Introduction

A Computational Fluid Dynamics (CFD) simulation was conducted to investigate the vibrational issue in multiple evaporator and super heater modules of a tail gas-fired boiler. The primary objective is to evaluate the dynamic forces (Lift and Drag forces) acting on individual tubes of each heater module and the vortex shedding frequency of tubes at 100% Maximum Continuous Rating (MCR).

Challenges

The primary challenge in the CFD simulation included the selection of an optimal turbulent model to accurately capture the vortex formation at the trailing edge of individual pipes and its influence on flow dynamics. Additionally, to minimize the complexity of the transient simulation without compromising the accuracy of the results, and to exactly predict the constant cyclic forces acting on the tube buddle due to the flow separation phenomenon.

Solution

Transient CFD simulation is carried out for the reduced domain of the tail gas boiler. High-quality fine mesh is generated to capture the boundary layer and resolve the vortices shed accurately. The turbulent flow dynamics are modeled with the Large Eddy Simulation Turbulence Model, which reduces complexity by focusing on turbulence on larger length scales and larger time scales. Sufficiently fine meshing and time resolution are adopted for the solution strategy. 

Benefits

The Vortex Shedding Frequency for each column of the tube bundle is computed. Cyclic fluctuation of the wake region formed behind the tubes is observed, resulting in marginal variation in frequency. The simulation also gives insight into the dynamic forces acting on the tube surface, which stimulate vibrational motion.