1
Work
Heat
Adiabatic Process
Reversible Process
Irreversible Process
Isobaric process
Isochoric process
Isothermal process
Polytropic process
Modes of Heat Transfer
Chapter 4 : Work and Heat
Work – the energy transfer associated with a force acting through a distance
- a form of energy
- has unit such as kJ
- the work done during a process between states 1 and 2 is denoted 1W2 or simply W
The work done per unit mass of a system is denoted w and is defined as
(kJ/kg)
The work done per unit time of a system is called power and
denoted as . The unit of power is kJ/s, or kW
WORK
m
Ww
W.
Work Sign Convention
work done by a system : +ve
e.g. Work produced by car engines, hydraulic, steam, or gas
turbines
Work done on the system : -ve
e.g. Work consumed by compressors, pumps, and mixers : negative
System System
Surroundings
(+)
(+)
(-)
(-) W
Q
Work done by system
• Therefore;
• or in terms of the specific volume, v:
where m is the mass of the system.
Work = Area under the curve
LETS DO
Quiz 1
Quiz 3
• If data PV is not available but an analytical
expression is given, we can integrate directly;
• Example: PV = constant
P1V1 = P2V2 = PV = constant
P = V
VP 11
• Substitute,
1W2 = P1V1
1W2 = P1V1
2
1V
dV
1
2lnV
V
• A common PV relation in a process is called a polytrophic process.
PVn = constant or P = nV
constant
•Substitute into work equation,
1W2 = n
VPVP
11122 (4.4)
Eq(4.4) is not applicable for n=1
Polytrophic
process ,
PVn = k
Q 5
For ideal gas,
n=0 Isobaric n=1 Isothermal
n= γ Adiabatic n= α Isohoric
Exercises
Lets look at tutorial problem 5
Exercises
Lets look at tutorial problem 5
Exercises
Please try out Quiz 9
• Equation of state for thermally perfect gas
pV = nRT
n = number of moles
R = Universal gas constant (8.314J/mol.K)
V = total system volume
also for T = constant, p1V1 = p2V2 , work done
by the system is
or in terms of the specific volume and the system
mass
Isothermal
process
Isothermal
process
Combined Gas Law
PV
R T
PV
R T
1 1
1
2 2
2
Chapter 3
More Exercises
Lets look at tutorial problem 4 Cengel text book 3-7 compressibility factor
Quiz 8
Quiz 10
Heat, Q
– Heat is energy transferred between a system
and its surroundings by virtue of a
temperature difference only.
• "Adiabatic" means no heat is transferred (Q=0).
• Unit : Joule, Calorie, BTU etc
Heat Sign convection
(+) heat transferred to the system
(-) heat transferred from the system
System System
Surroundings
(+)
(+)
(-)
(-)
W
Q
Reversible process
• Defined as a process which can be
reversed – both the system and
surrounding are returned to their initial
state at the end of the reverse process
Irreversible process
• Process that are not reversible
Quasi-equilibrium process
• A process during which the system
remains nearly in equilibrium at all times
• A slow process that allows system to
adjust itself internally (1-7)
Isobaric process
• Constant pressure
• W12 = P (V2-V1)
Isochoric process
• Constant volume, V1 = V2
• W12 = P (V2-V1)
• W = 0
Heat transfer mode
• Heat transfer by conduction: Heat transfer
by energy exchange between adjacent
molecules.
Given by Fourier’s Law: Q = -kA dx
dT
Heat transfer mode
Heat transfer by convection:
HT by the bulk motion of fluid
over or near surface with a
different tempt
Given by Newton’s Law of Cooling:
Q = Ah∆T
Where h is heat transfer coefficient,
∆T is temperature difference between solid surface and fluid.
Combined conduction and convection:
Q = UA∆Toverall
Heat transfer mode
• Heat transfer by radiation: heat transfer by
the radiation of hot surface
•Energy is transmitted as electromagnetic wave in
space.
Given by:
Q = εσATs4
Where ε - emissivity
σ - Stefan-Boltzman constant
Ts - surface temperature.
More Exercises
Quiz 5
Quiz 4
Quiz 2
Quiz 6
Quiz 7