(Expanded Abstract to the NASA Test Report
Of The Micro-Combustion Fluid Engine)
A Basic Explanation of Cavitation Ignition Bubble Combustion
Quang-Viet Nguyen
NASA Glenn Research Center
Cleveland OH 44143
Cavitation-ignition bubble combustion (CIBC), or ‘bubble combustion’ for short is a process that occurs when a combustible gaseous mixture is ignited by the high temperatures found inside a rapidly collapsing bubble.
A simple analogy of bubble combustion is the common diesel-cycle engine (although it is more closely related to the HCCI engine – see below). In diesel engines, air is heated by the rapid compression provided by the piston as it forces the air into a smaller volume. Similarly, in bubble combustion, the collapsing bubble wall heats the gases within the bubble interior much like a diesel engine piston heats the air inside the cylinder during the compression stroke.
However, the length scales in bubble combustion are very small, ranging from micrometers to nanometers, and the time scales are very brief, ranging from microseconds (10-6 seconds) to femtoseconds (10-15 seconds). Furthermore, the walls of the bubble are provided by the liquid-gas interface, and the shape of the volume is spherical rather than cylindrical.
At the point of maximum piston compression in a diesel engine, the gases reach their peak temperature and fuel is injected which then ignites and burns to release heat. The heat (or energy) in a diesel engine is then extracted from the expansion of the gases contained within the piston-cylinder volume.
In bubble combustion, the situation is quite different in that the gases undergoing compression already contain both the oxidizer (air) and fuel vapor (evaporation from the bubble wall) in a mixture which ignites and then expands to augment the natural expansion the bubble after it has reached its point of minimum radius.
In this way, bubble combustion is closer to an emerging new engine technology called homogeneous charge compression ignition (HCCI) where the contents of cylinder-piston are premixed fuel and air and are ignited by the piston-cylinder compression.
When the bubble undergoes cavitation-ignition and combustion, the extra heat generated by the conversion of the fuel vapor, through its heat of combustion, causes the bubble to expand to a radius that is larger than its original radius.
It is this difference in radius, and hence, bubble volume expansion that can be used to extract the energy release from bubble combustion.
References:
Q.V. Nguyen and D. A. Jacqmin, “A Study of Cavitation Ignition Bubble Combustion,” NASA TM-2005-213599 (2005).
11/15/2005 v2
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