Computational fluid dynamics modeling of bread baking process

A computational fluid dynamic (CFD) model was developed to study the temperature and browning profile of bread. This study differs from previous work of CFD modeling reported in literature in that phase change during evaporation as well as evaporation-condensation mechanism during baking process was incorporated in this model. Simulation results were validated with experimental measurements of bread temperature at three different positions. In this study crumb temperature does not cross 100 °C due to incorporation of evaporation-condensation mechanism in this model. Baking process completes within 25 min of processing time once the temperature of crumb becomes stable at 98 °C. Formation of crust and browning of bread surface were studied using earlier reported kinetic model and it predicted more browning at bread edges than the surfaces. However, predicted browning index was well within the range (< 52).     

化纤技术中的计算流体力学

计算流体力学有助于节省工艺开发时间和资源,因此具有很大的经济效益。有多少工艺,这种方法就可以有多少种应用,并根据不同问题,使用不同详细设计的模型,研究简单的层流及复杂的相互作用。概述了计算流体力学目前在化纤技术中的应用,也介绍了一些实验验证了的可能性。     

Computational fluid dynamics (CFD) modeling of an electrical heating oven for bread-baking process

Radiation is the most dominant heat transfer mode in an electrical heating oven. A 3D CFD model for an electric heating baking oven was developed. Three different radiation models namely, discrete transfer radiation model (DTRM), surface to surface (S2S) and discrete ordinates (DO) were employed for the simulation of the electrical baking oven. All models predicted almost similar results, which tallied well with the experimental measurements. During the full heating cycle, the oven set-point temperature was reached after 360 s. Lower temperature zones occurred near oven wall due to lower air flow. Based on preliminary evaluation of applicability, the DO radiation model was selected for bread baking simulation and validated with the experimental measurement of bread temperature. Bread simulation was carried out to study the profiles of temperature and starch gelatinization of crust and crumb of the product. This study indicated the baking process to be complete at 1500 s when the temperature of bread-center reached 100 °C.     

Computational fluid dynamics modeling of mass transfer for aroma compounds recovery from aqueous solutions by hydrophobic pervaporation

In this work, a mass transfer study on aroma compound recovery by pervaporation process was performed using a model based on solution–diffusion theory. CFD method was employed to solve the governing mass transfer equations by considering the flux coupling. Concentration profiles of penetrants inside the membrane as well as permeation flux and aroma permeate concentration through the membrane were determined. The modeling results were validated by the experimental data obtained for pervaporative recovery of isopentyl acetate and n-hexanol from their binary aqueous solutions with composite PDMS membrane. The influence of key operating parameters such as feed aroma concentration and temperature on the flux and permeate concentration was also investigated theoretically and experimentally. The results showed that the aroma permeate concentration as well as the total and partial fluxes increased with an increase in the feed aroma content and feed temperature. The predicted results were in good agreement with experimental data.     

Computational fluid dynamics studies on pasteurisation of canned milk

Pasteurisation is a vital unit operation to inactivate the spoilage organisms and enzymes present in the milk. Computational fluid dynamics (CFD) modelling can be used as a tool to determine the temperature distribution pattern inside canned milk during thermal processing. In this study, a CFD model was developed for the pasteurisation process of canned milk at 74°C and 85°C and validated with the experimental measurements. Moreover, the effect of can rotation (5, 50 and 100 rpm) on processing time and pasteurisation value was investigated. This study predicted rotation of can with 5 rpm was more effective than stationary positioned can during pasteurisation.     

Computational fluid dynamics investigation of turbulence models for non-newtonian fluid flow in anaerobic digesters

In this paper, 12 turbulence models for single-phase non-newtonian fluid flow in a pipe are evaluated by comparing the frictional pressure drops obtained from computational fluid dynamics (CFD) with those from three friction factor correlations. The turbulence models studied are (1) three high-Reynolds-number k-ε models, (2) six low-Reynolds-number k-ε models, (3) two k-ω models, and (4) the Reynolds stress model. The simulation results indicate that the Chang-Hsieh-Chen version of the low-Reynolds-number k-ε model performs better than the other models in predicting the frictional pressure drops while the standard k-ω model has an acceptable accuracy and a low computing cost. In the model applications, CFD simulation of mixing in a full-scale anaerobic digester with pumped circulation is performed to propose an improvement in the effective mixing standards recommended by the U.S. EPA based on the effect of rheology on the flow fields. Characterization of the velocity gradient is conducted to quantify the growth or breakage of an assumed floc size. Placement of two discharge nozzles in the digester is analyzed to show that spacing two nozzles 180° apart with each one discharging at an angle of 45° off the wall is the most efficient. Moreover, the similarity rules of geometry and mixing energy are checked for scaling up the digester.     

Computational fluid dynamics (CFD) analysis of pipeline in the food pellets cooling system

Pellet cooling-drying machine with a pneumatic conveying system is used to transfer heat and moisture of hot and moist pellets in the food and animal feed plants. During heat transfer along the pipeline, very fine pellets and flour is sucked in to the pipe and settling phenomenon is accrued. The sediment of fine particles and pellets leads to the erosion and wear in the pipeline especially at the warm and humid condition. In the present research, the reported problem in Faradane Feed Processing Company, consisting of the erosion of facilities and settling in the pipeline of pellet cooling-drying system was analyzed. A computational fluid dynamics modeling (CFD) based on the Euler-Euler method was developed to investigate the particle distribution and settling inside the vertical and horizontal pipeline as a function of pipeline diameter, particle size, flow velocity, and inlet volume fraction of particles. The results indicated that, the particle sediment along the vertical pipeline was lower than the horizontal pipeline and particle size had the greatest influence on the distribution and position of settling. Increasing of particle size from 62 μm to 250 μm was leaded to the rise in the particle sediment up to 54%.     

Computational fluid dynamics analysis for process impact assessment during thermal pasteurization of intact eggs

Transient temperature and albumen velocity profiles during thermal pasteurization of intact eggs were studied using a commercial computational fluid dynamics (CFD) package. Simulated temperature profiles were in close agreement with experimental data for eggs of different sizes. Convective heat transfer only occurred in the egg white fraction, and conductive heat transfer only occurred in the yolk. For process assessment, a generally accepted kinetic inactivation model for Salmonella Enteritidis was incorporated into the CFD analysis. Minimum process times and temperatures needed to provide equivalent pasteurization at 5-log reductions of the target microorganism were obtained on a theoretical basis. The combination of CFD analysis and inactivation kinetics can be very useful for assessing pasteurization of intact eggs and can enable processors to gain a better understanding of these processes and to establish process conditions for consumer-safe eggs.     

三维流体力学预测风机不同布置形式对冷库气流的影响

我国冷库的建设数量近年来增长迅速,可多数冷库中的气流分布并不合理,而风机的布置形式是影响冷库中气流分布的一个重要因素。运用SIMPLE算法,结合波兴涅克（Boussineq）假设,对目标冷库中不同的冷风机安装形式进行三维数值模拟。模拟结果显示目标冷库中采用两个三出风口的风机吹风比采用三个两出风口的风机吹风所引起的气流场均匀性更好;同时针对后者造成的射流偏移,对风机位置进行改变后得出了冷库中的流场较改变前均匀性提高。数值模拟的结果对中型冷库和大型冷库中风机的安装形式和安装位置提供了参考依据。     

新型搅拌桨组合在非牛顿流体中的传氧性能研究

为了研究新型搅拌桨组合在非牛顿流体中的传氧性能,在水和黄原胶溶液中,通过对体积传氧系数、气含率和功率的测定,对4种搅拌桨组合的传氧、气体分散和功率消耗性能进行比较。结果表明：在非牛顿流体中,抛物线圆盘桨＋翼型轴流桨＋翼型轴流桨耗能最少（Np≈1．7）,节能66％;在输入相同功率下传氧性能和气体分散性能明显好于其他三组桨,相对于小直径平直叶圆盘桨＋小直径平直叶圆盘桨＋小直径平直叶圆盘桨,体积传氧系数和气含率分别提高了45％、35％。因此,适用于非牛顿流体的组合桨,此研究结果可为工业用多层组合桨在非牛顿流体中的应用提供参考。     

固定床生物反应器气流场的仿真模拟

针对固定床生物反应器通风制曲的实际,运用连续方程和纳维-斯托克斯方程,建立流场数学模型并简化为二维模型,在GAMBIT中建立实体模型,选择FLUENT5/6求解器划分网格,网格选用四边形网格,在模型的入口与出口处对网格进行细化,把网格导入FLUENT中,选择隐式分离求解器、标准的K-ε湍流模型,经过200次迭代,使模型收敛。根据仿真结果,讨论了固定床生物反应器风道入口的截面形状对气流场的影响,对固定床生物反应器的结构设计提供了理论基础。     

Degradation of waste gas containing toluene in an airlift bioreactor

Suspension microorganisms in an internal-loop airlift bioreactor were utilized to treat waste gas containing toluene. The working volume of the reactor was 35 L, and the biomass concentration was 3 kg/m3. The gas pollutant flowed into the reactor from the bottom; it then transferred from the gas phase to the liquid phase and was degraded by the microorganisms suspended in the liquid phase. The microorganisms were able to degrade 50-90% of the inlet toluene when its concentration was from 0.5 to 10 g/m3, and the superficial gas velocity ranged from 0.15 to 1.23 cm/s. A comprehensive mathematical model was also developed to describe the overall degradation process of toluene in the internal-loop airlift bioreactor. The overall degradation process included gas flow, gas-liquid mass transfer, flow and dispersion of the liquid phase, and microbial kinetics. The hydrodynamic properties including the gas and liquid superficial velocities, the gas holdup, the volumetric mass transfer coefficients of toluene and oxygen, and the microbial kinetics were obtained for this model. The substrate inhibition theory was used to simulate the microorganism growth kinetics, and its kinetic constants were obtained experimentally. The penetration theory was used to predict the volumetric mass transfer coefficient. And the drift-flux theory was used to predict the hydrodynamic properties in each section (riser, gas-liquid separator, downcomer, and bottom) of the airlift bioreactor. The concentration distributions of toluene and oxygen in the airlift bioreactor and the removal efficiency of toluene predicted by the mathematical model agreed well with the experimental data.     

Effect of mixing time on taxoid production using suspension cultures of Taxus chinensis in a centrifugal impeller bioreactor

The effects of fluid mixing on the cell growth and secondary metabolite production of plant cells were investigated in a low-shear centrifugal impeller bioreactor (CIB) system. Suspension cultures of Taxus chinensis cells producing taxuyunnanine C (Tc), a physiologically active secondary metabolite, were used as a model system for this investigation. The mixing time (t(m)) and volumetric oxygen transfer coefficient (k(L)a) in the bioreactor were characterized at various cell densities and operating conditions. A constant t(m) of 5 s or 10 s was maintained during cultivation by adjusting the impeller agitation speed with no detrimental effect on the cultured cells. A higher cell density, Tc content and total Tc production were obtained under the shorter mixing time of 5 s. The favorable effect of more rapid mixing on Tc production was also confirmed when the Tc accumulation was significantly increased through culture elicitation using 100 microM methyl jasmonate (MJA). The lower Tc production at the longer t(m) of 10 s was mainly attributed to oxygen transfer limitation in the dead zones and larger cell aggregates resulting from poor mixing.