The durability of gas turbine engines is strongly dependent on the component temperatures. For the combustor and turbine airfoils and endwalls, film cooling is used extensively to reduce component temperatures. Film cooling is a cooling method used in virtually all of today's aircraft turbine engines and in many power-generation turbine engines and yet has very difficult phenomena to predict. The interaction of jets-in-crossflow, which is representative of film cooling, results in a shear layer that leads to mixing and a decay in the cooling performance along a surface. This interaction is highly dependent on the jet-to-crossflow mass and momentum flux ratios. Film-cooling performance is difficult to predict because of the inherent complex flowfields along the airfoil component surfaces in turbine engines. Film cooling is applied to nearly all of the external surfaces associated with the airfoils that are exposed to the hot combustion gasses such as the leading edges, main bodies, blade tips, and endwalls. In a review of the literature, it was found that there are strong effects of freestream turbulence, surface curvature, and hole shape on the performance of film cooling. Film cooling is reviewed through a discussion of the analyses methodologies, a physical description, and the various influences on film-cooling performance.

Gas Turbine Film Cooling

A detailed chemical kinetic model has been used to study dimethyl ether (DME) oxidation over a wide range of conditions. Experimental results obtained in a jet-stirred reactor (JSR) at I and 10 atm, 0.2 < 0 < 2.5, and 800 < T < 1300 K were modeled, in addition to those generated in a shock tube at 13 and 40 bar, 0 = 1.0 and 650 :5 T :5 1300 K. The JSR results are particularly valuable as they include concentration profiles of reactants, intermediates and products pertinent to the oxidation of DME. These data test the Idnetic model severely, as it must be able to predict the correct distribution and concentrations of intermediate and final products formed in the oxidation process. Additionally, the shock tube results are very useful, as they were taken at low temperatures and at high pressures, and thus undergo negative temperature dependence (NTC) behavior. This behavior is characteristic of the oxidation of saturated hydrocarbon fuels, (e.g. the primary reference fuels, n-heptane and iso- octane) under similar conditions. The numerical model consists of 78 chemical species and 336 chemical reactions. The thermodynamic properties of unknown species pertaining to DME oxidation were calculated using THERM.

/pdf/wide-range-modeling-study-of-dimethyl-ether-oxidation-pnag57ojwu.pdf

Wide range modeling study of dimethyl ether oxidation

Review on active thermal protection and its heat transfer for airbreathing hypersonic vehicles

The current work is an experimental investigation of the dependence of film cooling effectiveness on the injection Mach number, velocity, and mass flux. The freestream Mach number is 2.4, and the injection Mach numbers range from 1.2 to 2.2 for both air and helium injection. The adiabatic wall temperature is measured directly. The injection velocity and mass flux are varied by changing the total temperature and Mach number while maintaining matched pressure conditions between the injected flow and that of the freestream. The total temperature of the freestream is 295 K, and for the injection it ranges from 215-390 K. The results indicate an increase in film cooling effectiveness as the injection rate is increased. With the exception of heated helium runs, larger injection Mach numbers slightly increase the effective cooling length per mass injection rate. The results for helium injection indicate an increase in effectiveness as compared to that for air injection. Heated injection, with the injectant to freestream velocity ratios greater than 1, exhibit a rise in wall temperature downstream of the slot resulting in effectiveness values greater than 1. The experimental results are also compared with earlier studies in the literature.

Influence of injectant Mach number and temperature on supersonic film cooling

Recent advances in the shock wave/boundary layer interaction and its control in internal and external flows

A study has been made to investigate the flowfield of supersonic slot injection and its interaction with a two- dimensional shock wave. Air and helium were injected at Mach numbers of approximately 1.3 and 2.2 into an airstream of Mach 2.4. Measurements of the total pressure profiles perpendicular to the wall were made at sev- eral axial locations, the farthest being at 90 slot heights. The profiles provided details of the structure of the flow for the different injection conditions. With heated gas injection, experiments were conducted to determine the adiabatic wall temperatures and the wall static pressures. These measurements were then repeated with the impingement of two-dimensional shock waves at 60 slot heights downstream of the slot. The shock strengths were chosen to illustrate the differences between separated and attached flows. The shock strength that produced incipient separation was found to be smaller when helium was injected than when no film coolant was present. Conversely, the shock strength that produced incipient separation with air injection was slightly larger than that obtained without film cooling. develop relations for effectiveness as a function of downstream position divided by slot height x/s and the ratio of mass flux for the injected flow to that in the freestream, A, = (pw)//( pu)^. The phys- ical basis for these relations has also been motivated from simple integral analysis of the flowfield.5 The variation in the effectiveness with downstream position is accompanied by changes in flowfield structure. Some subsonic film-cooling experiments6 and other subsonic experiments involv- ing wall jets with moving freestream7 have shown that the flow- field can be divided into three regions: a potential core region, a wall-jet region, and a boundary-layer region. The potential core region, like a freejet, contains a viscous layer that emanates from the lip and ends when it meets the slot-flow boundary layer. In this region the wall temperature remains at a constant value equal to that of the injected fluid (in the case of subsonic injection) or equal to the recovery value (in the case of supersonic injection). Thus, the effectiveness in the potential core region is unity. The wall-jet region starts when the viscous layer emanating from the lip merges with the injectant boundary layer. In this region intense mixing takes place, and the wall temperature increases toward the freestream value. In the boundary-layer region, the flow then relaxes to that of a boundary layer. Consequently, the effectiveness decreases from unity near the injector and approaches zero far downstream. Thus, film-cooling flows combine different types of familiar flows: a freejet flow, a wake, a shear layer, and a bound- ary layer. The different hydrodynamic features of the flow in each region suggest using different scaling laws to predict effectiveness. This approach was attempted with some success in low-speed flow,8 where empirical data from jet flows and boundary-layer flows were applied for near and far regions, respectively. These approxi- mate flowfields were used in the energy equation to solve for the distribution in wall temperature and the film-cooling effectiveness. In these incompressible analyses, the thermodynamic properties were considered invariant within the flowfield.5 The film-cooling effectiveness in high-speed flow is often defined as T| = T -T (2)

Khalid A. Juhany

Papers

Influence of injectant Mach number and temperature on supersonic film cooling

Flowfield measurements in supersonic film cooling including the effect of shock-wave interaction

Statistical analysis of viscous hybridized nanofluid flowing via Galerkin finite element technique

Laminar burning velocity and flame structure of DME/methane + air mixtures at elevated temperatures

Effects of CO2/N2 dilution on laminar burning velocity of stoichiometric DME-air mixture at elevated temperatures.