(depth of feld) Focal
Film cooling is performed almost exclusively through the use of
discrete holes and rows of evenly spaced holes. All film-cooling
holes are either round or shaped. Shaped holes are composed of
round metering or throat sections with a uniform and symmetric
expanded exit region on the hot gas surface. Most commonly, all
shaped holes applied in practice have fan diffuser exits with divergence angles between 10° and 15° on each lateral side, as well as
on the side into the surface.
One major advance has been the change from round holes to
fan-shaped or diffuser holes. Although the use of round cooling
holes is prevalent, aerospace designers increasingly use more
complex geometries such as diffuser holes. To enhance air flow,
diffuser holes vary in shapes and depths to blend into the 3D shape
of the part detail (FIGURE 4). Their geometries can range from tapered
round cones to squares or rectangles. The final through-hole is
often not centered to the outer diffuser shape.
These differing holes may have a specific form and function, either
of limited or widespread potential, but each must also ultimately face
the challenges of manufacturing, operability, and cost-effectiveness.
In the past, manufacturing constraints have limited the geometry
of film holes. These factors included film hole length-to-diameter
ratio, film hole axis angle to the external surface tangent, and the
specification of the hole exit shaping. Laser machining with five-axis computer numerical control (CNC) has enabled these limitations to be overcome. FIGURE 3. An example of a turbine blade with coolant holes.
FIGURE 2. Traditional dry laser (a) and LMJ wet laser (b) focusing
mechanisms are compared.