Non-zero temperature gradients in temperature-sensitive paint (TSP) cause the apparent temperature (the temperature measured by the TSP) to correspond to the temperature somewhere inside the TSP, which does not equal the top surface temperature. Treating the apparent temperature as the average temperature across the TSP layer is not always accurate, especially when there is a large temperature gradient in the TSP. In this paper, the apparent temperature is theoretically derived by integrating the luminescent intensity across the TSP layer and numerically determined using a Monte Carlo ray-tracing method. The results of a simulation show that the difference between the apparent and average temperatures increases with the temperature gradient in the TSP layer, which leads to a non-negligible error when determining the surface heat flux based on the average temperature. A modified Levenberg–Marquardt algorithm is used to accurately recover the surface heat flux based on the apparent temperature for typical optical conditions in the measurement system. The results are compared with those obtained using the average temperature of the TSP. The effects of heat flux, TSP thickness and base material on the errors in the determination of the heat flux are investigated in detail.
The adiabatic film cooling effectiveness behind a single row of circular holes fed by internal crossflow was measured by fast-response pressure-sensitive paint technique. During the experiment, the coolant flow was discharged from the coolant holes via either plenum or crossflow channel. The test model has a row of circular holes with 3D spacing, 6.5D entry length, and 35° inclination angle. Two blowing ratios (M = 0.40 and 0.80) were tested with a density ratio of 0.97. A numerical steady-state RANS simulation, using SST k-ω and Realizable k-ε turbulence models, was conducted to understand the internal crossflow behaviors. The unsteadiness caused by the flow structures (counter-rotating vortex pair (CRVP) and horseshoe vortex) was quantified by the root mean square and the cross-correlations. In addition, the proper orthogonal decomposition was used to identify the large-scale unsteady coherent structures and their contributions. The fluctuations of the crossflow feed were asymmetric, which were significantly weaker compared with the plenum case. The CRVP, as the most significant coherent structures, were demonstrated to play the main role in the unsteadiness of the crossflow feed.
Practical strategy for the thermal evaluation of film-cooled blade is of great importance to the gas turbine community. Due to the physical or methodology limitations, it is difficult to evaluate the blade's thermal performance at simulated engine conditions. The present study proposed novel focal-sweep-based phosphor thermometry for blade cooling inspection. While Mg4FGeO6:Mn (MFG) served as the temperature sensor to quantify the blade temperatures as well as simulated the TBC effect, the focal sweep method was adopted to overcome the optical constraints in cascade testing. The obtained MFG results of microstructures, jet impingement, and anti-erosion test demonstrated that the MFG phosphor is robust enough to simulate the thermal insulation effect of TBC and can withstand high-speed flow erosion. Furthermore, the proposed strategy clearly captured the blade temperature distributions (mainstream at T_(0,8)=~850 K) with high spatial resolution, which was then successfully remapped onto the three-dimensional twisted blade. Additional comparisons with the thermocouples demonstrated that the simulated-TBC has a thermal insulation effect of about 68K. This study addressed the common problems of phosphor thermometry in blade cooling evaluation, offering a practical strategy for future thermal diagnostics of the gas turbine.
An experimental study was conducted to investigate the oscillating freemstream effect on the Spatio-temporal distributions of leading-edge film cooling effectiveness. The investigation utilized Fast-PSP technique at high frame rate. During the experiment, coolant was discharged into three rows of cylindrical holes. Various blowing ratios were tested under the steady (i.e., f = 0 Hz) and oscillating (i.e. f = 7 Hz and 25 Hz) conditions. The measured instantaneous effectiveness was analyzed in terms of time-averaged and phase-averaged results. The results revealed that the mainstream oscillation, consisting of simultaneous pressure and velocity oscillation, significantly influences the behavior of the film cooling effectiveness. The time-averaged effectiveness significantly decreased at high oscillating frequency (i.e., 13.0 - 19.8% reduction at M = 0.50, f = 25 Hz compared with f = 0 Hz), especially at low blowing ratios (i.e., M = 0.50 and 0.75). The phase-averaged results captured significant decay in the film distributions associated with backflow caused by negative pressure gradients in coolant holes at certain phases. However, the oscillation effect was relatively insignificant at high blowing ratios, which revealed the robustness of coolant coverage at low coolant Strouhal number under the same oscillating frequency. Furthermore, the unsteady coolant intermittency showed highly unstable film coverage at high coolant Strouhal number. The coolant decay associated with backflow at high coolant Strouhal number should be considered by the gas turbine designers in order to improve the lifecycle of turbine blades.
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