THE EFFECTS OF AIR INLET GEOMETRY AND SPRAY CONE ANGLE ON THE WALL DEPOSITION RATE IN SPRAY DRYERS

The use of a numerical simulation to minimise the wall deposition rate in a spray dryer is demonstrated, with the numerical simulation solving the equation of continuity and the Navier-Stokes equations inside the dryer using the k - epsilon model for turbulence. To validate the model, a solution of sodium chloride containing 20% by mass of the salt has been sprayed at the rate of 0.0012 kg s-1 from a two-fluid nozzle into a 0.935 m diameter, 1.69 m high cylinder-on-cone chamber. The simulation has predicted the wall deposition rate (measured as 0.000044 kg s-1) within 16% at an inlet air temperature of 245-degrees-C when turbulence constants in the k - epsilon model for recirculating flows as recommended by Abujelala and Lilley (1984) have been employed. This numerical simulation has been used to explore methods for decreasing the wall deposition rate, including simple modifications to the air inlet geometry (to eliminate swirl in the inlet air) and a reduction in the spray cone angle from 60-degrees to 45-degrees. Within the constraints imposed by the experimental equipment, this work has suggested that the maximum spray cone angle (60-degrees) and the maximum amount of swirl in the inlet air (62-degrees) tend to minimise the wall deposition rate. The measured trends in the wall deposition rate caused by decreasing the amount of swirl in the inlet air (0.000093 kg s-1 measured, an increase) and the included angle of the spray cone (0.000099 kg s-1 measured, an increase) have been predicted by the simulation, which suggested deposition rates of 0.000053 kg s-1 for the no-swirl case and 0.000056 kg s-1 for the reduction in the spray cone angle respectively. This work demonstrates the potential for using this type of numerical stimulation to refine the operation of spray dryers.