Thermodynamics Chapter Summary

CHAPTER ONE
OVERVIEW OFTHERMODYNAMICS
1/28/2018By: Mesfin M.
1
By Mesfin Megra

OUTLINE
• Introduction
• The first law of thermodynamics
• The second law of thermodynamics
• The Carnot cycle
2

INTRODUCTION
• The design, operation and performance of electric generating power
plants are largely dependent up on the science of thermodynamics.
• Thermodynamics is the study of the effects of work, heat and energy
on a system.
• Thermodynamics is only concerned with macroscopic (large scale)
changes and observation.
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CONT.…
➢Thermodynamics can be expressed in terms of four quantities.
• Temperature (T)
• Heat (Q)
• Internal energy (U)
• Entropy (S)
➢Energy can be transferred to or from a system in three forms: Mass,
Heat andWork.
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MASS TRANSFER
➢The mass entering a system carries energy with it and the energy of the
system increases.
➢The mass leaving a system decreases the energy content of the system.
➢When a fluid flows into a system at a mass flow rate of ሶ𝑚 (kg/s), the rate
of energy entering is equal to mass times energy of a unit mass of a
flowing fluid
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HEAT TRANSFER
➢Heat is the thermal form of energy.
➢Heat transfer takes place when a temperature difference exists
within a medium or between different media.
➢If there is no heat transfer involved in a process (∆Q=0), it is called
an adiabatic process.
6

WORK
➢Work is the energy that is transferred by a difference in pressure or
under the effect of a force of any kind.
➢Work is subdivided into shaft work and flow work.
➢Shaft work is mechanical energy used to drive a mechanism
➢Flow work is the energy transferred into a system by fluid flowing
into, or out of, the system.
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FIRST LAW OFTHERMODYNAMICS
➢The first law of thermodynamics is an extension of the law of
conservation of energy.
➢The net change in the total energy of a system during a process is
equal to the difference between the total energy entering and the
total energy leaving the system:
𝐸𝑖𝑛 − 𝐸 𝑜𝑢𝑡 = ∆𝐸𝑠𝑦𝑠𝑡𝑒𝑚
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CON….
➢The increase of the internal energy of a system is equal to the sum
of the heat added to the system plus the work done on the
system.
𝛥𝑈 = 𝛥𝑄 + 𝛥𝑊
➢The change in internal energy of a system is equal to the heat
added to the system minus the work done by the system.
𝛥𝑈 = 𝑄 − 𝑊
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CON….
➢Note that internal energy is a function of state.
➢The internal energy of the Body increases by 𝛥𝑄 + 𝛥𝑊, while the
internal energy of the Rest of the Universe (the Surroundings)
decreases by the same amount.
➢Thus the internal energy of the Universe is constant.
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SECOND LAW OFTHERMODYNAMICS
➢A system which is initially not in equilibrium always changes towards
equilibrium. while the opposite direction does not happen.
➢In the first law there is nothing that prevent from moving in
opposite direction, since energy is conserved
➢But some processes are irreversible.
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CON….
➢Reversible process:A process in which it is possible to return both
the system and surroundings to their original states.
➢Irreversible process:A process in which it is impossible to return
both the system and surroundings to their original states.
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CON….
➢The inadequacy of the ﬁrst law to identify whether a process can
take place is treated by introducing another general principle, the
second law of thermodynamics.
➢The second law also asserts that energy has quality as well as
quantity.
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CON….
➢There are numerous forms of second law statements.Two classical statements
are as follows.
➢The Kelvin–Plank statement: It is impossible to construct a device,
operating in a cycle that accomplishes only the extraction of heat energy from
some source and its complete conversion to work.
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CON….
• The Clausius statement: It is impossible to construct a device,
operating in a cycle that transfers heat from the low-temperature
side (cooler) to the high-temperature side (hotter), and producing
no other effect.
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ENTROPY
➢Entropy is the measure of the disorder of the system
➢Entropy tells us that how much energy is available for doing useful
work.
➢The entropy of the universe increases in all natural processes.
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CON…. 1/28/2018By: Mesfin M.
17

CON….
➢The change in entropy of a system, ΔS, is equal to the heat, ΔQ,
flowing into the system as it changes from one state to another
divided by absolute temperature.
∆𝑆 =
∆𝑄
𝑇
➢In adiabatic process the change in entropy becomes zero which
means it is reversible process.
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HEAT ENGINE
➢A device which transforms heat into work is called a heat engine
➢Heat engines require a hot reservoir to supply energy (QH) and a
cold reservoir to take the excess energy
➢In a cyclic process, work is converted into heat and in the end of
each cycle the system returns to its original state.
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WORKING PRINCIPLE OF HEAT ENGINE 1/28/2018By: Mesfin M.
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➢Part of the heat that is transferred to
the system from a heat bath with
temperature T1, Q1 is converted into
work and the rest Q2 is delivered to
a second bath with T2 < T1

21

THERMAL EFFICIENCY
➢The thermal efficiency is the index of performance of the a work
producing device or heat engine.
➢It is given by the ratio of the net work output (the desired result)
to the heat input (the cost to obtain the desired result)
η 𝑡ℎ =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑟𝑒𝑠𝑢𝑙𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡
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CON….
➢For the heat engine the desired result is the network done and the
input is the heat supplied to make the cycle operate.
➢Thermal efficiency is always less than 1 or 100%.
η 𝑡ℎ =
𝑊𝑛𝑒𝑡,𝑜𝑢𝑡
𝑄𝑖𝑛
Where 𝑊𝑛𝑒𝑡,𝑜𝑢𝑡 = 𝑊𝑜𝑢𝑡-𝑊𝑖𝑛 and 𝑄𝑖𝑛 ≠ 𝑊𝑛𝑒𝑡
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EXAMPLE
1. A steam power plant produces 50 MW of net work while burning
fuel to produce 150 MW of heat energy at the high temperature.
Determine the cyclic thermal efficiency and the heat rejected by
the cycle to the surrounding.
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THE CARNOT CYCLE
➢Carnot laid the foundations of the second laws of thermodynamics,
introducing the concept of reversibility and cycle.
➢It also introduces the thermal efficiency of a reversible cycle is
determined by the temperature of heat source and heat sink.
➢Also postulated efficiency is never 100% since heat has to be
rejected to the heat sink.
25

CON….
➢Carnot demonstrated how PMM of the first and the second kind
cannot exist.
26

27

CON….
➢1 → 2: adiabatic reversible (isentropic) compression (Q = 0) from
TL toTH
➢2 → 3: isothermal reversible expansion atTH
➢3 → 4: adiabatic reversible (isentropic) expansion (Q = 0) fromTH
toTL
➢4 → 1: isothermal compression atTL
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CON….
➢All processes in the cycle are either adiabatic or isothermal and
operate in a reversible cycle (cycle where system & surroundings
return to initial state)
➢Adiabatic – complete combustion of fuel; no lingering ΔQ input in
other parts of cycle (no exchange of heat with surroundings)
29

CON….
➢Isothermal – no heat loss to walls; no turbulence of fluids; no
friction; heat transfer is doneVERY slowly in order to retain
isothermal quality
η 𝑐𝑎𝑟 = 1 −
𝑇𝐿
𝑇 𝐻
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QUIZ 1 (10%) 10MIN
2. In each cycle, a heat engine absorbs 375 J of heat and performs 25
J of work.
A. Find the efficiency of the engine
B. Find the heat expelled in each cycle
3. The exhaust temperature of a Carnot heat engine is 300°C. What
is the intake temperature if the efficiency of the engine is 30%?
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Thermodynamics Chapter Summary

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Thermodynamics Chapter Summary