Physics
Heat Transfer
Heat transfer is the movement of thermal energy from a hotter object to a cooler one. It occurs through conduction, convection, or radiation. Conduction involves direct contact between materials, convection involves the movement of fluids, and radiation involves the transfer of energy through electromagnetic waves.
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eBook - PDFPhysics 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.
eBook - PDF- 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.
- Gerald Miller(Author)
- 2022(Publication Date)
- Springer(Publisher)
Thermal conduction is the process by which Heat Transfer occurs by molecular interaction. It can occur in gases, liquids and solids. An example is Heat Transfer across a closed window or across a wall. In the human body, the example of conductive Heat Transfer is across tissue from the core towards the periphery (skin). Thermal convection is the process by which Heat Transfer occurs via bulk motion of a fluid. An example might be forced air flow from an air conditioner vent or wind chill on a windy day. In the body, examples of convective Heat Transfer include blood flow from the core towards the periphery or air flow in the lungs from the alveoli though the trachea and out the mouth and nose. Both conduction and convection require that there be a material involved, although convection does not occur in solids, whereas conduction can. The third method of Heat Transfer is thermal radiation, which occurs as a result of electromagnetic transport processes. An example is heat gain from the sun to the earth (and your own skin). Radiation is a surface to surface phenomenon and does not require a material interaction. The greenhouse effect is an example of thermal radiation. 42 3. BIOMEDICAL HEAT TRANSPORT As an example, Figure 3.1 depicts the three types of Heat Transfer from a campfire. Conduction Radiation Convection Figure 3.1: The three types of Heat Transfer: conduction, convection and radiation (Wikipedia). 3.2 THERMAL CONDUCTION Conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and it acts to equalize temperature differences. Conduction takes place in all forms of matter including solids, liquids, and gases, but it does not require any bulk motion of matter. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.
eBook - PDF- Nelson Bolívar(Author)
- 2020(Publication Date)
- Arcler Press(Publisher)
Heat Transfer by Conduction, Convection, and Radiation 8 CONTENTS 8.1. Introduction .................................................................................... 222 8.2. Approaches Of Heat Transfer .......................................................... 223 8.3. Conduction ..................................................................................... 224 8.4. Convection ..................................................................................... 229 8.5. Biot Number ................................................................................... 233 8.6. Radiation ........................................................................................ 235 8.7. Combined Heat Transfer ................................................................. 239 References ............................................................................................. 241 Chapter General Physics 222 8.1. INTRODUCTION It is known that heat is the energy associated to the motion of molecules. Roughly, molecules having higher heat energy will move faster and molecules having less heat energy will move slower. It is also known that as the molecules heat up and start moving faster, the molecules spread apart, and the bodies expand. This is known as thermal expansion (Heijnen and Van’t Riet, 1984; Chang and You, 1997) (Figure 8.1). Figure 8.1. (a) Heat Transfer from the warmer object to cooler one. (b) The temperature of cooler object increases slowly. Heat is always considered moving. If two substances or objects are taken which are at different temperatures, the heat will move out of warmer substance or objects, and into the cooler substance or object. This transfer of heat will continue until the objects or substances are the same temperatures (Kou, 1996; Kalb and Seader, 1972). So how does exactly heat move out of the object and into another object? This is known as Heat Transfer.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
LEARNING OBJECTIVES After reading this module, you should be able to... 13.1 Define convection. 13.2 Solve conduction problems. 13.3 Solve radiation problems. 13.4 Analyze Heat Transfer applications. Radius Images/Getty Images CHAPTER 13 The Transfer of Heat Igloos are constructed from ice and snow to provide protection from wintery conditions. One reason that igloos do their job so well is that the ice and snow act as thermal insulation and minimize the loss of heat from the inside. The physical process called conduction plays the primary role in how thermal insulation works, and it is one of the three main processes by which heat is transferred from place to place. 13.1 Convection When heat is transferred to or from a substance, the internal energy of the sub- stance can change, as we saw in Chapter 12. This change in internal energy is accompanied by a change in temperature or a change in phase. The transfer of heat affects us in many ways. For instance, within our homes furnaces distribute heat on cold days, and air conditioners remove it on hot days. Our bodies con- stantly transfer heat in one direction or another, to prevent the adverse effects of hypo- and hyperthermia. And virtually all our energy originates in the sun and is transferred to us over a distance of 150 million kilometers through the void of space. Today’s sunlight provides the energy to drive photosynthesis in the plants that provide our food and, hence, metabolic energy. Ancient sunlight nurtured the organic matter that became the fossil fuels of oil, natural gas, and coal. This chapter examines the three processes by which heat is transferred: convection, conduction, and radiation. When part of a fluid is warmed, such as the air above a fire, the volume of that part of the fluid expands, and the density decreases. According to Archimedes’ principle (see Section 11.6), the surrounding cooler and denser fluid exerts a buoyant force on the warmer fluid and pushes it upward.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
394 CHAPTER 13 22 24 26 28 30 32 33.4°C 21.6°C LEARNING OBJECTIVES After reading this module, you should be able to... 13.1 Define convection. 13.2 Solve conduction problems. 13.3 Solve radiation problems. 13.4 Analyze Heat Transfer applications. All objects with a temperature above absolute zero (−273.15 °C) emit infrared or thermal radiation. The image above, known as a thermal image, or thermogram, is made with a special camera that is sensitive to this type of radiation. A colored scale represents the relative temperature of the objects in the image. Warmer objects, such as people, most animals, and other sources of heat, stand out in contrast to the cooler background and colder objects. The image above shows a snake wrapped around a human hand and wrist. The snake, being a coldblooded animal, matches its temperature to its environment and appears much cooler than the warm hand that is holding it. In this chapter, we will study radiation, as well as the other processes of Heat Transfer. The Transfer of Heat 13.1 Convection When heat is transferred to or from a substance, the internal energy of the substance can change, as we saw in Chapter 12. This change in inter- nal energy is accompanied by a change in temperature or a change in phase. The transfer of heat affects us in many ways. For instance, within our homes, furnaces distribute heat on cold days, and air conditioners remove it on hot days. Our bodies constantly transfer heat in one direc- tion or another, to prevent the adverse effects of hypo- and hyperther- mia. And virtually all our energy originates in the sun and is transferred to us over a distance of 150 million kilometers through the void of space. Today’s sunlight provides the energy to drive photosynthesis in the plants that provide our food and, hence, metabolic energy. Ancient sun- light nurtured the organic matter that became the fossil fuels of oil, nat- ural gas, and coal.
eBook - PDF- Richard W. Hartel, D.B. Hyslop, D.B. Hyslop, T.A. Howell Jr.(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
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. Heat Transfer by fluid motion is much more rapid than conduction, since fluid flow can be quite rapid. Especially in turbulent flow, heat is transferred from a hot source to colder environments quite readily. Convection can either be forced, where the fluid is pumped or circulated to promote Heat Transfer, or natural, where density variations with temperature result in fluid circulation currents. When fluid is pumped through a plate heat exchanger in pasteurization, forced convection occurs, and Heat Transfer is quite rapid. Fluid heating in an unstirred pot or kettle will undergo natural convection, where the warmer fluid near the heat source is less dense than the colder fluid farther away from the heat source. Fluid will flow from hot to cold due to the density difference, and this natural convection distributes the 154 Math concepts for food engineering heat throughout the container. Circulation in water being heated on a stove is caused by natural convection. The third mechanism of Heat Transfer is radiation, which occurs when a hot object radiates heat in the form of electromagnetic radiation. The electro-magnetic radiation emitted by a hot object is partially absorbed by a neigh-boring object, causing that body to heat up. Browning of toast in a toaster is accomplished in part through radiation Heat Transfer. Heat Transfer can occur under steady-state or unsteady-state conditions. In steady-state Heat Transfer, the temperature at any point in space is con-stant.
eBook - PDF- Jean-Maurice Vergnaud, Iosif-Daniel Rosca(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
21 2 General Study on Heat Transfer The laws of heat transmission are of controlling importance in the design of equip-ment and the operation of many diverse instruments in many different industries. This applies particularly to the rubber industry. 2.1 VARIOUS MEANS OF Heat Transfer When different parts of a body are at different temperatures, heat flows from the hotter parts to the cooler. There are three distinct ways by which this transference of heat takes place: (i) conduction, in which the heat passes through the substance of the body itself, (ii) convection, in which heat is transferred by relative motion of portions of the heated body, and (iii) radiation, in which heat is transferred directly between distant portions of the body by electromagnetic radiation. 2.1.1 H EAT C ONDUCTION Conduction in a homogeneous opaque solid is the transfer of heat from one particle to another, under the influence of a temperature gradient, without appreciable displace-ment of the particles. Conduction involves the transfer of kinetic energy from one molecule to an adjacent molecule; it is the only mechanism of heat flow in an opaque solid. With gases and liquids, conduction may be supplemented by convection and radiation; within a fluid (motionless or in streamline motion), heat is transferred by conduction at right angles to the direction of fluid flow. Thus, heat is transferred by conduction either through the rubber materials or through the mold. 2.1.2 H EAT C ONVECTION Convection involves the transfer of heat by mixing one parcel of fluid with another. The motion of the fluid may be entirely the result of differences of density caused by temperature differences, as in natural convection, or it may be produced by mechani-cal means, as in forced convection. Energy is also transferred simultaneously by molecular conduction and, in transparent media, by radiation.
eBook - PDFWeather
A Concise Introduction
- Gregory J. Hakim, Jérôme Patoux(Authors)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
CONTENTS 4.1 Conduction 51 4.2 Convection 52 4.3 Radiation 53 4.4 Radiative Interactions 55 4.5 Radiation and Weather 66 Summary 73 CONTENTS CHAPTER 4 Heat and Energy Transfer Energy enters the Earth system in the form of solar radiation, preferentially heating the ground and the tropical latitudes. The resulting temperature contrasts set the atmosphere in motion, as heat is transferred upward and poleward. Weather is largely the result of this transfer of heat by atmos- pheric motions. In this chapter, we will explore the nature of heat and radiation, the origin of temperature contrasts on Earth, the mechanisms by which energy is transferred, and the implications for weather. Although we might tend to think about weather in terms of wind and rain, weather is in fact largely the result of the redistribution of heat in the atmosphere. A heat imbalance, i.e., a disequilibrium in the distri- bution of heat between different regions of Earth, causes a transfer and redistribution of heat in the atmosphere. The resulting movement of air masses creates wind, storms, clouds, and rain. Therefore, if we are to understand the development of weather sys- tems, we need first to understand the source of the heat imbalance and the nature of Heat Transfer in the atmosphere. It is accepted in thermodynamics (the field of sci- ence that is concerned with the energy of systems) that heat does not normally flow from cold to warm objects (unless some other process is at work): it necessarily flows from warm to cold, i.e., in a direction that will bring the system to a state of equilibrium – technically, a state of lower potential energy. In the atmosphere, this transfer of heat can be achieved by three pro- cesses, conduction, convection, and radiation, which we will now explore in more detail. 4.1 Conduction Earlier we defined heat as the kinetic energy of atoms and molecules, or their energy of motion. In solid matter, motion is reduced to the vibrations of the atoms and molecules.
eBook - PDFWeather
A Concise Introduction
- Gregory J. Hakim, Jérôme Patoux(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
In the atmosphere, this transfer of heat can be achieved by three processes, conduction, convection, and radiation, which we will now explore in more detail. 4.1 Conduction Earlier we defined heat as the kinetic energy of atoms and molecules, or their energy of motion. In solid matter, motion is reduced to the vibrations of the atoms and molecules. In a liquid or a gas, molecules can move around and their kinetic energy describes both their vibrations and their displacement. When very energetic molecules (i.e., a hot substance, such as hot tea) are in contact with slower molecules (e.g., a cold spoon placed in the tea), the more energetic molecules necessarily impart some of their energy to the slower molecules when they bump into them (Figure 4.1). In the process, the slower molecules gain energy (i.e., the spoon warms up) while the faster molecules lose energy (i.e., the tea cools down). As a result, energy is redistributed more uniformly throughout the system. Following the same reasoning, the atoms in the bottom part of the spoon, being now more energetic than those in the top part of the spoon, will impart kinetic energy to the slower atoms by collision, in the form of faster vibrations, and heat will rise up the spoon. Note that the atoms in the spoon do not need to be displaced to transfer their heat up the spoon: they merely need to bump into adjacent atoms and share their extra energy of motion by contact. We refer to this transfer of heat by contact, collision, and transfer of molecular vibrations as conduction. By its very nature, conduction always transfers heat from warm to cold regions. Thus, when you touch a cold object, even though you might be under the impression that “cold- ness” is conducted into your hand, it is really your own heat that is conducted into the cold object. While conduction is very efficient in solids (in par- ticular in metals), that is not the case for fluids such as liquids and gases.
eBook - PDF- Carey Merritt(Author)
- 2022(Publication Date)
- Wiley(Publisher)
The equation below shows how thermal conduc- tivity is calculated and has units of btu/(h-F-ft) or W/(m-K). k Q L A T = × × ( ) / Δ The Table 3.2 below shows the variation of thermal conductivity with temperature for various common materials. CONDUCTION-TYPE Heat Transfer 19 Air has a particularly low thermal conductivity and this is why insulating mate- rials often have lots of air spaces. In addition to air, water, scale, dry steam, and glass all have relatively low thermal conductivities. It is worth noting that not all metals have equal thermal conductivities. This is important to consider for heat exchanger design and selection. Another way to consider thermal conductivity is to realize it is the inverse of resis- tivity (R-factor) multiplied by the material thickness. Therefore, thermal conduc- tivity = d/thermal resistivity or k = d × 1/R, where d is the material thickness and R is the resistivity. In a steam system, conduction Heat Transfer occurs mainly through the boiler pressure vessel and the steam/product wall boundary. Conduction also occurs to a lesser extent through the hot flue gases in the furnace area, the boiler water and through insulation of hot surfaces. Like radiation Heat Transfer, conduction is augmented by convection Heat Transfer. The first area where conduction takes place is in the boiler furnace area. In addition to radiant heat energy transfer from the burner flame, the hot burner flue gases will conduct heat energy through the gas molecules to the boiler pressure vessel wall as the flue gases travel through the boiler furnace section. When the hot flue gas molecules contact the metal boiler pressure vessel, their energy is transferred quickly through the vessel wall to either the boiler water or to an insulation barrier.
eBook - PDF- Albert Ibarz, Gustavo V. Barbosa-Canovas(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
413 18 Heat Transfer by Radiation 18.1 INTRODUCTION Energy transfer by radiation is basically different from other energy transfer phenomena because it is not proportional to a temperature gradient, nor does it need a material medium to propagate. In addition, Heat Transfer by radiation is simultaneous with convective transfer. Any molecule possesses translational, vibrational, rotational, and electronic energy, and all of those are done on quantum states, that is, discrete values of energy. Passing from one energy level to another implies an energy absorption or emission. Passing to a higher energy state implies energy absorption by a molecule; on the contrary, a molecule emits energy as radiation when passing to a lower energy level. Since the energy levels are quantized, the absorption or emission of energy is in the form of photons where the duality wave particle becomes relevant. Any body at a temperature higher than absolute zero can emit radiant energy, and the amount of energy emitted depends on the temperature of the body. As the temperature of a body increases, energy levels are excited first, followed by the electronic level changes. A tem-perature increase implies that the radiation spectrum moves to shorter wavelengths or are more energetic. The corpuscle theory states that radiant energy is transported by photons and also that it is a function of its frequency ν , according to the expression E = h ν (18.1) in which the proportionality constant h is the so-called Planck’s constant, whose value is h = 6.6262 × 10 −34 J s. The wave theory considers radiation as an electromagnetic wave, relating frequency to wave-length according to the following equation: ν λ = c (18.2) where λ is the wavelength of the radiation c is the value of light speed under vacuum (2.9979 × 10 8 m/s) So-called thermal radiation, which includes the ultraviolet, visible, and infrared spectra, corre-sponds to wavelengths of 10 −7 –10 −4 m.
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