Instability of subcooled boiling film on a vertical wall

Consider a vertical plate with a leading edge. The temperature of the plate is above the spontaneous nucleation temperature, so that vapor completely covers the plate. Two-dimensional quasi-parallel theory is used to examine the stability of the two-phase system. Numerical calculations show that the main parameter determining the thickness of the vapor film and its stability is the difference between the temperature of the heated vertical plate and the saturation temperature of the liquid. As the thickness of the vapor film is made smaller, the nose of the neutral curve approaches that of the corresponding one-phase liquid. The overall temperature difference between the plate and liquid bulk does not strongly influence stability properties.     

On the atmospheric water vapor transmission function for solar radiation models

A new expression for the integral transmission of atmospheric water vapor has been developed. This expression is based on the latest known water vapor spectral absorption data, on a new vertical atmospheric profile and finally, on the Neckel and Labs model (1981; 1984), incorporating the most recent corrections of the extraterrestrial solar spectrum. The proposed expression can be easily used by solar radiation models in order to predict the beam, diffuse, and global solar radiation, especially in regions where the vertical atmospheric profile is similar to that in Athens.     

西北地区近地面水汽特征及其与区域蒸发关系

针对现有水文循环要素研究中较少关注近地面水汽的问题,以西北地区为例,应用NCEP/NCAR再分析资料分析了水汽垂直分布特征,采用趋势分析法研究了近地面水汽分布特征及其与区域蒸发的响应关系。分析结果界定了西北地区的近地面水汽层,水汽空间分布受地形地势影响十分显著,近50年来近地面水汽含量呈减少的趋势,并与区域蒸发量的变化呈正相关关系,蒸发量减少是造成近地面水汽含量减少的主要因素。     

Effects of Working Fluid,Tubeside Enhancement and Bundle Depth on the Optimized Fin Geometry of a Horizontal Condenser Tube

A theoretical study has been made to optimize the fin geometry of a horizontal finned tube which is to be usedfor condensers that handle the vapor load of a liquid phase change cooling module.Systematic numerical calcu-lations of the vapor to coolant heat transfer have been performed for parametric values of fin height,fin spacing,vertical bundle depth and tubeside heat transfer coefficient.Three dielectric fluids (R-113,FC-72,and FC-87)at atmospheric pressure were selected as the working fluids.For a single tube with optimized fin geometry,theaverage heat flux increased in the order of FC-87,R-113 and FC-72.Both the optimum fin height and optimumfin spacing increased with increasing vertical bundle depth.     

Modeling the development of hydrogen vapor cloud considering the presence of air humidity

A three-dimensional CFD model for large-scale liquid hydrogen spills is developed and validated by the experiments carried out by NASA. The effect of humidity on the development of hydrogen vapor cloud is emphasized, with the modified expressions of Lee model accounting for the phase changes of water and hydrogen. The results show that the numerical prediction is more consistent with the experiment considering the presence of air humidity. The condensation of water in the atmosphere increases the buoyancy of the vapor cloud, and promotes the diffusion of the cloud in vertical direction. The dimension of the cloud in streamwise direction changes little under different humidity, due to the balance between the height-dependent wind speed and the induced buoyancy. The scope of visible cloud indicated by the condensed water vapor expands with the increasing air humidity, and still lies within the flammable domain when the relative humidity approaching to 75%. Water vapor condensation induces the cloud temperature rise under the same concentration, and the leeward part is more influenced compared with the upwind part.     

A three-dimensional thermal-fluid analysis of flat heat pipes

A detailed, three-dimensional model has been developed to analyze the thermal hydrodynamic behaviors of flat heat pipes without empirical correlations. The model accounts for the heat conduction in the wall, fluid flow in the vapor chambers and porous wicks, and the coupled heat and mass transfer at the liquid/vapor interface. The flat pipes with and without vertical wick columns in the vapor channel are intensively investigated in the model. Parametric effects, including evaporative heat input and size on the thermal and hydrodynamic behavior in the heat pipes, are investigated. The results show that, the vertical wick columns in the vapor core can improve the thermal and hydrodynamic performance of the heat pipes, including thermal resistance, capillary limit, wall temperature, pressure drop, and fluid velocities due to the enhancement of the fluid/heat mechanism form the bottom condenser to the top evaporator. The results predict that higher evaporative heat input improves the thermal and hydrodynamic performance of the heat pipe, and shortening the size of heat pipe degrades the thermal performance of the heat pipe.     

Numerical Study of the Forced Convective Condensation on a Short Vertical Plate

Numerical investigation was conducted on the effects of gravity, surface tension, and wall adhesion upon condensation on a short vertical plate. The volume of fluid method was applied to model the interaction between the liquid and vapor phases and to capture the interface. The surface tension was implemented by employing the method of continuum surface force model. A modified phase-change model, derived from basic equations related to the kinetic gas theory, was proposed and verified based on the cases of Nusselt film condensation of water vapor on a vertical flat plate, the forced convection film condensation on a horizontal flat plate, and the capillary blocking due to condensation in a horizontal miniature circular tube. The predicted results showed that a laminar capillary wavy flow regime exists and the waves enhance the heat transfer of condensation on the plate. The mean film thickness increases and the heat transfer performance becomes worse with decrease of gravity. A high value of surface tension or contact angle, representing a large surface free energy difference, leads to an enhancement of heat transfer on the plate with large-amplitude waves.     

Heat transfer research on vapor‐gas mixture with condensation in a vertical tube

The convection‐condensation heat transfer of vapor‐gas mixtures in a vertical tube was studied theoretically and experimentally. The effects of the condensation of a small amount of water vapor (8 to 20%) on heat transfer in a vertical tube were discussed. Comparisons show that theoretical solutions obtained through modified film model and experimental results are in good agreement. The results show that the condensation heat transfer of a small amount of water vapor and single‐phase convection heat transfer in the vapor‐gas mixtures are of the same order of magnitude, and these two modes of heat transfer could not be neglected. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 531–539, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10055     

Film condensation in horizontal microchannels: Effect of channel shape

The paper gives a progress report on a theoretical study of film condensation in microchannels. The model takes account of surface tension, vapor shear stress and gravity. The effect of channel shape is investigated for condensation of R134a in channels with cross sections: square, triangle, inverted triangle, rectangle with longer side vertical, rectangle with longer side horizontal and circle. The case considered here is where the channel wall temperature is uniform and the vapor is saturated at inlet. For a given mass flux, the local condensate film profile around the cross section is calculated together with the mean heat-transfer coefficient at different distances along the channel. Results are presented here for one vapor mass flux, one vapor temperature and one wall temperature.     

Condensation heat transfer enhancement in the presence of non-condensable gas using the interfacial effect of dropwise condensation

Heat transfer characteristics of dropwise condensation (DWC) were experimentally studied on a vertical plate for a variety of non-condensable gas (NCG) concentration, saturation pressure, and surface sub-cooling degree. As the heat transfer performance was dominated by the vapor diffusion process near the interface of the gas–liquid within the gas phase, the additional thermal resistance of the coating layer may not be strictly limited, a fluorocarbon coating was applied to promote dropwise condensation mode. Compared with the traditional filmwise condensation (FWC), heat and mass transfer with NCG can be enhanced with the dropwise condensation mode. In the present paper, the effect of condensate liquid resistance should not be regarded as the most vital factor to explain the results, but the vapor diffusion process. This is attributed to the liquid–vapor interfacial perturbation motion caused by coalescence and departure of condensate droplets. The results also demonstrated that the feature of droplets departure is the dominant factor for the steam–air condensation heat transfer enhancement.     

An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture.     

Heat and mass transfer in the liquid film on a vertical wall in roll-wave regime

The investigation of thin liquid film flowing down a vertical wall in the roll-wave regime in presence of heat and mass transfer through the free surface is presented. The roll-wave equation taking into account heat and mass transfer through the liquid–vapor interface has been derived. The self-similar solutions of the progressive-wave type for film thickness have been obtained. The families of discontinuous solutions have been constructed, where the progressive waves are conjugated with each other or with “residual” film thickness through the strong discontinuity. As an example, the calculations of the condensate water film flowing down a vertical surface are presented.     

Experimental identification of the phenomenon triggering the net vapor generation in upward subcooled flow boiling of water at low pressure

Observation of the bubble behavior was made using a high-speed camera to investigate the mechanisms to cause the net vapor generation in subcooled flow boiling. In the experiments, water was used as the test fluid, the flow direction was vertical upward, and the pressure was kept close to the atmospheric pressure. At high liquid subcooling close to the condition of the onset of nucleate boiling, all the bubbles were lifted off the heated surface immediately after the nucleation to disappear quickly in the subcooled bulk liquid due to condensation. It was found that the void fraction did not increase significantly unless the liquid subcooling became low enough for some bubbles to be reattached to the heated surface after the lift-off. When the reattachment took place, the bubble lifetime was substantially elongated since the bubbles slid up the vertical heated surface for a long distance after the reattachment. The reattachment therefore contributed to an increase in the void fraction. It was concluded that in the experimental conditions tested in this work, the bubble reattachment to the heated surface was a key phenomenon to cause the sharp increase of the void fraction at the point of net vapor generation.     

Experimental assessment of the effects of body force, surface tension force, and inertia on flow boiling CHF

The interfacial instabilities important to the modeling of critical heat flux (CHF) in reduced-gravity systems are sensitive to even minute body forces, especially for small coolant velocities. Understanding these effects is of paramount importance to both the reliability and safety of two-phase thermal management loops proposed for future space and planetary-based thermal systems. Unfortunately, reduced gravity systems cannot be accurately simulated in 1g ground-based experiments. However, ground-based experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1g flow boiling experiments at different orientations. Boiling experiments were performed using FC-72 in vertical and inclined upflow and downflow, as well as horizontal flow, and with the heated surface facing upward or downward relative to gravity. CHF was very sensitive to orientation for flow velocities below 0.2 m/s and near-saturated flow; CHF values for downflow and downward-facing heated surface were much smaller than for upflow and upward-facing surface orientations. Increasing velocity and subcooling dampened the effects of flow orientation on CHF. For saturated flow, the vapor layer characteristics fell into six different regimes: wavy vapor layer, pool-boiling, stratification, vapor stagnation, vapor counterflow, and vapor concurrent flow. The wavy vapor layer regime encompassed all subcooled and high-velocity saturated conditions at all orientations, as well as low-velocity upflow orientations. Prior CHF correlations and models were compared, and shown to predict the CHF data with varying degrees of success.     

Total and partial condensation of binary mixtures under gravity driven film flow

Laminar film condensation at a vertical flat plate is considered under the aspects of total and partial condensation. With the aid of a perturbation approach the heat and mass transfer correlations for total condensation are derived from the governing balance equations, thus avoiding the complex iterative procedure of numerically solving the balance equations. This approach is particularly valuable during partial condensation where the more volatile component accumulates in the vapor phase. If this vapor is also the lighter component, which is most often the case, buoyancy forces lead then to an upward directed vapor flow, whereas the liquid flows down. Only the buoyancy forces of such vapor mixtures, as for example methanol–water, where the more volatile component is specifically heavier, are downward directed as is the liquid film. Buoyancy forces of the vapor reach a maximum for a given suction rate depending on the relevant properties of the mixture. These effects are illustrated on the basis of practical examples.     

Condensation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger

Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow.     

Approximate equations for film condensation in the presence of non-condensable gases

Non-condensable gases greatly influence vapor condensation, resulting in a substantial reduction in the condensation heat transfer coefficient. Although extensive analytical and numerical investigations of condensation heat transfer in the presence of non-condensable gases have been done, most of the solutions are quite complicated. Based on a thermodynamics analysis, when the vapor is not close to its critical state and the mass fraction of the non-condensable gas in the main stream is less than 0.1, an equation which relates the vapor/gas-liquid interface parameters and the main stream parameters was developed in the present work. For forced convection film condensation heat transfer on the outside surface of a horizontal tube, the present equation combining with an existing analytical solution as well as a heat transfer correlation given by previous investigators, gives the heat flux and the interfacial parameters of the water vapor-air mixture. The results show that the predicted heat flux is in good agreement with experimental data available in the literature and that even a small amount of air substantially reduces the heat flux. An algebraic equation set is given to calculate free convection film condensation on a vertical flat surface, which associates the interfacial and main stream parameters, an integral solution and an analytical solution given by previous investigators. The calculated results are in good agreement with experimental data in the literature.     

Study of a Vertical Boiling Flow in Rectangular Mini-Channels

Mini-channel heat exchangers with boiling flows present optimal performances: they are highly efficient and compact and require low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient and the eventual premature dry-out in mini-channels. Therefore, new studies are needed to provide better knowledge on flow boiling phenomena in small, confined spaces. The proposed paper presents an experimental study of vertical flow boiling in mini-channels. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapor quality, and mass flow rate values have been tested. A critical dry-out vapor quality depending on the mass flow rate has been found. Nevertheless, the superficial velocity appears to be much more appropriate than the vapor quality or the mass flow rate for the dry-out occurrence prediction. A clean dependence with a single critical velocity value has been found.     

Evaluation of the performance of the empirical correlations used to predict R134a's boiling frictional pressure drop inside smooth and corrugated tubes

Owing to the generalization problem, there aren't sufficient empirical correlations for two-phase flows. So as to investigate the thermal features of the two-phase flow in smooth and enhanced tubes, a suitable procedure of the models and correlations related with the heat transfer coefficients, friction factors and two-phase multipliers are needed because a significant variation in thermal properties happens during phase-change. Comparison of frictional pressure drop of R134a during flow boiling phenomena occurred in a smooth and 5 enhanced tubes with well-known empirical correlations were performed in this study. The apparatus has 0.85 m long double tube for vertical configuration as a test section that includes smooth and corrugated copper tubing having inner diameters of 0.0087 m, and the range of mass fluxes are between 200 and 400 kg m^− 2 s^− 1. The average vapor qualities vary from 0.14 to 0.86, and saturation pressure interval is between 4.5 and 5.7 bar. The mean boiling heat transfer coefficient of R134a is determined via energy balance in the test section. The estimation performance of 36 empirical correlations in literature proposed for convective boiling flows in smooth and corrugated tubes are evaluated by means of authors' database (350 data points for vertical tubes). Boiling trend lines have been plotted for the change of vapor quality, liquid phase Reynolds numbers with gas phase ones. In addition, the most successful correlations are confirmed their predictabilities for the vertical adjusted evaporator having smooth and corrugated tubes using the database of authors' earlier publications in open sources.     

Studies on pool boiling heat transfer: Macrolayer thickness in transition boiling

Macrolayer thicknesses in transition boiling were determined from the energy balance relation q_tr = ρ_lH_fgδ_l·f , based on measurements of q_tr (the time-averaged heat flux in transition boiling) and f (the detachment frequency of vapor masses) for water and ethanol boiling on vertical and horizontal 15-mm-diameter surfaces under atmospheric pressure. The macrolayer thickness for the vertical surface, designed to prevent liquid contact with the periphery of the surface during the vapor mass hovering, agreed well with the correlation proposed previously by the present authors, when the heat flux at macrolayer formation is obtained from a nucleate boiling curve extrapolated to the superheat of transition boiling. The macrolayer on the horizontal surface was apparently thickened due to the inflow of bulk liquid beneath the growing vapor masses. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 568–583, 1998