Velocity Gradient Calibration of Jar‐Test Equipment

The jar-test procedure is widely used to simulate the water-pretreatment process in the laboratory to produce data for process control, yet few carefully controlled jar-test techniques are found in related literature. Jar-testing has depended upon the approach of each investigator.1 3 However, the interpretation of jar-test data must be founded on unvarying and well-calibrated techniques if they are to be quantitatively meaningful. One of the important variables in the procedure is the mixing intensity, which is related to the rotational speed and the configuration of the agitator as well as the geometry of the mixing vessel. The purpose of this study was to determine the mixing intensity, expressed as the mean velocity gradient "<7," throughout the applicable speed range, using various jar-test configurations. The resulting data should prove useful for application of laboratory data to water-treatment-plant design. Camp1 has called attention to the facts that (1) the fluid condition in full-scale plant mixing and flocculation basins is always turbulent, even when G values are relatively low; and (2) at speeds commonly used in jar-test machines, laminar flow conditions may occur. One object of this study was to evaluate the minimum threshold speeds above which turbulence always occurs in jar-testing. Camp and Stein5 applied Stokes' theory6 to relate the total energy input to what they called a root-mean-square velocity gradient G (Stokes' theory states that the velocity gradient equals the square root of energy dissipation at a point, divided by the absolute viscosity of the fluid): G= du = [W~ dz V fl (1)