Thermal shock and CFD stress simulations for a turbine blade.

Abstract

A 2-D CFD / FEM model to simulate thermal stresses in a turbine blade has been set up using the

software FLUENT and FIDAP. The model was validated against the data of Bohn et. al. (1995)

and was used to simulate 5 test cases. The numerical model was set up for a single Mark II nozzle

guide vane (NGV) and utilised the appropriate boundary conditions for the surrounding flow

field. A commercially available software code, FLUENT, was used to resolve the flow field, and

heat transfer to the blade. The resulting surface temperature profile was then plotted and used as

the boundary conditions in FIDAP (a commercial FEM code) to resolve the temperature and

stress profile in the blade. An additional solver within FLUENT essentially superimposes an

additional flow field as a result of the NGV vibration in the flow field.

The pressure, temperature and heat transfer coefficient distribution, from FLUENT, were

compared to those from Bohn et. al. (1995). The model predicted the distributions trends

correctly, with an average over-prediction for temperature, of 10 % on the suction side and 6 %

on the pressure side. This was restricted to the region from leading edge to 40 % chord on both

sides of the blade. The blade temperature and equivalent stress contour trends were also correctly

predicted by FIDAP. The blade temperature was over-predicted by and average of 1.7 %, while

the equivalent stress magnitude was under-predicted by a worst case of 43 %, but the locations of

maximum stress were correctly predicted.

The reason for the differences between the stresses predicted by FLUENT / FIDAP and the data

given in Bohn et. al. (1995), is believed to be the results of the temperature dependence of the

material properties for the blade (ASTM 310 stainless steel), used in the two studies, not being

identical. The reasoning behind this argument is because the distribution trends and contour

variation, predicted by the model, compared favourably with the data of Bohn et. aI., and only the

equivalent stress magnitude differed significantly. This completed the validation of the FLUENT

/ FIDAP model. The model was used to simulate test cases where temperature (i.e. turbine inlet

temperature or TIT), at the model inlet (Le. the pressure inlet boundary in FLUENT), was set up

to be time varying.

Four simplified cases, viz single shock, multiple shocks, simplified cycle and multiple cycles, and

a complex cycle (a mission profile) were simulated. The mission profile represented typical gas turbine operational data. The simulation results showed that stress was proportional to TIT.

Changes in TIT were seen at a later time in the stress curve, due to conduction through the blade.

Steep TIT changes, such as the shock loads, affected stress later than gentler TIT changes - the

simplified and multiple cycles. These trends were consistently seen in the complex cycle.

The maximum equivalent stress was plotted against TIT to try and develop a loose law that gives

maximum equivalent stress as a function of TIT. A 4th order polynomial was fitted through the

maxima and minima of the maximum equivalent stress plot, which gave the maximum and

minimum stress as a function of TIT. This function was used calculate the maximum and

minimum and mean equivalent stress using the TIT data for the mission profile. Thus, the

FLUENT I FIDAP model was successfully validated, used to simulated the test cases and a law

relating the equivalent stress as a function of TIT was developed.