CFD studies of dynamic gauging

Abstract

Dynamic gauging is a non-contact technique for measuring the thickness of soft deposit layers on solid surfaces immersed in liquid environments, in situ and in real time. The technique works by inducing a flow into a nozzle located close to, and normal to, the deposit surface; the relationship between pressure drop and mass flow rate yields a measure of the distance between the nozzle and the deposit, whence the thickness of the deposit can be deduced. Computational Fluid Dynamics (CFD) studies were performed to illuminate the fluid dynamics of this technique, with particular focus on the flow patterns and on the stresses imposed on the surface. The governing Navier-Stokes equations were solved using the Augmented Lagrangian Method implemented by the commercial partial differential equation solver, Fastflo TM. The code was first tested successfully against previous studies in the literature featuring confined, slow Homann flows, where fluid flowed out of a nozzle. Then, simulations of gauging flows, where fluid enters a nozzle from a confined entry region, were compared with experimental data; good agreement was observed. Laminar Newtonian flows have been investigated, with Reynolds number at the nozzle throat in the range 0 < Ret < 2200. The shear and normal stresses on the gauged surface were predicted using the output from the CFD simulations. An initial comparison of experimental results for power-law fluids (aqueous CMC solutions) demonstrated the versatility of the technique and implied its 2 applicability to more complex fluids, which would be useful for industrial application. The success of this study will enable (i) use of the gauge to measure the strength of deposits, (ii) optimization of the shape of the nozzle for different tasks and (iii) extension of the technique to power-law fluids.

Structure and shear strength of microbial biofilms as determined with confocal laser scanning microscopy and fluid dynamic gauging using a novel rotating disc biofilm reactor

“…CFD studies performed by Chew et al (2004a) showed the applicability of an analytical solution for a centripetal flow to compute the shear stress in the gap between nozzle and surface:…”

“…In this study fluid dynamic gauging (FDG) as described by Chew et al (2004a) was used for measuring the mechanical strength (i.e., shearing yield strength or cohesive strength) of biofilms. This method provides the advantage that the forces acting on the biofilm derive from fluid flow tangential to substratum surface.…”

A Method for Measuring the Strength of Scale Deposits on Heat Transfer Surfaces

“…ms indicates that the flow below the rim of this nozzle design can be solved analytically for steady radial flow between parallel discs. The flow rate can be estimated using the discharge coefficient correlation published by Tuladhar et al (2000), whose accuracy has been confirmed by CFD (Chew et al, 2004a).…”

“…the clean condition. For the experiments performed here with Newtonian liquids, the flows lie in the laminar regime so the stresses imposed on the surface can be calculated by numerical solution of the governing Navier-Stokes and continuity equations (see Chew et al 2004a). Ths combination of thickness measurement, in response to imposed stress, yields the deformation behaviour of the fouling layer and has been applied to soft layers of food materials by Chew et al (2004b).…”

In situ investigation of soft cake fouling layers using fluid dynamic gauging

“…This value represents the greatest shear stress applied during FDG readings, which occurs at the inner edge of the nozzle rim (a radial distance of dt/2 from the centreline) (Chew et al, 2004). The test section was m ounted in the flow loop shown in Fig.…”