Local Mass Transfer Coefficients Research Articles

Heat transfer enhancement for fin-tube heat exchanger using vortex generators

Vortex generators are fabricated on the fin surface of a fin-tube heat exchanger to augment the convective heat transfer. In addition to horseshoe vortices formed naturally around the tube of the fin-tube heat exchanger, longitudinal vortices are artificially created on the fin surface by vortex generators. The purpose of this study is to investigate the local heat transfer phenomena in the fin-tube heat exchangers with and without vortex generators, and to evaluate the effect of vortices on the heat transfer enhancement. Naphthalene sublimation technique is employed to measure local mass transfer coefficients, then analogy equation between heat and mass transfer is used to calculate heat transfer coefficients. Experiments are performed for the model of fin-circular tube heat exchangers with and without vortex generators, and of fin-flat tube heat exchangers with and without vortex generators. Average heat transfer coefficients of fin-flat tube heat exchanger without vortex generator are much lower than those of fin-circular tube heat exchanger. On the other hand, fin-flat tube heat exchanger with vortex generators has much higher heat transfer value than conventional fin-circular tube heat exchanger. At the same time, pressure losses for four types of heat exchanger is measured and compared.

Modeling of transient particle–liquid mass transfer in liquid and liquid–solid systems

Models based on the film, penetration and surface renewal theories were developed for predicting the local, dynamic behavior of liquid–solid mass transfer in a fixed bed. The influence of velocity pulses of frequency less than 1 Hz in the range 0.005–0.03 m/s has been studied. A model based on the surface renewal theory estimates the local instantaneous mass transfer coefficient from the instantaneous velocity. Experimental validation is provided. Potential applications are the determination of local transient velocity in fixed beds from mass or heat transfer measurements and the assessment of mass transfer enhancement in pulsating systems.

Numerical simulation of flow in an array of in-line blunt boards: mass transfer and flow patterns

The airflow patterns around an array of in-line truncated boards and the sublimation process of thin naphthalene layers over the in-line boards have been simulated numerically. The simulation used the renormalization group k– ε turbulence model and the two-layer zonal model for the near-wall treatments. When only one of the in-line boards is coated with naphthalene, as in the experiments by previous authors, the predicted local mass-transfer coefficients over that board show a substantial increase at the leading edge of the board and are in good agreement with the measurement data. However, when all the in-line boards are naphthalene coated, the calculated mass-transfer coefficients decrease in an approximately asymptotic manner from a maximum value near the leading edge of the first board. These results indicate that the small gaps between in-line boards have an insignificant effect on the mass-transfer coefficients. Vortex shedding from in-line boards separated by small gaps has also been investigated numerically. It is shown that in a wind tunnel with a single layer of boards, all the recirculating flow eddies at different locations around the in-line boards are characterized by vortex shedding, in agreement with the experimental observations by previous authors, while in a kiln stack with multiple layers of in-line boards, only the eddies in the near wake of the stack are formed periodically.

Global and local mass transfer coefficients in waste water treatment process by computational fluid dynamics

Gas–liquid mass transfer is a currently used process in the waste water treatment industry. The objective today is to optimize the design and have a better control on the operation of these processes by using the CFD code ASTRID in two-phase flow reactors. A source term which represents interfacial mass transfer is introduced, accounting for two-phase flow hydrodynamics, it permits to predict accurately dissolved gas concentration in reactors such as laboratory airlift internal loop reactor and full scale activated sludge basin.

Distribution of Local Mass Transfer Coefficients on Walls of Complex-Shape Channels

Distribution of Local Mass Transfer Coefficients on Walls of Complex-Shape Channels

Measurement of local and overall mass-transfer coefficients to a sphere in a quiescent liquid using a limiting current technique

The mass-transfer rate to a sphere in a quiescent ferri–ferrocyanide solution was determined from measurement of the limiting current through the spherical cathode. Mass-transfer rates to local areas on the sphere, without mass transfer to the rest of the sphere, were also measured. Convective movement of the electrolyte was observed visually although the experimental apparatus was jacketed for maintaining a constant temperature. Under this condition, the experimental Sherwood number for mass transfer to the whole sphere was 110 rather than 2 as for pure diffusion. The local Sherwood number for a 2 mm spot on the sphere was 275, which was about 1.7 times that of a 8 mm spot on the sphere. The local mass-transfer rate, in the absence of mass transfer to the rest of the sphere surface used in the present study, did not change significantly with the angular position to the upward vertical as it does when mass transfer occurs over the whole sphere simultaneously. The transient transfer approached steady state at the dimensionless time ( tu/ d) of about 20. The mass-transfer rate did not vary significantly with the distance between the two electrodes varied from 2.5 to 11 radii of the spherical cathode.

LOCAL OVERALL GAS–LIQUID VOLUMETRIC MASS TRANSFER COEFFICIENTS IN A GAS–LIQUID TWO-PHASE REVERSED FLOW JET LOOP REACTOR

Local overall gas-liquid volumetric mass transfer coefficient profiles at the specified point were experimentally investigated in a gas–liquid two-phase reversed flow jet loop reactor with Newtonian and non-Newtonian systems. It was observed that the local overall gas-liquid volumetric mass transfer coefficient profiles of this reactor with Newtonian and non-Newtonian systems increase with increase in gas jet flow rates and liquid jet flow rates, and with decrease in nozzle diameter and CMC concentration.

Humidification of air with warm water at elevated pressure in wetted-wall column

A comprehensive analysis of pressurized air humidification in wetted-wall column has been carried out. A developed mathematical model consists of mean flow conservation equations with a full account of variable properties and gas-liquid interfacial thermodynamics, in which a low-Reynolds number к − ε turbulence model in conjunction with combined heat and mass transfer analysis. The influence of various high-pressure phenomena (including property variation, thermodynamic non-ideality etc.) on gas-liquid heat and mass transfer is numerically examined in detail. Results are presented for air-water under different conditions. Particular attention is paid to investigating the variation of the local Nusselt and Sherwood number of gas. The results show that buoyancy force caused by properties variation will reduce the local heat and mass transfer coefficients, and that the extent of reduction in local heat and mass transfer coefficients increases with operate pressure and inlet temperature difference between gas and liquid.

Extraction of tylosin from its aqueous solutions by Rotating Film-pertraction

Removal of tylosin from its dilute aqueous solutions was studied applying the Rotating Film-pertraction technique. Non-toxic and practically water insoluble alcohols as n-octanol and n-decanol were used as intermediate, membrane liquid. A 0.001 mol/l solution of HCl was used as a stripping phase. Effects of temperature, feed phase pH-value and solute concentration on antibiotic distribution coefficients were obtained for both systems. It was found that the first two parameters have important effect, while the solute concentration in the studied range (0.02–0.1 g/l) did not modify these coefficients. Results obtained show that the first pertraction step – solute extraction from the feed – is a very fast process, while the second one – the alcohol stripping – is relatively slow, in particular when octanol was used as membrane phase. A mathematical model was proposed and used for evaluation of corresponding local mass transfer coefficients.

Effect of Gas Recirculation Rate on Mass Transfer within Mechanically Agitated Vessels

The effect of gas recirculation on the mass transfer and gas dispersion within mechanically agitated vessels was examined by measuring the local mass transfer coefficients and estimating the gas recirculation rates through a developed CFD approach. Since the recirculated gas as well as the sparged gas affects the power drawn by the impeller, it also exerts a strong effect on the mass transfer. With the same rotational speed, as the gas is completely dispersed, the pitched blade impeller obtains a larger gas recirculation rate due to its stronger circulating flow, and can Provide 80 % of the mass transfer rate which the Rushton turbine impeller can produce. However, with a higher gassing rate, the Rushton turbine impeller demonstrates a better gas dispersion and recirculates more gas back into the impeller region, resulting in about a 50% higher mass transfer rate than the pitched blade impeller. Summarizing the calculated data shown in this study, the gas recirculation rate QR can be correlated with N, Qs and the D/T ratio as: QR/QS=1.3810-18N14.78VS-1.13(D/T)4.01 for a single Rushton turbine impeller system, and QR/QS=7.2110-19N14.21VS-1.25(D/T)5.11 for a single pitched blade impeller system, for the gas dispersion status beyond stage(c). Comparing the plots of (KLa) vs. the total gassing rate Qt, and the plot of (KLa) vs. the sparging gas rate Qs, it is found that ＜ KLa) always increases monotonically with Qt while a leveling-off value of (KLa is seen in the plot of (KLa ＞ vs. Qs. This finding implies that (KLa) is more appropnately correlated with Q, than with the sparging gas rate Qs. By corre-laring (KLa) with the rotational speed N and Qt, it is found that the Rushton turbine impeller and the pitched blade impeller perform similarly if the value of the modified aeration number N A' (= Qt/ND3) is the same, and the values of (KLa) for both systems can be estimated by a single equation as: ＜KLa＞=0.314(Qt/ND3)+0.00339 where the deviation of this correlation is always less than 15%.

Local Mass Transfer Correlations for Nonaqueous Phase Liquid Pool Dissolution in Saturated Porous Media

Local mass transfer correlations are developed to describe the rate of interface mass transfer of single component nonaqueous phase liquid (NAPL) pools in saturated subsurface formations. A three-dimensional solute transport model is employed to compute local mass transfer coefficients from concentration gradients at the NAPL-water interface, assuming that the aqueous phase concentration along the NAPL-water interface is constant and equal to the solubility concentration. Furthermore, it is assumed that the porous medium is homogeneous, the interstitial fluid velocity steady and the dissolved solute may undergo first-order decay or may sorb under local equilibrium conditions. Power-law expressions relating the local Sherwood number to appropriate local Peclet numbers are developed for both rectangular and elliptic/circular source geometries. The proposed power law correlations are fitted to numerically generated data and the correlation coefficients are determined using nonlinear least squares regression. The estimated correlation coefficients are found to be direct functions of the interstitial fluid velocity, pool dimensions, and pool geometry.

Advective mass transfer from stationary sources in porous media

We study mass transfer to a flowing fluid in porous media from stationary sources of various geometries. Effects of the porous medium and the source geometry are explored using a modification of the local mass transfer coefficients. We develop exact solutions for mass transfer in flow over a flat plate and asymptotic and numerical solutions in flow over sources of various geometries, including self‐similar surfaces, such as a Koch surface and a percolation cluster. For the latter, as well as for sources distributed at the pore‐network scale, a pore‐network representation of the porous medium is used. The dependence of the overall mass transfer rates on various parameters, and particularly on the flow rate, is analyzed. The analysis allows for macroscopic coefficients in equations, which lump the process in terms of an effective, first‐order reaction, to be interpreted in terms of the microstructure of the porous medium and the source geometry.

Gas–liquid mass transfer in an internally finned monolith operated countercurrently in the film flow regime

Gas–liquid mass transfer has been determined in internally finned monoliths (IFM) by desorbing oxygen from a falling water film. Measurements were performed for various monolith lengths to quantify the inlet effects caused, among others, by the development of the concentration profile. The local volumetric mass transfer coefficients were found to vary from approximately 0.04 (s −1) at the inlet to about 0.01 (s −1) in the downstream sections. A theoretical model has been derived, assuming that the water flow can be treated as a film falling down in a corner. By averaging the radial mass transfer perpendicular to the film surface, mass transfer can be predicted within reasonable accuracy. On comparing the film in the IFM with a film along a vertical plane, it was found that in an IFM the concentration profile develops faster while mass transfer is less effective in the developed sections.

Detection of near-wall hydrodynamic effects by electrochemiluminescence

The imaging of hydrodynamic conditions at an electrode surface by a new method based on the electrochemiluminescence of luminol has been demonstrated. The technique allows the mapping of local mass transfer coefficients, localised hydrodynamic disturbances and the effects of approaching gas bubbles to electrode surfaces by light emission contrasts obtained with a transparent electrode. These can be used to assess the evolution of hydrodynamic instabilities and the effects of bubble penetration in the near-wall region.

Local mass transfer coefficients around a cylinder in a fluidized bed

Local mass transfer coefficients from a horizontal cylinder immersed in a fluidized bed were investigated experimentally. An aluminum cylinder coated with naphthalene, immersed in a fluidized bed of glass beads, was used to obtain mass transfer coefficients. The test results indicated that the local mass transfer was strongly influenced by the angular position and air flow rate. The data obtained show that minimum and maximum Sherwood are found, respectively, at the top and bottom regions of the immersed cylinder.

Visualization and simulation of non-aqueous phase liquids solubilization in pore networks

The design of in-situ remediation of contaminated soils is mostly based on a description at the macroscopic scale using averaged quantities. These cannot address issues at the pore and pore network scales. In this paper, visualization experiments and numerical simulations in pore networks are carried out to understand basic aspects of mass transfer during the solubilization of residual non-aqueous phase liquids (NAPL). The experiments are carried out in 2-D etched-glass micromodels with randomly distributed pore sizes. The evolution of the configuration of the residual phase, the pressure drop across the micromodel and the concentration of the effluent are continuously recorded under various displacement conditions. A pore network numerical model, based on the convection–diffusion equation using appropriate modifications for the local mass transfer coefficients, is developed to simulate mass transfer during the solubilization of a residual phase. The pore network simulator is found to match well the experimental results, provided that the local mass transfer coefficients are properly modeled. Under the assumption of spatially uniform local mass transfer coefficients, an asymptotic power-law scaling between the effluent concentration and the Peclet number is obtained, the exponent of which is related to the local mass transfer expression.

Dissolved oxygen transfer at the sediment‐water interface in a turbulent flow

Laboratory experiments have been conducted in an effort to elucidate the details of the dissolved oxygen transfer mechanism at the sediment‐water interface. Near‐bed liquid turbulence and dissolved oxygen concentrations were measured for a range of Reynolds numbers from 2700 to 10,600 over a smooth bed. The results show that the shear stress velocity controls the dissolved oxygen diffusive layer thickness and therefore the local mass transfer coefficient at the sediment‐water interface. From a dynamic point of view the results suggest the possibility that streamwise vortices control the interfacial dissolved oxygen transfer process. The research results will provide an opportunity to validate the existing conceptual models as well as enhance the development of new models that can improve our ability to predict dissolved oxygen and transport related parameters in lakes, reservoirs, coastal waters, and rivers.

Microelectrode measurements of local mass transport rates in heterogeneous biofilms.

Microelectrodes were used to measure oxygen profiles and local mass transfer coefficient profiles in biofilm clusters and interstitial voids. Both profiles were measured at the same location in the biofilm. From the oxygen profile, the effective diffusive boundary layer thickness (DBL) was determined. The local mass transfer coefficient profiles provided information about the nature of mass transport near and within the biofilm. All profiles were measured at three different average flow velocities, 0.62, 1.53, and 2.60 cm sec-1, to determine the influence of flow velocity on mass transport. Convective mass transport was active near the biofilm/liquid interface and in the upper layers of the biofilm, independent of biofilm thickness and flow velocity. The DBL varied strongly between locations for the same flow velocities. Oxygen and local mass transfer coefficient profiles collected through a 70 micrometer thick cluster revealed that a cluster of that thickness did not present any significant mass transport resistance. In a 350 micrometer thick biofilm cluster, however, the local mass transfer coefficient decreased gradually to very low values near the substratum. This was hypothetically attributed to the decreasing effective diffusivity in deeper layers of biofilms. Interstitial voids between clusters did not seem to influence the local mass transfer coefficients significantly for flow velocities of 1.53 and 2.60 cm sec-1. At a flow velocity of 0.62 cm sec-1, interstitial voids visibly decreased the local mass transfer coefficient near the bottom.

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for staggered tube arrangement by mass transfer measurements

Local heat transfer coefficients on the outer surface of tubes in shell-and-tube heat exchangers with staggered tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients are transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature.

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for in-line tube arrangement by mass transfer measurements

The local heat transfer coefficients on the shell-side of shell-and-tube heat exchangers for in-line tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients were transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature.