O. Schäfer

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.

Predictions of Transient Fuel Spray Phenomena in the Intake Port of a Si-Engine

The present study addresses the numerical prediction of the two-phase flow in the intake port of a SI-engine. Particular emphasis is put on transient phenomena, as well as secondary effects, such as droplet breakup and droplet wall interaction. These phenomena have a significant influence on the fuel air mixture characteristics and cannot be neglected in the numerical prediction. The numerical methodology, presented in this paper, is based on a 3D body-fitted Finite Volume discretization of the gas flow field and a Lagrangian particle tracking algorithm of the disperse fuel phase. The Unsteady Reynolds Averaged Navier-Stokes equations (URANS) are solved by a time-implicit three level scheme. In the Lagrangian particle tracking algorithm, the spray is modeled by superposition of a large number of droplet trajectories. Two advanced numerically effective models are presented for the prediction of droplet breakup and droplet wall interaction. Special emphasis is put on the correct reproduction of the droplet statistics. In the present study the fuel injection and spray preparation process within the intake port of a SI-engine is investigated. Spray preparation is dominated by atomization processes like droplet breakup and wall interaction which predominantly take place at the valve seat. In order to find the principal characteristics of fuel preparation in a SI-engine, a parametric study has been carried out focusing on the influence of the gap sizes of the intake valve which strongly affects the complete fuel preparation process. The study is concluded by an analysis of qualitative and quantitative results of the predicted flow field.

Combined “Fluorescence” LDV (FLDV) and PDA Technique for Non‐ambiguous Two Phase Measurements Inside the Spray of a SI‐Engine

Laser velocimetry measurements in the vicinity of reflecting surfaces are still a major problem in many fluid mechanical applications such as measuring close to walls or wall film surfaces, respectively. Moreover, in any kind of two phase flow an unambiguous separation of the gas and the liquid phase is of particular interest. Commonly used techniques like Phase Doppler Analyzers (PDA) with size discrimination are limited to two phase flows where the smallest particle of the dispersed phase is significantly larger than the seeding particles. This condition can rarely be fulfilled in technically relevant spray/air systems for instance in automobile engines or gas turbines. One of the most promising approaches to overcome this problem is a correct phase discrimination using fluorescent tracer particles for the gas phase. In this paper different laser based velocimeters have been compared using the spray of a gasoline injection nozzle as a typical example. The working principle of the “fluorescence” LDV (FLDV) will be explained in detail. Moreover, the quality of the fluorescence signals and of the standard bursts received from Mie-scattering particles will be compared. Finally, the capabilities of combined FLDV and PDA measurements inside the spray of a SI-engine at unsteady conditions will be presented. The pros and cons of this technique will be discussed against the background of discriminatory two phase PIV measurements applied to the same spray.

Visualization and phase doppler particle analysis measurements of oscillating spray propagation of an airblast atomizer under typical engine conditions.

Abstract: Propagation of a kerosene spray formed by a prefilming airblast atomizer within a pulsating flame has been investigated. The measurements were performed in a 400 kW single combustor rig at typical engine conditions of up to 0.8 MPa inlet pressure and 673 K inlet temperature. The homogeneity of the fuel spray propagation in the reacting zone substantially influences the local temperature distribution in the reaction zone and, therefore, the formation of thermal NOχ. Phase locked visualization of the spray propagation and the flame luminescence was applied to the pulsating combustion process in order to analyze the complex interaction between the time dependent flow field, the spray propagation, and the combustion process. The spray characteristic was additionally investigated by means of time resolved PDPA measurements in the reacting flow.

A Combined Eulerian and Lagrangian Method for Prediction of Evaporating Sprays

Polydisperse sprays in complex three dimensional flow systems are important in many technical applications. Numerical descriptions of sprays are used to achieve a fast and accurate prediction of complex two-phase flows.The Eulerian and Lagrangian methods are two essentially different approaches for the modeling of disperse two-phase flows. Both methods have been implemented into the same CFD - package which is based on a 3D body-fitted Finite Volume method. Considering sprays represented by a small number of droplet starting conditions, the Eulerian method is clearly superior in terms of computational efficiency. However, with respect to complex polydisperse sprays, the Lagrangian technique gives a higher accuracy. In addition, Lagrangian modeling of secondary effects such as spray-wall interaction enhances the physical description of the two-phase flow. Therefore, in the present approach the Eulerian and the Lagrangian methods have been combined in a hybrid method. The Eulerian method is used to determine a preliminary solution of the two-phase flow field. Subsequently, the Lagrangian method is employed to improve the accuracy of the first solution using detailed sets of initial conditions. Consequently, this combined approach improves the overall convergence behavior of the simulation.In the final section, the advantages of each method are discussed when predicting an evaporating spray in an intake manifold of an IC-engine.Copyright

Modelling of shear-driven liquid wall films: effect of accelerated air flow on the film flow propagation

Liquid wall films that are driven by the shear stress exerted from a co-current air stream occur in many technical systems, e.g. in rocket nozzles, heat exchangers and on steam turbine blades. They are also present in prefilming airblast atomisers which are used for the fuel preparation in modern aviation gas turbines. In many cases, an acceleration of the co-current air flow is imposed in order to improve the performance or because of the geometrical constraints in complex configurations. The film flow characteristics are strongly influenced by the additional pressure gradient and the associated increase of the interfacial shear forces at the gas-liquid interface. In order to predict the two-phase flow field, a model has been developed at the Institut fur Thermische Stromungsmaschinen (ITS), University of Karlsruhe (TH) which allows a fully coupled computation of the gas flow field and the liquid film. In the present paper the modifications are discussed which are required to take into account the effect of an imposed pressure gradient in the air flow on the film flow dynamics. It will be shown that the numerical approach is capable to predict the film propagation with a high accuracy, providing a powerful tool for the design and the improvement of technical applications where liquid film phenomena play an important role.

Phase discrimination inside a spray: LDV measurements using fluorescent seeding particles (FLDV)

Laser Velocimetry measurements in the vicinity of reflecting surfaces are still a major problem in many fluid mechanical applications such as measuring close to walls or wall film surfaces, respectively. Moreover, in any kind of two phase flow an unambiguous separation of the gas and the liquid phase is of particular interest. Commonly used techniques like Phase Doppler Analysers (PDA) with size discrimination are limited to two phase flows where the smallest particle of the dispersed phase is significantly larger than the seeding particles. This condition can rarely be fulfilled in technically relevant spray/air systems. One of the most promising approaches is a phase discrimination using fluorescent tracer particles for the gas phase. In this paper the working principle of the “fluorescent” LDV (FLDV) will be explained. Moreover, the applicability of different fluorescent dyes will be discussed. Finally, a comparison between PDA results using size discrimination and FLDV results inside a hollow cone spray will be presented.

Disintegration of Oil Films Emerging From Radial Holes in a Rotating Cylinder

A fundamental study has been performed to examine the disintegration of oil films emerging from radial holes in a rotating hollow cylinder. The configuration investigated is an abstraction of one of the droplet generation sources in an aero-engine bearing compartment; similar configurations may also occur inside gearboxes. The paper aims to contribute to both the determination of directly applicable droplet characteristics and the establishment of a data-base that can be used for the development of droplet generation models. Similar to a prior paper on droplet generation processes at the rim of a rotating disk (Glahn et al, 2000), the near-term objectives of the study are (i) to determine droplet sizes under relevant aero-engine bearing compartment operating conditions, and (ii) to measure individual droplet diameter/velocity relationships. The long-term objective is to incorporate this information into advanced CFD-based design tools. Therefore, special emphasis has been directed towards a correlation of test results that enables determination of boundary conditions for a two-phase (oil droplets/air) simulation of lubrication system components. Based on the results of the present paper, droplet flow boundary conditions in terms of mean diameter, standard deviation of the diameter distribution, starting velocity, and flow angle are available for oil droplets generated by disintegration of oil films emerging from rotating radial holes and rotating disks.Copyright

Disintegration of Oil Jets Emerging From Axial Passages at the Face of a Rotating Cylinder

A fundamental study has been performed to examine the disintegration of oil films emerging from axial passages at the face of a rotating cylinder. The investigation has been conducted in parallel to a similar study on atomization processes at rotating radial holes (Glahn et al. 2001) and has used the same approaches in simulating one of the droplet generation sources in aero-engine lubrication systems. Both papers aim to contribute to the establishment of a database that can be used for the development of droplet generation models directly applicable to engine conditions. As with the parallel investigation, the near-term objectives of fundamental oil film disintegration studies are (i) to determine droplet sizes under relevant aero-engine bearing compartment operating conditions, and (ii) to measure individual droplet diameter/velocity relationships. The long-term objective is to incorporate this information into advanced design systems such as CFD-based tools.In the present study, flow visualization has been used to identify the dominant disintegration processes. Droplet diameters and velocities have been obtained for relevant engine operating conditions. Data are presented in terms of both characteristic diameters and size-class resolved droplet velocities and flow angles. A comparison of droplet sprays measured in the present study with those generated by disintegration of oil films at the rim of a rotating disk (Glahn at al. 2000) has been enabled by introducing non-dimensional parameters for atomization products and operating conditions.© 2001 ASME

Flashback in Lean Prevaporized Premixed Combustion: Non-Swirling Turbulent Pipe Flow Study

A fundamental study has been performed on the upstream flame propagation of a turbulent kerosene flame, stabilized in a confined stagnation flow at atmospheric pressure. Besides temperature and equivalence ratio, mixture properties and fluid dynamic parameters have been varied. The flashback phenomenon is discussed in terms of critical mean velocities and additionally based on detailed LDV data at the outlet of the premixing duct.The largest critical velocities uc for flashback are found for the “perfectly” premixed case and equivalence ratios close to stoichiometric, which is in accordance with the theory on laminar flame propagation. In the case of a homogeneous mixture, flashback is determined by the velocity distribution at the outlet of the premixing section. In the undisturbed pipe flow the flame propagates through the wall boundary layer. The data for this case are compared with the theory of side-wall quenching in terms of a critical Peclet number and critical velocity gradients at the wall. Both are deduced from the experimental data. Reducing the velocity on the axis forces the flame to propagate through the center at a velocity predicted by correlations on turbulent flame velocity.Copyright