Heiko Rosskamp

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.

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.

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

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