancements were found to be approximately 2.5 times the corresponding active-area enhancements, and this finding was in line with earlier data for R-113. An increase in the vapor-side heat transfer enhancement is noticed with the decreasing values of pin spacing. The best performing pin-fin tube gave a heat transfer enhancement about 14% higher than the “equivalent” two-dimensional integral-fin tube (i.e., with the same fin root diameter, longitudinal fin spacing, and thickness and fin height).

Effect of vapour velocity on condensate retention between fins during condensation on low-finned tubes

The paper reports experimental results using simulated condensation on horizontal finned tubes in a vertical wind tunnel. Condensation was simulated using three liquids (water, ethylene glycol and R113) supplied to the tube via small holes between the fins along the top generator of the tubes. Eight tubes with different fin dimensions were used. The results indicate that when the retention angle (measured from the top of the tube to the position where the interfin space is completely filled with liquid) is less than about π/2 at zero air velocity, it increases with increasing air velocity. On the other hand, for cases where the retention angle is greater than about π/2 at zero air velocity it decreases with increasing air velocity. In all cases (all fluids and all tubes tested) the retention angle approaches a value near π/2 with increasing air velocity. Satisfactory agreement was found with observations taken during condensation.

Condensation of ethylene glycol on pin-fin tubes: Effect of circumferential pin spacing and thickness

New experimental data are reported for condensation of ethylene glycol at atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The only geometric parameters varied were circumferential pin spacing and thickness, since these have been shown to have a strong effect on condensate retention on pin-fin tubes. Heat-transfer enhancement was found to be strongly dependent on the ‘unflooded’ area enhancement i.e. on the parts of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapour-side, heat-transfer enhancements were found to be approximately 2.5–3 times the corresponding unflooded area enhancements. An increase in the vapour-side, heat-transfer enhancement is noticed with decreasing values of pin spacing. The best performing pin-fin tube gave a heat-transfer enhancement of 4.9; equal to those obtained from an ‘optimised’ two-dimensional fin-tube reported in the literature and about 20% higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with same fin-root diameter, longitudinal fin spacing and thickness and fin height).

Enhancement of condensation on a vertical plate (2nd report, prediction of condensation on a dispersed finned surface)

AbstractIn this study, a prediction model for condensation heat transfer on a vertical dispersed finned surface was proposed, utilizing the Adamek‐Webb model for condensation heat transfer outside a horizontal finned tube. The prediction model was based on two main experimental observation results. One is the phenomena of the condensate retention at the bottom of each row of the dispersed fin. Another is the offset phenomena of the condensate flow between each row of the dispersed fin. Given the results by the present model, it is predicted that the dependence of the condensation heat transfer coefficient for the dispersed finned surface on the fin pitch is controlled mainly by the dispersed fin length, not the total fin length. On the contrary, for a different fin pitch, the effect to the condensation heat transfer by dispersing the fin is different. From comparison with the experiment results, it is confirmed that the present model was able to predict the condensation with extremely good precision when the fin pitch was larger. Further, when the fin pitch was smaller, the predicted values were higher than the experimental values, but the tendency of the condensation heat transfer with dispersing the fin was nearly predicted. In addition, this condensing model can predict the experimental values with an error of 25% at the maximum in a range of fin pitch 0.6 mm to 1 mm. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20288

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.

Air-Side Surface Wettability Effects on the Performance of Slit-Fin-and-Tube Heat Exchangers Operating Under Wet-Surface Conditions

A study of condensate retention and the attendant thermal-hydraulic effect associated with changes in air-side surface wettability is presented for a series of slit-fin-and-tube heat exchangers of identical geometry with controlled wettability covering a wide range of contact angles. An experiment in which the retained mass of air-side condensate was measured under dynamic conditions is described, and the results are analyzed using conventional thermal-hydraulic measurements of j and f. The data demonstrate that for the heat exchangers used in this study, the j factor is not strongly influenced by condensate retention, but the friction factor is significantly reduced for surfaces of increased wettability. Hydrophilic heat exchangers retain much less air-side condensate than do the hydrophobic heat exchangers, and the amount of retention is found to depend on the air-side Reynolds number (Redh) and the rate of latent heat transfer (Ql). Based on an assumption of filmwise condensation, a new model for predicting the mass of retained condensate is described and compared with the steady-sate retention data. The model is successful in predicting retained condensate over a wide range of tested conditions. The potential of this new approach and possible refinements that will add engineering value are discussed.

A surface embossing technique to create micro-grooves on an aluminum fin stock for drainage enhancement

Recent advances using a surface embossing technique allow us to inexpensively impart micro-scale surface features on heat exchanger construction material, which we exploit to reduce condensate retention. The retention of condensate is important to the performance of heat exchangers in a broad range of air-cooling and dehumidification applications. We report a study of wetting behavior and drainage performance on a series of embossed surfaces with different micro-groove dimensions. Static contact angles, critical sliding angles, droplet aspect ratios, etc, are reported with detailed surface topographical information. Our results show that unless the spacing between grooves is very large (>100 µm), the parallel-grooved surface feature normally increases the apparent contact angle of a droplet on the surface. The micro-groove structure causes anisotropic wetting behavior of the droplets, and apparent contact angles measured by viewing along with the micro-grooves (θ⊥) were found to be larger than those measured from the other direction (θ‖) (by viewing perpendicular to the grooves). A consistent reduction of a critical sliding angle was observed on surfaces after embossing (the micro-grooves are aligned to be parallel with gravity). This may be due to contact line discontinuities and contact-line pinning induced by a groove structure of the surface. Water droplets exhibit an elongated shape along the micro-grooves, which is in contrast to the nearly circular base contour observed on an isotropic surface. Smaller groove spacing, larger depth and steeper sidewalls are observed to be favorable for drainage enhancement and recommended as design guidelines in the future.

Experimental analysis of ammonia condensation on smooth and integral-fin titanium tubes

This paper reports on the experimental research conducted to study the condensation of ammonia on smooth and integral-fin (32 fpi) titanium tubes of 19.05 mm outer diameter. Experiments were carried out at saturation temperatures of 30, 35, 40 and 45 °C and wall subcoolings from 1 to 8 °C. The results show that the condensation coefficients on the smooth tubes are well predicted by the Nusselt theory with an average error of +0.66% and within a deviation between −6.6% and +8.3%. The enhancement factors provided by the integral-fin tubes range from 0.77 to 1.22. The low enhancement factors are due to the high condensate retention between fins, which brings about flooded fractions of the tube perimeter from 62.9% to 73.2%, and the low thermal conductivity of titanium. The Briggs and Rose [1994. Effect of fin efficiency on a model for condensation heat transfer on a horizontal, integral-fin tube. Int. J. Heat Mass Transfer 37, 457–463.] model, which accounts for the conduction in the fins, predicts the experimental data with a mean overestimation of 20%. The analysis of the partial thermal resistances in the overall heat transfer process points out the convenience of enhancing the outside ammonia condensation when high water velocities are considered inside the tubes.

Condensation enhancement on a vertical plate (1st report, heat transfer characteristic on a dispersed finned surface)

AbstractIn this study, a condensation heat transfer experiment on vertical continuous and dispersed finned surfaces using FC5312 was carried out. Experimental parameters were the pitch and height of the fin, and the dispersed fin length. In the results, the phenomena of condensate retention were observed in the bottom of each row of the dispersed fin. The condensate flow from the upper row was concentrated into the valley of the fin and then flowed down into the valley of the next fin. Moreover, it was found from the experiment that the heat transfer coefficient on the dispersed finned surface was lower than the one on the continuous finned surface as the fin pitch was smaller, but was larger than that of the continued finned surface for a larger fin pitch. Furthermore, the heat transfer enhancing effect became more significant for the higher fin with the larger fin pitch, and the heat transfer reducing effect became more significant for the lower fin with the smaller fin pitch. These special characteristics of condensation mentioned above were caused by the phenomena of condensate retention in each row of the fin and the flow pattern of the condensate between two adjacent fins on the dispersed finned surface based on experimental observations. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20221

Enhancement of Condensation on a Vertical Plate (1st Report, Heat Transfer Characteristic on a Dispersed Finned Surface)

In this study, a condensation heat transfer experiment on vertical continued and dispersed finned surfaces using FC 5312 was carried out. Experimental parameters were the pitch and height of the fin, and the dispersed fin length. As the results, the phenomena of condensate retention were observed in the bottom of each row of the dispersed fin. The condensate flow from the upper row was concentrated into the valley of the fin and then flew down into the valley of next fin. Moreover, it was found from the experiment that the heat transfer coefficient on the dispersed finned surface was lower than the one on the continued finned surface as the fin pitch was smaller, but was larger than the one on the continued finned surface for the larger fin pitch. Further, the heat transfer enhancing effect become more significant for the higher fin with the larger fin pitch, and the heat transfer reducing effect become more significant for the lower fin with the smaller fin pitch. These special characteristics of condensation mentioned above were caused by the phenomena of condensate retention in each row of the fin and the flow pattern of the condensate between two adjacent fins on the dispersed finned surface based on experimental observations.

Condensate retention on a louver-fin-and-tube air cooling coil

This paper presents a study of condensate retention on a louver-fin-and-tube air cooling coil, which is commonly used in air conditioning (A/C) systems. Compared to previously related work focusing on the influence of condensate retention on the heat and mass transfer between air and a cooling coil, the present study emphasizes the impacts of operating parameters on condensate retention on a cooling coil. A new method to describe the steady-state condensation has been suggested and a new mathematical model to represent the force balance of retained condensate developed. The mass of condensate retained has been measured experimentally under various operating conditions of a direct expansion (DX) air cooling and dehumidification system. The influences of air dry-bulb temperature, moisture content and Reynolds Number on condensate retention are discussed. The model developed relates the mass of condensate retained to condensing rate, and is successful in predicting the trends of condensate retention under normal operating conditions for air cooling applications.

A Model for Condensate Retention on Plain-Fin Heat Exchangers

A model has been developed for predicting the amount of condensate retained as drops on the air-side of heat exchangers operating under dehumidifying conditions. For a coil with a given surface wettability, characterized by the advancing contact angle, the maximum diameter for a retained drop is obtained from a balance between gravitational and surface tension forces. A logarithmic function is used to describe the size-distribution of drops on fins, based on the fraction of fin-area covered by liquid. The volumes of individual drops are calculated by a geometric method for approximating the three-dimensional shapes of drops on vertical and inclined surfaces. The total volume of condensate accumulated on a coil is then found by multiplying the size-distribution and volume functions and integrating over all drop diameters. The model is successful in predicting measurements by other researchers of the mass of condensate retained on plain-fin heat exchangers. The critical fin spacing to avoid the formation of condensate bridges is also predicted.