Aerothermal Analysis of Film Cooling Flows

Flow field and thermal film cooling parameters are analysed and compared for different blowing ratios for a 10°-10°-10° laidback fan-shaped film cooling hole geometry with an inclination angle of 35° and a wide spacing of P/D = 8 to ensure the absence of jet interaction. The influence of the coolant ejection on the aerodynamics of the hot gas is investigated and stationary as well as non-stationary vortex structures are identified. The present study uses an existing test rig at the Institute of Thermal Turbomachinery (ITS) at the Karlsruhe Institute of Technology (KIT) designed for generic film cooling studies with high spatial resolution applying stereoscopic particle image velocimetry (SPIV) as well as infrared thermography (IRT). Operating conditions of hot gas and cooling air inlet and exit are uniquely compliant with scaled engine-realistic conditions, including temperature ratio, turbulence intensity and coolant flow configuration

Towards Quantitative Wall Shear Stress Measurements: A Comparative Study on the Impact of RGB-to-Hue-Conversion Algorithms for Liquid Crystal Diagnostics

Liquid crystal diagnostics is a capable tool for determining quantitative wall shear stress distributions with high spatial resolution, which can be applied to almost any surface shape. A standard consumer camera is typically used to record the scattered light of the liquid crystals as red, green, and blue RGB data. This RGB data has to be converted to a hue-based color space in order to perform a state-of-the-art calibration procedure. Algorithms for this purpose are numerous in the literature. However, a considerable number of them show a wide range of resulting hue values due to different trigonometric relations. This renders some conversion algorithms unsuitable for calculating physical wall shear stresses, as their magnitude and distribution depend on the conversion algorithm used. For this reason, the choice of an inappropriate conversion algorithm may compromise the measurement accuracy and subsequent comparability significantly. The main objective of this paper is to give recommendations for the use of appropriate algorithms to determine physical wall shear stresses. In a first step, synthetic liquid crystal data is converted using algorithms described in the literature. The preselected algorithms are then applied to liquid crystal data from a flat plate wind tunnel experiment to illustrate their influence on the determined uncalibrated wall shear stress distribution. The final discussion serves as guidelines for the post-processing of liquid crystal data and their subsequent comparability

Analysis of Heat Flux Distribution during Brush Seal Rubbing Using CFD with Porous Media Approach

This paper discusses the question of heat flux distribution between bristle package and rotor during a rubbing event. A three-dimensional Computational Fluid Dynamics (3D CFD) model of the brush seal test rig installed at the Institute of Thermal Turbomachinery (ITS) was created. The bristle package is modelled as a porous medium with local non-thermal equilibrium. The model is used to numerically recalculate experimentally conducted rub tests on the ITS test rig. The experimentally determined total frictional power loss serves as an input parameter to the numerical calculation. By means of statistical evaluation methods, the ma in influences on the heat flux distribution and the maximum temperature in the frictional contact are determined. The heat conductivity of the rotor material, the heat transfer coefficients at the bristles and the rubbing surface were identified as the dominant factors

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the Short Helical Combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultra-low NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In the present work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the Large Eddy Simulation method, the shape and lift-off height of the flame is adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected towards the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. The present study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications

Numerical Investigation of the Low-Swirl Flow in an Aeronautical Combustor With Angular Air Supply

Civil air traffic is predicted to further grow in the near future. Hence, the development of aeronautical combustors will face major challenges to meet future stringent environmental regulations. In the present study, an innovative gas turbine combustor with angular air supply called Short Helical Combustor (SHC) is investigated. The main feature of this concept is the helical arrangement of the fuel injectors around the turbine shaft. Aiming at the implementation of a lean-burn concept, a low-swirl lifted flame is adopted. This flame is lifted off and not anchored to the injector which opens the potential of low NOx emissions due to a high degree of premixing within the combustor. In this work, isothermal flow characteristics of such a generic SHC combustor are studied by use of RANS predictions with special emphasis on the interaction of adjacent low-swirl flows. For evaluating the influence of injector parameters on the flow field, a parametric study based on single sector simulations is performed. It is shown that the asymmetrically confined swirling jet flow is strongly deflected towards the sidewall of the staggered SHC dome. The deflection of the flow is associated with an asymmetric pressure field in the vicinity of the burner which is generally known as Coandă effect. As a consequence of the deflected flow, the angular momentum flux at combustor outlet is increased. The interaction of the low-swirl jet and the SHC sidewall is investigated with regards to backflow momentum and residence time in the recirculation zone. It is concluded that by modifying the momentum of the air flow through the injector, the amount of recirculating air flowing back along combustor walls is strongly affected. The present work establishes an understanding of the underlying aerodynamics of the SHC concept which is essential for matching the requirements of lean lifted flames

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the short helical combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultralow NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In this work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the large eddy simulation method, the shape and liftoff height of the flame are adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected toward the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. This study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications

Similarity solutions of a Blasius flow with variable fluid properties and viscous dissipation

An analytical model of the Blasius flow is studied including temperature-dependent fluid properties and viscous dissipation. The friction coefficient and Nusselt number at the wall are calculated from the resulting dimensionless velocity and temperature fields. The variable properties model is compared to a constant properties model to verify if and under which conditions this simplification is valid. Air, water and oil are analyzed as fluids over a representative operating regime, respectively. For air, the variable properties do not influence the friction coefficient and the Nusselt number. For water, the influence of the variable properties is present for both parameters but limited since no large temperature difference can occur in water without a phase change. New correlations for the friction coefficient and Nusselt number were derived for water and oil over a large range of operating conditions. Viscous dissipation does not significantly affect these parameters for air and water because of their relatively low Prandtl numbers. The high Prandtl number of oil in combination with a viscosity that is strongly decreasing with increasing temperature, leads to a more complex behavior. The friction coefficient as well as the Nusselt number are strongly dependent on the fluid properties. Dissipation effects cannot be neglected above an Eckert number of around 0.01. The superposition principle to evaluate wall heat flux in experiments is based on the assumption of constant fluid properties. It can be used without restrictions for air but should be thoroughly checked for all other fluids, especially liquids, using the presented methodology

Microscopic Imaging Spray Diagnostics under High Temperature Conditions: Application to Urea–Water Sprays

The quantitative investigation of droplet laden turbulent flows at high temperature conditions is of great importance for numerous applications. In this study, an experiment was set up for investigation of evaporating urea-water sprays, which are relevant for the effective reduction of nitrogen oxide emissions of diesel engines using Selective Catalytic Reduction. A shadowgraphy setup is pushed to its limits in order to detect droplet diameters as small as 4µm and droplet velocities up to 250m/s. In addition, the operating conditions of the gaseous flow of up to 873K and 0.6MPa are an additional challenge. Due to the high temperature environment, image quality is prone to be compromised by schlieren effects and astigmatism phenomena. A water-cooled window and an astigmatism correction device are installed in order to correct these problems. The results to be presented include characteristics of the turbulent gas flow as well as detailed spray characteristics at different positions downstream of the atomizer. It is demonstrated that the velocity of the gas can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. Furthermore, the statistical analysis of velocity fluctuations provides data for predicting the turbulent dispersion of the droplets

Prediction of Heat Transfer in a Jet Cooled Aircraft Engine Compressor Cone Based on Statistical Methods

The paper presents the setup and analysis of an experimental study on heat transfer of a jet cooled compressor rear cone with adjacent conical housing. The main goal of the paper is to describe the systematic derivation of empirical correlations for global Nusselt numbers to be used in the design process of a jet engine secondary air system. Based on the relevant similarity parameters obtained from literature, operating points are deduced leading to a full factorial design experiment to identify all effects and interactions. The varied similarity parameters are the circumferential Reynolds number, the non-dimensional mass flow, the non-dimensional spacing between rotor and stator, and the jet incidence angle. The range of the varied similarity parameters covers engine oriented operating conditions and is therefore suitable to predict Nusselt numbers in the actual engine component. In order to estimate measurement uncertainties, a simplified model of the test specimen, consisting of a convectively cooled flat plate, has been derived. Uncertainties of the measured quantities and derived properties are discussed by means of a linear propagation of uncertainties. A sensitivity study shows the effects of the input parameters and their interactions on the global Nusselt number. Subsequently, an empirical correlation for the global Nusselt numbers is derived using a multivariate non-linear regression. The quality of the empirical correlation is assessed by means of statistical hypotheses and by a comparison between measured and predicted data. The predicted values show excellent agreement with experimental data. In a wide range, accuracies of 15% can be reached when predicting global Nusselt numbers. Furthermore, the results of the sensitivity study show that pre-swirled cooling air does not have a positive effect on heat transfer