Numerical Studies on The Effect of Swirl and Transient Temperature Data on Flat Surface Film Cooling

Abstract

Higher inlet temperatures to increase the efficiency of gas turbine engines require improved component cooling. In this regard, the shape of the film cooling hole proved to impact the cooling performance considerably. Plenty of research are being carried out to develop a novel film cooling hole concept. In film cooling flows, the counter-rotating vortex pair (CRVP) is of paramount significance due to its inherited characteristics in lifting the coolant away from the wall. Most of the advanced hole design concepts are aimed at weakening the strength of this vortex, there by reducing jet penetration and enhancing film cooling effectiveness. The present thesis is intended to study the film cooling behavior in the presence of a twisted tape insert inside a film hole through computational fluid dynamics simulations. The twisted tape insert imparts a swirl to the coolant flow. The presence of serpentine passage channels and impingement angles are the sources of swirl generators on the coolant flow during internal cooling. The twisted tape insert can further be used to control such swirls systematically. This method allows controlling the coolant swirl intensity by varying the pitch of the twisted tape. The present study investigates swirl numbers of 0.0289, 0.116 and 0.168 at the beginning. Area-averaged effectiveness and heat transfer coefficient are evaluated as the measure of performance parameters for blowing ratios of 0.5, 1.0, 1.5 and 2.0. Results revealed a significant improvement in the effectiveness in the presence of swirl. Coolant swirl predominantly modifies the jet trajectory resulting in a reduced jet penetration and increased lateral expansion. Further investigation on the effect of twisted tape thickness on the coolant distribution is found to be negligible. But inserting a twisted tape created higher pressure losses across the hole with a nearly 11% reduction in discharge coefficient, indicating additional pumping power requirements. Further, the study is extended to higher swirl numbers of 0.2, 0.4,0.6 and 0.7. For blowing ratios less than 1.0, peak effectiveness occurred at a swirl number of around 0.4, enhancing effectiveness up to 180%. While at higher blowing ratios, the effectiveness peaks at higher swirl numbers. The spatially averaged effectiveness for ithe case of blowing ratio 2.0 and swirl number 0.6 has increased by almost 2500%. The case with swirl number of 0.7 resulted in high cooling efficiency in the immediate downstream of injection at high blowing ratios while completely ignoring the far downstream region. Mean while, the average net heat flux reduction due to swirl is achieved as high as 500% when blowing ratio and swirl numbers are at 1.5 and 0.6, respectively. The swirl effect on CRVP is apparent at high swirl numbers creating a highly asymmetrical structure. On the other hand, heat transfer coefficients are seen barely affected by the presence of swirl. A parametric study by varying injection angle and hole length at a particular swirl number and blowing ratio revealed enhancement in the effectiveness with the angle, a contrary feature to the absence of swirl. However, the hole length effect was moderate on the swirled film cooling behavior. It also attempted to obtain an optimized geometrical combination of swirl number, injection angle and hole length, employing a Radial Basis Function Neural Network as a surrogate model and Genetic algorithm as an optimizing tool. At a blowing ratio of 1.0, the optimized parameters of geometry were obtained as a swirl number of 0.32, injection angle of 19.3° and hole length to diameter ratio of 4. In the last part of this thesis, an inverse heat conduction based data reduction technique is proposed for the simultaneous estimation of film cooling effectiveness and heat transfer coefficient from transient temperature measurements. This method employs an optimization technique known as the Levenberg-Marquardt Algorithm to estimate the unknown thermal boundary parameters of film cooling. The objective function for the inverse algorithm is constructed using the analytical solution of a transient one-dimensional semi-infinite body. The transient surface temperature data required for the analysis is obtained through a conjugate numerical simulation. Laterally averaged effectiveness and heat transfer coefficient for blowing ratios of 0.5, 0.8, and 1.0 are analyzed using the present technique and compared against the steady-state simulation results to demonstrate the methodology. An average deviation of around 7% for the estimated effectiveness and 4% for the heat transfer coefficient values are observed. This method avoids the existing two-test strategy and yields unknown parameters with short duration measurements.