احتراق

اللهب ينتج من احتراق (اشتعال) وقود

الاحتراق (Combustion) هو تفاعل كيميائي بين مادتين ينتج عنه حرارة و انبعاثات و يصحبه لهب و غالبا ما يكون أحد المادتين هو الأكسجين.

وتحدث عملية الاحتراق عاده برفع درجة حرارة مادة إلى درجة الاشتعل في وجود كمية وافرة من الأكسجين أو الهواء فتحترق المادة احتراق تام ، وتنطلق كمية من الطاقة الحرارية تعتمد عل كمية المادة المحترقة وتسمة حرارة الاحتراق.

ويمكن تعريفها بأنها : كمية الحرارة المنطلقة عند احتراق مول واحد من المادة في وجود كمية وافرة من الاوكسجين او الهواء الجوي عند الظروف القياسية.

CH 4 + 2O 2CO 2 + 2H 2O
CH 2S + 6F 2CF 4 + 2HF + SF 6

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المعادلة الكيميائية

Generally, the chemical equation for stoichiometric burning of hydrocarbon in oxygen is as follows:

For example, the burning of propane is:

The simple word equation for the combustion of a hydrocarbon in oxygen is:

If the combustion takes place using air as the oxygen source, the nitrogen can be added to the equation, although it does not react, to show the composition of the flue gas:

For example, the burning of propane is:

The simple word equation for the combustion of a hydrocarbon in air is:

Nitrogen may also oxidize when there is an excess of oxygen. The reaction is thermodynamically favored only at high temperatures. Diesel engines are run with an excess of oxygen to combust small particles that tend to form with only a stoichiometric amount of oxygen, necessarily producing nitrogen oxide emissions. Both the United States and European Union are planning to impose limits to nitrogen oxide emissions, which necessitate the use of a special catalytic converter or treatment of the exhaust with urea.


الوقود

الوقود السائل

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

الوقود الصلب

The act of combustion consists of three relatively distinct but overlapping phases:

  • Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
  • Distillation phase or gaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. Flames are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
  • Charcoal phase or solid phase, when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore but just glows and later only smoulders.

آلية التفاعل

الاحتراق في الاكسجين هو radical chain reaction حيث يشارك العديد من distinct radical intermediates .


اللاتوازنات=

لا توازنات الاحتراق هم نمطياً تأرجحات الضغط العنيفة في غرفة الاحتراق. تأرجحات الضغط تلك يمكن أن ترتفع إلى 180dB, والتعرض لفترة طويلة لهذا الضغط الدوري والأحمال الحرارية يخفض عمر مكونات المحرك. ففي الصواريخ, مثل F1 المستخدم في برنامج ساترن V, أدت اللاتوازنات إلى ضرر بالغ في غرفة الاحتراق والمكونات المحيطة. This problem was solved by re-designing the fuel injector. In liquid jet engines the droplet size and distribution can be used to attenuate the instabilities. Combustion instabilities are a major concern in ground-based gas turbine engines because of NOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the NOx emissions; however, running the combustion lean makes it very susceptible to combustion instabilities.

The Rayleigh Criterion is the basis for analysis of thermoacoustic combustion instabilities and is evaluated using the Rayleigh Index over one cycle of instability.[بحاجة لمصدر]

When the heat release oscillations are in phase with the pressure oscillations the Rayleigh Index is positive and the magnitude of the thermo acoustic instability increases. Consecutively if the Rayleigh Index is negative then thermoacoustic damping occurs. The Rayleigh Criterion implies that a thermoacoustic instability can be optimally controlled by having heat release oscillations 180 degrees out of phase with pressure oscillations at the same frequency. This minimizes the Rayleigh Index.[بحاجة لمصدر]


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