Effect of the Modification of the Start-Up Sequence on the Thermal Stresses for a Microgas Turbine

Oscar Tenango-Pirin,Oscar De Santiago,Laura Castro-Gómez,J C García,F Sierra,J M Rodríguez-Lelis,J A Rodríguez

doi:10.1155/2016/5834172

Abstract

Microgas turbines (MGT) are an alternative for small-scale energy production; however, their small size becomes a drawback since it enhances the heat transfer among their components. Moreover, heat transfer drives to temperature gradients which become higher during transient cycles like start-up. The influence of different start-up curves on temperature and thermal stresses of a microgas turbine was investigated. Stationary and rotational blades of the turbine were numerically simulated using CFD and FEM commercial codes. Conjugated heat transfer cases were solved for obtaining heat transfer from fluid toward the blades. Changes of temperature gradients within the blades during the start-ups were calculated under transient state with boundary conditions according to each curve to assess accurate thermal stresses calculations. Results showed that the modification of the start-up curves had an impact on the thermal stresses levels and on the time when highest stresses appeared on each component. Furthermore, zones highly stressed were located near the constraints of blades where thermal strains are restricted. It was also found that the curve that had a warming period at the beginning of the start-up allowed reducing the peaks of stresses making it more feasible and safer for the turbine start-up operation.

Highlights

It is well known that thermal stresses in materials are caused by temperature gradients and mechanical constraints
That problem becomes stronger in microgas turbines because their small size makes it complicated to employ cooling methods like internal cooling holes, which enhances the heat transfer among the components and limits the material strain
For the analysis of the blades, transient conjugated heat transfer (CHT) and static structural analyses were conducted by using Computational Fluid Dynamics (CFD) and FEA; Fluent and Ansys were implemented, respectively

Summary

Introduction

It is well known that thermal stresses in materials are caused by temperature gradients and mechanical constraints. Turbomachinery is often subjected to that condition, with the gas turbines being the most affected due to they work with high temperature gases [1, 2]. This is because for higher turbine inlet temperatures the efficiency of the turbine is improved; the turbine components life in creep is reduced. As a result of energy transformation, temperature and pressure of fluid change giving rise to nonuniform temperature distributions. This condition, together with the constraints on the bases of each component, drives to thermal stresses development. That problem becomes stronger in microgas turbines because their small size makes it complicated to employ cooling methods like internal cooling holes, which enhances the heat transfer among the components and limits the material strain

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