Hans-Jörg Bauer

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.

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

Soot Formation in a Shock Tube under Elevated Pressure Conditions

Abstract High pressure soot formation from methane, ethylene, acetylene, propane and n-heptane was studied at rich burning conditions applying the shock tube technique. Pressure behind reflected shock was varied between 15 and 100 bar. Time resolved measurements of soot particle diameter and number density were carried out using an extinction-scattering technique at 488 nm. It could be shown that soot formation at high pressures is characterized by particle diameters below 30 nm that decrease with pressure. The corresponding high particle number densities in the range of N≈1012 — 10131/cm3 turned out to be considerably higher than at atmospheric conditions. This behavior has to be attributed to reduced coagulation coefficients in the transition regime between free molecular and continuum flow. It was found that an increase in carbon concentration has a strong promoting influence on soot volume fraction. Total pressure, however, does significantly enhance soot yield at pressures up to 30 bar and loses its ...

A novel calibration method for an infrared thermography system applied to heat transfer experiments

In heat transfer measurements with highly non-uniform wall heat fluxes, high spatial resolution of wall temperatures is required to fully capture the complex thermal situation. Infrared thermography systems provide that spatial resolution. To meet the thermal accuracy, they are usually calibrated in situ using thermocouples embedded in the test surface, which have to cover the complete temperature range of interest. However, thermocouples which are placed in regions of high temperature and heat flux gradients often cannot be used for the calibration and the overall accuracy of the calibration decreases significantly. Therefore, in the present work a novel in situ calibration method is presented which does not require thermocouples over the complete surface temperature range. The number of free parameters of the calibration function is reduced by an optimized insensitivity of the system with respect to changes in operating conditions. Reference measurements demonstrate the advantages of the new method.

Experimental Investigation of the Total Temperature Increase and Swirl Development in Rotating Labyrinth Seals

Labyrinth seals are widely used as reliable components in many areas of turbo machines, e.g. the cooling air system in gas turbines. While the discharge behavior is generally well predictable, the uncertainty predicting the exit circumferential velocity (exit-swirl) and the total temperature increase due to internal losses (windage heating) is comparably large. In order to evaluate analytical correlations and for the validation of numerical simulations convergent and divergent stepped labyrinth seals were investigated experimentally. The change in total temperature across the labyrinth seal was measured in a test rig capable to establish different rotational speeds, pressure ratios and various inlet swirls. In an engine, honeycomb abrasive liners on the stator protect the seal fins. To simulate real engine conditions honeycombs were applied in the test setup, too and the influence of these liners on the windage heating was compared to smooth stator configurations. Detailed velocity profiles within the seal chambers were determined using a 2D Laser-Doppler-Velocimeter. Additionally, the ability of axisymmetric numerical k-e simulations to predict the data was evaluated. The present study provides important data for the design of future turbo machines, because the exact knowledge of the labyrinth seal exit swirl and temperature is expected to further improve the design of downstream components such as the pre-swirl system. Additionally, more accurate boundary conditions for the thermal analysis will be available and the rotor dynamic stability of the seal can be estimated better.Copyright

Numerical Study of Bicomponent Droplet Vaporization in a High Pressure Environment

The understanding of multicomponent droplet evaporation in a high pressure and high temperature gas is of great importance for the design of modern gas turbine combustors, since the different volatilities of the droplet components affect strongly the vapor concentration and, therefore, the ignition and combustion process in the gas phase. Plenty of experimental and numerical research is already done to understand the droplet evaporation process. Until now, most numerical studies were carried out for single component droplets, but there is still lack of knowledge concerning evaporation of multicomponent droplets under supercritical pressures. In the study presented, the Diffusion Limit Model is applied to predict bicomponent droplet vaporization. The calculations are carried out for a stagnant droplet consisting of heptane and dodecane evaporating in a stagnant high pressure and high temperature nitrogen environment. Different temperature and pressure levels are analyzed in order to characterize their influence on the vaporization behavior. The model employed is fully transient in the liquid and the gas phase. It accounts for real gas effects, ambient gas solubility in the liquid phase, high pressure phase equilibrium and variable properties in the droplet and surrounding gas.It is found that for high gas temperatures (T = 2000 K) the evaporation time of the bicomponent droplet decreases with higher pressures, whereas for moderate gas temperatures (T = 800 K) the lifetime of the droplet first increases and then decreases when elevating the pressure. This is comparable to numerical results conducted with single component droplets. Generally, the droplet temperature increases with higher pressures reaching finally the critical mixture temperature of the fuel components. The numerical study shows also that the same tendencies of vapor concentration at the droplet surface and vapor mass flow are observed for different pressures. Additionally, there is almost no influence of the ambient pressure on fuel composition inside the droplet during the evaporation process.© 1996 ASME

Understanding the Twin Scroll Turbine: Flow Similarity

The current study investigates the flow conditions of a twin scroll asymmetric turbine. This is motivated by the operating conditions of the turbine at a heavy-duty reciprocating internal combustion engine with exhaust gas recirculation. The flow conditions of the turbine at the engine can be described best with the turbine scroll interaction map. Standard hot gas measurements of a turbocharger turbine are presented and discussed. Due to the strong interaction of the turbine scrolls, further hot gas measurements are performed at partial admission conditions. The turbine inlet conditions are analyzed experimentally, in order to characterize the turbine performance. The turbine scroll pressure ratio is varied, leading to unequal twin turbine admission conditions. The flow behavior is analyzed regarding its ability for further extrapolation. Beyond scroll pressure ratio variations, unequal temperature admission conditions were studied. A way of characterizing the representative turbine inlet temperature, regarding the reduced turbine speed, is presented. The different scroll parameter ratios are evaluated regarding their capability of describing flow similarity under different unequal turbine admission conditions. In this content, turbine scroll Mach number ratio, velocity ratio and mass flow ratio are assessed. Furthermore, a generic representation of the turbine flow conditions at the engine is presented, based on standard turbine performance maps.

A Finite Element Treatment of the Angular Dependency of the Even-Parity Equation of Radiative Transfer

The present article introduces a new method to solve the radiative transfer equation (RTE). First, a finite element discretization of the solid angle dependence is derived, wherein the coefficients of the finite element approximation are functions of the spatial coordinates. The angular basis functions are defined according to finite element principles on subdivisions of the octahedron. In a second step, these spatially dependent coefficients are discretized by spatial finite elements. This approach is very attractive, since it provides a concise derivation for approximations of the angular dependence with an arbitrary number of angular nodes. In addition, the usage of high-order angular basis functions is straightforward. In the current paper, the governing equations are first derived independently of the actual angular approximation. Then, the design principles for the angular mesh are discussed and the parameterization of the piecewise angular basis functions is derived. In the following, the method is applied to one-dimensional and two-dimensional test cases, which are commonly used for the validation of approximation methods of the RTE. The results reveal that the proposed method is a promising alternative to the well-established practices like the discrete ordinates method (DOM) and provides highly accurate approximations. A test case, which is known to exhibit the ray effect in the DOM, verifies the ability of the new method to avoid ray effects.

Modeling of Rough-Wall Boundary Layer Transition and Heat Transfer on Turbine Airfoils

A new model for predicting heat transfer in the transitional boundary layer of rough turbine airfoils is presented. The new model makes use of extensive experimental work recently published by the current authors. For the computation of the turbulent boundary layer, a discrete element roughness model is combined with a two-layer model of turbulence. The transition region is modeled using an intermittency equation that blends between the laminar and turbulent boundary layer. Several intermittency functions are evaluated in respect of their applicability to rough-wall transition. To predict the onset of transition, a new correlation is presented, accounting for the influence of freestream turbulence and surface roughness. Finally, the new model is tested against transitional rough-wall boundary layer flows on high-pressure and low-pressure turbine airfoils.

Analysis and Comparison of Primary Droplet Characteristics in the Near Field of a Prefilming Airblast Atomizer

As the dominating parameters influencing the Sauter mean diameter of the spray produced by a prefilming airblast atomizer, the air velocity, liquid surface tension and atomizing edge thickness could be identified.Correlations for the prediction of the droplet sizes produced by prefilming airblast atomizers are compared to droplet sizes measured close to the atomizing edge. The measurements were performed using three geometrical variants of a planar atomizer over a wide range of operating conditions. The diagnostics are based on a particle and ligament tracking technique, that enables simultaneous measurement of the liquid blobs and ligaments formed at the atomizing edge and the droplets in the primary breakup region of the atomizer.The comparison between measured and calculated droplet diameter indicates, that most of the correlations are capable of reproducing the correct tendency. However, since the measurement data of most correlations were collected in a region where secondary atomization effects can obscure the initial droplet sizes in the primary breakup region, the droplet sizes are generally predicted too small.Copyright