ICC-2: heat transfer characterization

ICC-1: Fluid dynamics characterization

As gas turbine inlet temperatures can nowadays reach values as high as 1800°C in modern engines, vanes and blades have to be efficiently cooled to operate without failure. Internal cooling can provide the large forced convection heat transfer rates which are needed in the airfoil inner surfaces to prevent the metal from overheating. Obstacles and turbulators are provided inside the cooling channels of the turbine blades for this purpose. The ICC-1 and ICC-2 facilities are designed to reproduce these configurations to investigate the aerodynamic and thermal characteristics of these highly turbulent flow fields.

The experimental facility dedicated to the fluid dynamics characterization (ICC-1) consists of five sections specifically designed to employ water as working fluid: settling chamber, development duct, test section, outlet channel and discharge chamber. Steady boundary conditions are maintained by first accumulating water in the settling chamber equipped with an adjustable overflow channel. By setting the height of the overflow channel, the water column upstream of the inlet is kept constant during the experiments. This allows the fine tuning of the driving pressure and the Reynolds number in the test section, avoiding the unsteadiness that the pump may introduce in the inlet flow.

The mass flow set during the experiments corresponded to a typical Reynolds number observed in internal cooling channels, low enough to eventually allow corresponding LES computations. The maximum Reynolds number that the facility can reach is about 30000.

The settling chamber discharges the water in the inlet duct, presenting a length equal to 2250 mm to promote flow development ahead of the measurement section. With the aim of suppressing any possible swirling motion generated in the settling chamber, a honeycomb is placed at the entry of the development duct.

The test section is placed just downstream of the inlet duct, and it is made of acrylic glass to obtain full optical access to the area of interest. The test section can be equipped with turbulators placed on the inner surface. Stereoscopic Particle Image Velocimetry (S-PIV) was applied to measure the three components of the mean and turbulent velocities in the symmetry plane of the channel.

ICC-2 is the experimental facility dedicated to the heat transfer characteristics of ribbed cooling channels. The mass flow is generated by a centrifugal fan placed at the outlet of the facility, being operated in suction mode. In this way, the inlet flow is not affected by the highly turbulent flow that the fan produces. The working fluid (air) first enters the inlet duct, of 150 mm diameter and 3000 mm length.

Thus, the ratio of the inlet pipe length to the diameter is equal to 20, which allows developing a turbulent boundary layer at the inlet of the test section, located just downstream of the inlet duct. The test section is an acrylic tube of 150 mm internal diameter and 980 mm length divided into two halves, to allow the preparation of the heating surface and the calibrations required before the tests. A layer of TLCs is applied first on the inner wall of the pipe, followed by the black matt paint and the Inconel foil. In this way, the color distribution of the TLC layer can be observed from the exterior when the Inconel foil is heated, as shown in Fig. 9. Two thermocouples are placed at the inlet and outlet of the test section to obtain the reference temperature to calculate the transfer coefficient at the area of the measurement area, which is located 600 mm downstream of the inlet of the test section. The mass flow exiting the test section is directed through a U-bend towards the exit pipe. An orifice plate is placed in the exit pipe at a distance of 30 pipe diameters downstream the U-bend to monitor the mass flow in the experimental facility. The rotational speed of the fan is varied to adapt the pressure drop across the orifice plate to the required Reynolds number (max value of Re=80000). Downstream of the mass flow meter, the fluid is finally discharged into a settling chamber, to which the blower is directly connected. The inner surface temperature distribution (T_w) of the test section has been measured by using the same type of TLCs as in the previous case, active from 35 to 55oC (308.15 to 328.15 K).

To increase the wall temperature and to activate the TLCs, a DC voltage is applied to the Inconel foil, inducing the heat transfer between the foil and the fluid. The transparency of the Plexiglas and the black paint allows an SLR camera to acquire RGB intensities scattered by the TLCs. The SLR camera is focused on the test section from a distance of about 850 mm with an angle of 45° concerning the horizon. The part of the image processed is called Region of Interest (ROI).

ICC-2: heat transfer characterization

ICC-1: Fluid dynamics characterization