(Excerpt, in general, taken from the NASA Test Report
of the Micro-Combustion Fluid Engine)
Producing Energy Through "Bubble Cavitation"
Bubble Combustion Principle
Comparative Analysis
The basic concept behind the Micro-Combustion-Chamber Heat (MCCH) engine is a “bubble” combustion rather than a “droplet” combustion. In a traditional gas turbine combustion engine, liquid fuel is injected as droplets into high-pressure air from a compressor stage, the fuel droplets evaporate and mix with the air and the precisely controlled fuel/air mixture is ignited to release the thermal energy stored in the fuel. The working fluid, in this case, hot combustion products, is then expanded through a turbine stage to extract the thermal energy as mechanical energy. The mechanical energy (or power) is then used to drive the compressor stage (in a self-sustaining engine) and used as net-torque output from the engine.
The MCCH engine, on the other hand, can be thought of as the inverse of the traditional gas turbine combustion engine in that bubbles or vapor pockets are injected into a liquid, which acts as both the working fluid and the fuel. The liquid fuel-vapor interface serves to transfer fuel vapor into the interior of the air bubble, thereby providing a combustible mixture that is then compressed by a bubble implosion. The implosion is induced by a set of “combustor” vanes, which compress the bubbles in the working fluid to a very high pressure. This compression heats the mixture inside the bubble to a high temperature, thereby, igniting the fuel/air mixture inside the bubble. The thermal energy released by the combusting bubble releases pressure waves which then drive a set of turbine blades. The idea of combusting fuel vapor inside bubbles has been previously studied by Likhachev et al., Kitano and Shibata, and Carbonell and Guirardello. However, these studies used bubble combustion in the context of a flow-reactor rather than as a heat producing energy source for a turbine engine.
In contrast, the MCCH engine not only produces it’s own unique fuel/air bubble, but produces this bubble on a continuous stream basis. This allows the special designed turbo-machinery to ignite and burn a series of bubbles on a continuous cycle, which then can be translated to a shaft in the form of torque.
The mode of combustion of the MCCH engine is also fundamentally different from a gas turbine engine. A gas turbine engine relies on turbulence induced fluid re-circulation to sustain a continuous combustion process by feeding hot combustion products into the unburned reactants. The fact that the MCCH does not rely on turbulence to sustain combustion has an additional advantage: the design can be scaled-down to very small sizes without sacrificing the ability to sustain combustion. As combustors are reduced in size, the flow becomes increasingly laminar, consequently, turbulence is diminished and sustaining a continuous burn in a conventional gas-phase combustor becomes difficult. Furthermore, the efficiency of gas-phase turbo-machinery drops dramatically as the size is scaled-down. Since the MCCH engine impellers are used in the liquid phase, their efficiency does not suffer so drastically as they are scaled down in size.
The potential advantageous characteristics of the MCCH bubble combustion engine derives from the fact that the combustion is initiated by very high temperature phenomenon. Recent simulation of bubble sonoluminescence, in which an acoustic wave is used to initiate a cavitation bubble which implodes on itself, predict temperatures as high as 10,000K. The high ignition temperatures inherent with the MCCH engine reduces the requirement for expensive, highly refined fuels such as gasoline or kerosene used in traditional combustion engines. High ignition temperatures also permits the burning of ultra fuel-lean mixtures to reduce the final combustion temperature, thereby reducing NOX emissions from the thermal NO mechanism.
Another advantage of the MCCH engine is the fact that the high temperature combustion occurs in localized volume that is surrounded by the working fluid, which thermally isolates the high temperature of combustion from the engine’s mechanical components. This means that the MCCH engine can be mass-produced using low-cost conventional materials and techniques. Gas turbine engines are not in widespread consumer use since they are expensive. Their high cost results partly, from the requirement of expensive high temperature alloys and complicated active cooling designed. Additionally, the bubble combustion process may be cleaner burning (depending on the fuel used) resulting in reduced unburned hydrocarbon (HC) emissions. Also, several emission discharge tests conducted to date has produced no noticeable Carbon Dioxide, (CO2) being emitted. Fluid analysis by NASA Stennis of different processed samples indicates the carbon is being left behind.
Reference:
NASA REVIEW REPORT
Of the Micro-Combustion Heat Engine
Quang-Viet Nguyen and Tibor Kremic
John H. Glenn Research Center
National Aeronautics and Space Administration
Lewis Field
Cleveland, Ohio 44135
May 14, 1999
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