Boundary layer transition and convective heat transfer of the high-pressure turbine vane LS89

Boundary layer transition and convective heat transfer of the high-pressure turbine vane LS89 by Tânia Sofia CAÇÃO FERREIRA

Pour l’obtention du grade de Docteur en sciences de l’ingénieur et technologie

Boundary layer transition from laminar to turbulent generates high levels of shear stresses and heat transfer rates, influencing the overall efficiency of the aircraft engine. This is especially detrimental to the performance of the high-pressure turbine, which is subjected to elevated levels of flow turbulence and temperature gradients from the upstream combustor, and to strong stream-wise pressure gradients. Hence, a more complete understanding of the transition phenomenon and combustor turbulence would help in its prediction for the design and analysis of more efficient blades and vanes.

Our main goal is to improve the limited knowledge on the boundary layer transition physics in the high-pressure turbine. Since turbine experimental testing in facilities is usually on the lower range of turbulence intensity, how does increasing the cascade inlet turbulence to the levels found downstream of a combustor chamber affect the boundary layer state and heat transfer? And are the thermal effects negligible for bypass transition? Our research focus is in understanding the weight and consequence of high free-stream turbulence and gas-to-wall temperature ratio on boundary layer transition, and to extend our high-pressure turbine heat transfer database to account also for these effects. Our experimental boundary layer studies are based on time-resolved wall heat flux measurements performed in the Isentropic Compression Tube (CT-2) facility at the von Karman Institute. This facility simulates the flow conditions found in an aircraft engine turbine in terms of free-stream turbulence, Mach and Reynolds numbers, and gas-to-wall temperature ratios (TR), which are independently variable. The test model is a linear five vane cascade of the VKI LS89 highly loaded turbine guide vane profile.

We first developed a new turbulence grid to generate turbulence levels in excess of 10%, for more engine-representative inlet flow turbulence. The resulting passive square/diamond bar grid induces turbulence intensities between 10-26% and integral length scales in the order of 1-2 cm.

The TR survey shows that there is a small effect on the stability of the boundary layer. Increasing the flow temperature led to a later/longer transition, but only when the pressure gradients were mild. A clear effect on the transition length was observed at a free-stream turbulence of 6%.

The high levels of turbulence intensity led to strong changes in boundary layer transition and heat transfer when comparing to a natural turbulence case. Between turbulence levels, however, the pressure gradients mostly drive transition, i.e., delaying transition onset with acceleration and triggering it with deceleration. Increasing the turbulence intensity could shift transition for a fully subsonic velocity distribution, but for transonic, steeper pressure gradients, the effect was limited to a reduction of a shock-induced separation bubble, and slightly earlier transition onsets, still suppressed by strong accelerations. Varying the facility Reynolds number had a small coupling with the Mach number, but it was still clear the overall increase of heat flux, and the earlier transition.

Implicit LES in collaboration with Cenaero led to the high-fidelity simulation of the LS89 cascade. By means of their numerical code Argo and cluster Zenobe, we simulated an important test case (MUR235), with both bypass transition, relaminarization, and shock-induced transition. While the heat transfer profile could not be perfectly matched, we could observe a high sensitivity to a numerical parameter, the artificial viscosity (AV). Depending on the level of AV, which could not be decoupled from the shock and boundary layer, an almost perfect match is obtained for MUR235 up to the turbulent boundary layer, which remained under-predicted. However, increasing the resolution, consequently, reducing the AV, led to a very distinct transition behavior. The resulting heat transfer profile indicates a laminar boundary layer up to the shock, which is instead in line with the present measurements.

In sum, experimental and numerical endeavors imply that boundary layer transition is even more sensitive to the environment than expected, with special emphasis on the pressure gradients.

Jury members :

• Prof. Vincent Legat (UCLouvain), supervisor
• Prof. Tony Arts (UCLouvain), supervisor
• Prof. Laurent Delannay (UCLouvain), chairperson
• Prof. Francesco Contino (UCLouvain), secretary
• Prof. Sergio Lavagnoli (von Karman Institut, Belgium)
• Prof. Koen Hillewaert (Université de Liège, Belgium)

Pay attention :

The public defense of Tania Sofia Cação Ferreira scheduled for Thursday 29 April at 5:00 p.m will take place in the form of a video conference Teams