Syngas ash deposition for a three row film cooled leading edge turbine vane
Coal gasification and combustion can introduce contaminants in the solid or molten state depending on the gas clean up procedures used, coal composition and operating conditions. These byproducts when combined with high temperatures and high gas stream velocities can cause Deposition, Erosion, and Corrosion (DEC) of turbine components downstream of the combustor section. The objective of this dissertation is to use computational techniques to investigate the dynamics of ash deposition in a leading edge vane geometry with film cooling.
Large Eddy Simulations (LES) is used to model the flow field of the coolant jet-mainstream interaction and the deposition of syngas ash in the leading edge region of a turbine vane is modeled using a Lagrangian framework. The three row leading edge vane geometry is modeled as a symmetric semi-cylinder with a flat afterbody. One row of coolant holes is located along the stagnation line and the other two rows of coolant holes are located at ±21.3° from the stagnation line. The coolant is injected at 45° to the vane surface with 90° compound angle injection. The coolant to mainstream density ratio is set to unity and the freestream Reynolds number based on leading edge diameter is 32000. Coolant to mainstream blowing ratios (B.R.) of 0.5, 1.0, 1.5, and 2.0 are investigated.
It is found that the stagnation cooling jets penetrate much further into the mainstream, both in the normal and lateral directions, than the off-stagnation jets for all blowing ratios. Jet dilution is characterized by turbulent diffusion and entrainment. The strength of both mechanisms increases with blowing ratio. The adiabatic effectiveness in the stagnation region initially increases with blowing ratio but then generally decreases as the blowing ratio increases further. Immediately downstream of off-stagnation injection, the adiabatic effectiveness is highest at B.R.=0.5. However, in spite of the larger jet penetration and dilution at higher blowing ratios, the larger mass of coolant injected increases the effectiveness with blowing ratio further downstream of injection location.
A novel deposition model which integrates different sources of published experimental data to form a holistic numerical model is developed to predict ash deposition. The deposition model computes the ash sticking probabilities as a function of particle temperature and ash composition. This deposition model is validated with available experimental results on a flat plate inclined at 45°. Subsequently, this model was then used to study ash deposition in a leading edge vane geometry with film cooling for coolant to mainstream blowing ratios of 0.5, 1.0, 1.5 and 2.0. Ash particle sizes of 5, 7, 10μm are considered. Under the conditions of the current simulations, ash particles have Stokes numbers less than unity of O(1) and hence are strongly affected by the flow and thermal fields generated by the coolant interaction with the main-stream. Because of this, the stagnation coolant jets are successful in pushing and/or cooling the particles away from the surface and minimizing deposition and erosion in the stagnation region. Capture efficiency for eight different ash compositions are investigated. Among all the ash samples, ND ash sample shows the highest capture efficiency due to its low softening temperature. A trend that is common to all particle sizes is that the percentage capture efficiency is least for blowing ratio of 1.5 as the coolant is successful in pushing the particles away from the surface. However, further increasing the blowing ratio to 2.0, the percentage capture efficiency increases as more number of particles are transported to the surface by strong mainstream entrainment by the coolant jets.