- Email: [email protected]
Cooling of Rounded Seeds over Moving Belts
0960–3085/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part C, March 2004 Food and Bioproducts Processing, 82(C1): 73–77
www.ingentaselect.com=titles=09603085.htm
SHORTER COMMUNICATION
COOLING OF ROUNDED SEEDS OVER MOVING BELTS D. VALENTINI1, A. A. BARRESI1* and G. FISSORE2 1
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Torino, Italy 2 GIEMME, Bra, Italy
C
ooling of a single layer of rounded particles with air is investigated; this operation is of interest in many industrial processes, particularly in the food industry. Simulations with commercial CFD software have been carried out to evaluate the heat transfer phenomena and calculate the cooling time required to obtain different final temperatures in the particles. The influence of different geometrical configurations, cooling air velocity and thermal conductivity of seeds and nuts has been investigated, with particular reference to conditions adopted for quick cooling of hazelnuts. Keywords: spherical particles; cooling; heat transfer; rounded seeds; hazelnuts.
INTRODUCTION
conditions adopted for the hazelnuts process have been considered first, but the influence of physical properties and operating parameters has been investigated in order to extend the validity of the results to other seeds and nuts.
In many industrial processes, it is necessary to cool round particles as a single-layer over grid belt conveyors. To design the equipment properly it is necessary to know the heat transfer coefficient between the cooling air and the particles or the time necessary for cooling particles to the temperature desired. Correlations available in literature are limited to a single sphere and thus do not consider the influence of the interaction between the fluid flow and the single layer of particles over the heat transfer coefficient; as fluid dynamics and hence heat transfer is strongly influenced by the geometrical disposition of the spheres in the layer, these correlations are not suitable. An example of this operation in the food industry refers to cooling hazelnuts coming out of the roasting oven, to avoid alterations in organoleptic properties; in fact, exposure to high temperatures for long times can decrease the quality of the hazelnuts (Lopez and Pique, 1997). The equipment considered for the cooling process is composed of a tunnel in which a single layer of hazelnuts is carried by a thin metallic grid belt conveyor. Hot hazelnuts are cooled by a cross flow of cold air emitted from a distribution system located over the conveyor. The hazelnuts must be cooled down to room temperature in a short time, in order to avoid a burnt taste. As no experimental data are available, a CFD approach seems the more powerful tool to address the problem, considering the geometrical configuration and its lack of symmetry and the transient nature of the process. The
THE MODEL A commercial computational fluid dynamics (CFD) code (Fluent1 6.0) has been employed using the k–e turbulence closure model and the enhanced wall treatment option, which ensures more realistic results for problems involving simultaneously fluid dynamics and heat exchange. In fact, no assumption is made on the wall functions, an approach that is valid only at high Reynolds numbers, but the velocity profile is solved up to the wall. As in our simulations the mesh is fine enough to be able to resolve the laminar sub-layer (Fluent 6.0 User’s Guide, paras 10.8.1–3), the enhanced wall treatment corresponds to the traditional two-layer zonal model and is valid also at low Reynolds numbers, where the turbulence is not completely developed. In this approach, the whole domain is subdivided into a viscosity-affected region, in which the one-equation model of Wolfstein is employed, and a fully turbulent region, solved with the turbulence model adopted. This approach is particularly relevant in our simulations because near-wall effects (flow through small gaps) and recirculation zones are simultaneously present, and thermal exchange is taken into account; this ensures both accuracy and reliability of the code. The air distribution system produces an homogeneous flow because the air passes through a grid, but inlet turbulent properties are not specified. The turbulence intensity (variable between 1 and 5%, depending on the cooling air velocity value) and a length scale have been specified.
*Correspondence to: Dr. A. A. Barresi, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail: [email protected]
73
74
VALENTINI et al.
Even if the flow of hazelnuts can follow many different geometrical configurations and many irregularities can be present in the system, it has been decided to model them as spheres and to consider the centres of the spherical particles disposed in the vertices of a triangular scheme to take into account different configurations in a simple way. Different distances between the surfaces of the particles have been considered, with d=D varying from 0.038 to 0.38. The geometry considered has been obtained through successive simplifications (Figure 1), considering symmetry planes present in the system as shown in the figure. The last configuration can give information about the complete system, but the number of cells required by the simulations is reduced to 1=36 with respect to the complete geometry. A mesh with variable cell size was adopted, as shown in Figure 2. To minimize the cell number (and consequently the computational time required), an unstructured hybrid mesh was chosen, using tetrahedral cells to mesh the solid particle and the region near the solid wall where complex transport phenomena occur, and hexahedral cells of greater dimension in the region far from the solid particle. The system has been modelled introducing some simplifications: the effect of the metallic grid under the hazelnuts layer was neglected because the thin metallic wires are believed
to cause minimal flow disturbance, and it is downstream of the nuts; the velocity of the belt conveyor was considered negligible with respect to the vertical velocity component of the air flow; as a consequence the cooling air flow was considered perpendicular to the particles layer; a uniform velocity profile over the layer of particles was assumed; constant physical properties of air (density, viscosity, thermal conductivity and specific heat) were assumed; the solid was considered homogeneous.
For the simulations concerning the roasted hazelnuts, a diameter of 13 mm and a density of 1060 kg m3 were assumed. Heat capacity was measured at two different temperatures (1046.5 J kg1 K1 at 293 K and 2093 J kg1 K1 at 443 K) and taken as a function of temperature with piecewise linear law obtained interpolating the two values; an experimental value of 1475 J kg1 K1, measured in the range 5–40 C was reported by Demir et al. (2003). Typically the moisture content in the roasted hazelnuts ranges between 1 and 3%; the relatively large variantion in the apparent specific heat can be due to water capillary=evaporative effects. The thermal conductivity is influenced by different parameters such as temperature, water and oil content,
Figure 1. Simplification of the geometry considered using symmetry planes.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
COOLING OF ROUNDED SEEDS
75
Figure 2. Complete mesh and particular of the central zone.
composition: an experimental value of 0.157 W m1 K1 has been reported (Demir et al., 2003). Two different values of 0.230 and 0.115 W m1 K1 were considered in this work, to cover the possible range. Other data available in literature, for rice and corn, are shown in Table 1. Different air velocities in the range from 0.09 to 1 m s1 were considered; the dependence on the particle size was also investigated. In order to favour numerical convergence, the initial conditions of the flow field (velocity, pressure and temperature profile) to be used for the transient simulation were obtained from a preliminary steady-state simulation in which the solid particle was modelled at the constant temperature of 413 K. The unsteady simulation provided information about temperature profile in the solid particle, heat flux and heat transfer coefficient on the solid surface, velocity and temperature profile of the cooling air, and the time necessary to obtain the desired temperature in the solid particle.
The upper part of the particle was always cooler than the lower one, but increasing the cooling air velocity the symmetry of the temperature profile in the spherical particle
RESULTS AND DISCUSSION The influence of different parameters (cooling air velocity, particle diameter and distance between particles) was investigated and will be discussed in the following paragraphs. The influence of solid thermal conductivity was also considered.
Figure 3. Effect of different cooling air velocity and thermal conductivity values over the temperature profiles inside the particles: (a) v ¼ 0.09 m s1, k ¼ 0.23 W m1 K1; (b) v ¼ 1 m s1, k ¼ 0.23 W m1 K1; (c) v ¼ 1 m s1, k ¼ 0.115 W m1 K1.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
76
VALENTINI et al. Table 1. Thermal conductivity of various seeds. Thermal conductivity (W m1 K1) Material
Minimum value
Maximum value
Rice
0.0860
0.1580
Corn ‘Stauffer’ Corn ‘Bo Jac’ Corn ‘Beck 65’ Intermediate moisture foods (up to 50%)
0.1628 0.1632 0.1100 0.0900
0.1850 0.1860 0.2200 0.4100
between the upper and lower part also increased, and for high values of cooling air velocity the warmest part of the particle was in the centre (see Figure 3). Higher values of thermal conductivity smoothed temperature gradients in the particle. Even if it is possible to evaluate from the results of the simulations the heat flux from the nuts, considering the transient nature of the process and the nonuniformity and lack of symmetry in the temperature profile, it is difficult to obtain general correlations for heat transfer useful for equipment design. Therefore in the following the time required for cooling down the nuts from 140 C to the specified temperature will be shown. The distance between particles determined the open section for the air, influencing the velocity through the bed and thus the temperature profiles, similarly to the air flow rate. Increasing cooling air velocity significantly reduced cooling time (that is the time required to bring all the solid temperature below a fixed value) and the reduction was particularly relevant, increasing the value of the velocity from 0.09 up to 0.5 m s1 as shown in Figures 4–6. The cooling time was also reduced when the solid conductivity is higher (see Figure 5). A set of simulations was carried out to investigate the influence of the solid particle diameter over the cooling time, maintaining constant the ratio d=D, that is the open section (see Figure 6). A thermal conductivity value of 0.17 W m1 K1, corresponding to a mean value for hazelnuts, and in the range of values for other seeds, as reported in Table 1, was considered.
Source Sreenarayanan and Chattopadhyay (1986) Chang (1986) Chang (1986) Fortes and Okos (1980) Sweat (1987)
Figure 5. Influence of the thermal conductivity value (k) on cooling time for different final temperatures (shown in the label); D ¼ 0.13 mm, d ¼ 0.5.
Figure 6. Influence of solid particles diameter (D) on cooling time (to 293 K); k ¼ 0.17 W m1 K1, d=D ¼ 0.038.
CONCLUSIONS Simulation results have shown that highly non-symmetric temperature profiles can develop during cooling of solid particles in cross flow, depending on air velocity and solid
thermal conductivity. Thus for design purposes it is better to refer to cooling time required to lower the temperature in all the particles below a fixed value. High values of cooling air velocity are required to obtain a fast cooling, but this requires a larger amount of cold air; in these conditions the temperature profiles become more symmetric. In addition, it is important to take care of the distribution system of the spherical particles over the belt conveyor, to obtain a regular distribution with the maximum density of particles, to avoid the phenomenon of local channelling. NOMENCLATURE
Figure 4. Influence of distance between particles (d) on cooling time (to 293 K); D ¼ 13 mm, k ¼ 0.23 W m1 K1.
D d k v
diameter of solid particles, mm minimum distance between particle surfaces, mm thermal conductivity of solid particles, W m1 K1 cooling air velocity, m s1
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
COOLING OF ROUNDED SEEDS ADDITIONAL INFROMATION The complete series of data and related charts are available on request. REFERENCES Chang, C.S., 1986, Thermal conductivity of wheat, corn, and grain sorghum as affected by bulk density and moisture content, Trans ASAE, 29(5): 1447–1450. Demir, A.D., Baucour, P., Cronin, K. and Abodayeh, K., 2003, Analysis of temperature variability during the thermal processing of hazelnuts, Innov Food Sci Emerg Technol, 4: 69–84. Fortes, M., and Okos, M.R., 1980, Changes in physical properties of corn during drying, Trans ASAE, 23(4): 1004–1008.
77
Lopez, A. and Pique, M.T., 1997, Influence of the drying conditions on the hazelnut quality: III. Browning, Drying Technol, 15(3–4): 989–1002. Sreenarayanan, V.V. and Chattopadhyay, P.K., 1986, Thermal conductivity and diffusivity of rice bran, J Agric Eng Res, 34: 115–121. Sweat, V.E., 1987, Thermal properties of low and intermediate moisture foods, ASHRAE J, February, 44 [quoted from Rahman, S., 1995, Food Properties Handbook (CRC Press, Boca Raton, USA)]
ACKNOWLEDGEMENTS The authors wish to thank Dr Marco Vanni for valuable suggestions. The manuscript was received 20 June 2003 and accepted for publication after revision 7 January 2004.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
www.ingentaselect.com=titles=09603085.htm
SHORTER COMMUNICATION
COOLING OF ROUNDED SEEDS OVER MOVING BELTS D. VALENTINI1, A. A. BARRESI1* and G. FISSORE2 1
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Torino, Italy 2 GIEMME, Bra, Italy
C
ooling of a single layer of rounded particles with air is investigated; this operation is of interest in many industrial processes, particularly in the food industry. Simulations with commercial CFD software have been carried out to evaluate the heat transfer phenomena and calculate the cooling time required to obtain different final temperatures in the particles. The influence of different geometrical configurations, cooling air velocity and thermal conductivity of seeds and nuts has been investigated, with particular reference to conditions adopted for quick cooling of hazelnuts. Keywords: spherical particles; cooling; heat transfer; rounded seeds; hazelnuts.
INTRODUCTION
conditions adopted for the hazelnuts process have been considered first, but the influence of physical properties and operating parameters has been investigated in order to extend the validity of the results to other seeds and nuts.
In many industrial processes, it is necessary to cool round particles as a single-layer over grid belt conveyors. To design the equipment properly it is necessary to know the heat transfer coefficient between the cooling air and the particles or the time necessary for cooling particles to the temperature desired. Correlations available in literature are limited to a single sphere and thus do not consider the influence of the interaction between the fluid flow and the single layer of particles over the heat transfer coefficient; as fluid dynamics and hence heat transfer is strongly influenced by the geometrical disposition of the spheres in the layer, these correlations are not suitable. An example of this operation in the food industry refers to cooling hazelnuts coming out of the roasting oven, to avoid alterations in organoleptic properties; in fact, exposure to high temperatures for long times can decrease the quality of the hazelnuts (Lopez and Pique, 1997). The equipment considered for the cooling process is composed of a tunnel in which a single layer of hazelnuts is carried by a thin metallic grid belt conveyor. Hot hazelnuts are cooled by a cross flow of cold air emitted from a distribution system located over the conveyor. The hazelnuts must be cooled down to room temperature in a short time, in order to avoid a burnt taste. As no experimental data are available, a CFD approach seems the more powerful tool to address the problem, considering the geometrical configuration and its lack of symmetry and the transient nature of the process. The
THE MODEL A commercial computational fluid dynamics (CFD) code (Fluent1 6.0) has been employed using the k–e turbulence closure model and the enhanced wall treatment option, which ensures more realistic results for problems involving simultaneously fluid dynamics and heat exchange. In fact, no assumption is made on the wall functions, an approach that is valid only at high Reynolds numbers, but the velocity profile is solved up to the wall. As in our simulations the mesh is fine enough to be able to resolve the laminar sub-layer (Fluent 6.0 User’s Guide, paras 10.8.1–3), the enhanced wall treatment corresponds to the traditional two-layer zonal model and is valid also at low Reynolds numbers, where the turbulence is not completely developed. In this approach, the whole domain is subdivided into a viscosity-affected region, in which the one-equation model of Wolfstein is employed, and a fully turbulent region, solved with the turbulence model adopted. This approach is particularly relevant in our simulations because near-wall effects (flow through small gaps) and recirculation zones are simultaneously present, and thermal exchange is taken into account; this ensures both accuracy and reliability of the code. The air distribution system produces an homogeneous flow because the air passes through a grid, but inlet turbulent properties are not specified. The turbulence intensity (variable between 1 and 5%, depending on the cooling air velocity value) and a length scale have been specified.
*Correspondence to: Dr. A. A. Barresi, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail: [email protected]
73
74
VALENTINI et al.
Even if the flow of hazelnuts can follow many different geometrical configurations and many irregularities can be present in the system, it has been decided to model them as spheres and to consider the centres of the spherical particles disposed in the vertices of a triangular scheme to take into account different configurations in a simple way. Different distances between the surfaces of the particles have been considered, with d=D varying from 0.038 to 0.38. The geometry considered has been obtained through successive simplifications (Figure 1), considering symmetry planes present in the system as shown in the figure. The last configuration can give information about the complete system, but the number of cells required by the simulations is reduced to 1=36 with respect to the complete geometry. A mesh with variable cell size was adopted, as shown in Figure 2. To minimize the cell number (and consequently the computational time required), an unstructured hybrid mesh was chosen, using tetrahedral cells to mesh the solid particle and the region near the solid wall where complex transport phenomena occur, and hexahedral cells of greater dimension in the region far from the solid particle. The system has been modelled introducing some simplifications: the effect of the metallic grid under the hazelnuts layer was neglected because the thin metallic wires are believed
to cause minimal flow disturbance, and it is downstream of the nuts; the velocity of the belt conveyor was considered negligible with respect to the vertical velocity component of the air flow; as a consequence the cooling air flow was considered perpendicular to the particles layer; a uniform velocity profile over the layer of particles was assumed; constant physical properties of air (density, viscosity, thermal conductivity and specific heat) were assumed; the solid was considered homogeneous.
For the simulations concerning the roasted hazelnuts, a diameter of 13 mm and a density of 1060 kg m3 were assumed. Heat capacity was measured at two different temperatures (1046.5 J kg1 K1 at 293 K and 2093 J kg1 K1 at 443 K) and taken as a function of temperature with piecewise linear law obtained interpolating the two values; an experimental value of 1475 J kg1 K1, measured in the range 5–40 C was reported by Demir et al. (2003). Typically the moisture content in the roasted hazelnuts ranges between 1 and 3%; the relatively large variantion in the apparent specific heat can be due to water capillary=evaporative effects. The thermal conductivity is influenced by different parameters such as temperature, water and oil content,
Figure 1. Simplification of the geometry considered using symmetry planes.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
COOLING OF ROUNDED SEEDS
75
Figure 2. Complete mesh and particular of the central zone.
composition: an experimental value of 0.157 W m1 K1 has been reported (Demir et al., 2003). Two different values of 0.230 and 0.115 W m1 K1 were considered in this work, to cover the possible range. Other data available in literature, for rice and corn, are shown in Table 1. Different air velocities in the range from 0.09 to 1 m s1 were considered; the dependence on the particle size was also investigated. In order to favour numerical convergence, the initial conditions of the flow field (velocity, pressure and temperature profile) to be used for the transient simulation were obtained from a preliminary steady-state simulation in which the solid particle was modelled at the constant temperature of 413 K. The unsteady simulation provided information about temperature profile in the solid particle, heat flux and heat transfer coefficient on the solid surface, velocity and temperature profile of the cooling air, and the time necessary to obtain the desired temperature in the solid particle.
The upper part of the particle was always cooler than the lower one, but increasing the cooling air velocity the symmetry of the temperature profile in the spherical particle
RESULTS AND DISCUSSION The influence of different parameters (cooling air velocity, particle diameter and distance between particles) was investigated and will be discussed in the following paragraphs. The influence of solid thermal conductivity was also considered.
Figure 3. Effect of different cooling air velocity and thermal conductivity values over the temperature profiles inside the particles: (a) v ¼ 0.09 m s1, k ¼ 0.23 W m1 K1; (b) v ¼ 1 m s1, k ¼ 0.23 W m1 K1; (c) v ¼ 1 m s1, k ¼ 0.115 W m1 K1.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
76
VALENTINI et al. Table 1. Thermal conductivity of various seeds. Thermal conductivity (W m1 K1) Material
Minimum value
Maximum value
Rice
0.0860
0.1580
Corn ‘Stauffer’ Corn ‘Bo Jac’ Corn ‘Beck 65’ Intermediate moisture foods (up to 50%)
0.1628 0.1632 0.1100 0.0900
0.1850 0.1860 0.2200 0.4100
between the upper and lower part also increased, and for high values of cooling air velocity the warmest part of the particle was in the centre (see Figure 3). Higher values of thermal conductivity smoothed temperature gradients in the particle. Even if it is possible to evaluate from the results of the simulations the heat flux from the nuts, considering the transient nature of the process and the nonuniformity and lack of symmetry in the temperature profile, it is difficult to obtain general correlations for heat transfer useful for equipment design. Therefore in the following the time required for cooling down the nuts from 140 C to the specified temperature will be shown. The distance between particles determined the open section for the air, influencing the velocity through the bed and thus the temperature profiles, similarly to the air flow rate. Increasing cooling air velocity significantly reduced cooling time (that is the time required to bring all the solid temperature below a fixed value) and the reduction was particularly relevant, increasing the value of the velocity from 0.09 up to 0.5 m s1 as shown in Figures 4–6. The cooling time was also reduced when the solid conductivity is higher (see Figure 5). A set of simulations was carried out to investigate the influence of the solid particle diameter over the cooling time, maintaining constant the ratio d=D, that is the open section (see Figure 6). A thermal conductivity value of 0.17 W m1 K1, corresponding to a mean value for hazelnuts, and in the range of values for other seeds, as reported in Table 1, was considered.
Source Sreenarayanan and Chattopadhyay (1986) Chang (1986) Chang (1986) Fortes and Okos (1980) Sweat (1987)
Figure 5. Influence of the thermal conductivity value (k) on cooling time for different final temperatures (shown in the label); D ¼ 0.13 mm, d ¼ 0.5.
Figure 6. Influence of solid particles diameter (D) on cooling time (to 293 K); k ¼ 0.17 W m1 K1, d=D ¼ 0.038.
CONCLUSIONS Simulation results have shown that highly non-symmetric temperature profiles can develop during cooling of solid particles in cross flow, depending on air velocity and solid
thermal conductivity. Thus for design purposes it is better to refer to cooling time required to lower the temperature in all the particles below a fixed value. High values of cooling air velocity are required to obtain a fast cooling, but this requires a larger amount of cold air; in these conditions the temperature profiles become more symmetric. In addition, it is important to take care of the distribution system of the spherical particles over the belt conveyor, to obtain a regular distribution with the maximum density of particles, to avoid the phenomenon of local channelling. NOMENCLATURE
Figure 4. Influence of distance between particles (d) on cooling time (to 293 K); D ¼ 13 mm, k ¼ 0.23 W m1 K1.
D d k v
diameter of solid particles, mm minimum distance between particle surfaces, mm thermal conductivity of solid particles, W m1 K1 cooling air velocity, m s1
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77
COOLING OF ROUNDED SEEDS ADDITIONAL INFROMATION The complete series of data and related charts are available on request. REFERENCES Chang, C.S., 1986, Thermal conductivity of wheat, corn, and grain sorghum as affected by bulk density and moisture content, Trans ASAE, 29(5): 1447–1450. Demir, A.D., Baucour, P., Cronin, K. and Abodayeh, K., 2003, Analysis of temperature variability during the thermal processing of hazelnuts, Innov Food Sci Emerg Technol, 4: 69–84. Fortes, M., and Okos, M.R., 1980, Changes in physical properties of corn during drying, Trans ASAE, 23(4): 1004–1008.
77
Lopez, A. and Pique, M.T., 1997, Influence of the drying conditions on the hazelnut quality: III. Browning, Drying Technol, 15(3–4): 989–1002. Sreenarayanan, V.V. and Chattopadhyay, P.K., 1986, Thermal conductivity and diffusivity of rice bran, J Agric Eng Res, 34: 115–121. Sweat, V.E., 1987, Thermal properties of low and intermediate moisture foods, ASHRAE J, February, 44 [quoted from Rahman, S., 1995, Food Properties Handbook (CRC Press, Boca Raton, USA)]
ACKNOWLEDGEMENTS The authors wish to thank Dr Marco Vanni for valuable suggestions. The manuscript was received 20 June 2003 and accepted for publication after revision 7 January 2004.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C1): 73–77