Characterization of local mass transfer rate downstream of an orifice

Abstract

<p>Flow accelerated corrosion(FAC) results in wall thinning of pipes, tubes or vessels from exposure to flow due to corrosion. If FAC is not detected, it can lead to sudden failure of piping components. Orifices are used in piping systems to monitor and control the flow. Flow separation and reattachment downstream of an orifice can enhance the mass transfer of the pipe wall. In this thesis, the effect of Reynolds numbers and surface roughness on the mass transfer rate downstream of an orifice was investigated. A dissolving wall method was used to measure the wall mass transfer. The test sections were cast from gypsum with water as the working fluid. Multiple destructive tests were performed for different test times in a 2.5 cm diameter flow loop, and the wear topology measured by a laser scanner to obtain the progression of wear with time over the pipe surface. None-destructive tests were performed in a 20 cm diameter flow loop using test section with an inner gypsum lining, and measured online at selected locations using an ultrasonic method. Experiments were performed at Reynolds numbers of 80000, 140000 and 200000 in the 2.5 cm diameter flow loop, and at 180,000 in the 20 cm diameter flow loop with an orifice to pipe diameter ratio of 0.5. The results show that different surface roughness patterns are developed at different Reynolds numbers from the initially smooth surfaces. The different surface roughness patterns have a significantly different effect on the mass transfer rate downstream of an orifice. A larger population of scallops developed from the smooth pipe surface, as the Reynolds number was increased, which enhanced the mass transfer rate. The mass transfer rate in the 20 cm diameter test section was much smaller than in the 2.5 cm diameter test section at a similar Reynolds number. The pattern of the roughness in the 20 cm diameter test section was formed as isolated roughness which is similar to the roughness pattern in 2.5 cm diameter test section at much lower Reynolds number.</p>