Thermodynamics I MECN 4201

1. Thermodynamics I MECN 4201 Professor: Dr. Omar E. Meza Castillo omeza@bayamon.inter.edu http://facultad.bayamon.inter.edu/omeza Department of Mechanical Engineering Inter American University of Puerto Rico Bayamon Campus

2. Course Information • Catalog Description: Analysis of the basic concepts of thermodynamics. Includes the study of the properties of pure substances and the equation of the ideal state of gas. Analysis of the transfer of energy by heat, work and mass. Application of the first and second law of thermodynamics. Analysis of the Carnot Cycle and entropy. • Prerequisites: PHYS 3312 – Physics for Engineers IICHEM 2115 – General Chemistry for Engineers. • Course Text: Cengel, Y. A. (2008). Introduction to Thermodynamics and Heat Transfer. 2nded. McGraw-Hill. • Absences:On those days when you will be absent, find a friend or an acquaintance to take notes for you or visit Blackboard. Do not call or send an e-mail the instructor and ask what went on in class, and what the homework assignment is.

3. Course Information • Homework assignments:Homework problems will be assigned on a regular basis. Problems will be solved using the Problem-Solving Technique on any white paper with no more than one problem written on one sheet of paper. Homework will be collected when due, with your name written legibly on the from of the title page. It is graded on a 0 to 100 points scale. Late homework (any reason) will not be accepted. • Problem-Solving Technique: • Known • Find • Assumptions • Schematic • Analysis, and • Results

4. Course Information • Quiz: There are four or more partial quizes during the semester. • Partial Exams and Final Exam: There are three partial exams during the semester, and a final exam at the end of the semester.

5. Course Grading • The total course grade is comprised of homework assignments, quizes, partial exams, and final exam as follows: • Homework (6 or more ) 20% • Quiz (4 or more) 20% • Partial Exam (3) 20% • Final Exam 25% • Final Project 15% 100% • Cheating: You are allowed to cooperate on homework by sharing ideas and methods. Copying will not be tolerated. Submitted work copied from others will be considered academic misconduct and will get no points.

6. Course Materials • Most Course Material (Course Notes, Handouts, Homework, Final Project, and Communications) on Web Page • Power Point Lectures will posted every week or two • Office Hours: Tuesday and Thursday @ 5:50 to 7:20 PM • Email: mezacoe@gmail.com

7. Course Outline • Introduction and Overview • Basic Concepts of Thermodynamics • Properties of Pure Substances • Energy Transfer by Heat, Work and Mass • The First Law of Thermodynamics for Closed System • The First Law of Thermodynamics for Open System • The Second Law of Thermodynamics • Entropy.

8. Introduction to Thermodynamics Introduction

9. Course Objective • To explain the fundamental concepts of thermodynamics such as system, state, equilibrium, process, and cycle

10. Introduction • What is Thermodynamics? Thermodynamics can be defined as the science of energy. The name thermodynamics stem from the Greek words: THERME (heat) and DYNAMIS (power). Initially at early 1900’s: the capacity of heat to produce work. Today the scope is larger including all aspect of energy and its transformation. Engineers are interested to studySystem <-> Surrounding. • Involves the study of work and heat interactions with matter and its properties • Two important concepts in Thermodynamics are Energy and Entropy. • Energy is always conserved. • Entropy determines whether a process is possible, i.e. a process which produces entropy is possible, one that destroys entropy is impossible.

11. Thermodynamics and Energy • Why do Engineers study Thermodynamics?

12. Introduction • Applications: Thermodynamics in Engineering Systems • Power Generation • Refrigeration and Heat Pumps • Internal Combustion Engines • HVAC Systems • Jet Propulsion • Supersonic Flows • Fuel Cells • Reacting and non-Reacting Processes

13. Introduction • One of the greatest inventions ever – the Steam Engine!

14. Introduction • How one invention changed the world

15. Thermodynamics Basic Concepts

16. Importance of Dimensions and Units • Engineering Units Three systems: • SI (from Le Système International d’ Unités) System of Units [M,L,t,T] – force is a secondary unit • British Gravitational System [F,L,t,T] – force is a primary unit • English Engineering System [F,M,L,t,T] - both mass and force are primary units

17. Importance of Dimensions and Units • English Engineering Units • When is a pound not a pound? • In English Engineering Units of course! • In the EEU system, the unit of force is the pound force (lbf or lbf) and the unit of mass is the pound mass (lbm or lbm). Length is in (ft), time in (s), and temperature in (R). Both forceand mass are primary. Absolutely brilliant!!! • A force of one pound is the force that gives a one pound mass an acceleration equal to that of the earth’s gravity g= 32.174 ft/s2. i.e. 1 lbf= 1 lbm x 32.174 ft/s2 • Newton’s Law then becomes:

18. Importance of Dimensions and Units • The constant gc= 32.174 (lbm ft)/(lbf s2) is required in any relationships derived from Newton’s law. • This constant of proportionality has both units and a value that is not equal to unity. • Care must be taken when working in the EEU system, to understand when a lbf or lbm are specified. The course text adopts that (lb) is (lbm). So be careful. • We will primarily use the SI system. Occasionally, we will do an example in the EEU system. • Finally, note:

19. Importance of Dimensions and Units • British Gravitational Units • In this system, force is defined in (lbf), length in (ft), time in (s), and temperature in (R). The unit of mass the slug is secondary, such that: • The weight of one slug in earth’s gravity is then: • This leads to, using Newton’s Law from EEU, as:

20. Example • Let’s say a person who weighs 150 lbf in earth gravity is weighed on the moon where g=5.348ft/s2. What is their new weight? On earth we also say they have a mass of 150 lbm! Thus: • In the BGU system, this person has a mass of 4.662 slug, such that:

21. Example (continued) • Now this same person has a mass of 68.04 kg in SI units • On earth we would find this person weighs: • On the moon, this person now weighs: • In summary, SI is simpler. The BGU system is a little more practical than the EEU, as the pound is force and not mass as well. But you should see the equality in BGU and EEU, since lbm/gc

22. Systems and Control Volumes • Thermodynamic System: Three-dimensional region of space which is bounded by an arbitrary surface or a quantity of mass chosen for study.. • Control Surface: or Boundary may be real or imaginary, may be at rest or in motion, and may change its size or shape. It neither contains matter nor occupies a volume in space. Thickness of a boundary is mathematically zero. Depending of its properties it can be adiabatic boundary: not allowing heat exchange; rigid boundary: not allowing exchange of work. • A system can be very simple as a cylinder-piston, or as complex as portions of an oil refinery. • Surrounding or Environment: All physical space which lies outside the boundary.

23. Systems and Control Volumes • A Closed System or Control-mass consists of a fixed amount of mass, its analysis involves no mass transfer across the boundaries (mass cannot enter or leave). However, energy transfer, in the form of heat and work is allowed, as well as a change in chemical composition within the system. A special type of closed system that does not interact in any way with its surroundings is called a isolated system. A common example is the piston-cylinder device. • A Control Volume or Open System is a properly selected region in space where mass and energy may cross the boundary. It usually encloses a devices that involves mass flow such as a compressor, turbine, mixing chamber or nozzle.

24. Systems and Control Volumes System Boundary CLOSED SYSTEM m=constant MASS -> NO Fixed Boundary ENERGY -> YES Thermodynamic Surrounding Moving Boundary Real Boundary CONTROL VOLUME CONTROL VOLUME Imaginary Boundary

25. Example • Closed System The air/fuel mixture in a piston of an Internal Combustion (IC) engine is considered the control mass. The system boundary or control surface is placed around the gas mixture.

26. Example • Open System An IC engine can be analyzed as an open system. In this case, air, fuel, and exhaust streams flow through the control volume. Heat (in the exhaust stream) and work also cross the control surface.

27. Properties of a System • Any characteristic that describe a system is called a property. A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure and temperature. The properties can be: • directly measured • defined by laws of thermodynamics, and • defined by mathematical combinations of other properties. • The properties can be either intensive or extensive. A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided (mass, volume, energy, etc). The extensive properties depend on the size or extent of the system and can change with time.

28. Properties of a System • Intensive properties are not additive in the sense previously considered. They are those that are independent of the size or extent of a system, and may vary from place to place within the system at any time (temperature, pressure and density). When an extensive property is divided by mass, the resultant is called specific property. It becomes an intensive property. Extensive properties Intensive properties

29. Properties of a System • Consider a block of material of mass (m), volume (V) and uniform temperature (T). If we cut it up into pieces, the total mass of the system and total volume are the sum of the pieces, but the temperature is not the sum of the temperatures of the pieces, nor is the density the sum of the densities, i.e.

30. Properties of a System • Continuum: Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum. The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities. The continuum idealization is implicit in many statements we make, such as “the density of water in a glass is the same at any point.”

31. Density and Specific Gravity • Density is defined as mass per unit volume. • The density of a quantity of matter is defined as: • V’ is the smallest volume containing enough particles such that statistical averages are significant. It is also the smallest volume that we can consider the region a “point” and still maintain the continuum hypothesis. • Density can vary from point to point within the system.

32. Density and Specific Gravity • The reciprocal of density is the specific volume v, which is defined as volume per unit mass. That is, • Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative density, and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which ρH2O =1000 kg/m3). That is,

33. Density and Specific Gravity • The weight of a unit volume of a substance is called specific weight and is expressed as • Where g is the gravity acceleration

34. State, Equilibrium, Processes and Cycle • State is defined as the description of the condition of a system at a given instant. The properties are defined only when a system is in equilibrium (this imply a state of balance). Any transformation of a system from one equilibrium state to another is called a process. The path of a process is the series of states through which the system passes. A process is described by its initial and final states, the followed path and the interaction with its surrounding. • When a system is infinitesimally close to equilibrium at all times during a process, the process is calledquasistatic. • Cycle is a sequence of processes that begin and end at the same state.

35. State, Equilibrium, Processes and Cycle Gas is compressed from state 2 to state 1 or expanded from state 1 to state 2. The process occurs from 1 to 2 or 2 to 1.

36. State, Equilibrium, Processes and Cycle A simple cycle for power generation. Water flows through the pump, is heated in a boiler, steam expands through a turbine, and then the condensed water flows through the 1 pump again.

37. State, Equilibrium, Processes and Cycle • The Steady-Flow Process: The terms steady and uniform are used frequently in engineering. The term steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region. • A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Processes involving such devices can be represented reasonably well by a somewhat idealized process, called the steady-flow process, which can be defined as a process during which a fluid flows through a control volume steadily.

38. Temperature and Zeroth Law of Thermodynamics • What is temperature? • A definition of temperature in terms of concepts that are independently defined or accepted is difficult to give, despite the fact that we are aware of it through our senses. • Temperature is a perception that is associated with the notions of “hotness” and “coldness” • It is easier to obtain an objective understanding through the equality of temperature, by using the fact that when the temperature of an object changes other properties also change • Temperature is an intensive property.

39. Temperature and Zeroth Law of Thermodynamics If we had two blocks of copper each at a different temperature and each in contact with a mercury thermometer, which are suddenly brought into contact,several things will happen: • As the warm block cools, its electrical resistance and volume, measurably decrease, and the mercury level drops • As the cooler block warms, its electrical resistance and volume, measurably increase, and the mercury level increases • Eventually the two blocks come into thermal equilibrium and the changes in properties cease and the two thermometers read the same level

40. Temperature and Zeroth Law of Thermodynamics • Zeroth Law of Thermodynamics • States that when two bodies have equality of temperature with a third body, they in turn have equality of temperature with each other. • Why is it the Zeroth Law? • While this principle seems obvious, it is not derivable from other laws, and because it precedes the First and Second laws of thermodynamics in the logical presentation of fundamentals, it has come to be known as the Zeroth Law! • This is the basis for measurement of temperature.

41. Temperature and Zeroth Law of Thermodynamics • Temperature Scales • 4 - Temperature Scales • Celsius Scale (oC) Ice point H20 = 0oC Boiling point H20 = 100oC • Fahrenheit Scale (oF) Ice point H20 = 32 oF Body temperature = 98.6oF Boiling point H20 = 212oF • Kelvin (K) - Absolute T(K) = T(oC) + 273.15 • Rankine (R) - Absolute T(R) = T(oF) + 459.67

42. Pressure • What is pressure? • Pressure is the normal or compressive force per unit area exerted by a fluid at a point. • Consider a small infinitesimal area A in medium of fluid at rest. At some point on this surface, a normal force is exerted by the fluid on the top and bottom of this area. The pressure is defined in the limit as this area becomes smaller, until it is the smallest area that can be considered measureable: • Pressure is an intensive property.

43. Pressure • For a fluid at rest, pressure is the same in all directions at this point. But can vary from point to point, e.g. hydrostatic pressure. • For a fluid in motion additional forces arise due to shearing action and we refer to the normal force as a normal stress. The state of stresses in a fluid in motion is dealt with further in Fluid Mechanics. • In the context of thermodynamics, we think of pressure as absolute, with respect to pressure of a complete vacuum (space) which is zero. • In Fluid Mechanics we often use gage pressure and vacuum pressure.

44. Pressure • Absolute Pressure Force per unit area exerted by a fluid • Gage Pressure Pressure above atmospheric Pgag=Pabs - Patm • Vacuum Pressure Pressure below atmospheric Pvac=Patm - Pabs

45. Pressure • Common Pressure Units are: Pa (Pascal), mmHg (mm of Mercury), atm(atmosphere), psi (lbf per square inch) • 1 Pa = 1 N/m2(S.I. Unit) • 1 kPa =103Pa • 1 bar = 105Pa (note the bar is not an SI unit) • 1 MPa = 106Pa • 1 atm = 760 mmHg = 101,325 Pa = 14.696 psi

46. Variation of Pressure with Depth • The pressure of a fluid at rest increases with depth (as a result of added weight).

47. Variation of Pressure with Depth To obtain a relation for the variation of pressure with depth, consider a rectangular fluid element of height Δz, length Δx, and unit depth (into the page) in equilibrium. Assuming the density of the fluid ρ to be constant, a force balance in the vertical z-direction gives Where W=mg= ρgΔxΔz is the weight of the fluid element

48. Variation of Pressure with Depth If we take point 1 to be at the free surface of a liquid open to the atmosphere, where the pressure is the atmospheric pressure Patm, then the pressure at a depth h from the free surface becomes

49. Variation of Pressure with Depth A consequence of the pressure in a fluid remaining constant in the horizontal direction is that the pressure applied to a confined fluid increases the pressure throughout by the same amount. This is called Pascal’s law, after Blaise Pascal (1623–1662). The area ratio A2/A1is called the ideal mechanical advantage of the hydraulic lift.

50. The manometer The elevation change of Δz in a fluid at rest corresponds to ΔP/ρg, which suggests that a fluid column can be used to measure pressure differences. A device based on this principle is called a manometer, and it is commonly used to measure small and moderate pressure differences. A manometer mainly consists of a glass or plastic U-tube containing one or more fluids such as mercury, water, alcohol, or oil.