Principles of Heat Transfer

10 Key excerpts on "Principles of Heat Transfer"

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  Physics of Thermal Therapy

  Fundamentals and Clinical Applications

  • Eduardo Moros(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  3 1.1 Introduction The science of heat transfer deals with the movement of ther-mal energy across a defined space under the action of a tem-perature gradient. Accordingly, a foundational consideration in understanding a heat transfer process is that it must obey the law of conservation of energy, or the first law of thermody-namics . Likewise, the process must also obey the second law of thermodynamics, which, for most practical applications, means that heat will flow only from a region of higher tempera-ture to one of lower temperature. We make direct and repeated use of thermodynamics in the study of heat transfer phenom-ena, although thermodynamics does not embody the tools to tell us the details of how heat flows across a spatial temperature gradient. A more complete analysis of heat transfer depends on fur-ther information about the mechanisms by which energy is driven from a higher to a lower temperature. Long experi-ence has shown us that there are three primary mechanisms of action: conduction , convection , and radiation . The study of heat transfer involves developing a quantitative representa-tion for each of the mechanisms that can be applied in the context of the conservation of energy in order to reach an overall description of how the movement of heat by all of the relevant mechanisms influences changes in the thermal state of a system. Biological systems have special features beyond inanimate systems that must be incorporated in the expressions for the heat transfer mechanisms. Many of these features result in effects that cause mathematical nonlinearities and render the analytical description of bioheat transfer more complex than more routine problems. For that reason, you will find numerical methods applied for the solution of many bioheat transfer prob-lems, including a large number in this book.

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  Heating Services Design

  • Ronald K. McLaughlin, R. Craig McLean, W. John Bonthron(Authors)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The Fundamentals of Heat and Mass Transfer and Thermodynamics 2.1 INTRODUCTION To the services engineer dealing with thermal environmental problems a thorough understanding of the basic principles of heat and mass transfer and elementary thermodynamics is of vital importance. This chapter is intended as a suitable introduction to the fundamental theory of these subjects. The application of this theory to practical design problems is covered extensively in later chapters. 2.2 CONDUCTION 2.2.1 Nature of conduction Conduction is the term given to the process within a medium whereby heat is transmitted from a region of higher temperature to a region of lower temperature, without appreciable displace-ment of the particles of the medium. Conduction involves the direct transfer of kinetic energy at a molecular level and may have several distinct operating mechanisms associated with it, e.g. the elastic collision of molecules in a fluid medium or the motion of free electrons in metals. Regardless of the exact mech-anism, a knowledge of which is unimportant anyway when con-duction is considered as an engineering problem, the observable effect of the transfer between neighbouring regions will be the elimination of the temperature difference between them. Alter-natively, if the temperatures of these regions are maintained constant by the addition and removal of heat at appropriate points, the result will be a continuous flow of heat from the higher temperature region to the lower temperature one. Conduction is the only mode of heat transfer which occurs in opaque solids. It is also an important process in fluids but in these 39 2 40 The Fundamentals of Heat and Mass Transfer and Thermodynamics media it is usually combined with convection, and in some cases with radiation also. Conduction is the dominant heat transfer mechanism in fluid regions where laminar flow conditions exist, e.g. in the laminar sub-layer immediately adjacent to a solid boundary.

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  Solar Energy

  Renewable Energy and the Environment

  • Robert Foster, Majid Ghassemi, Alma Cota(Authors)
  • 2009(Publication Date)
  • CRC Press

    (Publisher)

  55 3 Fundamentals of Engineering Thermodynamics and Heat Transfer 3.1 INTRODUCTION This chapter provides an introduction to heat transfer and engineering thermodynamics. The sci-ence of thermodynamics deals with energy interaction between a system and its surroundings. These interactions are called heat transfer and work. Thermodynamics deals with the amount of heat transfer between two equilibrium states and makes no reference to how long the process will take. However, in heat transfer, we are often interested in rate of heat transfer. Heat transfer processes set limits to the performance of environmental components and systems. The content of this chapter is intended to extend the thermodynamics analysis by describing the different modes of heat transfer. It also provides basic tools to enable the readers to estimate the magnitude of heat transfer rates and rate of entropy destruction in realistic environmental applications, such as solar energy systems. The transfer of heat is always from the higher temperature medium to the lower temperature medium. Therefore, a temperature difference is required for heat transfer to take place. Heat trans-fer processes are classified into three types: conduction, convection, and radiation. Conduction heat transfer is the transfer of heat through matter (i.e., solids, liquids, or gases) with-out bulk motion of the matter. In other words, conduction is the transfer of energy from the more energetic to less energetic particles of a substance due to interaction between them. This type of heat conduction can occur, for example, through the wall of a boiler in a power plant. The inside surface, which is exposed to gases or water, is at a higher temperature than the outside surface, which has cooling air next to it. The level of the wall temperature is critical for a boiler. Convection heat transfer is due to a moving fluid. The fluid can be a gas or a liquid; both have applications in an environmental process.

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  Heat Transfer Basics

  A Concise Approach to Problem Solving

  • Jamil Ghojel(Author)
  • 2023(Publication Date)
  • Wiley

    (Publisher)

  1.2 Heat Conduction 3 proper analysis requires the use of tools that are provided by other branches of science, such as fluid dynamics, physics, and mathematics. In heat transfer, the first law of thermodynamics is widely used, albeit without the work compo- nent and is generally known as the principle of conservation of energy (in short, energy or heat balance). Applied to the control volume in Figure 1.1, the heat balance can be expressed as Rate of heat transfer into the control volume, Q in  = Rate of heat removed by the cooling water, Q cool  + Rate of heat loss to the surrounding media,  Q out = + Q Q Q in cool out    (1.2) More generally, taking into account that heat transfer processes may be accompanied by internal energy generation Q g  and/or energy storage (accumulation) within the control volume Q s  , the energy balance equation for a control volume can be written as − + = Q Q Q Q in out g s     (1.3) Internal heat generation could be due to electrical heating, chemical processes, or nuclear reactions. The field of applications of heat transfer is wide and diverse covering, for example, energy genera- tion, transport, process industry, building industry, agriculture, medical industry, and aviation and space industries. In the following sections, the three modes of heat transfer will be discussed sepa- rately but it should be kept in mind that they continually interact and rarely occur independently. 1.2 Heat Conduction Consider a solid layer or static liquid layer contained between two long vertical plates at two dif- ferent temperatures T 1 and T 2 (Figure 1.2). The molecules of the medium between the plates are depicted as small circles, with the molecules at higher temperature in black. The molecules exist in continuous agitated random motion indicated by the small arrows, with the degree of agitation being directly proportional to the temperature.

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  Introduction to Thermal and Fluid Engineering

  • Allan D. Kraus, James R. Welty, Abdul Aziz(Authors)
  • 2011(Publication Date)
  • CRC Press

    (Publisher)

  19 Introduction to Heat Transfer Chapter Objectives • To introduce and describe the three modes of heat transfer: conduction, convection, and radiation. • To define thermal conductivity, a heat transfer property. • To develop the concept of thermal resistance. • To consider the overall heat transfer coefficient. 19.1 Introduction The remaining eight chapters of our study deal with heat transfer. The quantity, heat, was defined in earlier chapters and it was quantitatively related to work and system behavior through the first law of thermodynamics in Chapter 5. The first law remains a fundamental precept as we now devote our attention to the rate of heat exchange. As discussed earlier, heat will be exchanged between systems when they are at differ-ent temperatures. The second law of thermodynamics stipulates that this exchange will be from the higher-temperature body or system, toward the bodies or systems at lower temperatures. Practical considerations regarding heat transfer processes involve the rate at which heat transfer occurs. All decisions that involve equipment and material specifications require that heat transfer rates and process temperatures be determined. Our goal, in this chapter, is to examine the mechanisms of heat transfer and to introduce the equations that are fundamental to the evaluation of rates at which energy transfer, due to temperature difference, occurs. 19.2 Conduction Energy transfer by conduction is accomplished via two mechanisms. The first mechanism is by molecular interaction, in which the greater motion of a molecule at a higher-energy (temperature) level imparts energy to adjacent molecules at lower-energy levels. This type of energy transfer is present, to some degree, in all systems in which a temperature gradient exists and in which molecules of a solid, liquid, or gas are present. The second mechanism of conduction heat transfer is by “free” electrons.

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  Engineering Problem Solving

  A Classical Perspective

  • Milton C. Shaw(Author)
  • 2001(Publication Date)
  • William Andrew

    (Publisher)

  Thermal Engineering 269 269 1.0 INTRODUCTION Important topics to be considered in this chapter are thermodynamics, thermal transformation systems, and heat transfer. Thermodynamics in-volves fundamental relationships between heat, work, and the properties of a system. It is concerned with the transformation of one form of energy into another and the basic laws that control such transformation. Of particular importance is the transformation of thermal energy into mechani-cal energy, which is the first step in the conversion of the energy associated with fossil fuels into electrical energy as discussed in Ch. 10. Thermal transformation systems are systems that transform thermal energy into mechanical energy. This includes steam power plants, steam engines, steam turbines, gas turbines, and internal combustion engines. Heat trans-fer is concerned with the transfer of thermal energy from one medium to another by: • Radiation • Conduction • Convection 11 Thermal Engineering 270 Engineering Problem Solving: A Classical Perspective With radiation, heat is transferred by electromagnetic waves ema-nating from a hot body to a cold body where radiation waves are absorbed resulting in a temperature rise. Conductive heat transfer involves the passage of heat through a solid from a region of high temperature to one of lower temperature. Convective heat transfer involves the transport of thermal energy from a hot body to a fluid flowing across the hot body. 2.0 HISTORICAL BACKGROUND Before the 17 th century, little attention was given to thermal energy. The Phlogiston Theory of heat championed by Stahl (1660–1734) was the first generally accepted. This proposed that all combustible materials contain a massless material (phlogiston) that escapes on combustion. Some materials like sulfur were considered rich in phlogiston while others con-tained very little.

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  Therapeutic Hypothermia

  • Stephan A. Mayer, Daniel I. Sessler, Stephan A. Mayer, Daniel I. Sessler(Authors)
  • 2004(Publication Date)
  • CRC Press

    (Publisher)

  9 Thermodynamics and Heat Transfer THOMAS L. MERRILL Biomedical Engineering Department, Wyeth Research, Princeton, New Jersey, U.S.A. BACKGROUND Thermodynamics and heat transfer provide the language and the concepts to understand the fundamental nature of temper- ature and how it can be controlled (1–5). For scientists and engineers, these concepts are necessary tools to analyze and develop therapeutic temperature modulation technologies. Examples of temperature control are ubiquitous, from the ice cream freezers in your grocery store to the comfortable pas- senger cabins inside jet airliners. Stripped to its core, thermodynamics studies energy from four perspectives: generation, storage, transfer, and dissipa- tion. More specifically, the first law of thermodynamics, a 265 natural law, provides the starting point for all thermal prob- lems. From a first law perspective, the problem of jet airliner cabin temperature control is no different than patient temper- ature control. Thermodynamics, however, inherently deals with equi- librium states—states without temperature gradients. For example, although the first law can tell you the rate or the amount of heat transfer needed to maintain comfortable living conditions in a building, it is silent on what insulation should be selected. The science of heat transfer provides an essential missing link—linking the rate of heat transfer to temperature gra- dients and material properties. For example, consider the heating load for a home. Two important temperatures are needed: the desired indoor temperature and the range of outdoor temperatures. Taking these temperatures and the building design and materials into consideration, heat transfer concepts enable us to calculate the expected heat losses and the resultant internal heat generation needed for adequate tem- perature control. These loss calculations are based on four different modes of heat transfer: conduction, convection, radi- ation, and advection.

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  Elements of Heat Transfer

  • Ethirajan Rathakrishnan(Author)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  That is, heat transfer can predict the temperature of both the metal bar and the water as a function of time. In other words, unlike ther-modynamics, heat transfer can answer the transient energy transfer questions such as the following: • Can heat be supplied to a system without employing high temperature difference? • How long would it take to supply a certain amount of heat energy to the 1 2 Basic Concepts and Definitions system? • How much heat energy is transferred between two specified instances of time during a process? • What sort of temperature distribution exists in the system? • How large an area is necessary to transfer the desired heat energy? Heat transfer finds application in all processes involving energy transfer. In this introductory text we will discuss heat transfer briefly, highlighting the basic principles of the subject. Basically there are three modes of heat transfer: conduction, convection and radiation. • Conduction is an energy transfer process from more energetic particles of a substance to the adjacent, less energetic ones as a result of the interaction between the particles. • Convection is the mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion. • Radiation is a heat transfer mode in which the energy is 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, dictated by their temperature.

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  Math Concepts for Food Engineering

  • Richard W. Hartel, D.B. Hyslop, D.B. Hyslop, T.A. Howell Jr.(Authors)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  153 chapter ten Heat transfer Heat transfer is arguably the most important transport process in the food industry, since processing of most foods requires either an increase or a decrease in temperature. Thermal processing, or heating to destroy unde-sirable microorganisms, is one of the main techniques a food technologist utilizes to ensure safety of the food supply. Sterilization and pasteurization require that a food be heated to a certain temperature and held there for suf-ficient time to destroy microorganisms before being cooled again. Both the heating and cooling steps require an understanding of heat-transfer prin-ciples. There are numerous other examples of heat transfer in the food indus-try, from baking a food in an oven, to solidification of chocolate in a cooling tunnel. Nearly every food processing system utilizes heat transfer in some form or another. Thus, knowledge of the Principles of Heat Transfer is critical for the food technologist. Furthermore, using the Principles of Heat Transfer together with enthalpy balances provides a powerful tool for the food tech-nologist to solve complex food-processing problems. Heat transfer occurs in one of three modes: conduction, convection, or radiation. Sometimes all three modes of heat transfer may occur at the same time. Conduction heat transfer involves transfer of thermal energy from one molecule to another. A metal rod placed in proximity to a heat source will begin initially to heat up on the end near the heat source. However, after some time, the other end will also be hot, as the molecules of the metal trans-fer the heat along the length of the rod. Materials that are good heat conduc-tors have high thermal conductivity, k ; whereas insulators, materials with low k , do not transfer heat very well by conduction. Convection heat transfer occurs when a fluid carries thermal energy from one place to another.

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  Extraction Techniques for Food Processing

  • Reddy, Y S(Authors)
  • 2021(Publication Date)
  • Genetech

    (Publisher)

  11 Heat Transfer Theory Heat transfer is an operation that occurs repeatedly in the food industry. Whether it is called cooking, baking, drying, sterilising or freezing, heat transfer is part of the processing of almost every food. An understanding of the principles that govern heat transfer is essential to an understanding of food processing. Heat transfer is a dynamic process in which heat is transferred spontaneously from one body to another cooler body. The rate of heat transfer depends upon the differences in temperature between the bodies, the greater the difference in temperature, the greater the rate of heat transfer. Temperature difference between the source of heat and the receiver of heat is therefore the driving force in heat transfer. An increase in the temperature difference, increases the driving force and therefore increases the rate of heat transfer. The heat passing from one body to another travels through some medium which in general offers resistance to the heat flow. Both these factors, the temperature difference and the resistance to heat flow, affect the rate of heat transfer. As with other rate processes, these factors are connected by the general equation: Rate of Transfer = Driving Force/Resistance For heat transfer: Rate of Heat Transfer = Temperature Difference/ Heat Flow Resistance of Medium During processing, temperatures may change and therefore the rate of heat This ebook is exclusively for this university only. Cannot be resold/distributed. transfer will change. This is called unsteady state heat transfer, in contrast to steady state heat transfer when the temperatures do not change. An example of unsteady state heat transfer is the heating and cooling of cans in a retort to sterilize the contents. Unsteady state heat transfer is more complex since an additional variable, time, enters into the rate equations. Heat can be transferred in three ways: by conduction, by radiation and by convection.

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