Film cooling is one of the methods used to cool gas turbine components. The coolant is air taken from the gas turbine compressor and injected through holes, which are drilled in blades and/or vanes. The ¯lm cooling holes can be produced by means of laser drilling. Small variations in laser settings can generate imperfections inside the hole. These imperfections are mostly discrete and can be produced anywhere inside the hole. The goal of this thesis is to resolve how the discrete production imperfection a®ects ¯lm cooling. To this end an experimental study was conducted, in which ¯lm cooling is modelled as a jet in a cross-°ow over a °at plate. This work was carried out as part of the STW project "Air ¯lm cooling through laser drilled nozzles" (EWO:5478). The e®ect of imperfection size even up to 25% of the hole diameter can be neglected if the imperfection is located deep inside the hole (more than 2 diameters). Imperfections located near the hole exit can have either a positive or negative in°uence on ¯lm cooling. The shape and position play a crucial role in this region. Measurements have been conducted in a water channel. The set-up was con- structed and scaled using the Reynolds number (Rex = 3:23 x 105 and Red ¼ 104). The °ow was visualised by laser induced °uorescence and the velocity ¯eld was mea- sured with particle image velocimetry. The temperature on the plate wall was mea- sured by means of thermochromic liquid crystals. To investigate the in°uence of the imperfection position, a half-torus was placed at di®erent depths inside the jet hole. Besides the imperfection position, its size and shape were also changed. Both symmet- rical and asymmetrical imperfections have been studied. The experiments performed with a perfect hole are analysed, compared to literature and used as a benchmark. The velocity ratio and turbulence intensity were also varied. Experiments conducted with the perfect hole show that the jet cross-°ow interac- tion with the ¯lm cooling characteristics generates coherent structures, which are the most responsible for the momentum and heat transfer. Four large vortical structures are formed: the counter rotating vortex pair, windward, lee and spiral vortices. The experiments performed with an imperfect hole show that a discrete imperfection inside the hole induces an instability of the jet cross-°ow system. This instability, generated by the symmetrical imperfection placed at the hole exit, forces the counter rotating vortex pair to oscillate and it stays in the vicinity of the plate wall. This mecha- nism reduces the exchange of the momentum and heat between the jet and cross-°ow °uids. Therefore, the ¯lm cooling e®ectiveness is increased. The imperfection, ¯xed 1:2 diameters inside the hole, produces larger windward and lee vortices than the 148 Summary perfect hole. These vortices are big and they lead to the breakdown of the jet. This bursting mechanism lifts the cooled volume and enlarges the counter rotating vortex pair, which as a result has the smaller e®ectiveness than the perfect hole. The symmetrical imperfection at the hole exit improves ¯lm cooling at moder- ate and large blowing ratios while the symmetrical imperfection ¯xed 1:2 diameters inside the hole reduces the e®ectiveness. It is remarkable that the windward asym- metrical imperfection at the same position (1:2 diameters inside the hole) enhances the e®ectiveness. To resolve heat transfer, taking into account an e®ect of compressibility, experi- ments have also been conducted in a Ludwieg tube. Besides the parameters investi- gated in the water channel the Mach and Reynolds numbers can be changed indepen- dently. The heat °ux was measured on the °at plate with the thin-¯lm gauges. It is found that an increase in the Mach number (0:11 <M <0:47) reduces a normalised heat °ux. The ¯lm cooling e®ectiveness and normalised heat transfer coe±cients are calculated and analysed. The imperfection placed 1:2 diameters inside the hole de- creases the normalised heat transfer coe±cients and ¯lm cooling e®ectiveness. There- fore, the normalised heat °ux is enhanced. Qualitatively, conclusions gained from the Ludwieg tube measurements agree with the results obtained from the water channel measurements.
|Qualification||Doctor of Philosophy|
|Award date||5 Oct 2006|
|Place of Publication||Eindhoven|
|Publication status||Published - 2006|