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Effect of Subsonic Microjets on Reacting Flow in Dump

Combustors

K. Russell,* H. Kanchi,

* and F. Mashayek

†

University of Illinois at Chicago, Chicago, IL 60607, USA

In order to improve the efficiency and performance of air-breathing liquid-fuel

combustors, advanced control strategies must be developed. The backward-facing step

dump combustor is a conventional flame holder which causes flow separation after the step.

Novel design concepts are necessary for controlling and manipulating the fluidic shear layer

that develops inside these chambers. The dynamic mixing and burning that occurs within

this design are key mechanisms for turbulent combustion. Because these systems carry a

substantial drag penalty, control techniques are developed to create a compact, low drag

combustor for high-speed propulsion applications. Microjets, which are traditionally used

for noise reduction techniques in aerospace systems, are used in this work to generate higher

turbulence and three-dimensionality within the turbulent reacting shear layer of a dump

combustor. The focus of this work is to use numerical simulations to study the effects of

microjets on the reacting flow-field for improving flame stabilization. The three-

dimensional simulations for turbulent combustion are conducted using the Reynolds-

averaged Navier-Stokes (RANS) method.

Nomenclature

Djet = Microjet diameter

H = Combustor step height

k = Turbulent kinetic energy

= Turbulent dissipation rate

To = Reference temperature

Tw = Wall temperature

U = x-velocity component

Uo = Centerline velocity of primary flow

Ujet = Microjet velocity

I. Introduction

A compact, low drag combustor is vital for the proper functioning of high-speed propulsion systems. Backward-

facing step dump combustors have been traditionally used as flame holders in ramjet engines. These flame holders

consist of bluff-bodies that provide a protective environment for fuel burning, yet, this chamber is also known to

carry a substantial drag penalty. In dump combustors, flow separates over the step causing vigorous mixing and

burning, which are important parameters for turbulent combustion. For an engine to become more efficient and

economical, proper burning of fuel is essential, as this also reduces environmental pollutants.

Microjets are commonly used to reduce noise generated from high-speed jets. In research, microjets were shown

to eliminate large-scale structures and decrease turbulent kinetic energy in the mixing region inside supersonic twin

jets1. Microjets have also been shown to suppress turbulence in subsonic axisymmetric jets

2 and reduce shock noise

by using aqueous microjets in high-speed jets3. Our group has previously used the implementation of subsonic

microjets as a shear layer control strategy in dump combustors4-6

. Microjets with subsonic speeds were found to

greatly affect turbulence and heat release in the compact chamber.

* Graduate Student, Department of Mechanical and Industrial Engineering, 842 W. Taylor St., Chicago, IL 60607

† Professor, Department of Mechanical and Industrial Engineering, 842 W. Taylor St., Chicago, IL 60607, AIAA Associate Fellow

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-6704

Copyright © 2010 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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In the shear layer region of a dump combustor, the structures are mainly two-dimensional and cause thermo-

acoustic instabilities during combustion. Generating three-dimensionality in the mean flow will increase the flame

surface area and produce greater volumetric heat release. Previous work4-6

done by our group has shown that

microjets can achieve these characteristics. The size, location, speed, and number of the microjets are important

parameters when looking to improve the function of a combustor. One study found that the momentum ratio for a

single microjet dump combustor determines the penetration distance into the shear layer and creates more

dissipation5. Increasing the velocity of the microjets directly affects the production of shear layer turbulence.

Multiple microjets placed on the horizontal wall of the step were found to alter the turbulent properties of the shear

layer without directly exciting it. The use of microjets injecting air upward at 45º was studied for an axisymmetric

dump combustor6. The results in this study showed that the flow injecting at an upward angle greatly increased the

heat release rates and produced higher temperatures throughout the length of the combustor.

Numerical simulations for turbulent reacting flow are conducted within the Reynolds-averaged Navier-Stokes

(RANS) framework. The simulations are performed for a three-dimensional (3D) planar dump combustor with

multiple microjets. The effect of microjet speed on the reacting fluid properties is the center of this work. The cases

presented will serve to show the benefits of using multiple subsonic microjets for efficient mixing and heat release

in this combustor design.

II. Numerical Modeling Approach and Problem Setup

Numerical simulations for the 3D dump combustor with microjets are conducted using the commercial code

FLUENT. The steady-state, incompressible flow is solved within the RANS framework utilizing the two-equation

standard k- model for turbulence.

There is research that presents some known disadvantages to using two-equation models for simulating

configurations that involve jets injecting into a cross-flow. RANS simulations with two-equation models have been

shown to produce results with overly simplistic predictions of complex flow-fields7. This same study also found

that using nonlinear and Reynolds Stress Models (RSM) does not significantly improve predictions. In another

study, using linear two-equation models in a direct numerical simulation (DNS) was found to have limitations

attributed to the assumption of isotropy in the flow8. For the study presented in this work, the flow of the microjets

is parallel to the direction of the main flow and we do not expect the same level of inaccuracies that were reported in

the cross-flow studies. Nevertheless, because of the complexity of the design problem featured in this work, the

numerical methods used here will only serve as engineering design tools.

RANS modeling is less computationally expensive compared to DNS and large-eddy simulation (LES) and

therefore, will be the method adopted for this study. An eddy-dissipation combustion model for a single-step

reaction mechanism for premixed propane-air is used to model the chemistry. The computational grid has

approximately 3.25 million cells.

Figure 1. Schematic of the backward-facing step dump combustor with multiple

horizontal microjets (not drawn to scale)

A schematic of the backward-facing step dump combustor with multiple microjets is shown in figure 1. The step

height (H) is 2 cm. The length of the combustor before the step edge is 2 cm (2H) and the length after the step is 20

cm (10H). For our 3D simulations, only half of the combustor width is modeled at 16 cm (8H) wide. A symmetry

boundary and wall boundary conditions are applied in the spanwise directions. The diameter of the microjets is 1

z

y

x

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mm located 1 cm (0.5H) up from the combustor floor. Four microjets are used in this configuration, spaced 2 cm

(H) center-to-center. Spacing from the center of the microjets to the wall or symmetry boundary is 1 cm (0.5H). The

microjets deliver air at 45º angle to the x-axis in the x-z plane. The velocity profile for the premixed propane-air

mixture at the main inlet was taken from experiments and has a centerline velocity of 12.5 m/s. Turbulent intensity

is at 5% for the inlet velocity. The premixed propane-air mixture has an equivalence ratio of 0.5 and a reference

temperature (To) of 298 K. The walls are isothermal with a temperature of 300 K. The entire domain is initialized

with a temperature of 1000 K. Outflow boundary conditions are applied at the outlet.

III. Results and Discussion

The dump combustor without microjets will be referred to as the “baseline” case throughout this work. To

properly study the interaction between the microjets and how they affect the flow-field, profiles for the x-velocity

and turbulent kinetic energy are analyzed at two locations. One location is directly in front of one of the microjets to

study the impact of the 45º airflow on the flame. This is the second microjet located away from the symmetry

boundary to avoid the effects of the wall. The second location is in-between two microjets.

The normalized x-velocity profiles along the axial direction of the combustor are shown in figure 2. Results are

shown for the baseline case and for jet velocities of 50 m/s and 100 m/s. For the 50 m/s case, profiles taken in front

of the microjet exhibit higher velocities in the shear layer compared to the baseline case, but this effect is only

evident near the step wall (x/H = 0.25). For 100 m/s, the profiles taken directly in front of the microjet exhibit

penetration of the flow-field until approximately x/H = 2.5, and have higher regions of negative velocities in the

shear layer region for x/H 1. The disturbance from the high speed jets on the main flow-field cause the formation

of turbulent structures to increase. This gives more energy to the recirculation zone and causes it to have a larger

bubble height with higher regions of negative velocities near the step wall. When comparing the profiles taken

directly in front of the jets with the profiles taken in-between the jets, we see that there is non-uniformity and

mixing, especially for the 100 m/s case. Profiles for the 50 m/s case behave similar to the baseline case for x/H 5.

Yet, the profiles for the 100 m/s case take on a more uniform profile with higher velocity values compared to the

baseline case for x/H 5. The higher levels of turbulence generated in the reacting shear layer from the 100 m/s jets

produce a faster and more uniform flame speed near the exit.

Figure 2. Comparison of normalized x-velocity profiles at various axial locations

The normalized turbulent kinetic energy profiles along the axial direction are shown in figure 3. For microjet

speeds of 50 m/s and 100 m/s, the peak turbulent kinetic energy values are higher than for the baseline case for x/H

2.5. For x/H 2.5, the profiles taken directly in front of the microjets show that kinetic energy in this region

affects both the shear layer and the main flow. The mixing and non-uniformity that is generated by the microjets

causes higher values of turbulent kinetic energy to occupy a larger region of the combustor. This increase in kinetic

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energy produces higher burning velocities. Profiles taken in-between two microjets show that the kinetic energies

on this plane dominate in the shear layer near the bottom wall (x/H 2.5). The mixing that occurs in-between two

microjets cause the flow in this region to move downwards producing higher turbulence near the floor of the

combustor. Whereas, the flow that is directly in front of the microjets infuses and mixes with the primary flow-

field, causing higher turbulence above the shear layer. At x/H = 5 and 7.5, the baseline case dominates in the lower

half of the combustor because the microjets primarily affect the turbulence in the shear layer region. Overall,

increasing the velocity of the 45º microjets leads to higher turbulence levels and lifts the flame upward to promote

faster burning of fuel in the combustor.

Figure 3. Normalized turbulent kinetic energy profiles at various axial location

The temperature contours for the y-z plane at x/H = 1 are shown in figure 4. Near the step, the 100 m/s jet

velocity case has higher flame temperatures compared to the 50 m/s case. The jet airflow increases the flame speed

and flame surface area so that more fuel from the primary flow region will burn. The 50 m/s case has a peak

temperature of approximately 1400 K, whereas, the 100 m/s case has a peak value of approximately 1600 K. This

happens because the higher velocity jets produce more turbulent structures within the combustor which causes the

fuel to burn at a higher rate. It should be noted that the lower velocity jets generate high temperatures near the wall

boundary (right edge of figure 4(a)). For 100 m/s, the high peaks of temperature stay within the combustor and

away from the wall.

Figure 4. Temperature contours at x/H = 1 plane for (a) Ujet = 50 m/s and (b) Ujet = 100 m/s

In figure 5, the temperature contours are shown at the exit (y-z plane) of the combustor. Looking at figure 5(b),

we see that the higher velocity microjets continue to affect the burning throughout the entire combustor. The 100

m/s case maintains high levels of burning and the flame penetrates upward even at the exit. This occurrence helps to

minimize the unburned fuel exiting the combustor; however, it produces an adverse pattern factor. In figure 5(a),

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the burning is occurring only in the lower half of the combustor and at lower temperatures because the 50 m/s speed

jets do not produce the high levels of mixing necessary to bring fuel from the unburned region to the flame. The

addition of microjets may be an important design consideration for applications where burning near the exit needs to

be achieved. There is also an absence of high temperatures near the wall boundary in figure 5(b) because the angle

of the airflow from the microjets causes the flame to lift and cluster in-between the regions of the jets. This is also

an ideal condition when considering methods for cooling the combustor walls.

Figure 5. Temperature contours at the exit plane for (a) Ujet = 50 m/s and (b) Ujet = 100 m/s

The volumetric heat release and the average exit temperature for various microjet velocities are listed in table 1.

Here, the mass ratio is defined as the ratio of the total mass flow rate through the microjets to the mass flow rate at

the inlet of the combustor. For the 100 m/s case, the mass contribution of the four microjets is equal to the mass

contribution of the primary inlet. In this case, the volumetric heat release rate increases by approximately 140%

over that of the baseline case. This shows that high microjet speeds have favorable effects on burning in the

combustor. At the combustor exit, the average temperatures for jet speeds of 25 m/s and 50 m/s decrease compared

to the baseline case because the mixing generated at these speeds are not able to sustain the turbulent structures

necessary to promote faster burning. However, for the 75 m/s and 100 m/s cases, the average exit temperatures

significantly increases because the jet flow is able to lift the flame to mix with and burn more fuel.

Ujet

[m/s]

Mass ratio

[%]

Volumetric heat release

[MW/m3]

Average exit temperature

[K]

0 n/a 2.752 683

25 0.26 2.965 (7.8%) 639 (-6.4%)

50 0.52 3.144 (14.2%) 676 (-1.0%)

75 0.78 4.776 (73.5%) 821 (20.2%)

100 1.04 6.615 (140.8%) 955 (39.8%)

Table 1. Comparison of combustion properties for various microjet velocities

The average mass flow rates for propane species along the axial direction of the combustor for various microjet

velocities are shown in figure 6. For the 25 m/s and 50 m/s cases, the species burn at a rate similar to the baseline

case because of the low levels of mixing generated by the jets. The 75 m/s and 100 m/s cases have lower mass flow

rates near the step wall (x/H 1.5) because the high speed microjets generate high levels of turbulent structures in

this region which causes the fuel to burn at a higher rate. The continuous mixing of the flow-field increases the

surface area of the flame which allows the flame to burn a larger quantity of fuel. Throughout the length of the

combustor, the flame surface area for the 75 m/s and 100 m/s cases continues to grow and burn more fuel in the

primary flow region. This leads to the large decrease in the species mass flow rate for the higher jet velocities for x

= 2.5 until the combustor exit.

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Figure 6. Average mass flow rate for C3H8 species for various microjet velocities along the

axial direction

The average heat release rates along the axial direction for various microjet velocities are shown in figure 7.

These values are analyzed from the step wall to the combustor exit. For the 25 m/s case, the heat release rates

exhibit slightly lower values compared to the baseline case because this jet velocity is not able to maintain the high

levels of turbulent kinetic energy necessary to facilitate fast fuel burning throughout the combustor. Compared to

the baseline case, the 50 m/s case has higher heat release within the shear layer region (x/H 3.5), but levels fall

below the baseline case outside of this region. For low jet speeds, the fuel does not mix at a high rate outside of this

shear layer region and is not able to energize the flame for increased burning. Ideally, high heat release values

should exist throughout the combustor to maximize burning. The 75 m/s and 100 m/s cases have peak heat release

rates at approximately x/H = 2 and x/H = 1.5, respectively. As the velocities of the jets increase, the jet flow is able

to lift the flame higher to burn a larger amount of fuel near the step. The high rates of mixing that the higher speed

jets produce causes continuous formation of turbulent structures throughout the length of the combustor. These

structures generate a larger flame surface area that is able to find and burn larger quantities of fuel compared to the

baseline case. For the 100 m/s case, the jet initiates high levels of turbulence that are maintained even at the

combustor exit.

Figure 7. Comparison of average heat release rates for various microjet velocities along

the axial direction

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The average temperatures along the axial direction for various microjet velocities are shown in figure 8. For the

microjet velocity of 25 m/s, the average temperature values are lower than for the baseline case. The 50 m/s case

exhibits higher temperatures near the wall (x/H 1.5) compared to the baseline case, but these values decrease

outside of this region. For the lower jet speeds, small levels of mixing occur near the step wall and less energy is

available to sustain a high-speed flame. The flow structures that are generated from the higher jet velocities aid in

producing a larger and faster flame. For 100 m/s, the average temperatures in the combustor continuously increase

because the dynamic mixing of the flow increases the surface area of the flame so that more fuel will burn. The

average temperatures for the 100 m/s case decrease for 0.75 x/H 2 because the direct effects of the jet flow

decrease. But for x/H 2, the turbulent structures gain the momentum necessary to increase the flame surface area

which increases the fuel burning until the exit.

Figure 8. Comparison of average temperatures for various microjet velocities along the

axial direction

IV. Conclusions

Proper selection of microjet speeds can have either an undesirable or favorable effect on the flame properties in a

dump combustor. The use of multiple subsonic microjets was shown to promote three-dimensionality and

turbulence within the shear layer and the primary flow-field. Low speed microjets do not generate enough mixing

and turbulence to sustain a flame speed that is higher than that of the baseline combustor. For the high jet speed of

100 m/s, the jet flow was shown to increase the turbulent structures in the shear layer which causes the flame surface

area to grow and the flame to lift upwards into the primary region to burn fuel more efficiently. Ideally, combustors

of the future will be designed to not only provide a more stable flame anchor but also achieve maximum burning of

fuel to eliminate environmental pollutants and alleviate combustor wear-and-tear.

Acknowledgements

The support for this work was provided by the U.S. Office of Naval Research with Dr. G.D. Roy as the Program

Officer.

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