Vapor Velocity Research Articles

Mechanism of transition from elongated bubbles to wavy-annular regime in flow boiling through microchannels

Transition from elongated bubbles to annular flow regime is one of the most intricate phenomena in flow boiling in microchannels. To identify this transition, flow regime maps have been introduced primarily based on visualizations and characterized using average flow properties. However, understanding the mechanism responsible for this transition demands accurate determination of the hydrodynamic characteristics of liquid films around the vapor core. But, the challenges associated with measurements of the thickness and velocity of the liquid films have made it difficult to determine the flow hydrodynamics. Here, we utilize a measurement technique to determine the liquid film thickness along with velocities of the two phases. These measurements have enabled accurate calculation of the cross-sectional void fraction, vapor quality, interfacial shear stress and superficial velocities, which form the coordinates of the existing flow regime maps. The results show that upon transition to the annular flow regime, a remarkable increase in the liquid film thickness occurs due to a substantial increase in the liquid-vapor interfacial shear stress. This shear stress, which is a function of the liquid film thickness, vapor velocity, and channel aspect ratio, is introduced as a criterion for transition from elongated bubbles (EB) to wavy-annular (WA) flow regime.

Computational Fluid Dynamics (CFD) Modeling and Analysis of Hydrocarbon Vapor Cloud Explosions (VCEs) in Amuay Refinery and Jaipur Plant Using FLACS

Process safety helps prevent the unexpected and unplanned release of flammable and toxic chemicals, leading to poisonous gas clouds, fires, and explosions. Vapor cloud explosions (VCEs) are among the most severe hazards to humans and the environment in process facilities. Therefore, process safety demands to use best and reliable techniques to model VCEs in process industries and storage tanks of flammable chemicals. In this regard, the Computational Fluid Dynamics (CFD) models are more appropriate, as these models provide three-dimensional (3D) modeling of all sequences of events in an accident. In this study, CFD is used to model VCE in two industrial accidents: the Amuay refinery disaster (happened in 2012) and the Indian Oil Corporation’s (IOC) Jaipur terminal (2009). This work studies 3D CFD modeling of flammable cloud explosion in the real-time configuration for both accidents. FLACS (FLame ACceleration Simulator), a CFD software, is used to simulate the loss of hydrocarbon containment, cloud formation, and explosion in both industrial case studies. The ignition locations and grid sizes were varied to analyze their influence on explosion overpressure, temperature, vapor velocity, and fuel mass. This work also investigated the effect of geometry complexity on the explosion. Results showed that, as opposed to the coarse grid, the fine grid provides more precision in the analysis. The study also reveals an explosion overpressure of the order 4–15 bar (g) for the given case studies. This study’s results can help perform a qualitative and quantitative risk assessment of the Amuay refinery accident and Jaipur fire. The simulation of different scenarios can help develop and improve safety guidelines to mitigate similar accidents.

Marangoni condensation of steam-ethanol mixtures on a horizontal smooth tube

Experiments were conducted to measure condensation heat transfer of steam-ethanol mixtures on a horizontal smooth tube. Heat flux and vapor-to-surface temperature difference were accurately measured over a wide range of mass concentration of ethanol (0.001%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.5% and 1.0%.) at atmospheric pressure. The present work extends earlier data of Murase et al. (2007) and Ali et al. (2013) to lower mass concentration of ethanol and higher vapor-to-surface temperature difference. Visual observations, the appearance of condensate, show the condensation modes and their transitions of pseudo-dropwise, rivulets and filmwise. Heat flux and heat-transfer coefficient are found to increase, reach to a peak and decrease as vapor-to-surface temperature difference increases for all mass concentrations of ethanol and vapor velocities. The maximum heat-transfer coefficients appear in the range of mass concentration of ethanol from 0.01% to 0.025%. The highest heat-transfer coefficient values are 112.4 kW/m2 K and 93.9 kW/m2 K at ΔT = 3.43 K and 4.46 K, respectively. The maximum enhancement ratio of 5.5 is achieved at vapor-to-surface temperature difference of 4 K and mass concentration of ethanol 0.01%. The results also show that heat-transfer coefficient of filmwise eventually approaches to a value slightly higher than that of pure steam.

Code development and analysis on the operation of liquid metal high temperature heat pipes under full condition

Heat pipe cooled reactor (HPR) is an innovative solid-core reactor which utilizes heat pipes to extract heat from the core to the energy conversion system. In this paper, a mathematical model for high-temperature heat pipes from a frozen state to a steady state is developed based on experimental data. The model is simplified and solved numerically. The two-dimensional transient heat conduction model of the wall and wick and the one-dimensional quasi-steady compressible vapor flow model are coupled to simulate the operation of a heat pipe when vaporization and condensation occur at the liquid–vapor interface. And the equivalent heat capacity model and the rarefied vapor model are introduced to simulate the process of heat pipe startup from a frozen state. Numerical results indicate that the model of heat pipe can predict the operation of high-temperature heat pipe with an error of less than 1.04% at steady state and 23.4% during the startup. Based on the simulation results, the velocity of vapor decreases with the heat transfer power in the steady state and the velocity of potassium vapor is 58.4% of sodium vapor velocity under the same operation condition. During the startup, when the rarefied-continuum interface located in the end of the evaporator section, the maximum vapor velocity occurs which is 1.3 times of steady velocity. This work makes it possible to predict the operation of a high-temperature heat pipe and provides a reference for the design and application of HPR.

Phase distribution of two-phase refrigerant flow over an impacting horizontal T-junction

Two-phase flows of refrigerants commonly occur in a multitude of systems in the power and process industries, including heat pumps, organic flash cycle,... These flows often have to pass through T-junctions to divide the inlet stream into two outlet streams. Herein, phase maldistribution can occur, meaning that the vapour qualities in the outlet are not equal to the inlet vapour quality. Undesired splitting could lead to unwanted effects, such as to much superheat after evaporation if less liquid phase entered the corresponding exit branch than was expected. Or liquid impact in turbines when to much liquid was allowed to flow to the turbine. Therefore, gaining insight into phase maldisdribution for refrigerants is important to be able to predict the amount of each phase in each exit branch.In literature, data is mostly available for air-water flows and models are built and validated solely on these datasets. The present work extends the air-water data of phase distribution over a horizontal impacting T-junction with data on different refrigerants. For this purpose an experimental setup was developed which can test refrigerant flows with mass flux G up to 700 kg.m−2.s−1 at a saturation temperature between 10 ∘C and 20 ∘C and with a vapour quality x between 0 and 1. In total 551 experiments were performed with four different refrigerants: R32, R134a, R125 and R1234ze.For inlet superficial liquid velocities equal or higher than 0.2m/s it was found that with increasing inlet superficial vapour velocities, the amount of liquid phase flowing to the branch with the lowest mass flow rate decreased. This trend is consistent with literature.A discontinuity in this trend was observed when a flow regime transition occured. For the refrigerant R32 and an inlet superficial liquid velocity equal or lower than 0.1 m/s, the liquid phase followed an opposite trend by preferring the branch with the highest mass flow rate.The models from literature do not predict the refrigerant data with the necessary accuracy. They only work for specific refrigerants and for inlet flow regimes similar to the flow regimes of the air-water data used to construct the model. For this reason, the provided work also presents a new phase distribution model created upon the data gathered.

Dual-wavelength locking technique for coherent 2-µm differential absorption lidar applications

A dual-wavelength locking technique for coherent 2-µm differential absorption lidar (DIAL) applications has been developed for simultaneously measuring water vapor (${{\rm{H}}_2}{\rm{O}}$) and radial wind velocity profiles. The two wavelengths for DIAL measurement were stabilized by sidebands of an electro-optic modulated laser, which was locked to the ${{\rm{CO}}_2}$ R30 absorption line. We selected wavelengths of 2050.550 and 2051.103 nm to minimize the effects of height-dependent temperature and pressure variations. The long-term wavelength stability of the two locked lasers of ${\lt}{0.2}\;{\rm{pm}}$, which corresponds to 14 MHz, was achieved. The DIAL instrument using the developed technique meets the requirement of measuring the ${{\rm{H}}_2}{\rm{O}}$ concentration with a systematic error ${\lt} {{5}}\%$ below an altitude of 5 km.

TEMPERATURE PREDICT OF AERATION TANK WALLS FOR BIOLOGICAL WASTEWATER TREATMENT SUBSYSTEM OF HAMDAN STATION

A heat transfer modeled for predicting the wall temperature of aeration tank in a biological wastewater treatment subsystem of Hamden station is presented. The method to treatment employed depending upon the pollutants in wastewater and extent to which it is desired to eliminate them in order to congregate required standards of water quality. Several heat gain and loss mechanisms concerned to develop of the model of temperature computer includes heat gains through conduction and radiation. While the heat losses referred to convection and radiation. It classified radiation heat transfer and biological reaction as a gained heat, while classifying the rate of evaporation, aerator, and wind velocity as lost heat.
 This study relied on a previous study, and based on the assumptions that have been identified so that a model development can be obtained to calculate the surface temperature of the wall of the aeration tank in a biological treatment system. The operational, weather and temperature data were to be registered from Iraqi weather forecast- Basra Airport. To obtain reliable results, the model was simulated using the STELLA software v.9.02, which gave accurate results in determining the parameters that affect the tank wall temperature changes. The STELLA software is Model calibration and considers as a dynamics language because of STELLA is software for graphic and dynamic simulation for the wall temperature of aeration tank. The results have shown a good accuracy and increment between the production walls temperatures with average ranged about (0.2 %) of present work. The model shows the sensitivity through set of precious five parameters like organic removal rate, ambient air temperature, wind velocity, air relative humidity, and the wall effective area of the aeration tank

Su Esaslı Seyreltik Fe+ZnO Hibrit Nanoakışkanının Düşük Isı Akısı Şartında Havuz Kaynama Isı Transferi Özellikleri: Sayısal Bir Çalışma

Thermophysical properties of the working fluid of a fluid-driven heat transfer system is the key parameter in determination of heat transfer performance of the system they were used. The nanofluids, nanoparticles containing colloidal suspensions, have been extensively used for performance enhancement in many applications, however, the nanoparticles cannot have both high thermal conductivity and good stability simultaneously. To provide these characteristics together, hybrid utilization of nanoparticles has emerged. In this numerical study, pool boiling heat transfer characteristics of dilute Fe+ZnO/deionized water hybrid nanofluid were investigated under low heat flux condition. The hybrid nanofluid suspension at the volumetric rate of 0.5% and Fe+ZnO combination of (50:50) were taken into account. For numerical simulations, Computational Fluid Dynamics approach and Volume-of-Fluid multiphase model were employed. Vapor volume fractions and velocity vectors in fluid medium were obtained for deionized water, and the hybrid nanofluid aforementioned. The start-up of the bubbles and departures with time were investigated for each working fluid. Under the same conditions, the start-up time of the boiling for deionized water and dilute hybrid nanofluid were observed as 468. and 441. seconds, respectively. The numerical findings also displayed that hybrid nanofluids can be used for pool boiling implementations in order to provide improved heat transfer characteristics.

Computational fluid dynamics parametric investigation for two-phase flow of ammonia-water mixing in bubble pump tube

The 2-D numerical simulation of two-phase NH3-water flowing under uniformly heated tube is used. The ANSYS FLUENT is used to predict the time evolution of thermal and hydrodynamic parameters of the bubble pump. Phase-dependent turbulent models are used to calculate the turbulent viscosity of each phase. Through user-defined functions, different interfacial force models and the wall boiling model are implemented in the code. The simulation results show a slow oscillation of hydrodynamic parameters such as: pressure, mass flux, vapor velocity, and liquid velocity during the initial stage of operation. However, a vigorous oscillation is detected for the temperature behavior. The amplitude and period of oscillation decrease with the heat input increasing. By using the void fraction contour, it is possible to predict the flow regime along the bubble pump at different times of the operation. The domination of flow regime is the function of heat flux too. It is bubbly to slug for heat fluxes less than 5 kW/m? and transits from churn to annular for 15 kW/m? and 50 kW/m? of heat flux.

Numerical Investigation on the Dielectric Working Fluid Effect on the Flow and Thermal Parameters of an Electrohydrodynamic Flat Heat Pipe

The present work highlights the impact of the working dielectric fluid on the flow and the thermal parameters of an axially grooved flat mini heat pipe (FMHP) submitted to Electrohydrodynamic (EHD) effects. Three dielectric working fluids are considered: pentane, R123, and R141b. A model is developed by considering the Laplace-Young, mass, momentum, and energy balance equations. The numerical results show that the electric field affects the liquid distribution along the heat pipe and helps the condensate to flow back to the evaporator section. Moreover, under the electric field conditions, the vapor pressure drop increases, however, the liquid pressure drop decreases. The effect of the electric field on the liquid velocity depends on the FMHP zone, and the vapor velocity is hardly affected by the EHD effects. Furthermore, lower capillary driving pressures are required to provide the necessary capillary pumping under EHD conditions. Besides, pentane allows for higher vapor pressure drops compared to those obtained with R123 and R141b, while the liquid pressure drops are highest for R123. It is found that with R123, the liquid velocity is higher than that reached with R141b and pentane. It is also demonstrated that the capillary limit increases under EHD conditions, and for R141b, the capillary limit is the highest either in zero-field and EHD conditions. Best heat pipe thermal performances are observed for wide and deep grooves with R141b. Finally, the optimum fill charge allowing the maximum heat transfer capacity is determined for each working fluid and different groove dimensions. It is shown that the optimum fill charge is hardly affected by the electric field whatever the working fluid. R123 requires the highest optimum fill charge, however, the heat transport capacity of the FMHP is the lowest when using this working fluid.

Modeling Study on Heat Transfer in Marangoni Dropwise Condensation for Ethanol-Water Mixture Vapors

In this paper, a model was developed to predict the heat transfer characteristics of Marangoni dropwise condensation. In accordance with the feature of Marangoni condensation, condensation was treated as dropwise condensation of mixture vapors. The condensation space was divided into two parts: the vapor diffusion layer and the condensate layer. For the condensate layer, the classical heat transfer calculation method of dropwise condensation was imitated to obtain the heat transfer characteristics. For the vapor diffusion layer, the heat transfer characteristics were achieved by solving the conservation equations. These heat transfer characteristics were coupled through the conjunct boundary, which was the vapor-liquid interface. The model was applied to the condensation of water-ethanol mixture vapors. A comparison with the existing experimental data showed that the developed model could basically reflect the influences of vapor-to-surface temperature difference, vapor concentration, vapor pressure, and vapor velocity on heat transfer characteristic of Marangoni condensation. Results showed that some differences existed between the calculation results and experimental results, but the prediction deviation of the model could be acceptable in the range of vapor-to-surface temperature difference where the condensation heat transfer coefficients reached peak values.

LES investigation on vortex dynamics of transient sheet/cloud cavitating flow using different vortex identification methods

The sheet/cloud cavitating flow always contains complex multiscale vortex structures generated by the cavity cloud shedding and collapsing. In this study, the transient sheet/cloud cavitating flow around a Clark-Y hydrofoil is numerically investigated using the Large Eddy Simulation (LES) method coupled with the Zwart–Gerber–Belamri (ZGB) cavitation model. The simulation accurately reproduces the unsteady cavitation evolution process, and the predicted time-averaged lift and drag coefficients, total vapor volume variation and velocity distribution agree fairly well with the experimental measurements. The cavitation vortex dynamics are studied in detail with different vortex identification methods including the vorticity method, the [Formula: see text]-criterion method, the [Formula: see text] method, the [Formula: see text] method and the Liutex method. The vortex identification ability of the different methods in the transient sheet/cloud cavitating flow is also discussed. Generally, the Liutex method combines the advantages of the other methods and can accurately identify both the vortex position and strength. Further analysis of cavitation-vortex interactions demonstrates that the cavity cloud shedding and collapsing have a pronounced influence on the vortex structure.

Effect of steam velocity during dropwise condensation

Dropwise condensation (DWC) is a complex phenomenon involving droplets nucleation, coalescence and motion. Starting from the nanoscale up to the macroscale, DWC involves millions of droplets per square meter. The maximum dimension assumed by a droplet before sliding is characterized by the departing radius: it is the radius at which the droplet starts to sweep through the surface as a consequence of the acting forces (gravity force, adhesion force, drag force induced by the flowing vapor). In the literature, very few works investigate the effect of vapor velocity on the heat transfer coefficient (HTC) and on the droplet departing radius during DWC. Furthermore, the effect of vapor velocity is not accounted for in available DWC models. In the present paper, DWC of steam has been promoted on an aluminum sol-gel coated surface. Heat transfer coefficients and droplets departing diameters have been measured at 107 °C saturation temperature, heat flux of 335 kW m−2 and average vapor velocity between 2.7 m s−1 and 11 m s−1. A method for the estimation of the droplet departing radius in presence of non-negligible vapor velocity is here proposed. The equation accounting for vapor velocity has been included in the model by Miljkovic et al. [1] for heat transfer coefficient prediction during DWC and has been assessed using the present data and two other datasets from independent laboratories.

Effect of Operating Conditions on the Ceramic Particles Drying Process by Superheated Steam in the Packed Bed Dryer

A novel model of ceramic particle drying by superheated steam in the packed bed dryer is applied to examine the effects of operating conditions on the drying process. It is shown that the drying kinetic has two drying stages: the evaporation flux firstly increases to the maximum value while particle temperature is remained as saturation temperature, then the evaporation flux decreases to the zero, and particle temperature rises to the equilibrium temperature. Results also illustrate that the drying process is faster at the thinner bed layer, smaller particle diameter, and higher initial vapor velocity and temperature.

Analytical Study on Gas-Oil Separation of a Heat Pump System under Lunar Gravity

A heat pump in the aerospace industry can significantly reduce the area of radiator by elevating the rejection temperature. Especially for a Lunar base, the heat pump can improve the heat rejection capability of the thermal control system to adapt the high-temperature environment. However, gravity on the Lunar (about 1/6 g) may have an adverse impact on a gas-oil separator of the heat pump, and solving this problem is the key for a heat pump used on Lunar base. At present, the gas-oil separator all based on gravity separation theories, the researches under low or micro gravity were blank. In this work, a gravity separation model based on a single-particle principle was built, and the effects of the vapor velocity, the oil droplet initial velocity, and the oil droplet diameter were investigated under normal gravity. Then the variations of the separation efficiency under Lunar gravity were discussed and the numerical calculation results showed that the separation efficiency was reduced when the vapor velocity or droplet initial velocity increased in a certain height of the separator whenever under normal or Lunar gravity. Particularly, the separation efficiency under Lunar gravity was reduced from 99% to 55% than it under normal gravity.

Ground heat flux determination based on near-surface soil hydro-thermodynamics

Near-surface soil hydro-thermodynamics plays an important role in heat and water transfer between land and atmosphere. However, it is difficult to quantify these processes in field campaigns, such as movement of liquid water and water vapor. This has affected determination of ground heat flux, a key component of surface energy budget. This study aims to quantify soil heat and water processes utilizing in situ measurements, and to improve ground heat flux estimation and surface energy imbalance. A new model has been proposed based on three main physical processes, including thermal conduction, convection of heat by moving water, and convection of latent heat. The model was tested at a grassland site in Colorado, USA. Results show that the model can capture high values of soil water content during rain events, low values under intense solar radiation, and high-frequency fluctuations under intermittent cloudy conditions. Also, the model produces reasonable vertical velocity and mass flux of water vapor. For the estimation of ground heat flux, the method that considers vertical variation of soil thermal conductivity and the contribution of water vapor gives better energy balance ratio than methods that ignore these conditions. In spite of this, the energy balance ratio is still low. Footprint analysis further shows the seasonal variation of ground cover is a potential reason responsible for this.

Influence of co-current vapor flow on falling film over horizontal tube

In the multi-effect evaporator (MED), the seawater falls on the outside of the tube, and the heat is transmitted from the steam flowing inside the tube. As a result, part of seawater evaporates and vapor is generated. Vapor flows in either co-current direction, cross direction or combination of both. The vapor flow exerts external force on falling film and affects film hydrodynamics, heat and mass transfer. This study focuses on analyzing the effects of co-current vapor flow on falling film distribution and hydrodynamics. A two-dimensional (2D) computational fluid dynamics model is developed and validated, in which sea water enters at 65 °C, which represents the maximum limit of current MED plant, and falls on a tube of 25.4 mm diameter from a 2 mm orifice. First, the liquid load is varied between 0.02 and 0.05 kg/(m·s), and its effects on the film thickness and the wetting time are calculated, presented and discussed. Afterwards, the impact of co-current vapor flow on film distribution, wetting time, minimum thickness and its location are quantified and examined by increasing the vapor velocity up to 6 m/s with 2 m/s intervals. It is found that the vapor flow significantly affects the film distribution on the upper half of the tube. The average film thickness and wetting time is reduced by 14.1% and 18.5%, respectively for the vapor velocity of 6 m/s compared to the no vapor flow, for 0.02 kg/(m·s). In addition, the minimum film thickness decreases by 23.1% for 0.02 kg/(m·s). The high decrement in film thickness can lead to dry patches and scale deposition in case of evaporation.

Jumping droplet condensation in internal convective vapor flow

Condensation is an important process in the Rankine cycle that significantly affects overall efficiency. Condensate typically forms a liquid film due to the high surface energy of industrial condenser materials; by engineering the condenser surface with a superhydrophobic layer, however, we can increase condensation heat transfer by an order of magnitude with the jumping droplet mode of condensation. While the basic phenomenon of jumping droplet condensation has been explored in depth, its effects on heat transfer and pressure drop in confined vapor flow inside a condenser tube, as in power plant condensers, have not been considered. Here, we report an experimental study of internal forced convective condensation with hydrophilic, hydrophobic, and superhydrophobic surfaces to study condensation in the filmwise, dropwise, and jumping droplet modes, respectively. The condenser tube samples were tested in a closed system internal flow condensation setup, and the heat transfer and pressure drop behavior were characterized over various operating conditions. In the jumping droplet mode, the heat transfer coefficient was highest at lower condensation heat flux and condenser surface subcooling, but a transition to the flooded mode at higher subcooling resulted in a heat transfer coefficient comparable to filmwise condensation. For dropwise condensation in the hydrophobic tube, the condensation heat transfer coefficient increased with the vapor velocity, similar to observations in past work. In addition to a large heat transfer coefficient, the pressure drop with the superhydrophobic tube samples was the lowest. These experimental results demonstrate the viability of harnessing the jumping droplet mode of condensation to enhance heat transfer and reduce pressure drop for internal forced convective flow condensation in industrial condensers.

A Numerical Study on Performance Improvement in Boiling Heat Transfer via Dichloromethane-Based Nanofluid Utilization

Nanofluids are colloidal suspensions and have been utilized frequently in heat transfer implementations ranging from heating to cooling. In this study, for numerically specifying the influence of nanoparticle addition inside the base fluid on pool boiling heat transfer, a series of numerical analysis were performed by using hexagonal boron nitride and dichloromethane as nanoparticle and base fluid material, respectively. The nanofluid solution was prepared by doping hexagonal boron nitride nanoparticles into the dichloromethane at the rate of 1.0% (vol.) and Sodium Dodecyl Benzene Sulfonate was added into this solution to prevent nanoparticle compilation during operation. Computational Fluid Dynamics (CFD) approach was preferred and ANSYS Fluent software was used for numerical analysis. In order to be able to make comparison, analysis was performed both dichloromethane and hexagonal boron nitride nanoparticles containing dichloromethane, i.e. nanofluid, solutions. Vapour volume fractions, velocity vectors and contours for each working fluid were determined and discussed.

Convective filmwise condensation on the outer surface of a vertical tube: A theoretical analysis

In this study, laminar filmwise condensation on the outer surface of a vertical tube in the presence of a flowing vapor is theoretically investigated. Analytical solution is obtained for the condensation on an isothermal tube wall when the film thickness is much smaller than the tube radius. The presence of vapor shear stress acting on the liquid-vapor interface was found to enhance the condensation heat transfer by accelerating the condensate film flow. The shearing enhancement factor increases almost linearly with the increase in vapor velocity. In addition, the influence of the vapor shear also increases with the reduction in the tube radius. The combined effect of surface tension and vapor velocity on the condensation heat transfer characteristics of the vertical tube was also investigated. From our investigation, it was determined that the surface tension has negligible effect on the condensation heat transfer performance of mini- and macro-size tubes. However, the influence of the surface tension becomes prominent for condensation on micro-size tubes. The influence of the Kelvin effect was also studied and it was found that the Kelvin effect is insignificant for condensation on micro-size tubes or tubes of larger sizes. A conjugate heat transfer analysis was also performed with cooling water flowing in the internal tube channel and flowing vapor condensation on the outer tube surface. In this analysis, the variation in the tube wall temperature was considered. A good comparison between our theoretical model and experimental results was achieved. The condensation heat transfer coefficient decreases as the cooling water velocity increases. The condensation heat transfer coefficient predicted for countercurrent flow configuration is larger than that for cocurrent flow configuration. The condensation heat transfer coefficients of both cocurrent and countercurrent configurations are larger than those where the tube wall is isothermal. For both cocurrent and countercurrent flow configurations, the thermal resistance lies mainly on conduction through the tube wall and convection of cooling water at the inlet condensing region whereas the dominant thermal resistance gradually shifts to the condensate film as the distance increases from the top of the tube. The tube wall temperature is affected by a combined effect of the thermal resistance distribution and the variation in the overall temperature difference.