Airflow blockage and guide technology on energy saving for spiral quick-freezer

The effect of airflow blockage and guide technology on energy saving for spiral quick-freezers were investigated by simulating and analyzing the airflow field and measuring of the velocity distribution in the freezing zone for different designs. The k– turbulence model was used. The velocities and temperatures of the air in the freezing zone for different designs of airflow blockage and guide boards were measured. The study shows that the airflow pattern plays a key role on energy efficiency, freezing time, and production rate. In the study case, through the optimization of the airflow blocking boards and the guide boards, the average air velocity in the freezing zone would be enhanced to 2.5–2.7 times compared with the original design. Correspondingly for bean curds in a stationary condition, the freezing time would be shortened by 78–85%, energy efficiency and the production rate would be increased by approximately 18–28% individually.     

Predicting local surface heat transfer coefficients by different turbulent k-ε models to simulate heat and moisture transfer during air-blast chilling

A numerical investigation using a computational fluid dynamics (CFD) code is carried out to predict the turbulent flow field, and heat and moisture transfer in a three-dimensional air-blast chiller with cooked meats of cylindrical and elliptical shapes. Three turbulent models [standard, low Reynolds number (LRN) and RNG k-ε model] have been used in these simulations. Based on local heat transfer coefficients on the surface of the meat calculated by CFD code, the unsteady heat and mass transfer were simulated which took into account of the effects of conduction within the meat, forced and natural convection, radiation and moisture evaporation on the surface of the cooked meat joint. The model allowed the simultaneous CFD prediction of both temperature distribution and weight loss of the meat throughout the chilling process. Good agreement with experimental results was obtained. The effect of using different models on the accuracy of the simulation of local heat transfer coefficient is presented.     

Analysis of the air flow in a cold store by means of computational fluid dynamicsAnalyse du débit d'air dans un entrepôt frigorifique à l'aide de la dynamique des fluides informatisée

Airflow inside a cold store is investigated using computational fluid dynamics. The airflow model is based on the steady state incompressible, Reynolds-averaged Navier–Stokes equations. The turbulence is taken into account using a k− model. The standard as well as the Renormalisation-Group (RNG) version of the k− model is investigated. The forced-circulation air cooler unit is modelled with an appropriate body force and resistance, corresponding to the characteristics of the fan and the tube-bank evaporator. The finite volume method of discretisation is used. The validation of the model has been performed by a comparison of the calculated time-averaged velocity magnitudes with the mean velocities measured by means of a hot-film type omni-directional velocity sensor. A relative error on the calculated air velocities of 26% was observed. The RNG k− model does not help to improve the prediction of the recirculation. Both a finer grid and enhanced turbulence models are needed to improve the predictions.     

CFD modeling of dilute phase pneumatic conveying in a horizontal pipe using Euler–Euler approach

ABSTRACT

Computational fluid dynamics simulations were performed to investigate the behavior of dilute phase pneumatic conveying of plastic pellets in a horizontal circular pipe. The pellets are 200?µm in diameter and 1000?kg/m³ in density. A parametric study was performed to investigate the effects of turbulence model and model collision parameters on pressure drop, solid’s volume fraction and velocity profiles. Among model collision parameters, specularity coefficient has considerable effect on the pressure drop. Moreover, the results from simulations carried out for different solid loadings and velocities were compared with experimental data found in the literature. The air velocities range from 6 to 15?m/s and solids to air mass flow ratios range from 1 to 3. At higher air velocities, the pressure drops predicted by the standard k-omega turbulence model are higher than the pressure drops predicted by the standard k-epsilon model. In contrast, at lower gas velocities, the standard k-epsilon model predicts higher pressure drops compared to the standard k-omega turbulence model. However, no significant difference in solids and air velocity profiles is observed for the two different turbulence models.     

Effect of the airflow rate of a ceiling type air-conditioner on ventilation effectiveness in a lecture room

We performed a study on the effect of the discharge airflow rate of the ceiling type air-conditioner on ventilation performance in the lecture room with the mixing ventilation. The experiments and CFD were conducted for analyzing ventilation performance. The concepts of mean air age and indoor CO₂ concentration were used for evaluating ventilation performance. We made the CO₂ generation model in the simulation and calculated a lot of cases with respect to the airflow rate of air-conditioner and the mechanical ventilation rate. And the selected experimental measurements were performed in the lecture room of the same layout as the numerical one for verifying simulation results. Mean air age is gradually increased, but CO₂ concentration is oppositely decreased in the occupied zone with the increment of the discharge airflow rate of the ceiling type air-conditioner. This result shows that both mean air age and residual life time must be considered for evaluating ventilation performance when the contaminants are generated indoors. And the increment of discharge airflow of the ceiling type air-conditioner can induce the piston effect and push the contaminants out of the occupied zone. From this result, it is found out that ventilation performance can be increased when the momentum source like an air-conditioner is used in the room with the mixing ventilation.     

Calculations of the temperature and flow field in a room ventilated by a radial air distributor

In ventilated or air-conditioned rooms optimal conditions of temperature, humidity and air velocity are required. In the present study the behaviour of a jet emerging from a radial plate distributor and the resulting air flow in the room were investigated. To predict the behaviour of the air flow a numerical scheme was used to solve the conservation equations for mass, momentum and energy with the k/ε-turbulence model. The numerical results are compared with available experimental data.     

Airflow patterns in an enclosure loaded with slotted pallets

A reduced scale model and CFD predictions are used to investigate experimentally and numerically the airflow patterns within a refrigerated truck loaded with slotted but empty pallets. Air velocity measurements are carried out on a reduced scale model with a Laser Doppler Anemometer above and inside the pallets. The numerical predictions obtained with Computational Fluid Dynamics package using the RSM turbulence model showed a satisfactory agreement with experimental data in high velocity zones. An approach was developed to evaluate the local ventilation efficiency and the fresh air within the pallets. The flow rate through the last pallet is about 35 times smaller than for the five first pallets. In terms of fresh air the difference is not so high; the equivalent fresh airflow rate through the last pallet is about six times smaller than for the first pallets.     

CFD model of the airflow, heat and mass transfer in cool stores

A transient three-dimensional CFD model was developed to calculate the velocity, temperature and moisture distribution in an existing empty and loaded cool store. The dynamic behaviour of the fan and cooler was modelled. The model accounted for turbulence by means of the standard k-ε model with standard wall profiles. The model was validated by means of velocity, air and product temperature. An average accuracy of 22% on the velocity magnitudes inside the empty cold store was achieved and the predicted temperature distribution was more uniform than predicted. In the loaded cold store, an average accuracy of 20% on the velocity magnitudes was observed. The model was capable of predicting both the air and product temperature with reasonable accuracy.     

Frost formation on a cold surface under turbulent flow

This study presents a mathematical model to predict the frosting behavior on a cold surface under turbulent flow. The model consists of the standard κ–ε model for turbulent flow and the diffusion equation for the frost layer. The numerical results show that turbulent flow promotes the growth of the frost layer on the cold surface, compared to the laminar flow. Increase in air velocity has little effect on mass transfer under turbulent flow, while frost growth under laminar flow is influenced by the air velocity. With constant air humidity, the frost layer thickness increases with decreasing air temperature, while the relationship for the frost density is reversed. The effect of the air temperature on the mass flux is negligible, compared to the other frosting parameters.     

Analysis of Turbulence Models and Investigation of the Structure of the Flow in a Hydrocyclone

A numerical investigation of the structure of the flow in a hydrocyclone has been carried out on the basis of Reynolds equations with the use of various models of turbulence: the k– model, the k– RNG (ReNormalization Group) model, the k– model corrected for the Richardson number Ri, and the k– model. It is shown that the distributions of the velocities and pressure in a hydrocyclone obtained with the k– Ri model, in which the influence of the rotation of the flow on the processes of generation/dissipation of turbulence and the anisotropy of the turbulent pulsations are taken into account, coincide most closely with the experimental ones.     

Surface Oscillations of an Electromagnetically Levitated Droplet

The natural oscillation frequency of freely suspended liquid droplets can be related to the surface tension of the material, and the decay of oscillations to the liquid viscosity. However, the fluid flow inside the droplet must be laminar to measure viscosity with existing correlations; otherwise the damping of the oscillations is dominated by turbulent dissipation. Because no experimental method has yet been developed to visualize flow in electromagnetically levitated oscillating metal droplets, mathematical modeling can assist in predicting whether or not turbulence occurs, and under what processing conditions. In this paper, three mathematical models of the flow: (1) assuming laminar conditions, (2) using the k−ɛ turbulence model, and (3) using the RNG turbulence model, respectively, are compared and contrasted to determine the physical characteristics of the flow. It is concluded that the RNG model is the best suited for describing this problem when the interior flow is turbulent. The goal of the presented work was to characterize internal flow in an oscillating droplet of liquid metal, and to verify the accuracy of the characterization by comparing calculated surface tension and viscosity values to available experimental results.     

Large eddy simulation of a nonisothermal turbulent jet flowing out into submerged space

Large eddy simulation is performed of a subsonic nonisothermal turbulent jet flowing out from a round nozzle into submerged space. The Navier-Stokes equations filtered over space and the RNG model of subgrid-scale viscosity are used for describing the flow. The calculations are performed for different values of the degree of jet preheating. The processing of the results of numerical simulation enables one to obtain the distributions of correlation moments of fluctuations of density, velocity, and temperature along the axis and in cross sections of jet flow. The calculation results are compared with the available data obtained using the solution of Reynolds-averaged Navier-Stokes equations and equations of the k-? model of turbulence, as well as with the data of physical experiment.     

空调房间四种通风方式对污染物分布影响数值模拟与分析

采用RNG k-ε模型对4种侧送风方式下的空调房间进行数值模拟,对室内污染物衰减结果及污染物浓度场进行分析,并通过示踪气体衰减方法计算室内通风效率.结果表明当送回风口位于同侧且在同一垂直面上时室内空气品质最好.     

Application of a sensitivity equation method to the <Emphasis Type="Italic">k</Emphasis>–<Emphasis Type="Italic">ε</Emphasis> model of turbulence

In this paper, we present some examples of sensitivity analysis for flows modeled by the standard k–ε model of turbulence with wall functions. The flow and continuous sensitivity equations are solved using an adaptive finite element method. Our examples emphasize a number of applications of sensitivity analysis: identification of the most significant parameters, analysis of the flow model, assessing the influence of closure coefficients, calculation of nearby flows, and uncertainty analysis. The sensitivity parameters considered are closure coefficients of the turbulence model and constants appearing in the wall functions.     

某洁净车间流场的数值模拟与分析

对某电子厂洁净车间流场进行了数值模拟和测试。采用标准k-ε两方程的湍流模型和SIMPLE算法,模拟了洁净车间内的温度分布;将计算结果与测试值进行了对比,验证了模型。对洁净车间气流分布细节和温度分布进行了预测,为洁净室空调系统气流组织的优化设计提供参考。     

Comparison of CFD analysis to empirical data in a commercial vortex tube

This paper presents a comparison between the performance predicted by a computational fluid dynamic (CFD) model and experimental measurements taken using a commercially available vortex tube. Specifically, the measured exit temperatures into and out of the vortex tube are compared with the CFD model. The data and the model are both verified using global mass and energy balances. The CFD model is a two-dimensional (2D) steady axisymmetric model (with swirl) that utilizes both the standard and renormalization group (RNG) k-epsilon turbulence models. While CFD has been used previously to understand the fluid behavior internal to the vortex tube, it has not been applied as a predictive model of the vortex tube in order to develop a design tool that can be used with confidence over a range of operating conditions and geometries. The objective of this paper is the demonstration of the successful use of CFD in this regard, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the context of new applications.     

CFD Studies of Indoor Airflow and Contaminant Particle Transportation

This article presents a numerical study of indoor airflows and contaminant particle transportation in three ventilated rooms. The realizable k ? ε model is employed to model the air-phase turbulence, while the Lagrangian particle tracking model is utilized for the particle-phase simulation. The predicted air-phase velocities and contaminant particle concentrations are validated against the experimental data obtained from the literature. In the first case, the realizable k ? ε model successfully captures the flow trend and reasonably predicts the airflow velocity. The realizable k ? ε model under-predicts the vertical air velocities along the vertical inlet jet axis by 11% at x = 0.219 m, which is slightly better than the standard k ? ε model error of 17%. In a two-zone room case, the realizable k ? ε model, combined with a Lagrangian particle tracking model, predicts the particle concentration decay with the highest normalized difference being 24%. In the third case, the influence of particle size, location of particle resource, and particle-wall collision on the particle concentrations is investigated by the realizable k ? ε model and the Lagrangian model. It is found that for relatively small particles (diameter ≤ 10 μm), the particle concentration may be insensitive to the particle diameter. In addition it has been observed that the particle-collision model may have considerable effect on the particle concentration prediction.     

Interaction parameter, k12, for non-azeotropic refrigerant mixtures

The interaction parameter, k₁₂, is determined from the experimental equilibrium data obtained by other authors. Vapour-liquid equilibria for binary mixtures of halocarbon refrigerants are predicted using the Redlich-Kwong-Soave equation of state. The mixtures considered are: R14-R23, R23-R12, R13-R12, R13-R11, R13B1-R22, R13B1-R152a, R22-RC318, R12-RC318, R12-R11.     

Application of non-linear k–ω model to a turbulent flow inside a sharp U-bend

Simulation of the complex flow inside a sharp U-bend needs both refined turbulence models and higher order numerical discretization schemes. In the present study, a non-linear low-Reynolds number (low-Re) k–ω model including the cubic terms was employed to predict the turbulent flow through a square cross-sectioned U-bend with a sharp curvature, R _C/D = 0.65. In the turbulence model employed for the present study, the cubic terms are incorporated to represent the effect of extra strain-rates such as streamline curvature and three-dimensionality on both turbulence normal and shear stresses. In order to accurately predict such complex flowfields, a higher-order bounded interpolation scheme (Song et al., 1999) has been used to discretize all the transport equations. The calculated results by using both the non-linear k–ω model and the linear low-Reynolds number k–ɛ model (Launder and Sharma, 1974) have been compared with experimental data. It is shown that the present model produces satisfactory predictions of the flow development inside the sharp U-bend and well captures the characteristics of the turbulence anisotropy within the duct core region and wall sub-layer.     

Optimal shape of the multi-passage branching system in a single-phase parallel-flow heat exchanger

A numerical simulation is performed to examine the heat and fluid flow characteristics of the branching system in a single-phase parallel-flow heat exchanger (PFHE) and to obtain its optimal shape. The relative importance of the design parameters [injection angle of the working fluid (Θ), inlet shape and location (Y_c), and height of the protruding flat tube (Y_b)] is determined to decide the optimization sequence. The optimal geometric parameters are obtained as follows: Θ=−21°, Type A, Y_c=0 and Y_b=0. The heat transfer rate of the optimum model compared to that of the reference model is increased by about 55%. The optimal values of the parameters can be applicable to the Reynolds number ranging from 5000 to 20,000.