Experimental and numerical studies on film cooling with reverse/ backward coolant injection Kuldeep Singh, B. Premachandran * , M.R. Ravi Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi,110016, India article info Article history: Received 3 February 2016 Received in revised form 21 September 2016 Accepted 22 September 2016 Keywords: Film cooling Forward hole Reverse hole Injection angle Discharge coefficient abstract The conventional forward injection for film cooling with cylindrical holes, where the axial component of the coolant velocity is aligned with mainstream flow direction creates kidney vortices. This results in quick mixing of the coolant with the mainstream. The conventional anti-kidney vortices cooling holes require shaping or branching which adds to the cost and complexity of the system. In this paper, reverse/ backward injection is proposed to improve film cooling. In the case of reverse/backward injection the secondary air is injected such that its axial velocity component is in the reverse direction to that of the mainstream. Film cooling is studied experimentally and numerically on a flat plate with forward and reverse injection. The injection angle of the cooling hole is varied from 30  to 60  in both forward and reverse directions at five blowing ratios ranging from 0.25 to 3.0 at a fixed density ratio of 0.91. The length to diameter ratio of the cooling hole is kept at 5 and the mainstream Reynolds number is maintained at 3.75  10 5 . Film cooling effectiveness obtained with the reverse holes is found to be much higher than that of the forward holes. Improvement in the area weighted average values of film cooling effectiveness for blowing ratio, M ¼ 1 is 170%, 78% and 186% for injection angles 30  , 45  and 60  respectively. Coefficient of discharge obtained from reverse injection is found to be smaller than that of forward injection. The film cooling effectiveness in the case of reverse injection is found to be less sensitive to the injection angle. © 2016 Elsevier Masson SAS. All rights reserved. 1. Introduction Film-cooling is used extensively in gas turbine engines for cooling of components exposed to hot gases. In film-cooling, rela- tively cold secondary fluid is injected into the hot flow through holes on the surface of the component. The injected cold fluid displaces the hot fluid and forms a layer between the surface to be protected and the hot gases. A coolant layer extends in the down- stream direction for a distance determined by the mixing of coolant with the hot gases [1]. In film-cooling, the holes from which the secondary fluid or the coolant is injected are inclined with reference to the surface to be cooled. The flow separates from the wall fluid just downstream of the injection hole and splits into counter rotating vortices, popu- larly known as kidney vortices [2]. These vortices are influenced by the operating parameters and hole design. Operating parameters such as blowing ratio, density ratio and momentum flux ratio affect the generation and growth of kidney vortices [3]. Out of the design parameters, the hole inclination, orientation and shape influence the growth of kidney vortices [4]. The presence of kidney vortices increases the mixing of secondary fluid with the hot mainstream. Hence, kidney vortices must be minimized or eliminated to main- tain maximum coverage of the surface with coolant film and hence better film cooling. In order to suppress the generation of kidney vortices, to avoid the lift off of secondary fluid jet and the associated undesirable effects, shaped holes are used in the film cooling. The study of Goldstein et al. [5] is recognized as the first investigation of shaped holes in film cooling studies. The shaped holes have circular cross section which acts as throat or metering section, while the outlet end of the cooling hole is shaped as a diffuser with a divergence angle 10  e15  in the lateral direction as well as in the flow direction [6]. Based on the expansion of the hole, shaped holes are classified as: ‘fan-shaped’, if the expansion is in the lateral direction, ‘laidback’ if the expansion is in the direction of the surface. The purpose of expansion of the hole is to reduce the momentum of the secondary fluid which in turn decreases the penetration and hence mixing of the secondary fluid into * Corresponding author. E-mail address: prem@mech.iitd.ac.in (B. Premachandran). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2016.09.027 1290-0729/© 2016 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 111 (2017) 390e408