MHD effect on flow structures and heat transfer characteristics of liquid metal-gas annular flow in a vertical pipe

The magnetohydrodynamic (MHD) effect on the flow structures and heat transfer characteristics was studied numerically for a liquid metal-gas annular flow under a transverse magnetic field. The side layers, in which the velocity was increased, appeared near the eastern and western sidewalls in an annular MHD flow as in a single-phase liquid metal MHD flow. Temperature distribution in the liquid film, and the Nusselt number distribution in the angular direction were influenced by the flow structures with the side layers. Consequently heat transfer rate was higher at the eastern/western sidewalls than that at the southern/northern walls. The pressure drop in the MHD annular flow is of the same order of magnitude as in the single-phase MHD pipe flow under similar liquid metal flow condition.     

Interfacial friction factors in countercurrent stratified two-phase flow in a nearly-horizontal circular pipe

The interfacial shear stresses for a countercurrent stratified two phase flow in a nearly horizontal circular pipe were determined from a momentum balance using gas and liquid flow rates, wall shear stresses and liquid holdups. The interfacial shear stresses were approximated to be a function of interfacial friction factors, gas and liquid velocities. An empirical correlation for predicting the interfacial friction factors has also been developed for practical applications.     

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.     

Steady flow and heat transfer of a Sisko fluid in annular pipe

The flow and heat transfer problem of a Sisko fluid in an annular pipe is considered. The governing nonlinear equation of an incompressible Sisko fluid is modelled. Both analytical and numerical solutions of the governing nonlinear problem are presented. The analytical solutions are developed using homotopy analysis method (HAM) and for the numerical solutions the finite difference method in combination with an iterative scheme is used. A comparison between the analytical and the numerical solutions is presented. Moreover, the shear-thinning and shear-thickening behaviors of the non-Newtonian Sisko fluid are discussed through several graphs and a comparison is also made with the Newtonian fluid.     

Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998     

Hydrodynamics and mass transfer in decaying annular swirl flow

This paper reports an electrochemical study of local mass transfer behaviour in decaying annular swirl flow. Initially, flow visualisation experiments were conducted to observe the general behaviour of the flow. It was found that the swirl angle decays exponentially along the tube. Measurements of pressure drop across the vaned swirl generators were correlated in terms of the inverse of the square of the tangent of the vane angle and approximately the square of the mean fluid velocity for vane angles of 30° and over. Measurements of the axial distribution of local mass transfer coefficient for the inner rod were carried out using the electrochemical limiting diffusion current technique and results were correlated for each swirl generator, in the Reynolds number range 3300–50000. It was found that the relative enhancement of mass transfer in swirl flow increases as the vane angle increases and Reynolds number decreases.     

Radiative heat transfer to flow of a combustible mixture in a vertical pipe

The flow of a combustible gas in a vertical cylinder, in the presence of radiative heat transfer, affords the closest model to biomass moving bed gasifier operating at temperatures between 750 and 1500 K. This problem forms the subject matter of the paper under the simplistic assumption of a binary reaction A → B. Under the general differential approximation for radiation, the temperature is perturbed about the wall temperature, and the nonlinear differential equations are subsequently integrated in a closed form. Consequences of the effect of the Arrhenius activation energy are discussed quantitatively.     

Post-CHF heat transfer during two-phase upflow boiling of R-407C in a vertical pipe

This paper reports a study of heat transfer in the post-critical heat flux (post-CHF) regime under forced convective upflow conditions in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length. Experiments were conducted with non-azeotropic ternary refrigerant mixture R-407C for reduced pressures ranging from 0.37 to 0.75, mass flux values from 1200 to 2000 kg/m² s and heat flux from 50 to 80 kW/m². Data shows a considerable effect of system pressure on the post-CHF heat transfer coefficient for specified mass and heat fluxes. The post-CHF heat transfer coefficients for R-407C are compared with three existing correlations which are found to over predict the current data. A modified correlation to represent the experimental data for R-407C is presented.     

Momentum and mass transfer on sandpaper-roughened surfaces in pipe flow

A series of three electrochemically obtained nickel pipes with different surface roughness degree has been used in this study. In respect to momentum transfer, these rough surfaces exhibit similar patterns in terms of friction factor, friction similarity function, and rough-to-smooth friction factor ratio. The mass transfer is investigated with the electrochemical technique which exploits the red-ox reactions of potassium ferri- and ferro-cyanides. The gain in mass transfer enhancement is positive against the friction development in the range 3000-30,000 for the Reynolds number.     

Mixed free convection and mass transfer flow along a vertical needle

The combined free convection and mass transfer flow in a plume over a vertical needle is studied. This mixed type of flow is produced from a point heat-mass source at the tip of the needle. A numerical solution of the similarity equations of the problem under consideration is obtained. The velocity, temperature and concentration profiles are shown for different values of the dimensionless parameters entering into the problem under consideration. The flow field is greatly influenced by the dimensionless parameters α (heat-source strength) and β (mass-source strength).     

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     

Assessment of heat transfer and flow characteristics of a two-phase closed thermosiphon

In this study, comprehensive modeling and simulations were developed and carried out to perform the investigation of the thermal performance of the enclosed thermosiphon through pool boiling in the evaporator sector and the condensation of the liquid film in the condenser part. To simulate these phenomena, the volume of fluid model was utilized. The simulation modeling using the computational fluid dynamics (CFD) technique was validated with existing experimental results, and a good agreement was reached. The simulation results were presented and evaluated in terms of temperature profiles and contours, the volume of fraction contours, and velocity vector distribution. Moreover, the thermal performance (ie, the heat transfer coefficient and thermal resistance) through the thermosiphon operation was analyzed. From the simulation results, it is found that the thermosiphon performance can be improved by the tilt angle and fill ratio. The results indicated that the optimal performance (ie, a high heat transfer coefficient and a low thermal resistance) was attained at a power input of 250 W, tilt angle of 90°, and fill ratio of 0.5. The established CFD simulations effectively predicted the formation of two-phase flow pattern and boiling and condensation zones with water at a low power input, termed as geyser boiling.     

Condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe

Condensation heat transfer and pressure drop data of R-134a in annular helical pipes is of significant importance to the effective design and reliable operation of helical pipe heat exchangers for refrigeration, air-conditioning, and many other applications. This paper presents the experimental investigation on condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe. The average condensation heat transfer coefficients and pressure drops were experimentally determined for R-134a at three different saturated temperatures (35 °C, 40 °C, and 46 °C). The experimental results are compared with the data available in the literature for helical and straight pipes.     

Interfacial area concentration in annular two-phase flow

Accurate prediction of the interfacial area concentration is essential to successful development of the interfacial transfer terms in the two-fluid model. The interfacial area concentration in annular flow and annular-mist flow is especially relevant to the transition process to the liquid film dryout, which might lead to fatal problem in the safety and efficient operation of boiling heat transfer system. However, very few experimental and theoretical studies focusing on the interfacial area concentration in annular flow region have been conducted. From this point of view, measurements of annular flow parameters such as one-dimensional interfacial area concentration of liquid film and local interfacial area concentration profile of liquid film were performed by a laser focus displacement meter at 21 axial locations in vertical upward annular two-phase flow using a 3-m-long and 11-mm-diameter pipe. The axial distances from the inlet (z) normalized by the pipe diameter (D) varied over z/D = 50–250. Data were collected for preset gas and liquid flow conditions and for Reynolds numbers ranging from 31,800 to 98,300 for the gas phase and 1050 to 9430 for the liquid phase. Axial development of the one-dimensional interfacial area concentration and the non-dimensional local interfacial area concentration profile of liquid film were examined with the data obtained in the experiment. Total interfacial area concentration including liquid film and droplets was also discussed with help of the existing drift-flux model, entrainment correlation, and droplet size correlation.     

Numerical simulation of heat transfer to separation air flow in an annular pipe

Separation and reattachment of air flow through a sudden expansion in an annular passage are considered in this study. Backward facing steps play a vital role in the design of many heat related applications where heat transfer is concerned. In the present work, numerical simulation is performed using computer fluid dynamics package (Fluent) to study the effect of step flow in an annular passage. The results are compared with the preliminary experimental findings. In the study, the flowing fluid was considered heated uniformly from the beginning of the expansion. Constant heat flux approach was also considered for the heat transfer investigation. Annular pipe flow system having a step ratio of D/d = 1.8 was considered where d and D are representing the diameter of the pipe before and after expansion. Numerical simulation review shows that the reattachment point extends further with the increase of velocity for different occasions. Finally, the local Nusselt number (Nu) in separation flow increases with the increase of Reynolds number (Re).     

Heat and mass transfer in a countercurrent moving bed dryer

The objective of this work is to study the heat and mass transfer between air and soybean seeds in a countercurrent moving bed dryer, based on the application of a two-phase model to the drying process. The numerical solution of the model is obtained by using a computational code based on BDF methods (Backwards Differentials Formulas). The experimental data of air humidity and temperature and of seed moisture content and temperature at the dryer outlet are compared to the simulated values, showing a good agreement.     

Experiments for liquid phase mass transfer rate in annular regime for a small vertical tube

The double film extraction technique was used to measure the deposition rate and the entrainment rate of droplets for vertical upward annular two-phase flow in a small diameter tube. The test section was a round tube of 5 mm in inside diameter, air and water were used as test fluids and the system pressure was varied within 0.14–0.76 MPa. It was shown in the present experimental conditions that the deposition rate was primarily influenced by the droplet concentration in the gas core and that the entrainment rate was correlated well with the dimensionless number denoting the ratio of interfacial shear force to surface tension force acting on the surface of liquid film. These results were consistent with available empirical correlations that were developed using the experimental data for larger diameter tubes.     

Single-phase and two-phase magnetohydrodynamic pipe flow

Experimental results are presented for a range of measured temperatures and other parameters of vertically downward flows, both single-phase (sodium) and two-phase (sodium-nitrogen), in a conducting-wall pipe in the presence of a transverse magnetic field. Existing MHD theory predicted, to within experimental error, all single-phase pressure differences for magnetic interaction parameter values of up to approximately 100, beyond which the single-phase normalized resistance coefficients were noticeably lower than the laminar-flow predictions. The magnetic interaction parameter at which such deviation occurred was governed by the conductivity ratio. Two-phase pressure differences were obtained across a range of void fractions, approximately 0.3-0.8, where two distinct flow regimes were encountered. For those two regimes, the normalized resistance coefficients of pressure difference were predicted to within experimental error by the corresponding two-phase MHD pressure-difference models. In half of the two-phase cases examined, decreases were observed in normalized resistance coefficients at high values of the magnetic interaction parameter, a trend similar to that found in single-phase flow. The wall-voltage profiles of single-phase flows were symmetric with respect to the center of the applied magnetic field region; two-phase wall-voltage profiles were asymmetric because of the expansion of the gaseous nitrogen along the length of the test section. The influence of temperature and other system parameters upon pressure differences and wall voltages, and the possible effect of ‘M-shaped’ velocity profiles in the two types of flow are discussed.     

Large eddy simulation of heated vertical annular pipe flow in fully developed turbulent mixed convection

The goal of the present study was to perform a large eddy simulation of vertical turbulent annular pipe flow under conditions in which fluid properties vary significantly, and to investigate the effects of buoyancy on the turbulent structures and transport. Isoflux wall boundary conditions with low and high heating are imposed. The compressible filtered Navier-Stokes equations are solved using a second order accurate finite volume method. Low Mach number preconditioning is used to enable the compressible code to work efficiently at low Mach numbers. A dynamic subgrid-scale stress model accounts for the subgrid-scale turbulence. Comparisons were made with available experimental data. The results showed that the strong heating and buoyant force caused distortions of the flow structure resulting in reduction of turbulent intensities, shear stress, and turbulent heat flux, particularly near the wall.