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International audience; Among innovative freezing processes, electromagnetic wave assistance during freezing has received great attention in recent years. The literature provides interesting studies indicating that smaller ice crystals may be obtained during freezing with the assistance of microwaves. In the companion paper, we designed a laboratory scale prototype to perform experiments of this type in well controlled conditions at 2.45 GHz. In the current study, we performed freezing experiments using this prototype and developed a protocol to measure ice crystal sizes using X-ray micro-tomography. The methodology was first validated by measuring ice crystal distribution in unidirectional conventional freezing. Then several microwave assisted freezing modalities were studied. Experiments were conducted with pulsed and continuously applied microwaves. The impact of microwave irradiation time was studied at a constant incident microwave power, and then at constant energy supplied to the system. We observed a significant reduction of up to 25% in ice crystal size when microwave assistance was used during freezing with both pulsed and continuously applied microwaves. These observations tend to disconfirm the hypothesis in the literature according to which ice crystal size reduction is due to temperature oscillations. The only remaining hypothesis in the literature is that ice crystal size reduction may be due to the perturbation of the H-bond network in water, which could be a precursor to crystalline structure. It appeared that at constant incident microwave power, the increase in irradiation time tended to decrease ice crystal size. No significant difference in ice crystal size reduction was observed for a constant energy supplied to the system. The amount of energy supplied by microwaves seems to be a factor influencing ice crystal size. Industrial relevance Microwave assisted freezing is an emerging process which permits reducing ice crystal size, and thus improving frozen product quality. The measurement of ice crystals in samples frozen in controlled conditions of both freezing and microwave irradiation allows studying the main parameters that induce the reduction of their size. Better understanding of the relevant factors influencing nucleation and/or crystal growth can help to optimize the process.

International audience; Free cooling technique using Phase Change Material (PCM) is a promising process to shave the peak load for the cooling system and reduce energy ventilation consumption. However it needs additional energy for the fan. In the way optimize the process we then proposed a first approach to estimate a priori the flow rate required to reach full freezing or thawing of PCM which is a useful indication of the process efficiency. The approach is based on the inverse method associated with sensitivity coefficient. The objective function was defined in terms of air temperature at the exit of the heat exchanger. The model was compared to experimental data, and the validation of numerical results shows the robustness and the consistency of this method. In the worst case, relative error does not exceed 510-03 .

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Researches about mathematical modeling of electrohydrodynamic (EHD) drying are rare. In this study, hot air combined with electrohydrodynamic (EHD) drying behavior of thin layer mushroom slices was evaluated in a laboratory scale dryer at voltages of 17, 19, and 21 kV and electrode gaps of 5, 6, and 7 cm. The drying curves were fitted to ten different mathematical models (Newton, Page, Modified Page, Henderson and Pabis, Logarithmic, Two-term exponential, Midilli and Kucuk, Wang and Singh, Weibull and Parabolic models) and a proposed new empirical model to select a suitable drying equation for drying mushroom slices in a hot air combined with EHD dryer. Coefficients of the models were determined by non-linear regression analysis and the models were compared based on their coefficient of determination ( R 2 ), sum of square errors (SSE) and root mean square error (RMSE) between experimental and predicted moisture ratios. According to the results, the proposed model that contains only three parameters provided the best fit with the experimental data. It was closely followed by the Midilli and Kucuk model that contains four parameters. Therefore, the proposed model can present comfortable usage and excellent predictions for the moisture content changes of mushroom slices in the hot air combined with EHD drying system.

An improved Eulerian model is proposed to predict particle deposition velocity in a typical mechanical ventilation system of a dairy food factory. For fully developed turbulent flow, the model is modified based on the three-layer model developed by Zhao and Wu (2006a), accounting for thermophoresis as well as turbophoresis, Brownian diffusion, turbulent diffusion, and gravitational settling. Based on in-situ measurements of the aerosol size distributions, particles discussed in this article were in the size range 0.3–20 μm. The measured mass concentration and the predicted particle deposition velocity were used to calculate the deposited particle mass flux in the ventilation duct. The results indicate that the effects of temperature gradients and of surface roughness should be considered in food factories ventilation ducts. For horizontal surfaces, it is shown that even a little difference between empirical and theoretical models can lead to a 2-fold difference in the predicted deposited particle mass flu...

Electrohydrodynamic (EHD) is a promising process for reducing the energy consumption of convective drying processes. In this study, a combined convective-EHD drying system was designed, constructed, and examined as an economical drying system for drying mushroom slices. The effects of two levels of velocities (0.4 and 2.2 m/s) and four levels of voltages (0, 20, 25, and 30 kV) on drying kinetics, drying rate, final moisture content, and specific energy consumption of EHD system (SECEHD), convective system (SECtunnel), and combined convective-EHD system (SECtotal) were studied. The results confirmed that by applying electric field at 0.4 m/s air velocity, the enhancement in the drying rate increased with the applied voltage. The combination of 30 kV voltage with the air velocity of 0.4 m/s could significantly increase the drying rate and simultaneously reduce the specific energy consumption of the combined convective-EHD system. However, a cross-flow with the high velocity of 2.2 m/s diminished corona wind effect. Results showed that increasing velocity alongside decreasing voltage reduced the SECEHD, but SECtunnel and SECtotal declined as velocity diminished. In fact, 30 kV-0.4 m/s treatment resulted in statistically similar averaged drying rate and final moisture content to 0 kV-2.2 m/s, 20 kV-2.2 m/s, 25 kV-2.2 m/s, and 30 kV-2.2 m/s treatments, but it represented considerably less SECtotal value in comparison to them. As such, the appropriate combination of electric field intensity with air velocity such as 30 kV-0.4 m/s treatment is a promising solution for reducing the substantial energy consumption of industries using convective drying technique.

The mass transfer and the influence of sudden external perturbations on the behavior of an air curtain are evaluated by means of experimental and numerical studies. The air curtain, consisting of a plane air jet, is combined with a laminar flow unit. They constitute an “open” protection device intended to protect a localized area against any external airborne contamination. The external perturbations are characterized by fast pressure changes, such as drafts which can be experienced in a room. Numerical and experimental results both show that the air jet is strongly perturbed by the pressure gradient in the lateral direction. The jet is subjected to a brutal back and forth motion, breaks, and generates a swirl which is responsible for pollution entrainment inside the protected area. Gas concentration measurements suggested that there is an inlet jet velocity for which the pollution is minimal. These experimental results, corresponding to a strongly unsteady flow, were qualitatively well-predicted by the s...

The thawing process is a unit operation that exposes the food to risk of microbial growth. To evaluate this risk, numerical modelling is very useful. The thawing process by means of a 3-D numerical model is investigated. Both heat diffusion with phase change inside the product and on the convection phenomenon in the heating medium is focused on. This coupled model does not require an a priori knowledge of the convective heat-transfer coefficient. The corresponding highly nonlinear equations were solved using a CFD software. Experimental investigations were carried out on a high-pressure vessel (temperature measurements), and on a specific device (velocity measurements). Comparisons between experimental and numerical results were very satisfactory, especially for transient temperature predictions. The model could then be exploited to assess process modification, and could be extended to investigate innovative processes at high-pressure. © 2006 American Institute of Chemical Engineers AIChE J, 52: 2006

Electrohydrodynamic (EHD) drying is a novel drying method used to enhance forced convective drying by using a wire-electrode to create an electrostatic field. In this study, it was hypothesized that an EHD enhanced forced convective drying process will not only increase the drying rate, but also the exergetic efficiency over time. A transient exergetic efficiency was defined as the ratio of the exergy use rate in the removing of moisture from the drying product, to the exergy rate of the drying air supplied. In the case of EHD enhanced forced convection, the exergy rate supplied by the wire electrode was also accounted for. Forced convection drying experiments were run on a test specimen simulating a food product (methylcellulose gel) using an air flow channel with and without EHD enhancement with varying air flow velocities. Initial results show that the moisutre loss rate of the methylcellulose gel increased with the application of the electrostatic field. In addition, for low velocities, the exergetic efficiency of EHD enhanced forced convection was higher for the first few hours of drying as compared to conventional forced convection. The exergetic efficiency of both conventional and EHD enhanced forced convection converged at greater air flow velocities.Copyright © 2014 by ASME

An assessment is made on the influence of external perturbations on an original minienvironment intended for food manufacturing. The minienvironment is a working unit where local displacement ventilation for clean room condition is applied. It consists of a unidirectional flow and an air curtain. Using the particle image velocimetry technique velocity measurements are initially carried out in order to describe the steady flow. Laser tomography and tracer gas experiments are then applied in order to explain the influence of sharp changes of pressure on the flow field and quantify the efficiency of the system. The corresponding results clearly indicate that the minienvironment and particularly the air curtain are strongly sensitive to perturbations. They highlight that perturbations, such as draughts or wind effects, must be taken into account in order to properly design ‘open’ minienvironments.

International audience; Solar drying is used to reduce post-harvest losses. An indirect solar dryer with oriented air is presented. It provides the ability to store energy. This work predicts the outlet temperature at the collector in the single-face and bifacial modes of collector. The heat balance applied to each element is determined taking into account the temporal term which characterises storage in the material. The simulation is done using Matlab software. Both models of the collector are validated by comparing the experimental and theoretical curves. The results show that the two curves of outlet temperatures have the same profile when the global heat transfer coefficient loss is 2.94 W.m –2 .K –1 , the contact resistance between the absorber and the insulator is 0.025 W –1 .m2.K and the velocity is 0.2 m.s –1. The contact resistance has a significant influence on the outlet air temperature but not accessible for controlling temperature compared to the velocity.

International audience; The topic of this study is related to airborne particle dynamics in indoor environments. Lab-scale experiments have been performed to investigate particle deposition velocity to six different surfaces orientations (with respect to gravity) for fully developed turbulent flow in horizontal large circular ventilation ducts. Monodispersed aerosol particles (1–6 μm) were used in the deposition experiments. A very low particle mass (40 ng) was measured reliably above background level on duct surfaces by a means of a nondestructive stencil technique associated with fluorescence analysis. For 2–6 μm particles (diffusion and impaction regime), deposition rates to floors were much greater than rates to the ceiling and greater than rates to the wall. For 1-μm particles, the effect of surface orientation to particle deposition was not significant. Results were compared to the very few similar and published studies. This work was conducted in the frame of the CleanAirNet project which aimed at producing new knowledge, models, and techniques to help controlling the safety food stuffs, through a better control of aerosol particle (bioaerosols) transport and deposition in the ventilation networks of the food industry.

Although plane air jets are often used as dynamic barriers to separate two environments, only a few works have explored their sensitivity to perturbations. We investigated the influence of sharp changes of pressure on the flow field of a device designed to avoid air-borne contamination. Laser tomography and tracer gas experiments clearly indicate that the air curtain is strongly sensitive to perturbations such as draughts. The results highlight that the control of air curtains used in open protection devices should be further investigated.

International audience; The quality of air within factory buildings is controlled by many food manufacturers. Ventilation system hygiene must be regularly cleaning to prevent the build up of dust, product or condensate that may provide a focus for microbial growth. In the present study we predict the particle deposition velocity in ventilation ducts of three different food factories. At these locations, during several days, aerosol particle number and mass size distribution were measured using optical particle counter and cascade impactor, respectively. The concentration of particulate matter of aerodynamic diameters in the size range 0.3-20 mu m varied from 0.2 to 1.7 mu g m(-3). The measured mass concentration and the predicted particle deposition velocity were used to calculate the deposited particle mass flux (DMF) in the ventilation ducts of each site. The results indicate that the DMF at the floor is about 2-20 times larger than that at vertical walls. Thus, the deposited particle mass on the floor is sufficient to check whether a ventilation duct should be cleaned. According to the chosen cleaning initiation criteria for ducts, ventilation systems would take approximately 1-9 years to meet the cleaning time. So, cleaning procedures and maintenance intervals are strongly influenced by several parameters, such as flow conditions, particle concentration, characteristics of the ventilation duct and the filter efficiency. The modeling approach combined with mass concentration measurements enables the analysis and the quantification of the influence of these parameters.

The existence of metastable phases of ice I and liquid in the domain of ice III permits the use of optimized paths for pressure shift freezing (PSF) and pressure induced thawing (PIT) processes, crossing the ice III domain, without actual crystallization of this ice polymorph. The use of higher pressures and lower temperatures before pressure release, in the case of PSF, and the use of higher pressures for PIT, increases the real temperature gradients of both processes, therefore reducing the total processing time. In this paper, a computational model has been developed to simulate heat transfer phenomena during the precooling (in PSF) and the heating (in PIT) steps of these optimized processes. The high-pressure-low-temperature (HPLT) processes are assumed to be conducted in a high-pressure tubular reactor designed to work in a semi-continuous mode on an industrial scale. Results showed that important vessel lengths (40–370 m) are needed when the sample velocity ranges from 1 to 6 m/min. These vessel lengths are feasible thanks to the modular structure of the suggested high-pressure system.

International audience; A numerical thermal model was previously developed to take into account heat transfers by convection, radiation and evaporation during the heating and cooling of a solid food product by a jet of air. It has been coupled with another model which describes the transfer of water inside the product. The response of the coupled model to a variation of the numerical parameters and the other inlet parameters has been tested. Temperature and water activity calculated at the surface of a piece of lean beef meat subjected to a constant air temperature treatment are discussed in relation to experimental results.

This work deals with the assessment of the airflow in a food-processing clean room. The flow pattern inside the working area of a pilot scale clean room was numerically investigated using a computational fluid dynamics code based on a finite volume formulation. Two versions of the k-e turbulence model were tested: the standard and the RNG version. The analysis of the velocity magnitude does not reveal sensitive differences between them. Moreover, both models well predict the main features of the flow and numerical results agree with experimental measurements. However, a further examination shows that the RNG k-e turbulence model predicts more swirls and more complex trajectories. As the standard k-e model overestimates the turbulent diffusion, the RNG version seems to be more suitable to calculate the airflow in clean rooms. The influence of initial turbulence intensity is also pointed out. Finally, the study of the airflow below a laminar flow unit confirms that the design of clean rooms can benefit from the numerical approach.

A finite difference scheme model which performs both atmospheric and high pressure thawing simulation is presented. The comparison between numerical simulation and experimental data shows a good level of agreement. This model has been used to predict the performance of the thawing process under a wide range of operation conditions. It clearly shows that high pressure thawing reduces the thawing time.