LESSON 1
RELATIONSHIP TO THERMODYNAMICS
HEAT is the form of energy that can be transferred from one system
to another as a result of temperature difference.
WHAT IS THE RELATIONSHIP OF HEAT TRANSFER TO
THERMODYNAMICS?
FIRST LAW OF THERMODYNAMICS
states that energy cannot be created nor destroyed. It can, however, converted
from one form to another.
What is the INTEREST of the
designer?
FOR HEAT TRANSFER
The FIRST LAW requires that the rate of energy transfer into a system be
equal to the rate of increase of the energy of that system.
FIRST LAW OF THERMODYNAMICS
[Initial Energy] + [Energy Entering] = [Final Energy] + [Energy Leaving]
For Closed System:
[Final Energy – Initial Energy] = [Energy Q and/or W Entering/Leaving
For Open System:
[Final Energy – Initial Energy] = [Energy Q and/or W Entering/Leaving] + [Energy Brought-in/Carried-away
by Fluid]
𝑬𝒊𝒏𝒊𝒕𝒊𝒂𝒍 + 𝑬𝑸&𝑾.𝒊𝒏 + 𝑬𝒇𝒍𝒖𝒊𝒅.𝒊𝒏 = 𝑬𝒇𝒊𝒏𝒂𝒍 + 𝑬𝑸&𝑾.𝒐𝒖𝒕 + 𝑬𝒇𝒍𝒖𝒊𝒅.𝒐𝒖𝒕
SECOND LAW OF THERMODYNAMICS
[1] It is impossible to construct a heat engine which will operate in a cycle,
receive heat from a reservoir, and convert all of it into work output
𝑄𝑆 = 𝑊𝑛𝑒𝑡 + 𝑄𝑅
SECOND LAW OF THERMODYNAMICS
Energy Source at
High Temperature
HEAT SUPPLIED
𝑄𝑆 = 𝑊𝑛𝑒𝑡 + 𝑄𝑅 HEAT
ENGINE
NET WORK OUTPUT, WNET
HEAT REJECTED
Energy Sink at Low
Temperature
SECOND LAW OF THERMODYNAMICS
[2] Heat cannot flow by itself from a low HIGH TEMPERATURE
RESERVOIR
temperature to a high temperature
HEAT SUPPLIED
REVERSED
CYCLE WORK
HEAT
ENGINE
INPUT
HEAT REJECTED
LOW TEMPERATURE
RESERVOIR
FOR HEAT TRANSFER
The SECOND LAW requires that heat be transferred in the direction of decreasing
temperature
FORMS OF ENERGY
100
Latent Heat of Fusion Latent Heat of Vaporization
335 kJ/kg 2257 kJ/kg
Temperature
Specific Heat Above Freezing
4.187 kJ/kg-K
Enthalpy
FORMS OF ENERGY
SENSIBLE HEAT
Heat Involved In A Change Of Temperature Of A Substance
LATENT HEAT
Heat Absorbed/Given Off When A Substance Changes Its State
100
Latent Heat of Fusion Latent Heat of Vaporization
Temperature
335 kJ/kg 2257 kJ/kg
Specific Heat Above Freezing
4.187 kJ/kg-K
Enthalpy
HEAT
U H
INTERNAL ENERGY ENTHALPY
ꟘU = mCvꟘT ꟘH = mCpꟘT
Microscopic energy of a non-flowing fluid Microscopic energy of a flowing fluid
UNITS: Joule OR Joule/kg
HEAT TRANSFER
• Heat is the transfer of energy due to a temperature
gradient.
• thermal energy in transit due to a spatial temperature
difference.
ꟘT
TEMPERATURE DIFFERENCE
HEAT TRANSFER RATE
• The amount of energy transferred per unit time (J/s) (BTU/hr)
HEAT FLUX
• The rate of heat transferred per unit area is normal to the heat
transfer's direction. (W/m2) (BTU/hr/ft2)
Q = Heat Rate
𝑄ሶ = Heat Rate per unit time
∆𝑡 = Change in time
ሶ
𝑄 = 𝑄𝑑𝑡
∆𝑡
𝑄 = 𝑄ሶ න 𝑑𝑡
0
ሶ
𝑄 = 𝑄∆𝑡
q= 𝑄ሶ
𝐴
q = Heat Rate
𝑄ሶ = Heat Rate per unit time
𝐴 = area
ሶ
𝑄 = 𝑄𝑑𝑡
∆𝑡
= 𝑄ሶ න 𝑑𝑡
0
𝑄 = 𝑄ሶ ∆𝑡
Q = amount of heat transfer
𝑄ሶ = Heat transfer rate
∆𝑡 = Change in time
q= 𝑄ሶ
𝐴
q = Heat flux
𝑄ሶ = Heat transfer rate
𝐴 = area
MODES OF HEAT TRANSFER
Conduction – is the transfer of energy from the more energetic particles of a
substance to the adjacent, less energetic ones as a result of interactions
between the particles.
Convection – is the mode of heat transfer between a solid surface and the
adjacent liquid or gas that is in motion, and it involves the combined effects of
conduction and fluid motion.
Radiation - is the energy emitted by matter in the form of electromagnetic
waves (or photons) as a result of the changes in the electronic configurations of
the atoms or molecules.
SAMPLE PROBLEM
10-cm diameter copper ball is to be heated from 100°C to an average
temperature of 150°C in 30 minutes. Taking the average density and
specific heat of copper in this temperature range to be 8950 kg/m3 and
Cp = 0.395 kJ/kg · °C, respectively, determine (a) the total amount of heat
transfer to the copper ball, (b) the average rate of heat transfer to the ball, and
(c) the average heat flux.
