Convective Mass Transfer Coefficients Research Articles

Drying Kinetics and Product Quality for Convective Drying of Apples (cvs. Golden Delicious and Granny Smith)

The convective drying of apples from two different cultivars, Golden Delicious and Granny Smith, in a range of temperatures from 30 to 60°C is studied. Some chemical and physical properties were determined fresh after drying: moisture, acidity and sugar content, color, and textural attributes, in order to evaluate the effect of drying and drying temperature on these properties. Furthermore, the drying kinetics were studied in terms of thin layer models and also by means of the Fick’s equation of diffusion, and some mass transfer properties were evaluated, such as effective moisture diffusivity and convective mass transfer coefficient. It was concluded that drying decreased both acidity and sugar content for both apple cultivars. Color was significantly affected by drying, resulting in high values of the total color difference, between 19.43 and 25.04. Drying turned the products less hard and less cohesive. Furthermore, it was found that mass diffusivity increased with temperature following an Arrhenius type function, where De0 was 5.4621 × 10−4 and 1.8401 × 10−4 m2/s, and E was 35.3 and 32.8 kJ/mol, respectively, for cultivars Golden Delicious and Granny Smith.

Formaldehyde emission behavior of building materials: On-site measurements and modeling approach to predict indoor air pollution

The purpose of this paper was to investigate formaldehyde emission behavior of building materials from on-site measurements of air phase concentration at material surface used as input data of a box model to estimate the indoor air pollution of a newly built classroom. The relevance of this approach was explored using CFD modeling. In this box model, the contribution of building materials to indoor air pollution was estimated with two parameters: the convective mass transfer coefficient in the material/air boundary layer and the on-site measurements of gas phase concentration at material surfaces. An experimental method based on an emission test chamber was developed to quantify this convective mass transfer coefficient. The on-site measurement of gas phase concentration at material surface was measured by coupling a home-made sampler to SPME. First results had shown an accurate estimation of indoor formaldehyde concentration in this classroom by using a simple box model.

Investigation of Evaporative Mass Transfer with Turbulent-Forced Convection Air Flow over Roughness Elements

AbstractThe effect of obstruction in the form of creation of additional turbulence caused by roughness elements on evaporation rates and hence on the convective mass transfer coefficient has been investigated under varying conditions of air velocities and water temperatures. The respective ranges of flow Reynolds number and water temperature for this study were 20,000–120,000 and 30–50°C, respectively. The range of relative roughness height was 0.011–0.075. It has been found that there is a significant effect of the presence of roughness elements, and this effect is a strong function of flow Reynolds number and the relative roughness height of its obstructions. An empirical correlation has been developed to relate Sherwood number as a function of Reynolds number and relative roughness height to facilitate prediction of mass transfer and evaporation rate under varying Reynolds number and relative roughness height.

CO2 long-term diffusive leakage into biosphere in geological carbon storage

Prediction of CO2 leakage into biosphere is very important for risk assessment in geological carbon storage projects. Underground CO2 can be transported into biosphere through short term leakage due to fractures of wellbores or cap rocks, which has been extensively investigated, and long term leakage due to diffusion, which has few relevant studies. This paper presents a diffusive model for CO2 gradual leakage into biosphere during a long period after CO2 injection. First, the paper describes a general diffusive model with long term secondary trapping effects for CO2 fluxes from underground into biosphere. Secondly, a simplified one-dimensional model is presented and solved for the CO2 concentrations in groundwater. The results show that the groundwater CO2 concentration will reach the maximum value at about 50th year after CO2 injection and then slowly decrease due to secondary trapping effects. Moreover, the partition coefficient is the dominant parameter for predicting the groundwater CO2 concentration while the convective mass transfer coefficient plays an insignificant role.

Effect of Size on the Convective Heat and Mass Transfer Coefficients during Natural Convection Greenhouse Drying of Khoa-A Heat Desiccated Milk Product

In this research paper, the convective heat and mass transfer coefficients are evaluated for khoa drying under natural convection greenhouse mode for different sizes of khoa samples for a given mass. The khoa pieces of dimensions 0.025 × 0.02 × 0.015 m 3 , 0.0375 × 0.03 × 0.015 m 3 , and 0.075 × 0.06 × 0.015 m3 with total quantity of 100 g are dried in the roof type even span greenhouse with a floor area of 1.2 × 0.8 m2. The khoa has been dried at atmospheric pressure till there is almost no variation in its mass is recorded. The experimental data are used to determine the values of the constants in the Nusselt number expression by simple linear regression analysis and, consequently the convective heat transfer coefficients are evaluated. The mass transfer coefficients have also been evaluated. The convective heat and mass transfer coefficients are observed to decrease with the increase in size of the khoa sample and are found more for the khoa pieces having dimension 0.025 × 0.02 × 0.015 m3. The experimental error in terms of percent uncertainty has also been evaluated.

<b>Experimental evaluation and computational simulation of the dissolution of sodium chloride particles in a brine fluid flow<b>

A mathematical model to predict the dissolution of salt particles suspended in a brine flow is provided. The model consists of a system of three partial differential equations (PDE) based on mass conservation of salt dissolved in the fluid phase, on mass conservation of salt particles in the solid phase and on the overall conservation of energy. A fluid flow experimental unit was built to determine the kinetics of the dissolution of salt particles in a brine flow. Fluid samples free from particled solid material retrieved through the flow line at several predefined points were collected to determine salt dissolution profile. The global convective mass transfer coefficient was evaluated based on the experimental data. Simulations validated the mathematical model and relative deviations between experimental data and simulations were less than 10%.

Intensity of convective mass/heat transfer in a rotary regenerator rotor with the transverse needle-fins

A forced convective mass transfer coefficient was electrochemically measured for a cylindrical bundle of transverse needle-fins ϕ1 × 10.9, applied as the rotor porous matrix of a rotary heat regenerator. The baffle inside the rotor was present. The technique based on the ferricyanide–ferrocyanide redox reaction controlled at the cathode, in the presence of a sodium hydroxide based electrolyte, was used in this experiment. A set of the six neighbouring fins, connected in parallel, was the cathode. The distribution of the mass transfer coefficient according to different static rotor angle position and the mean mass transfer Chilton–Colburn coefficient correlation j M = j M (Re) for rotation numbers, Ro: 0, 0.8, 1.6 and 2.0 were stated in the mean Reynolds number, Re, range 180–985. The comparison was made between the convective heat fluxes of the pin-fins and the sheet rotor, for Ro = 0.

Mathematical modeling of mass transfer in supercritical fluid extraction of oleoresin from red pepper

Supercritical fluid extraction (SFE) from red pepper (Capsicum frutescens) using CO2 was performed in order to evaluate mass transfer. Kinetic extractions were done at 15MPa and 40°C, varying solvent flow rate, particle diameter and extraction bed volume. The highest extraction rates were obtained for high solvent flow rates, low particle diameters and low extraction bed volume. These results can be explained by the high importance of the convective phenomenon under these conditions. A classical model based on the concept of intact and broken cells was applied to experimental SFE curves, and model parameters were obtained. Two modeling strategies were used: (1) simultaneous fitting creating a set of parameters for pairs of duplicates, (2) multiple fitting that adjusts a single value for the solute concentration in unbroken cell for curves with equal particle diameter. These fits allowed estimating the convective mass transfer coefficient for each condition, and the respective values of the experimental Sherwood number. Experimental data was used to calculate dimensionless numbers of Reynolds and Schmidt for each condition. A second series of mathematical modeling was performed using these dimensionless numbers, which allowed proposing new mass transfer correlations. These new equations were based on the existence of forced and free convection, although the importance of the latter was small. The convective coefficients calculated with the correlations were compared to those obtained from the model, and a good coherence was found between both.

Supercritical fluid extraction of spent coffee grounds: Measurement of extraction curves, oil characterization and economic analysis

This work addresses scientific and techno-economic aspects of supercritical fluid extraction (SFE) of spent coffee grounds which are of interest under the biorefinery context. Soxhlet experiments were carried out and SFE curves were measured at 190bar and 40°C/55°C. The extracts were characterized by GC-FID, and the triacylglycerides profiles were almost constant along an extraction curve, and similar to the n-hexane Soxhlet results: linoleic and palmitic acids content in both extracts are 44.5 and 37.5% (wt.), respectively.The oil solubility, intraparticle diffusion, convective mass transfer coefficient, and oil removal flux (this one estimated for infinite dilution) were calculated and discussed in detail in the ranges of operating conditions under analysis. It was concluded that solubility is the chief parameter behind the distinct cumulative extraction curves, and that all experiments are subjected to equivalent mass transfer resistances.An economic analysis involving cost of manufacturing (COM) and net income calculations were performed for distinct operating conditions and unit arrangements. The optima results were obtained for an arrangement of 3 beds of 1m3, extraction time of 2.0h, 300bar, 50°C and 30 kgCO2 kgSCG−1 h−1. Under these conditions production can reach 454tonyear−1, a cost of manufacturing (COM) around 2.4M€, and process net income of 56.6M€. A sensitivity analysis varying the unit capacity, extraction time and precipitation pressure (extract vessel), showed the process economics to remain viable.

Maxwell–Stefan based modelling of ion exchange systems containing common species (Cd2+, Na+) and distinct sorbents (ETS-4, ETS-10)

Cadmium(II) is a toxic hazardous cation, whose presence in the environment causes great concern because of its bioaccumulation in organisms and bioamplification along food chain. Hence, the removal of cadmium compounds from industrial waters and wastewaters is particularly essential, which requires intensive experimental and modelling studies to deal with the problem. In this work, the ion exchange of Cd2+ ions from aqueous solution using microporous titanosilicates (ETS-4 and ETS-10) has been modelled using adapted Maxwell–Stefan equations for the ions transport inside the sorbent particles. The fundamentals of the Maxwell–Stefan equations along with correlations for the convective mass transfer coefficients have been used with advantage to reduce the number of model parameters. In the whole, the model was able to represent successfully the kinetic behaviour of 11 independent and very distinct curves of both studied systems (Cd2+/Na+/ETS-4 and Cd2+/Na+/ETS-10). The predictive capability of the model has been also shown, since several uptake curves were accurately predicted with parameters fitted previously to different sets of experimental data.

Air to air fixed plate enthalpy heat exchanger, performance variation and energy analysis

The thermal performance of a Z shape enthalpy heat exchanger utilising 70 gsm Kraft paper as the heat and moisture transfer surface has been investigated. Effects of different inlet air humidity ratio conditions on the heat exchanger effectiveness and on the energy recovered by the heat exchanger have been the main focus of this investigation. A typical air conditioning cooling coil which incorporates an enthalpy heat exchanger has been modelled for tropical climate. Under test conditions, results have shown that latent effectiveness and the moisture resistance coefficient have strong dependency on the inlet air humidity ratio. Moreover, the latent effectiveness has been found to be strongly dependent on the moisture resistance coefficient rather than the convective mass transfer coefficient. Finally, annual energy analysis for Singapore weather conditions have also shown that energy recovered under variable inlet air conditions is 15% less than that recovered under constant inlet air conditions for the same heat exchanger.

Numerical simulation of sorption/desorption processes in metal-hydride systems for hydrogen storage and purification. Part I: Development of a mathematical model

The pressure drops and the interfacial convective heat and mass transfer coefficients in packed beds saturated with a gaseous mixture containing hydrogen are studied by direct numerical simulation in the Darcy regime flow (Red⩽1) for both random and structured packings of the beds. For conditions with the Knudsen number Kn⩽0.01, the effective thermal conductivity of the random packings was calculated for various filling gases (H2, He, N2, and Ar). Additional correlations of the mathematical model for variable heat and mass transfer in porous hydrogen-absorbing media and based on the approximation of interpenetrating continua were specified from the received results. The developed mathematical model was used in numerical simulations of the heat and mass transfer processes in practical hydrogen storage devices. These results will be presented in the second part of the paper.

CFD SIMULATION OF WATER EVAPORATION IN CLASS-A PAN WITH A TRANSIENT ANALYSIS

A CFD-based methodology is proposed to derive convective mass-transfer coefficients (wind functions) which are required for estimating evaporation of water bodies with the mass-transfer method. The CFD-derived wind functions were very similar to those empirically derived from a Class-A pan evaporimeter. The evaporation rates calculated with the synthetic wind functions were in good agreement with hourly and daily evaporation measurements. The proposed CFD-approach can be generalised and is cost effective, since it has low input data requirements. Besides, it provides the additional capability of modelling the spatial distribution of the evaporation rate over the water surface. Although the application of CFD to water bodies evaporation modelling is still in the making it looks very promising.

Analytical two-dimensional analysis of the transport phenomena occurring during convective drying: Apple slices

The two-dimensional analysis of transport phenomena occurring during drying of a rectangular object is performed employing analytical method. The external flow is predicted numerically and then the convective heat transfer coefficient is determined during convective drying. The convective mass transfer coefficient is obtained through the analogy between the thermal and concentration boundary layers. After calculating average heat and mass transfer coefficients, the heat and mass transfer equations within the object are analytically solved and temperature and moisture distribution is obtained. The present study is contemplated to fill a gap in analytical modeling of two-dimensional heat and mass transfer equations; since most of analytical solutions are one-dimensional, they cannot show the effect of front and rear faces of the moist object on the drying, so the motivation to do this study is to show mentioned effect. Results demonstrated that front and rear faces have significant influences on the drying which should be considered in modeling. It is expected that the model can be applied for other food products and processes involving similar phenomenon. The analytical results are compared to the numerical ones, presenting a reasonable adjustment.

Descrição da secagem de placas cerâmicas por meio de um modelo de difusão

O objetivo deste trabalho foi estudar o transporte de água, através do modelo da difusão líquida, em placas cerâmicas que podem ser usadas como revestimento de alvenaria, bem como apresentar um algoritmo de otimização baseado no método inverso. Para atingir os objetivos o algoritmo apresentado foi acoplado à solução analítica da equação de difusão, possibilitando determinar os parâmetros de processo a partir dos dados experimentais da secagem das placas. A condição de contorno empregada foi a de terceiro tipo e a geometria considerada foi a de um paralelepípedo. Uma vez determinados os parâmetros de processo, a cinética de secagem das placas cerâmicas foi simulada para cada temperatura. Os revestimentos cerâmicos foram moldados com argila vermelha, que também é usada na fabricação de tijolos e telhas. Amostras do produto com teor de umidade inicial em ~ 0,110 (bs) foram colocadas dentro de uma estufa a 50,0, 60,0, 70,0 e 90,0 ºC, e os dados da cinética de secagem foram coletados. A análise dos resultados obtidos, por meio dos gráficos das cinéticas de secagem e dos indicadores estatísticos, permite concluir que o modelo de difusão líquida descreve de forma satisfatória a cinética de secagem do produto. Os valores obtidos para o coeficiente de transferência convectivo de massa e para a difusividade efetiva da água foram consistentes com valores disponíveis.

Thermal modeling of the forced convection Sandwich Greenhouse drying system for rubber sheets

In this paper, a novel “Sandwich Greenhouse” for rubber sheet drying is proposed. Using solar energy as the only heat source instead of traditional smoke house that requires firewood, it eliminates shortcomings such as skilled labor monitoring requirement, possible fire hazard, and darken-color rubber sheets due to soot particle contamination. Our greenhouse is specially designed to retain solar energy within, while minimizing the heat loss to the outside environment. The mathematical models are developed to predict the convection mass transfer coefficient and to study the thermal behavior during the drying of rubber sheets under our proposed greenhouse design. Validated with experimental observations, the models show good agreement with the actual experimental data. The experiment demonstrates an effectiveness of our proposed Sandwich Greenhouse, as the temperature of the rubber sheet is 15°C and 5°C higher than the ambient temperature during the daytime and nighttime, respectively. As a result, the moisture content of the rubber sheets can decrease from 36.4% to 2.8% in fewer than 2days.

Modelling ion exchange kinetics in zeolyte-type materials using Maxwell-Stefan approach

In this essay, the Maxwell-Stefan (MS) formalism was adopted to model the removal of cadmium(II) and mercury(II) ions from aqueous solutions using microporous titanosilicate ETS-4. The embodied transport mechanism is surface diffusion, since the small pore diameters of such zeolite-type materials imply that counter ions never escape from the force field of the matrix co-ions, mainly owing to the strong and long range electrostatic interactions. The parameters of the global model are the MS diffusivities of ion–ion and ion–solid pairs, and a convective mass transfer coefficient. The average absolute relative deviations (AARD) achieved for Cd2+/Na+/ETS-4 and Hg2+/Na+/ETS-4 systems were only 3.47 and 7.34%, respectively. The model calculates concentration profiles and their evolution along time under transient regime, being able to represent the initial steep branches of removal curves and subsequent transition to equilibrium, where kinetic curves are frequently very difficult to fit. The well-known and frequently used pseudo-first and pseudo-second-order equations were also chosen for comparison, and provided large deviations: AARD(Cd2+) = 48.9% and AARD(Hg2+) = 26.6 % (first order), and AARD(Cd2+) = 29.0% and AARD(Hg2+) = 54.6% (second order).

A rigorous analysis of simultaneous heat and mass transfer in the pasta drying process

This study presents a two dimensional analysis of coupled heat and mass transfer during the process of pasta drying. Velocity and temperature distributions of air flowing around the pasta are predicted in steady state condition. Using these profiles and the similarity between heat and mass boundary layers, local convective heat and mass transfer coefficients were determined on different points of pasta surface. By employing these values, the solution of coupled heat and mass transfer equations within the pasta object in unsteady state condition was obtained. Furthermore the effects of operating conditions such as velocity, temperature and relative humidity of air flow on drying rate of pasta were studied. Sensitivity analysis results show that the effects of air temperature and relative humidity on the rate of drying are more important than the effect of air velocity. Finally, the results obtained from this analysis were compared with the experimental data reported in the literatures and a good agreement was observed while, no adjustable parameter is used in the presented model.

Osmotic dehydration and convective drying of coconut slices: Experimental determination and description using one-dimensional diffusion model

Mass migrations in coconut slices during osmotic dehydration and drying are described using a diffusion model with boundary condition of the third kind. The osmotic dehydration experiment was performed at 35°Brix (water and sucrose) and 40°C. The convective drying experiments were performed at 50, 60 and 70°C. The one-dimensional solution of the diffusion equation for an infinite slab was coupled with an optimizer to determine the effective mass diffusivities D and convective mass transfer coefficients h of the five processes studied. The analyses of the obtained results indicate that there is a good agreement between each experimental dataset and the corresponding simulation using D and h determined by optimization.

Correction: A Novel Method for Measuring the Diffusion, Partition and Convective Mass Transfer Coefficients of Formaldehyde and VOC in Building Materials

Correction: A Novel Method for Measuring the Diffusion, Partition and Convective Mass Transfer Coefficients of Formaldehyde and VOC in Building Materials