Radiation Heat Transfer

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View Factor Orientation (or View factor or shape factor) plays an important role in radiation heat transfer. View factor is defined as, "fraction of radiation leaving surface 'i' and strike 'j' ". Summation Rule (View Factor) If there is are similar surfaces 'i' and 'j' , then: Blackbody Radiation Exchange Radiation Exchange between Opaque, Diffuse, Gray surfaces in an Enclosure 1. Opaque 2. Surfaces 3. Two surface enclosure Radiation Shield It is used to protect surfaces from radiation act like a reflective surface. References: Material from Class Lectures + Book named Fundamentals of Heat and Mass Transfer by Theodore L. Bergman + My knowledge.  Photoshoped pics  are developed.  Some pics and GIF from Google.   Videos from YouTube ( Engineering Sights ).
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Combustion

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Combustion

It is defined as, "in which fuel is oxidized and a large quantity of energy is released".
    • Fuel  ↣  "Any material that can be burned to release thermal energy".
    • Mostly hydrocarbons.

Proportion of Air:

Air contains 21 % Oxygen, 79 % Nitrogen by volume. The molar ratio of N2/O2 = 79/21 = 3.76.
So, 
Combustion Process
Reactants + O2   ↣   combustion   ↣   Products
  • Simplest combustion process  ↣  known as Stoichiometric Combustion (all carbon from CO2, hydrogen from H2O).

Equivalence Ratio or Excess Air Ratio


It is defined as, "ratio of actual fuel-air ratio to the theoretical fuel-air ratio for complete combustion".
  • Theoretical air (or Stoichiometric Air)  ↣  refers to the minimum amount of air that is required for providing sufficient oxygen for complete combustion of fuel (C, H2, S).
    • If equivalence ratio < 1  ↣  Lean Mixture.
    • If equivalence ratio > 1  ↣  Rich Mixture.
  • Excess Air  ↣  any air supplied in excess of theoretical air (generally 25 to 100 %) to ensure better and complete combustion.
50 % Excess Air  =  150 % Theoretical Air

Wet and Dry Analysis of Combustion

Wet analysis is defined as, "combustion analysis when carried out considering water vapor". Dry analysis is defined as, "combustion analysis made on the assumption that vapor is removed after considering it".
  • Volumetric Analysis  ↣  based on percentage by volume of reactants and products.
  • Gravimetric Analysis  ↣  based on the percentage by mass of reactants and products.
  • Enthalpy of Reaction hR  ↣  difference between the enthalpy of products at a specified state and the enthalpy of reactants at the same state.
  • Enthalpy of Combustion hC  ↣  represents the amount of heat released during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure.
  • Heating Value  ↣  the amount of heat released when a fuel is burned completely in steady-flow process and products are returned to the state of reactants.
Heating Value = Absolute value of Enthalpy of Combustion of Fuel
  • Standard Condition  ↣  25 ℃, 1 atmospheric pressure.

Analysis of the Products of Combustion

Combustion occurs at elevated temperature and we assume that all the products of combustion (including the Water vapor) behave as ideal gas.

Quest: Ethane (C2H6) is burned with atmospheric air, the volumetric analysis of the dry products of combustion yields the following 10% CO2, 1% CO, 3 % O2 and 86 % N2. Develop the combustion equation, the percentage of excess air and the air fuel ratio.

References:

  • Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
  • Photoshoped pics are developed. 
  • Some pics and GIF from Google.  
  • Videos from YouTube (Engineering Sights).

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