Michael Willmann

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.

Experimental and theoretical study of one- and two-component droplet vaporization in a high pressure environment

Abstract A new experimental set-up is introduced where the evaporation of free falling, non-interacting droplets is investigated. Detailed measurements with one- and two-component droplets are presented for different pressures ( p → 40 bar) and gas temperatures (T → 650 K). The experimental results are compared with numerical calculations based on the Conduction Limit model and the Diffusion Limit model.

Comparative mass flux measurements in sprays using a patternator and the phase-Doppler technique

The phase-Doppler technique (PDA) has become one of the most important instrumental methods in two-phase flow research in recent years because of its capability of simultaneously determining the diameter and velocity of spherical particles. However, the shortcomings of this technique with respect to volume flux measurement have become increasingly apparent. In order to study the performance of different PDA instruments, comparative flux measurements in two characteristic sprays using a patternator and three phase-Doppler systems were performed at the Institut fur Thermische Stromungsmaschinen of the University of Karlsruhe (ITS) in cooperation with the Lehrstuhl fur Stromungsmechanik of the University of Erlangen-Nurnberg (LSTM) and DANTEC/invent Measurement Technology GmbH in Erlangen. The results of this study clearly indicate that especially in dense sprays, mass flux determination by the phase-Doppler technique is still critical. Further efforts to improve the reliability of the system and to identify the possible sources of errors are necessary. However, the dual-mode technique shows a significant improvement compared with the other instruments under investigation.

Heat up and evaporation of shear driven liquid wall films in hot turbulent air flow

Abstract Evaporating shear driven liquid wall films play an important role in the fuel preparation process of advanced gas turbine combustion chambers. In extending earlier studies of Sill, K.H., 1980. Sammelband der VGB-Tagung, May 1980, pp. 232–238 and Himmelsbach, J., Noll, B., Wittig, S., 1994. Int. Journal of Heat and Mass Transfer 37, 1217–1226, the present analysis of shear driven liquid films in a rectangular model duct is directed into the boundary layer flow at the gas-liquid interface and the wall film itself. The experimental conditions range from 314 to 373 K at atmospheric pressure giving duct flow Reynolds numbers of 81 000–162 000. The liquid mass flux for the film is varied between 33.3 g/sm and 100 g/sm. The experimental analysis includes spatially resolved measurements of interfacial shear stress, gas phase velocity and temperature profiles as well as the wall conditions. The film itself is investigated regarding its thickness and evaporation behaviour. Finally, a reference data set for comparison with CFD-data is obtained. The results obtained suggest that the theoretical models which are currently in use should be modified with regard to the interfacial shear stress development in the axial direction.

Prediction of the Three-Dimensional Reacting Two-Phase Flow Within a Jet-Stabilized Combustor

Numerical calculations of the two-phase flow in an experimentally well-investigated research combustor are presented. The comparison between measurements and calculations demonstrates the capabilities of the state-of-the-art Euler/Lagrange method for calculating two-phase flows, when applied to a complex reacting liquid-fueled combustor. The governing equations for gaseous and liquid phase are presented, with special emphasis on the control of the coupling process between the two phases. The relaxation method employed, together with a convergence history, shows a suitable way to achieve a fast and accurate solution for the strongly coupled two-phase flow under investigation. Furthermore, methods are presented to simulate the stochastic behavior of the atomization process caused by an air-blast atomizer. In addition to the numerical methods, experimental techniques are presented that deliver detailed information about droplet starting conditions.

EXPERIMENTAL AND THEORETICAL STUDY OF DROPLET VAPORIZATION IN A HIGH PRESSURE ENVIRONMENT

Due to the continuous increase of pressure ratios in modern gas turbine engines the understanding of high pressure effects on the droplet evaporation process gained significant importance. The precise prediction of the evaporation time and the movement of the droplets is crucial for optimum design and performance of modern gas turbine combustion chambers. Numerous experimental and numerical investigations have been done already in order to understand the evaporation process of droplets in high pressure environments. But until now, all high pressure experiments were carried out with droplets attached to a thin fiber resulting in the impairment of the droplet evaporation process due to the suspension unit.In the present study, a new experimental set up is introduced where the evaporation of free falling droplets is investigated. Monodisperse droplets are generated in the upper part of the test rig and fall through the stagnant high pressure gas inside the pressure chamber. Due to the relative velocity between droplet and gas, convective effects have to be considered in this study which are taken into account by experimental correlations. The droplet diameter and the droplet velocity are measured simultaneously by means of video technique and a stroboscope lamp. Detailed measurements with heptane droplets are presented for different pressures (p = 20 bar, 30 bar and 40 bar), gas temperatures (T = 550 K and 650 K) and initial diameters (d0 = 680 μm, 780 μm and 840 μm). The experiments were carried out with single component droplets.The experimental results are compared with numerical calculations. For this a theoretical model was developed based on the Conduction Limit model and the Uniform Temperature model. Good agreement for all conditions investigated is observed when using the Conduction Limit model. The Uniform Temperature model predicts incorrectly the evaporation process of the droplet.Copyright

Development and Optimization of Advanced Atomizers for Application in Premix Ducts

Lean premixed prevaporized combustion (LPP) is a promising approach for the reduction of emissions in gas turbine combustion. Typically, modern gas turbines operate at high pressure and high temperature flow conditions, giving rise to very short self ignition times. Consequently, the residence time of the mixture in the premix duct has to be minimized. Complete fuel vaporization and mixing can only be achieved by an improvement of the atomization process towards a fine droplet size spectrum over the whole range of operating conditions. In this paper an air assisted pressure swirl atomizer is introduced and analyzed. The special design of the nozzle under investigation enhances the interaction between the liquid sheet of the atomizer and the co-flowing air around the nozzle. The visualization of the atomization process gives detailed insight into the fundamental atomization phenomena of the atomizer. In addition, extensive measurements of droplet size distributions describe the dependence of the atomization process on liquid flow rate and air velocity, respectively. All phenomena occurring are explained in detail by means of a theoretical analysis of the flow pattern. With the help of this kinematic model, it can be shown why co-rotating swirls of atomization air and liquid cone lead to better atomization than counter rotating swirls.Copyright

Computation of Two-Phase Flows in Low-NOx Combustor Premix Ducts Utilizing Fuel Film Evaporation

For aircraft gas turbines as well as for industrial gas turbines current and future developments aim at the implementation of lean premixed-prevaporized (LPP) combustor techniques. For the development and optimization of these combustors powerful CFD-codes are required. A new code developed at the Institut fur Thermische Stromungsmaschinen (ITS), University of Karlsruhe, provides detailed information on the gas flow as well as on the propagation and evaporation characteristics of liquid wall films inside combustors. The flow characteristics of the gas phase are predicted using a Finite-Volume 3D-Navier-Stokes code with k-e turbulence modeling. To calculate the evaporation characteristics of a propagating wall film, a two-dimensional wall film model based on the boundary layer equations is proposed.The present paper comprises a comparison between calculations and experiments for the verification of the code and a detailed study on the evaporation characteristics of fuel films. The results obtained allow judgement to be made on the risk of coke formation on the prefilming surface and suggest that in some operating points a LPP combustor can be operated utilizing solely film evaporation. In addition, the computer code developed also accounts for many familiar types of shear driven film flows such as internal prefilming air blast atomizer flows for example.Copyright

Feasibility Study on Oil Droplet Flow Investigations Inside Aero Engine Bearing Chambers: PDPA Techniques in Combination With Numerical Approaches

The present paper deals with oil droplet now phenomena in aero engine bearing chambers. An experimental investigation of droplet sizes and velocities utilizing a Phase Doppler Particle Analyzer (PDPA) has been performed for the first time in bearing chamber atmospheres under real engine conditions.Influences of high rotational speeds are discussed for individual droplet size classes. Although this is an important contribution to a better understanding of the droplet flow impact on secondary air/oil system performance, an analysis of the droplet flow behaviour requires an incorporation of numerical methods because detailed measurements as performed here suffer from both strong spatial limitations with respect to the optical accessibility in real engine applications and constraints due to the extremely time consuming nature of an experimental flow field analysis. Therefore, further analysis is based on numerical methods. Droplets characterized within the experiments are exposed to the flow field of the gaseous phase predicted by use of our well-known CFD code EPOS. The droplet trajectories and velocities are calculated within a Lagrangian frame of reference by forward numerical integration of the particle momentum equation.This paper has been initiated rather to show a successful method of bearing chamber droplet flow analysis by a combination of droplet sizing techniques and numerical approaches than to present field values as a function of all operating parameters. However, a first insight into the complex droplet flow phenomena is given and specific problems in bearing chamber heat transfer are related to the droplet flow.Copyright

EFFECT OF THE SHEAR DRIVEN LIQUID WALL FILM ON THE PERFORMANCE OF PREFILMING AIRBLAST ATOMIZERS

Advanced prefilming airblast atomizers are widely used for low emission combustors since they deliver a fine spray almost independently of the fuel flow rate. The droplet spectrum produced by this type of atomizer results from the aerodynamic forces at the atomizer edge and from the fuel properties prior to the film disintegration. Therefore, the wall film temperature is an important parameter affecting the fuel properties and in turn the atomization quality. Even though this atomizer type became well investigated (Lefebvre 1989, Rizk et al. 1987, Sattelmayer et al. 1989), still no general quantitative relationship between atomizer design and spray quality could be established since the fuel state at the atomizer edge cannot be precisely predicted yet.In extending earlier experimental and theoretical work on airblast atomizers (Sattelmayer et al. 1989, Himmelsbach et al. 1994, Willmann et al. 1997) and recent advances in the numerical modeling of wall film flows (Rosskamp et al. 1997a), this paper presents a numerical approach to judge the effect of fuel mass flow, air flow and the film length (i. e. length of atomizer lip) on the temperature of the liquid at the atomizer edge. The computer code developed provides detailed information on the wall film flow and the nozzle wall temperature. For the prediction of heat transfer to the film a new model has been developed which is based on measurements of the internal film flow (Elsaser et al 1997).This new numerical approach can serve as a design tool to evaluate the effects of design modifications during atomizer development with view to their effect on atomization performance. The paper includes the theory for two-phase flow modeling and a generic parameter study that points out that the liquid loading and the length of the atomizer lip are important parameters in atomizer design. The calculations presented in the paper emphasize the necessity of coupled two-phase flow calculations because the film strongly interacts with the gas phase and the predicted atomizer performance is sensitive to changes in the air flow.Copyright