Dependence of Film Cooling Effectiveness on 3D Printed Cooling Holes

Film Cooling Effectiveness is closely dependent on the geometry of the hole emitting the cooling film. These holes are sometimes quite expensive to machine by traditional methods so 3D printed test pieces have the potential to greatly reduce the cost of film cooling experiments. What is unknown is the degree to which parameters like layer resolution and the choice among 3D printing technologies influence the results of a film cooling test. A new flat-plate film cooling facility employing oxygen sensitive paint (OSP) verified by gas sampling and the mass transfer analogy and measurements both by gas sampling and OSP is verified by comparing measurements by both gas sampling and OSP. The same facility is then used to characterize the film cooling effectiveness of a diffuser shaped film cooling hole geometry. These diffuser holes are then produced by a variety of additive manufacturing technologies with different build layer thicknesses. Technologies used include Fused Deposition Modeling (FDM), Stereo Lithography Apparatus (SLA) and PolyJet with build layer thicknesses ranging from 0.001D (25 μm) to 0.12D (300 μm). These are compared with an aluminum coupon manufactured by traditional machining methods. The objective is to determine if cheaper manufacturing techniques afford usable and reliable results. Tests are carried out at mainstream flow Mach number of 0.30 and blowing ratios (BRs) from 1.0 to 3.5. The coolant gas used is CO₂ yielding a density ratio of 1.5. Surface quality is characterized by an Optical Microscope that measures surface roughness. Test coupons with rougher surface topology generally showed delayed blow off and higher film cooling effectiveness at high blowing ratios compared to the geometries with lower measured surface roughness. At the present scale, none of the additively manufactured parts consistently matched the traditionally machined part, indicating that caution should be exercised in employing additively manufactured test pieces in film cooling work.