Measurement of mass transfer coefficients with point electrode and LES analysis of scalar transport between different wall transfer conditions in 90-degree elbow

Abstract

It is necessary to evaluate the geometry factor for predicting the flow accelerated corrosion (FAC) in the plant piping. Geometry factor is defined as the ratio of the wall mass transfer coefficient in the piping systems such as elbow to that in a straight pipe. In this study, the mass transfer coefficient in 90-degree elbow with curvature radius of 1.5 times the pipe diameter is computed by using large eddy simulation (LES) and is also measured with electrochemical method. Numerical simulations and experimental measurements were conducted at Reynolds number of 15000. In order to simulate the mass transfer coefficient, we adopt the analogy between mass transfer and heat transfer, and calculate the unsteady temperature field using the numerical data of LES. The experimental measurement is conducted with point electrode. However, concentration boundary layer developed over the surface of point electrode is different from that developed for whole surface of pipe wall such as FAC in the plant piping. To cope with this problem, we calculated the temperature field in the two different boundary conditions. As a typical case, whole pipe wall is heated uniformly (in this paper, referred as ‘overall heat condition’). In addition, localized area is heated to simulate working point electrode (referred as ‘point heat condition’). Geometry factor obtained from the heat transfer rate in point heat condition agrees well with that measured with point electrode, however, both of them are qualitatively different from that obtained from the heat transfer rate in overall heat condition. Analysis of heat transport revealed that heat conduction is dominant form of the wall heat transfer in point heat condition, in contrast, turbulent heat flux affects strongly the wall heat transfer in overall heat condition.