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Is this an error in the text? (Gas Turbine Combustion Chamber)

  1. Jan 6, 2016 #1
    Hello

    I am teaching physical science at an online school, and came across this issue in the textbook - I'm looking to double check with more knowledgeable people before I bring it up and potentially make a fool of myself.

    In one chapter of the physical science text it discusses various engines. It includes this text about gas turbines:

    THE GAS TURBINE

    Another type of internal-combustion engine is called a gas turbine. The main parts of this engine are represented in Figure 26. Air is fed through the inlet pipe into a set of rotating blades in the compressor. The compressed air passes through the pipe to the oil-burning chamber. Fuel oil from the pump is furnished to the oil-burning chamber, where it’s mixed with some of the compressed air from the pipe. This mixture is then ignited and burned in the chamber. The rest of the compressed air is fed through an air passage to the combustion chamber. In the combustion chamber, the air mixes with the gaseous products of combustion and cools these gases to a suitable temperature for use in the turbine wheel.

    The turbine wheel consists of a large number of blades mounted on a shaft. As shown, the turbine blades and the compressor blades are mounted on the same shaft. The gases, which are under high pressure in the combustion chamber, are allowed to expand and are forced to strike the blades of the turbine wheel at a high velocity. The impact of the gases against the blades causes the shaft to rotate. The rotation of the shaft drives a machine such as the generator, which is used for generating electricity. An electric motor must be provided to start the turbine.

    Gas turbines are commonly used in aircraft. The most suitable fuels for gas turbines are aviation-grade gasoline and a proper blend of gasoline, kerosene, and fuel oil.

    with this diagram:
    https://drive.google.com/file/d/0Bzpz5UtYxWMbUUNzdWo4NS1MQ1MyM2lKOWM5SWhwYXEyZXkw/view?usp=sharing

    The text and diagram and even a test question states that the combustion chamber is not for burning anything but is separate from the oil burner and is used for cooling combustion products to a suitable temperature. Is this correct? Diagrams from other sources show the combustion actually happening in the combustion chamber. Is my textbook wrong or just unclear?

    Thank you for your help!
     
  2. jcsd
  3. Jan 6, 2016 #2

    JBA

    User Avatar

    The statement regarding the combustion chamber is somewhat correct but definitely confusing and poorly presented.
    In reality, the engine's combustion chamber consists of both an inner fuel burning core and a surrounding annular outer cooling air flow section that is separated from the combustion core chamber by a perforated sleeve.
    During turbine operation a portion of the compressed air entering the combustion chamber directly enters the core burner section where it is mixed with the fuel and burned. Meanwhile, the remaining compressed air flows into the annular outer chamber surrounding the inner burner core of the chamber where, by way of the perforations in the wall separating the inner and outer regions of the combustion chamber, it then flows into the core chamber where it blends with and cools the combustion air; and then, the combined air flows out the back of the core combustion chamber onward to drive the turbine section of the engine.
    I hope this clarifies what actually takes place in the combustion chamber segment of the turbine engine.
    P.S. I realize this then a somewhat run-on description of what occurs but it is the best way I can describe what actually happens.
     
  4. Jan 16, 2016 #3
    Thank you very much!!

    Your description was very clear and helpful. I am not confident in editing or removing the test question on the basis of the text being unclear, and requesting a change to the text.
    Much appreciated!
     
  5. Jan 17, 2016 #4
    A somewhat simpler version of a gas turbine is a turbofan engine as used on "jet" airplanes. The extra compressed air that is not used to burn the fuel contributes to the output power, thereby enhancing fuel efficiency. There is a decent explanation and drawings on wikipedia. Compare their drawings with the one you referenced.

    https://en.wikipedia.org/wiki/Bypass_ratio
     
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