Sigmar Wittig

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.

Droplet evaporation modeling by the distillation curve model: accounting for kerosene fuel and elevated pressures

The emission of jet engines is strongly affected by the fuel preparation process. Due to stringent emission standards, the development of low emission combustor concepts, like lean premixed prevaporized combustion or rich quench lean burn, is an important goal. For the design process of advanced combustors, numerical methods become more and more important. In order to provide an accurate prediction of the fuel preparation process, an exact numerical prediction of thermophysical processes is crucial. A numerically effective fuel droplet evaporation model is presented in the present paper which accounts for the description of multi-component fuels like kerosene. Fractional boiling is described by a single process variable: the molar weight. This way, the fractional distillation process during evaporation of kerosene droplets is taken into account. In addition, a novel method for modeling the properties of the fuel is provided: the property data are supplied as a function of the molar weight. Real gas effects are also taken into account, in order to achieve an accurate prediction at elevated pressures. The major advantage of this new model is that algebraic expressions are derived for the multi-component droplet vaporization. Thus, the present model combines both numerical efficiency and accuracy. 2003 Published by Elsevier Ltd.

Measurements of the growth and coagulation of soot particles in a high-pressure shock tube

The advantage of well-defined experimental conditions in shock tubes has been used to investigate the growth and coagulation of soot particles at high pressures. The measurements have been made for fuel-rich oxidation and pyrolysis of different hydrocarbons behind the reflected shock at pressures between 10 and 60 bar and temperatures between 1500 and 2300 K. In addition to soot volume fraction, time-resolved scattering measurements yielded particle diameters and number densities; all these give insight into both surface growth and coagulation at enhanced pressures. The temperature behind the reflected shock was monitored by two-color pyrometry. Soot growth was characterized by induction periods and soot growth rates. At low final soot yields, the growth rate of soot depends on the square of the carbon concentration. At high soot yields, reduced growth rates of soot volume fraction were observed and can be attributed to a lack of growth species. At constant carbon concentration no pressure dependence of soot volume fraction could be found. Particle diameters between 15 and 40 nm were measured. The number density of particles was found to increase strongly with soot volume fraction. Calculated and measured particle number densities agree well during early soot growth. However, at longer times the experiments reveal coagulation rates which are significantly smaller than predicted. This behavior indicates that collisions of deactivated soot particles are characterized by sticking probabilities lower than unity. A correlation for the sticking probability has been established to match both the experimental results and calculations.

Discrete ordinates quadrature schemes for multidimensional radiative transfer

Abstract The fundamental problem of applying the method of discrete ordinates to radiative transfer predictions is the selection of the discrete directions and their associated weights. Both the accuracy of the solution and the computational effort depend on the angular discretization. This paper provides a sound mathematical methodology for the derivation of angular quadratures. By applying the collocation principle, the errors introduced by a quadrature are analysed and the constituting equations of angular quadratures are identified. Special emphasis is placed on the rotational invariance of the quadrature schemes. Multidimensional radiative transfer in participating media with isotropic and anisotropic scattering is accounted for throughout the analysis. A major goal of the present study is the construction of a new principle for multidimensional angular quadratures which is essentially a generalization of the principles employed for the well-known S n quadratures. The new construction principle has two major advantages. First, it enables a very flexible tailoring of quadratures according to the actual requirements. Second, compared to the S n quadratures, the new types of quadratures provide a higher accuracy while using the same number of nodal points.

Diffusion controlled evaporation of a multicomponent droplet: theoretical studies on the importance of variable liquid properties

Abstract A well-known multicomponent droplet vaporization model, the Diffusion Limit Model, has been extended to account for property variations in the liquid phase. The model has been tested for typical conditions of modern gas turbine combustors. The results for a hexane/tetradecane droplet show that the temperature- and concentration-dependence of the liquid properties affect the vaporization process, especially with regard to a reduced diffusional resistance. Additionally, remarkable variations of the refractive index are observed yielding helpful information for the estimation of errors in optical particle sizing techniques. Regarding comprehensive spray calculations, the use of the constant property formulation is recommended with improved reference values based on variable property calculations.

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.

CFD Analysis of Spray Propagation and Evaporation Including Wall Film Formation and Spray/Film Interactions

Abstract Addressing the numerical simulation of complex two-phase flows in gas turbine combustors, this study features a comprehensive approach to the coupled solution of the interacting flow fields of the gas phase, evaporating fuel spray and evaporating, shear-driven fuel wall film. The gas flow and wall-film flow are described in Eulerian coordinates and are both calculated in the same computational unit. A second, separate program is based on Lagrangian particle tracking to model spray dispersion and evaporation. To account for interaction effects, an iterative procedure is applied considering mutual mass, momentum and energy transfer between the three flow regimes. For a realistic modeling of spray/wall and spray/film interaction, the droplet-trajectory computation comprises a set of droplet-impact models covering a broad range of impact conditions. The results of a two-phase flow simulation in a schematic LPP combustor premix duct demonstrate the effects of phase interaction as well as spray/wall and spray/film interaction.