Condensation Of Pure Vapor Research Articles

Effect of hydrophobic coating on optimization of dropwise condensation of steam on hybrid surfaces

Dropwise condensation of pure vapor on hybrid surfaces, characterized by alternate hydrophobic-hydrophilic regions, is numerically investigated via phenomenological, Lagrangian modelization of dropwise condensation on hydrophobic regions. The drop size distribution over the hydrophobic domain, which knowledge is crucial for development of accurate simplified, statistically based models, is computed. Comparison with literature correlations shows that the theoretical correlation for size distribution of small droplets gives an inaccurate estimate, while the improved correlation, derived from the droplet population balance, better describes the numerical size distribution and, if incorporated in the statistically based model, allows to accurately predict the condensing flux.

VISUALIZATION OF DROPWISE CONDENSATION OF WATER VAPOR FROM EXPERIMENTS AND SIMULATION

Hydrophobic surfaces on which water vapor condensation is encountered in the form of droplets rather than a film is the preferred mode of condensation because of the large heat transfer coefficient and water production rate. The droplet pattern appearing on the surface depends on its orientation and the hydrophobicity characteristics. In the present work, patterns of dropwise condensation of vapor on various engineered surfaces were recorded from experiments and compared with simulations under various conditions of subcooling for pure vapor as well as moist air. Overall, condensation patterns reveal that droplet sliding frequency is higher for surfaces with large contact angles and small contact angle hysteresis. The growth rate of droplets of water vapor present in moist air is smaller than for pure vapor condensation.

Vapor condensation on micropillar structured surface with lattice Boltzmann method

Pure vapor condensation process and heat transfer on micropillar structured surfaces with various wettabilities are simulated numerically based on an improved three-dimensional phase-change lattice Boltzmann method (LBM). Condensed droplet dynamic behaviors containing nucleation, growth and departure are mainly explored. Two preferable droplet nucleation sites are detected successfully: at the corner between micropillar sides and subcooled substrate, and at the center of substrate. The effects of contact angle, micropillar spacing and height on droplet nucleation time, departure diameter and departure frequency are investigated quantitatively. Results indicate that as the contact angle increases, nucleation of condensate is delayed, while the departure diameter is reduced and departure frequency is increased. The increase in micropillar spacing, height and contact angle changes droplet from Wenzel state to partial wetting state. Moreover, it is also shown that the highest temperature and maximum local heat flux appear at the three-phase contact line. Despite the very complex roles of micropillar structures and wettability on condensation heat transfer, an optimal heat transfer surface with configurations of contact angle of 90°, micropillar spacing of 12 and height of 18 is presented. This study provides a fundamental understanding in regards to vapor condensation on microscale structured surface from numerical view.

Theoretical study on forced convection filmwise condensation inside converging channels in the presence of a non-condensable gas

In this paper, a theoretical study is performed on forced convection filmwise condensation inside a vertical converging channel in the presence of a non-condensable gas. Two solution schemes are developed with consideration of the turbulence effect in gas mixture and a laminar condensate film flow. Various heat transfer mechanisms are revealed by examining condensation heat transfer inside converging channels. In the present study, steam and air are taken as the vapor and non-condensable gas, respectively. It is found that the film velocity transits from a linear profile to a parabolic profile for a straight channel along the channel axial direction under the combined influence of gravitational force and interface shear stress. However, for a converging channel, the transition process from linear to parabolic velocity profiles is substantially delayed along the axial direction due to the enhanced interfacial shear stress in the latter condensing section. The film thickness decreases along the channel axial direction at the latter condensing section for both natural and forced convection condensation due to the substantially decreased local condensation rate and/or the enhanced interfacial shear stress in a converging channel. The combined influence of the curvature effect and enhanced interfacial shear stress leads the local heat transfer enhancement to increase substantially at the latter condensing region for forced convection condensation inside a converging channel. A uniform enhancement on the normalized local heat transfer coefficient in the inlet condensing section was found for both natural and forced convection condensation. However, the reduced condensation area has led the normalized local heat transfer coefficient to decrease substantially along the axial direction for converging channel as compared to straight channel in the case of natural convection condensation. Comparatively, the increased interfacial shear stress results in the normalized heat transfer coefficient along the entire channel to be enhanced. The average heat transfer coefficient increases as the converging angle or the Reynolds number increase. The turbulence effect can enhance the condensation heat transfer. However, the presence of air can cause a significant decrease in the condensation heat transfer. For the case of pure vapor condensation, the enhancement ratio increases as the Reynolds number increases. When the air is present in the system, an insufficient vapor supply is observed when the Reynolds number is low. Therefore, the enhancement ratio increases substantially as the converging angle increases. When the air mass fraction is high (W=0.4), the enhancement ratio appears to be insensitive to the Reynolds number in the laminar and turbulent regime, respectively. In addition, the enhancement ratio generally increases as the air mass fraction increases.

Theoretical and experimental study of water vapour condensation with high content of non-condensable gas in a vertical tube

This article deals with the possibility of separating water vapour from flue gases after oxyfuel combustion using condensation processes. Those processes can generally be described as condensation of water vapour in the presence of non-condensable gases. Hence, the effect of noncondensable gas (NCG) on the condensation process has been theoretically and experimentally analysed in this study. The theoretical model was developed on the basis of the heat and mass transfer analogy with respect to the effect of the NCG, the flow mode of the condensate film, the shear stress of the flowing mixture, subcooling and superheating. Subsequently, an experimental analysis was carried out on a 1.5m long vertical pipe with an inner diameter of 23.7mm. The mixture of vapour and air flowed inside the inner tube with an air mass fraction ranging from 23% to 62%. The overall heat transfer coefficients (HTC) from the theoretical model and experimental measurement are significantly lower than the HTC obtained according to the Nusselt theory for the condensation of pure water vapour. The overall HTC decreases along the tube length as the gas concentration increases, which corresponds to a decrease in the local condensation rate. The highest values of the HTC are observed in the condenser inlet, although a strong decrease in HTC is also observed here. Meanwhile, there is a possibility for an HTC enhancement through turbulence increase of the condensing mixture in the condenser outlet. Results also showed that the heat resistance of the mixture is several times higher than the heat resistance of the condensate film. The developed theoretical model based on heat and mass transfer analogy is in good agreement with experimental results with the standard deviation within +25% and −5%. The model is more accurate for lower NCG concentrations.

Comprehensive correlation for the prediction of the heat transfer through a single droplet in dropwise condensation regime

Numerical simulations have been performed to determine the conduction heat transfer in a sessile droplet for a large range of dynamic contact angle θ and Biot number B i . The substrate is set at a constant and uniform temperature, while a convective heat transfer is set at the liquid–vapor interface. In such a configuration, the heat flux is concentrated in the triple line region, so that numerical results can become inaccurate as the Biot number increases. A reference case in which the heat flux can be determined analytically has thus be established to derive an empirical criterion on the local mesh refining needed to obtain accurate numerical results. To consolidate the results obtained with a finite elements code, calculations have been performed with a completely independent tool using Monte Carlo method on a set of cases. A correlation has then been derived from the numerical results data with a maximum deviation of less than 4% in the considered range of θ and B i , that covers conditions encountered in all the studies dealing with dropwise condensation of pure vapor. Comparisons with other laws available in literature have then been performed, evidencing some important discrepancies. • Available correlations lead to huge discrepancies when they are compared to each other. • Elements of the mesh must be smaller than the inverse of the local Biot number. • An unbiased correlation is provided to calculate the conduction thermal resistance. • In dropwise condensation this law is valid whatever the Biot number and contact angle.

Exploring the limits of condensation heat transfer: A numerical study of microscale-confined condensation between parallel surfaces having wetting contrast

Dropwise condensation has received extensive interest due to its potential to enhance the efficiency of power generation, water harvesting, and thermal management systems. The degree of heat transfer enhancement during dropwise condensation is hindered by the delicate balance between nucleation, droplet growth rate, and condensate shedding. Condensation between parallel surfaces having wetting contrast, termed microscale-confined condensation (MCC), presents a promising method to break the fundamental limits of dropwise condensation by decoupling droplet nucleation and growth from condensate removal. The key mechanisms governing MCC enable spontaneous condensate transfer from the condensing surface to a highly wetting absorbing surface to limit the maximum droplet size. Here, we developed and explored numerical simulations of MCC to elucidate the upper limits of the condensation heat transfer rate. To optimize MCC with respect to heat transfer, we examined the effects of surface gap or maximum droplet size, and condensing surface contact angle on droplet size distribution and overall condensation heat flux. We then developed a regime map in terms of surface wettability and shedding droplet size, and showed that smaller surface gaps and smaller condensing surface contact angles are preferential to achieve a higher heat flux in pure vapor condensation conditions. The optimized MCC outperforms state-of-art condensation including classic dropwise condensation and jumping-droplet condensation, with a 250% enhancement of heat transfer coefficient at a steam temperature of 100 °C. Through numerical simulations, our study not only calculates the upper bound of condensation heat transfer, but also provides guidelines for further enhancement of condensation via decoupling droplet shedding from nucleation.

Dropwise condensation heat transfer on nanostructured superhydrophobic surfaces with different inclinations and surface subcoolings

Due to the excellent heat transfer efficiency, dynamic behaviors of coalescence-induced droplet jumping on superhydrophobic surfaces have been extensively investigated to facilitate the technological applications in power generation, refrigeration, and water harvesting. Despite numerous experiments and simulations of vapor condensation, the droplet dynamics and heat transfer performance affected by the combination of initial wetting state and inclination during pure vapor condensation are still poorly understood. Combining visual experiments and lattice Boltzmann simulations, the droplet dynamics, size distribution and heat transfer on the nanostructured surface at various condensing conditions are analyzed. As the subcooling increases from 0.5 to 3.5 K, the jumping frequency of droplets at α = 90° decreases significantly from 173 to 36 cm−2 s−1 and the average droplet diameter is increased by ∼300%. Meanwhile, the jumping diameter ranges from 20 to 280 μm, with a ∼60% reduction in maximum jumping height. The critical sliding diameters at α = 30° are 1.5, 1.4 and 1.5 times higher than those at a 90° inclination as the subcoolings are 5.0 K, 6.5 K and 8.0 K, respectively. Heat transfer coefficient is enhanced by 21.1% at α = 30°, 49.2% at α = 60°, and 72.4% at α = 90°. In addition, the LB results demonstrate that if the energy conversion efficiency is below 2.1%, condensate droplets are stuck to the substrate rather than jumping away spontaneously.

Laminar Film Condensation outside an Inclined Tube.(Dept.M)

The heat transfer in laminar film condensation of pure saturated stagnant vapour outside an inclined tube is analyzed by applying the principles of the laminar-boundary-layer theory. The analysis results in a partial differential equation for the local film thickness, which in the vertical case, yields the result found by Nusselt. However, for the horizontal case, this equation reduces to an ordinary differential equation which verifies that the film thickness at the top point of a horizontal tube is not zero as assumed by Nusselt, and certain value has been determined. This value is of order of the critical thickness of the stable film; defined by another author studied the characteristic of transient, and steady condensate film. Consequently, a modified Nusselt formula has been determined for calculating the average heat transfer coefficient, in which the constant coefficient is 0.65 instead of O. 728 in the Nusselt solution. This modification may be in the right direction in view of some authors proposed such a modification for better agreement with measured data. Then, the main partial differential equation has numerically been solved for the inclined tube, and results obtained for inclination angles with the horizontal; from 0 to 90. The exact solution of the vertical, and that of the horizontal tube are used to test the numerical solution. The results indicate that small inclination angle with the horizontalis is) results in an increase in the heat transfer coefficient over the horizontal value. While for a vertical tube, small inclinations from the vertical position, (SS) results in a significant increase in the heat transfer coefficient above the vertical value. Comparison of present results with available correlations has been made.

Numerical study of gas-dynamic and condensation processes in an ejector device used to purify gas-air emissions

The article presents a numerical study of vapor condensation appearing in an ejector. The ejector is a part of a device developed by the authors and used to purify air emissions. The article considers condensation of pure water vapor and condensation of vapor in a mixture of vapor and carrier gas. The system of axisymmetric Navier-Stokes equations and the system of moment equations are considered as a mathematical model for describing two-dimensional non-stationary gas-dynamic processes and processes of phase transitions in the device. The general system of equations is numerically solved using the finite volume method.

Condensation of water vapor in the presence of fluocarbons in the turbulent stream of liquid

The paper presents an experimental study of condensation of pure water vapor and water vapor in the presence of octafluoropropane (C3F8) in a vapor-gas mixture (VGM) in a turbulent jet of liquid.

Numerical modeling of vapor condensation over a wide range of non-condensable gas concentrations

This paper presents a numerical model to study the process of vapor condensation on surfaces characterized by film-wise condensation with the presence of Non-condensable gases (NCG). State variables in both the condensate film and the diffusion layer were solved separately and the condensation interface was used to couple the two solutions. The solution of the condensate film was obtained using well-established solutions of laminar film condensation of pure vapor. In contrast to other models surveyed, this work provides a inexpensive and accurate predictions of heat and mass transfer characteristics. We validated the work against two classical condensation problems. The model was first validated against empirical correlations and experimental work, resulting in a very good agreement. We then assessed the applicability of ignoring the condensate film effect, as performed in previous models, on the condensation processes by observing the thermal resistances of both the condensate film and diffusion layer. Results indicated that for the studied cases of NCG mass fractions above 20%, the condensate thermal resistance was at least an order of magnitude lower than that of the diffusion layer. However, the two thermal resistances seem to approach each other as NCG mass fraction becomes smaller. On another front, we observed that models that ignore the condensate film thermal resistance underestimate the interfacial temperature albeit accurately predicting the overall heat transfer rate. To simulate even lower NCG mass fractions, we validated our model to the classical analytical work of Sparrow and co-workers. Results showed a striking agreement between the two solutions at different NCG mass fractions (0.5%–10%) and subcooling degrees (5∘F–40∘F). Finally, we found a good agreement between results of our model and the heat/mass transfer analogy. The heat/mass transfer analogy is a semi-empirical method therefore, is limited to the existing correlations and their uncertainties. On the other hand, our model does not use any empiricism and relies on the available solutions of laminar condensate film of pure vapor in predicting the liquid side heat transfer coefficient.

Effects of stagnant air on condensation heat transfer over a horizontal tube

The effects of stagnant air on the condensation heat transfer of vapor over a horizontal tube were investigated by experiments. The condensation wall temperatures along the circumference and the cooling water temperature were measured at various cooling water and vapor mass fluxes as well as vapor pressures. The heat transfer coefficients and Nusselt number were calculated, and the effects of stagnant air on several parameters, including the wall temperature of the condensation tube, Nusselt number, heat transfer coefficient, and flow pattern transition point of liquid film, were analyzed. Results showed that stagnant air was a main factor that affected the heat and mass transfer uniformity of vapor. Unlike in pure vapor condensation, the temperature difference between the top and bottom parts of the condensation tube increased when the mole fraction of stagnant air was less than 0.17, whereas it decreased when the mole fraction of stagnant air was larger than 0.47. The increment in the mole fraction of stagnant air reduced the temperature difference along the circumference of the condensation tube wall. As the mole fraction of stagnant air increased, the heat transfer coefficient and Nusselt number decreased. However, Nusselt number was insensitive to the effect of air when the mole fraction of stagnant air was larger than 0.25. As the circumferential angle of horizontal condensation tube increased from 0° to 180°, the Reynolds number of liquid film transformed from the laminar state to the turbulence state and then changed into the laminar state. Reynolds number reached a maximum value within the circumferential angles of 45°–90°. As the circumferential angle changed from 90° to 180°, a large amount of condensate fell, and the liquid film attached to the condensation tube wall became thinner, which resulted in an intensive reduction in the Reynolds number of the liquid film. As the mole fraction of stagnant air increased, a transition point of liquid film moved to a larger circumferential angle, which delayed the transition of liquid film from laminar flow to turbulence. The present experimental results were compared with the data from Sherkriladze and Gomelauri (1966), Rose (1984), Fujii et al. (1972), and Nusselt (1916).

Numerical study of condensation flow regimes in presence of non-condensable gas in minichannels

A numerical research on flow regime of R134a in horizontal minichannels was conducted. The model was on the basis of the volume of fluid approach, and inserted user-defined routines which cover condensation mass and heat transfer. The observed annular flow, injection flow and intermittent flow of pure steam condensation, and annular (annular-wavy) flow, injection flow, annular-intermittent flow and intermittent flow of gas mixture condensation were qualitatively compared against experimental data. Flow patterns of lower non-condensable gas mole concentration show similar with that of pure steam, while the fluctuation of gas-liquid interface in annular flow and the noticeable annular-intermittent flow of higher non-condensable gas mole concentration were noted. Besides, injection flow characters with larger head volume and longer neck, and intermittent flow characters with larger slug sizes in existence of non-condensable gas were found. Furthermore, the influence of mole concentration of non-condensable gas and operating conditions on flow pattern transition lines were analyzed after validating the flow pattern transition. The results showed that the injection flow transition point has a relative fixed point at pure vapor condensation, while various in presence of NCG. Besides, lower non-condensable gas mole concentration, smaller inlet gas mass flux and larger wall heat flux could lead to earlier flow pattern regime.

Effects of millimetric geometric features on dropwise condensation under different vapor conditions

In this work, we investigated the effects of millimetric surface structures on dropwise condensation heat transfer under two different environments: pure vapor and an air-vapor mixture. Our experimental results show that, although convex structures enable faster droplet growth in an air-vapor mixture, the same structures impose the opposite effect during pure vapor condensation, hindering droplet growth. We developed a model for each case to predict the heat flux distribution along the structured surface, and the model shows reasonable agreement with experimental results. This work demonstrates that the effects of geometric features on dropwise condensation are not invariable but rather dependent on the scenario of resistances to heat and mass transfer in the system. The fundamental understanding developed in this study provides useful guidelines for condensation applications including power generation, desalination, dew harvesting, and thermal management.

The influence of surface-active agents in gas mixture on the intensity of jet condensation

The report presents: the methodology of calculation of contact condensation of steam from the steam-gas mixture into the stream of water, taking into account: the mass flow of steam through the boundary phase, particularly the change in turbulent transport properties near the interface and their connection to the interface perturbations due to the surface tension of the mixture; the method of calculation of the surface tension at the interface water - a mixture of fluorocarbon vapor and water, based on the previously established analytical methods we calculate the surface tension for simple one - component liquid-vapor systems. The obtained analytical relation to calculate the surface tension of the mixture is a function of temperature and volume concentration of the fluorocarbon gas in the mixture and is true for all sizes of gas molecules. On the newly created experimental stand is made verification of experimental studies to determine the surface tension of pure substances: water, steam, C3F8 pair C3F8, produced the first experimental data on surface tension at the water - a mixture of water vapor and fluorocarbon C3F8. The obtained experimental data allow us to refine the values of the two constants used in the calculated model of the surface tension of the mixture. Experimental study of jet condensation was carried out with the flow in the zone of condensation of different gases. The condensation process was monitored by measurement of consumption of water flowing from the nozzle, and the formed condensate. When submitting C3F8, there was a noticeable, intensification condensation process compared with the condensation of pure water vapor. The calculation results are in satisfactory agreement with the experimental data on surface tension of the mixture and steam condensation from steam-gas mixture. Analysis of calculation results shows that the presence of surfactants in the condensation zone affects the partial vapor pressure on the interfacial surface, and the thermal conductivity of the liquid jet. The first circumstance leads to deterioration of the condensation process, the second to the intensification of this process. There is obviously an optimum value of concentration of the additive surfactants to the vapour when the condensation process is maximum. According to the developed design methodology contact condensation can evaluate these optimum conditions, their practical effect in the field study.

Numerical analysis of different fluted fins for condensation on a vertical tube

A numerical analysis of laminar film condensation of pure vapor on an isothermal vertical fluted tube surface is presented using a surface-fitted orthogonal curvilinear coordinate system. Based on the assumptions employed in Nusselt's analysis on a flat plate, a liquid film thickness equation for film-wise condensation on fluted surface is obtained by simplifying the Navier-Stokes equations. By defining the fluted fin surface curvatures, different fin shapes can be obtained. A relationship between curvature parameters of the sinuous fluted tube such as fin height and fin arc length is obtained. A general condensation performance comparison method for different fin geometries which is set up using the Fourier's series approximation is presented by defining the fin surface curvature parameters. By solving the liquid film thickness equation, the curvature variation effects on a condensing surface and the stable film profiles are investigated. The calculations show that the standard sinuous fin's crest region area is crucial for total condensation heat transfer. It is demonstrated that the effect of surface tension because of curvature change on fluted sinuous fins is important in heat transfer enhancement as compared to other rectangular, triangular and other sinuous fin surfaces. For a constant base diameter tube, it was determined that heat transfer enhancement of the sinuous fins can be achieved by increasing the number of fins and the fin length. However, it is not a valid method to lengthen the fin height for rectangular or triangular fin when the liquid flow does not reach flooding. A more useful outside tube fin arrangement, which takes full advantage of fin tip effect and flooding flow characteristic, is presented.

Development of an analytical model for pure vapor downflow condensation in a vertical tube

In-tube condensation of pure vapor are widely adopted by passive safety systems of advanced nuclear reactors to remove the decay heat. Accurate calculation of the in-tube condensation heat transfer coefficient is important for evaluating the heat removal ability. A better understanding of the mechanism of pure vapor condensation heat transfer and its modelling are essential for the design and assessment of those passive safety systems. An analytical model for pure vapor condensation in a vertical tube is developed based on mass, momentum and energy conservation equations. The total axial pressure drop is calculated through the momentum equation of the vapor core assuming a uniform radial pressure distribution. An optimized transition area between the laminar and turbulent film flow is defined to avoid discontinuity of heat transfer coefficients. Based on the sensitivity analyses, Kay’s turbulent eddy diffusivity model and his turbulent Prandtl number model were recommended for the newly proposed model. The predicted results were compared with results of Shah (2016a,b), Chen et al. (1987) and Cavallini et al. (2002) correlations as well as Kuhn’s and KAIST experimental data. It was found that the proposed model predicts the heat transfer coefficient well with a mean absolute deviation of 11.99% for Kuhn’s data and 8.08% for KAIST’s data.

Computational Fluid Dynamics Simulations of Convective Pure Vapor Condensation Inside Vertical Cylindrical Condensers

This paper presents the results from computational fluid dynamics (CFD) simulations of heat and mass transfer of pure vapor flowing and condensing in a vertical cylindrical condenser system at various inlet temperatures, mass flow rates, and operating pressure for the case where the vapor condensation is not completed inside the condenser tube. The heat and mass transfer inside the condenser tube is simulated as single phase flow, and the thin condensate film on the condensing surface is replaced by a set of boundary conditions that couple the CFD simulations inside the condenser tube and the coolant channel. The CFD results are compared with the experimental results, and good agreement has been found for the various measured temperatures. It is found that both the wall temperature and the heat flux vary significantly along the condenser tube, and it is necessary to consider the conjugate problem that consists of the whole condenser system (condenser plus coolant flow) in predicting the pure vapor condensation in a condensing system. The CFD results show that the heat flux along the condenser tube can be increasing for counter-flow condenser, and the condensate film may not be the main limiting factor in the pure vapor condensation. The results from the CFD simulations also show that the estimation of the interface shear stress cannot be based on the bulk velocity of the water vapor alone.

Effect of air on condensation in a non-vacuum gravity heat pipe

An experimental and theoretical investigation was performed to show the effects of air on condensation in a non-vacuum gravity heat pipe at heat loads of 0.8–5.3kW. Parameters involving local condensation heat transfer coefficients, air mole fractions, and air storage capacity in a reservoir mounted below the condensation tube, were calculated. A degradation factor method was applied to solve vapor condensation length. Results showed that local air mole fraction increased to over 95% in the condensation tube downstream, where a saturated moist air column was formed. The reservoir effectively alleviated the adverse effects of air on the condensation section. At an operating pressure of 0.24MPa, 68% of the reservoir was filled with air. In the condensation tube downstream with a low heat load, air seriously affected condensation heat transfer, and the average degradation factor was only 0.26. By contrast, air slightly affected condensation heat transfer in the condensation tube upstream with a high heat load, and the average and local degradation factors were 0.7 and 0.76, respectively. Vapor condensation length with air was 1.32 times as much as pure vapor condensation length at a vapor mass flux of 1.8g/s and an operating pressure of 0.36MPa.