thermo

# thermo - Review of Relevant Thermodynamics Students have...

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1 Review of Relevant Thermodynamics Students have all had a course – but contents and performance vary significantly Review can be one lecture or ten!!! Thermodynamics is the study of energy Our interest is engineering thermodynamics Chemical Thermodynamics is important in: fuel cells, combustion analysis beyond our interest here

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2 the Total Stored Energy Molecular configuration (Internal) Motion (Kinetic) Position in a field (Potential) Gravity field Electrical potential Magnetic potential –S u r f a c e t r a i n
3 Simple Compressible Substances Types of substances relate how many independent thermodynamic variables there can be – Relates to transfer of energy – For simple compressible substance: energy transfer modes = work and heat – Results in two independent thermodynamics variables Air, steam are simple compressible substances Equations of state have two independent variables

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4 PdV x d A Pd x d F w = = = r r r r δ Mechanical Work – Mechanical work is the result of a force acting through a distance. – Force is a vector, Work has a sign, but is scalar – Energy of the control mass is increased when volume is decreased (thus the sign) Like compressing a spring
5 Heat Heat is energy transfer due to a temperature difference between the system and the surroundings. Sum of a variety of microscopic work modes

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6 1st Law for control mass Change of energy within a system is equal to transfers of energy across a system Sign conventions vary - I always draw figure showing positive energy transfers PdV Q dW dQ dE = + = δ
7 1st Law for control mass Extensive variables (capital letters) are dependent on mass Solve control mass problems with simple integrations For constant pressure process between states 1 and 2: ) ( 1 2 2 , 1 2 , 1 2 , 1 1 2 V V P Q W Q E E + = + =

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8 1st Law for control mass Or we can make it a rate equation If we are solving a piston problem with constant cross sectional area: dt dV P Q W Q dt dE + = + = & & & dt dx PA Q W Q dt dE + = + = & & &
9 1st Law for control VOLUME Not mass attached Most common way to look at thermodynamic systems Considers a system with – Hard walls No work going through (dx = 0) Heat transfer can occur – Internal energy sources Shaft work Internal heat generation (combustion) – Flow crossing CV boundary “advective energy changes” Flow work

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10 Control Volume First Law Flow work results from pushing fluid into a system or having it push its way out
11 1st Law for CV (cont.) A V m v where Pv m V PA dt dx PA dt FlowWork d r & & r ρ = = = = = and 1 ) (

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12 Control Volume First Law dE/dt = (a) rate of external heat transfer + (b) rate of internal energy gain + (c) advective energy + (d) rate of flow work out in out in shaft Pv m Pv m d e m e m c W b Q a ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( & & & & & & = = = =
1st Law for CV (cont.) Or, dividing through by mass flow rate yields an equation which is scaled per unit mass (and introduces enthalpy h = u+Pv) 13 out in shaft pe ke h pe ke h w q dt de ) ( ) ( + +

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## This note was uploaded on 09/13/2011 for the course MECH 462 taught by Professor Mul during the Spring '11 term at Rutgers.

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thermo - Review of Relevant Thermodynamics Students have...

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