Determination Of Transfer Coefficient Research Articles

Heat Transfer Coefficient Determination during FC-72 Flow in a Minichannel Heat Sink Using the Trefftz Functions and ADINA Software

This work focuses on subcooled boiling heat transfer during flow in a minichannel heat sink with three or five minichannels of 1 mm depth. The heated element for FC-72 flowing along the minichannels was a thin foil of which temperature on the outer surface was measured due to the infrared thermography. The test section was oriented vertically or horizontally. A steady state heat transfer process and a laminar, incompressible flow of the fluid in a central minichannel were assumed. The heat transfer problem was described by the energy equations with an appropriate system of boundary conditions. Several mathematical methods were applied to solve the heat transfer problem with the Robin condition to determine the local heat transfer coefficients at the fluid/heated foil interface. Besides the 1D approach as a simple analytical method, a more sophisticated 2D approach was proposed with solutions by the Trefftz functions and ADINA software. Finite element method (FEM) calculations were conducted to find the temperature field in the flowing fluid and in the heated wall. The results were illustrated by graphs of local heated foil temperature and transfer coefficients as a function of the distance from the minichannel inlet. Temperature distributions in the heater and the fluid obtained from the FEM computations carried out by ADINA software were also shown. Similar values of the heat transfer coefficient were obtained in both the FEM calculations and the 1D approach. Example boiling curves indicating nucleation hysteresis are shown and discussed.

Experimental Study on Oxygen Mass Transfer Characteristics by Plunging Hollow Jets

Aeration by plunging water jets is an effective and economical method of transferring oxygen into the receiving pool of water. A dispersion of air bubbles occurs underneath the pool surface as a high-velocity liquid jet plunges on the water surface after being ejected from a particular fall height. The present study aims to investigate the impact of jet velocity, jet length, water depth and jet thickness of plunging hollow jets in oxygenating the water in the aeration tank. Empirical correlations are proposed for the determination of volumetric oxygen transfer coefficient based on jet velocities and jet kinetic powers. Moreover, the oxygen transfer capacity of single hollow jet, double hollow jet and a dual jet (combined hollow and solid) is tested by keeping a nearly constant jet flow area through the jet openings. The observations recorded from the experimental analysis lead to the development of predictive relationships for estimating the values of volumetric oxygen transfer coefficient in terms of basic jet variables.

Experimental and numerical study of liquid–liquid interphase mass transfer in a pilot-scale extraction column

Liquid–liquid interphase mass transfer was experimentally and numerically studied in a pilot-scale pulsed disc-and-doughnut column (PDDC). The mass transfer coefficients, which were determined by analyzing experimental data through a transient axial dispersion model, increased from 2.27 × 10–6 to 3.06 × 10–6 m/s with an increase in the pulsation intensity from 0.0065 to 0.0105 m/s. The Euler–Euler two-fluid model was applied to simulate the interphase mass transfer in the PDDC. Three key issues, including the formulation of governing equations, selection of the mesh size, and determination of the mass transfer coefficient, which significantly influence the numerical accuracy, were discussed in detail. The simulated concentration profiles accorded well with that predicted by the experimentally verified mass transfer model. Local mass transfer performance was uncovered based on simulation results to elucidate the spatial and temporal variation of the solute concentrations, hold-up, and mass transfer coefficients in the PDDC.

Removal of methylene blue from aqueous solution by ozone microbubbles

In this work, ozone microbubbles (OMBs) technique was used to remove methylene blue dye (MB) from water in a semi- batch reactor. The removal efficiency of methylene blue dye were investigated under various reaction conditions such as effect of initial solution pH, ozone generation rate, initial methylene blue dye concentration and determination of mass transfer coefficient. The removal of methylene blue by Ozonation microbubbles were very high at the acidic media and upon increasing ozone generation rate from 0.498 to 0.83 mg s−1, the removal efficiency dramatically increased from 8 to 98%.The overall rate of the oxidation reaction fitted well a second order kinetic model. The results demonstrated that ozone microbubbles were effective in terms of the elimination of methylene blue concentration and its complete mineralization.

Experimental determination of gas holdup and volumetric mass transfer coefficient in a jet bubbling reactor

The hydrodynamics and mass transfer characteristics of a lab-scale jet bubbling reactor (JBR) including the gas holdup, volumetric mass transfer coefficient and specific interfacial area were assessed experimentally investigating the influence of temperature, pH and superficial gas velocity. The reactor diameter and height were 11 and 30 cm, respectively. It was equipped with a single sparger, operating at atmospheric pressure, 20 and 40 °C, and two pH values of 3 and 6. The height of the liquid was 23 cm, while the superficial gas velocity changed within 0.010–0.040 m·s −1 range. Experiments were conducted with pure oxygen as the gas phase and saturated lime solution as the liquid phase. The liquid-side volumetric mass transfer coefficient was determined under unsteady-state oxygen absorption in a saturated lime solution. The gas holdup was calculated based on the liquid height change, while the specific interfacial area was obtained by a physical method based on the bubble size distribution (BSD) in different superficial gas velocities. The results indicated that at the same temperature but different pH, the gas holdup variation was negligible, while the liquid-side volumetric mass transfer coefficient at the pH value of 6 was higher than that at the pH = 3. At a constant pH but different temperatures, the gas holdup and the liquid-side volumetric mass transfer coefficients at 40 °C were higher than that of the same at 20 °C. A reasonable and appropriate estimation of the liquid-side volumetric mass transfer coefficient ( k l a ) in a pilot-scale JBR was provided which can be applied to the design and scale-up of JBRs. • k l a is determined experimentally for O 2 absorption into Ca(OH) 2 solution in the JBR. • k l a is calculated for SO 2 absorption into Ca(OH) 2 solution in the JBR. • Specific surface area, is experimentally obtained by a physical method based on BSD. • Effects of temperature, solution pH, and superficial gas velocity are studied on k L a . • Gas holdup as a hydrodynamic parameter is investigated for the JBR.

ОСОБЕННОСТИ ОБРАБОТКИ ЭКСПЕРИМЕНТАЛЬНЫХ ДАННЫХ ПРИ ОПРЕДЕЛЕНИИ КОЭФФИЦИЕНТА ТЕПЛООТДАЧИ В МАСЛЯНОЙ ПОЛОСТИ ОПОРЫ ГТД

The development of gas turbine technology is accompanied by an increase in temperatures, pressures, and airflow velocity in the gas path. Increasing operating cycle parameters for gas turbine engine complicates the tasks of ensuring the permissible temperature state of engine parts, requires improving the methods of their calculation and design. This fact fully applies bearing assemblies, especially those operating in a hot environment, and causes interest in the study of thermohydraulic processes in the bearing chamber, which determines the temperature state of the rotor parts. The necessity of pressurizing the seals leads to the presence of the oil-air mixture in the bearing chamber. A wide range of operating parameters, flow inhomogeneity, phase disequilibrium, and phase separation significantly complicate the mathematical description of processes in the bearing chamber, including the use of CFD-modeling. Therefore, considerable attention is paid to experimental research. The experimental results are used not only to verify mathematical models but also to obtain generalizing dependencies. Most often, the desired value is the heat transfer coefficient in the oil cavity of the support. The article deals with the heat transfer features in the near-wall zone of the gas-turbine engine bearing chamber which were associated with the presence of oil-air flow. Also, approaches to the experimental determination of the heat transfer coefficient were analyzed and an appropriate system for measuring the local temperatures of the media was formed. The values of the error of the experimental heat transfer coefficient and the degree of influence of the determining factors were estimated. The contribution of the non-uniformity of the temperature field in the walls of the chamber and the uncertainty in the value of the temperature of the flow core was determined. The advantages of using the averaged heat transfer coefficient for engineering calculations and the significant influence of the averaging method on its value were also shown. Averaging over the heat flux density corresponds most accurately to the tasks of such calculations, at which the total heat flux through the chamber walls does not change.

Determination of mass transfer coefficients in high-pressure two-phase flows in capillaries using Raman spectroscopy

This study presents a method for the experimental determination of local volumetric mass transfer coefficients kLaL in a high-pressure two-phase flow of water (H2O) and carbon dioxide (CO2) in a micro-capillary using Raman spectroscopy. H2O and CO2 are continuously and co-currently fed at high pressure (8, 9 and 10 MPa) and moderate temperature (303 K) into a fused silica micro-capillary. A segmented two-phase flow is obtained therein and the fraction of CO2 in the water-rich phase is measured in-situ at different points along the capillary using Raman spectroscopy. A modified Henry’s law is used to compute the equilibrium compositions of the water-rich phase at the desired pressure and temperature. A mixture density model is used to convert the fraction of CO2 in the water-rich phase into a CO2 concentration. The volumetric liquid mass transfer coefficient is computed at various axial locations along the capillary, from the contacting zone to the end of the capillary. Experimentally derived kLaL values range between 2.10−3 and 5.10−3 s−1.

A modeling study on the effects of pH and partial wetting on the removal of ammonia nitrogen from wastewater by membrane contactors

Abstract A one-dimensional mass transfer model, which includes two vital parameters on the performance of the membrane contactors for the ammonia nitrogen removal from wastewater, namely wastewater pH and membrane partial wetting, was developed. The degree of membrane wetting was estimated as a function of wastewater velocity and included in the determination of the overall mass transfer coefficient based on the resistance-in-series model. A modified Henry’s law constant was employed to take into account the effect of pH on the mass transfer. The calculated overall mass transfer coefficients based on the proposed model fitted well with experimental results in various ranges of wastewater inlet pH and velocities. The contribution analysis of resistance in each phase to the overall mass transfer resistances showed that the wastewater phase was still the dominant phase, although the partial wetting was included in the consideration. Comparing with the carbon dioxide and methane desorption, the membrane phase resistance of the ammonia desorption was much more significant to the mass transfer. The determination of the membrane area required to remove the dissolved ammonia revealed that the area was greatly influenced by the wastewater inlet pH. The K OV along the length of membrane module was gradually decreased due to the NH3 desorption. Thus, the membrane area could be significantly much lower when the wastewater pH was adjusted to the initial value in between the membrane modules connected in the series.

Determination of transfer coefficients of a tunnel type screenhouse for thermal modelling

Screenhouses could help the reduction of pesticide usage for sustainable agriculture production in the tropics. A screenhouse is inherently naturally ventilated by the air flowing through the screen that covers the structure. However, overheating is still a problem that often occurs inside screenhouses in the tropics. Therefore, it is important to develop a thermal model as design tool for tropical screenhouses. The objective of this study was to determine the screenhouse transmissivity and the overall heat transfer coefficient of a tunnel type screenhouse for thermal modelling. A thermal model has been developed to consider these transfer coefficients. The model was modified from thermal model proposed by Desmarais et al. Validation of the thermal model was done by comparing predicted air temperature inside the screenhouse to that of measured values. The average transmissivity of the screenhouse obtained was 0.42, while the heat transfer coefficient of the screenhouse averaged from measurement results was 51.6 W/m2 oC. The validation showed that predicted air temperature inside the screenhouse agreed well to that of the measured values. It was concluded that the thermal model could be used as a good design tool for tunnel type tropical screenhouses.

Experimental work on decontamination of wastewaters containing organic dye by liquid phase micro extraction method

ABSTRACT Nowadays, the innovative processes for refining industrial dye wastewater are often used to reduce the toxicity of these pollutants due to satisfy the treatment standards. This paper presents a study on the removal of methylene blue (MB) and thymol blue (TB) dyes from contaminated water using a mono-and di-ester mixture(MDEHPA) as the extractant in a Y-Y junction microchannel. Operating parameters such as aqueous solution pH, and extractant and residence time were optimized. The maximum rates of TB and MB extraction were achieved to be 97.9% and 98.4%, respectively. Mass transfer performance is surveyed using overall volumetric mass transfer coefficient determination ().

Determination of fluxes and mass transfer coefficients of polychlorinated biphenyls (PCBs)

This study evaluated a modified water surface sampler (MWSS) in terms of deposition characteristics of atmospheric polychlorinated biphenyls (PCBs). Water was employed as a collection surface for the atmospheric PCBs. The main difference between the MWSS and previous WSSs was its cover which prevented the deposition of particles and minimized wind effects. Therefore, lower particle-phase deposition fluxes were observed. Moreover, preventing wind from the water surface, which was the collection surface, caused lower dissolved phase fluxes and mass transfer coefficients (MTCs). Forty-eight flux and ambient air samples were taken by using an MWSS and a high volume air sampler (HVAS), respectively. The average dissolved phase PCB fluxes were 5.92 ± 5.37 ng/m2-day. Particle phase PCB fluxes were also determined by attaching a filter to the sampler. The particle phase flux ratio was 14% of the total flux, which was much smaller than previously reported ones. This difference was mainly due to the cover layer which minimized the wind effects. 4- CBs and lower chlorinated PCBs were dominant in the samples. Homolog groups obtained from both the MWSS and HVAS were highly correlated. This result indicated that both samplers collected similar atmospheric PCBs. Then, MTCs were calculated for each PCB congener. To calculate the MTCs, dissolved phase PCB fluxes were divided by gas-phase PCB concentration. The average MTC was determined to be 0.25 ± 0.23 cm/s which was smaller than the previously reported MTCs. This result illustrated that the MWSS configuration allowed us to determine the minimum MTCs and gas transfers to the water.

Establishing patterns in the temperature distribution within a deformation zone during thin strip rolling

To improve the strip rolling technology, it is important to know components of the thermal state of both the strip being rolled and the tool used, that is, rolls at each point of different layers of the strip and rolls, in any section of the deformation zone. It was established that for the numerical solution of thermal problems of heat transfer in the strip-roll system described by equations of unsteady heat conduction, the finite difference method is the most effective. For the further numerical solution of the problems of unsteady thermal conductivity of the strip and rolls during hot rolling, the sections of slabs and rolls were divided by a conditional mesh. Energy balance equations with subsequent finite-difference Fourier approximation for possible options of the mesh nodes occurring in solving the two-dimensional problem of unsteady heat conduction.When solving the heat balance problems for both the strip and the rolls, the performed transformations make it possible to switch from solving the nonlinear heat conduction problem to solving the linearized problem. It was also shown that when calculating the thermal state of the active zone in which cyclic temperature changes occur during one revolution, it becomes possible to switch from solving a problem in a cylindrical coordinate system to solving it in a rectangular coordinate system. Transition to solving a one-dimensional strip-roll system greatly simplifies the calculation. The solution of the III boundary-value problem for the roll and comparison of the obtained results with the results of solutions for the strip-roll system enables the theoretical determination of the heat transfer coefficient in the deformation zone.The study results can be used to determine temperature and speed mode of cooling a thin strip during its rolling as well as set tasks for designing special equipment for accelerated cooling in a production stream of strip rolling mills

Numerical simulations and modelling of mass transfer through random assemblies of catalyst particles: From dilute to dense reactive particulate regime

We study mass transfer through random assemblies of fixed spherical catalyst particles experiencing an external convective-diffusive fluid stream. Chemical species are transported through the array and are diffused from fluid to solid phase through particles surface. An internal first order irreversible chemical reaction takes place within the porous catalyst particles. We address the determination of mass transfer coefficient by performing direct numerical simulations with fully internal-external coupling using concentration and flux continuity boundary conditions at the solid-fluid interface. We derive a theoretical prediction of the profiles of cup-mixing concentration, average of mean surface and average of mean volume concentration of the particles along the height of the domain. The model for the dimensionless mass transfer coefficient (‘reactive’ Sherwood number) is accounting for the five dimensionless parameters that control the physics of the system: the Reynolds number Re, the Schmidt number Sc, the Damköhler number Da, the internal-to-external diffusion coefficient ratio γ and the solid volume fraction αs. We use a coupled Sharp Interface/ Discrete Lagrange Multiplier-Fictitious Domain Method (SIM-DLM/FD), thoroughly validated in our previous study (Sulaiman et al., 2019) to test the accuracy of the model over a wide range of dimensionless parameters and solid volume faction (from dilute αs=0.1 to dense regime αs=0.5). We show and discuss the limitations of the proposed model.

Kinetic Study of Reactive SO2 Absorption in a Jet Bubbling Reactor: Experimental Determination of Mass Transfer Coefficients and Absorption Rate

The reactive absorption kinetics for SO2 removal from flue gas was studied by the use of a saturated solution of Ca(OH)2 in a bench-scale jet bubbling reactor (JBR). Gas and liquid sides mass trans...

Prediction of the residual state of stress in a superduplex stainless steel produced by sand casting (using a coupled thermo-mechanical approach)

The present work aims at investigating the effect, in terms of residual stress prediction, determined by the adoption of different material constitutive models in the Finite Element simulation of the casting process of a superduplex stainless steel benchmark, from the cooling after the pouring phase to the subsequent heat treatment (heating and water quenching). A 3D thermo-mechanical Finite Element (FE) model was created with the commercial code Abaqus: the preliminary thermal problem, i.e. the determination of the heat transfer coefficients during both the mould cooling and the quenching, was solved by means of an inverse analysis approach by minimizing the difference between the numerical evolution of temperature and the experimental acquisition coming from real casting test. Two separate routes were followed according to the adopted constitutive equations: modelling the material as (i) elasto-plastic or (ii) elasto-viscoplastic. The numerical prediction of the residual state of stress in terms of casting relaxation was compared with the experimental data after the cut of the casting using an electro-discharge machine. The analysis revealed that when the elasto-viscoplastic modelling was adopted, the simulations underestimated the relaxation with an error larger than 50%; on the other hand, the elasto-plastic model leads to an overestimation with an error of about 30%.

Determination of the mass transfer coefficients in direct immersion solid‐phase microextraction

Diffusion of the analytes across the diffusion boundary layers and subsequently through the fiber coatings determines the extraction kinetics of solid-phase microextraction in aqueous matrices. Besides, the matrix effects can distort the behaviors of the analytes transferring across the diffusion boundary layers. However, these processes were always studied via certain simplification, which often left the mass transfer through the fiber coatings unconsidered and the matrix effects partially investigated. Herein, a comprehensive study on the mass transfer processes in direct immersion solid-phase microextraction was presented. Under different agitation speeds, it was determined that the mass transfer coefficients across the diffusion boundary layers were three to six orders larger than those through the fiber coatings. However, the mass transfer across the diffusion boundary layers was generally the major rate-limiting step. In addition, the shuttle effect and the barrier effect, which were responsible for accelerating and retarding the extraction kinetics, respectively, were found to be the dominant matrix effect alternately under different agitation speeds. This study comprehensively illustrated the major rate-limiting step and the dominant matrix effects through recording the mass transfer coefficients.

Online Determining Heat Transfer Coefficient for Monitoring Transient Thermal Stresses

This paper proposes an effective method for determining thermal stresses in structural elements with a three-dimensional transient temperature field. This is the situation in the case of pressure elements of complex shapes. When the thermal stresses are determined by the finite element method (FEM), the temperature of the fluid and the heat transfer coefficient on the internal surface must be known. Both values are very difficult to determine under industrial conditions. In this paper, an inverse space marching method was proposed for the determination of the heat transfer coefficient on the active surface of the thick-walled plate. The temperature and heat flux on the exposed surface were obtained by measuring the unsteady temperature in a small region on the insulated external surface of a pressure component that is easily accessible. Three different procedures for the determination of the heat transfer coefficient on the water-spray surface were presented, with the division of the plate into three or four finite volumes in the normal direction to the plate surface. Calculation and experimental tests were carried out in order to validate the method. The results of the measurements and calculations agreed very well. The computer calculation time is short, so the technique can be used for online stress determination. The proposed method can be applied to monitor thermal stresses in the components of the power unit in thermal power plants, both conventional and nuclear.

Numerical Solution of Axisymmetric Inverse Heat Conduction Problem by the Trefftz Method

In this paper, the issue of flow boiling heat transfer in an annular minigap was discussed. The main aim of the paper was determining the boiling heat transfer coefficient at the HFE-649 fluid–heater contact during flow along an annular minigap. The essential element of the experimental stand was a test section vertically oriented with the minigap 2 mm wide. Thermocouples were used to measure the temperature of the heater and fluid at the inlet and the outlet to the minigap. The mathematical model assumed that the fluid flow was laminar and the steady–state heat transfer process was axisymmetric. The temperatures of the heated surface and of the flowing fluid were assumed to fulfill energy equations with adequate boundary conditions. The problem was solved by the Trefftz method. The local heat transfer coefficients at the fluid–test surface interface were calculated due to the third kind boundary condition at the saturated boiling. Graphs were used to illustrate: the measurement of the heater surface temperature, 2D temperature distributions in the pipe and fluid, and the heat transfer coefficient as a function of the distance from the minigap inlet. The measurement uncertainties and accuracy of the heat transfer coefficient determination were estimated.

Determination of the mass transfer coefficient of ammonia emissions from dairy open lots using a scale model

Open lots are commonly used to provide outdoor free ranging spaces for dairy cows in many countries such as China and the USA. For this type of operation, the manure characteristics, floor types and environmental conditions, which potentially affect gaseous emissions from manure, are quite different from those of indoor systems. The estimation of NH3 emissions from manure depends greatly on the mass transfer coefficient, an empirical constant determined by environmental factors in the process-based emission model. Using a scale model, this study measured ammonia emissions to be expected from dairy open lots, based on different combinations of different surface air velocities (0.6–2.2 m s−1), air temperatures (15–35 °C) and relative humidity values (20–60%). Based on the two-film theory, the overall mass transfer coefficient (KOL) of NH3 was calculated from directly measured NH3 emission rates and the ammonium nitrogen content, temperature, and pH of manure. As an indicator of the driving potential of gaseous volatilisation, the vapour pressure deficit (VPD) was estimated using air temperature and relative humidity. As the VPD and surface air velocity increased, the measured NH3 emission rates ranged from 1.35 to 19.18 mg kg−1 h−1, and the estimated KOL of NH3 from dairy open lots was within the range of 1.48 × 10−6 – 7.86 × 10−6 m s−1. Using the air velocity and VPD as variables, a non-linear empirical model was developed to estimate the KOL for NH3 emissions from manure on dairy open lots (R2 = 0.69). The normalised mean error of the model was approximately 19.4%.

Determination of Mass Transfer Coefficients for Adsorption of Pb and Cd Onto Coir Pith and Statistical Analysis

Adsorption of Pb and Cd ions onto coir pith from single solute aqueous solutions was studied. Experimental data on variation of solute concentration in the solution with time were fitted to exponential kinetic model and model parameters were determined. New parameter, concentration decay constant, was introduced to comprehend the adsorption rate. Novel method was introduced to determine the liquid film mass transfer coefficient (kL) accurately using the exponential kinetic model equation. Liquid film mass transfer coefficient and Overall liquid phase mass transfer coefficient were calculated and the proposed method overcomes the errors due to approximation of initial rate of change of solute concentration and assumption of linear isotherm parameters. Statistical analysis of mass transfer coefficients showed that for all the parameters tested the variation of the parameters are significant with initial solution concentration for Pb.