VOL. 9, NO. 7, JULY 2014 ISSN 1990-6145 ARPN Journal of Agricultural and Biological Science © 2006-2014 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 250 EFFECTS OF NON-UNIFORM AIRFLOW DISTRIBUTION ON GRAIN MOISTURE CONTENTS DURING AERATION Desmond Essien, Richard Bani and Edward Sabi Department of Agricultural Engineering, University of Ghana, Legon, Ghana E-Mail: desmondessien@gmail.com ABSTRACT Maintaining the quality of grains in storage over long periods of time is dependent on several factors, including the provision of a functional aeration system and an adequate management strategy. The growth and activities of insects in stored grains is a function of time, grain moisture content and grain temperature, but this can be controlled with effective aeration. To ensure effective aeration the conditions of the grains in storage must be monitored. Though several models exist for the prediction of grain moisture contents during aeration, most of these models assume airflow to be uniform during aeration. Thompson’s model which is commonly used for predicting grain moisture content assumes uniform airflow during aeration. Conversely, airflow is non-uniform in hopper bottom silos and partially perforated floors. Therefore the objective of this study is to modify Thompson’s model used for natural drying so it could be used to predict the moisture contents of grains during aeration when non-uniform flow of air is considered. A hopper bottom silo of 3m radius filled to a grain height of 1.8m was used for this study. Four tests, each test under different testing conditions and lasting 120 hours, were carried out to investigate the non-uniform movement of air. The investigations revealed that the modified model presented in this study is useful for predicting moisture content to within 0.5% of measured moisture content when non-uniform airflow is considered. The coefficient of determination (R 2 ) between the measured and predicted values obtained for the moisture tests ranged from 0.97 to 0.98. Keywords: grain, aeration, non-uniform, hopper bottom. INTRODUCTION Airflow distributions in most grain structures is non-uniform due to the variations in material properties of the grain mass, the geometry of the storage structure and/or the design of the aeration system. Airflow is generally assumed to be uniform in silos with fully perforated floors and non-uniform in silos with aeration ducts, pads or partially perforated floors. The airflow distribution can also be non uniform in a silo with hopper bottom, peaked grain from overfilling, inverted grain from partial uploading and high fine material concentration in the core of the grain mass (Bartosik and Maier, 2006). According to Garg and Maier (2006), one of the primary causes of non uniform airflow distribution is variation in the material properties of the grain mass. Airflow resistance is a function of particle size and porosity of the grain. Therefore a number of material properties like the distribution of fine material, the loading method, moisture content and level compaction cause non uniform airflow distributions. Several investigators have investigated the non- uniformity of airflow and resistance in a grain mass; however the investigators focused on developing models to calculate pressure drop and consequently power consumption by aeration fans. Little has been reported on the effects of non-uniformity of airflow on moisture content. Ergun (1952) presented an equation to calculate the pressure drop of fluid flow based on Reynolds Theory. According to this equation, the total energy loss in a packed bed is the sum of the viscous and kinetic energy losses. Ergun’s equation has been modified by several investigators to model non-uniform airflow. Lai (1980) investigated three-dimensional axisymmetric (3D) non- uniform airflow in cylindrical grain beds. According to Lai (1980), the porosity of grains differs at different locations within the bin and therefore he divided the grain mass into two regions with different porosities (0.4 at the centre and 0.6 at the periphery). He then used Ergun’s equation to calculate the resistance to airflow. Smith (1996) modified Ergun’s (1952) non nonlinear momentum equation into curvilinear coordinates and used it to predict the pressure drop through a grain mass. Garg and Maier (2006) modified Ergun’s (1952) equation and inserted it into FLUENT (a Computational Fluid Dynamics software) to model non-uniform airflow distribution in large silos. The modified Ergun’s equation was applied to three scenarios: a peaked grain mass, a grain mass with a high fines concentration core, and a grain mass aerated from a ring duct around the bottom of the silo wall. They reported that non-uniform airflow resulted in the reduction of air velocity and volumetric airflow rate within grain layers during aeration. However, the model was not validated with measured data. Hukill and Shedd (1955) presented a model to predict the pressure drop over an airflow range between 0.01-0.20 m 3 /s m 2 . The American Society of Agricultural and Biological Engineers (ASABE) modified Shedd's equation and approved it for determining the static pressure drop of airflow through a grain bed (ASABE Standards, 2006) Kay et al. (1989) investigated airflow resistance in corn grains and reported that the horizontal airflow resistances through corn grains were about 58 and 45% of the vertical airflow resistance when airflow rates were above and below 0.lm 3 /s m 2 , respectively. Garg (2005) investigated the heat and mass transfer due to non-uniform airflow distribution in a grain