超燃冲压发动机燃料

Scramjet Fuels Hydrogen Vs Hydrocarbons Scramjets are engines designed to operate at high speeds usually only associated with rockets and are typically powered by hydrogen fuel. Scramjet is an acronym for Supersonic Combustion Ramjet. This suggests that before discussing scramjet fuels you must first have an understanding of ramjets. A ramjet has no moving parts. Air entering the intake is compressed using the forward speed of the aircraft. The intake air is then slowed from a high subsonic or supersonic speed to a low subsonic speed by aerodynamic diffusion created by the inlet and diffuser, as seen in figure 1. Fuel is then injected into the combustion chamber where burning takes place. The expansion of hot gases then accelerates the subsonic exhaust air to a supersonic speed. This results in a forward velocity. Figure 1 - Ramjet Scramjets on the other hand do not slow the freestream air down through the combustion chamber rather keeping it at some supersonic speed. This may appear mechanically simple however it is immensely more aerodynamically complex than a jet engine. Figure 2 - Scramjet Keeping the freestream flow supersonic enables the scramjet to fly at much higher speeds. Supersonic flow is needed at higher speeds to maximise efficiency through the combustion process. Scramjet top speeds have been estimated between Mach 15 to Mach 24, however at this early stage Mach 9.6 is the fastest recorded flight achieved during the third and final flight of the X-43A flown by NASA. This is three times the speed of the SR-71, officially the fastest jet-powered aircraft which achieved Mach 3.2. Scramjets are set to make it possible to fly from Sydney to London in approximately two hours. Shown below is an artist impression of what a commercial hypersonic transport aircraft may well look like in the future. They are also set to revolutionise the launch of small space payloads such as communication satellites as they will substantially reduce costs involved. Figure 4 - Artist impression of a commercial hypersonic transport aircraft Now that we have a simple understanding of what a scramjet is and how it differs from a ramjet we can discuss why hydrogen fuel is normally preferred over hydrocarbon fuel. As the oxygen needed by the engine for combustion is taken from the atmosphere, instead of a tank like onboard a rocket (refer figure 5), there is huge reduction in the amount of fuel carried by the aircraft. As a result the scramjet propelled aircraft can be smaller, lighter and faster. Fuel is still needed to mix with air to get the thrust required to propel the aircraft at such high speeds. This is just one challenge faced when trying to fly a scramjet at such high speeds. The problem is getting the fuel to mix properly with the air while combusting and expanding before it exists the exhaust through the tail of the aircraft. This process can take as little as 0.001 seconds. Not only does the fuel have to mix well with air, it needs to burn rapidly to generate large amounts of thrust to propel the scramjet. Hydrogen has these properties which will be discussed later. What is hydrogen? Hydrogen is the simplest, lightest element in the universe. It consists of one proton and one electron. Some argue that because of its simplicity, hydrogen is the root of all elements. In fact, hydrogen makes up 90% of all mater. In its normal state, gas, it is colorless, odorless, tasteless, and is nontoxic. As a result it is different from every other common fuel we use. What is hydrocarbon? Hydrocarbon is an organic chemical compound consisting of hydrogen and carbon, called petroleum. This molecular structure of hydrocarbon compounds varies from the simplest, methane (CH4), a constituent of natural gas, to the very heavy and very complex. Octane, for example, a constituent of crude oil, is one of the heavier, more complex molecules. The rationale for using liquid hydrogen for scramjet fuel rests with its high specific impulse (change in momentum per unit mass of propellant) and its potential for cooling parts of the vehicle. This has the desired effect of increasing the thrust by adding the hydrogen to the flow beyond the stoichiometric burning ratio, which is when all the fuel is combined with all the free oxygen. Figure 7 - Hydrogen V Hydrocarbon Mach Number Similarly, hydrogen has a good Lower Heating Value (LHV). The LHV describes the amount of energy released when a fuel is combusted and all of the remaining products remain in gaseous form. Hydrogen has a LHV of 119,600 kJ/kg. Compare that to JP-8, another commonly used fuel in aircraft, which only has 43,190 kJ/kg and is less than half that of hydrogen. Hydrogen is also extremely flammable. This property means it only takes a small amount of energy to ignite and make it burn. As it has a wide flammability range it can burn when it occupies between 4 – 74% of the air volume. Also, as it is a gas it mixes very easily with air and this allows for very efficient combustion. Another advantage over hydrocarbon based fuels like JP-8 is that hydrogen is a clean fuel as it doesn’t produce any harmful pollutants like carbon monoxide (CO), carbon dioxide (CO2) or particulate matter during the combustion process. When it is burnt the only by product is water, which can be safely exhausted into the atmosphere. To increase the performance of solid hydrogen fuel, researchers have proposed side wall injection into the supersonic airstream. The primary advantages of solid hydrogen fueling are: 1) the fuel can be injected much farther into a flow which covers a much larger volume in a shorter time, 2) rapid dispersion of hydrogen across the pellet trajectory in the combustion chamber will result from wake mixing behind a small particle and liquid ablation which will improve mixing we, 3) wall injection would eliminate protruding injector drag, 4) it may be possible to inject the solid at high velocity and use its kinetic energy to augment thrust, and 5) the fuel density is higher and beneficial additives can be injected with the fuel. The performance can only be fully assessed by considering the effects, not only on engine thrust, but the aerodynamics and structure of the vehicle. For the duration of a flight to orbit the vehicle will experience a number of aerodynamic conditions. During the final stages it will experience high mach and low Reynolds numbers. However the scramjet’s maximum height is limited by the intakes ability to capture sufficient air. It typically requires a dynamic pressure of 70kPa, and at Mach 10 this translate into a height of ~30kms and a Reynolds number of about 107 per meter. At high altitudes the air becomes less dense. It is for this reason that scramjets won’t enter space unless a rocket is used to propel it on its final phase. Although it may appear that hydrogen is the idea fuel for scramjet propulsion it does present some disadvantages. Hydrogen is not a dense fuel, having a density of only 0.09 kg/m3 at standard temperature and pressure. JP-8 on the other hand, has a density of 800 kg/m3 in similar conditions. Having a low density does save weight, however a large volume is needed in order to store enough chemical energy for practical use. To increase its density hydrogen gas is stored under pressure, but even at ~70 000 kPa it will still only contain one quarter of that stored by an equivalent amount of JP-8. Hydrogen’s density can also be increase by cooling the pressurized tanks until the substance becomes a liquid however this process is expensive and can be dangerous. It is for these reasons that aircraft flying in the skies today continue to use hydrocarbon based fuels as opposed to hydrogen. The denser hydrocarbon fuels allow aircraft to fly longer distances than those burning hydrogen. Even with the above limitations hydrogen has been the clear choice for many scramjet researchers due to its versatility and performance. The X-43A was part of NASA’s Hyper-X program in which they developed new air-breathing propulsion systems to reach supersonic speeds. As previously mentioned, the X-43A has had excellent results using hydrogen fuel. Apart from flying at Mach 9.6 it also flew autonomously under its own power and achieved positive acceleration while climbing at Mach 7 for approximately 10 seconds. One of the most recent hydrogen fueled experiments is Australia’s very own scramjet HyShot. HyShot is a research experiment of the University of Queensland (UQ) designed to develop a correlation between measurements made of supersonic combustion in their T4 shock tunnel and results gather from their observed flight. To this day, there have been four HyShot launches. In each launch the fuel used was gaseous hydrogen. The combustion process in the scramjet engine is dependent on the ambient pressure. This lead to a highly parabolic trajectory with a near vertical decent and ensured a correlation could be developed over an envelope of ambient pressures.  Apart from the scientific merits, a vertical trajectory is also more cost efficient as there are less structural difficulties resulting from the lower heat and dynamic loads.   Seen above are the four HyShot launches taking place at Woomera, South Australia on 30 Oct 2001, 30 Jul 2002, 25 Mar 2005 and 30 Mar 2006. The approach taken by the University is new and the results have sparked a new era in the flight testing of hypersonic air-breathing engines. After intense discussion focusing on hydrogen based fuels, it is clear that hydrogen will be the preferred fuel for future projects and developments using scramjet technology. It might actually be possible that in our lifetime, we will be able to travel in aircraft at speeds once only dreamt about, giving us the ability to fly half way round the world in only a few hours. Bibliography David Bartecchi, ‘What is Hydrogen’ http://www.hydrogennow.org/Facts/whatishydrogen.htm, viewed 02 Jun 07 John D. Anderson Jr, Hypersonic and High-temperature Gas Dynamics 2nd Edition, Virginia, 2006, p. 457. Len Krenzler, ‘The Ramjet/Scramjet Engine’ http://www.aviation-history.com/engines/ramjet.htm, viewed 02 June 07 Matt Walker, ‘Hydrogen and the Scramjet’ http://www.aerospaceweb.org/question/propulsion/q0170.shtml, viewed 31 May 07 Pike, J, The choice of propellants: a similarity analysis of scramjet second stages, APECS Ltd, Unit Kingdom 1999 Stephen C Bates, ‘Solid Hydrogen Fueling of an Air Breathing Supersonic Combustor’ Thoughtventions Unlimited LLC, Glastonbury University of Queensland, ‘HyShot’ http://www.uq.edu.au/hypersonics/?page=19501, viewed 03 Jun 07 University of Surrey, ‘The World’s Fastest Air-Breathing Engine Will Fly in Australia in 2005’ http://www.azom.com/details.asp?newsID=1406, viewed 02 Jun 07 Yvette Smith and Brian Dunbar, ‘NASA’s X-34A hypersonic scramjet’ http://www.nasa.gov/missions/research/x43-main.html, viewed 02 Jun 07 Figure � SEQ Figure \* ARABIC �3� Left - Artist impression of X-34A. Right X-34A mounted to NASA's B52B Figure � SEQ Figure \* ARABIC �5� - External fuel tanks seen on Discovery Figure � SEQ Figure \* ARABIC �6� - Graphical representation of a hydrogen atom Figure 8 – Inert gases table Figure � SEQ Figure \* ARABIC �8� - Qantas Commercial A380 Figure � SEQ Figure \* ARABIC �9� - HyShot scramjet experiment developed by University of Queensland Figure 10 - HyShot Launch 1, 2, 3 and 4. � Len Krenzler, ‘The Ramjet/Scramjet Engine’ � HYPERLINK "http://www.aviation-history.com/engines/ramjet.htm" ��http://www.aviation-history.com/engines/ramjet.htm� � John D. Anderson Jr, ‘Hypersonic and High-temperature Gas Dynamics 2nd Ed’ (Virginia, 2006), p. 457. �Yvette Smith and Brian Dunbar, ‘NASA’s X-34A hypersonic scramjet’ � HYPERLINK "http://www.nasa.gov/missions/research/x43-main.html" ��http://www.nasa.gov/missions/research/x43-main.html� � University of Surrey, ‘The World’s Fastest Air-Breathing Engine Will Fly in Australia in 2005’ � HYPERLINK "http://www.azom.com/details.asp?newsID=1406" ��http://www.azom.com/details.asp?newsID=1406� � Pike, J, The choice of propellants: a similarity analysis of scramjet second stages, APECS Ltd, Unit Kingdom 1999 p. 2357 �David Bartecchi, ‘What is Hydrogen’ http://www.hydrogennow.org/Facts/whatishydrogen.htm � Pike, J, The choice of propellants: a similarity analysis of scramjet second stages, APECS Ltd, Unit Kingdom 1999 p. 2358 � Matt Walker, ‘Hydrogen and the Scramjet’ � HYPERLINK "http://www.aerospaceweb.org/question/propulsion/q0170.shtml" ��http://www.aerospaceweb.org/question/propulsion/q0170.shtml� � Stephen C Bates, ‘Solid Hydrogen Fueling of an Air Breathing Supersonic Combustor’ Thoughtventions Unlimited LLC, Glastonbury � Pike, J, The choice of propellants: a similarity analysis of scramjet second stages, APECS Ltd, Unit Kingdom 1999 p. 2358 � ibid. � University of Queensland, ‘HyShot’ � HYPERLINK "http://www.uq.edu.au/hypersonics/?page=19501" ��http://www.uq.edu.au/hypersonics/?page=19501� � ibid. 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