Numerical analysis of the effect of natural convection heat transfer on the gas turbine axial compressor casing

Different events result in casing distortion of gas turbines. Rubbing between rotating and stationary components, unequal thermal mass between the thick flanges and the thin turbine casing and uneven convection heat transfer coefficient around the casing are main causes of this distortion. Distortion of casing results in compressor efficiency decreases via increase of tip leakage loss. In this study, the effect of natural convection on gas turbine axial flow compressor casing has been investigated. For this purpose, a two-dimensional model, including the shaft, inner casing, outer casing, main gas and fluid between casings has been developed. Transient conjugate heat transfer analysis is conducted by applying different boundary conditions. The results show that the top of casing temperature increases initially to reach a maximum value, then the temperature decreases. On the other side, the temperature of lower part of casing generally decreases over time. According to the casing rate of heating and cooling, natural convection heat transfer causes a significant temperature gradient in the casing at gas turbine shutdown. Also, due to the high value of the Rayleigh number, turbulence in the fluid is high. Although the Rayleigh number increases with increasing the convection heat transfer coefficient in enclosure, the temperature gradient between the upper and lower parts of the casing is reduced locally. To reduce casing distortion practical solutions were presented such as control of the convection coefficient of turbine enclosure, changing of bleed valve logic and improving casing insulation, related to operating conditions.