Computational Fluid Dynamics analysis of flow patterns in a thermal tray dryer

Abstract

Industrial tray air-dryers are increasingly used for the drying of agricultural products. The main drawback of these dryers is the non-uniform velocity distribution in the drying zone resulting in a non-uniform drying of the product. Computational Fluid Dynamics (CFD) software was implemented to predict and decrease the non-uniform velocity distribution of various dryer configurations. Tunnel dryers in commercial use were used to obtain experimental data. The CFD results were correlated with the test data. Trolley and tray tunnel dryers provide a relatively simple, low capital intensive and versatile method for drying a wide range of products. Artificial drying has the advantage of controlled drying conditions compared to traditional sun drying. The main focus of every tunnel design should be the improvement of the quality of the product in terms of colour, texture and aroma. Increasing the evaporation rate without increasing the energy required to do so, should always be done in-line with this main objective. Many studies focus on the mango structure and food dehydration principles that influence the uniform drying product with the assumption that the airflow over the produce is uniform. Few have been conducted on the air movement inside industrial dryers. CFD analysis predicts the airflow without influencing the airflow pattern compared to the measuring equipment inside test dryers. The experimental data were obtained from an empty dryer without a flow diverter. This was compared to dryer with the flow diverter included and compared to a dryer with the trolleys, trays and mango slices included. The test results showed that turbulence created by this configuration, still played a major role in the nonuniform velocity distribution along the drying zone of the tunnel. The inclusion of a flow diverter did however dampen the swirl effect of the main fan. Measuring the velocity distribution was practically difficult with the handheld devices used, which influenced the accuracy of the measurements taken. This justified the CFD analysis in order to better visualise and predict the airflow pattern inside the dryer. The total average speed CFD results of the sections in the drying zone (without mangoes and trolleys) of the dryer without a flow diverter was 11.2% higher compared to the test results. It was 14% higher for the dryer with the flow diverter included. The dryer with the mangoes, trays, trolleys and flow diverter showed a large difference where the total average speed of the CFD analysis was 49% higher compared to the test results. The main reason for the difference of the CFD analysis compared to the measured results are the factors that influenced the uncertainty of the experimental set up. The CFD analysis showed that the coefficient of variance (CV) of the dryer with the flow diverter (mangoes and trolleys included) was 3% lower compared to the dryer without one. Various dryer configurations were analysed using the CFD software to investigate what the best combination of flow diverter, vanes and blanking-off plates would be. A dryer configuration where flow diverters (Up-and-downstream of the main fan) above the false ceiling and inside the drying zone was analysed. A 16% decrease in terms of the CV value was obtained compared to the dryer with just the flow diverter downstream of main fan above the false ceiling. There was however a large region of swirl upstream of the main above the false ceiling resulting in a larger loss of heated air through the outlet fan before it reached the drying zone. The cost of manufacturing a simple vane and flow diverter for an existing dryer is 4% of the initial building costs (excluding the initial cost of the trolleys). The overall drying uniformity of this dryer is improved according to the CFD analysis by 7%. A cost analysis (taking into account the 15 year life cycle of a dryer) in terms of the energy requirement to evaporate water from the drying zone, showed that the dryer with the flow diverter was 6% less expensive to run on a yearly basis. Labour costs will be lower due to man-hours saved in terms of sorting out the wet slices from the dried product. Resources (dryers and trolleys) that would have been used for re-drying the wet produce, could now be implemented to increase the production rate of the plant. Copyright