Computational Analysis of Heat Recovery From Simple Cycle Gas Turbine Exhaust Stacks Through the Silencer Semi-Circular Sections

The challenge with exhaust heat recovery is that placing additional heat transfer surfaces in the exhaust flow path leads to pressure drop; which lowers the gas turbine (GT) performance. One way to overcome the increase in the pressure drop is to enlarge the exhaust flow path. However, that is associated with similar capital and space constraints as combined cycle (CC) conversion. This paper investigates the potential of integrating heat transfer channels within the existing semi-circular section of the exhaust silencer baffles of simple cycle (SC) gas turbines, in order to avoid such an exhaust flow path enlargement. A heat transfer fluid in the channels absorbs GT exhaust waste heat flow across the silencer. The heated fluid was utilized in the fuel gas pre-heater, leading to reduced fuel gas usage at the same GT output.

In this paper, a computational model of the integrated heat exchanger was developed, and compared to an existing mathematical model; showcasing the computational fluid dynamics (CFD) results of the model.

After describing the system, the paper describes the computer aided design (CAD) setup of the heat transfer model and flow channel details. The mesh setup and mesh independence study for optimal CFD analysis were conducted. The CFD setup, methodology and results using Ansys Fluent are described.

The computational model was successful in estimating the convective heat transfer coefficient, heat exchanger outlet temperatures and flow rates. The proposed system resulted in 2.3 MW heat recovery rate. It was found that while the existing mathematical model over-predicted the heat recovery, the system still provides significant net power gain and reduction in CO2 emissions.

The CFD flow visualization allowed for a greater understanding of the exhaust flow parameters enabling the localization of areas with maximized heat transfer coefficient potential. Utilizing this knowledge, areas of improvement for the heat transfer passages were identified; and future work proposed.