H2 in Aircraft

8/14/2019 H2 in Aircraft

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American Institute of Aeronautics and Astronautics

A Review of Hydrogen as a Fuel for Future Air Transport

Bhupendra Khandelwal1, Paulas R. Sekaran2,Adam Karakurt3, Vishal Sethi4, Riti Singh5

Dept. of Power and Propulsion, School of Engineering, Cranfield University, UK, MK43 0AL

Innovations in propulsion system have been the key driver for the progress in air

transportation and it is expected to grow at a rapid pace. This incurs challenges in aircraft

noise reduction and regulation of hazardous emissions. This paper address the issues

associated to reduction in hazardous emissions by investigating the properties and traits of

hydrogen. Hydrogen as a fuel is most likely to be the energy carrier for the future of

aviation due to its potential zero emissions. A historical review has been carried out on

hydrogen usage in aerospace industry till today. The challenges of using hydrogen as a fuel

for aero applications have been laid down. The paper also shows various strategies

analyzed in order to evaluate hydrogen's feasibility which includes production, storage,

engine configurations and aircraft configurations.

Nomenclature

AAN = army after next

CH4 = methane

CO = carbon-monoxide

CO2 = carbon-dioxide

GE = general electric

GH2 = gaseous hydrogen

GWP = global warming potential

HALE = high altitude long endurance

H2O = water

LDI = lean direct injection

LH2 = liquid hydrogen

MLI = multi layer insulationNOx = oxides of nitrogen

O3 = ozone

UAV = unmanned aerial vehicle

UHC = unburned hydrocarbons

I. Introductionccording to the leading experts the aviation industry is expected to grow continuously, at a rapid pace in the

coming few decades. Commercial sectors are projected to increase in the order of 5% and more than that for

cargo transportation, despite the downturn in the current world economy1-4

. This increase is due to the tendency in

developing countries now requiring addition travel and cargo. Hence aviation appears to be the fastest growing

industry for the next two to three decades. Also, there is a requirement to limit the dependency on fossil fuels.

Although opinions about the exact date of perilously low levels may vary but the supply of fossil fuels areexpected to be exhausted sometime in this century.

1Researcher, Dept. of P&P, Cranfield University, UK, MK43 0AL, Student member2Student, Dept. of P&P, Cranfield University, UK, MK43 0AL,[email protected],non-member3Student, Dept. of P&P, Cranfield University, UK, MK43 0AL, [email protected], non-member4Research Fellow, Dept. of P&P, Cranfield University, UK, MK43 0AL, [email protected], non-member5Emeritus Professor, Dept. of P&P, Cranfield University, UK, MK43 0AL, [email protected]

A

48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit30 July - 01 August 2012, Atlanta, Georgia

AIAA 2012-426

Copyright 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
mailto:[email protected]:[email protected]


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Innovations in propulsion systems have been the primary driver for the progress in air transportation. Due to

advances in propulsion performance and efficiency, aircraft have the ability to travel at higher speeds over longer

distances with the capability of carrying larger payloads4. Present air traffic contributes to about 3% to the

anthropogenic greenhouse effect. This number may alter due to an increase of air traffic in the near future and

additionally the strategy to decrease major CO2

producers of today will vary this figure as well. Figure

1 shows the challenges involved with future propulsion

systems. Pollutants and particles that are emitted into

the environment have a negative effect on our global

climate. Whilst pollution is created directly from the

combustion of fuel it is also formed by power

production and consumption. The power produced is

sourced from energy in fuel that is extracted from

reserves, refined and then transported. During the

extraction, refinement and transportation pollutants are

being discharged before the fuel has even been

combusted. This is usually over looked when

comparing typical carbon based fuels with hydrogen.

Hydrogen is a suitable energy storage medium that

is free of carbon and other impurities; it is also the

most abundant element in the universe allowing it to beeasily sourced. Hydrogen was enthusiastically studied

during the last fuel crisis, with the current trend of increased fuel prices together with environmental

considerations; hydrogen is again being examined as an answer for a long term energy solution. Although

hydrogen cannot be the answer alone, it must be utilized with current day technology to truly go into operation.

II. Historical review of Hydrogen AircraftHydrogen was used for the first time in aeronautics for the inflation of balloons. Earlier balloons flew using

hot air as a lifting medium. On December 1, 1783 just two weeks after the ground breaking Montgolfier flight, the

French physicist Jacques Charles and Nicolas Robert flew the maiden gas balloon using hydrogen. This model

(Charlire Hydrogen Balloon) was 26 ft in diameter was launched and carried two passengers6.Early in the 20th

century, a German Count Ferdinand von Zeppelin pioneered a type of rigid airship known as A Zeppelin which

was the first airship to fly with hydrogen as fuel. It was based on his design outlined in 1874 and was detailed in1893

7-8. Figure 2 shows the evolution of hydrogen in aviation as a fuel.

Von Ohain is the pioneer, in using hydrogen as an alternate fuel for aero-derivative gas turbine. In 1937, he

ran successfully a gas turbine fuelled by hydrogen. It was tested in a rig and named as Heinkel-Strahltriebwerk 1

(HeS-1) experimental engine. The engine was a turbojet which produces 250 lbs of thrust8. A couple of decades

later, in 1956 Pratt & Whitney were asked by US Air Force to find the feasibility of liquid hydrogen fuel for aero

engines. The research was done by modifying the J57 engine for hydrogen fuelled injection. Further research work

performed altitude test for 3 turbojet engines (J-47, J-65-B-3 and J71-A-11). As laboratory testing was not

adequate to establish the reliability of using LH2 in aircraft, flight tests were executed establishing in-flight

performance statistics9. A modified J-65 turbojet engine with a separate hydrogen supply system was installed in a

US Air force B-57 twin-engine bomber for the flight tests. It was the first aircraft to fly using liquid hydrogen

pressurized with helium in one of its engines10

.

Since that time, the US has started several other projects like the CL-400 airplane, the US Space Program and

the Space Shuttle Program all utilized liquid hydrogen. In the 1970s hydrogen as a fuel renewed its interest due toconsequences of the oil crisis. Since the beginning of 1970s, several studies were carried out by General Electric

(GE) and NASA to explore the procedure of hydrogen as an alternate fuel for use in gas turbines. GE evaluated

unconventional cycles, using hydrogen for aircraft propulsion system11

.

In 1988, the Soviets tested hydrogen with a modified TU-154 aircraft (renamed TU-155), with one engine

operating on hydrogen. During 1991, the Soviet Union and Germany announced their agreement to work together

on liquid hydrogen fuelled commercial prototype, which is similar to the A310 at an estimated range of 500

miles15

. There are two different projects of Cryoplane designs for subsonic aircraft adopted by NASA-Langley

Figure 1.Key challenges for a propulsion system5


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Research Centre and the Russian-German Cooperative Venture on the basis of an existing Airbus A310. The

Russian-German Cooperative Venture established a design with hydrogen tanks on the top of the fuselage and

small amount of fuel on the wings, which intently reduces wing size. The NASA project was to have two spherical

tanks for hydrogen, in order to reduce the surface to volume ratio. It was designed for 400 passengers, at a cruise

speed of Mach 0.85 and a range of 5500 nautical miles12

.

Figure 2. Evolution of hydrogen in aviation industry5

In 1998, the Army After Next (AAN) annual report states, An absolute imperative exists to develop

alternative fuels for AAN-era forces. Furthermore, it is stated that The development of hydrogen-based vehicles

are a national necessity for AAN platforms. In 2000, the European Commission funded a consortium of 35

partners from the aviation sector, led by Airbus Deutschland called CRYOPLANE project, for the system analysis

of air craft fuelled by LH2. The investigation aim was to have a strong platform for initiating large scale activity

for the development of alternate fuel and the introduction of LH2 fuel for aviation. Different configurations of

aircraft were studied and a transition of aviation fuel was investigated during these 26 months of study lead by

Airbus Deutschland13

. The fuel tanks must be 4 times larger when compared to conventional aircraft fuel storage.

Due to this excessive surface area of the tanks, consumption of energy would increase from 9 to 14%. Overall

operating costs of hydrogen fuelled aircraft would increase from 4% to 5% based on fuel alone 13-14.

AeroVironment built and tested the worlds first LH2 powered UAV (Unmanned Ariel Vehicle) successfully

during 2005. The prototype built, demonstrated the robustness and the practicality involved in enabling the

concept of a Global Observer Operation System15

.

III. Hydrogen ProductionThere are several different processes of hydrogen production available broadly, divided into three categories

named from renewable resources, nuclear energy and fossil fuel16

. A summary of all the processes have been

shown in Figure 3. Various chemical methods have also been studied by researchers for production of hydrogen17

.

There are two main methods which are being used today for the production of hydrogen, from current technology

to state of the art. For the majority (97%) of hydrogen production natural gas steam reforming is used18

. This is

mainly due to the economic benefit of production by this method, which is unlikely to change in near future. The

other major production method is electrolysis of water. The major benefit of hydrogen production with electrolysis


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Figure 3. Hydrogen production paths21.

of water is use of almost all sources of primary energy. Production of hydrogen from sunlight offers large

environmental benefits, especially if the cost of production could be decreased and efficiency of production could

be improved19

. Electrolysis of water is a process where water is decomposed into oxygen and hydrogen by forcing

electric current flow through the water. Production of hydrogen by electrolysis could be renewable or non-

renewable depending on the source of electricity production. Electricity could be produced from renewable

sources as solar, wind, geo-thermal or from non-renewable sources as gas, oil and coal. Normally hydrogen

production by fossil fuel is the most efficient method, considering hydrogen production and delivery. The overall

efficiency is estimated around just over 50 % for fossil fuel, 40 % for biomass and 53 % for gases20

.

Sarigiannis and Kornberger21

studied different renewable energy

based methods for the production of

hydrogen based on Life Cycle

Assessments of technology.

Production of electricity to fabricate

hydrogen was considered being

produced from renewable sources.

They concluded that low emissions

could be achieved by using wind

and hydropower energy sources

even for long distance transports.

Production of hydrogen by biomass

can also result in low emissions on

the condition that biomass is

produced locally for avoiding

transportation.

For production of hydrogen

through electrolysis, large amounts

of desalted water and electricity

would be required. Kronberger22

studied production of hydrogen by electrolysis. He found that for producing

50,000 kg/day of hydrogen, 105 MW of electricity and 28 m3/hr desalt water would be required considering an

efficiency of 80 %. At the same time a liquefaction plant would consume 25,400 kW of electricity for main

electrical power, 155 kW for control electrical power; for an output of 50,000 kg/day of hydrogen23

. Kronberger22

also studied uses of biomass for hydrogen. He found that for producing 50,000 kg/day of hydrogen 490,000 kg of

dry biomass would be required resulting in 179x106 kg biomass per year. Sevenson24 compared the amount ofenergy, in terms of biomass, that would be required to power all aviation refueling in Sweden with hydrogen, with

the potential of biomass supply in Sweden. He concluded that the amounts required for aviation are not

unreasonably large. However, it requires that the biomass use would be enlarged.

Production of hydrogen by fossil fuel is a good option, but it again leaves the problem of emissions and

depleting fossil fuel resources. Looking at the short term goal and cost effectiveness, hydrogen produced from

fossil fuel reformation is an effective method. Under this transition phase it might be reasonable to employ this

method to reduce production costs, particularly if the CO2is extracted and sequestered in reservoirs or utilized19

.

Electrolysis of water and gasification of biomass are promising technologies for future. Considering the progress

in research and development of hydrogen production methods it can be said that in the long run there are likely to

be sustainable hydrogen production methods. Hydrogen productions in photo-chemical and photo-biological

systems using sunlight are examples that probably will offer large environmental benefits in the future if

successfully developed24

. It is clear that civil aviation with hydrogen will not necessarily indicate that emissions of

greenhouse gases are eradicated, since greenhouses gases may be released during the hydrogen production.Production of hydrogen by nuclear energy is also a substantially important production method for low emissions.

IV. Fuel tank ConfigurationsTank Configuration

The key driver in tank configuration will be to maximize the hydrogen available in a lightweight and low

boil-off system. Storage of hydrogen can be of pressurized gas as a hydride or in as liquid hydrogen (LH2). Other


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forms of storage forms appear to be impossible for airborne application because of their excessive weight or

volume25, 26

. During selection of tank configuration tank shape and tank insulation plays a vital role.

Tank ShapeLH2has a very low density, with respect to other liquids which results in its larger volumes. Hence the weight

of LH2required to be carried on-board for a particular mission will be less than any other cooling fluid. The tank

shape depends upon several issues mainly, fuselage and the type of tank matters a lot. Non-integral tanks act as afuel container and have to be mounted in a conventional fuselage/skin/frame structure. Hence this type of

configuration has to resist the loads associated with fuel containment. In this method, the tank is kept outside the

fuselage. The main constraints in designing non-integral tanks are the aerodynamics effect and the integration

problem. Integral tanks forms as an integral part of the airframe structure. Hence, it should resist stresses such as

fuselage axial, bending and shear forces resulting from aircraft loading. The main constraint in designing an

integral tank is that the fuselage drives the geometry (diameter) of LH2storage tank. Integral tank configuration

seems to be only feasible configuration for wide body aircraft.

In Cryoplane project, the tanks were kept over the fuselage and across the wings. This gave the chance to

increase the LH2carrying capacity but this lead to a thick and heavy tank wall and many stiffeners since the tank

shape was not adopted well according to the pressurization requirement. Therefore, possible shapes are spherical

and cylindrical tank shapes with diameter equal to the fuselage diameter27

. Spherical LH2tanks are being used in

space application because it requires less surface area for the given volume. The boil-off rate is less because the

there is lesser passive heat flux into the spherical tank shape. Given that advantages provided by spherical tanks it

has a problem in manufacturing and it has a higher frontal area compared to cylindrical shaped tanks 25. On the

other side it is easier to manufacture a cylindrical tank shapes but the drawback it has higher surface area to

volume ratio which results in higher passive heat load into the tank. Cylindrical tanks are easier to be integrated

inside the fuselage and they give higher volumetric efficiency. Hence, the space inside the fuselage can be used in

an optimum way26

.

Tank InsulationThere are two types of insulations that can be applied for a tank. They are internal insulation and external

insulation. In case of internal insulation, the insulation is exposed always to LH 2 which is at cryogenic

temperature but due to heat transfer effect the LH2changes its state to GH2will occur. The GH2causes diffusion

into the tank wall which will inturn increase the thermal conductivity of the insulation, thereby crippling its

effectiveness. Another problem involved with this insulation method is the system must be impermeable to GH 226

.

When the tank is insulated on the outside then it is known as external insulation. In this case, primarily there will

be an expansion and contraction of tank, as LH2is filled and utilised. Secondly, there is a problem of attachmentbecause of structural support system and the dimension of the tank increases as well. External insulation is easily

subjected to mechanical damage and it has to withstand the impact load38

. In the Cryoplaneproject, Air Liquide

also adopted the external insulation method27

. But these drawbacks can be solved when compared to the problems

with internal insulation system. The key challenge involved in LH 2storage is mass of boil-off which leads to loss

of hydrogen. Boil-off is the phenomena that occur when liquid boils and changes its state into its gaseous form

because of heat transfer and escapes by permeation. LH2 boil-off depends upon thermal insulation and tank

geometry28,29

. Tanks need to be equipped with effective insulation in order to minimize boil-off30

. The design

parameters for LH2 storage tanks are determined by LH2 temperatures, operating pressure and insulation

thickness. Different types of insulations which could be potentially suitable for carrying LH 2 for airborne

applications are illustrated below.

Multilayer insulationMultilayer insulation system uses a number of thermal radiation shields perpendicular to the direction heat

flow31. MLI usually consists of reflective foil over the outer side of inner tank wall to minimize the transport of

radiation heat. Generally, the radiation shields are alternate layers of metal foil and a thin insulating material like

glass fiber, polyester etc. to avoid metal to metal contact. With increase in number of heat shields, additional heat

transfer takes place due to conduction. The optimal numbers of layers that can be used are about 60 and 100

layers. MLI insulations performance depends on the pressure and type of residual gas in the insulation. The

thermal behavior of MLI degrades quickly for pressures higher than 0.001 mbar32

. MLI insulations are very

sensitive to the layer density so that local compression must be avoided during manufacturing. The most important


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parameter for an insulation system to be used in aeronautical applications is low thermal conductivity, low

emissivity and a low density. Overall the finished product of a LH 2storage tank with MLI layers will be heavier in

weight.

Vacuum insulation

The vacuum process seems to be a perfect solution for minimizing the mass of boil-off. But practically it is

impossible to attain a vacuum and therefore venting equipments are necessary in the vacuum region

33

. Theinteraction between LH2and the air has to be avoided by sealing materials to prevent air entering and freezing

inside the tank system. If the air freezes in the flow lines, LH2 flow will be blocked33

. The tank wall thickness

must be sufficient enough to withstand the buckling, since the vacuum jacket is subjected to external pressure.

Additional stiffeners are required between the (vacuum jacket shell) outer wall and the inner wall which increases

the weight of the tank34

. External equipment is required to suck the air and maintain the pressure at the vacuum

chamber. The Vacuum insulation technique is adopted in space applications for the storage of LH2. Several

research activities have taken place for vacuum insulation and it seems to be a promising solution for a LH2

storage tank. This type of concept is well established but heavier tank walls are required, which are expensive to

implement and to maintain the temperature and pressure in the vacuum35

.

Foam insulation

Generally, the materials used for foam insulation have very low density and thermal conductivity. The rigid

foam insulation is applied outside the inner tank wall and a thin metal wall required to be surrounded around the

foam to maintain its structural stability to withstand and protect it from external forces36

. The foam insulation

concept is more resistant to catastrophic failure than the vacuum-jacketed insulation28

. The insulation thickness of

the tank depends upon the insulation material properties, tank size, allowable boil-off and overall allowable tank

weight. Foam insulation is low cost, easy to implement and light weight. Vacuum-jacketed and multilayer

insulation has been investigated for quite some time and in the case of loss of vacuum it might cause catastrophic

failure whereas in foam insulation the chances of catastrophic failure are less.

V. Hydrogen Aircraft ConfigurationsHydrogen powered aircraft must comply with some practical configurations so that the typical performance

and handling requirements of airline operation could be met. This has to be done without undue need for

infrastructure and ground equipment inconsistent with current aviation industry. Sefain18

studied various

configurations of hydrogen powered aircrafts to incorporate large LH2 fuel volume with minimum penalty and

optimum performance benefits. In his study medium range aircrafts were considered. A team of researchers and

experts worked in this study to work-out several configurations. Twin Tail-Boom and Tail-Tank Concepts as

shown in Figure 4 are selected as most appropriate configurations from a pool of various configurations studied.

Figure 4.Twin Tail-Boom and Tail-Tank Concepts23

It is proposed that in Twin-Boom configuration external slender booms were utilised as hydrogen fuel tanks

and also as a structural booms interconnecting the wing and tail surface together. In a Tail-Tank configuration, a


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tank was placed above the fuselage, physically separated it from the aircraft. Tank is interconnected by an above-

fuselage pylon and the tail plane. Three different concepts have been shown in Figure 5 which explains the

different configuration of aircraft for LH2storage. In Figure 5 (a & b) hydrogen tank storage has been shown at

the top and the end of fuselage, whereas in Figure 5 (c) it is proposed that hydrogen is stored in front and end of

the fuselage13

.

VI. Hydrogen Aircraft EngineHydrogen has a tendency to flash back and produce high temperature flames which in turn lead to higher NO x

emissions. Figure 6 represents a comparison between Hydrogen and Kerosene flame stability limits. Whilst

Hydrogen has a much higher temperature at its stoichiometric ratio, it can burn stable at significantly leaner ratios.

Leaner equivalence ratios will attain low temperature flames avoiding the high temperatures associated with

stoichiometric conditions. With leaner equivalence ratios the mixing intensity is required to increase, as to

eliminate local hotspots and to enable the fuel to

effectively mix producing a balanced flame

profile. Burning the fuel requires changes in the

combustor to avoid high temperatures and to

provide effective mixing to take full advantage

of hydrogen's attributes at leaner conditions.

Although if we take into account hydrogen's

attributes such as its high flame speeds, its low

temperature from its cryogenic state and itsmajor concern of flashback with flame

propagation then we must consider systems to

establish redundancy and reliability.

Considering hydrogen's journey right from

the cryogenic tanks, from startup it can be given

that the fuel lines are unquestionably void of H2

due to its likely hood to escape over time and

there will most likely be ambient air present.

With ambient air present in the fuel lines a large risk of flask back is presented immediately, during the start up

cycle of the engine. To eliminate this risk, the fuel lines must be purged with an inert gas, cheaply nitrogen can be

used to flush the lines, although the solidification of gases via liquid hydrogen must be considered which will

cause fuel flow issues. This will also be required during shut down, flushing the fuel lines will eliminate this

flashback risk entirely, Dahl and Suttrop37have tested this method proving its reliability. Although before the fuelhas even entered the combustion chamber it

must be preheated to ensure the fuel has

fully vaporized under its maximum flow

rate from the cryogenic storage tank.

To perform this effectively and safely a

heat exchanger is required to provide extra

energy. Placing fuel lines in hot sections of

the engine are not recommended as fuel

leaks will immediately ignite causing a

large flammability risk. A heat exchanger

will transport heat away from hot sections

of the engine, be it the exhaust spokes,

turbine section, combustion chamber or thehigh pressure compressor stage. Taking

heat away from the hot engine sections will

aid in reducing the amount of energy

required for combustion, ensuring the fuel

is completely in a gaseous state before

injection. This will provide more

advantages increasing thermal efficiency, increasing component life and resilience to temperature abuse, taking

full advantage of the heat sink potential. During start up the engine will require an electrical heater to provide an

Figure 6. Temperature Characteristics Hydrogen and

Kerosene38

Figure 5. Hydrogen Aircraft with different hydrogentank configurations

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Figure 8. CFD analysis of micro mix injectors42

increase in fuel temperature and once the engine has reached idle rotational speed the heat exchange can resume

operations. A metering system will be required to alter the liquid and gas fuel flow rate under different power

conditions of the engine. A metering system was also described by Dahl and Suttrop37 that provided reactive

hydrogen feed to the engine under fluctuating power conditions.

VII. Hydrogen CombustorsAlthough the combustion of hydrogen is more complicated than just an adjustment of the air to fuel ratio and

is predominantly dependent on combustor geometry39. The use of hydrogen addition to conventional fuels produce

improved results although pure hydrogen use in conventional combustors has results that are inferior to

conventional fuels37.This is due to the combustors geometry being inadequate to effectively mix the fuel and air.

When hydrogen is combusted in a conventional chamber large diffusive flames are formed where stoichiometric

ratios appear around the flame causing very high temperatures and in turn high NO x emissions40. To elude

inadequate combustion all flame attributes of combustion must be considered such as flame stability, combustion

efficiency, acoustics and other vital diagnostics. For this reason studies have been performed to design and

research novel combustor concepts to effectively

combust hydrogen realizing its full potential.

There are two concepts of hydrogen combustors

that pose the most likely designs to be further

adapted into combustor configurations. The twoconcepts are the Lean Direct Injection (LDI)

investigated by NASA with Marek et al.41 and the

Micro mix concepts investigated by Dahl and

Suttrop37. Both designs have been proven as concepts

with actual combustion tests performed. The two

concepts are quite similar in their methodology. Both

concepts have established that flashback is of

primary concern with the desire to increase fuel

mixing. The mixing intensity of hydrogen and air is

greatly increased in both designs to avoid large

diffusion flames forming that result in higher NOx

emissions. By increasing the mixing intensity, the

flame length will be reduced having completedcombustion earlier with reduced residence time. As

NOx is both dependant on

temperature and residence time,

increasing the mixing intensity will

enable very low NOx emissions. It

was established that the injection

methods of H2 are crucial in

acquiring effective combustion, for

this reason further investigations

have been performed to illustrate the

attributes of alternative fuel

injection. The LDI method resulted

in reduced NOx levels with noflashback or auto ignition

phenomena occurring41.

Studies of micro mix

combustors were formerly

performed by Dahl and Suttrop37

establishing hydrogen combustion,

demonstrating safe combustion of

the fuel with an effort to minimize NO xproduction. The objective of the investigations was to convert an A320

Figure 7. Micro-mix cross-section40


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APU GTCP 36-300 to function on Hydrogen safely37

. Miniaturized diffusive combustion was established to

counteract the results of H2combustion with large diffusive flames, which result in very high temperature zones.

As thermal NOxis dependent on residence time, the amount of oxygen and nitrogen present and temperatures that

increases emissions exponentially above 1800K. It is very important to avoid these conditions in combustor

design. The micro mix combustor avoids this by the use of heterogeneous mixing, which generates numerous

small diffusive flames. This is done by utilizing miniaturized diffusion by having a large number of fuel/air inlets,

whilst taking the standard of 3-4% PD into consideration limiting the amount of fuel and air inlets. The

improved mixing has taken advantage of turbulent formations and eddy breakdown reducing the local residence

times of the flame and avoiding stoichiometric conditions43

. A schematic of a micro mix combustor is shown in

Figure 7.

Further investigations were performed by Dahl and Suttrop37

examining the performance of various

configurations of micro mix combustion. Dahl and Suttrop40

were able to show that further reductions of NOxare

possible. This is accomplished with modifications to micro mix combustion. Other micro mix injectors have also

been studied with various inlet geometries42, 44

. Figure 8 displays the three different models of varrious micro mix

injectors studied by Murthy et al42

. Model 1 is a conventional micro mix combustor which has much lower NOxemissions than hydrogen combustion in conventional cans. Although it is clear from Figure 18 that high

temperature hot spots are present in the flame profile. These high temperature zones are primariliy responsible for

the thermal NOxproductions. Further improvements of injectors have moved the H2 injector locations along the

main flow conduit as seen in model 2. Lower NOx emissions are present with this design, although critically the

high temperature location is at the injector face. This will present a large potential risk. The high temperatures atthe injector of model 2 will ineveitably lead to failure resulting in a large flammability risk. Utilising a frustum

incoroprated into the design is present in model 3. The small hot spot is now present further down stream in the

flame profile. This will enable a relible and durable ocombustor for further experimentation. The NO x emissions

from this design are substantially reduced 80% lower than model 1 used in the study42

.

VIII. SafetyHydrogen has shown to be a very advantageous fuel for the future of aviation, enabling reduced emissions

whilst providing exceptional performance. Concern associated with the hydrogen is its persistence to escape from

enclosures. This is unwanted for obvious reasons as this is a primary loss of energy. Whilst the slow dissipation of

hydrogen does not pose a large flammability risk when correctly ventilated. This is due to the fact that hydrogen is

the lightest element, allowing it to be quickly dissipated

as it rises into the atmosphere. This is very similar when

fuel leaks occur. Investigations performed at the

University of Miami45

have compared a hydrogen fuel

leak with a kerosene fuel leak. Both fuels are ignited

after a certain period of time, enabling the fuel to

spread. Whilst kerosene is a liquid that will pour and fill

out as much space as possible, hydrogen is a gaseous

fuel that will be localized to its leak, rising into the

atmosphere in a controlled manor.

Although a large hydrogen liquid leak will spread more

than a gas leak, but it still will be the least hazardous

when compared to other fuels as can be observed from

Figure 9. Hydrogen is a much safer fuel than

conventional fuels when provided with the effective systems.

IX. SummaryHydrogen is the most likely energy carrier for the future energy economy. Hydrogen provides a clean energy

system that will supply energy for mobile applications. There are no issues of CO2 associated with hydrogen

combustion and PEM fuel cells. This allows all other pollutants to be eliminated from backup generators and

power production etc. Hydrogen combustion has emissions of only H 2O and NOx which must be considered. NOx

Figure 9. Danger Zone of Spilled Liquid Gas46

.


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has the most potential to cause damage to the environment, whilst water contrails can be reduced easily by

decreasing altitude.

Although with hydrogen as an energy carrier the fuel must be stored in a liquid cryogenic state to attain a

reasonable energy density. This provides one of the largest challenges with the fuel. Further investigations are

required to establish the most feasible materials and insulation that provide the ability to contain high pressures for

ground and aero applications. As weight is a very important issue with aero applications the design of the aircraft

and cryogenic storage of hydrogen must been established with the least amount of boil off for the minimal weight.

The NOx emissions are a major factor and must be considered. The only way to reduce the NO xemissions with

hydrogen combustion is to improve the local mixing intensity. This is dependent on combustor design and the

equivalence ratio of the fuel. As seen in Figure 16 we can burn hydrogen at significantly leaner ratios. Although if

we combust hydrogen at low equivalence ratios hot spots will still form in the flame profile if the mixing intensity

is inadequate. The LDI and micro mix concepts of combusting hydrogen provide the best methods avoiding

flashback, having the fuel and air feed to the chamber along separate paths and utilizing small but rapid mixing

channels proving an effective solution.

There are risks associated with hydrogen like any other fuel. Although as hydrogen is a gas it will react

differently to liquid fuels. If a leak has ignited in an aircraft the flames will be localized to the leak and the flames

will rise. This makes hydrogen a very safe gas removing the large fire risks associated with conventional fuels that

will flow to the ground and consume the vehicle spreading in all directions. This gives the incentive to locate the

fuel tanks above the passengers as to further protect them from fire. Propelling our future aircraft by hydrogen is a

viable option considering various constraints.

X. ConclusionsThree times more efficient than oil but four times bulkier even in its liquid state, hydrogen has already

powered several prototype Cryoplanes around the world. Unlike normal aircraft, which use wings for storing fuel,

hydrogen powered aircraft will be usually designed with the liquid hydrogen fuel carried inside the fuselage, in

order to minimize surface-area and reduce boil-off which lead to design a radical bulky aircraft. But in a reputable

consensus hydrogen seems to be the strongly viable option to address the long term solution to environmental

concerns and energy dependency.

Hydrogen is no stranger to aircraft it has successfully been used in flight test such as the Hes1 tests and

Tupolev TU-155 tests. All flight tests performed were successful with no complications. Although it was

established that hydrogen has a very high specific energy its energy density is quite low. This required cryogenic

storage during flight tests, maintaining the fuel in a liquid state. It was recognized that to enable hydrogen use

effectively in aviation, viable cryogenic storage is required to provide low weight, whilst concerning boil off andstorage densities. Further studies with cryogenic storage are required to establish light structures that are safe,

capable of withstanding high loads experience during take-off and landing.

Hydrogen combustion only regards NOxand water vapor. Water vapor that escapes high into the atmosphere

will contribute to global warming; its effect in aero applications can be eliminated by reducing the altitude of

aircraft producing no contrails. NOxemissions on the other hand are a difficult pollutant to reduce. The primary

properties of hydrogen enable fuels to be combusted stably at leaner equivalence ratios, reducing the residence

times. Thus reduced emissions are acquirable with hydrogen combustion

The two most viable methods of hydrogen combustion for gas turbines are LDI and micro mix methods. Both

of these concepts establish improved local mixing intensities that enable lower equivalence ratios to combust

absent of local hot spots that would otherwise form in the flame profile resulting in high NO xemissions. The two

concepts have been tested and show effective methods of eliminating flashback risks associated with hydrogen's

high flame speeds. Hence in principle it is possible to fly with hydrogen but for the moment producing enough

hydrogen in an environmentally friendly manner for aviation is a challenge.

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