Thermal and hydraulic characteristics of brazed plate heat exchangers Part I: Review of single phase and two phase adiabatic and flow boiling characteristics

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Thermochemistry of fuel air mixtures
Dr. Primal Fernando primal_at_eng.fsu.edu Ph
[phone]
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Combustion process

• Thermodynamic aspects of particular type process
  involving chemical reactions, is called
  combustion
• Usually occurs between fuel and an oxygen carrier
  (air)
• Energy stored in the bonds between constituent
  atoms of fuel and air (form of internal energy)
  and in the combustion process it will transformed
  to new molecules of lower energy level combustion
  products plus release heat (exothermic reaction)

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Combustion process

• Controls the engine power
• Efficiency
• Controls the emissions
• Different for SI and CI engines

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Flames

• A flame is a combustion reaction propagate
  subsonically through space motion of gas
  relative to unburn gas is important.
• The existence of flame motion implies that the
  reaction is confined to a zone which is small in
  thickness compared to the engine combustion
  chamber.
• The reaction zone is usually called the flame
  front
• Flames can be categorized as premixed and
  diffusion flame (mixed together at same place
  where the reaction takes place)
• Flames also categorized as laminar (mixing and
  transport done by molecular process) and
  turbulent (enhanced by eddies and lumps)
• Flames also categorized by whether the flow is
  steady or unsteady

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Fuel - fossil fuels mainly consists of H and C
By mass
By mass
By volume
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Chemical equation and conservation of mass
Consider a simple equation
Note all gasses occupy equal volume for kmol
when they are at same pressure and temperature
(exactly true for perfect gases, but for other
gasses substantially true). Volume occupied by
liquid and solids are negligibly small compared
to gasses.
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Chemical equation and conservation of mass
If insufficient O2 presents
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Necessary Oxygen is mainly obtained by mixing
fuel with are
Note Molar mass of N2 is 28 kg/kmol (28.16), and
that for air 29 kg/kmol (28.962)
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Example

• Determine the stoichiometric air/fuel ratio for a
  petrol approximating to hexane C6H14. Hence
  deduce the chemical equation if the petrol is
  burnt in 20 percent excess air, and the wet
  volumetric analysis of the products
• If all the water vapor is present
• If products are cooled to an atmospheric pressure
  and temperature of 1 bar and 15 C.
• Determine also the dry volumetric analysis.
• Estimate the chemical equation if only 80 of the
  air required for stoichiometric combustion is
  provided
• The partial pressure of saturated water vapor at
  15 C is 0.01704 bar

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Solution
Products
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Solution
Wet volumetric analysis of the Products including
N2
Amount-of-substance in the product
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Solution
Amount-of-substance in the product
The wet volumetric analysis
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Solution
If products are cooled to an atmospheric pressure
and temperature of 1 bar and 15 C.
Part of water will be condensed, if new amount of
water is y, total substance is 50.79 y
Volume fractionMole fractionpartial
pressure/total pressure
Above volumetric analysis repeats base on the
total volume of 51.67
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Solution
Determine also the dry volumetric analysis
Analysis is done by assuming no water present,
then the amount of substance becomes 50.79
Above volumetric analysis repeat base on total
volume of 50.79
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Solution
Chemical reaction with insufficient air (80)
When insufficient O2 is given there will be
unburned C and H2. H2, However, has a greater
affinity for O2. If mixture is not too rich in
fuel, it is reasonable to assume that all the H2
will be burnt. Some of C will be burnt to CO and
other to CO2.
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General combustion stoichiometry
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General combustion stoichiometry
Take, yb/a ratio of H2 to C
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Energy balance
Systems changes from reactants to products (since
mass constant, can apply first law for a close
system)