Achmed Schulz

Adiabatic Wall Effectiveness Measurements of Film-Cooling Holes With Expanded Exits

This paper presents detailed measurements of the film-cooling effectiveness for three single, scaled-up film-cooling hole geometries. The hole geometries investigated include a cylindrical hole and two holes with a diffuser-shaped exit portion (i.e., a fan-shaped and a laid-back fan-shaped hole). The flow conditions considered are the crossflow Mach number at the hole entrance side (up to 0.6), the crossflow Mach number at the hole exit side (up to 1.2), and the blowing ratio (up to 2). The coolant-to-mainflow temperature ratio is kept constant at 0.54. The measurements are performed by means of an infrared camera system, which provides a two-dimensional distribution of the film-cooling effectiveness in the near field of the cooling hole down to x/D = 10. As compared to the cylindrical hole, both expanded holes show significantly improved thermal protection of the surface downstream of the ejection location, particularly at high blowing ratios. The laidback fan-shaped hole provides a better lateral spreading of the ejected coolant than the fan-shaped hole, which leads to higher laterally averaged film-cooling effectiveness. Coolant passage cross-flow Mach number and orientation strongly affect the flowfield of the jet being ejected from the hole and, therefore, have an important impact on film-cooling performance.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Ma m =0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an enginelike density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio (M=2.5). In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally, the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

Assessment of Various Film-Cooling Configurations Including Shaped and Compound Angle Holes Based on Large-Scale Experiments

The demand of improved thermal efficiency and high power output of modern gas turbine engines leads to extremely high turbine inlet temperatures and pressure ratios. Sophisticated cooling schemes including film cooling are widely used to protect vanes and blades from failure and to achieve high component lifetimes. Besides standard cylindrical cooling hole geometry, shaped injection holes are used in modern film cooling applications in order to improve cooling performance and to reduce the necessary cooling air flow. However complex hole shapes may lead to manufacturing constraints and high costs. This paper evaluates some film-cooling injection geometry with different complexity. The comparison is based on measurements of the adiabatic film-cooling effectiveness and the heat transfer coefficient downstream of the injection location. In total, four different film-cooling hole configurations are investigated: a single row of fanshaped holes with and without a compound injection angle, a double row of cylindrical holes and a double row of discrete slots both in staggered arrangement. All holes are inclined 45 deg with respect to the models surface. During the measurements, the influence of coolant blowing ratio is determined. Additionally, the influence of cooling air feeding direction into the fanshaped holes with the compound injection angle is investigated. An infrared thermography measurement system is used for highly resolved mappings of the models surface temperature. Accurate local temperature data is achieved by an in-situ calibration procedure with the help of single thermocouples embedded in the test plate. A subsequent finite elements heat conduction analysis takes three-dimensional heat fluxes inside the test plate into account.

Correlation of Film-Cooling Effectiveness From Thermographic Measurements at Enginelike Conditions

Adiabatic film-cooling effectiveness on a flat plate surface downstream of a row of cylindrical holes is investigated. Highly resolved two-dimensional surface data were measured by means of infrared thermography and carefully corrected for local conduction and radiation effects. These locally acquired data are laterally averaged to give the streamwise distributions of the effectiveness. An independent variation of the flow parameters blowing rate, density ratio, and turbulence intensity as well as the geometrical parameters streamwise ejection angle and hole spacing is examined. The influences of these parameters on the lateral effectiveness is discussed and interpreted with the help of surface distributions of effectiveness and heat transfer coefficients presented in earlier publications. Besides the known jet in cross-flow behavior of coolant ejected from discrete holes, these data demonstrate the effect of adjacent jet interaction and its impact on jet lift-off and adiabatic effectiveness. In utilizing this large matrix of measurements the effect of single parameters and their interactions are correlated. The important scaling parameters of the effectiveness are shaped out during the correlation process and are discussed. The resulting new correlation is designed to yield the quantitatively correct effectiveness as a result of the interplay of the jet in crossflow behavior and the adjacent jet interaction. It is built modularly to allow for future inclusion of additional parameters. The new correlation is valid without any exception within the full region of interest, reaching from the point of the ejection to far downstream, for all combinations of flow and geometry parameters.

Film Cooling Effectiveness and Heat Transfer on the Trailing Edge Cutback of Gas Turbine Airfoils With Various Internal Cooling Designs

The present study deals with trailing edge film cooling on the pressure side cutback of gas turbine airfoils. Before being ejected tangentially onto the inclined cut-back surface the coolant air passes a partly converging passage that is equipped with turbulators such as pin fins and ribs. The experiments are conducted in a generic setup and cover a broad variety of internal cooling designs. A subsonic atmospheric open-loop wind tunnel is utilized for the tests. The test conditions are characterized by a constant Reynolds number of Re hg =250 000, a turbulence intensity of Tu hg =7%, and a hot gas temperature of T hg =500 K. Due to the ambient temperature of the coolant, engine realistic density ratios between coolant and hot gas can be realized. Blowing ratios cover a range of 0.20 <M<1.25. The experimental data to be presented include discharge coefficients, adiabatic film cooling effectiveness, and heat transfer coefficients in the near slot region (x/H<15). The results clearly demonstrate the strong influence of the internal cooling design and the relatively thick pressure side lip (t/H= 1) on film cooling performance downstream of the ejection slot.

High-Resolution Measurements of Local Effectiveness From Discrete Hole Film Cooling

Local adiabatic film cooling effectiveness on a flat plate surface downstream a row of cylindrical holes was investigated. Geometric parameters such as blowing angle and hole pitch, as well as the flow parameters blowing rate and density ratio, were varied in a wide range emphasizing engine relevant conditions. IR thermography was used to perform local measurements of the surface temperature field. A spatial resolution of up to seven data points per hole diameter extending to 80 hole diameters downstream of the ejection location was achieved. Since all technical wall materials have a finite thermoconductivity, a procedure for correcting the measured surface temperature data based on a Finite Element analysis was developed. Heat loss over the back and remnant heat flux within the test plate in lateral and streamwise directions were taken into account. The local effectiveness patterns obtained are systematically analyzed to quantify the influence of the various parameters. As a result, a detailed description of the characteristics of local adiabatic film cooling effectiveness is given. Furthermore, the locally resolved experimental results can serve as a data base for the validation of CFD codes predicting discrete hole film cooling.

Detached Eddy Simulation of Film Cooling Performance on the Trailing Edge Cutback of Gas Turbine Airfoils

The present study deals with the unsteady flow simulation of trailing edge film cooling on the pressure side cut-back of gas turbine airfoils. Before being ejected tangentially on the inclined cut-back surface, the coolant air passes a partly converging passage that is equipped with turbulators such as pin fins and ribs. The film mixing process on the cut-back is complicated. In the near slot region, due to the turbulators and the blunt pressure side lip, turbulence is expected to be anisotropic. Furthermore, unsteady flow phenomena like vortex shedding from the pressure side lip might influence the mixing process (i.e. the film cooling effectiveness on the cut-back surface). In the current study, three different internal cooling designs are numerically investigated starting from the steady RaNS solution, and ending with unsteady detached eddy simulations (DES). Blowing ratios M = 0.5; 0.8; 1.1 are considered. To obtain both, film cooling effectiveness as well as heat transfer coefficients on the cut-back surface, the simulations are performed using adiabatic and diabatic wall boundary conditions. The DES simulations give a detailed insight into the unsteady film mixing process on the trailing edge cut-back, which is indeed influenced by vortex shedding from the pressure side lip. Furthermore, the time averaged DES results show very good agreement with the experimental data in terms of film cooling effectiveness and heat transfer coefficients.Copyright

Film-cooling holes with expanded exits: near-hole heat transfer coefficients

Abstract This paper presents detailed measurements of local heat transfer coefficients in the vicinity of three film-cooling holes with different hole geometries including a standard cylindrical hole and two holes with a diffuser shaped exit portion (i.e. a fanshaped and a laidback fanshaped hole). Tests were conducted over a range of blowing ratios M =0.25…1.75 at an external crossflow Mach number of 0.6 and a coolant-to-mainflow density ratio of 1.85. Additionally, the effect of the internal coolant supply Mach number was addressed. Surface temperatures downstream of the injection location were measured by means of an infrared camera system and used as boundary conditions for a finite element analysis to determine surface heat fluxes and heat transfer coefficients downstream of the injection location. Furthermore, the superposition method was applied to evaluate the overall film-cooling performance of the hole geometries investigated by combining heat transfer and adiabatic cooling effectiveness data. As compared to the cylindrical hole, both expanded holes show significantly lower heat transfer coefficients downstream of the injection location, particularly at high blowing ratios. The laidback fanshaped hole provides a better lateral spreading of the injected coolant than the fanshaped hole which leads to lower laterally averaged heat transfer coefficients. Coolant passage crossflow Mach numbers affect the flowfield of the jet being ejected from the hole and, therefore, have an important impact on film-cooling performance.

Effect of a Crossflow at the Entrance to a Film-Cooling Hole

Understanding the complex flow of jets issuing into a crossflow from an inclined hole that has a short length-to-diameter ration is relevant for film-cooling applications on gas turbine blades. In particular, this experimental study focused on the effect of different velocities in a coflowing channel at the cooling hole entrance. Flows on both sides of the cooling hole (entrance and exit) were parallel and in the same direction. With the blowing ratio and the mainstream velocity at the hole exit remaining fixed, only the flow velocity in the channel at the hole entrance was varied. The Mach number at the hole entrance was varied between 0 < Mac < 0.5, while the Mach number at the hole exit remained constant at Ma∞ = 0.25. The velocity ratio and density ratio of the jet were unity giving a blowing ratio and momentum flux ratio also of unity. The single, scaled-up film-cooling hole was inclined at 30 deg with respect to the mainstream and had a hole length-to-diameter ratio of L/D = 6. Flowfield measurements were made inside the hole, at the hole inlet and exit, and in the near-hole region where the jet interacted with the crossflow at the hole exit. The results show that for entrance crossflow Mach numbers of Mac = 0 and 0.5, a separation region occurs on the leeward and windward side of the cooling hole entrances, respectively. As a result of this separation region, the cooling jet exits in a skewed manner with very high turbulence levels.

Transonic Film-Cooling Investigations: Effects of Hole Shapes and Orientations

The emphasis of the present study is to understand the effects of various flowfield and geometrical parameters in the nearfield region of a scaled-up film-cooling hole on a flat test plate. The effect of these different parameters on adiabatic wall effectivenesses, heat transfer coefficients, discharge coefficients and the near-hole velocity field will be addressed. The geometrical parameters of concern include several angles of inclination and rotation of a cylindrical film-cooling hole and two different hole shapes — a fanshaped hole and a laidback fanshaped hole. The fluid dynamic parameters include both the internal and external Mach number as well as the mainstream-to-coolant ratios of total temperature, velocity, mass flux, and momentum flux. In particular, the interaction of a film-cooling jet being injected into a transonic mainstream will be studied.This paper includes a detailed description of the test rig design as well as the measuring techniques. Firstly, tests revealing the operability of the test rig will be discussed. Finally, an outlook of the comprehensive experimental and numerical program will be given.Copyright