Berthold Noll

Diffusion controlled evaporation of a multicomponent droplet: theoretical studies on the importance of variable liquid properties

Abstract A well-known multicomponent droplet vaporization model, the Diffusion Limit Model, has been extended to account for property variations in the liquid phase. The model has been tested for typical conditions of modern gas turbine combustors. The results for a hexane/tetradecane droplet show that the temperature- and concentration-dependence of the liquid properties affect the vaporization process, especially with regard to a reduced diffusional resistance. Additionally, remarkable variations of the refractive index are observed yielding helpful information for the estimation of errors in optical particle sizing techniques. Regarding comprehensive spray calculations, the use of the constant property formulation is recommended with improved reference values based on variable property calculations.

GENERALIZED CONJUGATE GRADIENT METHOD FOR THE EFFICIENT SOLUTION OF THREE-DIMENSIONAL FLUID FLOW PROBLEMS

A generalized conjugate gradient (CG) method was examined for use in the numerical calculation of recirculating turbulent three-dimensional fluid flows. The proposed algorithm was found to be a very efficient solution procedure in a large number of practical applications. Furthermore, the results obtained from two other CG-like solution methods that are entirely free of recursion clearly indicate the advantages of the recommended partly recursive method even for vector computers.

Experimental and numerical studies of evaporating wavy fuel films in turbulent air flow

Abstract In extending earlier studies the present paper is concerned with the two phase flow inside prefilming airblast atomizers of gas turbine combustors at elevated temperatures. A numerical procedure is presented, which describes the internal flow of the nozzle, with the turbulent air flow and the shear driven evaporating wavy liquid film. The flow conditions of the film and gas at the atomizing edge as well as the pressure drop inside the nozzle are predicted. The theoretical results are compared with detailed measurements of the wavy liquid film and of the gas phase. For the experiments, the test section and the optical measuring techniques for evaporating wavy liquid films presented in a recent paper were employed. Using time-averaged values for the thickness, the velocity and the roughness of the film and assuming laminar film flow, the code leads to accurate predictions of the interaction of the liquid film with the gas phase. A variation of the dominating parameters such as the gas velocity, gas temperature, liquid flow rate, and pressure illustrates the accuracy of the numerical predictions with respect to the gas flow as well as to the film flow. The pressure drop as well as the film temperature, film thickness and velocity are accurately predicted under all conditions. Although the present study is focused on prefilming airblast atomization, the benefits of the calculational procedure are not limited to atomization processes. Quite similar flow conditions are found in intake pipes of internal combustion engines and in film vaporization employing combustors.

Evaporation of fuel droplets in turbulent combustor flow

Detailed measurements of a recirculating, droplet charged air flow within a model combustor are compared with predictions based on three different evaporation models. Similar results are obtained with the simplified d2-law, the uniform temperature model and thin skin model for relatively short droplet-heatup phases. Discrepancies, however, are observed under conditions where the droplet heating phase is relatively long, i.e. at low temperature conditions. Extended evaporation models, therefore, are necessary when the ignition performance is to be analysed.Copyright

Development of a Projection-Based Method for the Numerical Calculation of Compressible Reactive Flows

The ability to calculate compressible reactive flows enables the computation of thermoacoustic interactions in gas turbine combustor systems. A new projection-based numerical method able to compute compressible reactive flows is developed within this work. This computational scheme is based on a modified Helmholtz decomposition, by which an arbitrary vector field is split up into a field with a so-called divergence constraint and an irrotational field. This leads to a fractional step scheme which consists of a predictor and a corrector step. The Poisson equation solved for the pressure in case of incompressible flows is extended to a Helmholtz equation for the computation of compressible flows. After solving the corrector step, the mass and momentum balances are fulfilled. This results in a fast numerical scheme, since no further iterations need to be computed. Based on the modified Helmholtz decomposition, it is shown that the created method can be understood as an extension of the incompressible projection scheme. Moreover, the spatial and temporal order of accuracy of incompressible projection-based methods are discussed and the ones of the compressible scheme are determined. The created compressible projection-based method is further on validated against a one-dimensional linear acoustic test case from the literature, whereby an analytical solution can be derived.

Effects of variable liquid properties on multicomponent Droplet vaporization

A multicomponent droplet vaporization model, the Diffusion- Limit Model, is modified to account for the variation of liquid properties due to large temperature gradients as well as consid- erable concentration gradients within the droplet. The effects on the vaporization behavior are analysed for an iso- lated bicomponent droplet consisting of heptane and dodecane. The results are presented for both moderate and high gas tem- peratures excluding combustion. During the vaporization process the liquid phase properties vary considerably. For example, the Lewis number changes approx- imately one order of magnitude. The mass ratio of the liquid components seems to be rather sensitive to the variation of ther- mophysical property values, especially during the second half of the droplet lifetime, where about 50% of the droplet mass will still evaporate. The gas phase behavior is less affected by the use of constant liquid properties. For both gas temperature levels tested it was found that single component models cannot describe satisfactorily the whole va- porization process of multicomponent droplets. With regard to ignition the sharp rise of the vapor concentration in the begin- ning of the droplet vaporization is important. This behavior is caused by the more volatile component and cannot be achieved by the single component substitute.

The harmonical transmission model: a new approach to multidimensional radiative transfer calculation in gases under consideration of pressure broadening

Abstract A new method, the harmonical transmission model (HTM), has been developed accounting for the effect of the line structure of gaseous absorption in multidimensional radiative heat transfer calculations. The method is derived from a development of the absorption coefficient and the spectral intensity in Fourier series and a least-square fit over the equation of radiative transfer. For the one-dimensional case, an analytical solution for the spectral mean transmissivity is derived, which can be directly compared to band models. For the multidimensional case, a set of differential equations is presented, which is of the same mathematical form as the equation of radiative transfer. Therefore, any suitable numerical method for the solution of the radiative transfer equation can also be employed to solve the differential equation resulting from the HTM.

Correlations for the laminar flame speed, adiabatic flame temperature and ignition delay time for methane, ethanol and n-decane

This paper provides a detailed database for curve fitted polynomials to deliver information about the laminar flame speed, adiabatic flame temperature and ignition delay time depending on the equivalence ratio, temperature and pressure. The fuels under investigation are methane, ethanol and n-decane. A new aspect of this paper is the envisaged broad range of validity of the correlations, covering the range of 0.6 1.5 for the equivalence ratio, 293.15 K 593.15 K for the temperature, regarding the laminar flame speed and the adiabatic flame temperature and 1400 K 1800 K, regarding the ignition delay time and 0.5 bar 6 bar for the pressure range. As basis of the mathematical expressions, curves with the highest order of four have been chosen to best describe the behavior of the quantities under examination. Evaluation of well established experimental data sets for each respective fuel generates a first rudimentary groundwork for the derivation of the polynomial. The experimental results are further enhanced by obtaining a large amount of additional points through finite rate chemistry simulations with detailed reaction mechanism. By means of a least squares fit, the coefficients for the algebraic expression are inferred. An error analysis is contained subsequent to the calculation, helping the reader to assess the reliability and quality of the fitted curves.

Direct Combustion Noise Simulation of a Lean Premixed Swirl Flame using Stochastic Sound Sources

A lean, swirl-stabilized gas turbine model combustor is simulated with a stochastic approach for combustion noise prediction. The employed hybrid and particle based method, FRPM-CN (Fast Random Particle Method for Combustion Noise Prediction) reconstructs temperature variance based direct combustion noise sources from local CFD-RANS turbulence and flow field statistics. Those monopole sound sources are used as right hand side forcing of the Linearized Euler Equations. First, findings from steady state CFD simulations are validated with experimental results. It is shown that the employed RANS models accurately reproduce the experimental flow field and combustion. Turbulence is treated with a two equation model and a global reaction mechanism is utilized for combustion. Subsequently, the specifications of the CCA (Computational Combustion Acoustics) setup is introduced and selected pressure spectra of the acoustics simulations are compared to experimental results, showing that FRPM-CN is able to deliver absolute combustion noise levels for the investigated burner at low computational costs.

Numerical Analysis of Probe Microphones Used for Thermoacoustic Measurements

Acoustic measurements within combustion chambers are expensive due to high thermal loads applied on the measurement devices at operating conditions. As a more feasible substitute, probe microphones can be used to lead acoustic waves from combustion chambers to externally mounted microphones. Since these probe microphones are purged by nitrogen at atmospheric temperature, high thermal loads are avoided. However, the acoustic signal measured by the probe microphone is altered compared to the signal within the combustion chamber. This change in the acoustic signal can be characterised by means of the acoustic transfer function of the probe microphone, which mainly depends on the probe microphone geometry and the combustion chamber temperature distribution. Both impacts are studied in the present paper. The main subject is to analyse the inuence of the combustion chamber temperature distribution on the acoustic transfer function of probe microphones. In addition, the eect of changes in the probe microphone diameter is discussed. For this scope, experiments at standard conditions and transient CFD simulations for dierent temperature distributions have been carried out.