Experimental investigation of the boundary layer transition phenomenon in gas turbines : effect of the gas-to-wall temperature ratio

The transition of the boundary layer from a laminar to a turbulent state is considerably impacting heat transfer in gas turbines. The understanding of the transition phenomenon is, therefore, essential to achieve better performances and efficiencies. Although the influence of various parameters has already been studied, the effect of the gas-to-wall temperature ratio is still little investigated in the literature. The present study is focused on the influence of the gas-to-wall temperature ratio on the boundary layer transition. Experimental data was recorded using the isentropic compression tube facility of the von Karman Institute for Fluid Dynamics. The investigation was carried out with a linear cascade made of five turbine guide vanes. Twenty-seven platinum thin films, located along the suction side of the third (central) blade, are used to assess temperatures and heat fluxes thanks to the coupling with analogue circuits. Upstream and downstream flow conditions are determined by means of total temperature, total pressure and static pressure measurements. Tests were performed at freestream turbulence intensities ranging from 0.8% to 6%. Tested exit isentropic Mach numbers were between 0.55 and 1.05; Reynolds numbers, from one to 2.2 million. Results were obtained at gas-to-wall temperature ratios ranging from 1.1 to 1.55. Boundary layer behaviour and transition were analyzed based on the evaluation of the convective heat transfer coefficient, the root mean square deviation and the skewness of the wall heat flux. Under subsonic conditions, low turbulence intensities and at an exit isentropic Reynolds number of one million, increasing temperature ratios appear to stabilize the laminar boundary layer as the transition onset moves downstream. The opposite was observed at higher tested freestream turbulence, Mach and Reynolds numbers.