Hans-Jörg Bauer

Reliable and Accurate Prediction of Three-Dimensional Separation in Asymmetric Diffusers Using Large-Eddy Simulation

Large-eddy simulations (LES) and Reynolds-averaged Navier―Stokes (RANS) calculations of the flow in two asymmetric three-dimensional diffusers were performed. The setup was chosen to match an existing experiment with separation. Both diffusers possess the same expansion ratio but differ in performance. The aim of the present study is to find the least expensive method to reliably and with reasonable accuracy account for the impact of the change in geometry. RANS calculations failed to predict both the extent and location of the separation. In contrast, LES with wall-functions delivered results within the accuracy of the experimental data.

Heat Transfer Measurements on a Turbine Airfoil With Pressure Side Separation

Heat transfer measurements on a highly loaded low-pressure turbine airfoil with a separation bubble on the pressure surface are presented. The experiments were conducted in a linear cascade at various free-stream turbulence intensities (Tu1 = 1.6% to 10%) and Reynolds numbers of the inflow. The effect of both quantities on heat transfer, separation and laminar-turbulent transition is quantified. Particle-Image-Velocimetry has been performed to study the characteristics of the separation bubble. The results reveal a considerable influence of the boundary layer separation on the local heat transfer. The size of the separation region is strongly influenced by free-stream turbulence level and Reynolds number.Copyright

Investigation of the Effect of Incomplete Droplet Prevaporization on NOx Emissions in LDI Combustion Systems

The influence of incomplete liquid fuel prevaporization on the emissions of nitric oxides in a swirl stabilized model gas turbine combustor is investigated experimentally and numerically. The design of the model combustor enables the variation of the degree of prevaporization. This is achieved by using two liquid fuel injectors. One injector is located far upstream of the combustor and generates a fully prevaporized and premixed air fuel mixture. The second injector is located at the combustor inlet. Consequently, the liquid fuel mass flow split between the two injectors determines the fraction of nonprevaporized fuel present in the reaction zone. The NO/NO 2 measurements were performed with a chemoluminescence analyzer. In accordance to the findings of other researchers, the present experimental study revealed that the influence of prevaporization on nitric oxide emissions is of significance for practical applications. The experimental studies were accompanied by numerical studies of partially prevaporized lean combustion in an ed configuration. The purpose of this numerical study is to gain a detailed understanding of the influence of droplet slip on droplet flame position and peak temperature. The droplet slip velocity was found to have a significant impact oh the peak temperature of the droplet flame and, therefore, NO formation rates within the droplet flame. The combustion system used for the experimental investigation was characterized regarding droplet slip velocities with an extended laser Doppler anemometry technique. The comparison between numerical and experimental results shows that the droplet slip velocities in the macroscopic reaction zone are within the transition range from an envelope to a wake flame. It is concluded that small-scale mixing effects play a significant role in the formation of nitric oxides in spray combustion systems with incomplete prevaporization.

Performance of Prefilming Airblast Atomizers in Unsteady Flow Conditions

Within the context of lean premixed prevaporized combustion (LPP) which is considered as most promising technology for the next generation of low emission combustors for aero engines, combustion instabilities are a major issue. These combustion instabilities may compromise the pollutant emissions and even cause damage to the combustion chamber structure. In the literature, numerous phenomenological studies on combustion oscillation are available, but a comprehensive theory is still missing. One potential excitation mechanism is the interaction of strong air velocity fluctuations and pressure oscillations with the airblast atomizer leading to temporal fluctuations of the spray characteristics. This phenomenon was investigated experimentally at the Institute of Thermal Turbomachinery (ITS) within a parametric study. A duct with a prefilming surface was set up as an abstraction of a prefilming airblast atomizer. A mean air velocity up to 65 m/s can be reached, and periodic oscillations can be superimposed by means of a siren with a frequency up to 570 Hz. The disintegration process of the liquid fuel was studied downstream the atomizing edge of a plain airblast nozzle. Several optical diagnostics like phase resolved LDV (Laser Doppler Velocimetry) and an improved PTV technique (Particle Tracking Velocimetry) were used. The mean air velocity, the film load, the kinematic viscosity and the surface tension of the fluid as well as the pulsation frequency and amplitude of the siren were varied, and their effect on the temporal evolution of the droplet size and droplet rate was studied. It was found that the amplitude of fluctuations of the droplet size and the droplet rate is almost proportional to the air velocity fluctuations at low frequencies. At higher frequencies, however, both are nearly unaffected. In addition, the fluctuations of droplet diameter and rate increase strongly if the mean air velocity is increased. The phase shift between particle diameter, particle rate and air velocity fluctuations was found to increase at higher excitation frequencies.© 2006 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 analysed experimentally, in order to characterize the turbine performance. The turbine scroll pressure ratio is varied, leading to unequal twin turbine admission conditions. The flow behaviour is analysed 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.Copyright

Influence of Honeycomb Facings on the Temperature Distribution of Labyrinth Seals

As labyrinth seals with honeycomb facings become more usual, the present paper reveals part of the honeycomb parameters which have an influence on the temperature drop across the seal and the temperature distribution along the rotor and the stator. Numerical and first experimental analyses with varying honeycomb parameters, pressure ratios and seal clearances, typical for engine operating conditions, have been carried out on a non-rotating setup in order to exclude other influencing parameters, such as windage heating. As expected, the numerical analyse shows that the honeycomb diameter, the honeycomb height and the seal clearance are of great importance for the temperature distribution of the rotor and the stator and the temperature drop across the seal.Copyright

Modelling of an Aero-Engine Bearing Chamber Using Enhanced CFD Technique

This manuscript presents the application of an improved CFD methodology to simulate the scavenge system film flow phenomena in a real aero engine bearing chamber environment i.e. influence of seals and rotational shaft. Near the scavenge off-take, the usual thin film approach is not valid due to the occurrence of relative thick films (up to 5mm, comp. [1]) where film internal dynamics become very important. Therefore, other multiphase modelling techniques need to be explored. Young and Chew suggest in [2] that the Volume Of Fluid (VOF) method is the most suitable technique for air/oil system applications. Hashmi et al. reported in [3] that this free surface method for shear driven thick wall films in the bearing chamber environment needs additional provisions for turbulence modelling. Accordingly, a simple correction is made to the k–e RNG turbulence model to improve the simulation results. The improved CFD methodology is applied to an engine representative geometry and proves to be robust and computationally efficient. The test conditions in the simulation was chosen in a way to avoid any droplet stripping from the film surface. It is shown that the applied methodology together with the correction in the turbulence modelling prove to play a vital role for a good comparison with experimental data. After validation the simulation results are used to describe the flow phenomena which occur in the bearing chamber for the investigated condition. The introduced CFD modelling technique shows large potential for the development and trouble shooting purposes in the industrial environment.Copyright

CFD Methods for Shear Driven Liquid Wall Films

Shear driven liquid wall films or physically similar two phase flow phenomena can be found in a number of different industrial and engineering applications. Gear boxes, bearing chambers or combustors in aero engines, heat exchanger ducts, oil and gas production and transport in petrochemical industry are just a few examples where this phenomenon is present and has been studied for decades. The most common approach of modeling shear driven film flows consist of empirical correlations derived from simple experiments. This approach is reasonable but highly case dependent. The problem lies in the difficulty of achieving experimental data for cases of practical importance. For a more global approach in this respect, CFD can be a useful tool. Therefore the study presented in this paper is dedicated to explore the potential of modern CFD methods. All available multiphase flow models are analyzed for their applicability for subcritical shear driven wall films (no mass transfer, no droplet shedding/deposition from/to film). VOF is suggested to be the only available multiphase flow model applicable to shear driven flows. However, further investigations have revealed that VOF method in its original form is not suitable for the flow conditions leading to high interaction between the phases i.e. where the motion of slow moving heavier phase is dictated by the fast moving lighter phase. This shortcoming in the VOF method is explained by means of a false momentum transfer between the phases. The focus then turns to find the improvement possibilities in the VOF method. Two approaches can be found in literature addressing the improvement possibilities in VOF method. The approach of physically justified modification of the turbulence quantities at the gas-liquid interface is adopted in this paper and is referred to as interface treatment. The approach is applied to a simple test case where the liquid phase acts as a wall. The results achieved for this test case are compared to the validation data where remarkable improvements are observed when compared to the VOF method without interface treatment. The interface treatment is then applied to a case of more practical importance where improvements are clearly evident again. Due to the lack of quantitative information on the interfacial waves, outlet boundary conditions cannot be well defined at this point. Therefore the later case is only seen as a motivation for further investigation of this approach.Copyright

Experimental Investigation of Lubrication Oil Film Dynamics in a Typical Aero-Engine Bearing Chamber Environment

This paper deals with the wall film dynamics of lubrication oil in the vicinity of the scavenge port of a typical aero engine bearing chamber. Based on the major driving forces influencing the film dynamics, shear forces and gravity, two film flow regimes namely co & counter-current can be identified. The film flow in the bearing chambers is influenced by several factors which results in an undeveloped film thus making a comprehensive analysis of the flow field extremely difficult. A profound knowledge of the individual factors is required before a superposition can be performed. A simple test rig was designed and built to isolate and investigate the influence of major factors affecting the wall film dynamics in a typical bearing chamber environment. For the quantitative analysis, a parameter determined from the single phase pressure drop measurements (only in gas) is introduced to effectively analyze the undeveloped multiphase flow regime. It is shown that the momentum losses occurring in the counter-current regime become considerably greater than in the co-current regime when the shearing gas flow rate is increased beyond a certain value. For high gas flow rates (high shaft speeds in engine) the losses on the counter-current side (churning losses) can be several magnitudes larger than on the co-current side (no churning losses). It is also shown that for the conditions investigated and relevant to the bearing chamber, the possibility of waveforms leading to the droplet shedding on the co-current side is very unlikely. Significant droplet shedding occurs in the counter-current regime. Based on the results obtained, the possible characteristics of the near scavenge oil film in the absence of coexisting phenomena e.g. droplet interaction, offtake disturbances etc are outlined.© 2011 ASME

DNS of Stagnation Point Heat Transfer Affected by Varying Wake-Induced Turbulence

In the present study a tandem cylinder setup is simulated by means of embedded DNS. The influence of wake turbulence on the heat transfer in the stagnation region of the rear cylinder is investigated. The oncoming flow is varied by increasing the distance between the two cylinders, causing a change of the turbulent wake characteristics and the heat transfer. The data of both simulations show good agreement with an existing experimental correlation in the literature. For the small wake generator distance, a clear shift of the maximum heat transfer is observed away from the stagnation line. This shift is less pronounced for the larger distance.Copyright