Ali Rozati

Large Eddy Simulation of Leading Edge Film Cooling: Part I — Computational Domain and Effect of Coolant Pipe Inlet Condition

A numerical investigation is conducted to study compound angle leading edge film cooling with Large Eddy Simulation. The leading edge has two rows of coolant holes located at ±15° of the stagnation line. Coolant jets are injected into the flow field at 30° (span-wise) and 90° (stream-wise). Mainstream Reynolds number is 100,000 based on the free stream velocity and cylinder diameter. Jet to mainstream velocity and density ratios are 0.4 and 1.0, respectively. It is found that during startup the stagnation line at the leading edge is not stationary but moves on a timescale much larger than the characteristic turbulent scales generated by the jet-mainstream interaction. To alleviate the long time integration necessitated by this feature, only half the domain is calculated (fixed stagnation) by showing that there is very little correlation in the flow structures generated by the jet-mainstream interaction on either side of stagnation. A comparison is made between a laminar uniform profile at the coolant pipe inlet with a time-dependent turbulent profile extracted from an auxiliary turbulent pipe flow calculation. The former over-predicts the span-wise averaged effectiveness, while the latter promotes better mixing in the outer region of jet-mainstream interaction and lowers the adiabatic effectiveness showing good agreement with measurements. In both cases, a characteristic low frequency interaction between the jet and the mainstream is identified at a non-dimensional frequency between 0.79 and 0.95 based on jet diameter and velocity. Even in the absence of any free-stream and jet turbulence, a turbulent boundary layer is established within a diameter downstream of the jet due to the strong lateral entrainment downstream of injection. The entrainment is primarily driven by an asymmetric counterrotating vortex pair in the immediate wake of the coolant jet. The driving mechanism for the formation of these vortices is a low pressure zone in the wake which entrains mainstream flow laterally into this region.Copyright

Effects of Syngas Ash Particle Size on Deposition and Erosion of a Film Cooled Leading Edge

The paper investigates the deposition and erosion caused by Syngas ash particles in a film cooled leading edge region of a representative turbine vane. The carrier phase is predicted using large eddy simulation for three blowing ratios of 0.4, 0.8, and 1.2. Ash particle sizes of I μm, 3 μm, 5 μm, 7 μm, and 10 μm are investigated using La-grangian dynamics. The 1 μm particles with momentum Stokes number, St p =0.03 (based on approach velocity and leading edge diameter), follow the flow streamlines around the leading edge and few particles reach the blade surface. The 10 μm particles, on the other hand with a high momentum Stokes number, St p = 0. 03, directly impinge on the surface, with blowing ratio having a minimal effect. The 3 μm, 5 μm, and 7 μm particles with St p = 0.03, 0.8 and 1.4, respectively, show some receptivity to coolant flow and blowing ratio. On a number basis, 85―90% of the 10 μm particles, 70―65% of 7 μm particles, 40―50% of 5 μm particles, 15% of 3 μm particles, and less than 1% of I μm particles deposit on the surface. Overall there is a slight decrease in percentage of particles deposited with increase in blowing ratio. On the other hand, the potential for erosive wear is highest in the coolant hole and is mostly attributed to 5 μm and 7 μm particles. It is only at BR = 1.2 that 10 μm particles contribute to erosive wear in the coolant hole.

Heat-Mass Transfer and Friction Characteristics of Profiled Pins at Low Reynolds Numbers in Minichannels

Three pin fin array geometries (T60, T90, and T120) are investigated at low Reynolds numbers, Re D < 350, in a channel. The number in T60, T90, and T120 denotes the angle made by the pin surface with the end wall. Results show that the T120 pin is the most effective in facilitating momentum transport along the height of the pin and mitigates the undesired effect of low momentum and recirculating wakes. Additionally, pin T120 causes localized flow acceleration between pins near the end wall, which results in high heat transfer coefficients at the end wall. Overall, T120 has the highest heat transfer (augmentation ratio 2.9 at Re D = 325), without any increase in friction factor (augmentation ratio 8.3 at Re D = 325) from the baseline configuration of T90. However, T120 results in a large reduction in end-wall surface area, which reduces overall conductance, and in this respect T60 is superior in the range Re D < 150. A performance study of conductance under the constraint of the same pumping power in an equivalent plane channel shows that the profiled geometries T60 and T120 augment conductance between 40% and 250% over an equivalent channel.

Thermal Performance of Pin Fins at Low Reynolds Numbers in Mini-Micro-Channels

The computational study investigates different pin fin arrangements at low Reynolds numbers, which would typically be prevalent in mini-micro-channels used in enhancing heat as well as mass transfer. The effect of pin density, span-wise pitch, and stream-wise pitch is investigated on friction and heat transfer over a range 5 200 due to the formation of larger recirculating wakes. Overall it is concluded that a high density arrangement with a small span-wise pitch provides the best thermal performance.Copyright

Study of Flow Structures and Heat Transfer in Parallel Fins With Dimples and Protrusions Using Large Eddy Simulation

Flow field and heat transfer for parallel fins with dimples and protrusions are predicted with large-eddy simulations at a nominal Reynolds number based on fin pitch of 15,000. Dimple and protrusion depth and imprint diameter to channel height ratio are 0.4 and 2.0, respectively. The results show that on the dimple side, the flow and heat transfer is dominated by unsteady vorticity generated and ejected out by the separated shear layer in the dimple. The high turbulent energy which results from the unsteady dynamics is mostly responsible for heat transfer augmentation on the dimple side. A maximum augmentation of about 4 occurs in the reattachment zone of the dimple and immediately downstream of it. On the protrusion side, however, the augmentation in heat transfer is dominated by flow impingement at the front of the protrusion, which results in a maximum augmentation of 5.2. The overall heat transfer and friction coefficient augmentations of 2.34 and 6.35 are calculated for this configuration. Pressure drag from the dimple cavity and protrusion contribute 82% of the total pressure drop.Copyright

Large Eddy Simulation of Leading Edge Film Cooling: Part II — Heat Transfer and Effect of Blowing Ratio

Detailed investigation of film cooling for a cylindrical leading edge is carried out using Large Eddy Simulation (LES). Part-II of the paper focuses on the effect of coolant to mainstream blowing ratio on flow features and consequently on the adiabatic effectiveness and heat transfer ratio. With the advantage of obtaining unique, accurate and dynamic results from LES, the influential coherent structures in the flow are identified. Describing the mechanism of jet – mainstream interaction, it is shown that as the blowing ratio increases, a more turbulent shear layer and stronger mainstream entrainment occur. The combined effect, leads to a lower adiabatic effectiveness and higher heat transfer coefficient. Surface distribution and span-averaged profiles are shown for both adiabatic effectiveness and heat transfer (presented by Frossling number). Results are in good agreement with the experimental data of Ekkad et al. [12].Copyright

Analysis of Flow Physics and Jet-Mainstream Interaction in Leading Edge Filmcooling Using Large Eddy Simulation

A numerical investigation is conducted to study leading edge film cooling at a compound angle with Large Eddy Simulation (LES). The domain geometry is adopted from an experimental set-up (Ekkad et al. [14]) where turbine blade leading edge is represented by a semi-cylindrical blunt body. The leading edge has two rows of coolant holes located at ±15° of the stagnation line. Coolant jets are injected into the flow field at 30° (spanwise) and 90° (streamwise). Reynolds number of the mainstream is 100,000 and jet to mainstream velocity and density ratios are 0.4 and 1.0, respectively. The results show the existence of an asymmetric counter-rotating vortex pair in the immediate wake of the coolant jet. In addition to these primary structures, vortex tubes on the windward side of the jet are convected downstream over and to the aft- and fore-side of the counter-rotating vortex pair. All these structures play a role in the mixing of mainstream fluid with the coolant. A turbulent boundary layer forms within 2 jet diameters downstream of the jet. A characteristic low frequency interaction between the jet and the mainstream is identified at a non-dimensional frequency between 0.79 and 0.95 based on jet diameter and velocity. The spanwise averaged adiabatic effectiveness agrees well with the experiments when fully-developed turbulence is used to provide time-dependent boundary conditions at the jet inlet, without which the calculated effectiveness is overpredicted.Copyright