仓储粮堆内热湿耦合传递的数值模拟

本文通过理论分析与数值模拟相结合的方法,以典型吸湿性多孔介质—小麦为研究对象,根据小麦吸湿和解吸湿曲线,建立了吸湿性多孔介质内部热湿耦合传递的数学模型,通过与相关实验数据比较验证了数学模型的合理性。基于有限元的方法模拟分析了外界气温和小麦分别为273K(0℃)和293K(20℃),小麦初始水分为14%和18%时直径为10m高度为10m的充满小麦的圆柱仓内部热湿迁移过程,重点探究了近似冬季和夏季仓储粮堆内部温度和水分的动态变化规律。研究结果表明,冬季工况下水分从粮堆内部向顶部和右侧壁面迁移,聚集在相对狭窄的壁面边界附近,形成低温高水分区域,最大水分值出现在右上部侧壁面附近。夏季工况下水分从顶部和侧面向内迁移,而内部又朝右上部区域扩散,形成较宽的高温高水分区域(r <4.5m, 8.0m 相似文献   

Modelling and simulation of heat transfer by convection in aerogel treated nonwovens

Simulation and numerical modeling are becoming increasingly popular due to the ability to seek solutions for a problem without undertaking real-life experiments. For the problems of heat transfer, these techniques to generate relevant data by incorporating different changes to the input parameters. Heat transfer property of textile materials is a major concern since it influences comfort properties of clothing. In this paper, numerical simulation was applied to evaluate the heat flux, temperature distributions, and convective heat transfer coefficients of the fibrous insulating materials treated with aerogel. The computational model simulated the insulation behavior of nonwoven fabrics without and with aerogel. Ansys and Comsol were used to model and simulate heat transfer. The simulation was performed assuming laminar flow and since the Mach number was < 0.3, the compressible flow model with Mach number < 0.3 was used. The results of simulation were correlated to experimental measurements for validation. Furthermore, aerogel-treated fabric samples showed better thermal performance. Using this model, the heat transfer properties of the nonwoven fabrics treated with aerogel can be optimized further.     

Evaluation of convective heat transfer coefficient between fluids and particles in suspension as food model systems for natural convection using two methodologies

This study was conducted to evaluate the convective heat transfer coefficient between Newtonian and non-Newtonian fluids and food particles inside of a glass vessel and to assess the effect of controlled variables: convection phenomena, heating temperature, immersion fluid, and different food particles. The coefficient was evaluated from the sample temperature as a function of the heating time by two approaches: a lumped parameter method and an analytical methodology. High values corresponded to heating in Newtonian fluids (70–181, 57–248 W/m² K), and lower corresponded in non Newtonian fluids (31–169, 28–167 W/m² K). Similarly, major heat transfer coefficient corresponded to the heating of mushrooms (32–110, 28–132 W/m² K), followed by tomatoes (30–181, 35–183 W/m² K) and potatoes (31–148, 34–248 W/m² K), respectively. Fluid properties were more important than the thermal properties of the particles. This coefficient was higher at 85 °C in both types of fluids. Both methods generated satisfactory values, but the analytical approach showed more accuracy.     

Experimental study of heat transfer and air flow in a refrigerated display cabinet

The heat transfer and air flow in a vertical open refrigerated display cabinet loaded with packages of test product was studied in order to analyse unsteady state phenomena. The product and air temperatures in the curtain were measured using tiny thermocouples, air velocity using a hot wire anemometer and PIV (Particle Image Velocimetry). A comparison between the air velocities measured by these two methods shows a good agreement at the internal side of the air curtain. However, these two methods give quite different result at the external side of the air curtain which can be explained by the complex air flow in this region. The temporal variation of the air temperature inside the air curtain is due partly to the “on/off” compressor cycle and partly to the introduction of ambient air via vortices. The latter phenomena contribute to the rapid velocity fluctuations which are greatest in the mixing layer of the air curtain. The product position in the cabinet is a determining factor of its temperature.     

密闭圆筒仓内储粮自然对流及热湿耦合传递的研究

粮食作为吸湿性多孔介质,具有吸湿和解吸湿特性。一般粮食收获后,大部分时间在密闭非通风状态下自然存储。鉴于粮仓外气候条件的季节和昼夜变化,会使粮堆的温湿度发生周期变化,导致超出存储安全指数。该文将以仓储粮堆内局部热湿耦合传递过程作为研究对象,借助多物理场数值模拟软件(COMSOL)进行数值模拟,研究近似冬夏季工况下粮堆内温湿度的动态变化规律,充分考虑了仓储粮作为吸湿性多孔介质具有复杂的热源、湿源。     

仓储粮堆内部自然对流和热湿传递的数学分析及验证

本文基于仓储粮堆内部自然对流、热湿耦合传递的数学模型,采用数学分析的方法对模型中各个方程中的各项的物理意义和数量级大小进行了分析,探讨了仓储粮堆内部自然对流、热量传递和水分迁移过程的相互关系。提出了判断粮堆内部自然对流强弱的瑞利数及其影响因素,分析了仓型结构、粮种及仓外大气温度对粮堆内部自然对流、热量传递和水分迁移的影响,并通过数值模拟对数学分析结果进行了验证。结果表明,数学分析方法是分析仓储粮堆内部自然对流、热量传递和水分迁移过程的一种有效途径,数学分析的结果可以为仓型设计、储粮生态系统的模拟、仓储技术管理提供借鉴。     

Simulation of heat transfer for solid–liquid food mixtures in cans and model validation under pasteurization conditions

During processing of canned mixtures of solid–liquid foods, conduction and convection occur simultaneously. The literature lacks in a complete simulation study where a large number of solids are dispersed in the liquid phase, e.g. canned peas. Therefore, the objectives of this study were to determine temperature changes inside a can containing solid–liquid food mixtures. For this purpose, dispersed stationary solids (canned peas in water) in a 2D (axi-symmetrical) configuration were applied. Ansys V11 (Ansys Inc., Canonsburg, PA) was used to solve continuity, energy and momentum equations. For experiments, canned pea samples were prepared in 500 g cans, and heating process was conducted in a retort under pasteurization conditions at ≈98 °C. Temperature changes were measured using needle type thermocouples, and simulation results were validated against experimental data. This study is expected to be a significant contribution to the literature for further optimization studies and to form basis of an industrial project to improve canning process of solid–liquid mixtures.     

Study of water evaporation and condensation in a domestic refrigerator loaded by wet product

This study was carried out to gain a better insight into evaporation and condensation phenomena due to natural convection in a domestic refrigerator. An experiment was undertaken in a refrigerator loaded with 40 humid plaster cylinders (50 mm diameter). The weight variation of the cylinders was followed over 12 days and the rate of water evaporation (or condensation for some cylinders) was calculated. This measured experimental rate was compared with one obtained from a CFD simulation which took into account air flow by free convection, heat transfer (convection, conduction and radiation) and mass transfer (water evaporation and condensation). The position of high evaporation or condensation rate was well predicted despite the fact that the simulation most often underestimated the experimental values. Near the cold wall where condensation was observed (or near the warm wall where evaporation was observed), the cylinder temperature was lower (or higher) than that of air. This demonstrates the importance of the radiative exchanges between the walls and the products inside a refrigerator which in turn can explain condensation or dehydration phenomena.     

Prediction of thermal resistance of woven fabrics. Part II: Heat transfer in natural and forced convective environments

The aim of the paper is to observe the effects of convection on the total heat transfer of the fabric. Convective heat transfer through textiles can be simulated with the help of computational fluid dynamics (CFD). The first part of this paper has dealt with the mathematical model for predicting the conductive and radiative heat transfer. The second part of the paper describes the use of CFD to simulate the wind tunnel of the instrument. The fabric has been subjected to natural and forced convection. The convective heat transfer coefficients obtained from the simulation were used to find the resistance due to convection. The final values obtained from the model were compared with the values obtained from the thermal resistance measuring instrument fabricated to measure the thermal resistance in natural and forced convective modes. The mathematical model gave excellent prediction in forced as well as natural convective mode when coupled with CFD results.     

Investigation and modelling of natural convection and conduction heat exchange: study on food systems with modified starch by means of computational fluid dynamics

This study is focused on the effects of starch gelatinisation on heat exchange in food systems containing four modified starch concentrations (0, 2, 6 and 10%). Viscosity profiles of samples were experimentally measured: the effect of gelatinisation was evident, particularly at 6% and 10%, where the viscosity increased from 0.010 to 70 Pa s and to 1507 Pa s, respectively. The heat exchange rate showed a decrease with the increasing of starch concentration, and the effects were observed until 6%. Four Computational Fluid Dynamics (CFD) models were also developed, experimentally validated (RMSE < 1.5 °C) and used to study the heat exchange. Velocity profiles showed that the convective flows slowed down from 2.5 to 0 mms^?1 after the gelatinisation. Finally, the effects on the slowest heating/cooling zone (SHZ/SCZ) location in the CFD models were studied: at 0% and 2%, the SHZ settled 15 and 80 mm from the bottom of jars in heating and cooling phase, respectively. At 6% and 10%, before the starch gelatinisation phase, the SHZ was located in a similar position of the 0% and 2% while when the gelatinisation occurred the SHZ slightly moved towards the geometric centre (50 mm) as for an only‐conductive product.     

Progress of ice slurry in food industry: application,production, heat and mass transfer

Clean label foods have gained a lot of attention because it satisfies consumers' interest in minimally processed foods with minimal ingredients and no artificial additives. The growing concern about food quality has stimulated the integration of ice slurry technology in food processing. An ice slurry is a two-phase system colding medium composed of ice crystals, liquid water and some additives. Ice slurry with ice fraction of 5%–30% was 5–6 times than that of conventional chilled water under the same conditions. The appearance of the food is less likely to be damaged by the ice slurry while also being able to cool down quickly compared to traditional ice preservation methods. It further extends the shelf life of food products. The purpose of this review is to provide an understanding and practical suggestions for the application of ice slurry in the food industry. The methods of producing ice slurry and their advantages and disadvantages are reviewed, and the current applications and heat and mass transfer characteristics of ice slurry in the food industry are also discussed. Finally, the current challenges and prospects for ice slurry production and application were pointed out.     

Experimental study and modelling of the residence time distribution in a scraped surface heat exchanger during sorbet freezing

Scraped surface heat exchangers (SSHEs) are widely used for crystallization applications in several food processes (i.e. crystallization of margarine, tempering of chocolate, freezing of ice cream and sorbet). The final quality of these food products is highly related to crystal size distribution and apparent viscosity, both of which are determined by the operating conditions of the process. During the freezing of sorbet, the increase in the ice volume fraction leads to an increase in the apparent viscosity of the product. This effect modifies the fluid flow behavior, the residence time distribution (RTD) and the temperature profile inside the equipment. This work aimed at studying the influence of the operating conditions on the RTD and the axial temperature profile of the product in a SSHE, so as to characterize the product flow behavior. RTD experiments were carried out in a continuous laboratory pilot-scale SSHE by means of a colorimetric method. Experiments showed that high product flow rates led to a narrowing of the RTD and thus to less axial dispersion, due to the enhancement of the radial mixing with the decrease in the apparent viscosity of the product. Spreading of the RTD was obtained for lower refrigerant fluid temperatures, due to a higher radial temperature gradient between the wall and the center of the exchanger, leading to a higher gradient of the apparent viscosity. This effect increased the difference in axial flow velocities and thus the axial dispersion. These results can be useful for the optimization and modelling of crystallization processes in SSHEs.     

Pressure and temperature stability of water-soluble antioxidants in orange and carrot juice: a kinetic study

The stability of water-soluble antioxidant in orange and carrot juice was studied on a kinetic basis during thermal (75–120 °C) and high pressure (100–800 MPa; 30–65 °C) treatments. The water soluble antioxidant capacities of both orange and carrot juice, determined using the TEAC number, increased for the first 60 min and 90 min of thermal treatment respectively. These increases can be described by a first order reaction model. The increases in antioxidant capacity became sharper as the temperature was increased, and the estimated Ea values were 79.00±7.83 and 40.46±8.43 kJ mol^–1 respectively for orange and carrot juice.During the high pressure treatments, a decrease in TEAC number was noticed for the antioxidant capacity of orange juice, and an increase in TEAC number for carrot juice. A first order reaction model was applied to estimate the rate constants. At all of the temperatures studied, the antioxidant capacity of orange juice decreased more quickly as the pressure was increased. On the other hand, the increase in the antioxidant capacity of carrot juice was slowed down by a pressure increase at temperatures above 40 °C.     

An iterative sensory procedure to select odor-active associations in complex consortia of microorganisms: application to the construction of a cheese model

The aim of this study was to develop and validate an iterative procedure based on odor assessment to select odor-active associations of microorganisms from a starting association of 82 strains (G1), which were chosen to be representative of Livarot cheese biodiversity. A 3-step dichotomous procedure was applied to reduce the starting association G1. At each step, 3 methods were used to evaluate the odor proximity between mother (n strains) and daughter (n/2 strains) associations: a direct assessment of odor dissimilarity using an original bidimensional scale system and 2 indirect methods based on comparisons of odor profile or hedonic scores. Odor dissimilarity ratings and odor profile gave reliable and sometimes complementary criteria to select G3 and G4 at the first iteration, G31 and G42 at the second iteration, and G312 and G421 at the final iteration. Principal component analysis of odor profile data permitted the interpretation at least in part, of the 2D multidimensional scaling representation of the similarity data. The second part of the study was dedicated to 1) validating the choice of the dichotomous procedure made at each iteration, and 2) evaluating together the magnitude of odor differences that may exist between G1 and its subsequent simplified associations. The strategy consisted of assessing odor similarity between the 13 cheese models by comparing the contents of their odor-active compounds. By using a purge-and-trap gas chromatography-olfactory/mass spectrometry device, 50 potent odorants were identified in models G312, G421, and in a typical Protected Denomination of Origin Livarot cheese. Their contributions to the odor profile of both selected model cheeses are discussed. These compounds were quantified by purge and trap-gas chromatography-mass spectrometry in the 13 products and the normalized data matrix was transformed to a between-product distance matrix. This instrumental assessment of odor similarities allowed validation of the choice of G312 as the best 10-strain ecosystem.     

Mathematical modeling of the heat and mass transfer in a stationary potato sphere impinged by a single round liquid jet in a hydrofluidization system

The hydrofluidization (HF) is a method of chilling and freezing of foods which consists in a circulating system that pumps a refrigerating liquid upwards through orifices or nozzles, creating agitating jets. The objectives of this work were to develop a mathematical model to represent the combined heat and mass transfer between a food and a refrigerant liquid medium in a HF system and to validate the model using a single stationary sphere of food impinged by a single round jet of liquid. The food domain consisted of a rigid solid matrix, a liquid phase and the ice phase. Transport equations were applied to each phase and solved by a control-volume approach. The transport phenomena in the fluid domain were studied by computational fluid dynamics. The surface heat transfer coefficients obtained from fluid flow simulations were used to model the heat and mass transfer inside the food. Experimental central temperature and average solute uptake profiles were obtained when potato spheres were placed in a HF system using a NaCl-water as refrigerant and considering different temperatures (−10 and −15 °C), flow rates (1 and 3 L min⁻¹), and orifice-sphere distances (1 and 5 cm). The predicted values agreed well with the experimental ones, the maximum root mean square errors being 3.3 g NaCl kg⁻¹ potato and 2.9 K for the average solute concentration and temperature profiles studied, respectively. The simulations improved the understanding of the HF process and it may help to study and control different operation scenarios.     

Improving parlor efficiency in block calving pasture-based dairy systems through the application of a fixed milking time determined by daily milk yield and milking frequency

Adjusting end-of-milking criteria, in particular applying a maximum milking time determined by expected milk yield at an individual milking session, is one strategy to optimize parlor efficiency. However, this strategy can be difficult to apply practically on farm due to large differences in session milk yield, driven by milking interval, which affects milking routines and can be limited by in-parlor technology. The objective of this study was to test the hypothesis that a single fixed milking time (duration) could be applied at all milking sessions without compromising milk production or udder health for a range of milking intervals. To test the hypothesis, 4 experimental herds were established: (1) herd milked twice a day (TAD) using a 10- and 14-h interval, (2) herd milked TAD using an 8- and 16-h interval, (3) herd milked 3 times in 2 d using a 10–19–19-h interval, and (4) herd milked once a day (OAD). Herds consisted of 40 cows each, and were established for two 6-wk experimental periods, one in peak lactation and the other in mid-late lactation. Within each herd, half the cows had an end-of-milking criterion of 0.35 kg/min (Flow), and the other half had milking ended after a fixed period of time (FixedT) based on the average milking session yield, the daily milk yield divided by average number of milkings per day, irrespective of milking interval. We found no differences in daily milk yield between end-of-milking criteria due to residual milk from one milking likely increasing the proportion of milk in the udder cistern at the next milking session for the FixedT treatment. However, fat yield was compromised when the percentage of the herd with a truncated milking exceeded an estimated 33% at a milking session, which occurred in the TAD 8–16 herd due to the divergence from the average milking interval (in the case of TAD, 12–12 h). Applying a fixed milking time had no detrimental effects on udder health, except in the OAD herd in mid-late lactation, which had both a higher cell count and new intramammary infection rate. This warrants further investigation, although the majority of cultured bacteria were coagulase-negative staphylococci (CNS). Consequently, we conclude that, in general, with appropriate monitoring (e.g., weekly inspection) to ensure the proportion of the herd with truncated milkings does not exceed 33%, farmers in pasture-based dairy systems can use a fixed milking time to improve parlor efficiency.     

Potato peel extract as a natural antioxidant in chilled storage of minced horse mackerel (Trachurus trachurus): Effect on lipid and protein oxidation

The present work was undertaken to examine the utilisation of potato peel, a waste material, as a source of natural antioxidants for retarding lipid and protein oxidation in minced mackerel. Mackerel mince with two different concentrations (2.4 or 4.8 g/kg) of water or ethanol extracts of potato peel and a control with no added extracts were prepared. The samples were stored at 5 °C for 96 h and the sampling was done at time points 0, 24, 48 and 96 h. The ethanol extracts, which contained high amounts of phenolic compounds, was found to be very effective in retarding lipid and protein oxidation as it resulted in low levels of peroxide value, volatiles, carbonyl compounds and protected against the loss of α-tocopherol and tryptophan and tyrosine residues. Water extracts was less efficient especially at higher concentrations, which might be due to lower phenolic content or due to the pro-oxidative nature of some of the phenolic acids/co-extracted compounds.     

Experimental study of hydrodynamics in a flat ohmic cell—impact on fouling by dairy products

This study highlights the link between hydrodynamics and fouling phenomena in a continuous rectangular ohmic cell. The hydrodynamic study was carried out using flow visualisation techniques and particle image velocimetry (PIV). The distribution of deposits in the ohmic cell was investigated by heating an aqueous solution of β-lactoglobulin-xanthan gum mixture. Experimental results show that the deposit distribution on the electrode surfaces is directly related to the flow structures in the ohmic cell.     

Advanced topics in the application of the WLF/free volume theory to high sugar/biopolymer mixtures: a review

The vitrification properties of mixtures of several biopolymers with sugars have been investigated in this laboratory using the technique of small deformation dynamic oscillation. It was demonstrated that the free volume followed a reciprocal relationship with the number average molecular weight of the biopolymer, which should be associated with imperfect intermolecular associations around the ends of the molecules. The effect of free volume on the kinetics of vitrification is quantified by the friction coefficient (ζ_o) per monomer unit of the biopolymer. The magnitude of the coefficient ζ_o increases with the extent of intermolecular associations and enhanced network cohesion diminishes the free volume thus elevating the rheological glass transition temperature (T_g). The approach was also applied to high solids starch systems where moisture loss owing to heating above 100°C results in viscoelastic properties characteristic of the glass transition region. These can be described by the Williams-Landel-Ferry equation modified with a ‘moisture term’. For all samples, the temperature dependence of shear moduli was described by the method of reduced variables. The generated shift factors (a_T) were used to engineer a state of ‘iso-free-volume’ thus demonstrating that, within the glass transition region, viscoelastic rate processes are solely governed by free volume. Practical applications include the construction of a state diagram for food materials, which utilizes the rheological T_g to follow the concentration dependence of the metastable vitrification events.