The Experts below are selected from a list of 8826 Experts worldwide ranked by ideXlab platform

Denis Flick - One of the best experts on this subject based on the ideXlab platform.

  • Influence of operating conditions on residence time distributions in a scraped Surface Heat Exchanger during aerated sorbet production
    Journal of Food Engineering, 2018
    Co-Authors: Fatou-toutie Ndoye, Hayat Benkhelifa, Graciela Alvarez, Oscar Darío Hernández-parra, Denis Flick
    Abstract:

    The residence time distribution (RTD) inside a scraped Surface Heat Exchanger (SSHE) during simultaneous crystallization-foaming process for continuous production of an aerated sorbet was studied. The effect of mix flowrate, air flowrate and refrigerant temperature on the unfrozen liquid/ice crystals phase RTD was investigated using a dye tracer method. The experimental results revealed that both mix and air flowrates increase leads to a lessening of the minimum and of the mean residence time inside the SSHE. Flow diagnosis showed the presence of a nearly stagnant air pocket which volume increase with the air flowrate increase, resulting in an augmentation of the liquid phase velocity. More axial dispersion was observed at higher air flowrates and at lower refrigerant temperature due to greater radial temperature and axial velocity gradients. These conclusions were confirmed by the parameters of the fitted flow models (axial dispersion, tank-in-series and gamma distribution models) even if only the gamma distribution model succeeded to well describe the flow patterns observed. The findings of this pioneering work in the field of gas-liquid flow in SSHEs might be useful for several applications involving multi phasic flow in SSHEs.

  • Modeling flow and Heat transfer in a scraped Surface Heat Exchanger during the production of sorbet
    Journal of Food Engineering, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    A computational fluid dynamics model was implemented for studying the production of non-aerated sorbet in a scraped Surface Heat Exchanger under different operating conditions. The coupled problem of fluid flow and Heat transfer is solved by taking into account realistic values for the product physical properties as well as its complex rheology. The mathematical solution of the coupled problem represents a challenging task, among other reasons by the strong influence of temperature on product physical properties and rheology due to phase change. Fluid flow, Heat transfer freezing and viscous dissipation phenomena are analyzed locally in the Heat Exchanger. The modeling approach is consistent, as indicated by comparisons between model predictions of the temperature profile along the Heat Exchanger and available experimental results.

  • Effect of process parameters on ice crystals and air bubbles size distributions of sorbets in a scraped Surface Heat Exchanger
    International Journal of Refrigeration, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Denis Flick, Fatou-toutie Ndoye, Graciela Alvarez
    Abstract:

    Ice crystals and bubbles size distributions were characterized for aerated sorbets produced during a crystallization-foaming process in a scraped Surface Heat Exchanger. Size measurements were performed immediately at the exit of the Heat Exchanger by means of a low-temperature microscopy method especially developed in order to discriminate between ice crystals and bubbles and to make possible to measure both sizes simultaneously. The influence of process parameters such as refrigerant fluid evaporating temperature and air flowrate on both size distributions was investigated. Larger ice crystals were obtained when refrigerant fluid temperature increased, but this effect was minimized by the presence of air which retards crystal growth by insulation and hindrance effect. Low refrigerant fluid temperature led to small air bubbles size because of the enhancement of bubbles breakup by shear forces. Ice crystals size was reduced when air flowrate was increased, even if the effect was less pronounced at the lower refrigerant fluid temperature. Variations on air flowrate had a very small effect on air bubbles size.

  • Prediction of bubble fragmentation during sorbet production in a scraped Surface Heat Exchanger
    2017
    Co-Authors: Oscar Dario Hernandez, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    Complex phenomena occur during production of sorbet in a scraped Surface Heat Exchanger: fluid flow, Heat transfer, phase change (ice crystallization), rheological modification and fragmentation of bubbles. In a previous work, all these coupled phenomena except bubble fragmentation were simulated by CFD. The objective of the present work is to predict the bubble break-up. Locally, the critical radius for bubble break-up can be estimated as a function of the shear rate, the extension rate, the apparent viscosity of the 'continuous' phase, the viscosity of the gas and the Surface tension, by using correlations based on the capillary number. The 'continuous' phase surrounding the bubbles is in fact constituted by a concentrated liquid and ice crystals; its non-Newtonian rheology sharply depends on ice-fraction, which in turn depends on temperature. The CFD simulation allowed analyzing all the parameters influencing the critical radius. It appears that the smallest bubbles are generated a) near the contact between the blades and the scraped Surface where the highest shear rate is reached, and b) in the coldest region (near to the outlet) where the continuous phase has the maximum apparent viscosity. The order of magnitude of the smallest crystals predicted by CFD corresponds to the observed values.

  • coupling population balance and residence time distribution for the ice crystallization modeling in a scraped Surface Heat Exchanger
    Chemical Engineering Science, 2013
    Co-Authors: Marcela Arellano, Hayat Benkhelifa, Graciela Alvarez, Denis Flick
    Abstract:

    Abstract This paper presents the mathematical modeling of the ice crystallization process occurring during the freezing of sorbet in a continuous scraped Surface Heat Exchanger (SSHE). Two different modeling approaches have been used, both of which include the nucleation, growth and breakage phenomena of the ice crystals. For both models, the rate of ice crystal nucleation and growth were determined by the subcooling degree. The first model combines Heat transfer and population balance equations (PBE), assuming plug flow. The evolution of the values of product temperature, mean chord length, ice volume fraction and apparent viscosity were determined as a function of the residence time. The second model is a coupled model of Heat transfer and PBE combined with an empirical model of residence time distribution (RTD), which makes it possible to take into account the fact that the fluid fractions flowing throughout the SSHE do not have the same time–temperature history. The values of the variables (product temperature and mean chord length) were determined for each fraction of fluid exiting the SSHE, and the bulk values were then calculated using the RTD. Simulation results were compared to a set of experimental data obtained during the ice crystallization process of sorbet in a continuous SSHE at the laboratory pilot scale. With a first estimated set of model parameters, it has been shown that the experimental tendencies are represented very satisfactorily by the two models within a 10% error limit. These modeling approaches can then be considered as a promising tool for the understanding and the prediction of the ice crystallization process in SSHEs so as to identify new ways to improve the performance of the process.

Jack Legrand - One of the best experts on this subject based on the ideXlab platform.

  • a 3d cfd model thermal analysis within a scraped Surface Heat Exchanger
    Computers & Fluids, 2013
    Co-Authors: Mourad Yataghene, Jack Legrand
    Abstract:

    Abstract This paper deals with numerical study of the coupled fluid flow and Heat transfer within a scraped Surface Heat Exchanger “SSHEs”. The finite volume Fluent™ 6.3 code was used to solve continuity, momentum and energy equations in a real SSHE geometry using multiple rotating reference frame formulation. The mesh of a real SSHE geometry was achieved with Gambit™ 2.2.3, in order to take into account geometry singularities and their effect on the Heat transfer performance within SSHE. The steady laminar non-isothermal flow of pure Glycerin, 2% CMC solution and 0.2% Carbopol solution were investigated. The cooling process without phase change within SSHE was studied. The Heat correlation was established numerically and validated with the given literature Heat correlations. The obtained numerical results agree well with given Heat correlations in the literature. The numerical model was then used to examine in more details the Heat performance of SSHE. The effect of the rotating velocity on the SSHE Heat performance was investigated. For pure Glycerin, the increases on the rotating velocity significantly reduced the cooling process due to viscous Heating. In the case of considered non-Newtonian fluids, increases on the rotating velocity improved the thermal efficiency of SSHE. The temperature profile a long of tip of blade, have shown an important influence of blade fixation on the Heat performance within SHHE. Viscous Heating was investigated and occurred for Newtonian viscous fluids. The numerical results have shown, in the case of Glycerin when the rotating velocity was ω  = 9 rev s −1 , that the total energy can reach 25% more than without viscous Heating. Finally, the study of the mixing time have shown a best Heat performance when mixing time was equal to t mix  = 6.72 s.

  • Experimental and numerical analysis of Heat transfer including viscous dissipation in a scraped Surface Heat Exchanger
    Chemical Engineering and Processing: Process Intensification, 2009
    Co-Authors: Mourad Yataghene, Francine Fayolle, Jack Legrand
    Abstract:

    Viscous dissipation plays an important role in the dynamics of fluids with strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The Heat generated by viscous friction causes a local temperature increase in the high shearing zone with a consequent decrease of the viscosity which may dramatically change the temperature and velocity distribution. These processes are mainly controlled by the Brinkman number, the rotating velocity and the thermal boundary conditions. This work analyses forced convection Heat transfer including the viscous dissipation in a scraped Surface Heat Exchanger (SSHE). In this study the increase of the temperature due to the viscous dissipation is analysed both experimentally and numerically for Newtonian and non-Newtonian fluids. Heat transfer simulations including viscous dissipation were carried out by means of the CFD code of the software Fluent, version 6.3, with solving momentum and energy equations. Two thermal boundary conditions were considered: pseudo-adiabatic wall and constant temperature on the stator wall exchange. In the case of Newtonian fluid (pure HV45), for both considered thermal boundary conditions, an important increase of the temperature was obtained. In the case of non-Newtonian shear thinning fluid (2 wt% CMC solution), viscous dissipation is neglected. The developed numerical model agrees well with experimental results. The validated numerical model was then used to study the effect of index and consistency behaviour of shear thinning fluid using power-law rheological behaviour on the viscous dissipation, and correlation using dimensionless analysis expressed with different dimensionless process numbers is proposed for Newtonian and non-Newtonian shear thinning fluid.

  • cfd analysis of the flow pattern and local shear rate in a scraped Surface Heat Exchanger
    Chemical Engineering and Processing, 2008
    Co-Authors: Mourad Yataghene, Francine Fayolle, Jeremy Pruvost, Jack Legrand
    Abstract:

    Abstract A numerical investigation of a scraped Surface Heat Exchanger (SSHE) was undertaken using the commercial CFD code FLUENT to characterize the shear rates for Newtonian and Non-Newtonian fluids. Simulations are carried out in standard geometries of SSHE. The electrochemical method was first employed to achieve experimental measurements of the shear rates. A two-dimensional model was created to perform the simulation, keeping the dimensions and flow parameters of the experiment. Because of the symmetry of the geometry, a bidimensional resolution of the continuity and momentum equations was conducted. A hybrid mesh was retained with a grid refinement between the tip of the blades and stator where high shear rates occur. A single reference frame approach was then applied to obtain the laminar steady-state flow induced by the rotation of blades in the geometry. A grid refinement in the zone localized between the tip of the blades and the stator is used. A comparison of predictions with experimental measurements was carried out, showing relative agreement between shear rates ( S max ) predicted and measured for three fluids (HV45, CMC and guar gum). The little differences observed were principally explained by the 2D simulation which neglects Taylor vortices, when Taylor number ( Ta g ) exceeds a critical value. In the case of HV45, rotating velocity can influence the scraper angle position due to the floating blade. This can induce a change in the gap between the tip of the blade and stator assembly (from 90 to130 μm) that was investigated here.

  • Development of a time temperature integrator for quantification of thermal treatment in scraped Surface Heat Exchangers
    Innovative Food Science & Emerging Technologies, 2008
    Co-Authors: Jérôme Mabit, Francine Fayolle, Rachida Belhamri, Jack Legrand
    Abstract:

    Abstract When processing highly viscous fluids in scraped Surface Heat Exchangers, it is often difficult to follow the real Heat flux received by each part of the product. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some parts of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment of Newtonian or non Newtonian viscous fluids in an experimental scraped Surface Heat Exchanger in conditions similar to a pasteurisation treatment. A natural coloured tracer (Betanin, from red beets) which thermal degradation is well known is used as time temperature integrator. Thermal degradation of the tracer is determined experimentally and compared with a dispersion RTD model. First results show that Betanin degradation diluted in viscous fluids follows a classical first order reaction in a batch system. However, this degradation is enhanced when mechanical treatments occur in a well identified shearing channel as well as in the SSHE, for shear rates higher than 20 000 s − 1 . The major concern was then to identify the origin of this enhancement. Temperature increase due to viscous friction was measured in the SSHE and compared with the prediction conversion factors. It was shown that thermal treatment received by the Betanin was highly dependent on the mechanical treatment delivered during the thermal treatment. Industrial relevance When processing highly viscous fluids in scraped Surface Heat Exchangers, it is often difficult to follow the real Heat flux received by each part of the product, especially in industrial apparatus. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some part of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment in an experimental scraped Surface Heat Exchanger in conditions similar to a pasteurisation treatment. When this tracer is found to be appropriate, it will be used directly in industrial equipments, in order to help industrials in: first, understanding their processes; verifying that their actual operating conditions are feasible for a correct thermal treatment; if not, in optimising these operating conditions. Betanin, representative of sensitive food products treated in a scraped Surface Heat Exchanger was found to be, not only sensitive to temperature but also to mechanical treatment. This rather unexpected result has to be taken into account very carefully by industrials in order to insure a content product quality, whereas the mechanical treatment undergone by the product can lead to an accentuated degradation of thermally sensitive products.

  • Determination of heterogeneities in a scraped Surface Heat Exchanger using electrochemical sensors
    Journal of Applied Electrochemistry, 2005
    Co-Authors: Francine Fayolle, Jérôme Mabit, Jack Legrand
    Abstract:

    An experimental investigation of a scaled-down model of an industrial Exchanger, using an electrochemical technique, was undertaken in order to show the presence of hydrodynamic heterogeneities at low axial Reynolds number. Heterogeneities were revealed in the bowls with respect to the generalised Taylor number as the result of the perturbations added to the flow by blade rotation at both ends of the Exchanger. Shear heterogeneities associated to flow visualisations were correlated to temperature heterogeneities observed in the bowls. Shear fluctuations were revealed in the scraped part describing two distinctive zones at low rotation speed caused by varying viscosity in the flow field. A complex spiral flow was observed by flow visualisation characterising a mass transfer evolution comprised between these two distinctive zones at low Taylor number.

Hayat Benkhelifa - One of the best experts on this subject based on the ideXlab platform.

  • Modeling flow and Heat transfer in a scraped Surface Heat Exchanger during the production of sorbet
    Journal of Food Engineering, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    A computational fluid dynamics model was implemented for studying the production of non-aerated sorbet in a scraped Surface Heat Exchanger under different operating conditions. The coupled problem of fluid flow and Heat transfer is solved by taking into account realistic values for the product physical properties as well as its complex rheology. The mathematical solution of the coupled problem represents a challenging task, among other reasons by the strong influence of temperature on product physical properties and rheology due to phase change. Fluid flow, Heat transfer freezing and viscous dissipation phenomena are analyzed locally in the Heat Exchanger. The modeling approach is consistent, as indicated by comparisons between model predictions of the temperature profile along the Heat Exchanger and available experimental results.

  • Influence of operating conditions on residence time distributions in a scraped Surface Heat Exchanger during aerated sorbet production
    Journal of Food Engineering, 2018
    Co-Authors: Fatou-toutie Ndoye, Hayat Benkhelifa, Graciela Alvarez, Oscar Darío Hernández-parra, Denis Flick
    Abstract:

    The residence time distribution (RTD) inside a scraped Surface Heat Exchanger (SSHE) during simultaneous crystallization-foaming process for continuous production of an aerated sorbet was studied. The effect of mix flowrate, air flowrate and refrigerant temperature on the unfrozen liquid/ice crystals phase RTD was investigated using a dye tracer method. The experimental results revealed that both mix and air flowrates increase leads to a lessening of the minimum and of the mean residence time inside the SSHE. Flow diagnosis showed the presence of a nearly stagnant air pocket which volume increase with the air flowrate increase, resulting in an augmentation of the liquid phase velocity. More axial dispersion was observed at higher air flowrates and at lower refrigerant temperature due to greater radial temperature and axial velocity gradients. These conclusions were confirmed by the parameters of the fitted flow models (axial dispersion, tank-in-series and gamma distribution models) even if only the gamma distribution model succeeded to well describe the flow patterns observed. The findings of this pioneering work in the field of gas-liquid flow in SSHEs might be useful for several applications involving multi phasic flow in SSHEs.

  • Effect of process parameters on ice crystals and air bubbles size distributions of sorbets in a scraped Surface Heat Exchanger
    International Journal of Refrigeration, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Denis Flick, Fatou-toutie Ndoye, Graciela Alvarez
    Abstract:

    Ice crystals and bubbles size distributions were characterized for aerated sorbets produced during a crystallization-foaming process in a scraped Surface Heat Exchanger. Size measurements were performed immediately at the exit of the Heat Exchanger by means of a low-temperature microscopy method especially developed in order to discriminate between ice crystals and bubbles and to make possible to measure both sizes simultaneously. The influence of process parameters such as refrigerant fluid evaporating temperature and air flowrate on both size distributions was investigated. Larger ice crystals were obtained when refrigerant fluid temperature increased, but this effect was minimized by the presence of air which retards crystal growth by insulation and hindrance effect. Low refrigerant fluid temperature led to small air bubbles size because of the enhancement of bubbles breakup by shear forces. Ice crystals size was reduced when air flowrate was increased, even if the effect was less pronounced at the lower refrigerant fluid temperature. Variations on air flowrate had a very small effect on air bubbles size.

  • Prediction of bubble fragmentation during sorbet production in a scraped Surface Heat Exchanger
    2017
    Co-Authors: Oscar Dario Hernandez, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    Complex phenomena occur during production of sorbet in a scraped Surface Heat Exchanger: fluid flow, Heat transfer, phase change (ice crystallization), rheological modification and fragmentation of bubbles. In a previous work, all these coupled phenomena except bubble fragmentation were simulated by CFD. The objective of the present work is to predict the bubble break-up. Locally, the critical radius for bubble break-up can be estimated as a function of the shear rate, the extension rate, the apparent viscosity of the 'continuous' phase, the viscosity of the gas and the Surface tension, by using correlations based on the capillary number. The 'continuous' phase surrounding the bubbles is in fact constituted by a concentrated liquid and ice crystals; its non-Newtonian rheology sharply depends on ice-fraction, which in turn depends on temperature. The CFD simulation allowed analyzing all the parameters influencing the critical radius. It appears that the smallest bubbles are generated a) near the contact between the blades and the scraped Surface where the highest shear rate is reached, and b) in the coldest region (near to the outlet) where the continuous phase has the maximum apparent viscosity. The order of magnitude of the smallest crystals predicted by CFD corresponds to the observed values.

  • coupling population balance and residence time distribution for the ice crystallization modeling in a scraped Surface Heat Exchanger
    Chemical Engineering Science, 2013
    Co-Authors: Marcela Arellano, Hayat Benkhelifa, Graciela Alvarez, Denis Flick
    Abstract:

    Abstract This paper presents the mathematical modeling of the ice crystallization process occurring during the freezing of sorbet in a continuous scraped Surface Heat Exchanger (SSHE). Two different modeling approaches have been used, both of which include the nucleation, growth and breakage phenomena of the ice crystals. For both models, the rate of ice crystal nucleation and growth were determined by the subcooling degree. The first model combines Heat transfer and population balance equations (PBE), assuming plug flow. The evolution of the values of product temperature, mean chord length, ice volume fraction and apparent viscosity were determined as a function of the residence time. The second model is a coupled model of Heat transfer and PBE combined with an empirical model of residence time distribution (RTD), which makes it possible to take into account the fact that the fluid fractions flowing throughout the SSHE do not have the same time–temperature history. The values of the variables (product temperature and mean chord length) were determined for each fraction of fluid exiting the SSHE, and the bulk values were then calculated using the RTD. Simulation results were compared to a set of experimental data obtained during the ice crystallization process of sorbet in a continuous SSHE at the laboratory pilot scale. With a first estimated set of model parameters, it has been shown that the experimental tendencies are represented very satisfactorily by the two models within a 10% error limit. These modeling approaches can then be considered as a promising tool for the understanding and the prediction of the ice crystallization process in SSHEs so as to identify new ways to improve the performance of the process.

Graciela Alvarez - One of the best experts on this subject based on the ideXlab platform.

  • Influence of operating conditions on residence time distributions in a scraped Surface Heat Exchanger during aerated sorbet production
    Journal of Food Engineering, 2018
    Co-Authors: Fatou-toutie Ndoye, Hayat Benkhelifa, Graciela Alvarez, Oscar Darío Hernández-parra, Denis Flick
    Abstract:

    The residence time distribution (RTD) inside a scraped Surface Heat Exchanger (SSHE) during simultaneous crystallization-foaming process for continuous production of an aerated sorbet was studied. The effect of mix flowrate, air flowrate and refrigerant temperature on the unfrozen liquid/ice crystals phase RTD was investigated using a dye tracer method. The experimental results revealed that both mix and air flowrates increase leads to a lessening of the minimum and of the mean residence time inside the SSHE. Flow diagnosis showed the presence of a nearly stagnant air pocket which volume increase with the air flowrate increase, resulting in an augmentation of the liquid phase velocity. More axial dispersion was observed at higher air flowrates and at lower refrigerant temperature due to greater radial temperature and axial velocity gradients. These conclusions were confirmed by the parameters of the fitted flow models (axial dispersion, tank-in-series and gamma distribution models) even if only the gamma distribution model succeeded to well describe the flow patterns observed. The findings of this pioneering work in the field of gas-liquid flow in SSHEs might be useful for several applications involving multi phasic flow in SSHEs.

  • Modeling flow and Heat transfer in a scraped Surface Heat Exchanger during the production of sorbet
    Journal of Food Engineering, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    A computational fluid dynamics model was implemented for studying the production of non-aerated sorbet in a scraped Surface Heat Exchanger under different operating conditions. The coupled problem of fluid flow and Heat transfer is solved by taking into account realistic values for the product physical properties as well as its complex rheology. The mathematical solution of the coupled problem represents a challenging task, among other reasons by the strong influence of temperature on product physical properties and rheology due to phase change. Fluid flow, Heat transfer freezing and viscous dissipation phenomena are analyzed locally in the Heat Exchanger. The modeling approach is consistent, as indicated by comparisons between model predictions of the temperature profile along the Heat Exchanger and available experimental results.

  • Effect of process parameters on ice crystals and air bubbles size distributions of sorbets in a scraped Surface Heat Exchanger
    International Journal of Refrigeration, 2018
    Co-Authors: Oscar Darío Hernández-parra, Hayat Benkhelifa, Denis Flick, Fatou-toutie Ndoye, Graciela Alvarez
    Abstract:

    Ice crystals and bubbles size distributions were characterized for aerated sorbets produced during a crystallization-foaming process in a scraped Surface Heat Exchanger. Size measurements were performed immediately at the exit of the Heat Exchanger by means of a low-temperature microscopy method especially developed in order to discriminate between ice crystals and bubbles and to make possible to measure both sizes simultaneously. The influence of process parameters such as refrigerant fluid evaporating temperature and air flowrate on both size distributions was investigated. Larger ice crystals were obtained when refrigerant fluid temperature increased, but this effect was minimized by the presence of air which retards crystal growth by insulation and hindrance effect. Low refrigerant fluid temperature led to small air bubbles size because of the enhancement of bubbles breakup by shear forces. Ice crystals size was reduced when air flowrate was increased, even if the effect was less pronounced at the lower refrigerant fluid temperature. Variations on air flowrate had a very small effect on air bubbles size.

  • Prediction of bubble fragmentation during sorbet production in a scraped Surface Heat Exchanger
    2017
    Co-Authors: Oscar Dario Hernandez, Hayat Benkhelifa, Graciela Alvarez, Artemio Plana Fattori, Fatou-toutie Ndoye, Denis Flick
    Abstract:

    Complex phenomena occur during production of sorbet in a scraped Surface Heat Exchanger: fluid flow, Heat transfer, phase change (ice crystallization), rheological modification and fragmentation of bubbles. In a previous work, all these coupled phenomena except bubble fragmentation were simulated by CFD. The objective of the present work is to predict the bubble break-up. Locally, the critical radius for bubble break-up can be estimated as a function of the shear rate, the extension rate, the apparent viscosity of the 'continuous' phase, the viscosity of the gas and the Surface tension, by using correlations based on the capillary number. The 'continuous' phase surrounding the bubbles is in fact constituted by a concentrated liquid and ice crystals; its non-Newtonian rheology sharply depends on ice-fraction, which in turn depends on temperature. The CFD simulation allowed analyzing all the parameters influencing the critical radius. It appears that the smallest bubbles are generated a) near the contact between the blades and the scraped Surface where the highest shear rate is reached, and b) in the coldest region (near to the outlet) where the continuous phase has the maximum apparent viscosity. The order of magnitude of the smallest crystals predicted by CFD corresponds to the observed values.

  • coupling population balance and residence time distribution for the ice crystallization modeling in a scraped Surface Heat Exchanger
    Chemical Engineering Science, 2013
    Co-Authors: Marcela Arellano, Hayat Benkhelifa, Graciela Alvarez, Denis Flick
    Abstract:

    Abstract This paper presents the mathematical modeling of the ice crystallization process occurring during the freezing of sorbet in a continuous scraped Surface Heat Exchanger (SSHE). Two different modeling approaches have been used, both of which include the nucleation, growth and breakage phenomena of the ice crystals. For both models, the rate of ice crystal nucleation and growth were determined by the subcooling degree. The first model combines Heat transfer and population balance equations (PBE), assuming plug flow. The evolution of the values of product temperature, mean chord length, ice volume fraction and apparent viscosity were determined as a function of the residence time. The second model is a coupled model of Heat transfer and PBE combined with an empirical model of residence time distribution (RTD), which makes it possible to take into account the fact that the fluid fractions flowing throughout the SSHE do not have the same time–temperature history. The values of the variables (product temperature and mean chord length) were determined for each fraction of fluid exiting the SSHE, and the bulk values were then calculated using the RTD. Simulation results were compared to a set of experimental data obtained during the ice crystallization process of sorbet in a continuous SSHE at the laboratory pilot scale. With a first estimated set of model parameters, it has been shown that the experimental tendencies are represented very satisfactorily by the two models within a 10% error limit. These modeling approaches can then be considered as a promising tool for the understanding and the prediction of the ice crystallization process in SSHEs so as to identify new ways to improve the performance of the process.

Francine Fayolle - One of the best experts on this subject based on the ideXlab platform.

  • Cleanabilty study of a Scraped Surface Heat Exchanger
    Food and Bioproducts Processing, 2013
    Co-Authors: W. Blel, Patrick Legentilhomme, Thierry Bénézech, Francine Fayolle
    Abstract:

    Scraped Surface Heat Exchangers (Contherm SSHE model 6 x 3) are widely used industrially. In this kind of Heat Exchanger, the inlet and the outlet are difficult to clean due to their particular geometry and the presence of seals. A specific study was conducted on the inlet bowl of a SSHE, whose design has been optimized by the manufacturer in order to minimize risk of deposition by eliminating hydrodynamic dead zones. For this purpose, measurements of wall shear stress were made by an electrochemical method, for different hydrodynamic conditions. On the other hand, cleanability measurements were also performed. The bowl geometry tested presented no dead zones. However, the available space for flow significantly reduces the Reynolds number and turbulence intensity. As a consequence, three areas of increasing contamination appeared throughout the bowl due to the low mean and fluctuating shear stress values. The use of a pulsating flow increases these fluctuations, and thereby reduces the residual contamination. (C) 2012 The Institution of Chemical Engineers.

  • Cleanabilty study of a Scraped Surface Heat Exchanger
    Food and Bioproducts Processing, 2013
    Co-Authors: W. Blel, Patrick Legentilhomme, Thierry Bénézech, Francine Fayolle
    Abstract:

    Abstract Scraped Surface Heat Exchangers (Contherm SSHE model 6 × 3) are widely used industrially. In this kind of Heat Exchanger, the inlet and the outlet are difficult to clean due to their particular geometry and the presence of seals. A specific study was conducted on the inlet bowl of a SSHE, whose design has been optimized by the manufacturer in order to minimize risk of deposition by eliminating hydrodynamic dead zones. For this purpose, measurements of wall shear stress were made by an electrochemical method, for different hydrodynamic conditions. On the other hand, cleanability measurements were also performed. The bowl geometry tested presented no dead zones. However, the available space for flow significantly reduces the Reynolds number and turbulence intensity. As a consequence, three areas of increasing contamination appeared throughout the bowl due to the low mean and fluctuating shear stress values. The use of a pulsating flow increases these fluctuations, and thereby reduces the residual contamination.

  • Experimental and numerical analysis of Heat transfer including viscous dissipation in a scraped Surface Heat Exchanger
    Chemical Engineering and Processing: Process Intensification, 2009
    Co-Authors: Mourad Yataghene, Francine Fayolle, Jack Legrand
    Abstract:

    Viscous dissipation plays an important role in the dynamics of fluids with strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The Heat generated by viscous friction causes a local temperature increase in the high shearing zone with a consequent decrease of the viscosity which may dramatically change the temperature and velocity distribution. These processes are mainly controlled by the Brinkman number, the rotating velocity and the thermal boundary conditions. This work analyses forced convection Heat transfer including the viscous dissipation in a scraped Surface Heat Exchanger (SSHE). In this study the increase of the temperature due to the viscous dissipation is analysed both experimentally and numerically for Newtonian and non-Newtonian fluids. Heat transfer simulations including viscous dissipation were carried out by means of the CFD code of the software Fluent, version 6.3, with solving momentum and energy equations. Two thermal boundary conditions were considered: pseudo-adiabatic wall and constant temperature on the stator wall exchange. In the case of Newtonian fluid (pure HV45), for both considered thermal boundary conditions, an important increase of the temperature was obtained. In the case of non-Newtonian shear thinning fluid (2 wt% CMC solution), viscous dissipation is neglected. The developed numerical model agrees well with experimental results. The validated numerical model was then used to study the effect of index and consistency behaviour of shear thinning fluid using power-law rheological behaviour on the viscous dissipation, and correlation using dimensionless analysis expressed with different dimensionless process numbers is proposed for Newtonian and non-Newtonian shear thinning fluid.

  • cfd analysis of the flow pattern and local shear rate in a scraped Surface Heat Exchanger
    Chemical Engineering and Processing, 2008
    Co-Authors: Mourad Yataghene, Francine Fayolle, Jeremy Pruvost, Jack Legrand
    Abstract:

    Abstract A numerical investigation of a scraped Surface Heat Exchanger (SSHE) was undertaken using the commercial CFD code FLUENT to characterize the shear rates for Newtonian and Non-Newtonian fluids. Simulations are carried out in standard geometries of SSHE. The electrochemical method was first employed to achieve experimental measurements of the shear rates. A two-dimensional model was created to perform the simulation, keeping the dimensions and flow parameters of the experiment. Because of the symmetry of the geometry, a bidimensional resolution of the continuity and momentum equations was conducted. A hybrid mesh was retained with a grid refinement between the tip of the blades and stator where high shear rates occur. A single reference frame approach was then applied to obtain the laminar steady-state flow induced by the rotation of blades in the geometry. A grid refinement in the zone localized between the tip of the blades and the stator is used. A comparison of predictions with experimental measurements was carried out, showing relative agreement between shear rates ( S max ) predicted and measured for three fluids (HV45, CMC and guar gum). The little differences observed were principally explained by the 2D simulation which neglects Taylor vortices, when Taylor number ( Ta g ) exceeds a critical value. In the case of HV45, rotating velocity can influence the scraper angle position due to the floating blade. This can induce a change in the gap between the tip of the blade and stator assembly (from 90 to130 μm) that was investigated here.

  • Development of a time temperature integrator for quantification of thermal treatment in scraped Surface Heat Exchangers
    Innovative Food Science & Emerging Technologies, 2008
    Co-Authors: Jérôme Mabit, Francine Fayolle, Rachida Belhamri, Jack Legrand
    Abstract:

    Abstract When processing highly viscous fluids in scraped Surface Heat Exchangers, it is often difficult to follow the real Heat flux received by each part of the product. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some parts of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment of Newtonian or non Newtonian viscous fluids in an experimental scraped Surface Heat Exchanger in conditions similar to a pasteurisation treatment. A natural coloured tracer (Betanin, from red beets) which thermal degradation is well known is used as time temperature integrator. Thermal degradation of the tracer is determined experimentally and compared with a dispersion RTD model. First results show that Betanin degradation diluted in viscous fluids follows a classical first order reaction in a batch system. However, this degradation is enhanced when mechanical treatments occur in a well identified shearing channel as well as in the SSHE, for shear rates higher than 20 000 s − 1 . The major concern was then to identify the origin of this enhancement. Temperature increase due to viscous friction was measured in the SSHE and compared with the prediction conversion factors. It was shown that thermal treatment received by the Betanin was highly dependent on the mechanical treatment delivered during the thermal treatment. Industrial relevance When processing highly viscous fluids in scraped Surface Heat Exchangers, it is often difficult to follow the real Heat flux received by each part of the product, especially in industrial apparatus. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some part of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment in an experimental scraped Surface Heat Exchanger in conditions similar to a pasteurisation treatment. When this tracer is found to be appropriate, it will be used directly in industrial equipments, in order to help industrials in: first, understanding their processes; verifying that their actual operating conditions are feasible for a correct thermal treatment; if not, in optimising these operating conditions. Betanin, representative of sensitive food products treated in a scraped Surface Heat Exchanger was found to be, not only sensitive to temperature but also to mechanical treatment. This rather unexpected result has to be taken into account very carefully by industrials in order to insure a content product quality, whereas the mechanical treatment undergone by the product can lead to an accentuated degradation of thermally sensitive products.