Fluid dynamic gauging for measuring the strength of soft deposits

Abstract

Fluid dynamic gauging (FDG) is a technique for the measurement of the thickness of soft deposit layers on solid surfaces immersed in a liquid environment, in situ and in real time. The technique has been extended using computational fluid dynamics (CFD) simulations of the laminar gauging flow to quantify the stresses imposed on the material being gauged. CFD results were validated by comparison with experimental values for (i) the hydrostatic head, (ii) the nozzle discharge coefficient and (iii) the normal stresses acting on the gauged surface. The shear stress distribution predictions were subsequently used to determine the shearing yield strength of a soft food deposit. Layers of tomato paste (∼2 mm thick) on stainless steel discs were aged by oven drying at 100 °C, and the removal characteristics studied. The shearing yield strength increased with drying time up to 3.0 h, after which the strength (13 Pa) was almost constant. These results are consistent with micro-structural images, and data reported separately by Liu et al. [Trans. IChemE Part C 80 (2002) 286] using a micro-manipulation technique. The liquid velocities required to fully remove the pastes in pipe flows were calculated from the critical shear stresses and were found to be higher than, but of the same order as, the lower limit of the range of velocities suggested for cleaning-in-place installations. FDG is therefore capable of measuring both the thickness of deposit macro-layers and their mechanical properties in situ.