ENGINEERING AND CFD
Modulation of Turbulent Properties in a Spray Flame Burning n-Heptane: Direct Numerical Simulation
Principal Investigator:
Dominique Thévenin
Affiliation:
Lab. of Fluid Dynamics and Technical Flows, University of Magdeburg "Otto von Guericke", Magdeburg (Germany)
Local Project ID:
pr84qo
HPC Platform used:
SuperMUC of LRZ
Date published:
Spray evaporation and burning in a turbulent environment is a configuration found in many practical applications, such as diesel engines, direct-injection gasoline engines, gas turbines, etc. Understanding the physical process involved in this combustion process will help improving the combustion efficiency of these devices and, therefore, reduce their emissions. Direct numerical simulation (DNS) is a very attractive tool to investigate in all details the underlying processes since it is able to capture and resolve all scales in the system. In this project, evaporation, ignition, and mixing are investigated in both temporally- and spatially-evolving jets, using DNS.
Figure 1: Spray combustion in a spatially-evolving jet; the white spheres represent the liquid droplets (size is multiplied by a factor of 5 for visualization), the blue iso-surface is the Q-criterion showing the main turbulent structures, and the colored iso-volume represents the gas temperature (T > 2000 K).
Copyright: Lab. of Fluid Dynamics and Technical Flows (LSS), University of MagdeburgCentral aim of the project, conducted by scientists of the University of Magdeburg, is to investigate turbulent spray flames, quantifying possible modifications of the associated turbulent properties, and investigating ignition under shear conditions, considering both temporally- and spatially-evolving jets. Turbulence-droplets interaction, evaporation, auto-ignition, and the finally resulting flame structure are investigated by Direct Numerical Simulation (DNS), always considering n-heptane liquid droplets (representative of complex fuels as found in Internal Combustion Engines).
The droplets, being smaller than the grid resolution and the Kolmogorov length scale, are modeled as point droplets, while the Navier-Stokes equations are solved in the low-Mach number regime. Detailed models describe chemical reactions and molecular transport in the gas phase.
Figure 2: Spray combustion in a temporally-evolving jet; the gray spheres represent the liquid droplets (size is multiplied by factor of 5 for visualization), the yellow iso-surface surface is the Q-criterion showing the main turbulent structures, and the red iso-surface represents the temperature (T = 1800 K).
Copyright: University of Magdeburg, LSSIn the current DNS, the two-way coupling interaction between both phases is quantified via the exchange of mass, momentum and energy. The impact of different parameters has been investigated, in particular: initial temperatures, initial pressure, equivalence ratio/droplet mass fraction, droplet size, turbulence level, scalar dissipation rate, and mean shear. The Stokes drag force is dominant in the droplet momentum equation, whereas the evaporation process is computed by using a variable Spalding mass transfer number and an infinite conduction model inside the droplet. The in-house 3D DNS solver DINO [1-4] has been used for all simulations. All kinetic and transport properties are handled in DINO using the external libraries Cantera and Eglib. A skeletal mechanism accounting for 29 species and 52 reactions [5] is used for n-heptane combustion. Examples of such simulations in spatially- and temporally-evolving jets are shown in Fig. 1 and Fig. 2, respectively.
DNS of turbulent reacting flows requires huge computing power. The computational requirement increases when using detailed reaction schemes as it has been done in the current project. Additionally, all spatial and temporal scales controlling spray dynamics, evaporation, and ignition process, are considered as well. Simulating the combustion of thousands of droplets is thus very challenging.
In spatially-evolving jets, the most significant observation is (1) whether ignition occurs at all (for safety considerations) and (2) where it takes place. It has been found that, based on the initial conditions, the ignition starts either along the edges of the spray jet or at the tip of the jet, as seen in Fig. 1. The scalar dissipation, an essential quantity used in simplified models, shows a non-monotonic behavior in mixture fraction space, as seen in Fig. 3; more details can be found in [2]. These observations help improving further simplified spray combustion models, opening the door for affordable simulations of real installations.
Figure 3: Conditional mean of scalar dissipation rate versus mixture fraction at ignition time for different jet velocities [2].
Copyright: University of Magdeburg, LSSHydrodynamic simulations revealved that the non-woven spacer with α=60° reached steady state for all velocities. An increase to α=120° turns the flow into an unsteady periodic flow for an inflow velocity of 0.2 m/s and into a turbulent flow for 0.4 m/s resulting in a high pressure drop over the channel. For woven spacers with β = 45°, the flow reached steady state for all velocities, but with β = 90° the flow becomes unsteady and periodic for inflow velocities with at least 0.6 m/s. Thus, increasing α or β increase the pressure drop. Fig.4 shows the pressure drop over the channel length (20 cm) for different spacer configurations over inflow velocities.
Research Team:
Abouelmagd Abdelsamie, Cheng Chi, Timo Oster
References:
[1] Abdelsamie A., Fru g., Oster T., Dietzsch F., Janiga G., Thévenin D. Towards direct numerical simulations of low-Mach number turbulent reacting and two-phase flows using immersed boundaries. Comput. Fluids, 131: 123-141, 2016
[2] Abdelsamie A., Thévenin D. Direct numerical simulation of spray evaporation and autoignition in a temporally-evolving jet. Proc. Combust. Inst. 36:(DOI 10.1016/j.proci.2016.06.030), 2016.
[3] Abdelsamie A., Thévenin D. Modulation of turbulent properties in a spray flame burning n-Heptane using Direct Numerical Simulation. In 35th Symposium (International) on Combustion, San Francisco, Poster presentation, 2014.
[4] Abdelsamie A., Thévenin D. DNS of burning n-heptane droplets: auto-ignition and turbulence modulation mechanisms. In Proceedings of Direct and Large-Eddy Simulation X, Limassol, Cyprus, Springer, 2015.
[5] Patel A., Kong S. C., Reitz R. D. Development and validation of a reduced reaction mechanism for HCCI engine simulations. SAE Paper 2004; 97–128.
Scientific Contact:
Prof. Dr.-Ing. Dominique Thévenin,
Lab. of Fluid Dynamics & Technical Flows,
University of Magdeburg "Otto von Guericke",
Universitätsplatz 2, D-39106 Magdeburg (Germany)
Phone :+49 391 67 18 570
Email: thevnin@ovgu.de
http://www.lss.ovgu.de