PSI - Issue 5

Rui F. Martins et al. / Procedia Structural Integrity 5 (2017) 640–646 Author name / StructuralIntegrity Procedia 00 (2017) 000 – 000

643

4

Table 4. Thermal properties of the structural stainless steels considered in the assessments. Thermal conductivity (W/mm.K)

Thermal expansion coefficient (K -1 )

373 K

773 K

373 K

588 K

811 K

Material

AISI 316L and Cr-Mn austenitic stainless steels

16.2 ×10 -3 22.5 ×10 -3

21.5 ×10 -3 22.6 ×10 -3

15.9 ×10 -6 16.2 ×10 -6 17.5 ×10 -6 10.0 ×10 -6 10.5 ×10 -6 11.1 ×10 -6

445M2 ferritic marine stainless steel

In what concerns physical properties, the austenitic stainless steels herein considered possess a density of approximately 8 000 kg/m 3 and a Young´s Modulus of 193 GPa, whilst the 445M2 ferritic marine stainless steel has a density equal to 7 750 kg/m 3 and a Young´s Modulus of about 199 GPa ( Australwright ).

2.2. Finite Element Analyses (FEA)

The exhaust system presented in Fig. 1a has a length of 7.847 m, a rectangular inlet section of 4.247 m 2 and an outlet section of 5.113 m 2 , while the exhaust system represented in Fig. 1c is 7.630 m long, has an inlet section of 1.902 m 2 and an outlet section of 2.269 m 2 . As the ratio thickness to radius (t/R) was smaller than 0.05 (1/20), both exhaust systems were considered to be composed by thin shells. Hence, two finite element types were used in the structural and thermal FEA, namely SHELL63 and SHELL57, respectively. The material models representing the AISI 316L austenitic stainless steel and 445M2 ferritic stainless steel were assumed to be isotropic, homogeneous and temperature-dependent for the Young’s Modulus, the thermal expansion coefficient and the thermal conductivity values. Consequently, a nonlinear material behaviour, together with the use of nonlinear boundary conditions (springs or elastomers) were considered in the numerical analyses. The Poisson ratio was 0.33 and the thermal properties of the austenitic and ferritic stainless steels (Table 4) were defined using a reference temperature of 293ºK. The meshes used in the finite element analyses were defined using quadrilateral elements, and the influence of the mesh refinement was assessed through a sensitivity analysis; hence, deformations and stress results obtained for the exhaust system’s model using an automatic mesh ( Smartsize 1 ) were compared with the numerical results obtained with finite element meshes having parameterized elements with edge length of 75 and 50 mm (Fig. 2).

Fig. 2. Exhaust system with coarse and refined finite element meshes.

The temperature was measured with thermocouples placed at several points of the exhaust system shown in Fig. 1a, whilst the gas turbine was operating, namely at: 773 ºK at the inlet section, 623 ºK at the intermediate section, 373 ºK at the outside of the reinforcement ribs (both right and left sides) and 473 ºK at the outlet section of the