Development of semi-empirical models to measure mass flow rate of solids in an air seeder
The air seeder, which is primarily used for seeding, plays a significant role in the large-scale agricultural industry. Air seeding technology is based on the principles of pneumatic conveying where seed or fertilizer are conveyed by air from a reservoir to the land through pipe. Although air seeding technology has seen many developments since its arrival in the 1950s, it still lacks in the area of mass flow measurement of conveyed solids. An effective method of on-line seed flow monitoring is important to reduce product wastage and also, to anticipate plugging in the lines. The goal of this research has therefore been to develop methods to measure mass flow rate in horizontal gas-solid flow in a way that can be implemented in an air seeder. In order to do that, two novel methods have been described. Both of the methods develop relationships between the solids flow rate, the pressure drop in the pipeline and the average air velocity by conducting experiments with wheat in a laboratory prototype air seeder. Pressure drop and average air velocity are two quantities that can be measured without difficulty under all conditions that an air seeder operates. Hence, these two quantities were chosen as the independent variables and material mass flow rate was chosen as the dependent variable. An earlier empirical model for mass flow measurement was developed prior to this research. But that model was only valid under its test condition and did not provide any insight on the flow mechanism. Hence, this investigation developed models based on existing relationships for gas-solid flow. These models provide better understanding of the mechanism of pressure drop and also, show superior potential for adaptability from test to real-time conditions. The first model was developed by modifying a relationship described for horizontal gas-solid flow by Hinkle (1953), Cabrejos and Klinzing (1992) and a few other researchers. That relationship between the specific pressure drop and the mass loading ratio was valid for fully-developed flow and higher air velocities. It needed modification because generally air seeders have a straight horizontal section in the non-developed region of the flow. The modified model is the first of its genre that describes the relationship between the specific pressure drop and the mass loading ratio in the non-developed flow region for both higher and lower air velocities. Although it was developed to be implemented on an air seeder, it can be applied to any horizontal gas-solid flow. The second model for mass flow measurement of solids used the so-called “dimensionless” state diagram for horizontal flow. The primary relationship between the mass loading ratio and the Froude number described in the dimensionless state diagram remains unchanged for all products being pneumatically conveyed. Only a single parameter varies with the mass flow rate of solids. This varying parameter was correlated with specific pressure drop in this second model. Again, this model is one of the first models to use the dimensionless state diagram for solids mass flow measurement. Both models had errors less than 20% in the predicted mass flow rate when tested. The first model had less than 10% error for 73% of the total estimates. The second model had less than 6% error for 60% of the total estimates. For the rest of the estimates, the error values varied between 10% and 15%. These results indicate that both of the models have promising potential to be implemented into an air seeder.
Pneumatic conveying, gas-solid flow, multiphase flow
Master of Science (M.Sc.)