Below, our main research topics are described briefly. More and more new accomplishments will follow, and we will update this page occasionally.

Currently, we are mainly conducting research on thermo-fluid dynamic issues related to propulsion. These engines are used for automotives, airplanes and rockets, where thrust is generated by converting thermal energy into kinetic energy. It is important to enhance the fuel efficiency and reduce harmful emissions.


(From left; automotive engine (Mazda), aero-engine (Rolls Royce), rocket engine (Apollo mission))


    

Liquid Fuel Spray Atomization

Automotive or aircraft engines generally use liquid fuels because the energy density is sufficiently high and it is much easier to store and carry liquid fuels compared with gaseous or solid fuels. In the combustion chamber, the liquid fuel is injected at a very high speed to make a spray. This process is called atomization. By this, the total surface area can be increased significantly and subsequently, fuel vaporization and mixing can be enhanced. Liquid atomization has been used for a long time, but still, details of its physical processes have not been well revealed. Due to ever-increasing demands for greener combustion technologies, designing highly-efficient liquid-fuel systems is very important. Therefore, it is needed to understand the physical processes of liquid atomization in detail.

(1) Detailed mechanisms of liquid atomization (direct numerical simulation)

In this study, a Diesel fuel spray is investigated using DNS (Direct Numerical Simulation), in which the deformation and breakup of the liquid/gas interface is directly captured. The results have unveiled the jet head formation process due to liquid/gas impact, surface instability development leading to breakup (formation of ligaments), and droplet generation from the ligaments. Figures 1 and 2 show these processes.

Furthermore, in a subsequent study, evaporation has been incorporated to examine the process of fuel/air mixing, and to further correlate the turbulence/droplet interaction in a dense spray like this.

Currently, these valuable datasets are being used for modeling. We believe that the accuracy of spray combustion simulation could be much improved when our modeling is completed.


Fig. 1 Mushroom-like head formation due to liquid/air interaction. Ligament breakup is visible both from the head and from the liquid core surface.


Fig. 2 Zoomed image of ligaments and droplets. Surface tension plays a critical role in the final droplet generation process from these ligaments.

- Relevant papers (See "Publications".)
Shinjo et al., Proc. Combust. Inst. 2015
Shinjo & Umemura, Proc. Combust. Inst. 2013
Shinjo & Umemura, Proc. Combust. Inst. 2011
Shinjo & Umemura, Int. J. Multiphase Flow 2011
Shinjo & Umemura, Int. J. Multiphase Flow 2010

(2) Detailed mechanisms of liquid atomization (experiment)

This is a joint study with Nagoya University and Japan Aerospace Exploration Agency (JAXA). We participate as a co-investigation institute in the space experiment "Detailed validation of the new atomization concept derived from drop tower experiments: Aimed at developing a turbulent atomization simulator" led by Prof. Umemura (Nagoya University). In this study, the mechanism of surface tension and capillary waves in atomization, especially in droplet pinch-off from a ligament, will be investigated under the micro-gravity conditions onboard the International Space Station (ISS). So far, such experiments have been conducted on the ground, forming a slow liquid jet vertically. Inevitably, the effect of gravitational acceleration is added in such a situation. The gravitational acceleration makes the liquid surface deformation unstable and this is superimposed on the intrinsic unstable mechanism due to surface tension. The conventional concept of surface destabilization has been misleading, and Prof. Umemura has proposed a new theory of self-destabilization due to capillary waves. This experiment is aimed to confirm the proposed mechanism, utilizing the micro-gravity environment on the ISS.

The experiment is scheduled in 2017. Latest results will be uploaded, as soon as they are available.


Fig. 3: Example of droplet pinch-off. Droplet generation is repeated due to the self-destabilization of capillary waves induced by the tip effect (taken from Umemura, J. Fluid Mech. 2016).

- Relevant papers (See "Publications".)
Umemura, J. Fluid Mech. 2016
Umemura & Osaka, J. Fluid Mech. 2014
Umemura, Phys. Rev. E 2011
Umemura et al., Phys. Rev. E 2011
Shinjo & Umemura, Int. J. Multiphase Flow 2010
Umemura et al., J. Japan Soc. Aero. Space Sci. 2010
Umemura et al., J. Japan Soc. Aero. Space Sci. 2010
Shinjo et al., Atomization 2009
Shinjo et al., J. Japan Soc. Aero. Space Sci. 2007

(3) Spray modeling (spray simulation)

This is a joint study with Nagoya University, and also with Hokkaido University and Mazda. Simulations like the one in (1) (DNS) are accurate without models, but require a huge amount of computer resources, and the computational domain is inevitably limited. For spray simulations, DNS will not be a practical method. Modeled simulations, then, will be the best choice, but the modeling accuracy is a key issue. So far, the modeling accuracy of the primary atomization region (see (1)) has not been good, leading to a situation where parameter tuning is required using experimental reference data. In fact, this is a big problem that has hindered the full prediction capability of spray simulation.

In this study, we have incorporated a newly proposed turbulent atomization model, which has been proposed by Prof. Umemura (Umemura, Combust. Flame 2016)based on our past accumulation of physical knowledge on liquid atomization. Two atomization modes, turbulent resonant mode and Rayleigh-Taylor (RT) mode, are included. The spray code is a hybrid Euler-Euler and Euler-Lagrange large-eddy simulation (LES) code. Fortunately, the validity of the code has been demonstrated well.

The present method is novel in that
- the model does not contain any arbitrary model constants and no parameter tuning is needed, and
- it is physically consistent from the nozzle region to the downstream spray region.
Therefore, extension to further upstream, i.e. up to the inner-injector region, is possible.
Such a spray simulation code, with a fully closed atomization model, has not been possible so far. The present code leads to the drastically improved accuracy in predicting the spray and atomization characteristics, which is beneficial both for research and industrial purposes.


Fig. 4: Simulation of a Diesel jet (D=0.3mm, U=200m/s) using the present method. The entire spray and the liquid core are shown respectively. Transient spray development is well captured.

- Relevant papers (See "Publications".)
Umemura & Shinjo, submitted
Umemura, Combust. Flame 2016

Biofuel Droplet Heating, Puffing and Combustion

In this study, we focus on biofuels, which are derived from vegetable oils, animal fats and other bio-origin substances. In total, it is believed that the emission of carbon dioxide (CO2) is offset. The physical properties of such biofuels are typically similar to those of fossil fuels. However, in detail, they are not the same. Currently, biofuels are used in a mixed form with fossil fuels. Therefore, it is needed to thoroughly understand the characteristics of biofuels for a better and efficient use.

In this study, droplets of combustor spray scale (~30 µm) are studied to mimic realistic flow conditions. A combination of (fossil) Diesel, bio-ethanol and bio-Diesel is used, resulting in a form of emulsion where ethanol is dispersed as tiny sub-droplets in the continuous (parent) phase of Diesel. Since the boiling temperature of ethanol is much lower than that of Diesel, when heated, ethanol may become superheated and, under some conditions, may boil explosively. This is called puffing or microexplosion, depending on the strength of the explosion. If puffing/microexplosion can be used positively, this can enhance secondary breakup and thus fuel vaporization and mixing. Again, DNS is used for investigating the detailed dynamics.

The results in Figs. 5 and 6 exemplify the effectiveness of this approach, yielding a lot of data on unsteady behavior of puffing, droplet breakup and vapor mixing. The case of a droplet group shown in Fig. 6 provides further new findings on droplet interaction under puffing conditions. Figure 7 shows the puffing-flame interaction. Currently, a subsequent modeling study is underway and is expected to improve the accuracy of spray simulation for biofuels in the near future.


Fig. 5 Puffing and secondary breakup. Ejected vapor can be seen as red pockets.


Fig. 6 Inter-droplet interaction in an emulsion droplet group. (Note that coloring is not the same as that in Fig. 5.)


Fig. 7 Vapor/flame interaction. On the flame surface (shown as a blue iso-surface of T=1800K), the molar consumption rate of the ejected ethanol is superimposed.

- Relevant papers (See "Publications".)
Shinjo & Xia, Proc. Combust. Inst. 2017
Shinjo et al., J. Fluid Mech. 2016
Shinjo et al., Atom. Sprays 2016
Shinjo et al., Phys. Fluids 2014

Combustion Instability and Control

To reduce NOx emissions from a gas turbine combustor, lean premixed combustion is one of the effective methods since the flame temperature can be lowered. (The generation of thermal NOx is due to the oxidation of N2 in the air, and the temperature is the primary parameter to determine its generation rate.) However, lean premixed combustion sometimes tends to be unstable, which, in an extreme case, leads to flame blowoff or causes mechanical damage to the combustor.

In this study, an active combustion control method is investigated. Experimentally, a model combustor has been setup and investigated using lean methane (CH4) as a primary fuel (Tachibana et al. Proc. Combust. Inst. 2007). A small amount of secondary fuel (pure CH4) is directly injected at the flame base to actively control the local heat release rate. A numerical study is also conducted by us to further understand the flame dynamics under control. The results reveal that the instability-sustaining loop among the pressure, velocity and heat release has been modulated by the control and thus oscillations are attenuated accordingly.


Fig. 8 Reduction of sound pressure level (SPL) by active control.(Tachibana et al. Proc. Combust. Inst. 2007)


Fig. 9 Flame dynamics with active control.

- Relevant paper (See "Publications".)
Shinjo et al., Combust. Flame 2007

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