CFD model of the airflow, heat and mass transfer in cool storesModélisation par dynamique des fluides numérisée de l'écoulement de l'air et du transfert de chaleur et de masse dans les entrepôts frigorifiques

https://doi.org/10.1016/j.ijrefrig.2004.08.014Get rights and content

Abstract

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.

Introduction

Agricultural products are subjected to heat and mass transfer during cooling and storage. Uniform cooling and storage of fresh product is difficult to attain in industrial cooling rooms, owing to the existence of an uneven distribution of the airflow [1], [2], which affects the product quality, especially during long-term storage. Heat and mass transfer inside bins of products, particularly, becomes very important in maintaining good quality of stored products, and is mainly dependent on the interaction between the supply airflow and the bulk products. The variability of the cooling rate as well as the temperature of the product inside a cool store causes the product quality to deteriorate through either increased respiration at higher temperature or by chilling or freezing injury at lower temperature.

One of the main aims in designing storage enclosures is to ensure a uniform targeted temperature and humidity in the stored bulk products. The intricate transport mechanics and the complex geometry of a fully loaded cool store make it difficult to determine the optimal configuration and operation parameters of the store in an empirical way. A model-based approach can prove to be advantageous for design purposes with small added cost. With the increasing availability and power of computers together with efficient solution algorithms and processing facilities, the technique of Computational Fluid Dynamics (CFD) can be used to solve the governing fluid flow equations numerically.

A first step towards modelling cool stores loaded with agricultural products is representation of the heat and mass transfer inside bulk storages of agricultural products. Many models have been proposed with different levels of complexity such as uniform air-product temperature [3], [4], thermal equilibrium [3], [4], [5], [6], [7] and internal temperature gradient [3] with mass transfer incorporated [8], [9]. Airflow has been studied in ventilated enclosures for food preservation and processing [10], [11], [12]. To study the non-uniform temperature and moisture of a loaded cool store, only a few models have been proposed in the last 10 years. These models are limited to a two-dimensional one-phase model [13], or distributed dynamic model with only validation for temperature at two locations in the cool store [2]. Van Gerwen and Van Oort [14] used CFD to model 3D airflow and heat transfer in a refrigerated room for agricultural products and studied the effect of different configurations on the cooling effectiveness. However, no detailed information of the model, or the validation was reported. Mass transfer was not modelled in most of the cases. Hoang et al. [15] used CFD to model 3D airflow, heat and mass transfer in an industrial cool store for chicory roots. The latter was validated for the air temperature only (air velocity and product temperature were not validated).

The main objective of the present work was to model the transient three-dimensional airflow, heat and mass transfer in an empty and loaded cool store. The model was then validated using experimental data for velocity and temperature distributions of both air and product phases.

Section snippets

Model formulation

A transient two-phase model of heat and mass transfer in a cool store was proposed. The governing equation expressed in Cartesian coordinates xi (i=1, 2, 3) for the air phase read as follows:uixi=0ρatui+ρaxi(uiuj)=pxj+xiμTOT(ujxi+uixj)ρ0giβ(TT0)+fe,jρatH+ρaxi(uiH)=xi(λTOTcp,aHxi)H=cp,f(TT0)cp,f=Ycp,w+(1Y)cp,aρatY+ρaxiuiY=ρaxjDYxiwith μTOT, the sum of laminar and turbulent viscosity (kg m−1 s−1) and λTOT, the thermal conductivity including a turbulence

Empty cool store

The model reached equilibrium at a flow rate of 2080 m3/h, which was 2.8% less than the designed flow rate of 2140 m3/h. This is due to the pressure drop in the cool store, which caused the total pressure drop to increase resulting in a lower mass flow of air. The general flow pattern is illustrated in Fig. 3, which shows the velocity in a vertical cross section of the cool store. After leaving the cooler, the air was accelerated, reached the ceiling and moved to the door. The air flowed downward

Conclusions

A simplified model for 2-phase momentum, heat and mass transfer in an empty as well as loaded cool store with agricultural product was established to predict airflow around bins, air and product temperature as well as product weight loss. The model equations were solved and validated by means of experimental data from a pilot cool room. An error of about 20% for velocity magnitude prediction for both the empty and loaded cool store was achieved. The model was capable of predicting the cooling

Acknowledgements

The Flemish government and the Research Council of the K.U. Leuven are gratefully acknowledged for financial support. Author Pieter Verboven is a postdoctoral researcher with the Flemish Fund for Scientific Research (F.W.O. Vlaanderen).

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