Suppression of natural convection heat transfer along a vertical flat plate with concavities

To clarify the effect of the suppression of natural heat transfer, the local heat transfer coefficients on a vertical cooled flat plate with circular grooves were measured by a multi‐type thermocouple method. Two flat plates with and without periodic circular grooves were tested in this experiment. The characteristics of heat transfer along the plate for both plates were compared. The local heat transfer coefficients on the periodic grooved plate became smaller than that of the flat plate. The flow pattern was changed when it passed over the grooves, and circulation was generated in the grooves in the downstream. As a result, the thickness of the thermal boundary layer on the grooved plate was more developed than the normal flat plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20294     

Condensate drop movement in Marangoni condensation by applying bulk temperature gradient on heat transfer surface

The spontaneous movement of condensate drops by application of a bulk temperature gradient on the heat transfer surface in Marangoni condensation was investigated, with consideration for applications to heat transfer devices. In the Marangoni condensation process, the removal of condensate on the heat transfer surface is important to maintain good heat transfer. A heat transfer device, in which the liquid movement occurs without external forces such as gravity and vapor shear force, may be useful in various applications. As a result of experiments using a water–ethanol vapor mixture, the movement of droplets from the low‐temperature side to high‐temperature side could be observed on a horizontally arranged heat transfer surface. The relation between the velocity of drop movement and the gradient of surface tension was studied for different concentrations. Furthermore, the effect of inclination of the condensing surface was examined. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 387–397, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20218     

Liquid film flow rising along the outer surface of the rotating cone

We have investigated fluid flow characteristics of liquid film flow rising along the outer surface of a rotating cone. The fluid is given a centrifugal force due to the rotation of the cone, while strong surface tension maintains the centrifugal force at the outer surface of the liquid film flow. Therefore, the fluid does not spread outward from the cone, but goes up along it. We have visualized the interesting flow phenomena with a high‐speed video camera. In addition, we have measured the correlation between the wetted radius, rotation rate of the cone, and flow rate of the rising film flow. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 492–496, 2010; Published online 2 June 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20307     

Forced Convection Laminar Film Condensation of Downward Flowing Vapor on a Single Horizontal Elliptic Cylinder or a Bank of Elliptical Tubes

A numerical study is performed on the laminar film condensation of pure saturated vapor flowing in the direction of gravity on a single horizontal elliptic cylinder or a bank of elliptical tubes. Temperature, velocity distribution, and heat transfer coefficient of the fully developed flow are carried out with a fully implicit finite difference scheme. The equality of shear stress at the liquid-vapor interface is used as the coupling condition between the two phases. The inertia and convection term are retained in the analysis. Outside of the vapor boundary layer, the vapor phase velocity is obtained from potential flow. The method of source density distribution on the body surface is used for determination of the external vapor velocity in elliptical tube banks. The effect of inundation produced by condensate on upper ellipses is taken into account by assuming that the vapor velocity field is not affected by the condensate flow from one elliptic cylinder to another. Based on the obtained solutions of flow field, the effect of surface tension, the interaction because of the ellipse spacing, and the inundation on the heat transfer coefficient and the boundary layer separation point have been evaluated. The results of this analysis are discussed especially in function of eccentricity e (effect of the surface tension). The heat transfer in interellipse space is analyzed and compared with the theoretical and experimental results of other authors. Good agreement is shown.     

Size effect on two‐phase regime for condensation in micro/mini tubes

Analysis was conducted to predict the influence of tube size on two‐phase flow regimes for flow condensing in mini/micro tubes. According to the importance of the interfacial tension compared to the interfacial stress, the regimes were classified into three kinds: axial symmetrical, semisymmetrical, and asymmetrical. The results indicated that the surface tension of the fluid media obviously affects the flow regimes; for tubes with an inner diameter less than 100 to 600 mm, the flow regimes would be axially symmetrical depending on the media. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 65–71, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10076     

Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004     

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

In 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     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

A mechanism study of initial droplet formation for dropwise condensation

The mechanism of formation of initial condensate droplets for dropwise condensation in the nanometer scale is still undetermined. In this paper, magnesium was used as the condensation surface since it can react with condensate to produce magnesium hydroxide, which can be regarded as the marker of the initial condensate state on the surface. Then an electron probe microanalyzer (EPMA) and scanning electron microscope (SEM) were used to scan the variation of the chemical compositions on the surfaces in nanometer scale before and after the initial condensation. The results showed that the oxygen contents on the test surfaces increased obviously with subcooling and condensation time after the initial dropwise condensation. In order to further analyze the variation of oxygen content occurring on the whole surface or only at the partial locations, the reaction dynamic relationship of magnesium and the condensate was found. And the calculated result indicated that the area ratio occupied by condensate was less than 1. Meanwhile the detected results of EMPA showed that oxygen on the test surface was distributed non‐uniformly. As a result, the formation mechanism of the initial condensate drops for dropwise condensation agrees with the hypothesis of nucleation sites in nanometer scale. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20377     

Effects of uniform suction and surface tension on laminar filmwise condensation on a horizontal elliptical tube in a porous medium

This study performs a theoretical investigation into the problem of steady filmwise condensation flow over the external surface of a horizontal elliptical tube embedded in a porous medium with suction at the tube surface. The combined effects of the surface tension force and the gravitational force in driving the flow of the liquid film within the porous medium are modeled using Darcy's law. An effective suction function, f, is introduced to model the effect of the suction force at the wall on the thickness of the condensate film. The theoretical results presented in this study show that the heat transfer performance can be enhanced by applying a suction effect at the wall. Furthermore, it is shown that the surface tension force has a negligible effect on the mean Nusselt number.     

Evaporating characteristics of a microlayer under a bubble

Using the Hallinan‐Ervin model, the flow and evaporation in a bubble microlayer were theoretically analyzed, and the dryout characteristics and Staub's criterion were discussed in detail. It was revealed that the critical dryout radius is associated with the wettability of the heated surface, and that the dominant role for microlayer evaporation is disjoining pressure, not surface tension gradient. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 456–462, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10052     

Study on high performance evaporator with micro‐grooves (Measurement of capillary force in a single groove and modeling of heat and mass transfer)

A micro‐grooved evaporator is composed of µm‐wide grooves on a heat transfer plate in which the inter‐line regions at the liquid–vapor meniscus of coolant become identifiable. The high‐heat performance of the evaporator is realized by this inter‐line region (ILR) where the liquid thin film reduces the thermal resistance on the heat transfer surface. In this report, we propose a numerical simulation model of heat and mass transfer in a single groove to predict its capillary force and heat flux. The capillary force performance (capillary‐rise length in a groove) of a single groove was measured for samples of varying width, superheat, and inclination. The performance was found to be a maximum at a specific groove width of 200–400 µm, which is in good agreement with the predicted results calculated by the proposed model. For a better prediction of capillary‐rise length, the effective capillary force and the effective flow resistance were considered. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20257     

Unsteady steam condensation flow patterns inside a miniature tube

Unsteady steam condensation inside a single miniature tube has been studied. The visualization of different instantaneous and periodically two-phase flow is conducted for different experimental conditions. The two-phase flow characterization is obtained using the image processing. Annular, slug bubbly, spherical bubbly, and wavy flows are observed by varying the steam inlet pressure and cooling heat transfer. The cycle of the periodically flows are compared. It is shown that increasing the cooling heat flow rate reduces the number of the instabilities and the injected bubbles. The axial vapor velocity decreases during the waves growth. The local distribution of the condensate film thickness is analyzed. It is shown that the liquid film becomes thinner near the meniscus-like interface because of the surface tension effect. The reverse annular flow is observed at the end of each periodic flow when the bubbles leave the channel. It can be concluded from experimental results that the stratification effect is not significant during the condensation inside the miniature tube. The capillary pressure evolution is measured. The maximum values are obtained in the waves locations and near the meniscus of the annular flow.     

Experimental study of free convection heat transfer and condensation of vapor of humid air over an inclined cold tube

In this study condensation heat transfer on a cold inclined circular cylinder due to natural convection for various conditions is investigated experimentally. The cylinder is placed in an isolated test room to permit pure natural circulation of ambient air. Ambient temperature and humidity of the test room are controlled by a refrigeration cycle and humidifying. The ambient relative air humidity changed in the range of 30 to 50% and temperature from 25 to 35 °C. The ethylene‐glycol/water solution is used as a refrigerant to control and keep the temperature of the test section at a constant value. The cold surface temperature is varied from 2 to 6 °C. The condensation rate and heat flux are found to depend mainly on time, temperature difference between ambient air and cold surface, ambient relative humidity, and tube inclination. Results are plotted for various conditions with respect to time. The experimental results are used to propose a correlation to predict the condensate mass flow rate for free convection heat transfer. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21015     

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

In 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     

The formation of rising liquid thin film on the fluted surface of a horizontal tube

The purpose of this study is to investigate the mechanism of the formation of the rising liquid thin film and its flow characteristics on the fluted surface of a horizontal tube. By analyzing the wetting behaviors of the fluted tube, which was primarily responsible for the formation of the rising liquid thin film, a numerical model of one‐phase fluid was established to analyze the distribution of the velocity and thickness of the rising liquid thin film during its evaporation. The behaviors of the flow characteristics were discussed and the results showed that geometric properties of the fluted surface of a horizontal tube and surface tension of the fluid were essential for the formation of a continuous and uniform liquid thin film. Theoretical analysis suggested that the capillary force created by the fluid surface tension was a key value for the formation of the thin film. The heat and mass transfer characteristics of the formed thin film also had an effect on the formation of the rising film. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 396–406, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20075     

Characteristics of condensate drop movement with application of bulk surface temperature gradient in Marangoni dropwise condensation

The characteristics of the spontaneous movement of condensate drops when a bulk temperature gradient is applied to a horizontal condensing surface in Marangoni dropwise condensation of a water?ethanol vapor mixture were experimentally investigated over a wide range of bulk temperature gradients and for various mass fractions. Drops were observed to move from the low-temperature side to the high-temperature side of the heat transfer surface. When the initial drop distance was adopted as a parameter for the Marangoni force acting on the condensate drop together with the surface tension gradient corresponding to the surface temperature of the condensing surface, the drop velocity was highly correlated with both the surface tension gradient and the initial drop distance over a wide range of parameters. At relatively large initial drop distances, the condensate drop velocity increases as the initial drop distance is reduced and it subsequently decreases after the velocity reaches its maximum value under an almost constant bulk surface tension gradient. The drop velocity increases linearly with increasing bulk surface tension gradient for a constant initial drop distance.     

Condensation of vapour on sub‐cooled liquid columns‐application to the thermal design of shell‐and‐tube condensers

Energy and mass transfer to a column of condensate draining by gravity between two successive tubes has been analysed theoretically. The results of the model agree well with previous experimental data where the variation in condensate temperature with distance from the upper tube was measured. The analysis provides a basic tool for the thermal analysis of inundated tubes by eliminating the need to make assumptions about the condensate inundation temperature and mass flow rate on such tubes. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

The structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm²/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20176     

Long-Wave and Integral Boundary Layer Analysis of Falling Film Flow on Walls With Three-Dimensional Periodic Structures

Falling films exhibit very complex wavy patterns, which depend on the properties of the liquid, the Reynolds number, the wall inclination angle, and the distance from the film inlet. The film hydrodynamics governs the heat and mass transfer in the liquid films. Our vision is to control and enhance heat and mass transport by using walls with specific microscale topographies that influence the falling film flow, stability, and wavy pattern. In this work, long-wave theory and integral boundary layer approximation are used for modeling the falling film flow on walls with three-dimensional periodic microstructures. The wall topography is periodic both in the main flow direction and in the transverse direction. Examples of such microstructures are longitudinal grooves with sinusoidal path (or meandering grooves) and herringbone structures. The effects of the Reynolds number, the wall inclination angle, and the longitudinal and transverse periods of the structure on the shape of liquid–gas interface are investigated. It is shown that, as opposed to straight grooves in longitudinal direction, grooves with meandering paths may lead to significant interface deformations.