length l 2 of 15 5 inches and a lower diameter of 12 inches and extending downward at an angle of 16 u00b0 from the vertical a cold flow model of venturi 24 was used to simulate the operation of a fluidized bed gasifier the bed contained a uniform mixture of 400 pounds of acrylic particles and 100 pounds of alumina spheres the acrylic particles had a density of 72 pounds per cubic foot and an average diameter of 0 02 inches to simulate unburned coal particles the alumina spheres had a density of 157 pounds per cubic foot and an average diameter of 0 16 inches to simulate ash agglomerates when a venturi having a smooth internal surface was utilized the simulated ash particles discharged from the fluidized bed at a rate of 2 0 pounds per minute when the upward gas velocity in the venturi was 40 feet per second when a cylindrical section 34 having eight spiral ridges 44 having a width of 0 5 inches and a descent angle of 45 u00b0 positioned on the interior surface of the cylindrical section was utilized the simulated ash particles were discharged at a rate of 3 5 pounds per minute for a venturi velocity of 40 feet per second as determined by measuring the volumetric flow rate through the venturi and dividing this rate by the venturi cross sectional area less the cross sectional area of the sprial ridges positioned on the interior surface of the venturi the increase in discharge from 2 pounds per minute to 3 5 pounds.