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Thermodynamics in Engineering: A Lecture Overview, Lecture notes of Applied Thermodynamics

Pakistan Institute of Engineering and Applied Sciences, Islamabad (PIEAS)Applied Thermodynamics

Applied Thermodynamics Lecture

Typology: Lecture notes

2018/2019

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Chapter 2
ENERGY, ENERGY TRANSFER,
AND GENERAL ENERGY
ANALYSIS
Muhammad Ahmad Jamil
KFUEIT
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thermodynamics: An Engineering Approach
Seventh Edition in SI Units
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2011
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Chapter 2

ENERGY, ENERGY TRANSFER,

AND GENERAL ENERGY

ANALYSIS

Muhammad Ahmad Jamil KFUEIT Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Thermodynamics: An Engineering Approach

Seventh Edition in SI Units Yunus A. Cengel, Michael A. Boles McGraw-Hill, 20 11

Objectives

  • Introduce the concept of energy and define its various forms.
  • Discuss the nature of internal energy.
  • Define the concept of heat and the terminology associated with energy transfer by heat.
  • Discuss the three mechanisms of heat transfer: conduction, convection, and radiation.
  • Define the concept of work, including electrical work and several forms of mechanical work.
  • Introduce the first law of thermodynamics, energy balances, and mechanisms of energy transfer to or from a system.
  • Determine that a fluid flowing across a control surface of a control volume carries energy across the control surface in addition to any energy transfer across the control surface that may be in the form of heat and/or work.
  • Define energy conversion efficiencies.
  • Discuss the implications of energy conversion on the environment.

FORMS OF ENERGY .

  • Microscopic forms of energy : Those related to the molecular structure of a system and the degree of the molecular activity.
  • Internal energy, U : The sum of all the microscopic forms of energy.
  • The magnetic, electric, and surface tension effects are significant in some specialized cases only and are usually ignored. In the absence of such effects, the total energy of a system consists of the kinetic, potential, and internal energies and is expressed as

Mass flow rate Energy flow rate ENERGY IN CNTROL MASS OR CONTROL VOLUME Most closed systems remain stationary during a process and thus experience no change in their kinetic and potential energies. Closed systems whose velocity and elevation of the center of gravity remain constant during a process are frequently referred to as stationary systems. The change in the total energy E of a stationary system is identical to the change in its internal energy U. In this text, a closed system is assumed to be stationary unless stated otherwise. Control volumes typically involve fluid flow for long periods of time, and it is convenient to express the energy flow associated with a fluid stream in the rate form. This is done by incorporating the mass flow rate m which is the amount of mass flowing through a cross section per unit time. It is related to the volume flow rate V which is the volume of a fluid flowing through a cross section per unit time, by

  • The total energy of a system, can be contained or stored in a system, and thus can be viewed as the static forms of energy.
  • The forms of energy not stored in a system can be viewed as the dynamic forms of energy or as energy interactions.
  • The dynamic forms of energy are recognized at the system boundary as they cross it, and they represent the energy gained or lost by a system during a process.
  • The only two forms of energy interactions associated with a closed system are heat transfer and work.
  • The difference between heat transfer and work: An energy interaction is heat transfer if its driving force is a temperature difference. Otherwise it is work.

Mechanical Energy Mechanical energy: The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine. Mechanical energy of a flowing fluid per unit mass Rate of mechanical energy of a flowing fluid Many engineering systems are designed to transport a fluid from one location to another at a specified flow rate, velocity, and elevation difference, and the system may generate mechanical work in a turbine or it may consume mechanical work in a pump or fan during this process. These systems do not involve the conversion of nuclear, chemical, or thermal energy to mechanical energy. Also, they do not involve any heat transfer in any significant amount, and they operate essentially at constant temperature. Such systems can be analyzed conveniently by considering the mechanical forms of energy only and the frictional effects that cause the mechanical energy to be lost (i.e., to be converted to thermal energy that usually cannot be used for any useful purpose).

ENERGY TRANSFER BY HEAT Heat : The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

During an adiabatic process, a system exchanges no heat with its surroundings.

A process during which there is no heat

transfer is called an adiabatic process (Fig.

2 – 14 ). The word adiabatic comes from the

Greek word adia batos, which means not to

be passed. There are two ways a process can

be adiabatic: Either the system is well

insulated so that only a negligible amount of

heat can pass through the boundary, or both

the system and the surroundings are at the

same temperature and therefore there is no

driving force (temperature difference) for

heat transfer. An adiabatic process should not

be confused with an isothermal process.

Even though there is no heat transfer during

an adiabatic process, the energy content and

thus the temperature of a system can still be

changed by other means such as work.

ADIABATIC PROCESS

13 Historical Background on Heat

  • Kinetic theory : Treats molecules as tiny balls that are in motion and thus possess kinetic energy.
  • Heat : The energy associated with the random motion of atoms and molecules. Heat transfer mechanisms:
  • Conduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.
  • Convection: The transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.
  • Radiation: The transfer of energy due to the emission of electromagnetic waves (or photons).

14 ENERGY TRANSFER BY WORK

  • Work : The energy transfer associated with a force acting through a distance. ✓ A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions
  • Formal sign convention : Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative.
  • Alternative to sign convention is to use the subscripts in and out to indicate direction. This is the primary approach in this text. Specifying the directions of heat and work. Work done per unit mass Power is the work done per unit time (kW)

MECHANICAL FORMS OF WORK

  • There are two requirements for a work interaction between a system and its surroundings to exist: ✓ there must be a force acting on the boundary. ✓ the boundary must move. Work = Force  Distance When force is not constant

Shaft Work A force F acting through a moment arm r generates a torque T This force acts through a distance s The power transmitted through the shaft is the shaft work done per unit time Shaft work

Work Done on Elastic Solid Bars Solids are often modeled as linear springs because under the action of a force they contract or elongate, as shown when the force is lifted, they return to their original lengths, like a spring. This is true as long as the force is in the elastic range, that is, not large enough to cause permanent (plastic) deformations. Therefore, the equations given for a linear spring can also be used for elastic solid bars. The work associated with the expansion or contraction of an elastic solid bar by replacing pressure P by its counterpart in solids, normal stress. in the work expression:

Work Associated with the Stretching of a Liquid Film Consider a liquid film such as soap film suspended on a wire frame (Fig. 2 – 33 ). We know from experience that it will take some force to stretch this film by the movable portion of the wire frame. This force is used to overcome the microscopic forces between molecules at the liquid–air interfaces. These microscopic forces are perpendicular to any line in the surface, and the force generated by these forces per unit length is called the surface tension , whose unit is N/m. Therefore, the work associated with the stretching of a film is also called surface tension work. It is determined from