Wall Mass Transfer Coefficient Research Articles

Two-phase numerical simulation of thermal and solutal transport exploration of a non-Newtonian nanomaterial flow past a stretching surface with chemical reaction

Abstract Non-uniform heat sources and sinks are used to control the temperature of the reaction and ensure that it proceeds at the desired rate. It is worldwide in nature and may be found in all engineering applications such as nuclear reactors, electronic devices, chemical reactors, etc. In food processing, heat is used to cook such as microwave ovens, pasteurize infrared heaters, and sterilize food products. Non-uniform heat sources are mainly used in biomedical applications, such as hyperthermia cancer treatment, to target and kill cancer cells. Because of its ubiquitous nature, the idea is taken as our subject of study. Heat and species transfer analysis of a non-Newtonian fluid flow model under magnetic effects past an extensible moving sheet is modelled and examined. Homogeneous chemical reaction inside the fluid medium is also investigated. This natural phenomenon is framed as a set of Prandtl boundary layer equations under the assumed convective surface boundary constraint. Self-similarity transformation is employed to convert framed boundary layer equations to ordinary differential equations. The resultant system is solved using the efficient finite difference utilized Keller box method with the help of MATLAB programming. The influence of various fluid-affecting parameters on fluid momentum, energy, species diffusion and wall drag, heat, and mass transfer coefficients is studied. Accelerating the Weissenberg number decelerates the fluid velocity. The temperature of the fluid rises due to variations in the non-uniform heat source and sink parameters. Ohmic dissipation affects the temperature profile significantly. Species diffusion reduces when thermophoresis parameter and non-uniform heat source and sink parameters vary. The Eckert number enhances the heat and diffusion transfer rate. Increasing the chemical reaction parameter decreases the shear wall stress and energy transmission rate while improving the diffusion rate. The wall drag coefficient and Sherwood number decrease as the thermophoretic parameter increases whereas the Nusselt number increases. We hope that this work will act as a reference for future scholars who will have to deal with urgent problems related to industrial and technical enclosures.

Influences of Stefan blowing on unsteady flow of Casson nanofluid past a stretching surface

This paper deals with the flow of non-Newtonian nanoliquid past an unsteady stretched exterior in attendance of Stefan blowing/suction. Model for Casson liquid has been taken into account to represent the non-Newtonian liquid. Two phase fluid replica for nanofluid has been considered. Similarity alterations are applied to transform the leading pdes (partial differential equations) to ordinary ones. Runge–Kutta method of order four has been used to solve the altered equations numerically with the help of shooting technique. Wall shear stress, heat and mass transfer coefficients diminish with the rise in unsteadiness. Fluid's velocity and temperature are found to rise with the enhancement in Stefan blowing/suction parameter. Temperature of the fluid rises but fluid's velocity diminishes for the rising values of Casson parameter. Basically, the pertinent parameters affect the stream, warmth and accumulation transport significantly which can be viewed by the figures and tables presented here and also by their physical analysis.

Promotion of heat, mass transfer and energy utilization efficiency in a stirred tank reactor by closely spaced turbulence promoters

ABSTRACT The present study attempts to obviate the drawbacks of the stirred slurry catalytic reactor by using the wall of a square stirred tank reactor as catalyst support where a diffusion-controlled reaction occurs. The electrochemical technique was used to measure the rate of mass and heat transfer (by analogy) at the reactor wall which was fitted with closely-spaced semicylindrical turbulence promoters. The effect of various geometrical and hydrodynamic parameters on the mass transfer coefficient was quantified by dimensionless correlations. The promoters increased the wall mass transfer coefficients up to 80% and the volumetric mass transfer coefficients by an amount varying from 57 to 190% depending on the working conditions. Power consumption measurements reported that the turbulence promoters enhanced the reactormechanical energy utilization up to 303% depending on the working conditions. The additional roles of turbulence promoters in increasing the reaction surface area and cooling the reactor where exothermic diffusion-limited reactions may occur were indicated. Merits of the present reactor over the stirred slurry biochemical reactor such as protecting the catalyst against shear and thermal degradation were highlighted.

Mass and heat transfer intensification at the wall of a square agitated vessel by chemically active semicylindrical turbulence promoters

Rates of mass and heat transfer at the wall of a square agitated vessel fitted with semicylindrical turbulence promoters were measured using the electrochemical limiting diffusion current technique. Variables studied were promoter diameter, spacing between successive promoters, promoter orientation, impeller speed, and impeller geometry. The data were correlated by empirical dimensionless correlations. The promoters increased both the wall mass transfer coefficients and the volumetric mass transfer coefficients by factors ranging from 1.1 to 1.59 and 1.25 to 2.09, respectively depending on the operating conditions. Vertical turbulence promoters enhanced the rate of mass transfer by 3–29% more than the horizontal promoters. Confirming their economic worth, the turbulence promoters enhanced the mechanical energy utilization of the reactor up to 96%. Power consumption measurements revealed that the 45° impeller was more energy-efficient than the 90° impeller up to 54%. Besides the novelty of using turbulence promoters in agitated vessels rather than their common use in flow ducts, the present study used chemically active promoters as a catalyst support in conducting diffusion-controlled catalytic reactions at the wall. The novelty of using chemically active promoters qualified the present promoters to outperform the chemically inert promoters in enhancing the mass and heat transfer rates.

Flow accelerated corrosion rate on carbon steel pipe bend by thin layer activation technique and computational modeling: Under PHWR operating conditions

Carbon Steel (CS) piping plays a vital role in the heat transport circuits of power plants. These pipes undergo excessive degradation in some of the specific locations due to the flow dynamics variation and such corrosion is termed as the flow accelerated corrosion (FAC). The corrosion rate is usually estimated at a specific location based on one of the best available technique known as the Think Layer Activation (TLA) method. In the present study this method was adopted for the estimation of CS pipe corrosion with 50NB Sch 160, bend angle of 58° and 4D bend radius (D-pipe diameter). Irradiated location was chosen based on the turbulence patterns in the bend as mapped by CFD analysis. At the specified location the corrosion rate to be monitored was irradiated with the proton beam of 13.3 MeV energy aiming for the nuclear reaction of Fe56(p,n) Co56. This irradiated specimen was welded to the High Temperature High Pressure (HTHP) loop and operated at 290 °C/87 bar. The chemistry parameters like pH (10.2) and DO (< 5 ppb) in the loop were maintained by suitably adding LiOH and N2H4. Corrosion rate was assessed by measuring the radioactivity decrease due to corrosion. Throughout the experimental duration the radioactivity measurement was carried on the CS pipe surface at the fixed distance locations. The flow structure in the considered geometry is simulated using CFD. The flow field, velocity, is used to calculate the wall shear stress and mass transfer coefficient. The corrosion rate is calculated with the help of Chilton-Calbourn Equation. It is observed that the computed corrosion rate for this CS pipe under this chemistry and geometry conditions was 400um/y which coincides with that of the TLA method.

Numerical study of the hydrodynamic parameters influencing internal corrosion in pipelines for different elbow flow configurations

Oil–water two-phase flow is common in the ocean engineering, petroleum, and chemical industries, among others. In the transportation process, the elbow system – as a paramount component – usually suffers from internal corrosion. In order to investigate the corrosion-prone characteristics of elbow systems, the volume of fluid (VOF) method and the renormalization group (RNG) k-ε model were used to study the oil–water flow in three different elbow configurations. The results indicate that the maximum wall shear stress and mass transfer coefficient are located at the intrados of the elbow for all flow configurations. Nevertheless, for horizontal-upward and horizontal-horizontal elbow flows, water does not come into direct contact with the intrados of the elbow, meaning it is much less susceptible to corrosion. Thus, a multiparameter model which also takes into account the water-volume fraction is necessary for characterizing the corrosion process. When this was combined with an analysis of the water wetting process, the horizontal-downward elbow flow was found to exhibit the largest corrosion risk among the three configurations examined, the horizontal-upward elbow flow was the least susceptible and the horizontal-horizontal elbow flow was in the middle.

Computational Fluid Dynamics Simulations of Gas-Phase Radial Dispersion in Fixed Beds with Wall Effects

The effective medium approach to radial fixed bed dispersion models, in which radial dispersion of mass is superimposed on axial plug flow, is based on a constant effective dispersion coefficient, DT. For packed beds of a small tube-to-particle diameter ratio (N), the experimentally-observed decrease in this parameter near the tube wall is accounted for by a lumped resistance located at the tube wall, the wall mass transfer coefficient km. This work presents validated computational fluid dynamics (CFD) simulations to obtain detailed radial velocity and concentration profiles for eight different computer-generated packed tubes of spheres in the range 5.04 ≤ N ≤ 9.3 and over a range of flow rates 87 ≤ Re ≤ 870 where Re is based on superficial velocity and the particle diameter dp. Initial runs with pure air gave axial velocity profiles vz(r) averaged over the length of the packing. Then, simulations with the tube wall coated with methane yielded radial concentration profiles. A model with only DT could not describe the radial concentration profiles. The two-parameter model with DT and km agreed better with the bed-center concentration profiles, but not with the sharp decreases in concentration close to the tube wall. A three-parameter model based on classical two-layer mixing length theory, with a wall-function for the decrease in transverse radial convective transport in the near-wall region, showed greatly improved ability to reproduce the near-wall concentration profiles.

エルボ管におけるFAC減肉速度と物質移行係数の評価について

Flow accelerated corrosion (FAC) is one of the causes for wall thinning mechanisms in carbon steel pipes. Prediction of geometry factor is the key elements for estimation of FAC. Geometry factor is defined as the ratio of wall mass transfer coefficient in the piping systems such as elbow to that in a straight pipe. In order to simulate the mass transfer coefficient, we adopt the analogy between mass transfer and heat transfer. In this study, geometry factor of the elbow pipe is computed by using large eddy simulation (LES) for various Prandtl number and Reynolds number. When Prandtl number is close to unity, geometry factor changes depending on Prandtl number. However, as Prandtl number increases sufficiently, geometry factor becomes independent of Prandtl number. As the Reynolds number increases, geometry factor of the INSS elbow decreases, with reduction of the area in which large geometry factor is observed.

On flow structures associated with large wall mass transfer coefficients in orifice flows

In this study, the turbulent flow field is simultaneously measured by stereoscopic particle image velocimetry (SPIV) and the wall mass transfer coefficient using the limiting diffusion current technique (LDCT). SPIV measurements of the flow field behind an orifice in a round pipe are performed in the cross and longitudinal sections. One-point linear and quadratic stochastic estimations are employed to calculate the conditional average of the flow field associated with the large wall mass transfer coefficient. The results suggest that the counter-rotating vortex pairs play an important role in the large wall mass transfer coefficient. The flow structures associated with a large wall mass transfer coefficient in the cross sections are found to be similar to the flow structures enhancing the wall heat transfer obtained by a theoretical analysis and numerical study in literature.

形状係数予測における壁面伝達境界条件が及ぼす影響の数値的研究

Prediction of geometry factor is one of the key elements for estimation of the flow accelerated corrosion (FAC). Geometry factor is defined as the ratio of wall mass transfer coefficient in the piping systems such as elbow to that in a straight pipe. In this study, the mass transfer coefficient is computed in 3 types of elbow pipe by using large eddy simulation (LES) at Re=45000 and is also measured experimentally with electrochemical method. In order to simulate the mass transfer coefficient, we adopt the analogy between mass transfer and heat transfer. The experimental measurement is conducted with the pipe of the test section made of nickel, in which the whole pipe surface acts as electrode. Geometry factor based on the heat transfer coefficient in LES agrees well with that measured in overall electrode condition and their maxium values are found to locate at the inner side of elbow outlet.

高レイノルズ数エルボ管流れにおける熱伝達のLES解析

Flow accelerated corrosion (FAC) is one of the causes for wall thinning mechanisms in carbon steel pipes. Prediction of geometry factor is the key elements for estimation of FAC. Geometry factor is defined as the ratio of wall mass transfer coefficient in the piping systems such as elbow to that in a straight pipe. In this study, geometry factor of the elbow pipe is computed by using large eddy simulation (LES) for Re=45000, 180000, 540000. In order to simulate the mass transfer coefficient, we adopt the analogy between mass transfer and heat transfer. Geometry factor around outside of the elbow affected by high velocity drift decreased with increasing Re. On the other hand, maximum value of geometry factor around inside caused by unsteady flow is independent of Re.

オリフィス下流における物質移行係数の直接測定およびLESによる乱流スカラー輸送の解析

It is necessary to evaluate the geometry factor for predicting the flow accelerated corrosion (FAC) of carbon steel piping in a complicated flow field. Geometry factor is defined as the ratio of wall mass transfer coefficient in the piping systems such as orifice to that in a straight pipe. The mass transfer coefficient behind the orifice is computed by large eddy simulation (LES) and is also measured by electrochemical method. The experimental measurement is conducted with the pipe of the test section made of Ni, in which the whole pipe surface acts as electrode (referred as ’overall electrode condition’). The obtained results are compared with the previous experiments, in order to clarify the effect of the area of the electrode, with a point electrode embedded in an insulated pipe (referred as ‘point electrode condition’). Geometry factor obtained from the overall electrode condition is larger than that of a point electrode measurement. In order to simulate the mass transfer coefficient in pipe flow, we adopt the analogy between mass transfer and heat transfer, and calculate the unsteady temperature field using the numerical data of LES. Geometry factor predicted by heat transfer coefficient agrees well with that measured in overall electrode condition. The large scale motions with vortex structure of separated flow from the orifice influence the instantaneous scalar fields. Analyses of turbulent scalar transport and space-time correlation between wall scalar transfer and flow fields reveal that the mean wall scalar transfer is affected by not only near the wall region but also the coherent scalar fields away from the wall.

Experimental study on near wall transport characteristics of slug flow in a vertical pipe

In this work, the wall shear stress and the mass transfer coefficient of the gas–liquid two-phase upward slug flow in a vertical pipe are investigated experimentally, using limiting diffusion current probes and digital high-speed video system. In experiments, the instantaneous and averaged characteristics of wall shear stress and mass transfer coefficient are concerned. The experimental results are compared with the numerical results in previous paper of the authors. Both experiment and numerical simulation show that the superficial gas and liquid velocities have an obvious influence on the instantaneous characteristics of the two profiles. The mass transfer coefficient has characteristics similar to the wall shear stress. The instantaneous wall shear stress and mass transfer coefficient profiles have the periodicity of slug flow. The averaged wall shear stress and mass transfer coefficient increase with increased superficial gas velocity. However, there is inconsistency in the variation trends of the averaged wall shear stress and mass transfer coefficient with superficial liquid velocity between experimental result and numerical simulation result, which can be attributed to the difference in flow condition. Moreover, the Taylor bubble length is also another impacting factor. The experimental and numerical results all shows that the product scale can not be damaged directly by the flow movement of slug flow. In fact, the alternative forces and fluctuations with high frequency acting on the pipe wall due to slug flow is the main cause for the slug flow enhanced CO2 corrosion process.

Effects of Chemical Reaction on the Unsteady Free Convection Flow past an Infinite Vertical Permeable Moving Plate with Variable Temperature

Analytical solutions for the effect of chemical reaction on the unsteady free convection flow past an infinite vertical permeable moving plate with variable temperature has been studied. The plate is assumed to move with a constant velocity in the direction of fluid flow. The highly nonlinear coupled differential equations governing the boundary layer flow, heat and mass transfer are solved using two-term harmonic and non-harmonic functions. The parameters that arise in the perturbation analysis are Prandtl number (thermal diffusivity), Schmidt number (mass diffusivity), Grashof number (free convection), modified Grashof number, Chemical reaction parameter (rate constant), Skin friction coefficient and Sherwood number (wall mass transfer coefficient). The study has been compared with available exact solution in the literature and they are found to be in good agreement. It is observed that: The concentration increases during generative reaction and decreases in destructive reaction. The concentration increases with decreasing Schmidt number. The effect of increasing values of K leads to a fall in velocity profiles. The velocity decreases with increasing values of the Schmidt number. An increase in modified Grashof number leads to an increase in velocity profiles. The skin friction increases with decreasing Schmidt number. In generative reaction the skin friction decreases and in destructive reaction the skin friction increases.

MHD Mixed Convective Boundary Layer Flow of a Nanofluid through a Porous Medium due to an Exponentially Stretching Sheet

Magnetohydrodynamic (MHD) boundary layer flow of a nanofluid over an exponentially stretching sheet was studied. The governing boundary layer equations are reduced into ordinary differential equations by a similarity transformation. The transformed equations are solved numerically using the Nactsheim‐Swigert shooting technique together with Runge‐Kutta six‐order iteration schemes. The effects of the governing parameters on the flow field and heat transfer characteristics were obtained and discussed. The numerical solutions for the wall skin friction coefficient, the heat and mass transfer coefficient, and the velocity, temperature, and concentration profiles are computed, analyzed, and discussed graphically. Comparison with previously published work is performed and excellent agreement is observed.

CFD modeling for flow and mass transfer in spacer-obstructed membrane feed channels

The effect of spacer geometry on fluid dynamics and mass transfer in feed channels of spiral wound membranes has been investigated. Three-dimensional computational fluid dynamics (CFD) simulations reveal significant influence of spacer geometric parameters such as filament spacing, thickness and flow attack angle on wall shear rates and mass transfer coefficients. The spacers with filaments in axial and transverse direction induce higher shear stresses at the top membrane surface when compared to the bottom; the mass transfer rates are almost equal. The distribution of mass transfer coefficients become uniform when the spacing between axial filaments is increased or transverse filament thickness is decreased. For spacers with filaments inclined to the channel axis, the flow structure depends on spacing and flow attack angle. The fluid follows a zigzag path when spacing is greater while it begins to line-up with the filaments when spacing is reduced or flow attack angle is increased. The flow when aligned with the filaments increases the wall shear stress but confines the region of higher mass transfer coefficient values to a narrower portion. The zigzag flow movement increases these values on a major portion of membrane surface which enhances the mass transfer rates.

Comparison of Axisymmetric and Three-Dimensional Models for Gas Uptake in a Single Bifurcation During Steady Expiration

Reactive gas uptake is predicted and compared in a single bifurcation at steady expiratory flow in terms of Sherwood number using an axisymmetric single-path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM). ASPM is validated in a two-generation geometry by comparing the average gas-phase mass transfer coefficients with the experimental values. ASPM predicted mass transfer coefficients within 20% of the experimental values. The flow and concentration variables in the ASPM were solved using Galerkin finite element method and in the CFDM using commercial finite element software FIDAP. The simulations were performed for reactive gas flowing at Reynolds numbers ranging from 60 to 350 in both symmetric bifurcation for three bifurcation angles, 30 deg, 70 deg, and 90 deg, and in an asymmetric bifurcation. The numerical models compared with each other qualitatively but quantitatively they were within 0.4-8% due to nonfully developed flow in the parent branch predicted by the CFDM. The radially averaged concentration variation along the axial location matched qualitatively between the CFDM and ASPM but quantitatively they were within 32% due to differences in the flow field. ASPM predictions compared well with the CFDM predictions for an asymmetric bifurcation. These results validate the simplified ASPM and the complex CFDM. ASPM predicts higher Sherwood number with a flat velocity inlet profile compared to a parabolic inlet velocity profile. Sherwood number increases with the inlet average velocity, wall mass transfer coefficient, and bifurcation angle since the boundary layer grows slower in the parent and daughter branches.

Effects of chemical reaction on free convective flow of a polar fluid through a porous medium in the presence of internal heat generation

This paper is focused on the study of combined effects of free convective heat and mass transfer on the steady two-dimensional, laminar, polar fluid flow through a porous medium in the presence of internal heat generation and chemical reaction of the first order. The highly non-linear coupled differential equations governing the boundary layer flow, heat and mass transfer are solved by using two-term perturbation method with Eckert number E as perturbation parameter. The parameters that arise in the perturbation analysis are Eckert number E (viscous dissipation), Prandtl number Pr (thermal diffusivity), Schmidt number Sc (mass diffusivity), Grashof number Gr (free convection), solutal Grashof number Gm, chemical reaction parameter Δ (rate constant), internal heat generation parameter Q, material parameters α and β (characterizes the polarity of the fluid), C f (skin friction coefficient), Nusselt number Nu (wall heat transfer coefficient) and Sherwood number Sh (wall mass transfer coefficient). Analytical expressions are computed numerically. Numerical results for the velocity, angular velocity, temperature and concentration profiles as well as for the skin friction coefficient, wall heat transfer and mass transfer rate are obtained and reported graphically for various conditions to show interesting aspects of the solution. Further, the velocity distribution of polar fluids is compared with the corresponding flow problems for a viscous (Newtonian) fluid and found that the polar fluid velocity is decreasing.

CFD simulations of hydrodynamic characteristics in a gas–liquid vertical upward slug flow

Computational fluid dynamics (CFD) simulations are conducted using the volume-of-fluid (VOF) method to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO 2 corrosion. The hydrodynamic characteristics are significantly affected by the viscous, interfacial, and inertial forces. In inertia dominated flows, the velocity of fully developed falling liquid film is increased with increased Taylor bubble rising velocity. The developing falling liquid film is formed at about the length of 0.5 diameter from the Taylor bubble nose, the fully developed falling liquid film is reached at about the length of 1.5–2.1 diameter from the Taylor bubble nose. The average mass transfer coefficient in the falling liquid film is always higher than that in the Taylor bubble wake zone. The iron ion near wall mass transfer coefficient is higher than that of hydrogen ion. The wall shear stress is increased with increased Taylor bubble rising velocity in fully developed falling liquid film zone, and the wall shear stress has a large fluctuation due to the chaotic and turbulent vortexes in Taylor bubble wake zone. The formation and the damage mechanism of the corrosion product scale are proposed for the gas–liquid two-phase vertical upward slug flow induced CO 2 corrosion. It is found that the wall shear stress of upward gas–liquid slug flow is alternate with high frequency, which is the key factor resulting in the corrosion product scale fatigue cracking. The CFD simulation results are in satisfactory agreement with previous experimental data and models available in literature.

Gas Uptake in a Three-Generation Model Geometry During Steady Expiration: Comparison of Axisymmetric and Three-Dimensional Models

Mass transfer coefficients were predicted and compared for uptake of a formaldehyde–air gas system using an axisymmetric single path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM) in three-generation model geometry at steady expiratory flow. The flow and concentration fields in the ASPM were solved using Galerkin's finite-element method and in the CFDM using a commercial finite-element software, FIDAP. Numerical results were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at all flows and lower wall mass transfer coefficients for both 40° and 70° bifurcation angles. However, at higher wall mass transfer coefficients, quantitatively they were within 40% for both the bifurcation angles. Also, at higher flow and wall mass transfer coefficients, they were off qualitatively for a 70° bifurcation angle although the uptake compared qualitatively. This is due to the normalization of uptake within a bifurcation unit with the average of inlet and outlet average concentrations. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow and wall mass transfer coefficients. Also, the local values of the mass transfer coefficients compared closely at all conditions. These results validate the simplified ASPM and the complex CFDM. Mass transfer coefficients increase with bifurcation angles and with a flat inlet velocity profile compared to a parabolic velocity profile since the flow is non-fully developed and hence, the uptake increases.