Sarah M. Coulthard Ralph J. Volino e-mail: volino@usna.edu Karen A. Flack Department of Mechanical Engineering, United States Naval Academy, Annapolis, Maryland 21402 Effect of Unheated Starting Lengths on Film Cooling Experiments The effect of an unheated starting length upstream of a row of film cooling holes was studied experimentally to determine its effect on heat transfer coefficients downstream of the holes. Cases with a single row of cylindrical film cooling holes inclined at 35 deg to the surface of a flat plate were considered at blowing ratios of 0.25, 0.5, 1.0, and 1.5. For each case, experiments were conducted to determine the film-cooling effectiveness and the Stanton number distributions in cases with the surface upstream of the holes heated and unheated. Measurements were made using an infrared camera, thermocouples, and hot and cold-wire anemometry. Ratios were computed of the Stanton number with film cooling St f  to corresponding Stanton numbers in cases without film cooling St o , but the same surface heating conditions. Contours of these ratios were qualitatively the same regardless of the upstream heating conditions, but the ratios were larger for the cases with a heating starting length. Differences were most pronounced just downstream of the holes and for the lower blowing rate cases. Even 12 diameters downstream of the holes, the Stanton number ratios were 10–15% higher with a heated starting length. At higher blowing rates the differences between the heated and unheated starting length cases were not significant. The differences in Stanton number distributions are related to jet flow structures, which vary with blowing rate. DOI: 10.1115/1.2184355 Introduction Film cooling has been studied extensively in order to provide improved cooling of the airfoils in gas turbine engines and thus increase the life of the airfoils and allow for higher turbine inlet temperatures. Both the film cooling effectiveness and the enhance- ment of the heat transfer coefficient caused by the film cooling jets are of interest. Many studies have been done using simple flat plate geometries to gain more insight into the physics of the prob- lem. To determine the heat transfer coefficient, surface heaters are typically used to provide a known uniform heat flux at the plate surface. Most of the previous flat plate experimental studies of heat transfer coefficients with film cooling have included an un- heated starting length, with heaters only located downstream of the film cooling holes. An unheated starting length will result in a thinner thermal boundary layer and higher heat transfer. It is typi- cally assumed that the ratio, h f / h o , of the heat transfer coefficient with film cooling to the heat transfer coefficient in a similar flow without film cooling and the same surface heating will not be greatly affected by the presence of an unheated starting length. Examples of studies with unheated starting lengths include Sen et al. 1 and Schmidt and Bogard 2. Although it is certainly plau- sible that an unheated starting length will affect a film cooled and uncooled boundary layer similarly, there is surprisingly little veri- fication of the assumption in the literature. Mayhew et al. 3 conducted experiments in a facility with a heated region upstream of the film cooling holes. They compared their results to data from similar studies with unheated starting lengths and attributed dif- ferences observed in heat transfer coefficient ratios to unheated starting length effects. The heat transfer ratios were larger in the heated starting length cases. Mayhew et al. 3 noted that since the thermal boundary layer is thicker in these cases, the film cooling flow may have more of an opportunity to disturb the thermal boundary layer and increase heat transfer. The only known study of unheated starting length effects is by Kelly and Bogard 4. They considered full coverage film cooling on a flat plate with normal injection, presenting heat transfer coefficient ratios at three streamwise locations downstream of the first row of holes for one of their cases. At x =2D, directly downstream of the holes, h f / h o was 30% higher with a heated starting length than with an un- heated starting length. At x =4D downstream of the holes, the effect was reduced, and by x =10D downstream of the holes the heated and unheated starting length cases were indistinguishable. At the midspan between adjacent holes, the unheated starting length had no effect on h f / h o . No experimental results appear to be available in the literature for other geometries. In the present study, heaters were placed on the surface of a plate upstream, downstream, and between the holes in a single row of film cooling holes. Tests were run with only the down- stream heaters on, with the upstream and downstream heaters on, and, finally, with all the heaters on to determine the effect of the earlier start of the thermal boundary layer. The film cooling ge- ometry consisted of a single row of five round holes inclined at 35 deg to the surface and parallel to the streamwise direction. The holes were spaced 3D apart, center to center, with a length-to- diameter ratio L / D = 4. The geometry matches that used by Burd and Simon5, Gritsch et al. 6, Kohli and Bogard 7, Sinha et al. 8, and Pietrzyk et al. 9. It is similar to that used by Mayhew et al. 3, who used a 30 deg injection angle. Blowing ratios of 0.25, 0.5, 1.0, and 1.5 were investigated. Experimental Facilities and Techniques Experiments were conducted with an open loop subsonic wind tunnel with a test plate attached at the exit and a plenum to supply the film cooling jets. The wind tunnel, shown in Fig. 1, was com- prised of six sections: a blower, a diffuser with three screens, a heat exchanger to maintain air nominally at 20° C, a honeycomb, a settling chamber with three screens, and a nozzle with an 8.8 area reduction. The nozzle exit area is 0.38 m  0.10 m. The ex- iting mainstream air was uniform in temperature and velocity to within 0.1° C and 1%, respectively. The freestream turbulence in- tensity at the nozzle exit was 1%. This value is lower than typical intensity levels in an engine, which will depend on the location in Contributed by the Turbomachinery Division of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received December 19, 2005; final manu- script received January 16, 2006. Review conducted by D. Wisler. Journal of Turbomachinery JULY 2006, Vol. 128 / 579 Copyright © 2006 by ASME