MassConsvEqnsDiffusionLects11&12ME525SP2011

MassConsvEqnsDiffusionLects11&12ME525SP2011 - ME...

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School of Mechanical Engineering, Purdue University ME 525: Combustion Lecture 11 & 12: Conservation Equations for Reacting Flows, Mass Diffusion Prof. Robert P. Lucht Room 87, Mechanical Engineering Building School of Mechanical Engineering Purdue University West Lafayette, Indiana Email: Lucht@purdue.edu Phone: 765-494-5623 February 15 & 17, 2011 1/19
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School of Mechanical Engineering, Purdue University Lecture Topics Mass conservation equations, overall and species. Mass diffusion. Binary diffusion coefficients. Multicomponent diffusion. 2/19
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School of Mechanical Engineering, Purdue University Mass Conservation  0 V t   ± Most general form of overall mass conservation: Steady 1-D planar form (constant-area plug flow reactor): v constant x Most general form of species mass conservation equation:   1, 2, . ... i ii Y mm t for species i    ± 3/19
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School of Mechanical Engineering, Purdue University Species Mass Conservation vv ii i i i i mY m V m Y       We have already discussed the species chemical production term. The species mass flux term is given by: The bulk or average velocity of the fluid is V . The diffusional velocity and flux are given by: , , id i f f i i i diff i mm m Y V  4/19
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School of Mechanical Engineering, Purdue University Species Mass Conservation The diffusional flux and diffusional velocities are complicated functions. A non-zero diffusional flux can result from concentration gradients, temperature gradients, or pressure gradients. In combustion, the most important term is usually the flux due to concentration gradients, but when light species such as H or H 2 are present thermal diffusion can be very important also. 5/19
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School of Mechanical Engineering, Purdue University Mass Diffusion • Diffusion due to concentration gradients. Assume binary mixture of A and B:  ,, , 1 v/ v A diff B diff AB A BA B AB BA A diff A diff A A diff AB A A mm Y Y mY Y Y    
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MassConsvEqnsDiffusionLects11&12ME525SP2011 - ME...

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