Fundamentals of Thermal Fluid Sciences: Yunus A Cengel. • Fundamentals of ...
Thermal sciences or thermal-fluid sciences can be more broadly defined as the ...
References • Fundamentals Fundamentals of Thermal Fluid Sciences: Yunus A. Cengel, of Thermal Fluid Sciences: Yunus A Cengel Robert H. Turner • Engineering Thermodynamics work and heat transfer: G.F.C. Rogers, Y.R. Mayhew
Internal combustion Engines: History, History engine types and operation of 2 & 4 stroke engines Dr. Primal Fernando
[email protected] d@ d lk Ph: (081) 2393608 1
Introduction and Overview
2
Application areas of thermal‐fluid sciences pp
• Thermal Thermal sciences can be loosely defined sciences can be loosely defined as the sciences that the sciences that deal with heat. Many natural and engineering applications (human body, refrigerators, automotive or jet engines, water or gas fi t t ti j t i t transportations …………………….. …………………………………
• Thermal sciences or thermal‐fluid sciences can be more broadly defined as the physical sciences that deal with energy and the transfer, transport and conversion of energy. • Traditionally they studied under the subcategories of, o o o
Thermodynamics Heat Transfer Fluid Mechanics
3
4
Thermodynamics y
Basically 3 laws of thermodynamics y y
Thermodynamics can be defined as the science of energy.
• 1st law of thermodynamics (1850, out of the works of William , p , ) Rankine, Rudollph Clausius, Lord Kelvin) Simply an expression of the conservation of energy principle.
Ability to cause changers
• 2nd law of thermodynamics (1850, out of the works of William Rankine, Rudollph Clausius, Lord Kelvin)
One of the most fundamental law of nature energy is O f th tf d t ll f t i conservation of energy principle,
Asserts as energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.
o During an interaction, energy can be transform from one state to a another state, but total energy amount remain as constant. th t t b tt t l t i t t o Energy can not create or destroy.
• 0th law of thermodynamics (formulated by R.H. Flower 1931) basis for the validity of the temperature measurements. y p
5
0th law of thermodynamics (formulated by R H Flower 1931) (formulated by R.H. Flower 1931)
6
0th law of thermodynamics y
• 0th law of thermodynamics is states that if two bodies are in law of thermodynamics is states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
Body 1 Thermal equilibrium Thermal equilibrium
• Another way, if two bodies have equality of temperature with a third body, they in turn have equality of temperature with each y, y q y p other.
Body 3
Thermal equilibrium Thermal equilibrium Body 2
• Re Replacing the third body with a thermometer, la i the thi d body ith a the o ete the 0 the 0th law of la of thermodynamics can be re‐stated as, as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. di if th ti t t 7
8
0th law of thermodynamics y
1st law of thermodynamics (1850, out of the works of aw o t e ody a ics (1850, out o t e wo s o William Rankine, Rudollph Clausius, Lord Kelvin) • The The increase in the internal energy increase in the internal energy of a thermodynamic system of a thermodynamic system is equal is equal to the amount of heat energy added to the system minus the work done by the system on the surroundings.
Body 1 Body 1 150°C Body 1 90°C 90 C
• Mathematical representation, dU=δQ‐δW
Body 2 90°C
where dU is the infinitesimal increase in the internal energy of the system, δQ is the infinitesimal amount of heat added to the system, and δW is the infinitesimal amount of work done by the system on the surroundings. The infinitesimal heat and work are denoted by δ di Th i fi it i l h t d k d t d b δ rather th than d because, in mathematical terms, they are inexact differentials rather than exact differentials. In other words, there is no function Q or W that can be differentiated to yield δQ that can be differentiated to yield δQ or δW. or δW
Body 2 50°C Isolated enclosure
9
10
The Kelvin‐Planck Statement:
2nd law of thermodynamics (1850, out of the works of William Rankine, Rudollph Clausius, Lord Kelvin) There are two classical statements: Kelvin‐Planck statement and There are two classical statements: Kelvin Planck statement and Clausius statement.
It is impossible to construct a device that will operates in a cycle and produce no effect other than the raising of a weight and exchange of heat with a single reservoir heat with a single reservoir. This related to heat engine. This also states that, it is impossible to construct a heat engine that Thi l t t th t it i i ibl t t t h t i th t operates in a cycle, receives a given amount of heat from a high‐ temperature body, and does an equal amount of work. The only alternative is that some heat must be transferred from the The only alternative is that some heat must be transferred from the working fluid at a lower temperature to a lower temperature body. Thus, work can be done by the transfer of heat only if there are two p g temperature levels, and heat is transferred from the high‐ temperature body to the heat engine, and also from the heat engine to the low‐temperature body. It is impossible to build a heat engine that has a thermal efficiency of 100% f 100%
• The Kelvin‐Planck statement: It is impossible to construct a device that will operates in a cycle and produce no effect other than the raising of a weight and exchange of heat with a single g g g g reservoir. • The The Clausius statement: Clau iu tate e t It is impossible to construct a device It i i o ible to o t u t a de i e that operates in a cycle and produces no effect other than the transfer of heat from a cooler body to a hotter body.
11
12
The Clausius statement
The Kelvin‐Planck The Kelvin Planck Statement: Statement: High temperature body High temperature body QH
QH W=QH
This statement is related to a refrigerator or heat pump. This statement is related to a refrigerator or heat pump W=QH‐QL
Heat engine
thermal
• It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a cooler body to produces no effect other than the transfer of heat from a cooler body to a hotter body.
High temperature body High temperature body
Heat engine
It state that, it is impossible to construct a refrigerator or heat pump without an input of a work. ith t i t f k
QL
W 100% QH
This also implies that the coefficient of performance (COP) always less than infinity.
Low temperature body thermal
W 100% QH 13
Heat Transfer
The Clausius statement : The Clausius statement : High temperature body
W=0
• Heat (form of energy) transfer from warm medium to cold medium.
High temperature body
QL
QH
• Heat transfer is always from the higher temperature medium to the lower temperature one. h l
W=QH‐QL
Heat engine
QL
Low temperature body QL COP W( 0 )
14
Heat engine
There can not be net heat transfer between two mediums that can not be net heat transfer between two mediums that • There are at same temperature. Temperature difference is the driving force for heat transfer.
QL
Low temperature body
• Heat transfer stops when two mediums reach the same temperature.
Q H ( orQ L ) COP W ( QH QL ) 15
16
Problem solving techniques
Solving engineering problems g g gp
• Step 1 ‐ Problem statement : Briefly state the problem in your words (key information given and quantities to find). • Step 2 ‐ Schematic : Draw a realistic sketch of the physical system involved, and list the relevant information on the figure. • Step 3 ‐ Assumptions : state any assumptions made to simplify the problem. • Step 4 ‐ Physical laws : Apply all the basic relevant physical laws and principles, and simplify to their simplest forms by utilizing the assumptions. • Step 5 ‐ Properties : Determine and list the relevant properties. • Step 6 ‐ p Calculations :Substitute the known quantities to relations and q perform the calculations. • Step 7 ‐ Reasoning, Verifications, and Discussion : Check to make sure the results obtained are reasonable.
• Experimentally – Expensive. Expensive – Not always practical. – Deal with actual problem. – Time consuming y y • Analytically – Inexpensive – Fast – Results depends on the assumptions. Results depends on the assumptions
17
18
A potato baked in a oven p
Insulated (adiabatic) room heated by electric heater
• Heat transfer to the potato; energy of the potato increases (disregard moisture loss from potato). (disregard moisture loss from potato).
• Electric work done = increase in the energy of the house 0
• The amount of heat transfer = energy increase in the potato
dU = δQ – δW dU = δQ
0 dU = δQ – δW
dU = δW dU = δW
dU = δQ
19
20
Shaft work (shaft) done on an adiabatic system
Energy balance for control a volume gy
• Work done by the shaft = increase in the energy of the system
0 dU = δQ – δW Total energy Total energy Change in the total entering the system leaving the system energy of the system
dU = δW
E in E out E system or
E in E out E system
21
22
Energy balance for steady flow system
The change in energy content of a control volume g gy
Q in W in m in hin in gZ in Q out W out m out hout out gZ ot 2 2
Q
in
2
Energy balance,
Q in Win m in hin in gZ Z in Q out Wout m out hout out gZ Z ot Esystem 2 2
2
2
23
2 out
2
2
in2
2
gZ ot m in hin
gZ in
Q W m out hout out gZ ot m in hin in gZ in 2 2
General form
E in E out Qin Qout Win Wout E mass ,in E mass ,out E system
Q out W out W in m out hout
Energy can be transferred to/from a system: as heat, as work and with incoming and outgoing mass incoming and outgoing mass.
Ein Eout Esystem
2
2
Note: In the above equation, heat input to the system and work output from the system is positive (+) and heat output from the system and work input to the system is negative (‐). y g ()
24
Carnot Cycle y
Carnot cycle for heat engine Carnot cycle for heat engine
• The Carnot cycle is the most efficient thermodynamic cycle for operating a heat engine • It is a reversible cycle, that consists of two isotherms and two adiabats
25
Entropy py
26
Entropy (S) py ( )
• The second law leads to define a property call entropy. • A measure of the energy that is no longer available to do work A measure of the energy that is no longer available to do work • Entropy is nonconserved property. • Entropy is an extensive property of a system and sometimes is referred to as total entropy. Entropy per unit mass, designated s, is an intensive property and has the unit kJ/kg ∙ K is an intensive property and has the unit kJ/kg ∙ K..
Hot
Cold
Cool
• It is path independent. • A measure of the microscopic disorder of a system.
Lowest Entropy py 27
Highest g Entropy py 28
Energy tube gy
Clausius Inequality (I) Clausius Inequality (I) (first stated German physicist R.J.E Clausius 1833‐1888)
Clausius inequality q y • The cyclic integral of dQ/T is always less than or equal to zero.
• First Law: The amount of energy in the universe is always constant. • The Second Law Th S dL t ll tells us that the quality th t th lit of a particular amount of f ti l t f energy i.e. the amount of work, or action, that it can do, diminishes for each time this energy is used. • The usable energy in a system is called exergy, and can be Th bl i t i ll d d b measured as the total of the free energies in the system. • Entropy is a measure of the unavailability of a system’s energy t d to do work. k
• This inequality is valid for all cycles, reversible or irreversible. the cyclic integral of dQ/T can be viewed as the sum of all these y g differential amounts of heat transfer divided by the absolute temperature at the boundary.
29
Clausius Inequality (II) q y( )
30
Entropy change in a process py g p
Reversible vs irreversible e equa i y i e au iu i equa i y o fo o a y o ju i e a y • The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones.
• The The entropy change of a system during a process can be entropy change of a system during a process can be determined by:
A property! • Here we have a quantity whose cyclic integral is zero! Therefore, the H h i h li i li ! Th f h quantity (dQ/T)int rev must represent a property in the differential form. • Clausius chose to name this property entropy (here defines change in entropy instead of entropy).
• To perform the above integration, one needs to know the relation between Q and T during a process. For the majority of cases we have to rely on tabulated data for entropy. of cases we have to rely on tabulated data for entropy.
31
32
A Special Case: Internally Reversible Isothermal Heat Transfer Processes
Entropy generation (Sgen)
• Recall that isothermal heat transfer processes are internally reversible (no internal thermal gradients). ibl ( i l h l di ) That is some entropy is generated or created in irreversible process and we can rewrite the above equation as, Always positive or zero quantity, depends on process, not a property
• T0 is the constant absolute temperature of the system and Q is the heat transfer for the internally reversible process. • This equation is particularly useful for determining the entropy changes of thermal energy reservoirs that can absorb or supply heat indefinitely at a constant temperature heat indefinitely at a constant temperature. 33
34
Entropy generation py g
Isolated system Isolated system Isolated system y or simply adiabatic closed system py y
• The The entropy change of a system can be negative during a entropy change of a system can be negative during a process, but entropy generation cannot.
Q=0
• Don’t mix entropy change, entropy transfer and entropy generation. And therefore, And therefore,
The entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. This is known as the i increase of entropy principle. f i i l 35
36
Case a
• IIn both cases involve heat transfer through b h i l h f h h finite temperature difference; therefore both irreversible. • Entropy change in each process involves two reservoirs. Case b
• Entropy generation in the system is equal to sum of entropy change in each reservoir. • Reservoirs are not in contact, separated by partition; ; Spartition=0 p
37
38
39
40
Isentropic process p p • We mentioned earlier that the entropy of a fixed mass (closed system) can be changed by (closed system) can be changed by – (1) heat transfer – (2) irreversibilities. • Then it follows that the entropy of a fixed mass will not change during a process that is internally reversible and g g p i e a y e e i e adiabatic. A process during which the entropy remains constant is called an isentropic process.
T‐S diagram for Carnot cycle
Relative pressure and relative specific volume p p s0 exp 2 s s s s P2 R exp exp exp 0 P1 R R R exp s1 R
Adiabatic compression (isentropic compression)
0 2
Isothermal expansion
0 1
P2 P r2 P1 s cons tan t Pr1 v v 2 r2 v 1 s cons tan t vr1
0 2
0 1
Pr 2 Pr 1
Dimensionless quantity P q y r is called relative pressure, (in Tables)
Dimensionless quantity v i i r is called relative specific volume, (in Tables) (in Tables)
Isothermal compression Adi b ti Adiabatic expansion i 41
42
