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"Theoretical Investigation of the Primary Breakup of High-viscosity Liquids in Twin-fluid Nozzles"
This work is part of the "Programmorientierten Förderung" of the Helmholtz Association of German Research Centres (HGF)
"Energy Efficiency, Materials and Resources"
The conversion of low-grade fossil and biogenic fuels to
synthesis gas (syngas; CO + H2) via high pressure entrained
flow gasification (EFG) opens a wide variety of applications.
Thus, the syngas can be used for the synthesis of methane (SNG),
for the production of high quality liquid fuels via a synthesis
(CtL; BtL) or serve as fuel in an IGCC power plant.
To guarantee for complete conversion of the fuel as well as
for a high syngas quality (no soot or residual hydrocarbons) the
spray quality generated by the burner nozzle of the gasifier is of
major importance. Fuel conversion is driven by the evaporation
rate of the droplets and subsequent degradation of the evaporated
fuel. Mainly gas assisted burner nozzles are applied due to
the physical properties of the fuel - since typically featured viscosities
are up to 1000mPa s - furthermore those nozzles allow
to utilize the gasification agent as atomization medium. Due to
the specific operating conditions the atomization for EFG is connected
with several challenges: In addition to the high-viscosity
of the fuels, those fuels typically feature a complex rheological
behaviour. Moreover, the gasifiers aim for operation at elevated
reactor pressure (up to 80 bar) in oxygen-blown mode,
where the atomization agent also serves as gasification agent, to
increase the overall efficiency of the process. As the amount of
gasification agent is limited by the stoichiometry of the reaction,
the available amount of atomization agent is also limited. Since
spray quality decreases with the amount of available atomization
agent, a thorough understanding of the atomization process of
high-viscosity fuels is needed.
Such a twin-fluid nozzle is schematically depicted in in Figure 1. The liquid is supplied by a central tube to the nozzle orifice, whereas atomization medium flows through a concentric annular gap. The central liquid jet is then accelerated by the gas, which results in the breakup of the jet by the shear forces.
The atomization process of low-viscous liquids under atmospheric
conditions has been thoroughly studied in literature by experiments and simulations:
Amongst other things breakup regimes, liquid core length, primary
breakup frequencies as well as the influence of turbulence
on the disintegration process have been studied.
There, mostly water has been used. For this purpose, various correlations for the breakup length of the liquid jet
or classification schemes for the primary breakup mode are to be found in the literature, for example in (1).
But particularly, when converting biogenic raw materials into synthesis gas, the high viscosity of the feedstock has a decisive influence on
the primary breakup mode as well as on the droplet sizes after completion of the secondary decomposition.
As already mentioned, the previous experiments and theories on atomization have mainly been limited to low-viscosity fluids. The atomization process can generally seperated in three steps. In a first step, the central jet is accelerated by the surrounding gas flow, whereby small disturbances on the surface form in the form of waves or individual disturbance points. These are continuously increased in a second step, the so-called primary breakup. As the perturbations grow, the liquid jet structure is more and more deformed and finally deflected into the gas stream. In the third stage, the protruding liquid structures are further accelerated and subject to the onward deformation and destabilization, which results in the formation of ligaments and large droplets.
Due to the complexity of the atomization process and the rheological properties of the real suspension fuel, different model liquids such as, for example, glycerin-water mixtures were used instead of these.
Both the low and high viscosity model liquids showed, that on the one hand the standard approach for predicting the spray quality by means of the Weber number does not provide satisfactory results (2) and, on the other hand, the complex rheology as is customary for pyrolysis oils has a significant effect on the spray quality (3). Further investigations for highly viscous model fuels also showed an unexpected influence of the viscosity on the primary breakup mode of the liquid jet (4), which is not described in the previous classifications of primary breakup in two-fluid nozzles (Figure 2).
In Figure 2 (a) it can also be seen that the primary breakup is an oscillating phenomenon, for which a characteristic frequency could be determined depending on the operating point and the substance properties. As a result of this, there may be a fluctuation of the local oxidizer-to-fuel ratio in the further course of the jet breakdown until the termination of the drop event. This variation can influence the gasification kinetics occurring after the atomization.
Firstly, the experimental results of the atomization are repeated in a numerical simulation of the two-phase flow (three-dimensional, Navier-Stokes, compressible, non-stationary). Subsequently, the instationarity and oscillation of the liquid mass fraction during the primary breakup it to be captured and their influence on the local oxidator-to-fuel ratio determined. Furthermore, the influences of different properties of the liquid such as the surface tension or the viscosity on the primary breakup frequency are to be considered by means of virtual fluids.
The open-source software package OpenFOAM is the basis for performing of the numerical
simulations of the two-phase flow. For the description of the two phases the
"Volume of Fluid" method (VOF) established by Hirt and Nichols (5) is used.
For the description of the interface, the VOF method uses an indicator function α, which determines the volume fraction of a phase within the control volume and is convectively transported with the flow field. For example, a value of α=0 represents a control volume completely filled with gas, while a value of α = 1 corresponds to a volume filled with liquid. Therefore the exact position of the boundary surface is described by all cells with a value of 0<α<1.
Fig. 3: Schematic representation of the VOF method
The following illustrations and videos present the latest results of the simulation of the primary
breakup of high-viscosity liquids in two-fluid nozzles. Further information can be found in the
publications listed below.
|(1)||C. Dumouchel: "On the experimental investigation on primary atomization of liquid streams", Exp. Fluids 45 (2008) 371–422|
|(2)||T. Jakobs, N. Djordjevic, S. Fleck, N. Zarzalis, T. Kolb: "Influence of Ambient Pressure on Twin Fluid Atomization – R&D Work for high pressure entrained flow gasification" ICLASS 2012, 12th Triennial International Conference on Liquid Atomization and Spray Systems|
|(3)||A. Sänger, T. Jakobs, N. Djordjevic, N. Zarzalis, T. Kolb: "Basic investigations on burner design for high pressure entrained flow gasifier: Influence of pressure and fluid viscosity on twin fluid atomization" VDI–Berichte Nr. 2161, 26. Deutscher Flammentag, 10. – 12.09.13, Duisburg, 2013|
|(4)||A. Sänger, T. Jakobs, N. Djordjevic, T. Kolb: "Effect of primary instability of a high viscous liquid jet on the spray quality generated by a twin–fluid atomizer" ILASS – Europe 2014, 26th Annual Conference on Liquid Atomization and Spray Systems, 8–10 Sep. 2014, Bremen, Germany|
|(5)||C.W. Hirt, B.D. Nichols: "Volume of fluid (VOF) method for the dynamics of free boundaries" J. Comput. Phys. 39 (1981) 201–225.|
SCOPUS - ID
Zhang, Feichi; Müller, Thomas; Zirwes, Thorsten; Wachter, Simon; Jakobs, Tobias; Habisreuther, Peter; Zarzalis, Nikolaos; Trimis, Dimosthenis; Kolb, Thomas, (2019). Effect of elevated pressure on primary jet-breakup: Basic research for entrained flow gasification, in 29. Deutscher Flammentag, Deutsche Sektion des Combustion Institutes und DVV/VDI-Gesellschaft Energie und Umwelt, September, 17-18, Bochum., .
Müller, T.; Goßmann, A.; Kühn, J.; Etzold, M.; Stelzner, B.; Zarzalis, N.; Durst, F.; Trimis, D., (2018). A Low Power Liquid Fueled Burner using a Novel Atomization Concept, in Proceedings of Joint Meeting of the German and Italian Sections of the Combustion Institute, Sorrento, Italy, 23-26 May, p. X2, (ISBN 978-88-88104-22-5), .
Müller, T.; Kadel, K.; Habisreuther, P.; Trimis, D.; Zarzalis, N.; Sänger, A.; Jakobs, T.; Kolb, T., (2018). Influence of Reactor Pressure on the Primary Jet Breakup of High-Viscosity Fuels: Basic Research for Simulation-Assisted Design of Low-Grade Fuel Burner, in Proceedings of the ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition (GT2018), ASME, June 11-15, Oslo, Norway, p. GT2018-75950, (doi:10.1115/GT2018-75950).
Müller, T.; Kadel, K.; Habisreuther, P.; Trimis, D.; Zarzalis, N.; Sänger, A.; Jakobs, T.; Kolb, T., (2018). Simulation of the Primary Jet Breakup of Non-Newtonian Fuels: Basic Research for Simulation-Assisted Design of Low-Grade Fuel Burner, in Proceedings of the ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition (GT2018), ASME, June 11-15, Oslo, Norway, p. GT2018-75945, (doi:10.1115/GT2018-75945).
Goßmann, A.; Müller, T.; Etzold, M.; Stelzner, B.; Zarzalis, N.; Durst, F.; Trimis, D., (2017). Novel Atomization Approach for low Liquid Fuel Mass Flows, in Proceedings of the European Combustion Meeting – 2017, April 18-21, Dubrovnik, Croatia, p. ECM2017.0226, .
Müller, T.; Dullenkopf, A.; Sänger, A.; Habisreuther, P.; Jakobs, T.; Zarzalis, N.; Kolb, T., (2017). Influence of Nozzle Design upon the Primary Jet Breakup of High-viscosity Fuels for Entrained Flow Gasification, in Proceedings of the ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition (GT2017), ASME, June 26-30, Charlotte, NC, USA, p. GT2017-63198, (doi:10.1115/GT2017-63198).
Müller, T.; Sänger, A.; Habisreuther, P.; Jakobs, T.; Zarzalis, N.; Kolb, T., (2016). Investigation on Jet Breakup of High-viscous Fuels for Entrained Flow Gasification, in Proceedings of the ASME Turbo Expo 2016: Turbine Technical Conference and Exposition, ASME, June 14-16, Seoul, South Korea, p. GT2016-56371, (doi:10.1115/GT2016-56371).
Müller, T.; Sänger, A.; Habisreuther, P.; Jakobs, T.; Trimis, D.; Kolb, T.; Zarzalis N., (2016). Simulation of the Primary Breakup of a High-viscosity Liquid Jet by a Coaxial Annular Gas Flow. International Journal of Multiphase Flow, 87, 212-228.(doi:10.1016/j.ijmultiphaseflow.2016.09.008)
Müller, T.; Sänger, A.; Habisreuther, P.; Jakobs, T.; Kolb, T.; Zarzalis, N.; (2016). Vortrag: Investigation on the Jet Breakup of High-Viscous Fuels for Entrained Flow Gasification. Jahrestreffen der ProcessNet-Fachgruppe Hochtemperaturtechnik, 10.-11. März, Universität Erlangen-Nürnberg,
Publications of T. Müller can also be found in Literature.