scramjet combustion process-sakthikumar.r.c,

8/9/2019 Scramjet Combustion Process-sakthikumar.r.c,

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SCRAMJET


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COMBUSTION PROCESS

The central problem of the design of supersonic

combustor is the achieving of combustion in a

short residence time with minimum stagnation

ressure loss.

Short residence time & minimum stagnation

pressure loss are relevant for any combustor,

supersonic combustor are in special in demandingshort residence time since, the combustor would

be too long with high speed flow.


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TYPE GT MC AB/RJ LR SCRAMJET

PRESSURE, atm 2-40 2-5 20-200 0.5-1.5

TEMP, K 500-750 600-2000 2500-3500 1200-1500

MACH NO 0.2-0.4 0.3-0.5 0.5-0.7 1.5-3.5

MEAN 150-300 200-350 800-1000 700-1500

VELOCITY, m/s

REACTION

TIME,ms

0.3-1 3-4 1-2 1-1.5

RESIDENCE

TIME, ms

3.5-5 4-5 2-3 0.7-1

DAMOHLER NO 1-5 1-2 1-3 0.5-1

PRESSURE

LOSS, %

6-8 4-5 5-20 15-20


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Contn

Solid rockets are not included since, theresidence time is much larger than reaction time.

This because, the chamber holds the propellant

inside with its port that has a larger free volume

near the propellant surface.

Scramjet, the flight will high altitude & therefore,the atm pressure will be b/w 0.025-0.006 atm &

rise in static pressure through external & internalcompression process will be 0.5-1.5atm.


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Contn

The essential difference is that one cannot affordto place larger blockages in the flow since, thestagnation pressure loss will be substantial.

The suggestion arising out of this study is that the

shocks from surfaces ahead of the fuel injectionzone.

If a shock intersects a hydrogen jet flowing into

high speed streams, additional vorticity caused atinterface b/w the fuel & air leads to bettermixing.


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Contn

Combustion gets initiated, one can expect to high

temp zones separated from low temp zones &

these will provide the necessary density gradient.

waves, to enhanced mixing.

The change in pressure distribution from when

fuel-off case to that when fuel is injected into airfrom the additional mass flux,momentum,energy

from the fuel.


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Contn

If the combustion chamber is sufficiently long

so that air & fuel become fully mixed, then the

avg pressure rise or fall ,depending on the fuel

.

In general, there will be change in the avg

pressure of fuel-air mixture, which will

depend not only on the momentum &energy, fuel injection.


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Contn

To develop an understanding of the

effectiveness of different injection systems,

-Wall injection-Centra n ect on

-Port injection


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Contn

When injecting the fuel, first aim clearly is to mixthe fuel& air, &this can be achieved by eitherdiffusion process or penetrating mechanism.

If diffusion process is used, the combustion,combustion chamber skin-friction losses maysignificant.

To use the penetration mechanism usually results

in a shorter combustion chamber. Fuel enters the free stream either sonically or

supersonically.


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WALL INJECTION

Wall injection-injection from a downstream-facing step in the combustion chamber wall.

Wall injection will be classed as one that uses a

purely diffusion process to obtain mixing.Some of the advantage over central injection:

-It provides film cooling to the combustionchamber walls.

-Free stream pressure losses are reduced.

-There is no central stut.


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WALL INJECTION


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Contn

Obviously, the main difference b/w wall-and central-injection systems is that wall injected close to a cold wall,whereas centrally injected fuel is injected into the middleof the free stream.

Clearly, one disadvantages of the wall-injection systems

diffusion boundary whereas centrally injected fuel has two.

This would account for a possible slowing down ofcombustion but would not account for its completesuppression.

The presence of the cold wall adversely affects the diffusion& burning of the fuel.


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Contn

Morgan et al postulated that the cold wall reduces thetemperature of the fuel-air mixture close to the wall,which is not a problem with central injection.

In the case of wall injection, the majority of the fuel.reach its self ignition temperature.

To enhance the mixing process when using wallinjection, placed a number of mixing wedges

downstream of the injector.

Combustion chamber cross sectional area 27x54mm


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PORT INJECTION

Port injection-injection from a hole or holes in thecombustion chamber wall.

Port injection uses a purely penetration

mechanisms. Scramjet port injection would appear to be a

reasonable option for delivery of fuel to air.

If the dynamic pressure is sufficiently highcompared with that of the free stream will beachieved from a port in the side of the combustionchamber.


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PORT INJECTION


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Contn

The penetration can be relatively insensitiveto the injection angle is greater than 15 deg.

Hence, fuel need not be injecteder endicular to the wall but can be in ected

at to the flow, so that component of the fuelmomentum is directed downstream, whichwill add to the thrust.

Fuel was injected sonically or supersonicallyfrom hole that dia of 3.5to7mm.


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Contn

Some experiments a large number of small

holes were used as an injection system, and a

distinct lack of mixing was observed.

size of the holes was increased, better mixing

was observed.

The combustion chamber cross sectional area

is 25x50mm.


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CENTRAL INJECTION

Central injection-injection from a downstream

facing centrally located in the combustion

chamber.

of two.


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CENTRAL INJECTION


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IGNITION

A supersonic combustion ramjet engine whichis a leading candidate for the propulsionsystem of future hypersonic vehicles, requires

technologies to secure reliable ignition andstable flame holding in supersonic airflow.

At low flight Mach numbers in particular,some types of forced ignition systems are

necessary because the total enthalpy ofairflow is insufficient to induce autoignition.


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Contn

the torch consists of a copper anode and ahafnium tipped cathode, isolated from eachother by a 4 micrometer gap.

Nitrogen feedstock gas is fed through this gapinto the combustion chamber.

When sufficient voltage is placed across thetorch, dielectric breakdown occurs within the

nitrogen and plasma is formed. Injection of the plasma into the fuel-air mixture is

a robust means of ignition and flame-holding.


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Contn

When the hydrogen jet is injected into the lowerstream of the plasma torch, the combustion efficiencyis reduced with the increase of the interval betweenthe plasma torch and the hydrogen jet.

, ,the recirculation zone, which causes a significantincrease of combustion efficiency, is observed.

On the other hand, when the hydrogen jet is injected

into the upper stream of the plasma torch, a largerinterval generally results in larger combustionefficiency.


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FUEL

Hydrogen fuelled

High energy, low storage density

Operating range: Mach 5 to 15

Isp ~ 3000s

Hydrocarbon fuelled

Lower energy, high storage density

Operating range: Mach 5 to 10

Isp ~1200s


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Contn

Since the structure of compounds in kerosene

is complex with a no of carbon & hydrogen

atoms, the reaction process with kerosene is

.

This is because the no of bonds to be broken

or made as the chemical system moves from

reactants to products is large.


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LAUNCH


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PROSPECTIVES

Space application

Missile application


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BACKUPS


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scramjet combustion process-sakthikumar.r.c,

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