Surface Transfer Coefficients Research Articles

Measurements of Carbon Diffusivity and Surface Transfer Coefficient by Electrical Conductivity Relaxation during Carburization: Experimental Design by Theoretical Analysis

The diffusion coefficient (D) and surface transfer coefficient (β) of carbon are important parameters governing the kinetics of carburization, and some other heat treatment processes accompanied by redistribution of carbon in steel. Here, we propose to use an electrical conductivity relaxation (ECR) method for the in situ measurement of D and β of carbon. The feasibility of the method is discussed by the theoretical modeling of carburization for an infinitely long rectangular sample. The synthetic ECR data for the carburization is simulated by tracking the relaxation of electrical conductivity upon a sharp or a gradual change of carbon potential. Then, by Fourier transform, the synthetic ECR data is transformed to an impedance spectroscopy, which is used for estimation of D and β by fitting with a one-dimensional equivalent circuit model. The effects of the width-to-thickness ratio of the sample and the duration of carbon potential buildup on the accuracy of the estimated D and β are studied. The feasibility of the ECR method is verified, and rational guidance for experimental design is proposed.

Electrochemical supercapacitor performance of NiCo2O4 nanoballs structured electrodes prepared via hydrothermal route with varying reaction time

Nanostructured binary metal oxides have shown their potential for improved electrochemical performance due to their structural stability, electronic conductivity, and multiple oxidation states. Herein, uniform NiCo2O4 nanoballs were successfully prepared using the chemical hydrothermal process with various reaction times (5 h, 8 h, 14 h) followed by characterizations and investigation for supercapacitive properties. Nanoballs prepared at 14 h reaction time demonstrated the highest supercapacitor performance, owing to features microstructure and synergetic effect. Nanoballs have the highest electrochemical active surface area and transfer coefficient. The surface capacitive contribution of the NCO-SH-14h electrode is greater with a higher electrochemically active surface area. The contribution of surface capacitance is increased when the minimum relaxation period. The NiCo2O4 nanoballs have shown remarkable electrochemical performance with a specific capacitance of 1144 F/g and areal capacitance of 242 mF/cm2, and good stability of 5000 cycles. The NiCo2O4 nanoballs prepared at optimal hydrothermal reaction time (14 h) could be beneficial in energy storage applications with high energy and power density.

Surface transfer coefficients estimation for heat conduction problem using the Bayesian framework

This work deals with an inverse two-dimensional nonlinear heat conduction problem to determine the top and lateral surface transfer coefficients. For this, the Bayesian framework with the Markov Chain Monte Carlo algorithm is used to determine the posterior distribution of unknown parameters. To handle the computational burden, a lumped one-dimensional model is proposed. The lumped model approximations are considered within the parameter estimation procedure thanks to the Approximation Error Model. The experiments are carried out for several configurations of chamber ventilator speed. Experimental observations are obtained through a complete measurement uncertainty propagation. By solving the inverse problem, accurate probability distributions are determined. Additional investigations are performed to demonstrate the reliability of the lumped model, in terms of accuracy and computational gains.

Comparison of drying kinetics, thermal and performance parameters during drying guava slices in natural and forced convection indirect solar dryers

The performance of the dryer, thermal properties and the drying kinetics of guava were evaluated in natural convection indirect solar dryer (NCISD) and forced convection ISD (FCISD) and compared. The average final moisture content (MC) of guava slices (kept in all four trays) was reached from 5.5355 to 0.0244 kg/kg of db. The final MC was attained at 14, 18 and 24 h in FCISD, NCISD and open sun drying (OSD) methods, respectively. Effective diffusion coefficient (Def), surface transfer coefficients (h and hm), collector and dryer efficiencies (ηcoll and ηdry), specific energy consumption and activation energy were estimated for both setups. The Page models were the best models for indicating the drying curve of guava slices in NCISD, FCISD and OSD. The specific energy consumption (SEC) of guava slices was 2.108 and 1.675 kWh/kg in NCISD and FCISD, respectively. The average increment of 34.12% on Def, 55.59% of h, 55.55% of hm, 16.63% of ηcoll, 26.19% of ηdry and decrement of 20.54% on SEC were observed in FCISD than NCISD. The activation energy of guava slices was 136.98 and 116.49 kJ/mol in NCISD and FCISD, respectively. 16.63% and 26.19% improvements in ηcoll and ηdry, respectively, were noticed in FCISD compared to NCISD. The performance of ISD is improved in FCISD by attaching DC fans driven by PV modules.

Numerical investigation on solar air collector and its practical application in the indirect solar dryer for banana chips drying with energy and exergy analysis

In this paper, a numerical investigation was carried out to identify a proper solar air collector (SAC) for an indirect solar dryer (ISD). Corrugated absorber surfaces were used to enhance the heat transfer rate between air and absorber. The manuscript aims to develop a numerical model to identify proper SAC and to execute the proposed SAC model in an experimental setup hence it is an innovative study. The drying behavior and drying characteristics of banana slices were estimated by conducting the experiments in an ISD dryer which was developed based on the outcomes of numerical analysis. The velocity and temperature inside the dying cabinet were estimated for both cases. Better performance was noticed in triangular corrugated SAC and therefore, an ISD was developed with triangular corrugated SAC. The thermal performance of ISD was investigated. Numerical and experimental results of temperature at the outlet of SAC were compared. Also, the drying kinetics of banana samples was studied. Different corrugation angles (θ) (5 to 30°) and corrugation heights (e) (0.5 to 2 cm) were used to identify the maximum thermo-hydraulic performance parameters (THPP). The maximum THPP was 1.83 at θ = 20° and e = 1 cm which helped uniform distributions of velocity and temperature inside the drying chamber. From the investigation, surface transfer coefficients, diffusivity, and activation energy of banana slices were calculated. The average collector and dryer efficiencies were 64.5% and 55.30%, respectively. Banana samples were dried from 3.5566 to 0.2604 kg/kg on a dry basis (db). The experimental drying curve was fitted to six thin layer drying models and a fitting model was identified. Effective moisture diffusivity, surface transfer coefficients, and activation energy were estimated. Exergy and cost analyses were performed.

Shrinkage, effective diffusion coefficient, surface transfer coefficients and their factors during solar drying of food products – A review

Drying of food materials leads to an increase in shelf-life and gives quality end products. Drying with solar energy is an economical and environmentally friendly method than other drying methods. The food products are dried using solar energy in different methods such as open sun drying, direct, indirect, mixed-mode and hybrid solar drying. There are some parameters such as shrinkage, effective diffusion coefficient (De), surface transfer coefficients, etc. majorly influence the solar drying of food products and these elements are extensively reviewed in this work. The shrinkage of food products affects the drying rate and quality of the food products as it involves surface cracking, volume, porosity and hardness changes. The De gives the moisture transfer rate in the food products during solar drying. The values of surface transfer coefficients including mass transfer coefficient (hm) and heat transfer coefficient (h) give the mass and heat transfer rate from the surface of the food product as these parameters are surface parameters. The parameters such as shrinkage, De, hm, and h of food products and their influencing factors need to be analyzed to understand more about the physics of the solar drying process. An extensive review is made on the factors of shrinkage, De, hm and h such as volume of water, moisture content, air temperature, air velocity, the thickness of product and geometry of food. The values of shrinkage, De, hm and h of different solar drying processes are tabulated and discussed. A brief review was performed on different mathematical models used to determine the values of shrinkage, De, hm, and h during solar drying of food products. The influencing factors such as moisture content, air temperature, velocity, thickness and geometry of the product on these three factors shrinkage, De, hm, and h were discussed. Important results and recommendations of this topic are mentioned at the final because it is an innovative review trial in the solar drying area.

Experimental Investigation of Drying Kinetics of Green Chilli and Okra using Indirect Solar Dryer with Evaluation of Dryer Performance

ABSTRACT In this study, green chillies and okra were dried using an indirect type solar dryer ITSD dryer and their drying kinetics were evaluated. The initial moisture content (MC) of green chilli was 8.3984 kg/kg of dry basis (db) and it was reduced to final MC of 0.2218 kg/kg of db. Also, MC of okra reduced from 10.1234 to 0.3348 kg/kg of db. The efficiency of solar air collector and the drying efficiency were estimated during the drying of green chilli and okra. The activation energy was found to be 32.484 and 25.330 kJ/mol and effective moisture diffusion coefficient was found to be 1.464 × 10-−9 and 5.1798 × 10-−9 m2/s for green chilli and okra, respectively. The surface transfer coefficients (both heat and mass) of green chilli and okra were estimated. Uncertainty analysis was performed on all the measured and derived drying parameters.

Development of indirect type solar dryer and experiments for estimation of drying parameters of apple and watermelon

In the present paper, an indirect type solar dryer (ITSD) has been developed for apple and watermelon drying. The performance of ITSD and drying kinetics of watermelon and apple slices have been analyzed. From the experimental analysis, diffusion coefficient, surface transfer coefficients and activation energy of apple and watermelon were estimated. Experimental outcomes showed that the temperature inside the drying cabinet varied with time because of frequent variation in solar intensity. The average thermal efficiency of the collector and dryer was 54.5% and 25.39% during apple drying and 56.3% and 28.76% for watermelon drying, respectively. Moisture content of apple decreased from 6.16 to 0.799 kg/kg of dry basis (db) and that of watermelon reduced from 10.76 to 0.496 kg/kg of db. Drying curve was fitted with different models of existing studies. Average effective moisture diffusivity was estimated and it was 4.28 × 10−9 m2/s and 4.01 × 10−9 m2/s for apple and watermelon, respectively. Mass transfer coefficient is found to be in the range 1.584 × 10−4 to 3.158 × 10−3 m/s for apple and 5.17 × 10−4 to 4.98 × 10−3 m/s for watermelon. Heat transfer coefficient was in the range of 0.16 to 3.19 W/m2 K and 0.52 to 5.04 W/m2 K for apple and watermelon, respectively. Activation energy for apple and watermelon is 17.34 and 18.71 kJ/mol, respectively.

Experimental study of heat-transfer coefficient of Al-Zn-Mg-Cu ultra-thick hot plate during multi-stage quenching

Experiments were conducted to investigate the cooling manner of an ultra-thick hot aluminum alloy plate during multistage quenching. Cooling curves and heat flux curves of different rapid quenching flux varied from 23 to 40 L min−1 and were analyzed in detail. In this investigation, cooling process was divided into the following four steps: (I) starting step, (II) rapid cooling step, (III) slow cooling step, and (IV) stopping step. Based on the curves, the calculation method for surface transfer coefficient was provided, and the effects of coefficient on surface temperature and quenching flux were discussed. Results showed that the transfer coefficient disagreed with heat flux and that it is a nonlinear function of surface temperature. The highest coefficient was observed not in the rapid cooling step with the largest heat flux but in the slow cooling step with lower heat flux. The coefficient increased with surface temperature ranging from 480 to 150°C, and a coefficient peak appeared in the temperature range of 150–100°C. The coefficient also increased with quenching flux. Finally, a simulation was performed using the finite element method to verify the reliability of the coefficient results, which showed good agreement with the measurement values.

Comparison of surface sensible and latent heat fluxes over the Tibetan Plateau from reanalysis and observations

Surface sensible and latent heat fluxes (SH and LE) over the Tibetan Plateau (TP) have been under research since 1950s, especially for recent several years, by mainly using observation, reanalysis, and satellite data. However, the spatiotemporal changes are not consistent among different studies. This paper focuses on the spatiotemporal variation of SH and LE over the TP from 1981 to 2013 using reanalysis data sets (ERA-Interim, JRA-55, and MERRA) and observations. Results show that the spatiotemporal changes from the three reanalysis data sets are significantly different and the probable causes are discussed. Averaged for the whole TP, both SH and LE from MERRA are obviously higher than the other two reanalysis data sets. ERA-Interim shows a significant downward trend for SH and JRA-55 shows a significant increase of LE during the 33 years with other data sets having no obvious changes. By comparing the heat fluxes and some climate factors from the reanalysis with observations, it is found that the differences of heat fluxes among the three reanalysis data sets are closely related to their differences in meteorological conditions as well as the different parameterizations for surface transfer coefficients. In general, the heat fluxes from the three reanalysis have a better representation in the western TP than that in the eastern TP under inter-annual scale. While in terms of monthly variation, ERA-Interim may have better applicability in the eastern TP with dense vegetation conditions, while SH of JRA-55 and LE of MERRA are probably more representative for the middle and western TP with poor vegetation conditions.

An experimental investigation of convective heat transfer at evaporation of kerosene and water in the closed volume

An evaporation of kerosene and water was investigated based on convective heat transfer in the experimental setup simulating a typical volume of the fuel tank of the launch vehicle. Basic criteria of similarity used in choosing the design parameters of the setup, parameters of the coolant and model liquids, were numbers of Reynolds, Prandtl, Biot, and Nusselt. The used coolants were gases, including air and nitrogen; in addition, at the stage of preliminary experiments, products of combustion of hydroxyl-terminated polybutadiene (HTPB) were considered. Boundary conditions were taken for the liquid located on the plate in the form of “drop” and at its uniform film spread in the experimental model setup. On the basis of experimental investigations, the temperature values were obtained for the system “gas-liquid-wall”, and areas of mass transfer surface and heat transfer coefficients of “gas-liquid” and “gas-plate” were determined for coolants (air and nitrogen) and for liquids (water and kerosene). The comparative analysis of the obtained results and the known data was carried out. Proposals for experiments using coolants based on HTPB combustion products have been formulated.

Application of a novel biological nanofluid in a liquid block heat sink for cooling of an electronic processor: Thermal performance and irreversibility considerations

Hydrothermal characteristics and entropy generation of a biological nanofluid containing silver nanoparticles are evaluated in a liquid block heat sink for cooling of an electronic processor. The liquid block under study has 20 channels, and its bottom surface is placed on the processor. Nanoparticles synthesized through plant extract technique from green tea leaves are employed. The degree of improvement in cooling, pumping power, thermal performance and irreversibilities are examined for case of using the nanofluid instead of water. By increasing Reynolds number and particle concentration, temperature distribution becomes more uniform in processor surface and heat transfer coefficient also increases. Furthermore, the surface temperature decreases with increasing concentration and Reynolds number, such that it reduces by 2.21°C in case of using the nanofluid with concentration of 1% instead of water at Reynolds number of 500. Moreover, maximum temperature of the processor surface decreases by increasing Reynolds number and concentration and therefore, the possibility of hot spot formation diminishes. Results show that at a constant work consumption, the nanofluid also presents better cooling compared to water. Entropy generation analysis reveals that irreversibility in the whole liquid block decreases with increasing either concentration or Reynolds number, which is a positive result based on second law of thermodynamics.

Numerical Prediction and Analysis of Surface Transfer Coefficients on Moist Object During Heat and Mass Transfer Application

Convective surface transfer coefficients data for a moist object are not easily available from the literature and this coefficient is usually assumed to be constant in most studies. A three-dimensional (3D) numerical model is developed for prediction of heat and mass transfer coefficients on the surface of the rectangular moist object. A computational fluid dynamics tool is used for flow analysis at three different higher velocities. The spatial distribution of heat and mass transfer coefficients is estimated at the surfaces of the object. The effect of constant and variable surface transfer coefficient is analyzed through transient 3D simultaneous heat and mass transfer solutions of convective drying. The numerical results are compared with experimental results. Good agreement was found with the experimental results when the numerical solution is considered with variable heat transfer coefficient. The numerical solutions with constant heat transfer coefficient gives overestimated results. A new set of Nusselt number and Sherwood number correlations is developed from a numerical model, and these correlations are expected to be useful for drying industries.

What controls the size of a tropical cyclone? Investigations with an axisymmetric model

Control mechanisms of tropical cyclone size are investigated with the axisymmetric cloud model HURMOD. In agreement with preceding HURMOD studies, the model results exhibit the existence of a fixed‐point attractor associated with a tropical cyclone. From the non‐dimensionalized model equations, a similarity law is derived, which relates six model parameters to the horizontal extent of the tropical cyclone in a steady state. Each parameter is associated with one of the following processes: planetary rotation, condensation time‐scale, radiative relaxation time‐scale, horizontal diffusion, vertical diffusion, and surface transfer. Individual variation of the parameters reveals that the radius of maximum horizontal wind speed is very sensitive to the Coriolis parameter, the mixing lengths and the radiative relaxation time‐scale, whereas the radius of minimum tangential wind merely depends on the Coriolis parameter, the surface transfer coefficients, and the radiative relaxation time‐scale. The increase of the radius of maximum horizontal wind speed with vertical eddy diffusivity goes along with an enhancement of the overturning mass flux within the secondary circulation. This agrees with the Hadley cell theory by Held and Hou who also emphasized the relevance of vertical diffusion. The strengthening of the overturning mass flux follows a power law similar to that found in an ocean modelling study. Moreover, TC size is found to be sensitive to environmental conditions comprising the prescribed sea‐surface temperature, tropopause height and static stability, while the sensitivity with respect to relative humidity is relatively small in the model. When the torque due to the upper sponge layer is switched off, the model exactly fulfills in the long run the steady state angular momentum budget equation.

小型チャンバーにおける気流速度と水蒸気換算物質伝達率との関係

Using the newly developed airflow control unit (ACU), chamber performance was examined through measuring the air velocity (va) near the specimen surface and mass transfer coefficient (MTC). va and its distribution depending on fan revolutions were examined over nine points at a distance of 10mm from the specimen surface. MTC was investigated by tests with using filter papers and water vapor in the chamber installed with ACU. For MTC tests, seven conditions of fan revolution were set at less than 700rpm, and loading factor 0.11 m2/m3 was adopted to avoid saturating of humidity in the chamber instead of 2.2m2/m3 of usual emission test. As the results, va distributions were even and in a small deviation. When va ranges from 0.10 to 0.27 m/s, tested MTC were in 17.4 to 22.0m/h, while the values of JIS and previous studies in much lower levels. Chamber-tested MTCs should be much lower than those of theoretical calculation, because the given Q/A would make the potential of mass transfer decreased at the chamber test but theoretical calculation have the assumption of infinite Q. There, theoretical MTC depending on va were recalculated using the properties of dry and wet air. CFD simulations provided that steady state appeared at over L=0.44 m2/m3 at va = 0.1m/s and the MCT was 15.3m/h, which was close to the result of the chamber test at 300 rpm (much less than 0.1m/s), because the limited ventilation rate in the chamber test restrained the vapor transfer and consequently MTC. It was cleared that tested MTC always showed the higher values than JIS and theoretical ones, and could become close to other ones by compensation considering the velocity boundary layer and the loading factor.

Evaluation of Sub-Zonal Airflow Models for the Prediction of Local Interior Boundary Conditions – Natural and Forced Convection Cases

Currently, researchers are striving to advance the possibilities to calculate the integrated phenomena of heat, air and moisture flows in buildings, with specific focus on the interactions between the building zones and building components. This paper presents an investigation of the capability and applicability of the sub-zonal airflow model to predict the local indoor environmental conditions, as well as the local surface transfer coefficients near building components. Two test cases were analyzed for, respectively, natural and forced convection in a room. The simulation results predicted from the sub-zonal airflow models are compared to experimental data and numerical computational fluid dynamics (CFD) results. The study shows that sub-zonal models combined with an appropriate surface transfer coefficient model are able to give reliable predictions of the local indoor environmental conditions and surface transfer coefficients near the building component for the analyzed cases. The relatively short computation time and flexibility of the sub-zonal model makes the application attractive for transient simulation of heat, air and moisture transport in buildings. However, the availability of appropriate reference conditions, for example experimental or numerical results, is a prerequisite for the development of a reliable sub-zonal model.

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.

Near-wall turbulent transport of large-Schmidt-number passive scalars

Turbulent diffusion of a passive scalar with a large-Schmidt number (Sc >> 1) is considered in the viscous sublayer of a turbulent channel flow. Close to the wall, the corresponding eddy diffusivity coefficient is expanded as a power series in terms of the viscous distance to the wall y . The coefficients of the series depend on the Schmidt number and the analysis of recent numerical results allows to conclude that in the close vicinity of the wall (y << Sc(-1/3)) , the y(3) term is the dominant term; whereas, at distances relatively large from the wall (Sc(-1/3)<< y <<1) , the y(4) term becomes dominant. Accordingly, in this region the turbulent Schmidt number is not constant but follows a hyperbolic law in terms of the distance to the wall that matches the values taken in the vicinity of the wall, on the order of Sc(-1/3) , with the values of order unity in the rest of the viscous layer. The implications of this behavior on the surface-transfer coefficient are analyzed.

Influence of the Convective Surface Transfer Coefficients on the Heat, Air, and Moisture (HAM) Building Performance

Current models to predict heat, air, and moisture (HAM) conditions in buildings assume constant boundary conditions for the temperature and relative humidity of the neighboring air and for the surface heat and moisture transfer coefficients. These assumptions may introduce errors in the predicted HAM conditions. The paper focuses on the influence of the interior surface heat and moisture transfer coefficients, and investigates its effect on the hygrothermal performance. The parameter study showed that the magnitude of the convective surface transfer coefficients has a relatively large influence on the predicted hygrothermal conditions at the surface of a building component and on the heat and vapor exchange with the indoor environment.

Synthesis of Glycidyl Ether of Poly(bisphenol-A 1,1′-ferrocene dicarboxylate) and Its Electrochemical Behavior

A novel ferrocene-based epoxy compound of the glycidyl ether of poly(bisphenol-A 1,1′-ferrocene dicarboxylate) (GEPBFC) was synthesized and the electrochemical behavior of its films and solutions was investigated by cyclic voltammetry (CV). It was found that the GEPBFC films and solutions present different CV behavior in different solvents and the swelling ability of the solvent is the main factor that affects the CV behavior of GEPBFC films. The kinetic parameters of the film-electrode process, including the surface transfer coefficient, the apparent diffusion coefficient and the standard rate constant in LiClO4 ethanol solution, were calculated, and the corresponding electrode-process mechanism was discussed.