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AIRFLOW DISTRIBUTION ALONG PERFORATED DUCT DURING GRAIN AERATION
Abstract
Two mathematical models were developed to study the air distribution normal to and along an aeration duct. The airflow distribution normal to the duct was analyzed using a finite element model. Example simulations of airflow in grain piles were performed on triangular and trapezoidal pile cross sections because of their widespread use in flat storage. The equivalent radial grain depth representative of the triangular pile cross section was determined. A model to predict the air distribution along the aeration duct was developed from basic principles of fluid mechanics and from experimental data for static pressure regain coefficients reported in the literature. The model handles both negative and positive pressure systems. Experimental tests on a duct covered with grain were performed to verify the model for static pressure distribution along an aeration duct. The predicted results using the same input data of the experimental test agreed favorably with the experimental results for both negative and positive pressure systems. The effects of the various duct parameters on the uniformity of air discharge along grain aeration ducts were studied. The performed analyses indicate that the problems of the static pressure distribution along aeration ducts operated under positive or negative pressure should be looked at as two different mathematical problems. This is due to the effect of the air intake or discharge on the velocity distribution across the duct diameter. The total airflow delivered in a system is influenced by the duct length, duct diameter, air velocity in the duct, and grain depth. Increasing the air velocity at the duct entrance increases the airflow but contributes to the deterioration of the uniformity of air discharge along an aeration duct. The effect of the air velocity in the duct on the uniformity of air discharge is more pronounced on the negative pressure system than on the positive pressure one. To deliver equal volumes of air, a large diameter duct is preferred to a small diameter one with regard to air discharge uniformity and power requirement. Increasing the duct length while keeping the air delivered constant results in low airflow rate per metric ton of grain. It also results in deterioration of air distribution along a perforated duct operated under negative pressure. The air distribution along a duct operated under positive pressure may improve depending on the relative magnitudes of the friction loss and the static pressure regain. However, the airflow rate per ton of grain decreases with increase of duct length. An increase in wall roughness results in deterioration of uniformity of air discharge along a duct operated under negative pressure. It slightly improves the air discharge distribution along a duct operated under positive pressure. Excessive increase in wall roughness reverses the problem of the air distribution without solving it, the end result being waste of energy. The increase in roughness of both duct wall and the perforations increases the static pressure requirement sharply. Increasing the grain depth improves the air discharge uniformity for both negative and positive pressure systems. Delivering the recommended average airflow rate may be the limiting factor for increasing the grain depth beyond a certain depth using a particular duct diameter. The main concern when operating positive or negative pressure systems should be the total airflow delivered together with the uniformity of air discharge and the power requirement. Comparing the positive and negative pressure systems, it has been concluded that the positive pressure system gives better air uniformity at lower power requirement, assuming all other variables are the same.
Subject Area
Agricultural engineering
Recommended Citation
HUMMEIDA, MUSTAFA A, "AIRFLOW DISTRIBUTION ALONG PERFORATED DUCT DURING GRAIN AERATION" (1981). ETD collection for University of Nebraska-Lincoln. AAI8118161.
https://digitalcommons.unl.edu/dissertations/AAI8118161