Impact of High Exhaust Back Pressure on Power Turbine Secondary Flows and Disc Thermals

Abstract

Industrial and aeroderivative gas turbines use exhaust systems for flow diffusion and pressure recovery. The downstream balance-of-plant systems such as heat recovery steam generators or selective catalytic systems require, in general, a steady, uniform flow out of the exhaust system. One detrimental effect of having these downstream systems is the increased back pressure. These combined-cycle systems increase the back pressure on the free power turbine which results in decreased power output and efficiency. Aeroderivative gas turbines for mechanical drive application have a wide operational envelope. In general, at baseload, the exhaust back pressure ranges from 1.5 to 2.5 kPa above ambient pressure. Increased exhaust back pressure results in changes to power turbine secondary flows by changing the cavity flow dynamics, sealing flows, and rim seal ingestion. This impacts the thermal characteristics of turbine rotor discs and their lives. The primary motivation for this research and development work was to develop solution for secondary air system and investigate the impact of high exhaust back pressure on power turbine disc thermals. At first, 1-D system-level power turbine secondary flow analyses were carried out with normal back pressure (3.0 kPa) and with high back pressure (11.37 kPa). In addition, 3-D computational fluid dynamic simulations were performed to understand the cavity flow dynamics and disc heat transfer coefficient variations. These results were used in a high-fidelity 2-D thermal modeling of the power turbine to study the impact of back pressure on turbine disc thermal characteristics and their lives. The fluid and thermal predictions were validated using normal back pressure full-scale full-load test results. Cooling mass flow rate, static pressure, air temperature, and metal temperature predictions are compared with test results over a wide operating range. The numerical predictions are in good agreement with test results.