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Chapter 13 Practice Problems

Chapter 13 Practice Problems - ENGRD 2210 Thermodynamics...

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ENGRD 2210 Thermodynamics Cornell University Fall 2009 Chapter 13 Practice Problems for the Final Exam (13.9, 13.13, 13.55, 13.61, 13.70) Problem 13.9 Problem Statement: A fuel mixture with the molar analysis 40% CH 3 OH, 50% C 2 H 5 OH, and 10% N 2 burns completely with 33% excess air. Determine: a. the balanced reaction equation b. the air-fuel ratio, both on a molar and mass basis Engineering Assumptions: 1. The textbook assumption is that air is comprised of 21% O 2 and 79% N 2 where N 2 includes nitrogen gas along with all the other components of air, resulting in 3.76 moles of nitrogen accompanying every mole of oxygen. 2. Nitrogen is inert, meaning it does not react and remains at the same temperature on both sides of the equation. Solution: Assume 1 mole of fuel for simplicity, and based upon the given molar ratios of the fuel components, the reaction equation for complete combustion with the theoretical amount of air is: where Greek notation represents the unresolved coefficients of air (alpha), carbon dioxide (beta), and water (gamma). Note how the inert nitrogen gas terms and coefficients have simply transferred to the right side of the equation. Now we can solve for the coefficients:
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Now that we have our complete combustion with the theoretical amount of air , we want to determine the complete combustion with the problem prescribed 33% excess air condition. We represent this with an additional coefficient of 1.33 in front of the air term. Note that Greek notation has been replaced with the above values and also how the added coefficients translate to either side of the equation. Also
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