EFFECTS OF OSCILLATIONS IN THE MAIN FLOW ON FILM COOLING AT VARIOUS FREQUENCIES AND BLOWING RATIOS
Open Access
- Author:
- Baek, Seung Il
- Graduate Program:
- Acoustics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 21, 2018
- Committee Members:
- Savas Yavuzkurt, Dissertation Advisor/Co-Advisor
Victor Ward Sparrow, Committee Chair/Co-Chair
Stephen Clarke Conlon, Committee Member
Robert Keolian, Committee Member
Savas Yavuzkurt, Outside Member - Keywords:
- Film cooling
Turbulent flows
Numerical simulation
Film cooling effectiveness
Stanton number ratio - Abstract:
- The objective of this study was to investigate the effects of oscillations in the main flow on gas turbine film cooling at various single frequencies from 0 to 2144 Hz (non-dimensional frequencies F: 0 to 5.36). The single frequencies were identified as the dominant frequencies from a Fourier analysis of combustor instability data on combustor pressure fluctuations. Numerical simulations are carried out using ANSYS Fluent LES and URANS turbulence models. The results show that if the oscillation frequency of the main flow is increased from 0 to 180 Hz (F: 0 to 0.45) at low blowing ratio of M = 0.5, the film cooling effectiveness is decreased due to increase of jet lift off leading to entrainment of hot main flow under the jet. However, when the frequency goes from 180 to 268 Hz (F: 0.45 to 0.67), the film cooling effectiveness is dramatically increased because a thin coolant film near the wall is overlapped by large vortices of the coolant created during the higher velocity part of the cycle at 268 Hz. The large vortices prevent the thin film near the wall mixing with the hot main flow leading to decrease of the wall temperature. If the frequency changes from 268 to 1072 Hz (F: 0.67 to 2.68), the effectiveness drops because the large vortices start overlapping and moving away from the wall by the vertical component of the jet momentum resulting in entrainment of the hot main flow under the jet. If the frequencies exceed 1072 Hz (F: 2.68 to 5.36), the film cooling effectiveness is increased because the coolant jet cannot respond to these high frequencies and the coolant behavior returns to that at 0 Hz gradually. The trends of the effectiveness at high blowing ratio of M = 1.0 are similar with those at M = 0.5 except from 0 to 180 Hz. At M = 1.0, coolant jet lift off is created under the steady state conditions. However, if the oscillation frequency goes from 0 to 180 Hz, the coolant flapping is generated leading to lower wall temperature (also lower adiabatic wall temperature which is used in calculation of the film cooling effectiveness, this results in higher effectiveness) compared to the wall temperature or adiabatic wall temperature under the steady flow conditions. In terms of heat transfer coefficients, if the frequency is increased from 0 to 536 Hz (F: 0 to 1.34) at low blowing ratio of M = 0.5, spanwise-averaged Stanton number ratio is increased because of increase of the disturbances in the flows. However, if the frequency goes from 536 to 2144 Hz (F: 1.34 to 5.36), the spanwise-averaged Stanton number ratio is decreased since the coolant jet cannot respond to high frequency oscillations and return to that at 0 Hz. The trends of the spanwise-averaged Stanton number ratio at high blowing ratio are similar with those at M = 0.5 except from 0 to 180 Hz. If the frequency is increased from 0 to 180 Hz at M = 1.0, more entrainment of the hot main flow is induced leading to less mixing near the wall and decrease of the Stanton number ratio. Further, multi-frequency velocities are applied to the main and coolant flow inlets and the results are compared to those at single frequencies. Some representative results are: If the frequency goes from 0 to 2, 16, 32, 90 or 180 Hz at M = 0.5, the spanwise-averaged effectiveness drops about 11%, 12%, 45%, 62%, or 75% respectively. If the frequency changes from 0 to 2, 16, 32, 90 or 180 Hz at M = 1.0, the spanwise-averaged effectiveness is increased about 0.3%, 1%, 3.7%, 4% or 4.1%. Besides the sinusoidal waveform, the triangular or the rectangular waveforms are applied to the main flow and the coolant jet oscillations and the effects of changing oscillation waveform shape on film cooling are investigated. At the low blowing ratio, the rectangular waveform for the main flow oscillation at 2 Hz has a negative effect on adiabatic film cooling effectiveness, while the rectangular waveform for coolant oscillation at 32 Hz has a positive effect on adiabatic film cooling effectiveness. Additionally, the acoustic behavior of the film cooling hole is described by using the Rayleigh conductivity. It was found that the acoustic role of film cooling hole to mitigate the growth of thermoacoustic instabilities and to reduce the amplitude of the sound pressure oscillations is effective especially for low frequencies less than 500 Hz and low incident angles.