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  3. Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brillian

Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brillian

admin2010-04-30  51
问题    Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brilliant flash of an explosion.
    A typical combustion process is the burning of gasoline in an automobile engine. The vaporized fuel is mixed with air, compressed in the engine’s cylinder, and ignited by a spark. As the fuel flames up, the heat produced flows into the adjacent layer of unburned fuel and ignites it. In this way a zone of first spreads throughout the fuel mixture until the combustion wave.
    The speed at which such a combustion wave travels through a fuel mixture is called the burning velocity of the mixture. The burning velocity of a gas such as methane quietly burning in air is only about one foot per second.  By comparison, the burning velocity of more reactive combinations such as the rocket fuels, hydrogen and fluorine, can be hundreds of feet per second.
    If the fuel flows at the same speed as the combustion wave, the result is a stationary flame, like the one in your kitchen gas burner. In the kitchen burner a jet of gas mixed with airflows from the openings in the head of the burner. If the velocity of the fuel mixture flowing from the opening is greater than its burning velocity, the flame blows out.
    In jet engines speeding through the air at 500 to 600 miles per hour, the engine’s flame is sometimes blown out by the blast of air entering the combustion chamber at high speeds. Jet pilots call this condition "flameout."
    Combustion can sometimes occur very slowly. A familiar example of slow combustion is the drying of ordinary oil based paint. In this chemical reaction, called oxidation, the oxygen in the air reacts with the drying oil in the paint to provide a tough film. The linseed oil molecules link together, forming an insoluble coating. Another example is the hardening, and cracking of rubber with age. One way to avoid this is to incorporate certain chemicals called inhibitors into the compound.
    How can the chemical reaction involved in such a quiet process as the drying of paint also produce spectacular flames and explosions? The main difference between the two is the temperature at which they occur.
    At lower temperatures the reaction must take place over a long time. The heat, which is slowly produced, is dissipated to the surroundings and does not speed up the reaction. When the heat produced ’by the low temperature reaction is retained instead of being dissipated, the system breaks into flame. This is the process that accounts for a major fire hazard, spontaneous combustion, as when oily rags suddenly burst into flame.
    In a flame or explosion, the reactions are extremely fast. In many chemical processes, however, such a rapid oxidation process would be extremely destructive.
A steady flame in a gas range is the result of ______.
选项 A、a burning velocity equal to the combustion wave
B、fuel being supplied at the same rate as the combustion wave
C、fuel being supplied at a higher rate than that of burning velocity
D、a free flowing supply of air from the openings in the head of the burner
答案B
解析 细节定位题。A：燃烧的速度和燃烧传播的速度一致，B：添加燃料的速率与燃烧传播的速度相差无几；C：添加燃料的速率比燃烧速度要快得多；D；在给燃烧物升温的过程中，空气可以自由流人。由本文第4段第1句得知：If the fuel flows at the same speed as the combustion wave, the result is a stationary flame, like the one in your kitchen gas burner. 意思是：如果燃料流动的速度与燃烧传播速度一样，结果会是，燃烧火焰很稳定，厨房里煤气灶的火焰就是一个典型的例子。所以， B正确。
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