lectures-chapter1.pdf

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650:351 Thermodynamics

Chapter 1

Mahsa Mortazavi

Email: [email protected]

Oce hours: Tuesday, 2:00 - 3:30 pm

Summer 2015

650:351 Thermodynamics Mahsa Mortazavi 1

Overview of Course

Conservation of mass Conservation of momentum Conservation of energy Equation of state Property tables and charts

All problems in this course are solved using the above


Fundamental Units

All physical processes involve units The fundamental units in the SI system are

Dimension Unit

Length meter (m)

Mass kilogram (kg)

Time second (s)

Temperature kelvin (K)

Electric current ampere (A)

Amount of light candela (cd)

Amount of matter mole (mol)


Fundamental Units

The fundamental units in the English system are

Dimension Unit

Length foot (ft)

Mass slug (slug)

Time second (s)

Temperature rankine (R)

Electric current ampere (A)


Fundamental Units

The fundamental units can be converted between English and SI

1 foot = 0.3048 meters

1 slug = 14.5939029 kilograms

1 deg R = 0.55556 deg K


Fundamental Units

Derived unitsUnit English SI

Force pound force (lbf) Newton (Nt)

Energy British Thermal Unit (BTU) Joule (J)

Power Horsepower (HP) Watt (W)


Fundamental Units

Derived units can be converted between English and SI1 lbf = 4.44822162 Nt

1 BTU = 1055.05585 J

1 HP = 745.699872 W


Fundamental Units

What is pound mass (lbm) ?One lbm is the amount of mass that weighs one lbf on earth

Since weight is the force of gravity on an objectweight = 1 lbm gravity1 lbf = 1 lbm 32.17 ft/s21 slugft/s2 = 1 lbm 32.17 ft/s2

and therefore

1 lbm =1

32.17slug


Fundamental Units

The dierences between lbm and lbf and slug are important Failure to understand the dierences can lead to an error of nearlytwo orders of magnitude and thus . . .


Systems and Control Volumes

A system is a quantity of matter or region of space A closed system comprises a xed amount of mass An open system is a properly selected region of space

Closed system

Closed system

moving boundary

Open system


State and Equilibrium

Thermodynamics deals with equilibrium states An equilibrium state represents a condition wherein there are nounbalanced forces or potentials in the system

Thermal equilibrium implies that the temperature is the samethroughout the system

Mechanical equilibrium implies that the net forces and moments onthe system are zero

Phase equilibrium implies that the mass of each phase (e.g., solid,liquid and gas) remains constant

Chemical equilibrium implies that the chemical composition of thesystem remains constant


State Postulate

Once a sucient number of properties of a system are specied, theremaining properties can be determined

As an example, consider the ideal gas equation

pV = nRT

For a given volume V, knowing the pressure p, and temperature T issucient to determine

1

the number of moles n

The State Postulate

The state of a simple compressible system

is completely specied by

two independent, intensive properties

1R = 8.31447 kJ/kmoleK is the Universal Gas Constant


Processes and Cycles

The change of a system from one state to another is a process The series of states through which a system passes from its initial tonal state is the path

The path may be quasi-equilibrium or non-equilibrium

Processes

p V diagram


Steady Flow Processes

During a steady ow process, the ow variables at a xed locationdo not change in time; however, they may change with position


Zeroth Law of Thermodynamics

Two bodies in thermal equilibrium with a third body are in thermalequilibrium with each other (i.e., all have the same temperature)


Pressure

Pressure is the average normal force on a small element of uid,either at rest or in motion

The SI unit of pressure is the pascal (Pa)1 Pa = 1 Nt/m2

There are several other units for pressureUnit Denition

bar 10

5

Pa

atm 101, 325 Papsi 6, 894.75 Pa

The actual pressure is the absolute pressure The gage pressure is dened asp

gage

= p patm

where p

atm

is 1 bar (1 atm)

What does the pressure gage for an automobile tire measure ?


Variation of Pressure with Depth

In a uid at rest the pressure varies linearly with depthdp

dz

= g

where g = 9.8 m/s2, is the uid density and z is measured positiveupwards


Variation of Pressure with Depth

Under hydrostatic (i.e., uid at rest) conditions, the pressure is thesame at all positions on a horizontal plane regardless of geometry,

provided that the points are interconnected by the same uid


Pressure Measurement Devices

The barometer measures the atmospheric pressure provided that thegap at the top of the tube is a vacuum

p

C

+ gh = pB

= patm

thus p

atm

= gh if pC

= 0

Note: The pressure on the open uid surface is p

atm

. Since point B

and the open surface are connected by the same uid, p

B

= patm


Manometer

The manometer measures pressure in the same manner as thebarometer

p

1

= p2

= patm

+ gh

since the open end of the tube is subject to p

atm

The specic gravity of a uid is dened bySG = uid

/water


Manometer

Note that pA

= pB

(Why ?)

Therefore assuming the uid is the same tube is at rest

p

1

+ 1

g (a+ h) = p2

+ 1

ga+ 2

gh

and thus

p

1

p2

= (2

1

) gh


lectures-chapter1.pdf

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