Junji Shinjo

Physics of puffing and microexplosion of emulsion fuel droplets

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.

Large Eddy Simulation of LOX/GH2 Shear-Coaxial Jet Flame at Supercritical Pressure

The aim of the present study is to validate the accuracy of large eddy simulation (LES) framework against liquid oxygen (LOX) and gaseous hydrogen (GH2) coaxial jet flame at supercritical pressure. In the present LES Framework, a laminar flamelet model is employed for chemistry modeling and the real-fluid properties are also taken into account. In order to validate the LES code, numerical simulation is conducted for the supercritical LOX/GH2 combustion experiment by the P8 combustor at the DLR. By analyzing the obtained LES result, the flowfield features of the P8 combustor are explored. The integral length scale of turbulent coherent structures evaluated from the present LES is compared with the experimental data. The comparison shows fair agreement between simulated length scale and experimental data.

On acoustic damping of a cylindrical chamber in resonant modes

Acoustic damping of a cylindrical chamber with open and closed ends in resonant modes is analytically and numerically investigated to understand the low damping characteristic of the chamber without chocked nozzle. First, on the basis of the analytic solution of resonant acoustic modes inside a cylinder, the damping by radiation from the open end is calculated analytically using simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. The values of acoustic damping calculated for the first longitudinal and tangential modes are in good agreement with the corresponding values obtained using numerical simulation. The damping is also investigated for a configuration of the chamber with an injector installed off-center. Finally, we numerically and semi-analytically investigate the acoustic damping for a configuration that includes a hot-gas injection. The obtained mode is found to be a spinning tangential mode and the radiated wave also has a spinning feature. The damping for the spinning tangential mode is found to be larger than that for the symmetric dipole-like radiation under a uniform standing condition, but much smaller than the chamber with a chocked nozzle. Therefore, the chamber with an open end has the low damping characteristic suitable for intentionally generating oscillatory combustion.

LES of High-Frequency Combustion Instability in a Rocket Combustor

In the present work, results of attempts to capture the high-frequency combustion instability in a rocket combustor equipped with hydrogen (H2) / oxygen (O2) injector are shown. To enhance the coupling between heat release rate fluctuations and pressure oscillations, the injector element is installed at the off-center of the combustion chamber in the simulation. Two-dimensional simulations are conducted for a single-element combustor. The flow forcing by modulating the mass flow rate of O2 injection is attempted to excite the first transverse (1T) mode of chamber. The simulation results indicate that the amplitude of 1T mode is increased by the flow forcing. To clarify a driving mechanism for the growth of pressure oscillations, the simulation results are analyzed using the Rayleigh index. Simulations are then extended to three-dimensional LES for a combustor equipped with five shear-coaxial injector elements. The obtained results of pressure field and the Rayleigh index are compared with those of a single element combustor. The effects of multiple flames on the pressure field are examined.

LES of H2/O2 Coaxial Jet Flames in a Multiple-Injector Combustor

The interactions between multiple H2/O2 coaxial jet flames are investigated by a full three-dimensional LES approach. The numerical simulation under atmospheric pressure condition is conducted for a cylindrical combustor equipped with three coaxial injector elements. Although the three flames merge together at the downstream location, the combustion field observed in the current combustor configuration is dominated by diffusion combustion. The analysis of resolved kinetic energy spectrum reveals that turbulent structures are significantly affected by the collisions between turbulent shear layers. The comparison with the result of a single element combustor also reveals that the O2 jet injected from the injector positioned in the center tends to be longer due to the interactions between flames.

Theoretical and Numerical Estimation of Acoustic Damping of a Model Combustion Chamber

Acoustic damping of a cylindrical chamber with open and closed ends is analytically and numerically investigated. In the analysis, based on the analytic solution of resonant acoustic modes inside cylinder, the damping from the open end is calculated by a simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. Acoustic damping calculated for first longitudinal and tangential modes are in good agreement with that obtained by numerical simulation. When a longitudinal mode exists, the directivity of radiation becomes like monopole and the damping is large. On the other hand, when a tangential mode exists, it is found that the dipole like directivity is obtained and the damping is small compared to that for the longitudinal mode. A configuration of the chamber and an injector installed off-centered is also investigated. Under non-resonant condition between injector and chamber acoustics, it is found that the acoustic radiation from the open end of the injector is negligible but the viscous effect becomes important. Finally we investigate the acoustic damping with hot gas injection numerically and semi-analytically. The obtained mode is found to be a spinning tangential mode due to the asymmetry of the mean flow field. The radiated wave has also a spinning feature and the damping is found to be much larger than that for symmetric dipole like radiation under uniform condition. The simple acoustic radiation modeling applied for the analysis also works well for this general non-uniform mean flow condition.

LES of High-Frequency Combustion Instability in a Single Element Rocket Combustor

In the present work, results of numerical simulations to capture the high-frequency combustion instability in a two-dimensional combustor with hydrogen (H2) / oxygen (O2) injector are shown. Three sets of computations, in which the locations to install the injector element are varied with an aim to change the degree of coupling between heat release fluctuation and pressure oscillation, are conducted. In the first computation (named as RUN1), the injector is installed at the center of combustion chamber. In the second and third computations (named as RUN2 and RUN3), the injector is installed at the off-centered location of combustion chamber. In RUN3, the flow forcing is attempted by adding an energy source term in the energy conservation equation to generate the first transverse mode (1T) in the combustion chamber. Although the amplitudes of pressure oscillations in the first two computations are relatively small, the result of RUN2 shows that pressure oscillation at 4kHz, which corresponds to the 1T mode of combustion chamber, is significantly excited. In RUN3, the 1T mode of combustion chamber is successfully generated by the flow forcing. After switching off the flow forcing, however, the amplitude of pressure oscillation decays rapidly and self-sustained oscillation can not be established. The results of three cases are analized by using the balance equation for acoustic energy. The analysis indicates that the Rayleigh term works as a driving mechanism for instabilities, but the other damping term has a large negative value and prevents the glowth of pressure oscillations for all three cases. Finally, the present work is extended to three-dimensional LES and the preliminary result is shown.

Development of a hybrid multi-scale simulation approach for spray processes

This paper presents a multi-scale approach coupling a Eulerian interface-tracking method and a Lagrangian particle-tracking method to simulate liquid atomisation processes. This method aims to represent the complete spray atomisation process including the primary break-up process and the secondary break-up process, paving the way for high-fidelity simulations of spray atomisation in the dense spray zone and spray combustion in the dilute spray zone. The Eulerian method is based on the coupled level-set and volume-of-fluid method for interface tracking, which can accurately simulate the primary break-up process. For the coupling approach, the Eulerian method describes only large droplet and ligament structures, while small-scale droplet structures are removed from the resolved Eulerian description and transformed into Lagrangian point-source spherical droplets. The Lagrangian method is thus used to track smaller droplets. In this study, two-dimensional simulations of liquid jet atomisation are performed. We analysed Lagrangian droplet formation and motion using the multi-scale approach. The results indicate that the coupling method successfully achieves multi-scale simulations and accurately models droplet motion after the Eulerian–Lagrangian transition. Finally, the reverse Lagrangian–Eulerian transition is also considered to cope with interactions between Eulerian droplets and Lagrangian droplets.

Large Eddy Simulation of High-Frequency Combustion Instability of Supercritical LO X /GH 2 Flame

This paper presents an attempt to caputure the high-frequency combustion instability of a supercritical LOX/GH2 flame by Large Eddy Simulation (LES) framework. Four sets of computations, in which lengths of the coaxial injector and purge gas inlet are changed with a view to changing the frequency of acoustic mode, are conducted for the combustion experiment by the P8 combustor at the DLR. The high-frequency combustion instability corresponds to the first tangential (1T) mode of combustion chamber is successfully captured by the present LES when the length of purge gas inlet is set to 20mm. By the analysis of acoustic mode of purge gas inlet using the finite element based solver, a possible explanation of observed combustion instability is explored.

Numerical Analysis of Hydrogen / Air Jet Diffusion Flame

The structure and stabilizing mechanism of a subsonic hydrogen / air jet lifted flame are numerically investigated using DNS. The lifted flame consists of an inner premixed flame and outer diffusion flames. The flame base is premixed, which sustains the vigorously turbulent inner premixed flame. The outer diffusion flame consist of several island flames, which are produced from the inner premixed flame. The stabilization at the flame base is maintained by the balance between the axial velocity and the laminar flame velocity. Deviation of heat release layer from hydrogen consumption is observed in the inner premixed flame.