Steady flow analysis of reacting mixtures

S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-Mail : gunabalans@yahoo.com

Part - 2

Steady flow analysis of reacting mixtures

Analysis for a steady flow or ‘constant pressure’ combustion process

Adiabatic combustion or Flame temperature

A combustion process without heat loss or gain is adiabatic. For a combustion process that takes place adiabatically with no shaft work, the temperature of the products is referred to as the adiabatic flame temperature. This is the maximum temperature that can be achieved for given reactants. Heat transfer, incomplete combustion, and dissociation all result in lower temperature. The maximum adiabatic flame temperature for a given fuel and oxidizer combination occurs with a stoichiometric mixture The amount of excess air can be tailored as part of the design to control the adiabatic flame temperature.

Condition for Adiabatic combustion temperature

• reactants enter the combustion process at 25oC (77oF) and 1 atm pressure

• products leaves the process at 1 atm pressure • combustion is stoichiometric without any

excess air Excess air will reduce the adiabatic flame temperature and is often introduced to avoid flame temperatures exceeding limits sets by the materials in the combustion system. http://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html

The conservation of mass and conservation of energy principles is used for the calculation

the energy rate balance on a per mole of fuel basis,

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Second Law Analysis of Reacting Systems Second law for the open system

Taking the positive direction of heat transfer to the system, the entropy balance relation can be expressed for a steady-flow combustion chamber as

QT

S S S S kJ kk

kgen CV React Prod ( / )

Derive an expression for availability in steady state steady flow process (SSSF) involving chemical reactions ( Apr-2013)

http://ocw.kfupm.edu.sa/ocw_courses/user062/ME204001/Lecture%20Notes/Chapter15.ppt.

11

Second Law Analysis of Reacting Systems Second law for the open system

QT

S S S S kJ kk

kgen CV React Prod ( / )

For an adiabatic, steady-flow process, the entropy balance relation reduces to

S S Sgen adiabatic, Prod React 0

The third law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero temperature is zero. The third law provides a common base for the entropy of all substances, and the entropy values relative to this base are called the absolute entropy.

Reference • Moran, M. J. 2011. Fundamentals of engineering thermodynamics. Wiley,

[Hoboken, N.J.?]. • Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett

Publishers, Sudbury, Mass. • Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New

Delhi. • http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node1

11.html • http://www.engineeringtoolbox.com/adiabatic-flame-temperature-

d_996.html • http://ocw.kfupm.edu.sa/ocw_courses/user062/ME204001/Lecture%20N

otes/Chapter15.ppt.