Electrochemical Limiting Diffusion Current Technique Research Articles

Effect of distance between impeller blade tip and surface on mass transfer to a local electrode in a stirred vessel in a wide range of Sc number

ABSTRACT Stirred vessels are encountered in a wide scale of applications from bench to industrial processes, and the knowledge of the heat/mass transfer rates in these vessels is required for their design, operation and control. This work submits an investigation on the mass transfer to a small circular surface immersed in a stirred vessel in the parameter ranges of 267 < Re < 9437, 1473 < Sc < 61,422, and 0.022 < x/dK < 0.22 for the distance between the impeller blade tip and the transfer surface. The Reynolds number was based on the impeller diameter. The electrochemical limiting diffusion current technique having a potassium ferri-/ferrocyanide system was applied for mass transfer coefficient measurements. Aqueous glycerine solutions were prepared to attain a wide range of Sc numbers. The rate of mass transfer is enhanced with increasing rotation rate, decreasing blade tip-to-electrode surface distance, and decreasing glycerine concentration. The experimental data were well correlated by the relation .

Usability of alloys with different composition as electrode for electrochemical reduction of ferricyanide ion

An experimental work was performed to investigate the usability of 8 metallic alloys with different composition in electrochemical limiting diffusion current technique employing the cathodic reduction of ferricyanide. All the alloys exhibited a limiting current plateau region in their voltage–current behaviour, but not all gave a wide and smooth plateau, except one with almost pure Ni. With the alloys of including more than 50% Ni, as Ni ratio of the metal increased, smoother and wider plateau was attained, and the electrode containing 99% Ni gave the smoothest plateau. The other alloys tested can only be used as electrode in the limiting diffusion current technique in low flow velocities in which the transfer of the active ion to the electrode surface is as slow as that the mass transfer controls the rate of system.

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.

A new application area of electrochemical limiting diffusion current technique: measurement of mixing time and effect of some parameters

In this study, the electrochemical limiting diffusion current technique, which was employed first time for the mixing time measurement in the stirred tank system by the authors, was applied to determine the effect of some parameters on the global and local mixing times in the range of 3500 ≤ Re ≤ 10,000. The technique well sensed that increasing Reynolds number (Re), blade angle and blade number decreased the mixing time. The most effective parameter was observed to be Re, and the blade number and blade angle, in order. In addition, the mixing time was also investigated for solutions added in various volume fractions having different viscosity and density by using local mixing time measurement. It was determined that the mixing time decreased with the Reynolds number, the ratio of the initial solution volume to the total solution volume and the ratio of kinematic viscosity of the solution in the vessel initially to that of the added one viscosity. The ratio of initial solution volume to total solution volume was found to be a more effective parameter than the others in terms of mixing time.

A novel method for the measurement of mixing time: A new application of electrochemical limiting diffusion current technique

The purpose of this work is to investigate whether the application of the electrochemical limiting diffusion current technique to the measurement of the mixing time in stirred vessels is possible; and if it is, then to establish a new technique for the measurement of the mixing time in the containers including liquids. For this aim, first the electrochemical limiting current conditions were determined, by observing the voltage-current change with the addition of a small amount of concentrated solution including active ions, that is, tracer, into the stirred system. Then, mixing time was determined by local and global measurements, employing both a local probe and one of the baffles as sensor, respectively. The comparison of the results with those of obtained by conductivity, which showed very good agreement, proved that electrochemical limiting current technique can be used as a means of measuring local and global mixing times as a cheap, simple, fast, straight forward and effective method.

Limiting current technique in the research of mass/heat transfer in nanofluid

In the paper the authors focused on the application of the electrochemical limiting diffusion current technique to the study of mass transfer in nanofluid flow. As mass and heat transfer are analogical phenomena, analysing mass transfer helps understand heat transfer processes in nanofluids. The paper begins with a short review of the available literature on the subject followed by the authors' results of mass transfer coefficient measurements and the conclusions concerning mass/heat transfer enhancement in nanofluids.

Turbulent mass transfer of Al2O3 and TiO2 electrolyte nanofluids in circular tube

Experimental study was performed to investigate turbulent mass transfer in straight circular tube. Electrochemical limiting diffusion current technique was used to measure the mass transfer coefficient in fully developed hydrodynamics and under developed mass transfer region. TiO2 and γ-Al2O3 nanoparticles were added into the electrolyte solution (ES) to make electrolyte nanofluids (ENF). Measurements revealed that enhancement in mass transfer reaches 10 % in a 0.01 vol% γ-Al2O3/electrolyte nanofluid while 18 % in a 0.015 vol% TiO2/electrolyte nanofluid relative to the base ES. Mass transfer coefficients increased with nanoparticles concentration up to an optimum concentration (0.01 % in γ-Al2O3/electrolyte nanofluid and 0.015 % in TiO2/electrolyte nanofluid) while decreased by increasing nanoparticles concentration further. Enhancement ratio which is the ratio of the mass transfer coefficient of nanofluid to that of the base fluid was a function of nanoparticle concentration and was independent of Reynolds number. The mechanisms of nanoparticles Brownian motion and nanoparticles clustering were used to describe the behavior of the enhancement ratio in ENF.

Laminar forced convective mass transfer of γ-Al2O3/electrolyte nanofluid in a circular tube

Mass transfer of ferricyanide ions through electrolyte nanofluids in a circular tube was investigated experimentally. Nanofluids consisted of 40 nm γ-Al2O3 nanoparticles suspended in ferri-ferrocyanide and aqueous sodium hydroxide as base fluid. Well known electrochemical limiting diffusion current technique was used to measure mass transfer coefficients. Measurements showed that mass transfer coefficient increased with concentration of nanofluid up to 0.01%. For greater concentrations of nanoparticles in nanofluid mass transfer coefficient decreased with the concentration. The augmentation of mass transfer coefficient for 0.01% nanofluid, was about 16.8% at Re = 1260 relative to the base liquid. Enhancement ratio which is defined as the ratio of the mass transfer coefficient of the nanofluid to the mass transfer coefficient of the base fluid slightly decreases with Reynolds number.

Effect of suspended CuO nanoparticles on mass transfer to a rotating disc electrode

The effect of suspended CuO nanoparticles on the mass transfer to a rotating disc electrode was investigated experimentally, using the electrochemical limiting diffusion current technique. The particle volume fraction was from 0.39% to 1.94%. The rotating speed ranged from 100 to 1000 rpm, which yielded the Reynolds number between 10 and 110, based on the electrochemically active disc radius. The results showed that the addition of the suspended particles increased the limiting current and the plot of log I vs. log ω resulted in linear lines, of which slopes decreased with increasing particle volume fraction. The ratio of Sh/Sh o ranged from 1 to 1.5. The Sherwood number correlation as function of the Reynolds number and the particle volume fraction was also given.

Pressure drop and point mass transfer in a rectangular microchannel

This paper describes the pressure drop and local mass transfer in a rectangular microchannel having a width of 3.70 mm, height of 0.107 mm and length of 35 mm. The pressure measurements were carried out with distilled water as working fluid at Reynolds numbers in the range of 100–845, while mass transfer measurements with a chemical solution at Reynolds numbers in the range of 18–552 by using the electrochemical limiting diffusion current technique (ELDCT). Experimental friction factors were slightly higher than those calculated by theoretical correlation. The Sherwood number correlation was also obtained.

Experimental study of laminar forced convective mass transfer and pressure drop in microtubes

The investigation of laminar convective mass transfer and friction factor was performed experimentally for the circular tubes with the diameter of 0.20 mm and the L/ d values in the range of 100–500 for a Reynolds number range of 40–1400. The pressure drop experiments were conducted with distilled water, and the mass transfer experiments were carried out with an electrochemical solution by using the electrochemical limiting diffusion current technique. The friction factor results showed good agreement with the classical Poiseuille flow theory, while Sherwood numbers are smaller than those obtained by conventional correlations.

Electrochemical Mass Transfer in Impinging Swirl Jets

The local mass transfer coefficients between an impinging swirl jet flow and a flat circular surface in a cylindrical cell were measured by the electrochemical limiting diffusion current technique. Helical-type swirl generators were used to generate swirl flow with intensities in the range of S = 0−0.73. The experiments were carried out for a jet Reynolds number range of 10 950−43 800, a nozzle-to-surface distance range of 2−10, and a dimensionless radial distance range of 0−15. The measured local mass transfer distributions in the radial direction exhibited an increase first and then a decrease and a converging behavior as the radial distance increased. It was observed that increasing Reynolds number increased the mass transfer rate, but its effect was reduced with an increase in swirl intensity. Increasing swirl intensity decreased transfer coefficients but increased their uniformity over the disk surface. The change in the nozzle-to-surface distance affected mass transfer coefficients up to a dimension...

Electrochemical mass transfer between an impinging jet and a rotating disk in a confined system

This paper presents the mass transfer results from an impinging liquid jet to a rotating disk. The mass transfer coefficients were measured using the electrochemical limiting diffusion current technique (ELDCT). Rotational Reynolds number (Rer) in the range of 3.4×104–1.2×105, jet Reynolds number (Rej) 1.7×104–5.3×104 and non-dimensional jet-to-disk spacing (H/d) 2–8 were taken into consideration as parameters. It was found that the jet impingement resulted in a substantial enhancement in the mass transfer compared to the case of the rotating disk without jet.

A comparison of mass transfer between a plate and submerged conventional and multichannel impinging jets

This study presents a work on the comparison of mass transfer distribution on a plate subjected to impingement by a conventional impinging jet (CIJ) and multichannel conventional impinging jet (MCIJ). The local mass transfer coefficients were measured by the electrochemical limiting diffusion current technique (ELDCT). The jet Reynolds number varied in the range of 10,950–50,800 and jet-to-plate distance 2 d–10 d, where d is the nozzle diameter. The local mass transfer values were recorded beginning from the impingement point to a radial distance of approximately 15 d. The distribution of the local mass transfer coefficients (LMTC) was compared for both jet systems.

Free convective mass transfer at circular thin disk electrodes with varying inclination

The electrochemical limiting diffusion current technique was used to evaluate free convective mass transfer rates at free-standing circular disk electrodes of varying inclination and diameter. The electrochemical system used was copper deposition from acidified cupric sulphate solution. Experiments were carried with the single faces active and also with both faces simultaneously active. A Toepler–Schlieren system was used to photovisualise the ascending convection streams in order to obtain flow pattern information. For the upward-facing surface, the flow separates at a certain distance from the leading edge of the inclined disk, whereas the flow on the downward-facing surface always stays attached. The data correlations for the doubled disks show an exponent in the Sh– Ra correlation ranging from 0.25 (vertical case), indicating laminar flow, to 0.294 (horizontal case), indicating a mixed effect of turbulent separated flow on the upward-facing side and attached flow on the downward-facing surface. A universal correlation Sh d = f A ( Ra d ) f B was obtained for disk diameters d from 5 to 80 mm and inclination angle from 0° to 90° (from the vertical) where f A and f B are functions of inclination.

An electrochemical mass transfer investigation in the presence of simultaneous subcooled pool boiling

Mass transfer rates were investigated in the presence of simultaneous subcooled pool boiling at a vertical cylindrical electrode using the electrochemical limiting diffusion current technique. Various concentrations of equimolar ferri/ferrocyanide (0.01-0.20M) in 0.5M NaOH electrolyte were used. The initial metal temperature was 500° C and liquid subcooling was varied between 25 and 80° C. Transient temperature and limiting current histories were recorded following sudden immersion of the hot electrode and Schlieren photographs were taken for a range of electrode temperatures. In the nucleate boiling regime, the variation of mass transfer coefficient with temperature difference between electrode and electrolyte showed similar trends to the variation of heat transfer with temperature difference. Film boiling was briefly observed at low subcooling, eg. 27° C

Free convective mass transfer at down-pointing truncated cones

Free convective mass transfer at down-pointing truncated cones was experimentally studied using the electrochemical limiting diffusion current technique. The total mass transfer coefficient has the same dependence on height as that of a conical surface; however, the values are higher because of the high mass transfer at the up-facing horizontal surface. The flow development after sudden switch on of the limiting current potential was observed. The flow begins as a ring of fluid rising from the upper cone periphery followed by necking of the plume and a steady-state situation arises in which there is a complex flow structure consisting of a multiplicity of close spaced interacting convection streams. The experimental total mass transfer data were correlated using a method which includes an interference factor taking account of the fact that the conical and up-facing horizontal surfaces of the truncated cones are exposed to fluid which has already been depleted in cupric ions as it flows up from the down-facing horizontal surface.

Energetic Correlation of Local Mass Transfer in Swirling Pipe Flow

Mass transfer measurements in a decaying swirl flow in a pipe were carried out, using the electrochemical limiting diffusion current technique. The measurements were taken from local electrodes of 1 mm diameter without a developing concentration boundary layer around. Swirl was generated using short helical guide-vanes with vane angles between 15 and 60° at the outer edge of the vanes to the duct axis. The pressure drop measurements were also taken to perform an energy analysis. For a Reynolds number range of 1700−10000, the overall energy correlation for a representative length of 36.5 diameter were evaluated to be Shs(Sc)-0.33 = 0.3705(1 + tan θo)-0.120 X0.236, where θo is the vane angle at the outer edge of the helical swirler and X is the energy dissipation parameter.

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.

Natural convection mass transfer at a horizontal cylinder electrode with an opposed thermal buoyancy effect

Simultaneous heat and mass transfer at horizontal cylinder electrodes in free convection has been investigated experimentally using the electrochemical limiting diffusion current technique, in the situation where the concentration and thermal buoyancy effects are opposed. The ferri-ferrocyanide redox system was used for the mass transfer measurements, the electrolyte system being equimolar potassium ferri-ferrocyanide with 0.5 M NaOH as supporting electrolyte. The chosen concentrations were 0.01, 0.02, 0.05, 0.10 and 0.2 M. Two different nickel cylinders of diameter of 33.2 and 21.3 mm were used, each 100 mm in length. The surface temperature of the cylinder varied from 20°C (isothermal) to 85°C. Results for the opposed upward and downward convection was correlated using a combined Grashof number for mass transfer in the range 10 8 < GR m Sc < 10 11. The Schlieren technique of flow visualisation was used for observation of the flow structure.