Wall Subcooling Research Articles

Experimental investigation of tube diameter and inclination angle on steam condensation accompanying with air in tube bundles

Steam condensation accompanying with air is an important thermal-hydraulic phenomenon in reactor containment. Condensation outside tubes is an important method to remove heat out of the containment under an accident. Most of the previous studies have been conducted on single tube and vertical plates, where the findings may not reflect the heat transfer in tube bundle well. The experimental studies were firstly carried out based on 3 × 3 tube bundle with diameters of 12–19 mm and inclination angles of 0–90° in the parameter range of pressure 0.2–1.6 MPa, air mass fraction of 12.1–75.2% and wall subcooling of 28.3–114.1℃. The results indicate that the effect of diameter and inclination angle in the tube bundle varies significantly in different pressure ranges. When the pressure is less than 0.8 MPa, the condensation heat transfer coefficient (CHTC) generally increases with decreasing tube diameter and inclination angle. The CHTC of the heat transfer tube with a tube diameter of 12 mm and an inclination angle of 0° increased by a maximum of 29% compared to that of the heat transfer tube with a tube diameter of 19 mm and an inclination angle of 90°. When the pressure is 0.8–1.6 MPa, the overall CHTC decreases as the diameter of the tube decreases, and the maximum reduction is about 20%. It decreases first and then increases with the decrease of the inclination angle, and the CHTC is the smallest when the inclination angle is about 60°.

A neural network model for free-falling condensation heat transfer in the presence of non-condensable gases

Condensation heat transfer has been widely studied for various applications such as power generation, water desalination, data centers, chemical and pharmaceutical syntheses, heating and air conditioning systems, and especially, the passive containment cooling system (PCCS) in nuclear power plants. The PCCS is designed to reduce the risk resulting from a loss of AC power or compounding human error. In a hypothetical accident situation, one of the main safety systems, the PCCS, condenses water vapor with gravity-driven force to remove the heat from the containment vessel. Although the condensation heat transfer in the presence of non-condensable gases for predicting the heat transfer performance of the PCCS has been successfully investigated, there is still no generalized model or correlation available. In this work, we focused on the development of universal models for predicting the heat transfer coefficients of free-fall condensation heat transfer on the external surfaces of vertical tubes in the presence of non-condensable gases. For this, we created a consolidated database covering a broad range of geometric values and operating conditions, including tube hydraulic diameter Dh = 10–41.2 mm, tube length L = 0.3–3.5 m, total pressure Ptot = 0.15–2.0 MPa, wall subcooling temperature ΔTsub = 5–71 K, average condensation heat transfer coefficients of 90 ≤ ͞h ≤ 19,400 W/m2K, and Reynolds numbers of the film of 8 ≤ Refilm ≤ 7160. We analyzed the influence of varying the geometric and operational parameter values by using this database. Conventional machine learning techniques such as nonlinear regression and multilayer perceptron (MLP) neural network methods were adopted to predict the condensation heat transfer rate based on the consolidated database. Moreover, the prediction accuracies of the condensation heat transfer rates of the proposed prediction models and 12 relevant correlations were compared by using the consolidated database. The proposed nonlinear regression model exhibited good prediction accuracy with a mean absolute error (MAE) of 12.7 % for average ͞h, which is much lower than those achieved by previously proposed relevant correlations. In addition, the MLP neural network model showed excellent prediction accuracy with an MAE of 4.2 % for the consolidated database.

CFD analysis of steam condensation with air in the tubes bundle channel under natural convection conditions

The condensation heat transfer characteristics of steam with the non-condensable gases (NCGs) in the tubes bundle channel are quite different from those in the simple geometry structure, such as the single tube and flat plate. In the present work, to evaluate the thermal characteristics of steam condensation with NCGs in the vertical tubes bundle channel, the numerical model of steam condensation was established based on the CFD code ANSYS FLUENT, which combined the gas component transport equations and the diffusion balance model. And several conditions with different thermal parameters were simulated, the simulation results are in good agreement with the experimental data. The condensation heat transfer in a hexagonal 7-tubes bundle channel with various tube pitches (from 1.5d to 5d) is investigated. The results indicate that the distribution of typical thermal parameters, such as velocity, air mass fraction, temperature and density, is quite different from that in the single tube due to the cross and mixture in each condensation tube boundary layer. As the condensation length increases, the air mass fraction rises, the gas flow rate decreases and the sub-cooling decreases, resulting in a lower natural convection intensity. When the tube pitch is less than 4d, the condensation heat transfer capacity in the tubes bundle channel is weakened obviously, and the heat transfer coefficient with the tube pitch 1.5d is only about 32% of that in a single tube. The air mass fraction and pressure have a significant positive effect on the heat transfer coefficient, and the wall sub-cooling has a significant negative effect on that.

Experimental study of air–steam condensation on the influence of tube diameter and inclination angle

Steam-air condensation is one of the important physical phenomena after reactor accidents, which has direct impacts on the discharge of containment residual heat after the accident. Previously, many researchers have proposed experimental correlations for condensation of non-condensable gas, which include the influence of pressure, concentration, and wall subcooling. However, there are relatively few studies on the influence of tube diameter and inclination angle. In order to investigate the heat transfer characteristics of the condenser at different tube diameters (19 mm, 15 mm, 12 mm) and inclination angles (0 ~ 90°), experiments were carried out in the pressure range of 0.15 ~ 1.6 MPa, air mass fraction of 14 ~ 87%, and wall subcooling of 28 ~ 122 °C. It shows that the condensation heat transfer coefficient (HTC) increases with the decrease of the tube diameter and inclination angle. The condensation HTC with a tube diameter of 12 mm and an inclination angle of 0°(Horizontally) is around 32 ~ 38% larger than that of the heat transfer tube with a tube diameter of 19 mm and an inclination angle of 90°(Vertically). Based on a total of 2276 data points from the present work and related experiments, an experimental correlation containing the effect of tube diameter and inclination angle is proposed, and its deviation is within ± 15% with 95% confidence.

Steam condensation on tube-bundle in presence of non-condensable gas under free convection

In the presence of non-condensable gases, the effects of thermal-hydraulic parameters on steam condensation on a tube-bundle heat exchanger differ from those for a single-tube because the geometry of the tube-bundle enhances shadow and suction effects. In this study, experiments were performed to investigate these effects on three types of tube-bundle heat exchangers with pitch-to-diameter ratios of 2.0, 2.5, and 3.0, and pressures, air mass fractions, and wall subcooling of 200–500 kPa, 0.2–0.7, and 20–55 K, respectively. The local bundle factor was defined as the difference in condensation heat transfer coefficients between a single-tube and tubes in a tube-bundle. Therefore, a local bundle factor correlation, which accounts for the shadow and suction effects, was developed. The applicable ranges of the local bundle factor correlation for local pressure loss, Bird's suction factor, and blockage fraction were 0.267–0.734, 1.12–1.98, and 0.044–0.304, respectively. An average bundle factor was derived from the local bundle factor. The developed correlation applying the average bundle factor predicted the heat transfer coefficients for the obtained experimental data within an error of 8.50 %.

Experimental investigation of refrigerant condensation in circular and square micro- and mini- channels

Microchannel condensers have the potential to improve system level efficiency by increasing the heat transfer coefficient because of the small hydraulic diameters. In addition, they offer a high surface area-to-volume ratio, reducing the overall system size and fluid inventory. Understanding condensation in microchannels is essential for efficient component design, especially with the need for new low Global Warming Potential (GWP) working fluids. Condensation of refrigerants R134a, R245fa and R1234ze(E) in circular (Dh = 1.55 mm) and square (Dh = 0.98 mm) microchannels over a range of mass fluxes (50 < G < 200 kg m−2 s−1), saturation temperatures (30 < Tsat< 50°C), and wall subcooling (1.7 < ΔT < 5K) is investigated in this study. It is found that at high mass fluxes (G ≥ 150 kg m−2 s−1), R245fa has a higher heat transfer coefficient and pressure gradient than both R1234ze(E) and R134a, which have comparable heat transfer coefficients at similar conditions. In the circular channel, for a mass flux range of 50 < G < 150 kg m−2 s−1 and Tsat = 30°C, the heat transfer coefficients for R245fa, R134a, and R1234ze(E) varied between 2.3 and 6.7 kW m−2 K−1, 2.5 and 4.7 kW m−2 K−1, and 2.2 and 5.4 kW m−2 K−1, respectively.

Vapor condensation with low air mass fraction inside water seal branch tube under free convection

The pressure vessel has used a newly designed water seal structure, and the sealed water is from condensation at low hydrogen concentration. To get the condensation capacity of the water seal branch tube under the pressure vessel normal operation, the experimental loop named WASETEL was constructed, which is aimed at studying the heat transfer during the process of air-steam mixture condensation inside the inclined branch tube at the condition of low air mass fraction. The inner tube is diameter 0.134 m with length 0.68 m which is connected with diameter 1.4 m, high 2.5 m pressure vessel. A canon camera was set to record the images and videos of the condensate height through glass tube in communicating vessel liquidometer. The experiment was conducted under the pressure ranged from 0.2 to 0.6 MPa, air mass concentration ranged from 0.49 to 6.42% and wall subcooling ranged from 11 to 40 K. The test showed HTC (heat transfer coefficient) would sharply deteriorate with the small air fraction, compared with pure steam condensation, about 0.49% mass fraction of the air will reduce heat transfer coefficient by 70–80%. When air concentration is relatively small, those widely used relations can not predict the condensation well, therefore, a new heat transfer empirical correlation with air mass fraction, pressure and subcooling is proposed, covered all the experimental data within the error band ±13%.

Condensation of steam mixed with non-condensable gas on vertical heat exchanger tubes in circumstances with free convection

Condensation heat transfer is significantly influenced by the geometrical and thermal-hydraulic parameters of the heat exchanger. Therefore, in order to assess their effects, experiments were conducted focused on the steam condensation on the outer surface of a vertical tube in the presence of a non-condensable gas. The experimental data on the heat transfer coefficient (HTC) were produced using three different tubes, with outer diameters of 21.5 mm, 33.6 mm, and 42.4 mm. The pressure, air mass fraction, and wall subcooling in the experiments ranged from 200−600 kPa, 0.05−0.80, and 16.1−55.6 K, respectively. Based on the data set consisting of the experimental data from the present study and the data collected in previous studies, an empirical correlation was developed, which accounts for the effects of the geometrical and thermal-hydraulic parameters. The applicable ranges of the proposed correlation for the air mass fraction, Jakob number, diffusion Reynolds number, and aspect ratio are 0.049−0.890, 8.83 × 10−3−3.41 × 10−2, 1.61−2.37 × 102, and 2.65−17.0, respectively. The maximum error of the correlation was calculated to be 20% with a 95% confidence level, when compared against the experimental data.

Experimental study of steam and steam–air mixture condensation over vertical chrome-plated tube and polished tube exterior surface

In this article, a series of steam condensation experiments were performed on the outer surface of chrome-plated tube and polished tube under steam and steam–air mixed conditions to study their heat transfer characteristics and whether they could be used to optimize Passive Containment Cooling System in advanced nuclear reactors. The effects of total pressure, air mass fraction and subcooling on condensation heat transfer of the chrome-plated tube and polished tube were analyzed. Moreover, basing on the existing experiment, a smooth tube that has the same size as the polished tube and chrome-plated tube was tested under the same conditions. The experimental phenomena indicated that filmwise condensation and dropwise condensation co-exist on the tube exterior surface, and more droplets were observed on the exterior surface of the chrome-plated tube. Under the steam condition, with the fixed wall subcooling, condensation heat transfer coefficient of the polished tube and chrome-plated tube shows different change rules under different pressure: the condensation heat transfer coefficient of the polished tube increases with the increase of pressure, while the chrome-plated tube decreases with the increase of pressure. Compared with the smooth tube, the heat transfer performance can be improved by using the chrome-plated tube and polished tube. Under the steam–air condition, the existence of air deteriorates heat transfer performance seriously. The heat transfer performance of the chrome-plated tube and the polished tube are both improved compared with the smooth tube when the air mass fraction is below 60℃. When the air mass fraction is over 60℃, the heat transfer ability of these tubes are similar to the smooth tube. Besides, within the measurement parameters of our experiment, the increase in total pressure has a positive effect on steam condensation with air.

Lagrangian modeling of intermittent flow condensation in circular micro- and mini-channels

Very few models exist in the literature for predicting phase-change heat transfer in the intermittent flow regime during condensation in microchannels. Intermittent flow is characterized by a transient bubble-slug train, which is modeled here using a transient Lagrangian bubble tracking scheme. The compressibility of the bubbles and local parameters, such as the film thickness, bubble velocity and slug velocity are taken into account. The model also accounts for the phase change due to condensation of vapor in the bubbles into the surrounding liquid. Heat transfer is assumed to occur through conduction in the thin film around the bubble and through convection in the slug. The model is applied to the condensation of R134a in a circular microchannel of hydraulic diameter 1.55 mm at a saturation temperature of 30°C and a wall subcooling of 2.5 K. Hydrodynamic parameters show good agreement with studies in the literature, and condensation heat transfer coefficients are in good agreement with experimental results (AAD = 11%).

Numerical analysis of thermal-hydraulic characteristics of steam-air condensation in vertical sinusoidal corrugated tubes

ABSTRACT The thermal-hydraulic characteristics of steam-air in the vertical corrugated tubes with sinusoidal waveform condensing wall of various amplitudes and wavelengths under the different inlet velocities, air mass fractions and wall subcoolings are numerically investigated, in the cases that the mass, species, energy and momentum sink terms are added in the cells adjacent to the condensing wall to simulate the condensation process. The mechanisms of turbulent flow and heat transfer enhancement of steam-air in condensation are also explored. The results show that the detached vortex characteristics and accumulated air in waveform domain influence the condensation heat transfer greatly, which are closely related to the amplitude and wavelength of basic waveform as well as inlet velocity. To evaluate the thermal-hydraulic performance of steam-air fluid in the corrugated tubes, the thermal-hydraulic performance factor, φ, describing the relation of heat transfer performance (i.e., η) to pressure drop characteristic (i.e., β), is defined. The better thermal-hydraulic performance can be obtained in the corrugated tubes with longer wavelength and higher amplitude waveform condensing wall under a low inlet velocity as well as with lower amplitude waveform condensing wall under a high inlet velocity. Besides, the corrugated tubes with A=0.5, 2 mm at L=21 mm respectively in the inlet velocity range of v=2-4 m/s and v=5-12 m/s are the better design of geometric parameters.

Condensation Heat Transfer of R-407C in Helical Coiled Tube Heat Exchanger

The results of an experimental study to evaluate the characteristics of R-407C thermofluid during condensation in a helically coiled copper tube heat exchanger are presented. The effects of saturation temperature (Tsat), and mass and heat fluxes of refrigerant R-407C on thermal performance and pressure drop were determined. The relationship between the refrigerant wall subcooling and heat transfer coefficients was also investigated. This paper reports the effect of the temperature of the water used as cooling medium on the heat transfer rate of condensing R-407C. The study was conducted with mass flux of R-407C in the range of 100–450 kg/m2s, mass flux of the coolant water in the range of 500–5000 kg/m2s and Tsat of 31 °C, 35 °C, and 39 °C. Compared with a straight smooth tube, the use of the helical coiled (helicoidal) tube increased the condensation rate with a corresponding pressure drop that depended on the value of Tsat of the refrigerant and temperature of the coolant.

Analytical modeling of laminar annular flow condensation in an arbitrarily shaped microchannel

A model for laminar annular film condensation in micro- and mini- channels is developed. The model is first derived for a circular cross section, and then extended to an arbitrarily shaped microchannel using the local radii of curvature and the wall curvilinear coordinate. The film governing equation is applied to a channel with a general wall distribution rw = rw (φ). Using this model, condensation of R134a in square and circular channels with hydraulic diameters of 1 mm at a saturation temperature of 50 °C, a wall subcooling of 6 °C, and mass flux of 500 kg m−2 s−1 is analyzed. Based on the local film distribution, it is shown that in the laminar flow regime, square channels have higher condensation heat transfer coefficients than circular channels. Good agreement is obtained between the predictions of this model and experimental studies in the literature for the condensation of R134a and FC-72 in square microchannels, and R134a and Propane in circular microchannels. The model is also used to predict condensation in channels with elliptical, rectangular, triangular and inverted triangular and trapezoidal cross sections. In addition, a study of the condensation of ammonia in a photochemically etched rectangular microchannel with fillets in the corners due to manufacturing tolerances showed heat transfer coefficients similar to those of an ideal rectangular channel.

Condensation Enhancement on Hydrophobic Surfaces Using Electrophoretic Method and Hybrid Paint Coating

Condensation heat transfer on stainless steel tube utilizing superhydrophobic coatings was investigated. The electrophoretic deposition and spraying methods were employed to coat the tubes’ outer surface. The mixture that was synthesized for spray coating was a hybrid paint. It incorporated polyurethane matrix and a colloidal suspension containing organic nanoparticles. The hybrid paint had a proper adhesion to the substrate which caused more durability compared to the electrophoretic coating. The agglomeration of nanoparticles in the hybrid paint caused the formation of particles with larger size compared to the particles in the electrophoretic coating. Consequently, contact angle hysteresis over electrophoretic coating was much lesser than spraying method. Dropwise condensation regime was observed over both the coatings as a result of hydrophobicity. Nonetheless, the condensation heat flux declined over the hybrid paint at high subcooling temperatures due to transition to the flooding condensation regime. This was due to the accumulation of ultra-small droplets as a result of high nucleation sites. The continuation of droplets sweeping was hindered by sticking to the substrate. The dropwise condensation was maintained over the electrophoretic coating and the transition to the film-wise condensation happened at larger wall subcooling temperature. The ability of the electrophoretic coating in shedding droplets efficiently was caused by higher hydrophobicity, high roughness density, and minimum droplet pinning.

Effect of Surface Inclination on Filmwise Condensation Heat Transfer During Flow of Steam–Air Mixtures

Abstract Empirical/semi-empirical correlations are available in the literature to quantify the effect of several major parameters, like bulk pressure, non-condensable gas mass fraction, and wall subcooling, on condensation heat transfer coefficient (HTC). However, despite numerous applications of condensation on inclined flat plates, there is a lack of understanding of the effect of surface inclination on condensation heat transfer. Accordingly, a dedicated experimental program was undertaken to investigate the effect of surface inclination angle on filmwise steam condensation. Experiments were performed at different bulk pressures (1.7–4.2 bar absolute) and steam-air mass fractions (ranging from pure steam, i.e., 0% to 40% w/w air), with the steam-air mixture flowing over a flat test plate (Re range, 4200–4800). In each run, the inclination angle of the test surface was varied from −90 deg (condensation underneath the horizontal surface, facing downward) to +90 deg (condensation over the horizontal surface, facing upward) in increments of 15–20 deg (inclination angle θ measured from vertical). The results reveal an intriguing trend: for pure steam condensation, the HTCs decrease as the plate is inclined in either direction from the vertical, and the variation is nearly symmetric for both upward- and downward-facing configurations. On the other hand, for steam condensation in the presence of air, the HTCs decrease monotonically for upward-facing configurations, while they increase slightly (10–20%), and decrease subsequently (for θ &amp;lt; −70 deg) for downward-facing cases. Finally, the HTCs for inclined orientations are compared with the HTC in the standard vertical configuration to quantify the effect of inclination angle.

Condensation Heat Transfer Characteristics of Moist Air Outside Hydrophilic and Super-hydrophobic 3D Pin Fin Tube

Three dimensional pin fin tube is an effective heat transfer element. It has been widely used in industry process. However, condensation heat transfer characteristic of moist air outside it has not been experimentally studied. The wettability of tube surface has an important effect on this process. In this study, experiments were carried out, in which moist air condensing outside three dimensional pin fin tube with different wettability. Smooth tube is also tested to make comparison. Effects of steam mole fraction, moist air temperature, and wall subcooling are analysed in detail. Super-hydrophobic surface is obtained using oxidation and self-assembling monolayer method. The results show that heat transfer coefficient increases with the increase of steam mole fraction, wall subcooling and the decrease of moist air temperature. Heat transfer coefficient ranges from 55 to 2,035 W/m2K within the study of this paper, and hydrophilic three dimensional finned tube can achieve 2 times heat transfer coefficient compared with smooth tube and superhydrophobic three dimensional pin fin tube.

Dropwise condensation heat transfer on superhydrophilic-hydrophobic network hybrid surface

Superhydrophilic-hydrophobic (SHPi-HPo) network hybrid surface was designed to investigate the condensation heat transfer using stainless steel as substrate material. The SHPi-HPo surface was comprised of superhydrophilic network grooves and hydrophobic regions. Hydrophobic (HPo) surface was prepared with fluorocarbon coating using polytetrafluoroethylene (PTFE) as the matrix resin and micro-nano silicon dioxide (SiO2) as additive to control surface roughness. Three kinds of SHPi-HPo surfaces were tested, having a grid spacing of 1.5, 2.5 and 3.5 mm and named as SHPi-HPo-1, SHPi-HPo-2 and SHPi-HPo-3, respectively. To study the effects of the wall subcooling, steam mass flux, cooling water temperature, cooling water mass flow rate and grid spacing, a series of experiments were conducted and a high speed camera was used to visualize the condensation process. The results show that SHPi-HPo surface can well control condensate droplet diameters and its condensation heat transfer performance is better than that of smooth hydrophilic (HPi) and HPo surfaces. This is attributed to SHPi-HPo surface sucking away droplets in time and limiting the growth of large condensate droplets through the superhydrophilic grooves. At wall subcooling ΔTw = 6.3 K, the heat transfer coefficient of SHPi-HPo-2 surface is 2.7 and 3.4 times that of HPi and HPo surfaces, respectively. For SHPi-HPo surface, there is optimum grid spacing between superhydrophilic grooves to enhance condensation heat transfer. Among three SHPi-HPo surfaces, the heat transfer coefficient of SHPi-HPo-2 surface has the best condensation heat transfer performance, about 0–10% higher than that of SHPi-HPo-1 surface, and at ΔTw = 9 K, the heat transfer coefficient is 1.7 times that of SHPi-HPo-3 surface.

Numerical investigations on steam condensation in the presence of air on external surfaces of 3 × 3 tube bundles

Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure.

An investigation on vapor condensation on nanopillar array surfaces by molecular dynamics simulation

Molecular dynamics simulation is used to investigate condensation of Argon vapor on nanopillar array surfaces made of copper. It is shown that condensation in the pillar array is nonuniform with respect to height. Effects of pillar height, pitch distance of nanopillars, wall subcooling and wettability on condensation of Argon vapor are discussed. It is found that higher pillars, denser pillars and stronger wall surface wettability can suppress vapor condensation at the bottom of a nanopillar array, making vapor more likely to condense in the upper section of the array. It is also shown that the condensation peak in the nanopillar array can be shift from bottom to top with the increasing wall subcooling. The formation of Cassie state droplets is observed and discussed.

Experimental Investigation of Heat Transfer at Downflow Condensation of Refrigerant R-21 in Assemblage of Minichannels

Presented are results of experimental investigation of heat transfer at downflow condensation of refrigerant R-21 in a vertical assemblage of minichannels in a plate-fin heat exchanger with plane fins. Selection of refrigerant was caused by the fact that its physical properties at room temperature are similar to those of cryogenic liquids at low temperatures. A pattern of vapor-liquid downflow in the heat exchanger channels was plotted using the working section with a transparent wall. For the mass velocity in the range of 30 to 50 kg/m2s and the wall subcooling below −2◦C, the dependence of the heat transfer coefficient on the mass vapor content is determined and a method is proposed to calculate the heat transfer coefficient that is in good agreement with the experimental data.