Development of Enclosed Combustion Rig for SCRAMJET Fuel Studies

Group #51

Student: Dan Martin

Supervisor: Dr. Egolfopoulos

Due: 12/5/08

Abstract:

The opposed jet burner configuration that has been constructed in this experiment has been proven

capable of producing and sustaining a stable stagnation flame. Despite the presence of a double helix

shaped silicon carbide heating rod that resides in the top burner, the effect that it has on the flow and the

resulting imposed velocity gradient on the stagnation flame did not cause a deviation from previously

performed experimental results for a Methane stagnation flame at φ=0.7. However, this only holds true if

the heating rod is positioned in just the right location, in the center of the top burner and at the right angle

so it is parallel to the inner walls of the burner. Now that the combustion rig has been constructed and is

capable of sustaining stable stagnation flames, the groundwork has been laid to begin testing ignition of

fuel air mixtures through the heating of the air flow through the top burner. Future studies will need to be

done to quantify the effect of the increased flow rate that will be induced by heating the flow through the

top burner, and the necessary flow rate adjustments to ensure the momentum of the top and bottom flow

remain equal so that a stable stagnation flame can still be produced. Ultimately, once the ignition of

gaseous fuels has been achieved by this system and is calibrated to sustain stagnation flames consistent

with previously published experimental data of laminar flame speeds, the rig will once more be modified

to allow for the preheating of liquid fuels through the bottom burner, with the end goal of achieving

ignition of various cracked states of JP­7.

Introduction:

The development of a hydrocarbon­fueled SCRAMJET is one of the highest priorities for the US Air

Force. Specifically, the goal is for the SCRAMJET to fly at hypersonic speeds between Mach numbers of

4 and 8. When the vehicle is moving at such high velocities it becomes difficult for the complete

combustion to take place within the combustor given the short residence time scales within the relatively

small area of the jet engine. Additionally, these vehicles can get extremely hot due to both the skin

friction moving at these speeds, and the compressed air at the nose of the jet. A method to counteract this

issue is to circulate the fuel under the skin of the jet, which would serve two purposes: it would cool the

jet, and the heat absorbed by the fuel would cause the fuel to thermally crack. The fuel of choice is JP­7, a

heavy jet fuel developed by the Air Force specifically for the use of Hypersonic SCRAMJET vehicles.

JP­7 is a heavy endothermic fuel that can absorb significant amounts of heat before it cracks. Cracking of

the fuel results in a “soup” of smaller molecules dominated by hydrogen, methane, and ethylene. These

smaller and lighter molecules burn quicker than the heavier JP­7 jet fuel in its un­cracked state. In this

investigation the behavior of flames resulting from mixtures of hydrogen, methane, and ethylene will be

characterized experimentally. An enclosed combustion rig will be developed that will allow for accurate

experimental measurements of fundamental flame properties, and that is capable of having Laser Doppler

Velocimetry (LDV) experiments performed. The rig will also be developed with the goal of achieving

independent remote ignition.

There are three fundamental flame properties studied when examining these fuels, namely ignition, flame

propagation, and extinction rate. The most ideal fuel mixture for a SCRAMJET engine would be able to

achieve ignition as at a low temperature, have a fast rate of propagation and extinction would occur at a

high strain rate. Ultimately the different cracked states of JP­7 will be examined closely, whether by

heating the jet fuel to known cracked states, or by simulating the known cracked soup mixtures. The

Combustion Lab has done previous studies on the composition of various cracked states of JP­7, so these

simpler gases can be mixed to simulate these cracked states.

The ignition of a fuel is characterized by both the temperature at which it occurs and the strain rate of the

mixture. The propagation of a fuel mixture, or its flame speed, characterizes the speed at which a

premixed fuel air mixture would propagate. The third fundamental property, the extinction strain rate,

characterizes when the flame of the ignited fuel goes out, and the conditions at which flame extinction

occurs. For the purposes of this experiment, an opposed jet system will be built, and the ignition of JP­7

fuel will be characterized, laying the groundwork for examining the propagation and extinction of the

flames achieved.

The most ideal way to carry out combustion experiments for jet fuels is to build a full size engine and run

experiments with it. However, not only would it be very expensive to produce a full size jet engine, it

would be very cost ineffective to use the large amount of fuel that would be required to do a

comprehensive study. Fundamental information regarding the properties of flames in jet engines cannot

be garnered due to the chaotic nature of the combustion, so another method is used to study these

properties in a well controlled environment. By properly configuring the flow of opposed jets, as shown

in Figure (1), a simple flame can be formed for this purpose. This flat flame, or stagnation flame, closely

approximates a flame described by the Ideal Flame Model (IFM), a steady, planar, laminar, adiabatic

1­dimenstional flame. The only way to experimentally achieve a stable flame is by imposing a velocity

gradient, in this case an opposing convective velocity, yielding a reference flame speed Su ref . As a result

of this imposed velocity gradient the flame does not perfectly conform to the IFM, but if all the properties

are uniform at the nozzle exits, the flame exhibits “quasi­one dimensional” behavior since the properties

are uniform in the radial direction. A constant “aerodynamic strain rate”, dv/dr, is thus imposed by the

external flow field, which is proportional to the “global strain rate” of the flame, du/dx (where dv/dr is the

change in velocity of the flow in the radial direction and du/dx is the change in the stagnation flames

velocity in the separation distance direction). (Egolfopoulos)

Figure (1) Opposed jet configuration used to achieve a flat flame (Egolfopoulos)

When using an opposed jet configuration, by running a thermally cracked fuel air mixture through one

burner and by running a hot air flow through the other burner, ignition can be achieved. In this case, the

flame resulting from ignition of these fuels can be studied with the use of Laser Doppler Velocimetry

(LDV), allowing in­depth studies of the conditions at ignition, including flame propagation and

extinction. A flat flame is studied because it is one of the most fundamental and simplest flames to model.

The bottom burner with a fuel air mixture through it is attached to a chamber. Rather than injecting the

fuel mixture parallel to burner, it is injected perpendicular into the burner to reduce vortices in the flow.

To further assist in achieving laminar flow, glass beads are inserted in the chamber attached to the bottom

burner to reduce the vortices in the flow also. The flow of this fuel air mixture then travels through the

length of the burner, through a diffuser section and a straight section with screens to reduce turbulence

also. A silicon carbide heating rod inside the top burner will heat the flow impinging upon the fuel air

mixture from the bottom burner up to potentially 1000°C, with the temperature being adjusted until

ignition is achieved. A co­flow of inert gas is run along the edges of the mouth of the burner to create a

shroud around the main flow to reduce the effects of shear on the main flow. As the opposing flow from

the top burner is run into the resultant flow from the bottom burner, a radial pressure differential is

established by this imposed velocity gradient. The flame sits at a position whereby its propagation speed

matches the velocity of the bottom jet.

When studying the ignition of a fuel, four key properties are used to characterize the process: temperature,

strain rate, composition, and equivalence ratio. The strain rate (K) in the flame, which is given by the

equation:

, (1)

This is a measure of the change in velocity versus the change in position across the flame, where u is the

flow velocity and x is the position in relation to distance from the nozzle exits. From the continuity

equation this is equal to the velocity gradient in the radial direction. Physically this relates to the rate at

which each fluid element in the flame is being stretched apart. For a stagnation flame, the strain rate is

positive, meaning the flame is being pulled apart. For the purposes of this study a relatively low strain rate

will be desirable so as to prevent the extinction of the flame. The equivalence ratio, φ, of a flame

describes the ratio of premixed fuel and air, given by the equation

. (2)

When the fuel to air ratio of a premixed flame is balanced stoichiometrically, φ=1, and as the flame

strength is typically reduced below and above value and extinction occurs faster. The temperature, T, of

the fuel, flame, and air of the system can all be controlled with the silicon carbide heating rods in each

burner, and the composition of the flame can be controlled directly by the amount that the JP­7 fuel is

thermally ‘cracked’ before ignition. By holding 2 of these 4 parameters constant at any given time, the

effect of each of these parameters on each other can be experimentally defined for any range of ‘’soups’’

of fuels.

Experimental Setup:

Since the primary goal of the development of this functioning rig is ignition, there will be numerous test

runs to test the ignition capabilities. Many of these will result in ignition not being achieved, or if there is

ignition the fuels may not be completely consumed. The byproducts of burning these fuels which will be

released into the atmosphere will in some cases contain known carcinogens. For safety reasons, the

combustion rig built to carry out these experiments will eventually be enclosed with Plexiglas to minimize

any safety risks to the conductors of the experiments. For the initial phase of this experiment however

only simple fuels such as Hydrogen and Methane will be tested in this system, which would require heavy

concentrated doses to prove to be a lethal hazard, so the rig will not be enclosed until heavier fuels, such

as JP­7, are utilized. This rig has been built in Dr. Egolfopoulos’ Combustion Lab in RRB 111.

Figure (2) Basic configuration of system. Nitrogen co­flow for both burners. Water for copper cooling coil. Fuel­air mixture enters bottom burner after flowing through a chamber with glass beads to diffuse flow from turbulent to laminar. Three sides of the rig will eventually be enclosed by Plexiglas, and the fourth side enclosed by a metal

sheet, which also serves as an inlet panel for the tubing for each of the gases into the system. The framing and stands for the rig is comprised of perforated square aluminum tubing, and will ultimately

be enclosed with Plexiglas walls as aforementioned, with an exhaust duct at the top of the enclosure to

serve as a vacuum to remove any hazardous byproducts. A basic configuration of the rig is outlined in

Figure (2). The burners, whose design is shown in Figures (*) and (*), have been cut out of Quartz glass

since they need to withstand high temperatures of potentially 1000°C and higher. Despite the

unavailability of a second Quartz glass burner while it is machined in the glass shop here at USC, a

temporary straight burner with a slightly convergent nozzle has been fixed to the rig. The nozzle exit is

20mm in diameter as with the top burner, and allows for sufficient mixing of the fuel air­mixture to

produce a flow with a low enough Reynolds Number to achieve a laminar flame. To secure the top burner

to the rig a safety arm­claw clamp mounted to the framing holds a section of the burner, with glass fiber

mesh wrapped around the burner for insulation. The bottom burner is suspended in a fixed position by the

use of a sheet mounted to the framing of the rig to serve as a stand, and the two burners were carefully

adjusted to ensure the nozzle exits were concentric and parallel to each other to minimize any instability

in the produced flame. A cooling coil has been placed around the top burner to protect all tubing

connected to the system from the high operating temperature of the rig. The coil is suspended from a

vertically mounted metal sheet with a gap welded for it to fit around the mouth of the top burner. This

sheet also serves as a mounting stand for the top burner, with the bottom inlet sections resting on the stand

for further stability.

Figures (3) and (4) The design for the Quartz glass burner. Figure (*) shows the cross section of the burners while Figure (3) shows the isometric view. The inert gas co­flow is run through the bottom inlet tubes into the blue

highlighted section in Figure (4) which is separated from the air flow which is run through the top inlet tubes. A silicon carbide heating rod is inserted at the top of the burner to heat up the air before it exits through the mouth at

the bottom.

A heating rod was inserted in the top burner as highlighted in Figures (3) and (4). Since the ultimate goal

of the construction of the combustion rig is to achieve ignition through the use of hot air, the flow through

the top burner must be heated to considerably high temperatures. For this reason a silicon carbide heating

rod capable of heating to temperatures above 1000°C was installed, and Quartz Glass was subsequently

selected for the burner to tolerate this temperature range. The heating rod was positioned so as not to be in

contact with the inner walls of the burner and was centered as accurately as possible to prevent any

significant impedance of the flow through the burner.

Pressurized poly flow tubing for all gases, liquids, and fuels necessary for the experiment is run into the

enclosure through a metal sheet bolted to the back of the rig. Tubing with a tolerance level of 250psi was

chosen and was run to a flow panel comprised of six pressure gauges, which are used to control the flow

of every gas, liquid, and fuel used in the chamber. To achieve a flat laminar flame the flow through the

bottom burner must be laminar, so flows at a high velocity are necessary to lower the Reynolds Number

of the flow through the bottom burner.

Sonic nozzles are used to meter the flow of all gasses. As a gas flows through a sonic nozzle it reaches the

speed of sound. From gas dynamics it is then known that the mass flow rate through this orifice has a

linear relationship to its upstream pressure. This is the main reason for their use in the experimental

setup. Another benefit of using sonic nozzles is the mass flow rate through them is unaffected by

variations in the upstream pressure during the experiments once again due to the gas flow reaching sonic

conditions. Calibration of the sonic nozzles was done using wet gas meters.

Ignition of Methane with an equivalence ratio (the ratio of fuel to air in a fuel­air mixture) of φ=0.7 will

be carried out to prove the combustion rig is operational and that it is able to reproduce existing published

experimental data. This test was carried out with manual ignition rather than with the heated oxidizer

from the top burner to first establish that the combustion rig could be shown to be functional. To achieve

ignition of any fuel in an opposed burner experiment, the flow rates of both burners must be calibrated to

achieve laminar flow at a Reynolds number less than 1000. This value is chosen so that there is no room

for any error in the experimental data due to turbulent flow from either of the two burners. The

composition of the flow from the top burner is of little relevance to the flame though the momentum of

the flow from both burners should be equivalent to ensure that the stagnation plane is in the center of the

two burners and the flame remains at a stable position.

A flame speed study to prove that a flat flame characteristic of previously published experimental data

was carried out with the use of Laser Doppler Velocimetry (LDV) for data acquisition. In order to

measure the speed of the flame, the fuel air mixture is seeded with silicon oil. This is achieved by running

the total flow of the bottom burner through a glass nebulizer filled with silicon oil. The idea behind this is

to achieve micron sized silicon oil droplets which will then seed the flow and effectively follow the

velocity of the mixture. The nebulizer used can be seen in the Appendix.

After ignition has been achieved and a stagnation flame has been established, a Nd:YAG

(neodymium­doped yttrium aluminum garnet) laser is then positioned parallel to the nozzle exits of each

burner and two laser beams are aimed at the center of this stagnation flame so that the region where they

cross generates an alternating field with constant fringe spacing in the separation distance axis. As the

seeding particles in the premixed fuel­air mixture pass through this laser field they are illuminated and

captured by a reflective lens. The time that it takes for this particle to travel through this laser field is then

measured and its velocity is calculated, and by comparing the speed of the flow of the fuel­air mixture to

the stagnation flame, the strain rate of the system is calculated. The final flame speed is then recorded,

and by adjusting the mass flow rate of the fuel air mixture while ensuring a stagnation flame is

maintained, the flame speed at different strain rates is measured. By then plotting these strain rates versus

the respective yielded flame speeds, a trend­line can be fit to these data points to characterize the laminar

flame speed of a certain equivalence ratio of a fuel, in this case φ=0.7 of methane, at any given strain rate.

Results and Discussion

The rig was constructed to the original design as shown in Figure (5) with the exclusion of the Plexiglas

enclosure and with a previously used 20mm diameter burner from the Combustion Lab until the second

Quartz Glass burner is machined. Upon calibration of the mass flow rates of each gas into the system,

49psi of methane was run through a .2mm diameter sonic nozzle and mixed with 42psi of air which had

been run through a .5mm diameter sonic nozzle to achieve the desired equivalence ratio of φ=0.7.

Figure (5) Final opposed jet burner configuration. The bottom burner will eventually by replaced with a second Quartz Glass burner but still has a 20mm diameter nozzle for experimental accuracy.

One significant design issue that became apparent while testing the ignition of Methane was the effect of

the silicon carbide heating rod on the air flowing from the top burner. Unless positioned dead­on in the

center of the burner the flow would lose uniformity as it exited the nozzle, effectively skewing any flame

ignited since the imposed velocity gradients were weaker than others due to the impedance of the flow by

the heating rod. A future design improvement will be made to secure the heating rod in place so that this

sensitivity issue will be mitigated. Once the heating rod was positioned properly a flat flame was able to

be produced as shown in Figure (6) and studied. An example of a crooked flame can be found in the

Appendix.

Figure (6) Stable stagnation flame of Methane at φ = 0.7.

The flow rate from the top burner significantly affects the nature of the stagnation flame that is produced

and must be adjusted after ignition is first achieved in order to stabilize the flame. The velocity must be

high enough to impose the necessary velocity gradient to make the flame quasi­one dimensional

otherwise it would start to resemble a Bunsen flame, and to keep the flame position stable in the

separation distance plane. It also must be low enough so that the top flow does not overpower the flame

and force it to become turbulent and/or cause it to extinguish due to too high of a strain rate. It is crucial

that the flame remain at a constant separation distance for it to be considered stable, otherwise any

measurements taken through LDV will be skewed since the stain rate is a function of separation distance

and velocity. Once the flame was stabilized LDV was used to capture the velocity profile of the centerline

of the flow up to the beginning of the flame. At this point, the flow experiences thermal expansion,

subsequently causing the seeding particles to lose fidelity either through destruction or second­order

diffusion effects. The flame speed itself is never measured but can be extrapolated by plotting the strain

rate as a function of separation distance x versus velocity u, and finding the local minimum velocity point

(reference flame speed, ) and local maximum stretch rate, , as seen in figure(7).

Figure (7) Velocity profile of Methane at φ=0.7 prior to thermal expansion.

The flame speed was then measured at different strain rates by adjusting the mass flow of the fuel­air

mixture into the system with a bypass valve. These reference flame speeds were then paired with their

respective strain rates as seen in Figure (8) and after performing a linear extrapolation the laminar flame

speed of Methane at φ=0.7 is characterized. This is the speed that the flame would burn at if it

experienced no stretch at all, thereby perfectly conforming to the Ideal Flame Model. While this condition

is physically impossible to reproduce, a flame speed study cannot be considered valid if the extrapolated

data does not yield a laminar flame speed equal to or within the tolerance of this universally accepted

fundamental flame property.

Figure (8) Flame speeds of Methane at φ=0.7 at varying strain rates. A linear extrapolation is performed to find the laminar flame speed of the fuel.

To check the validity of this experiment the extrapolated laminar flame speed was compared to that of a

published laminar flame speed value of 15±1cm/s (Vagelopoulos). This published data has been provided

in the Appendix for reference. The laminar flame speed extrapolated from the experimental data of

16±1cm/s is within this range, showing that the combustion rig that has been constructed is capable of

reproducing published laminar flame speeds.

When future experiments are conducted utilizing the heating of the air from the top burner for ignition,

there will be a minimum mass flow rate that must be flowing through the top burner due the operating

conditions of the silicon carbide heating rod. In order to operate the silicon carbide heating rod, the

thermal energy that accumulates must be absorbed by the particles of a gas flowing across it. Dependant

on the specified heating temperature, if the mass flow of this gas across the heating rod is below a certain

minimum value there will be an overload of thermal energy beyond the heating rod’s maximum tolerance

and it will subsequently be damaged.

The temperature of the system must be carefully adjusted and the power output of the heating rod has to

be monitored at all times due to its fragile nature. For instance if the system is told to jump from ambient

room temperature to 300 °C, a Proportional Integral Derivative (PID) controller used to control the

temperature of the heating rod will attempt to heat so as to achieve this temperature as quickly as possible

without regard to how much power it is supplying, which could result in an overflow of power causing the

heating rod to crack. As the flow through the burner is heated, a Type­R Thermocouple positioned at the

nozzle measures the temperature of this flow which is then displayed on the Temperature Controller as

shown in Figure (9), and the power output of the heating rod can be monitored as well. The thermocouple

provides feedback to the temperature controller so that the controller is aware of the temperature of the

top air flow. The temperature controller can ultimately be programmed to automatically heat the flow

from the top burner without exceeding a specified power level. Until this power output limit has been

programmed the temperature is manually raised in increments of 10­20°C with the power output closely

monitored during the process. A temperature of 500°C has been achieved, and once the temperature

controller has been programmed as aforementioned for safety reasons, the maximum attainable

temperature of the system will be characterized.

Figure (9) The PID controller used to control the temperature of the heating rod. The top value is the reading from the thermocouple and the bottom value is the desired temperature.

Concluding Remarks:

The opposed jet burner configuration that has been constructed in this experiment has been proven

capable of producing and sustaining a stable stagnation flame. Despite the presence of a double helix

shaped silicon carbide heating rod that resides in the top burner, the effect that it has on the flow and the

resulting imposed velocity gradient on the stagnation flame did not cause a deviation from previously

performed experimental results. However, this only holds true if the heating rod is positioned in just the

right location, in the center of the top burner and at the right angle so it is parallel to the inner walls of the

burner. Even the slightest deviation in position will cause the flow exiting the burner nozzle to lose

uniformity, resulting in a turbulent flame and/or flame extinction. However, perturbations generated in the

flow through the top burner due to the geometry of the heating rod did not significantly affect the

formation and sustaining of a methane stagnation flame at φ=0.7.

Now that the combustion rig has been constructed and is capable of sustaining stable stagnation flames,

the groundwork has been laid to begin testing ignition of fuel air mixtures through the heating of the air

flow through the top burner. Future studies will need to be done to quantify the effect of the increased

flow rate that will be induced by heating the flow through the top burner, and the necessary flow rate

adjustments to ensure the momentum of the top and bottom flow remain equal so that a stable stagnation

flame can still be produced. Since numerous ignition experiments are going to be conducted, to optimize

the experimental technique the temperature controller for the silicon carbide heating rod will be

programmed so that it can safely heat to any preset temperature without drawing too much power, rather

than manually adjusting the temperature every experiment. A design change to better secure the position

of the silicon carbide heating rod will be formulated as well due to the sensitivity of the current

configuration. Ultimately, once the ignition of gaseous fuels has been achieved by this system and is

calibrated to sustain stagnation flames consistent with previously published experimental data of laminar

flame speeds, the rig will once more be modified to allow for the preheating of liquid fuels through the

bottom burner, with the end goal of achieving ignition of various cracked states of JP­7.

References:

Egolfopoulos, Fokion. “Premixed Flames.” Pages 46­50. AME513 Sp. 2007 Course Notes. University

of Southern California Viterbi School of Engineering.

Escudie, D. “Influence of strain rate on a premixed turbulent flame stabilized in a stagnating flow. “

Experiments in Fluids, Vol. 27 Issue 6, pp 553­541, 1999. Spring­Verlag Berlin Heidelberg.

1999.

Direct experimental determination of laminar flame speeds

Vagelopoulos, Christine M. (Univ of Southern California); Egolfopoulos, Fokion N. Source:

Symposium (International) on Combustion, v 1, 1998, p 513­519

Appendix

Figure (10) Examples of skewed stagnation flames due to un­centered silicon carbide heating rod.

Figure (11) Published laminar flame speed values for methane at φ=0.7.

Figure (11) The nebulizer used to seed the fuel­air mixture prior to flowing through the bottom burner.