Experimental study of soot particle fouling on ribbed plates : applicability of the critical local wall shear stress criterion

Abstract

This article examines the notion of critical wall shear stress as the key control parameter of the local fouling removal process. In this study, an experimental setup was developed for measuring fouling on enhanced surfaces. Specifically, the experimental configuration consists of a forced convection plate heat exchanger containing a one-pass rectangular channel with two ribbed plates arranged in a symmetrically staggered manner. The exhaust gases flow by the rib-roughened sides of the plates, and the flat sides can be cooled with water from the independent external coolant circuit. As a result of soot particle deposition from exhaust gases, a layer of fouling is deposited over the ribs. After asymptotic conditions were reached during the tests, detailed fouling thickness measurements were conducted. The dimensionless particle relaxation time during these tests was determined to be in the range of 0.3–10. The measurements were then complemented with a numerical analysis. In particular, the local wall shear stress was calculated using a commercial computational fluid dynamics (CFDs) software package. The fouling thickness profiles deposited over the ribs and the local critical shear stress values were compared and discussed for two different geometries. The results obtained clearly support the idea that critical wall shear stress is an appropriate criterion for facilitating the understanding of the local behaviour of fouling deposits.