ME200 - Lecture 291

ME200 - Lecture 291 - 11/13/2010 EntropyBalance OpenSystem...

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11/13/2010 1 Entropy Balance–Open System ME 200 Thermodynamics I Purdue University , Dr. Tim Pourpoint – timothee@purdue.edu Lecture 29 November 5 th , 2010 Last Lecture: entropy balance for closed system Outline j system gen j j Q S T     system j gen j j dS Q dt T  In closed systems: only 2 mechanisms for entropy change: 1. heat exchange at all relevant boundaries 2. entropy generation due to irreversibilities within the given system Also, no entropy transfer associated with work interactions Today: – Finish Example 28.1 ME 200 2 Study entropy balance for open systems –S o l v e examples with open systems Next week: –L e c t u r e s on Monday and Wednesday will be taught by Daniel Richardson as I will be in Aberdeen, MD for a project review.
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11/13/2010 2 •A n electrical water heater having a 200 liter capacity employs an electric resistor to heat water from 23 to 55ºC. The outer surface of the resistor remains at average temperature of 80ºC. •H e a t transfer from the outside of the water heater is negligible and Example 28.1 the states of resistor and the tank holding the water do not change significantly. • Modeling the water as incompressible, determine the amount of entropy produced, in kJ/K, for: (a) the water as the system (Answer: 9.95 kJ/K). ME 200 3 (b) the overall water heater including the resistor as the system (Answer: 85.13 kJ/K). Solutions for Example 28.1 ME 200 4
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11/13/2010 3 Solutions for Example 28.1 ME 200 5 Entropy Balance–Open System ME 200 6
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11/13/2010 4 •Fo r open systems, entropy change occurs due to three mechanisms: – entropy transfer due to heat transfer at the boundary entropy transfer due to mass flow in and out of the system Entropy Balance–Open System – entropy transfer due to mass flow in and out of the system – entropy generation due to irreversibilities within the system • Entropy balance for open system: ME 200 7 • Entropy balance for open system on rate basis: Entropy Balance–Open System (contd.) Rate of entropy gain due to Rate of entropy loss due to outgoing mass flow incoming mass flow Rate of entropy change of control volume j CV ii ee g e n j inlets exits j Q dS ms dt T       ME 200 8 Rate of entropy transfer due to heat exchange at all “j” boundaries Rate of entropy
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ME200 - Lecture 291 - 11/13/2010 EntropyBalance OpenSystem...

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