Computational Fluid Dynamics

Vortex-Wall Reaction

Wed, 16 Apr 2008 10:35:36 -0400

Vortices generated when a drop of water hits the wall are simulated using UNICORN.

Note the cascade of vortices.

Detonation in a Channel

Wed, 16 Apr 2008 10:33:42 -0400

Multidimensional detonation propagating in a channel is simulated. The results are shown on a moving co-ordinate system. The viewer is assumed to be moving with the detonation front at a velocity of 2544 m/s (Chapman-Jouguet Velocity of the H 2 /Ar mixture).

As the detonation propagates into the unburnt mixture, the transverse waves travel back and forth between the walls. The interaction of the shock waves results in high-pressure points.

Vortex-Flame-Vortex Interaction

Wed, 16 Apr 2008 10:31:06 -0400

The process of vortex formation in a counterflow burner and the simultaneous interaction of the vortices issuing from the fuel and air sides of the flame surface with the flame are shown here.

Note the quenching process, the formation of secondary vortices, and the flame broadening due to cascade of the vortices.

Ignition of Opposed-Flow Diffusion Flame

Wed, 16 Apr 2008 10:15:56 -0400

The ignition process of a propane fuel is simulated by heating the air flow in a counterflow burner. Fuel (propane) issues from the bottom nozzle, while heated air (1310 K) issues from the top nozzle. Initially, the air is preheated to 1300 K, which provides a steady mixing of the heated air and the propane fuel without generating a flame.

The temperature of the air is then raised by 10 degrees. This increase in temperature leads to ignition of the fuel-air mixture in the diffusion layer. The changes in temperature are shown at different instants after the air temperature is increased.

Vortex-Flame Interaction in Opposed-Jet Flame

Wed, 16 Apr 2008 10:12:06 -0400

The process of vortex formation in a counterflow burner and the interaction of the vortex and the flame are shown here.
Note the quenching process, entrainment of the flame into the wake of the vortex, and the formation of the secondary vortices.

Combusting Flow

Wed, 16 Apr 2008 10:09:18 -0400

UNICORN is used to simulate the unsteady combusting flow in a trapped-vortex combustor.
Velocity vectors are shown on the left side, and temperature is shown on the right.

Heated Axisymmetric Jet

Wed, 16 Apr 2008 10:06:36 -0400

A heated-air jet becomes unsteady because of the buoyancy force acting on it. The structure of a 1200-K heated jet is visualized here using a particle-tracking technique.
The vortical structures are generated automatically by the UNICORN code in the presence of the buoyancy force in the axial direction.
The instantaneous locations of the particles that are injected inside and around a 1200-K air jet are visualized.

Smoke Foil Record of Detonation

Wed, 16 Apr 2008 10:03:46 -0400

A popular method of recording a detonation-wave structure involves the use of smoke-coated walls. As the detonation wave propagates, the high pressure generated at the shock-shock or shock-wall interaction point scratches the smoke--leaving a trace of the path of the high-pressure point on the wall.

Propagation of a detonation wave captured by the smoke-foil technique is shown here. The detonation wave is propagating in a channel from right to left. The fuel/air mixture is doped with tracer particles. As the detonation passes through, it scratches the smoke foil and displaces the particles.

Shear Flow

Wed, 16 Apr 2008 10:00:22 -0400

Kelvin-Helmholz instability of a shear layer formed between two jets having different velocities manifests small disturbances into coherent vortical structures.

A shear flow is animated by visualizing particles at different instants.

Transitional Jet Diffusion

Wed, 16 Apr 2008 09:57:38 -0400

When the jet velocity of a vertically mounted diffusion flame is moderately high, buoyancy-induced outer vortices and shear-induced inner vortices develop simultaneously.
This double-vortex structure of a hydrogen jet flame is shown here by plotting the instantaneous locations of the particles that are injected into the flow from the jet exit. The flame location is indicated by black dots.
Note the long coherence length of the inner vortices and a change in the direction of rotation of the inner vortices in the upper half of the images.

Driven Jet Diffusion Flame

Wed, 16 Apr 2008 09:55:13 -0400

The vortex-flame interaction is studied in a methane jet diffusion flame. Vortices are introduced into the flame at a frequency of 30 Hz by driving the fuel jet with a loudspeaker.
As the vortex stretches the flame, a local hole appears on the flame surface. Re-ignition in the hole region occurs as the vortex convects downstream.
Visualization of particles is shown on the left, and visualization of temperature is shown on the right.

Flickering Flame

Wed, 16 Apr 2008 09:51:32 -0400

A low-speed jet diffusion flame flickers at ~ 15 Hz. This flame flickering is, in fact, associated with the bulging and squeezing of the flame by the vortices formed outside the flame surface.

The structure of a hydrogen buoyant jet diffusion flame is shown here. The vortical structures are generated automatically by the UNICORN code in the presence of the buoyancy force in the axial direction. Temperature, OH-concentration, and NO-concentration are shown in the left, middle, and right animations, respectively.

Note the increase in temperature and NO concentration when the flame is bulging outwardly. This phenomenon (resulting from non-unity Lewis number) was discovered by scientists at ISSI and AFRL.

Flow Behind a bluff body

Wed, 16 Apr 2008 09:49:10 -0400

Vortex shedding behind a bluff body is visualized by injecting color-coded particles into the flowfield.
This animation shows instantaneous locations of the particles that are injected from both sides of the bluff body. Blue particles originate from the left side of the bluff body, and Red particles orginate from the right side.

Cold Jet

Wed, 16 Apr 2008 09:43:08 -0400

The effect of heat release on the structure of inner vortices is investigated by simulating the transitional jet flame without considering the heat release.
Animation of the vortex motion in the cold flame is shown here. Note the vortex merging and loss of coherence of the shear vortices.

Heated Planar Jet

Wed, 16 Apr 2008 09:33:38 -0400

Transition to turbulence in a heated-jet flow is shown in this animation. Buoyancy plays a critical role in this transition.
The instantaneous locations of the particles that are injected inside and in the region adjacent to the 1200-K air jet are visualized.

Dynamics of propane jet diffusion flames

Wed, 16 Apr 2008 09:07:05 -0400

For engineering applications, there is a definite value, and in many cases, a necessity of thinking in terms of mean values of parameters. However, there is a danger to this line of thinking in that the mean and fluctuating quantities can, in many cases, mask the physics and chemistry that are germane to understanding the fundamental processes that give rise to the statistical results. This is particularly true for near laminar and transitional jet flames in which the impact of large-scale, organized, buoyancy-induced vortices on the air side of the flame and the Kelvin-Helmholtz type vortex structures on the fuel side of the flame dominate the flame characteristics.
For gaining an insight into these processes, Katta et al. (1997) used both experimental and numerical methods. In the case of experiments, reactive Mie scattering visualization technique was implemented and details of the flow structures for both reacting and non-reacting flows were captured. These images showed that the vortices inside the flame remain coherent for a long time and undergo structural changes as they convect downstream. Numerical simulations were performed to understand the mechanisms responsible for the observed differences between the reacting and cold jets. Based on the numerical results, an explanation was provided for the longer coherence lengths for vortices in flames.

References:
Katta, V. R., Goss, L. P., Roquemore, W. M., and Chen, L. D. 1997 "Dynamics of Propane Jet Diffusion Flames," Atlas of Flow Visualization III, edited by The Visualization Society of Japan, pp.181-198.

Modeling of two phase fuels

Wed, 16 Apr 2008 09:03:08 -0400

ISSI has developed a numerical model for the research of two-phase flows in spray combustion systems. This model was derived based on the concept of "Level-Set" implemented in a two-dimensional/axisymmetrical Arbitrary Lagrangian-Eulerian (ALE) grid system.

Since the grids are fixed in time and space, and the interface matching procedure is not required numerically, this model has the capability of handling very complicated topological changes of the interfaces and the mass/energy transports between the two phases. A variety of phenomena in spray systems, such as droplet forming process, liquid film breakup, droplet interaction/collision, evaporation/combustion of the interactive droplets, etc. are suitable for applying this model.

In the future, ISSI plans to extend this two-phase model to three-dimensional for more realistic problems. Also, more sophisticated models in combustion chemistry for various fuels will be added to enhance the applicability for the modeling of droplet combustion.

Numerical codes

Wed, 16 Apr 2008 08:44:39 -0400

•
Time-accurate direct numerical simulations (DNS) and time-averaged simulations with turbulence model
•
Fuels such as hydrogen, methane, propane, methanol, ethylene, and acetylene
•
Global, reduced, or detailed chemistries
•
Premixed and/or diffusion combustion
•
Detailed simultaneous predictions for NOx
•
Droplet burning and spray flames
•

Kl310.pdf for a copy of "Role of Flow Visualization in the Development of UNICORN"-Adobe Acrobat PDF format 1,335 kB

Click here to obtain the latest version of the free Acrobat
reader from Adobe Systems Incorporated.

Foul2D (Time-Dependent Fouling in Heat Exchangers)

•
Real-time simulation for deposition and its effects on heat transfer and fluid dynamics
•
Prediction of hot spots due to coking
•
Multiple fluids and multiple passages
•
Fuels such as Jet-A, JP-5, JP-8, and dodecane
•
Endothermic and supercritical fuels
•

Complex data rich in information is presented using ISSI-developed
Integrated Graphics and Animation Software (IGAS).