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.     

Distributed dynamic modelling of a refrigerated room

The prediction of velocity and temperature distributions in a refrigerated room generally requires the simultaneous solution of the equations governing the flow pattern (Navier-Stokes equations) and the energy equation. Basically these are interacting non-linear differential equations requiring simultaneous integration by an iteration process. The specific application of advanced computer packages for this purpose in refrigeration is still in the early stages; even for one steady-state simulation the computation times are high because of the very complicated shape of the air space in a refrigerated room loaded with produce. The method of simultaneous integration becomes unattractive, especially for non-steady-state thermal conditions, if many runs are to be made with different values of the input parameters. In instances where the air cooler fans dominate the flow pattern, the equations can be decoupled. Moreover, the energy equation can be linearized with reasonable model assumptions, and will then not require an iteration process for integration. The simulation described here includes a compression refrigerating machine with capacity control, and was validated by measurements taken from a laboratory-scale cold store.     

The use of CFD to improve the performance of a chilled multi-deck retail display cabinet

Maintaining food temperatures below critical values is the key to maximising the high quality display life of chilled foods. Studies were carried out to see if computational fluid dynamic (CFD) modelling could be used to rapidly identify the changes that would be required to an existing multi-deck display cabinet so that it would meet a higher test specification. Implementing the changes on a Pastorfrigor MV 200TP display cabinet reduced the average power consumption from 1.37 to 1.29 kW as well as significantly reducing the number of test packs which spent any time above 4 °C, from 12 to 1.     

A review of numerical models of airflow in refrigerated food applications

Temperature homogeneity in most food refrigeration systems is directly governed by the airflow patterns in the system. Numerical modelling of airflow provides an opportunity to develop improved understanding of the underlying phenomena influencing system performance, which can lead to reduced temperature heterogeneity and increased effectiveness and efficiency of refrigeration systems. With the rapid advances in computational power of recent years, the use of Computational Fluid Dynamics (CFD) techniques in this application has become popular. This paper reviews the application of CFD and other numerical modelling techniques to the prediction of airflow in refrigerated food applications including cool stores, transport equipments and retail display cabinets.     

Review of effective thermal conductivity models for foods

The literature associated with modelling and predicting the thermal conductivities of food products has been reviewed. The uncertainty involved in thermal conductivity prediction increases as the differences between the food components' thermal conductivities increase, which means that there is greater uncertainty involved with predicting the thermal conductivity of foods which are porous and/or frozen, than with unfrozen, non-porous foods. For unfrozen, non-porous foods, a number of simple effective thermal conductivity models that are functions only of the components' thermal conductivities and volume fractions may be used to provide predictions to within ±10%. For frozen and/or porous foods, the prediction procedure is more complicated, and usually requires the prediction of porosity and/or ice fraction, which introduces another source of error. The effective thermal conductivity model for these foods may require an extra parameter (in addition to the components' thermal conductivities and volume fractions) whose value must often be determined empirically. Recommendations for selecting models for different classes of foods are provided. There is scope for more research to be done in this area.     

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.     

Steady state CFD modeling of airflow, heat transfer and moisture loss in a commercial potato cold store

Storage loss beyond permissible limit is one of the most important problems in Indian potato cold stores, which has been hindering further growth of this industry. The losses in the stored potatoes have a direct relation to the intricate coupled transport phenomena of heat, mass and momentum transfer therein. Therefore, airflow, heat transfer and moisture loss was investigated in a potato cold store of commercial scale under steady state condition using the computational fluid dynamics technique. The developed CFD model was a two-dimensional simplification of the cold store. Heat and mass transfer at the cooling coils were not modeled, instead temperature and relative humidity in the air space were specified based on measured values. The model was validated in a commercial scale cold store and was found to be capable of predicting the air velocity as well as product temperature with an average accuracy of 19.5% and 0.5 °C, respectively and also the simulated average total moisture loss was found to be only 0.61% water (w.b.) higher than the experimental one for a storage period of 6 months. The main deficiencies of the airflow pattern which resulted in wide variations in temperature and moisture loss within the stored commodity can be investigated. The model located the probable zones of hot and cold spots, excessive product dehydration and moisture condensation within the storage facility, which might lead to qualitative and quantitative deterioration in stored product. This modeling tool could very well be applied to incorporate necessary design improvements with a view to improve the airflow distribution and heat transfer in order to limit the storage losses within the permissible limit.     

Two- and three-dimensional CFD applied to vertical display cabinets simulation

A commercial CFD code has been employed to simulate the air flow pattern and the temperature distribution in a frozen food vertical display cabinet. At first the choice of solver parameters has been investigated in a 2D modelisation. 3D simulations have been then performed, and the effects of the cabinet length, of the warm air curtain and of longitudinal ambient air movement have been investigated. The results show that, in short cabinets, 3D secondary vortices at the side walls provide the most important mechanism for hot air entrainment. Comparison with experimental results shows that a 2D simulation is totally inadequate for such configurations, while 3D computations predict refrigeration power within engineering accuracy. Furthermore, the computed refrigerating power shows that even low room air velocity, due to its interaction with the end-wall vortices, has a significant impact on cabinet performance.     

Effect of deflecting ring on noise generated by outdoor set of a split-unit air conditioner

In order to analyze the influence of the deflecting ring on the noise generated by the outdoor set of a split-unit air conditioner, the flow field in the outdoor set is simulated with the CFD software STAR-CD, the relative turbulent intensities are computed and the influence of the width and contoured duct of the deflecting ring on the noise generated by the outdoor set is analyzed. The results of computation and experiment show that there is an optimal width of the deflecting ring, corresponding to the minimum noise generated by the outdoor set. In addition, the influence of the contoured duct of the deflecting ring on the noise generated by the outdoor set is analyzed and a double contoured duct is designed. The results of computation and experiment verify that the deflecting ring with double contoured duct can improve the aerodynamic performance and reduce the noise generated by the outdoor set.     

CFD analysis of ejector in a combined ejector cooling system

One-dimensional ejector analyses often use coefficients derived from experimental data for a set of operating conditions with limited functionality. In this study, several ejector designs were modelled using finite volume CFD techniques to resolve the flow dynamics in the ejectors. The CFD results were validated with available experimental data. Flow field analyses and predictions of ejector performance outside the experimental range were also carried out. During validation, data from CFD predicted the entrainment ratios with greater accuracy on definite area ratios, although no shock was recorded in the ejector. Predictions outside the experimental range—at operating conditions in a combined ejector–vapour compression system—and flow conditions resulting from ejector geometry variations are discussed. It is found that the maximum entrainment ratio happens in the ejector just before a shock occurs and that the position of the nozzle is an important ejector design parameter.     

Numerical modelling of the temperature increase in frozen food packaged in pallets in the distribution chainModélisation numérique de l'augmentation de température des produits alimentaires congelés palettisés dans la chaîne de distribution

Frozen food can undergo temperature rise essentially during handling processes which are the weakest links of the cold chain: delivery, loading or unloading operations and temporary storage where pallets are generally handled in an ambience above 0°C. In this study, the temperature rise in a pallet is investigated numerically and experimentally. A three-dimensional finite-volume heat transfer model is developed using Phoenics Computational Fluid Dynamics (CFD) software. Food temperature within the pallet is predicted as a function of time of exposure, ambient conditions, product initial temperature, palletization and thermal characteristics of products and packaging. The experiments are carried out with packaged frozen fish pallets placed on a closed or open dock. The temperatures are recorded, at different levels in pallets over 25- to 85-min periods. The model shows good agreement with experimental results.     

CFD simulation of refrigerated display cabinets

The finite element method is employed for the analysis of velocity and temperature distributions in refrigerated open display cabinets. The CFD code is based on the streamfunction-vorticity formulation, and incorporates a LES turbulence model. As an example of application, a vertical multi-deck cabinet is investigated under different operating conditions. The numerical results have been validated by comparison with experimental tests performed in accordance with the EN441 Standard. The influence of various design parameters has been investigated.     

Generalized model for the simulation of food refrigeration. Development and validation of the predictive numerical method

A generalized mathematical model was developed to simulate food refrigeration in air. The model takes into account surface water evaporation. It allows food physical properties to be considered as functions of temperature and/or composition, while providing means for including internal heat generation, composite materials (for instance, flesh and skin) and time-varying external temperature and humidity. Wetted surfaces and packaging can also be accounted for by the model, which can be used for spheres, infinite or finite cylinders, slabs. The numerical method was developed using the Crank-Nicolson scheme, and compared with exact analytical solutions as well as with experimental data on the refrigeration of various foods.     

Quantitative prediction of microbial behaviour during food processing using an integrated modelling approach: a review

Microbiological safety of food relies on microbial examination of raw materials and final products, coupled with monitoring process parameters and hygiene standards. The concept of predictive microbiology was developed to evaluate the effect of processing, distribution and storage operations on food safety. The objective of this paper is to review the approaches proposed by researchers to quantify the effect of competitiveness or fluctuating conditions on bacterial behaviour. The main microbial models that quantify the effects of various hurdles on microbial kinetics are presented. To provide complementary information for microbial models, three areas have to be considered: process engineering that characterises and models mass and heat transfer; microbiology that characterises and models bacterial behaviour and metabolite production, and; applied thermodynamics that characterises and models the physico-chemical properties of a food product. Global modelling approaches, developed by integrating the previous models, are illustrated with recent results.     

Modelling heat and mass transfer in frozen foods: a review

This paper reviews mathematical methods for modelling heat and mass transfer during the freezing, thawing and frozen storage of foods. It starts by considering the problems in modelling heat transfer controlled freezing (the Stefan problem): release of latent heat, sudden changes in thermal conductivity. The author gives a unified overview of the common numerical methods: finite difference, finite element and finite volume. Mass transfer is then considered, involving different phenomena and approaches for dense and porous foods. Supercooling, nucleation and trans-membrane diffusion effects during freezing, and recrystallization during frozen storage are considered next. High pressure thawing and thawing are considered in view of their recent popularity. Finally, the paper offers a brief look at mechanical stresses during freezing, a much neglected area. It is concluded that while modelling heat transferred controlled freezing is a settled problem, much work remains to be done in modelling associated phenomena in order to gain the ability to predict changes in food quality at the micro-level.     

CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process

A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.     

A generalised mathematical modelling methodology for the design of horticultural food packages exposed to refrigerated conditions Part 2. Heat transfer modelling and testing

Sub-models required to implement the generalised simulation system, described in Part 1 for predicting in-package cooling rates of horticultural produce, are presented. These are based on heat transfer principles, but their derivation also involved application of heuristics derived by engineering judgement. Testing used two different products, eight package designs and data from four independent sources. Lack of fit between experiment and prediction was no larger than could be explained by likely data uncertainties so there is confidence that the simulation system will be accurate for a wide range of products and package designs. The object-oriented design of the simulation package allows individual sub-models to be updated without affecting any other sub-model, thereby ensuring the simulation package can remain up-to-date. In Part 3 the mass transfer sub-models are presented and test results reported.     

Modelling the chilling of the leg, loin and shoulder of beef carcasses using an evolutionary method

An evolutionary algorithm was used to adjust unknown parameters during the beef cooling process. These parameters are the equivalent diameter and the initial temperature profile, which are difficult to estimate given the irregular geometry, the elapsed time after slaughter and variations in both the air temperature and velocity. The adjusted parameters produced accurate predictions of the center and surface temperature profiles of the leg, loin and shoulder. The adjusted dimensions agreed very well with the measured carcass dimensions. Empirical equations were obtained to correlate this diameter with the weight and fat grade of beef carcasses.     

A generalised mathematical modelling methodology for design of horticultural food packages exposed to refrigerated conditions: Part 3, mass transfer modelling and testing

A modelling methodology developed to simulate a wide range of heat and mass transfer processes in refrigerated storage of horticultural foods was used for prediction of mass transfer of water vapour in packaged horticultural products. The mass transfer pathways, which are considered the most significant in industrial practice for a range of products and packaging systems, were modelled. Experimental testing was conducted for 10 product-package systems. Even though the model supports multi-zone package representation, a single zone was found to be adequate for all test data. Agreement between prediction and measurement was within likely data uncertainties for mass loss, packaging moisture uptake and relative humidity difference to the external environment. Sub-models for which further development may be justified are identified. Coupled with the heat transfer results presented in Part 2, the present work verifies the validity of the overall model structure presented in Part 1.     

Modeling of frost growth and frost properties with airflow over a flat plate

A physical model of frost layer growth and frost properties with airflow over a flat plate at subfreezing temperature was developed. Frost roughness was measured, and an empirical correlation for the average frost roughness was suggested. Heat and mass transfer coefficients were calculated using the modified Prandtl mixing-length scheme containing the effects of both frost roughness and turbulent boundary layer thickness. Frost thermal conductivity was theoretically analyzed by solving the combined equations of air equivalent conductivity and thermal conductivity of the frost inner layer. Based on the present model, heat and mass transfer coefficient, frost thermal conductivity, frost thickness, frost mass concentration and frost density with time and space were estimated. The model showed good agreement with the basic trends of the test data taken from other literature. Spatial and temporal changes of heat flux and frost surface temperature were also investigated.