Investigation Of The Film Cooling Characteristics Of Cylindrical Holes Integrated With Saw-tooth Slots

As is one of the most effective cooling techniques,film cooling means cool air bled from film hole attach to the surface of blades to prevent hot free-stream gas to heat blades.In this paper,the influence of slot depths、corner angles、hole pitches and blowing ratios on both film cooling effectiveness and heat transfer coefficient values of cylindrical holes integrated with saw-tooth slots were experimentally studied.The mechanism of improving film cooling performance by saw-tooth slot was numerically studied.In experimental study,the depths of slot to diameter ratio are 0,0.25 and 0.5,the corner angles of slot are 60,12 and 180,the pitches to diameter ratio are 2,3 and 4,the blowing ratios are 1.0、1.5 and 2.0.Both film cooling effectiveness and heat transfer coefficient values are obtained.The results show that compared with typical cylindrical holes,the saw-tooth slot improves the span-wise spread of film and prevents the cooling jet separate from the surface at higher ratios.At lower blowing ratio of 1.0,the surface averaged film effectiveness of saw-tooth slot is higher 53% than that of cylindrical hole.At higher blowing ratio of 2.0,the surface averaged film effectiveness of saw-tooth slot is higher 178% than that of cylindrical hole.Due to the saw-tooth slot,the heat transfer coefficient increases near the hole,but decreases at a long distance away from the hole.With the increase of the depth of slot,the film is more equally distributed and the heat transfer coefficient is higher.But the increase of the depth of slot results in the decrease of the span-wise averaged film cooling effectiveness.With the decrease of corner angles of slot,the film spread in span-wise direction more efficiently,but the heat transfer coefficient is higher near the hole,and get closer to the other geometry at a long distance away from the hole.With the decrease of the pitch of hole,the span-wise averaged film cooling effectiveness increase.But the cooling jet lift-off phenomenon occurs at blowing ratio 2.0 for the pitch to diameter ratio of 2.0.In order to explore the mechanism of improving film cooling by saw-tooth compared to transverse slot,the flow and the temperature distributions,film cooling effectiveness of these two structures which have the hole pitches of 2,3 and 4 respectively,were numerically investigated at blowing ratios of 0.5 and 1.5 respectively.The results show that the saw-tooth slot have a strong ability of span-wise spreading of the coolant compared to the transverse slot.For saw-tooth slot,the vortex pairs existing hole center line become weak and the additional vortex pairs easily appear at the mid-span region,resulting in that film cooling effectiveness is much higher than that of transverse slot.At blowing ratio of 0.5,the surface averaged film effectiveness of saw-tooth slot is higher 27%,35% and 42% respectively than that of transverse slot with hole pitches of 2,3 and 4.At blowing ratio of 1.5,the surface averaged film effectiveness of saw-tooth slot is higher 27%,95% and 151% than that of transverse slot with hole pitches of 2,3 and 4.