SIMULTANEOUS HEAT AND MOISTURE TRANSFER IN CHEDDAR CHEESE DURING COOLING. II. MRI TEMPERATURE MAPPING

ABSTRACT

Non-destructive Magnetic Resonance Imaging (MRI) techniques were developed to monitor simultaneous heat and moisture transfer in cheese cooling. The FLASH imaging sequence was used to achieve fast data acquisition for temperature images. MRI results of temperature distributions in cheese blocks during cooing are very closed to those from numerical simulation.     

SIMULTANEOUS HEAT AND MOISTURE TRANSFER IN CHEDDAR CHEESE DURING COOLING I. NUMERICAL SIMULATION

ABSTRACT

Numerical simulation method was developed and used to predict temperature and moisture distribution in large Cheddar cheese block during cooling based on a simultaneous heat and mass transfer model. The predicted results agreed well with the experimental results. The results suggest that cooling for 3 to 4 days would be sufficient to achieve fairly uniform distribution of temperature and moisture in a 640 lb Cheddar cheese block.     

SIMULTANEOUS HEAT AND MOISTURE TRANSFER IN CHEDDAR CHEESE DURING COOLING I. NUMERICAL SIMULATION

Numerical simulation method was developed and used to predict temperature and moisture distribution in large Cheddar cheese block during cooling based on a simultaneous heat and mass transfer model. The predicted results agreed well with the experimental results. The results suggest that cooling for 3 to 4 days would be sufficient to achieve fairly uniform distribution of temperature and moisture in a 640 lb Cheddar cheese block.     

SIMULATION OF THE HEAT AND MOISTURE TRANSFER PROCESS DURING DRYING IN DEEP GRAIN BEDS

Grain drying is a simultaneous heat and moisture transfer problem. The modelling of such a problem is of significance in understanding and controlling the drying process. In the present study, a mathematical model for coupled heat and moisture transfer problem is presented. The model consists of four partial differential equations for mass balance, heat balance, heat transfer and drying rate. A simple finite difference method is used to solve the equations. The method shows good flexibility in choosing time and space steps which enable the simulation of long term grain drying/cooling processes. A deep barley bed is used as an example of grain beds in the current simulation. The results are verified against experimental data taken from literature. The analysis of the effects of operating conditions on the temperature and moisture content within the bed is also carried out     

SIMULATION OF THE HEAT AND MOISTURE TRANSFER PROCESS DURING DRYING IN DEEP GRAIN BEDS

ABSTRACT

Grain drying is a simultaneous heat and moisture transfer problem. The modelling of such a problem is of significance in understanding and controlling the drying process. In the present study, a mathematical model for coupled heat and moisture transfer problem is presented. The model consists of four partial differential equations for mass balance, heat balance, heat transfer and drying rate. A simple finite difference method is used to solve the equations. The method shows good flexibility in choosing time and space steps which enable the simulation of long term grain drying/cooling processes. A deep barley bed is used as an example of grain beds in the current simulation. The results are verified against experimental data taken from literature. The analysis of the effects of operating conditions on the temperature and moisture content within the bed is also carried out     

HEAT AND MOISTURE TRANSFER IN BAKING OF POTATO SLABS

ABSTRACT

Heal and moss transfer in a slab of potato potato during baking were explained using a multiphase porous media model that includes internal evaporation and capillary, diffusive, and pressure driven transport of water and vapor. The model also includes the effect of the formation of a surface “skin” during baking. Model predictions were validated using experimental data. Most of the temperature drop in the material occurs in a relatively thin and dry surface region, while much of the interior stays at somewhat uniform temperature. Moisture content inside the material stays uniform except near the surface region where it drops to very low values. Evaporation occurs over a significant region, as opposed to at a sharp front. Baking time reduces significantly with thickness, however at d slower rate compared to conduction-only heating (no moisture transport). In addition, the surface temperature also reaches a higher value for a thinner material, which may help develop the often     

HEAT AND MOISTURE TRANSFER IN BAKING OF POTATO SLABS

Heal and moss transfer in a slab of potato potato during baking were explained using a multiphase porous media model that includes internal evaporation and capillary, diffusive, and pressure driven transport of water and vapor. The model also includes the effect of the formation of a surface “skin” during baking. Model predictions were validated using experimental data. Most of the temperature drop in the material occurs in a relatively thin and dry surface region, while much of the interior stays at somewhat uniform temperature. Moisture content inside the material stays uniform except near the surface region where it drops to very low values. Evaporation occurs over a significant region, as opposed to at a sharp front. Baking time reduces significantly with thickness, however at d slower rate compared to conduction-only heating (no moisture transport). In addition, the surface temperature also reaches a higher value for a thinner material, which may help develop the often     

TEMPERATURE,DISTRIBUTIONS AND HEAT TRANSFER DURING THE DRYING OF LUMBER

ABSTRACT

This paper describes techniques that have been developed for accurately measuring the surface temperature of drying lumber using a radiation thermometer, and interior temperatures at various depths using 30 gauge thermocouples. Methods for calculating heat transfer coefficients during the drying of lumber are also described. Experimental results, showing surface and interior temperatures, and moisture content, as functions of drying time are presented.     

SIMULTANEOUS HEAT AND MASS TRANSFER IN LAMINAR GAS STREAMS IN WETTED-WALL COLUMNS

In the previous papers (Hikita and Ishimi,1976b; 1978, Hikita et al., 1979), the results of theoretical analysis of the mass transfer rate in laminar gas streams in wetted-wall columns have been reported. The similarity of the forms of the basic differential equations for mass and heat transfer makes it possible to apply the results of the mass transfer analysis to the heat transfer analysis. The present study was undertaken to confirm the above analogy between mass and heat transfer experimentally, and to investigate the effect of the gas and liquid flow rates on the heat transfer rate in laminar gas streams in wetted-wall columns.     

SIMULTANEOUS HEAT AND MASS TRANSFER IN LAMINAR GAS STREAMS IN WETTED-WALL COLUMNS

In the previous papers (Hikita and Ishimi,1976b; 1978, Hikita et al., 1979), the results of theoretical analysis of the mass transfer rate in laminar gas streams in wetted-wall columns have been reported. The similarity of the forms of the basic differential equations for mass and heat transfer makes it possible to apply the results of the mass transfer analysis to the heat transfer analysis. The present study was undertaken to confirm the above analogy between mass and heat transfer experimentally, and to investigate the effect of the gas and liquid flow rates on the heat transfer rate in laminar gas streams in wetted-wall columns.     

ANALYSIS OF SIMULTANEOUS RADIATIVE AND CONDUCTIVE HEAT TRANSFER IN FLUIDIZED BEDS

In the bubbling regime of operation for fluidized beds, the major mechanism for heat transfer is transient conduction to periodic packets of densely packed particles at the heat transfer surface. The well known Mickley and Fairbanks model, with various subsequently proposed modifications, adequately describes this transient conduction mechanism. However, no adequate theory exists for heat transfer in high-temperature fluidized beds where radiative contribution becomes significant.

Analysis of the radiative contribution is complicated by the nonlinear interaction of radiation with conduction/convection. This paper describes a differential formulation of the combined radiative/ conductive heat transfer process. The discrete flux method used by Churchill et al. for radiative transport in heterogeneous media is applied here to the problem of transient heat transfer to packets in fluidized beds. Packets are modeled as radiatively participating media with absorption, scattering, and emission of radiation. Simultaneous solution of the governing differential equations for temperature and forward and backward radiation fluxes permits calculation of instantaneous heat flux at the heat-transfer surface. Radiative transfer during bubble contact is added as a time-weighted contribution.

Using experimental data on radiative cross sections (from packed media experiments) and experimental data on packet residence times (from fluidized bed experiments), the combined conductive/radiative heat transfer to packets was obtained for examples of fluidized beds at different fluidizing velocities and wall temperatures. The analytical results indicate that the relative importance of radiation is affected by particle size, average packet residence time, and the radiative attenuation cross sections. For operating conditions representative of fluidized bed combustion, the model estimates a 10 to 20 percent contribution by radiation to the total heat transfer. Comparison to limited experimental data from the literature shows reasonable agreement.     

BOUND MOISTURE REMOVAL: HEAT AND MASS TRANSFER IN CONTACT DRYING

Coupled heat and mass transfer in short-term contact of the moist material and the heating surface (the physical model of drying with agitation) is examined. Technological characteristics of the drying process: heating rate and drying rate, heat transfer coefficient, etc. have been determined based on solutions of the diffusion and diffusion-filtration heat and mass transfer. The usage of non-field method of determination of mass and heat fluxes on the phase interface allows calculation of the drying equipment efficiency without preliminary determination of the fields of required quantities. The results may be used for estimation of the influence of drying conditions and material properties on the moisture removal process.     

ANALYSIS OF SIMULTANEOUS RADIATIVE AND CONDUCTIVE HEAT TRANSFER IN FLUIDIZED BEDS

In the bubbling regime of operation for fluidized beds, the major mechanism for heat transfer is transient conduction to periodic packets of densely packed particles at the heat transfer surface. The well known Mickley and Fairbanks model, with various subsequently proposed modifications, adequately describes this transient conduction mechanism. However, no adequate theory exists for heat transfer in high-temperature fluidized beds where radiative contribution becomes significant.

Analysis of the radiative contribution is complicated by the nonlinear interaction of radiation with conduction/convection. This paper describes a differential formulation of the combined radiative/ conductive heat transfer process. The discrete flux method used by Churchill et al. for radiative transport in heterogeneous media is applied here to the problem of transient heat transfer to packets in fluidized beds. Packets are modeled as radiatively participating media with absorption, scattering, and emission of radiation. Simultaneous solution of the governing differential equations for temperature and forward and backward radiation fluxes permits calculation of instantaneous heat flux at the heat-transfer surface. Radiative transfer during bubble contact is added as a time-weighted contribution.

Using experimental data on radiative cross sections (from packed media experiments) and experimental data on packet residence times (from fluidized bed experiments), the combined conductive/radiative heat transfer to packets was obtained for examples of fluidized beds at different fluidizing velocities and wall temperatures. The analytical results indicate that the relative importance of radiation is affected by particle size, average packet residence time, and the radiative attenuation cross sections. For operating conditions representative of fluidized bed combustion, the model estimates a 10 to 20 percent contribution by radiation to the total heat transfer. Comparison to limited experimental data from the literature shows reasonable agreement.     

BOUND MOISTURE REMOVAL: HEAT AND MASS TRANSFER IN CONTACT DRYING

ABSTRACT

Coupled heat and mass transfer in short-term contact of the moist material and the heating surface (the physical model of drying with agitation) is examined. Technological characteristics of the drying process: heating rate and drying rate, heat transfer coefficient, etc. have been determined based on solutions of the diffusion and diffusion-filtration heat and mass transfer. The usage of non-field method of determination of mass and heat fluxes on the phase interface allows calculation of the drying equipment efficiency without preliminary determination of the fields of required quantities. The results may be used for estimation of the influence of drying conditions and material properties on the moisture removal process.     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

ABSTRACT

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

HEAT AND MOISTURE TRANSFER IN CAPILLARY-POROUS BODIES SOME EXPERIMENTAL METHODS OF INVESTIGATION

This paper presents a synthesis of our scientific activity in the area of heat and moisture transfers in capillary-porous bodies. The materials which have been under investigation are essentially in the field of Civil Engineering but our ways of experimentation may be generalized.

Transfers may be characterized by two independent variables : temperature and water content. We have been concerned by the achievement of measurement technics of these quantities and of perturbation technics of materials at equilibrium.

When comparing a fitting model and the temperature or water content response to such perturbations, it is possible to infer the values of parameters which are significant. Then, the experimental results must be compared with results obtained by other authors or by means of different methods.

In the future, the assumption of an unstrained porous skeleton will not be kept any longer and the coupling of transfers with mechanical phenomena will be considered.     

- VISCOELASTIC STRAIN-STRESS AND HEAT/MOISTURE TRANSFER -

ABSTRACT

The theoretical analysis was developed to predict the deformation characteristic of formed clay during a drying process in a ceramic production. The three dimensional strain-stress distribution as well as heat and moisture transfer in a slab shape of clay were simultaneously analyzed by a finite element method. Linear viscoelasticity was assumed for the strain-stress analysis to account the effect of creep. The calculated result agreed well with an experimental result performed for a slab clay heated by hot air flow during the preheating and constant drying rate periods. A large tensional stress, which may generate a crack, was observed initially around the surface area. It was also found that the time behavior of the volume change of the formed clay is significantly influenced on the drying conditions and/or the drying rate.     

- VISCOELASTIC STRAIN-STRESS AND HEAT/MOISTURE TRANSFER -

The theoretical analysis was developed to predict the deformation characteristic of formed clay during a drying process in a ceramic production. The three dimensional strain-stress distribution as well as heat and moisture transfer in a slab shape of clay were simultaneously analyzed by a finite element method. Linear viscoelasticity was assumed for the strain-stress analysis to account the effect of creep. The calculated result agreed well with an experimental result performed for a slab clay heated by hot air flow during the preheating and constant drying rate periods. A large tensional stress, which may generate a crack, was observed initially around the surface area. It was also found that the time behavior of the volume change of the formed clay is significantly influenced on the drying conditions and/or the drying rate.     

CONVECTTVE HEAT AND MASS TRANSFER IN MOISTURE DESORPTION OF OAK WOOD BY INTRODUCING CHARACTERISTIC TRANSFER COEFFICIENTS

This paper presents the effect of forced convective heat and mass transfer coefficients on the predictive ability of a dynamic isothermal set of coupled partial differential equations. The analysis showed that the two coefficient two-dimensional model, resulted in a very good prediction of the experimental data.

The concept of experiment involves: six temperatures, six relative air humidities and three values of circulation velocity. Dimensions of oak (Quercus petraea) wood samples were 250×80×25mm. The experimental installation, designed and constructed for this study, enables the drying process of relatively small samples to be similar to the drying process of real timber.