Heat transfer from a tube bundle in a bubble column

Heat transfer coefficient for an immersed five tube bundle in a two-phase (air-water) bubble column is measured as a function of air velocity and other relevant parameters. Average and local air holdup are also measured under identical conditions to characterize the system uniquely.     

Heat Transfer and Hydrodynamic Investigations of a Baffled Slurry Bubble Column

Heat transfer and hydrodynamic investigations have been conducted in a 0.108 m internal diameter bubble column at ambient conditions. The column is equipped with seven 19mm diameter tubes arranged in an equilateral triangular pitch of 36.5 mm. A Monsanto synthetic heat transfer fluid, Therminol-66 having a viscosity of 39.8 cP at 303 K, is used as a liquid medium. Magnetite powders, average diameters 27.7 and 36.6 μm, in five concentrations up to 50 weight percent in the slurry, are used. As a gas phase, industrial grade nitrogen of purity 99.6 percent is employed.Gas holdup in different operating modes and regimes have been measured for the two- and three-phase systems over a superficial gas velocity range up to 0.20 m/s in the semi-batch mode. Heat transfer coefficients are measured at different tube locations in the bundle at different radial and vertical locations over a range of operating conditions. All these data are compared with the existing literature correlations and models. New correlations are proposed.     

Heat and mass transfer during bubble growth in an alternating electric field

We studied mass and heat transfer during nonisothermal desorption of a solvable dielectric gas to a stagnant bubble from a surrounding gas-saturated dielectric liquid under the influence of the alternating electric field. Mass flux is directed from a host fluid to bubble, and the applied electric field causes a creeping flow around the bubble. The moving boundary problem is solved in the approximations of a thin concentration and temperature boundary layers and finite dilution of an absorbate in the absorbent. The nonlinear partial parabolic differential equations of mass and energy conservation are solved by means of a generalized similarity transformation, and the solution is obtained in a closed analytical form for all frequencies of the applied electric field. Numerical calculations show an essential (by a factor of 2÷3) enhancement of the rate of mass transfer in sulfur dioxide-water system under the influence of alternating electric field for a stagnant bubble.     

Unsteady-state mass transfer from a binary gas bubble with changing volume

Numerical methods are used to investigate the transient mass transfer from a binary gas bubble to an incompressible liquid. The bubble has two components: A – soluble and B – insoluble in the surrounding fluid. The concentration of A inside the bubble is considered spatially uniform but not constant in time. The maximum value of the initial volumetric fraction of A is 0.3. The mathematical model equations were solved numerically in spherical coordinates system. Creeping flow, moderate Re number flow, 10 ? Re ? 100, and potential flow around the bubble were assumed. The computations focused on the influence of the initial fraction of A and Henry number on the mass transfer rate for Pe ? 10⁴.     

Study on the mass transfer of bubble swarms in three different rheological fluids

The mass transfer of the absorption of carbon dioxide bubble swarms into three different rheological fluids (Newtonian fluids, shear thinning fluids and viscoelastic fluids) were investigated experimentally. The volumetric liquid-phase mass transfer coefficients under different operating conditions were determined by using a carbon dioxide probe. The influences of gas flow rate and liquid properties on volumetric liquid-phase mass transfer coefficient were studied. Results indicated that the volumetric liquid-phase mass transfer coefficient increased with gas flow rate but decreased with increasing liquid apparent viscosity. Moreover, the viscoelasticity of liquid resulted in a decrease of mass transfer rate. A semi-empirical model was developed, the prediction by this model showed a satisfactory agreement with the experimental data in all three types of liquids studied.     

Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor

An experimental study is described on mass transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed (i.d. 120 mm) of sand (average diam. 325 μm) with static heights of 0.10–0.21 m. The bubbling flow rate ranged from 2.5 to 5.0 times that at incipient fluidization. The coke particles were 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L_eq (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L_eq = 50 mm suggests that for shallow beds the mass transferred during bubble formation is a significant part of the total mass transferred. The measured mass transfer factor between phases during a bubble’s rise (x′ = X/L_mf) is independent of the bubbling air flow rate and substantially lower than the theoretical predictions of Kunii and Levenspiel [1]. This disagreement is explained by the fact that the theoretical model is for an isolated bubble and does not account for the strong interaction between consecutive bubbles; this increases a bubble’s velocity and induces their coalescence, leading to a decrease in mass transferred between phases.     

Mechanism of mass transfer between a bubble initially composed of oxygen and molten glass

The bubble removal from molten glass is an important problem in glass melting process. In this paper, the mass transfer undergone by a bubble rising in molten glass is studied, the multicomponent feature being taken into account. In order to identify the time scaling of the bubble shrinkage, a careful dimension analysis is performed.A characteristic time to describe the mass transfer for each gaseous species in a bubble is introduced with an alternative expression of the permeability. This new permeability has the dimension of a diffusion coefficient, which is useful to compare to other transport phenomena. From the physical data known for soda–lime–silica glasses, a fast equilibrium state of water between a bubble and molten glass is determined. The opposite situation is observed for nitrogen.Experimental results giving the bubble size versus time with a dimensionless form leads to a good match at short time whatever the glass nature and the temperature. Finally, a simple equation to determine bubble size as a function of time is given, based on the dimension analysis previously established.     

Fluid-dynamic experimental study in a bubble column with internals

The main objective of this work was to determine the influence of the air velocity, the solid load concentration and the average solid particle diameter in the gas hold-up and solid distribution inside a bed with internals. Pilot scale equipment conformed by a column of square traverse section was used. In the central part of this equipment, there is a bank of 49 tubes in an aligned arrangement. The three-phase system used was compressed air, tap water and sieved sand at average solid particle diameters of 130, 510 and 2300 μm. The velocities of air were varied in the range from 1.6 to 3.3 cm/s, and the solids load concentration from 0 to 30% w/w. Results obtained allows to conclude that when increasing the gas velocities and the solids load concentration, and when diminishing the solid particle diameter, the gas hold-up increases. Additionally, quasi-symmetrical and parabolic shapes were detected in the traversal solid concentration profiles, as well as decreasing with height in the axial direction. These profiles, axial and traverse ones, become more uniform as the air velocity increases.     

Direct numerical simulation of mass transfer from Taylor bubble flow through a circular capillary

In this work mass transfer during a Taylor bubble flow regime has been investigated by a volume of fluid (VOF) based numerical method. The hydrodynamics of Taylor bubble flow through a circular capillary has been simulated in a single unit cell by a moving reference frame. The validity of Taylor bubble hydrodynamics simulation has been checked by comparing the liquid film thickness and the relative bubble velocity obtained from computational fluid dynamics (CFD) simulations with reported empirical correlations and experimental results. The conservation equation of tracer has been solved in whole of flow domain for simulating mass transfer from Taylor bubble to surrounding liquid. The tracer concentrations in the cells that are either completely or partially filled with the gas phase are assigned the equilibrium concentration by employing the concept of internal boundary condition. By this concept an artificial diffusion of tracer has been appeared in the simulations. In order to eliminate this artificial diffusion, the advection scheme in the tracer conservation equation has been modified by using the two film mass transfer model. The simulation of mass transfer from a single bubble has been validated by comparing the CFD results with reported experimental data. Afterwards, the effects of capillary number, unit cell length and capillary diameter variation on mass transfer from Taylor bubble has been investigated.     

Influence of inert admixtures on the rate of mass transfer during bubble collapse in an alternating electric field

In this study, we consider mass transfer under the influence of an alternating electric field in a system comprising liquid dielectric medium and stationary dielectric gas bubble composed of a solvable and inert gases. Resistance to mass transfer in both phases is taken into account. The applied electric field causes Taylor circulation around the bubble. Bubble deformation under the influence of the electric field is neglected. The bulk of a bubble beyond the diffusion boundary layer is completely mixed, and the concentration of an absorbate is homogeneous and time-dependent in the bulk. The thermodynamic parameters of a system are assumed constant. The moving boundary problem is solved in the approximations of a thin concentration boundary layers in the gaseous and liquid phases and infinite dilution of an absorbate in the absorbent. The partial parabolic differential equations of mass conservation for gaseous and liquid phases with time-dependent velocity components and time-dependent boundary conditions are solved by combining generalized similarity transformation method with Duhamel's theorem, and the solution is obtained in the form of Volterra's integral equation of the second kind for all the frequencies of the applied electric field. The asymptotic behavior of the obtained solutions is discussed. Numerical calculations are performed for different values of distribution coefficient.     

Experimental and theoretical analysis of heat and mass transfer in a packed column dehumidifier/regenerator with liquid desiccant

This paper presents the experimental tests and the theoretical analysis on the chemical dehumidification of air by a liquid desiccant and desiccant regeneration in an absorption/desorption column with random packing.The experimental set-up is fully described together with measurements, procedures, data reduction and accuracy. The experimental tests include dehumidification and desiccant regeneration runs carried out with the traditional hygroscopic salt solutions H₂O/LiCl and H₂O/LiBr and the new salt solution H₂O/KCOOH in the typical operative ranges of air conditioning applications.A theoretical model of the packed column and the relative simulation computer code was developed to predict the performance of the system and to analyse the system sensitivity to the main operating parameters. A fair agreement was found between the experimental tests and the simulation computer code.The experimental tests and the theoretical analysis show that the chemical dehumidification of air by hygroscopic salt solutions ensures consistent reduction in humidity ratio, which is suitable for applications to air conditioning or drying processes. Moreover, desiccant regeneration requires a temperature level around 40–50 °C which can be easily obtained by using solar energy or heat recovered from an industrial process or from a thermal engine.     

Unsteady-state mass transfer accompanied by a first-order chemical reaction from a binary gas bubble with changing volume

The transient mass transfer from a binary gas bubble to an incompressible liquid accompanied by a chemical reaction into the liquid phase is analysed numerically. Only one of the bubble components transfers into the liquid phase where a first-order, irreversible chemical reaction occurs. The concentration of the transferring species inside the bubble is considered spatially uniform but not constant in time. Creeping flow was assumed. The mathematical model equations were solved numerically in spherical coordinates system. The computations focused on the influence of the Damkohler number and Henry number on the mass transfer rate for Pe = 10, 100, 1000 and Da/Pe = 0.1, 1, 10.     

Transient heat transfer and bubble dynamics in a pressurized fluidized bed

A flow visualization technique for studying the gas bubble dynamics in a pressurized fluidized bed was developed and used to quantify these dynamics at the surface of a vertical tube submerged in the bed. Transient heat flux measurements were made and correlated with bubble motion. As a result, it is concluded that the heat transfer process is strongly affected by bubble dynamics and is much more complex than any of the generally accepted models can predict. It is also shown that the overall bed operating conditions are the primary driver for local bubble/particle motion around the tube which significantly affects the time-dependent fluctuation in local heat transfer.     

Turbulent mass transfer in the mass transfer entry region for non-newtonian fluids

The effect of non-Newtonian flow behavior on turbulent mass transfer in the mass transfer entry region is investigated analytically. The predicted mass transfer rates in the mass transfer entry region are in good agreement with available experimental data and empirical correlations. The theoretical results for drag-reducing fluids show about 30 – 50% mass transfer rate reduction in the entry region which is less than that in the fully developed region.     

Surface heat transfer due to sliding bubble motion

The presence of a rising bubble in a fluid can greatly enhance heat transfer from adjacent heated surfaces such as in shell and tube heat exchangers and chemical reactors. One specific case of this is when a bubble impacts and slides along the surface. The result is heat transfer enhancement by two main mechanisms: first, the bubble itself acting as a bluff body, and second, the wake generated behind the bubble leads to increasing mixing. The current research is concerned with measuring the heat transfer from a submerged heated surface that is subject to a sliding bubble flow. An ohmically heated 25 μm thick stainless steel foil, submerged in a water tank, forms the test surface. An air bubble is injected onto the lower surface of the test plate, it slides along its length and the effects are monitored by two methods. Thermochromic liquid crystals (TLC’s) are used in conjunction with a high speed camera to obtain a time varying 2D temperature map of the test surface. A second synchronised camera mounted below the foil records the bubble motion. Tests are performed at angles of 10°, 20° and 30° to the horizontal. This paper reports on the enhancement of the heat transfer due to the bubble. It has been found that the angle made between the heated surface and the horizontal influences heat transfer by changing the bubble’s motion. In general, a steeper angle leads to a higher bubble velocity, which results in greater heat transfer enhancement.     

Transient heat and mass transfer in soils

Transient heat and mass transfer in soil surrounding a buried heat source are considered. One dimensional (spherical) models are developed to predict the coupled heat and moisture migration phenomena. Numerical solutions of the exact formulation are given. In addition, two types of closed-form solutions are derived employing approximate models. Experimental measurements are presented for two disparate soil types — a well-graded loam (slow hydraulic response) and sand (more rapid hydraulic response). Predicted and measured temperature variations at the heat source surface are compared. In all cases the numerical solution of the exact formulation agrees with the experimental data reasonably well. The closed-form solutions deviate somewhat from the measurements, but are shown to be useful for obtaining approximate predictions in a simple manner.     

Charge transfer and mass transfer reactions in the metal hydride electrode

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB₅-type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.     

Heat transfer and diffusion in wedge flows with rapid mass transfer

Boundary-layer solutions are given for the flow of binary constant-property mixtures over planes and wedges, with heat and mass transfer through the boundary surface. Exact numerical solutions are given for Prandtl and Schmidt numbers from 0.1 to 10. Asymptotic solutions are given for Prandtl and Schmidt numbers outside this range, and also for high rates of mass transfer toward the surface.