1 INTRODUCTION TO NAVAL ENGINEERING THERMODYNAMICS II 2 INTRODUCTION TO NAVAL ENGINEERING FIRST AND SECOND LAWS OF THERMODYNAMICS 3 FIRST LAW OF THERMODYNAMICS Energy can be neither created nor destroyed but only transformed 4 THE GENERAL ENERGY EQUATION
Energy In Energy Out
U2 - U1 Q - W
U1 internal energy of the system at the beginning
U2 internal energy of the system at the end
Q net heat flow into the system
W net work done by the system
5 ELEMENTS OF A THERMODYNAMIC CYCLE 6 ELEMENTS OF A THERMODYNAMIC CYCLE 7 (No Transcript) 8 THERMODYNAMIC CYCLES
Working fluid never leaves the cycle except through accidental leakage (ex steam cycle)
Working fluid is taken in used then discarded (ex internal combustion engine)
heat is added to the working substance in the engine itself (ex internal combustion engine)
the working substance receives heat in some device that is separate from the engine (ex steam turbines)
10 CLOSED UNHEATED 11 OPEN HEATED 12 FLOW PROCESSES
One in which the working fluid does not flow into or out of its container in the course of the process (ex air compressors internal combustion engines)
One in which a working substance flows steadily and uniformly (ex boilers turbines condensers)
13 USING THE G.E.E. (Non-Flow)
Q12 (U2 - U1) wk12/J
Q12 total heat transferred (Btu)
U2 U1 total internal energy at points 1 and 2 (Btu)
wk12 work done between states 1 and 2 (ft-lbs)
J constant of 778 ft-lbs/Btu
14 (No Transcript) 15 G.E.E. (Non-Flow) EXAMPLE
5 LBM of a fluid is compressed in the cylinder using 350 Btus of work. If internal energy initially was 100 Btu/lbm and 150 Btu/lbm at the end of the compression how much heat was added/lost
Q12 (U2 - U1) wk12/J
16 STEADY FLOW SYSTEMS AND THE GENERAL ENERGY EQUATION 17 ENTHALPY (H or h)
INTERNAL ENERGY (U)
Mechanical energy necessary to maintain the steady flow of the fluid
Flow work pV/J (Btu)
H pV/J U
18 STATE CHANGES
the pressure of and on the working fluid is constant
the enthalpy of the working fluid does not change (h1 h2)
temperature is constant
occurs in such a way that there is no transfer of heat to or from the system during the process
19 THE SECOND LAW OF THERMODYNAMICS
(1) All energy received as heat by a heat engine cycle cannot be converted into work (This means that no cycle can have a thermal efficiency of 100)
20 THE SECOND LAW OF THERMODYNAMICS (2) The transformation of heat to work is dependent on a temperature difference and on the flow of heat from a high temperature reservoir to a low temperature reservoir. (In other word heat must flow from hot to cold) 21 THE SECOND LAW OF THERMODYNAMICS (3) It is impossible to construct an engine that operating in a cycle will produce no effect other than the transfer of heat from a low temperature reservoir to a high temperature reservoir 22 THE SECOND LAW OF THERMODYNAMICS FRICTION HAPPENS 23 ENTROPY
A THEORETICAL MEASURE OF ENERGY THAT CANNOT BE TRANSFORMED INTO MECHANICAL WORK IN A THERMODYNAMIC SYSTEM.
The total amount of entropy in a system always goes up.
No thermodynamic process can occur without some losses.
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