Correlations for the mass transfer rate of droplets in vertical upward annular flow

New correlations for the deposition rate and entrainment rate of droplets in vertical upward annular flow were developed from simple models and available experimental data. In the correlation for the deposition rate, the superficial gas velocity was used as the parameter of primary importance at low droplet concentration while the droplet concentration itself at high droplet concentration. In correlating the rate of droplet entrainment, the ratio of interfacial shear force to the surface tension force acting on the surface of liquid film was the appropriate scaling parameter to correlate the experimental data measured in varied conditions. The experimental data for air–water annular flow were used in the development of the present correlations since extensive databases were available. It was however confirmed that the present model provides satisfactory agreements with the experimental data for high-pressure steam–water annular flow.     

Post-dryout heat transfer to high-pressure water flowing upward in vertical channels with various flow obstacles

Post-dryout heat transfer to high pressure water was investigated experimentally in vertical tubes and annuli containing various flow obstacles. The operational conditions during the experiments were as follows: mass flux from 500 to 1750 kg/m² s, pressure from 5 to 9 MPa, inlet subcooling from 10 to 40 K and heat flux up to 1.5 MW/m². Five different test sections were used in experiments: three annular test sections with inner diameter 12.7 mm and outer diameter 24.3 mm, containing cylindrical and grid flow obstacles in the upper part, and two tubular test sections with inner diameter 24.3 mm with and without pin flow obstacles. The heated length in all test sections was 3650 mm. The wall temperature was measured with 88 thermocouples located along the inner rod and the outer tube surfaces. Due to the presence of flow obstacles, only developing post-dryout heat transfer was observed. Selected post-dryout heat transfer correlations were compared to the experimental data. It has been concluded that all tested correlations predict significantly higher wall temperatures than those obtained in the present experiment. A simple correction function to the Saha model has been suggested which significantly improves the agreement between the correlation and the present data.     

微通道内气体-近壁颗粒流动传热数值模拟研究

数值模拟了微通道受限空间内气体-近璧颗粒流动与传热过程,所建模型考虑微尺度气体的可压缩与交物性特征,且在通道和颗粒壁面采用速度滑移和温度跳跃边界条件以考虑滑移区气体动量/能量非连续效应.在此基础上,计算分析了克努森数(Kn)和颗粒偏移比对颗粒表面拖曳力系数(CD)以及传热努塞尔数(Nu)的影响规律.研究结果表明:受气体...     

Interaction between turbulence and thermal radiation in combined heat transfer channel flows

The aim of the present work is to examine the effects of interaction between turbulence and thermal radiation on the fully developed turbulent channel flow with variable properties in the presence of combined mixed convection‐radiation heat transfer. The vertical and horizontal channels under study are formed by differentially heated flat parallel plates. Large eddy simulation and the low Mach number approach are used to solve the governing equations. Also, the radiative transfer equation is solved using the method. The results are achieved by developing a solver in an open‐source computational fluid dynamics toolbox. The main focus is to find out whether neglecting turbulence‐radiation interaction (TRI) is a valid assumption for such flows under consideration. The present results show that, in both configurations, the maximum values of emission TRI and incident TRI are 2% and 3%, respectively. These results are consistent with the previous findings suggesting that in the nonreactive flows, these two terms are negligible.     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Dynamic characteristics of molten droplets and hot particles falling in liquid pool

The dynamic characteristics of molten droplets and hot particles at the very beginning of their fall into coolant pools are presented. The falling course of a single droplet or a single hot particle was recorded by a high-speed camera and a curve of velocity vs. time was obtained. Emphasis was placed on the effects of the droplet’s size and temperature, the coolant’s temperature and properties, and the droplet’s physical properties on the moving behavior. The results for the all cases showed that the velocity of a falling droplet/particle decreased rapidly but rebounded shortly, at the beginning of droplet/particle falling in the coolant. Following such a V-shaped evolution in velocity, the droplet/particle slows down gradually to a comparatively steady velocity. An increase in either coolant temperature or droplet temperature results in a larger velocity variation in the “J-region”, but a smaller deceleration when it moves out of the “J-region”. The elevated volatility of a coolant leads to a steeper deceleration in the “J-region” and beyond. The bigger size of a particle leads to a greater velocity variation in the “J-region” and terminal velocity. A high melting point and thermal conductivity as well as lower heat capacity contribute to dramatic variation in the “J-region” and low terminal velocity.     

Study of injected water subcooling effect on falling water limitation in countercurrent two-phase flow in vertical channels

This study analytically investigated the subcooling effect of injected water on the falling water limitation in countercurrent two phase flow (CCFL) in vertical channels, by applying a new model of momentum balances for both liquid and gas phases over the entire length of the channel. The subcooling effect of injected water on CCFL, which is one of the dominant parameters, had been clarified neither analytically nor experimentally because the CCFL phenomena is very complicated thermodynamically. As a result of the present study, it has been clarified that the analytical model proposed here could give good predictions of the existing data on the subcooling effect of experiments simulating the performance of emergency core-cooling water injection during a loss-of-coolant accident in pressurized light-water reactors. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 25–38, 1996     

Predicting turbulence and heat transfer in 3-D curved ducts by near-wall second moment closures

This paper presents discussions on predicting turbulence and heat transfer in two types of square sectioned U-bend duct flows with mild and strong curvature by recent second moment closures. Batten et al.'s [AIAA J. 37 (1999) 785] modified version of Craft and Launder's [Int. J. Heat Fluid Flow 17 (1996) 245] two-component-limit (TCL) turbulence model and Shima's [Int. J. Heat Fluid Flow 19 (1998) 549] wall-reflection free model are presently focused on. They are low-Reynolds-number models totally free from geometrical parameters. The former model is realizable and called the TCL model. For turbulent heat flux, a higher order version of the generalized gradient diffusion hypothesis by Suga and Abe [Int. J. Heat Fluid Flow 21 (2000) 37] is applied along with the TCL model. The results suggest that although both second moment closures are generally good enough for predicting flow and heat transfer in the case of mild curvature, only the realizable TCL model is reliable in the strong curvature case.     

Interaction between instream axial flow hydrokinetic turbines and uni-directional flow bedforms

A series of experiments were completed to investigate the interactions between relatively large-scale sediment dunes and axial flow hydrokinetic turbines. Baseline experiments were completed under clear water conditions to assess local scour impacts of single and two-turbine installations. Spatio-temporal measurements of bed elevation were obtained simultaneously with turbine voltage output, from which a measure of the instantaneous rotor angular velocity was used as a proxy for turbine response to unsteady loads. These experiments were completed in a mobile bed of 1.8 mm coarse sand with migrating bedforms. A bedform tracking routine was used to analyze streamwise bed elevation profiles to estimate bedform geometric characteristics and their corresponding impact on turbine performance. Cross-correlation analysis was also performed, investigating critical relationships between approaching bedform crest location, height and the corresponding turbine voltage output. In parallel with the analysis on bedform effects on turbine performance, an extended region downstream of the rotor location was analyzed to investigate how bedform geometric characteristics changed along the channel after turbine deployment.     

Proper orthogonal decomposition (POD) analysis of flow structure in volatile binary droplets

In the present study we perform an in-depth analysis of the internal flow induced by concentration gradients in an evaporating binary ethanol–water droplet. The flow structure during the first stage of evaporation is characterised using micro particle image velocimetry (PIV) to investigate the flow field and analyse various modes of convection. Although PIV shows convection vortices, it does not help in identifying the various superimposed convection modes present. The proper orthogonal decomposition (POD) method is hence used in conjunction with PIV data to identify and characterise prevailing convection modes. During the first stage of evaporation many modes i.e. flow structures are found to co-exist, however the analysis reveals one dominant mode. In addition to the identification of this new mode of convection, the analysis quantifies the energetic contribution of each of these convection modes. The dominant mode is found to contribute to more than half of the total kinetic energy.     

Transient thermal structure, turbulence, and heat transfer in a reattaching slot jet flow

The role of turbulent fluctuations on mean heat transfer coefficient in a reattaching slot jet flow is studied experimentally. Convective heat transfer rate and near-wall fluid flow are examined in the recirculation, reattachment, and post-reattachment regions for two nozzle-to-surface spacings of 0.25 and 0.75 times the width of the nozzle bottom plate. In the reattachment region, results indicate a strong correspondence between variances of near-wall velocity fluctuation and peak heat transfer rate for both spacings. Thermal structures that vary in the spanwise direction are identified in the recirculation region from low-frequency transient infrared thermographs of the heated surface. While these thermal structures are confined to regions in the vicinity of nozzle bottom plate for the low nozzle spacing, they span the entire recirculation region at larger spacings. Thermal streaks are observed past reattachment for the larger nozzle spacing, suggesting a periodic breakup and re-formation of the jet curtain. The scaling of heat transfer distribution is affected by the flow structure in the geometrically non-similar area of the recirculating flow beneath the nozzle. A correlation for peak Nusselt number is presented.     

Analysis of heat and mass transfer between air and falling film in a cross flow configuration

Heat and mass transfer between air and falling solution film in a cross flow configuration is investigated. Effects of addition of Cu-ultrafine particles in enhancing heat and mass transfer process are also examined. A parametric study is employed to investigate the effects of pertinent controlling parameters on dehumidification and cooling processes and their subsequent optimization. It is found that low air Reynolds number enhances the dehumidification and cooling processes. An increase in the height and length of the channel and a decrease in the channel width enhance dehumidification and cooling processes. It is also found that an increase in the Cu-volume fraction increases dehumidification and cooling capabilities and produces more stable Cu-solutions.     

Large eddy simulation of passive scalar in complex turbulence with flow impingement and flow separation

In order to reveal unknown characteristics of complex turbulent passive scalar fields, large eddy simulations in forced convection regimes have been performed under several strain conditions, including flow impingement and flow separation. By using the simulation results, relations between the dynamic and scalar fields are carefully examined. It is then confirmed that the scalar is transported by a large vortex structure near the examined regions wherever the mean shear vanishes, although in the high‐shear regions, the scalar transport is governed by a coherent structure due to the high shear strain. In addition, a priori explorations are attempted by processing the data, focusing on the derivation of a possible direction for modeling algebraically the passive scalar transport in a complex strain field. The a priori tests suggest that an expanded form of the GGDH model introducing a quadratic product of the Reynolds stresses is promising for general flow cases. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 402–418, 2001     

A novel method for measuring the coarse water droplets in wet steam flow in steam turbines

IntroductionWetoess in last stages Of LP steam ~es has beenlmown for a long time as a cause of efficiency loss anderosion of bladeslll. It comprises fine dIDPlets (also fogdrOPletS or Primal droplets) less than about 2 11 m indiameter and coarse water (big drOPlets or secondsdropletS), with diameter ~ about 10 11 m up tO sevedhundreds inicmns.For the fine droplet, the global method, known aSthe extinction method has been chosen tO Obtain itS sizedistribution and concentalon (weiness). …     

Interaction between mean flow and thermo-hydraulic behaviour in inclined louvered fins

In this study the inclined louvered fin, a hybrid fin design based on the slit fin and louvered fin design is considered. The goal of the research program is to investigate the interaction between the flow behaviour (flow deflection and transition to unsteady flow) and the thermo-hydraulics of the fin design. This approach was selected in order to reveal the flow physics behind the transitions found in the thermo-hydraulic data. Through flow visualization (dye injection in a water tunnel) the flow deflection and transition to unsteady flow was studied in different configurations and for varying Reynolds number. The flow deflection was quantified through the ‘fin angle alignment factor’. Validated CFD simulations were used to further explore flow behaviour. In parallel, wind tunnel measurements were performed measuring the local heat transfer coefficients for the different louvers and the overall pressure drop. The impact of the fin pitch, fin angle and Reynolds number were studied. A comparison of both local and global parameters to the observed flow behaviour revealed the strong coupling between the flow and the thermo-hydraulics showing evidence of boundary layer driven flow.     

Interaction between struts and swirl flow in gas turbine exhaust diffusers

The increasing use of gas turbines in combined cycle power plants together with the high amount of kinetic energy in modern gas turbine exhaust flows focuses attention on the design of gas turbine diffusers as the connecting part between the Brayton/Joule and the Rankine parts of the combined cycle. A scale model of a typical gas turbine exhaust diffuser is investigated experimentally. The test rig consists of a radial type, variable swirl generator which provides the exhaust flow corresponding to different gas turbine operating conditions. Static pressure measurements are carried out along the outer diffuser walls and along the hub of the annular part and along the centerline of the conical diffuser. Velocity distributions at several axial positions in the annular and conical diffuser have been measured using a Laser Doppler Velocimeter (LDV). Pressure recovery coefficients and velocity profiles are depicted as a function of diffuser length for several combinations of swirl strength, tip flow and strut geometries. The diffuser without struts achieved a higher pressure recovery than the diffuser with struts at all swirl angle settings. The diffuser with cylindrical struts achieved a higher pressure recovery than the diffuser with profiled struts at all swirl angle settings. Inlet flows with swirl angles over 18?affected the pressure recovery negatively for all strut configurations.     

Comparative study between parallel and counter flow configurations between air and falling film desiccant in the presence of nanoparticle suspensions

A comparative numerical study is employed to investigate the heat and mass transfer between air and falling film desiccant in parallel and counter flow configurations. Nanoparticles suspensions are added to the falling film desiccant to study heat and mass transfer enhancements. The numerical results show that the parallel flow channel provides better dehumidification and cooling processes of the air than counter flow configuration for a wide range of pertinent parameters. Low air Reynolds number enhances the dehumidification and cooling rates of the air and high air Reynolds number improves the regeneration rate of the liquid desiccant. An increase in the channel height results in enhancing the dehumidification and cooling processes of air and regeneration rate of liquid desiccant. The dehumidification and cooling rates of air are improved with an increase in the volume fraction of nanoparticles and dispersion factor. Copyright © 2003 John Wiley & Sons, Ltd.     

Effects of turbulence model and interfacial shear on heat transfer in turbulent falling liquid films

An improved method is presented for the prediction of heat transfer coefficients in turbulent falling liquid films with or without interfacial shear for both heating or condensation. A modified Mudawwar and El-Masr's semi-empirical turbulence model, particularly to extend its use for the turbulent falling film with high interfacial shear, is used to replace the eddy viscosity model incorporated in the unified approach proposed by Yih and Liu. The liquid film thickness and asymptotic heat transfer coefficients against the film Reynolds number for wide range of interfacial shear predicted by both present and existing methods are compared with experimental data. The results show that, in general, predictions of the modified model agree more closely with experimental data than that of existing models.     

Characteristics of interfacial profiles in upward and downward gas-liquid two-phase plug flow

Gas-liquid interfacial profiles in plug flow for both upward and downward flows were obtained using semi-supermultiple point-electrode probes, comprising 67 sensing tips arranged on a tube diameter. Typical interfacial profiles are demonstrated for both flows. Close inspection of the profiles reveals that four zones exist in a pair of gas and liquid slugs for upward plug flow and a high slip velocity region in downward plug flow. The lengths of the swelling liquid front zone and the wake zone were determined. The length of the wake zone strongly depends on the relative velocity between the liquid film around the gas slug and the liquid phase in the liquid slug. Characteristic distributions of bubbles within liquid slugs were found, i.e., three types of radial distributions of void fraction, namely saddle-shaped, trapezoidal and bullet-shaped distributions, exist for upward flow. The two types for downward flow exclude the saddle-shaped distribution. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25 (8): 568–579, 1996     

Experimental study of post-dryout with R-134a upward flow in smooth tube and rifled tubes

A study of post-dryout heat transfer was performed with a directed heated smooth tube and rifled tubes using vertical R-134a up-flow to investigate the heat transfer characteristics in the post-dryout region. Three types of rifled tube having different rib height and width were used to examine the effects of rib geometry and compare with the smooth tube, using a mass flux of 70–800 kg/m² s and a pressure of 13–24 bar (corresponding to an approximate water pressure of 80–140 bar). Wall temperature distribution in all tubes was strongly dependent on pressure and mass flux. Wall temperatures of the rifled tubes in the post-dryout region were much lower than for the smooth tube at same conditions. This was attributed to swirl flow caused by the rib. Thus, the thermal non-equilibrium, which is usually present in the post-dryout region, was lowered. The empirical correlation of heat transfer in the smooth tube of the post-dryout region was obtained. The heat transfer correlation for rifled tubes was also obtained as a function of rib height and width with the modification of the smooth tube correlation.