Experimental and numerical investigations on aero-thermal interaction of cooling air ejected from multiple cooling holes

Abdullah, Mohammad Kamil (2013) Experimental and numerical investigations on aero-thermal interaction of cooling air ejected from multiple cooling holes. PhD thesis, lwate University.

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Abstract

"Film cooling is the introduction of secondary fluid (coolant) at one or more discrete locations along a surface exposed to a high temperature environment to protect that surface not only in the immediate region of injection but also in the downstream region," (Goldstien, 1971). The secondary air which is cool air were extracted from the compressor and injected near the blade surface (through holes or slots) to provide a layer of cool fluid between the hot gases and the blade surface thus reducing the heat transfer to the surface. Enormous numbers of experimental and computational investigations dealing with both the aerodynamic and the thermal aspect of film cooling have been made available ever since. At the early stage of its introduction, the experimental investigations were usually focused on either thermal or aerodynamic aspect of the study. However, with the introduction of the laser based measurement (PIV and LDV), which reduce the complexity of aerodynamic measurements, most of current literature involving film cooling will have both, the thermal and the aerodynamic aspect of the study. Most of the available literature involved a discrete hole or single row cooling hole with common hole inclination angle towards the streamwise direction at 30" and 35". Although earlier researches have proved that decreasing the hole inclination angle could provide better film cooling performance, there are very few literature available with regards the hole inclination angle lower than 30". Based on these facts, the present study intended to investigate the aero-thermal interaction of a multiple shallow hole angle. In addition to the experiments, computational investigation has been extensively used in particular to explore the potential of new hole geometries and configurations. The computational investigation has also been manipulates to provide the physical insight of the aero-thermal interaction of the study. The present study involves thermal and aerodynamic investigations of multiple cooling holes with shallow hole angle. Total of three test models are considered in the present study namely TMA, TMB and TMG. The base line test model; TMB is design to justify the commonly used cooling hole having 35" hole angle. The other two test models; TMA and TMG, are having a shallow hole angle of 20" with different lateral pitch distance of 6D and 3D, respectively. Total of twenty conventional cylindrical holes constituting a matrix composed of four rows with five holes in each row. Thermal and aerodynamic investigations are carried out involving all of the test models. The thermal investigations involve temperature measurement of test model surface by infrared thermography; NECIAvio H2640. The surface temperature is converted to film cooling effectiveness to represent the thermal performance of the given test model. In addition to the surface temperature measurements, the thermal investigations also involve measurement of thermal field downstream of each row cooling hole. The thermal fields are measured by a thermocouple rig which is mounted on a 2-D traverse system. The traverse system will navigate the rig according to a designated measurement grid to enable thermal field contour plot. 3-D Laser Doppler Velocimetry has been utilized for the aerodynamic measurements. The laser probes are mounted on a 3-D traverse system which will navigate the probes according to a designated measurement grid to enable velocity field contour plot. The measurements are carried out at single Reynolds number base on the hole diameter of 6200 at three different blowing ratios of 0.5, 1.0 and 2.0. The present study also includes the numerical investigation of the above mentioned cases. Simplified computational domains are constructed to include only single lateral pitch based on the test models. The meshes are generated by ANSYS ICEMCFD ver. 12. The analyses are carried out by ANSYS CFX ver.12 with the employment of steady and unsteady Reynolds Average Navier Stokes analyses using the shear stress transports turbulence model. The experimental aerodynamic results are presented in the form of contour plot of various variables including film cooling effectiveness, normalized u, v, and w velocities. The velocity distributions show that at BR = 1.0, the lift-off effects are more apparent in the case of TMB in comparison with the TMA. The vector plots between the two test models also reveals the different in terms of the positioning of the counter rotating vortex core which is observed to be further away from the wall surface in TMB. The velocity plots also reveal that downstream of the cooling hole at the centerline, TMA is having higher streamwise velocity in comparison with TMB. These observations can be directly associated with the different in the inclination angle of the cooling hole. Steeper angle in the case of TMA allowed more streamwise momentum component to be ejected through the cooling hole in comparison with the case of TMB. The computational fluid dynamics investigation also reveals the occurrence of different flow structure inside the cooling hole between TMA and TMB. In the case of TMB, the separation which occurs at the inlet of the cooling hole triggers the formation of counter rotating vortex inside of the cooling hole and persists through the cooling hole. An accomplish counter rotating vortices can be observed at various plane inside the cooling hole. The same observation cannot be made for TMA, where the vector plot reveals that the secondary air which has been separated at the cooling hole inlet then reattached to the downstream wall of the cooling hole. This phenomenon hinders the formation of counter rotating vortex inside of the cooling hole which will only be completed by the cross-flow phenomena after the secondary air exiting the cooling hole. In comparison between TMA and TMG, both the experimental and computational results reveal that the shorter lateral pitch in TMG allows the secondary air to dominate the lateral space between cooling hole. The domination of the secondary air within the lateral space leads to greater blockage not only at the hole vicinity but also at the lateral space. The numerical results also show that the greater blockage which occur in TMG resulting a different interaction between the upcoming and the concurrent vortex cores in comparison with TMA. At wider lateral pitch of TMA, the upcoming vortex cores diffuse directly into the concurrent vortex cores keeping the secondary air in-line. At shorter lateral pitch of TMG, the upcoming vortex cores are deflected in the lateral direction before diffuse at further downstream of the cooling hole. The deflection cause the secondary air to disperses at the lateral space. The velocity plots also confirmed the interaction between the neighboring counter rotating vortices in the case of TMG which hindering it own growth and leads to lesser lift-off effects. The thermal investigations results are presented in terms of the contour plot of film cooling effectiveness, distribution of laterally average film cooling effectiveness and area average film cooling effectiveness along the x-axis. Benchmarking between the present and previous study are made to validate the present measurement and analysis approach. Good agreement has been achieved between the present and previous study. The film cooling effectiveness provided by TMA is higher at all blowing ratio in comparison with TMB particularly at BR=1.0 and 2.0. The thermal fields show that at BR=1 .O, the lift-off effects are more evident in the case of TMB in comparison with TMA. At BR=2.0, the thermal fields show complete detachment of the secondary air from the wall for the case of TMB leaving no trace of film cooling coverage on the surface. The thermal fields also indicate a lesser dispersion of the thermal field in the case of TMA in comparison with TMB which can directly be associated with the interaction rate between the secondary air and the mainstream air. In comparison between TMA and TMG, similar distribution of film cooling coverage can be observed downstream the first and second row cooling hole. Downstream the third and fourth row cooling hole, TMG start to provide better film cooling coverage. The contour plot shows an establishment of full coverage film cooling effectiveness region accommodating the lateral space between the cooling hole centerline. This observation serves the flow phenomena which have been describe earlier in the writing. The thermal fields also show the domination of the secondary air in the lateral space which is more apparent downstream of the third and fourth row cooling hole. Conclusions of the present study are summarized as follows; a) shallow hole angle configurations are definitely improving the film cooling coverage in comparison to the commonly used hole angle at 30" or 35". The superiority of the shallow hole angle is more apparent at higher blowing ratio cases in which the cooling protection provided at BR = 2.0 of shallow hole angle is a match to the result of BR = 2.0 of the 35" hole angle; b) at shorter lateral pitch distance, full coverage film cooling effectiveness were prevail indicating the benefit of the interaction between the secondary air of the neighboring cooling holes; c) the superposition effect induces by the in-line hole arrangement of the present study help to improve the film cooling performance with greater benefit were obtained by the shallow hole angle in comparison to the 35 degree hole angle; d) the CFD results confirm the formation of a counter rotating vortex pair inside of the cooling hole which perseveres until downstream of the cooling hole in all cases. Increase in the length of the cooling hole will allow the separated secondary air to reattach thus hindering the formation of the kidney vortices so as to better the film cooling effectiveness in the case of TMA and TMG; e) the domination of the mainstream air at the lateral space between the cooling hole in the case at longer lateral pitch distance kept the secondary jet in-line as it travel towards the downstream direction. It will cause the upcoming vortex core to directly infuses thus amplify the jetting effect of the downstream vortices which will ultimately minimizing the utilization of the secondary air; f) at the shorter lateral pitch distance, the narrow lateral space between the cooling hole enables the secondary air jet to interact with each other and prevent the mainstream air to dominating the lateral space resulting the upcoming vortices to be deflected while it approaching the downstream cooling hole. At the same time, the deflection helps to spread the secondary air laterally hence to better the film cooling effectiveness.

Item Type:Thesis (PhD)
Subjects:T Technology > TJ Mechanical engineering and machinery > TJ181-210.1 Mechanical movements
ID Code:4666
Deposited By:Normajihan Abd. Rahman
Deposited On:24 Dec 2013 16:09
Last Modified:24 Dec 2013 16:09

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