A Study of Centrifugal Buoyancy and Particulate Deposition in a Two Pass Ribbed Duct for the Internal Cooling Passages of a Turbine Blade
In this thesis, the ribbed ducts of the internal cooling passage in turbine blading are investigated to demonstrate the effects of high speed rotation. Rotation coupled with high temperature operating conditions alters the mean flow, turbulence, and heat transfer augmentation due to Coriolis and centrifugal buoyancy forces that arises from density stratification in the domain. Gas turbine engines operate in particle laden environments (sand, volcanic ash), and particulate matter ingested by the engine can make their way into the blade internal cooling passages over thousands of operating hours. These particulates can deposit on the walls of these cooling passages and degrade performance of the turbine blade. Large-Eddy Simulations (LES) with temperature dependent properties is used for turbulent flow and heat transfer in the ribbed cooling passages and Lagrangian tracking is used to calculate the particle trajectories together with a wall deposition model. The conditions used are Re=100,000, Rotation number, Ro = 0.0 and 0.2, and centrifugal Buoyancy parameters of Bo=0, 0.5, and 1.0.
First, the independent effects of Coriolis and centrifugal buoyancy forces are investigated, with a focus on the additional augmentation obtained in heat transfer with the addition of centrifugal buoyancy. Coriolis forces are known to augment heat transfer at the trailing wall and attenuate the same at the leading wall. Phenomenological arguments stated that centrifugal buoyancy augments the effects of Coriolis forces in outward flow in the first pass while opposing the effect of Coriolis forces during inward flow in the second pass. In this study, it was found that in the first pass, centrifugal buoyancy had a greater effect in augmenting heat transfer at the trailing wall than in attenuating heat transfer at the leading wall. On the contrary, it aided heat transfer in the second half of the first pass at the leading wall by energizing the flow near the wall. Also, contrary to phenomenological arguments, inclusion of centrifugal buoyancy augmented heat transfer over Coriolis forces alone on both the leading and trailing walls of the second pass.
Sand ingestion is then investigated, by injecting 200,000 particles in the size range of 0.5-175μm with 65% of the particles below 10 μm. Three duct wall temperatures are investigated, 950, 1000 and 1050 °C with an inlet temperature of flow and particles at 527 °C . The impingement, deposition levels, and impact characteristics are recorded as the particles move through the domain. It was found that the Coriolis force greatly increases deposition. This was made prevalent in the first pass, as 84% of the deposits in the domain occurred in the first pass for the rotating case, whereas only 27% of deposits occurred in the first pass for the stationary case with the majority of deposits occurring in the bend region. This was due to an increased interaction with the trailing wall in the rotating case whereas particles in the stationary case were allowed to remain in the mean flow and gain momentum, making rebounding from a wall during collision more likely than deposition. In contrast, the variation of wall temperatures caused little to no change in deposition levels. This was concluded to be a result of the high Reynolds number used in the flow. At high Reynolds numbers, the particles have a short residence times in the internal cooling circuit not allowing the flow and particles to heat up to the wall temperature. Overall, 87% of the injected particles deposited in the rotating duct whereas 58% deposited in the stationary duct.