Technology & Engineering
Heat
Heat is a form of energy that can be transferred between objects or systems. It is typically generated by the movement of atoms and molecules within a substance. In engineering, heat is a crucial factor in the design and operation of various technologies, such as engines, HVAC systems, and industrial processes.
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11 Key excerpts on "Heat"
eBook - PDFSolar 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.
eBook - PDFEngineering 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 (16601734) 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.
eBook - PDF- Mike Tooley, Lloyd Dingle(Authors)
- 2010(Publication Date)
- Routledge(Publisher)
Technically only conduction and radiation are true Heat transfer processes, because both of these depend totally and utterly on a temperature difference being present. Convection also depends on the transportation of a mechanical mass. Nevertheless, since convection also accomplishes transmission of energy from high to low temperature regions, it is conventionally regarded as a Heat transfer mechanism. Thermal conduction in solids and liquids seems to involve two processes, the first is concerned with atoms and molecules ( Figure 5.79), and the second with free electrons. A modern idea of Heat is that it is energy in transition and cannot be stored by matter. Energy may be defined as the capacity to do work; more accurately it may be defined as the capacity to produce an effect. These effects are apparent during the process of energy transfer. Heat (Q) may be defined as transient energy brought about by the interaction of bodies by virtue of their temperature difference when they communicate. Matter possesses stored energy but not transient (moving) energy such as Heat or work. Heat energy can only travel or transfer from a hot body to a cold body, it cannot travel up hill. Figure 5.78 illustrates this fact. When the temperature between the two bodies illustrated in Figure 5.78 are equal and no change of state is taking place (e.g. water to ice), then we say that they are in thermal equilibrium. KEY POINT Heat flow when measured in joules per second is in fact thermal power in watts KEY POINT Heat energy can only travel from a hot body to a cold body Heat energy transfer Body ‘B’ 10C Body ‘A’ 20C Hot to cold Figure 5.78 Zeroth law of thermodynamics states that if two bodies are each in thermal equilibrium with a third body, they must also be in thermal equilibrium with each other. Mechanical Principles and Applications 425 UNIT 5 Atoms at high temperatures vibrate more vigorously about their equilibrium positions than their cooler neighbours.
eBook - PDF- Ethirajan Rathakrishnan(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
Chapter 1 Basic Concepts and Definitions 1.1 Introduction Heat transfer is the science of energy transfer due to a temperature differ-ence. We know that thermodynamics deals with energy balance in a variety of physical situations. In other words, thermodynamics deals with the amount of Heat transfer as a system undergoes a process from one equilibrium state to another. Heat transfer, on the other hand, deals with the rate at which Heat is transferred as well as the temperature distribution within the system as a function of time. Basically thermodynamics deals with systems in equi-librium. It may be used to predict the amount of thermal energy required to change a system from one thermal equilibrium state to another. But it can-not predict how fast this change from one equilibrium state to another will take place, because the system is not in equilibrium during the process. For example, consider the cooling of a hot metallic bar placed in a water bath. Thermodynamics may be used to predict the final equilibrium temperature of the metallic bar-water combination. But thermodynamics cannot answer the question, how long the process would take to reach this equilibrium or what would be the temperature of the bar after a certain time interval before the equilibrium is attained? Whereas, the science of Heat transfer can answer these questions. 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
eBook - PDFHeat Transfer Basics
A Concise Approach to Problem Solving
- Jamil Ghojel(Author)
- 2023(Publication Date)
- Wiley(Publisher)
1 Heat Transfer Basics: A Concise Approach to Problem Solving, First Edition. Jamil Ghojel. © 2024 John Wiley & Sons, Inc. Published 2024 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/ghojel/Heat_transfer 1 Basic Concepts of Heat and Mass Transfer 1.1 Heat Transfer and Its Relationship With Thermodynamics Heat transfer can be looked at as a branch of classical thermodynamics with specific distinguish- ing features. This can best be explained by considering the processes taking place in the cylinder of the internal combustion engine schematically shown in Figure 1.1a. The thermodynamic processes taking place in the cylinder are elegantly and succinctly stated by the first law of thermodynamics (principle of conservation of energy) in the form − = ∆ Q W U (1.1) where ● Q is the Heat generated by the combustion of the injected fuel in the cylinder at the end of the compression process, resulting in high-pressure, high temperature gaseous products ● W is the work done by the piston as the combustion products expand with the piston moving downwards ● ∆U is the balance of the Heat left from the combustion process to raise the internal energy and temperature of the gases inside the cylinder. The first law of thermodynamics states that energy can be converted (transformed) from one form (Heat) to another (work) but cannot be created or destroyed and that the process takes place between two states at equilibrium. The first law does not provide information on the amount of energy that can be transformed, energy transformation direction, and its effectiveness. This gap is filled by the second law of thermodynamics which stipulates that only part of the Heat will be con- verted to work, Heat will flow spontaneously from a high temperature source to a lower tempera- ture sink, and the flow will be accompanied by irreversible changes that degrade the resultant energy making it difficult to be utilized further.
eBook - PDF- 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.
If you have ever tried to warm a house during a cold winter day, you have practical experience of this fact. If you turn off the furnace, the energy in the house gradually leaks away to the outside, and the house begins to get cold. The only way you can keep the house warm is to keep adding more thermal energy. Similarly, our bodies constantly produce energy to maintain our Figure 4-1 • The second law of thermodynamics tells us that it is easier to tear something down than to build it. This law is shown dramatically in this sequence of photos showing the demolition of an apartment building. Paulo Carrico/EPA/NewsCom 80 | CHAPTER 4 | Heat and the Second Law of Thermodynamics core body temperature close to 98.6 degrees Fahrenheit (37°C). Both your furnace and your body produce energy on the inside—energy that will inevitably flow to the outside as Heat. You use that Heat on the fly, as it were. In order to understand the nature of Heat and its movement, we need to define three closely related terms: Heat, temperature, and specific Heat capacity. Heat AND TEMPERATURE In everyday conversation we often use the words temperature and Heat interchangeably, but to scientists the words have different meanings. Heat is a form of energy that moves from a warmer object to a cooler object; Heat is therefore energy in motion. Any object that is hot can transfer its internal energy to the surroundings as Heat. A gallon of boiling water contains more internal energy than a pint of boiling water and it can thus transfer more Heat to its surroundings. Heat is often measured in calories, a common unit of energy defined as the amount of Heat required to raise 1 gram of room-temperature water by 1° Celsius in temperature.
eBook - PDFEngineering Fundamentals
An Introduction to Engineering, SI Edition
- Saeed Moaveni(Author)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
You will also learn how to estimate Heat transfer rates for vari- ous situations, including the cooling of electronic devices and the design of fins Copyright 2020 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. CHAPTER 11 Temperature and Temperature-Related Variables in Engineering 396 for transformers or motorcycle and lawn mower engine heads and other Heat exchangers, like the radiator in your car or the Heat exchangers in furnaces and boilers. The intent of this section was to briefly introduce you to the concept of Heat transfer and its various modes. Answer the following questions to test your understanding of the preceding sections. 1. Explain what is meant by temperature and give examples of its important role in engineer- ing analysis and design. 2. Explain why an absolute temperature is defined and give its SI and U.S. Customary units. 3. Give examples of how temperature is measured. 4. What causes Heat transfer? 5. What are the modes of Heat transfer? Vocabulary—State the meaning of the following terms: Absolute Temperature Thermocouple Wire Conduction Convection Radiation R-Value Before You Go On LO 3 11.3 Thermal Comfort Human thermal comfort is of special importance to bioengineers and mechan- ical engineers. For example, mechanical engineers design the Heating, ven- tilating, and air-conditioning (HVAC) systems for homes, public buildings, hospitals, and manufacturing facilities. When sizing the HVAC systems, the engineer must design not only for the buildings’ Heat losses or gains but also for an environment that occupants feel comfortable within.
- Arun Datta(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
141 © 2010 Taylor & Francis Group, LLC chapter four Heat transfer Introduction Heat transfer is an important unit operation that is used in almost all chemical industries. In industries, Heat is either gained or lost by a typical fluid stream. Once a particular fluid loses Heat, there will be a stream that will gain Heat. There are different types of Heat-transfer equipment or Heat exchangers, starting from a simple double-pipe exchanger to the highly complex multi-pass shell and tube exchanger. Conductive Heat transfer Heat transfer per unit area through conduction is proportional to the tem-perature gradient and mathematically defined as [1]: Q kA dt dx = - (4.1) The temperature gradient − dt / dx is the change in temperature in the x direction. Consider a Heat conduction element in Figure 4.1 that receives Q x Heat through the left face and rejects Q x+dx Heat through the right face. Heat received and rejected by the element can be defined as Q k dz dy dT dx x = - (4.2a) Q k dz dy dT dx d T dx dx x dx + = -- 2 2 (4.2b) Heat gained by the element through the x direction will be Q Q k dz dy d T dx dx x x dx -= + 2 2 (4.3) 142 Process engineering and design using visual basic ® © 2010 Taylor & Francis Group, LLC Now, Heat gained by the element can also be defined as Q Q c dx dy dz dT d x x dx -= + 1 3 6 . ρ θ (4.4) From Equations 4.3 and 4.4 dT d k c d T dx θ ρ = 3 6 2 2 . (4.5) Equation 4.5 is Fourier’s general equation. The term 3.6 k / c ρ is called the thermal diffusivity, m 2 /h. Multiplication factor 3.6 is used to balance the unit. Equation 4.5 is the Heat flow through one direction and if all three directions are considered, the general equation of Heat flow will be dT d k c d T dx d T dy d T dz θ ρ = + + 3 6 2 2 2 2 2 2 . (4.6) Heat conduction through a composite wall Heat conduction through a composite wall is presented in Figure 4.2.
- Jonathan Wickert, Kemper Lewis, Jonathan Wickert(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
This temperature rise is an upper bound because we assumed that all the kinetic energy is converted to Heat and not lost in other forms. T 5 82.69 8C Example 7.9 | continued ▸ ▸ 7.5 Heat Engines and Efficiency O ne of the most important functions of engineering is developing machines that produce mechanical work by burning a fuel. At the simplest level, a fuel such as natural gas can be burned, and the Heat released can be used to warm a building. Of equal practical importance is the need to take the next step and produce useful work from that Heat. Mechanical engineers are concerned with the efficiency of machines in which a fuel is burned, thermal energy is released, and Heat is converted into work. By increasing the efficiency of that process, the fuel economy of an automobile can be raised, its power output can be increased, and the weight of its engine can be reduced. In this section, we will discuss the concepts of real and ideal efficiencies as applied to energy conversion and power generation. The Heat engine sketched schematically in Figure 7.11 represents any machine that is capable of converting the Heat supplied to it into mechanical work. As the input, the engine absorbs a quantity of Heat Q h from the high-temperature energy source, which is maintained at the temperature T h . A portion of Q h can be converted into mechanical work W by the engine. The remainder of the Heat, however, is Heat engine Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 7.5 Heat Engines and Efficiency 299 exhausted from the engine as a waste product.
- S. Bobby Rauf(Author)
- 2021(Publication Date)
- River Publishers(Publisher)
Temperature Temperature can be defned as a measure of the average kinetic en-ergy of the particles in a substance, where such energy is directly propor-tional to the degree of hotness or coldness of the substance. While temperature is one of the principal parameters of thermody-namics, it must be clear that temperature is not a direct measurement of Heat, Q. Temperature is, however, is a parameter that is instrumental in determining the direction of fow of Heat, Q. In that, Heat travels from bodies at higher temperature to bodies at lower temperature. This role of temperature comports with the laws of thermodynamics. From physics perspective, temperature is an indicator of the level of kinetic energy possessed by atoms and molecules in substances. In solids , at higher temperature, the atoms oscillate or vibrate at higher frequency. In atomic gases , the atoms, at higher temperatures, tend to exhibit faster translational movement. In molecular gases, the mole-cules, at higher temperatures, tend to exhibit higher rates of vibrational 5 Introduction to Energy, Heat, and Thermodynamics Table 1-1. Units for Pressure and Associated Conversion Factors and rotational movement. Even though, for a system in thermal equilibrium at a constant vol-ume, temperature is thermodynamically defned in terms of its energy ( E ) and entropy ( S ), as shown in Eq. 1-4 , unlike pressure, temperature is not commonly recognized as a derivative entity and, therefore, the units for temperature are not derived from the units of other independent entities. ∂ E T ≡ —— Eq. 1-4 ∂ S The universal symbol for temperature is: T. The unit for tempera-ture, in the SI, or metric, realm is °C or degrees Celsius. In the Celsius tem-perature scale system, 0°C represents the freezing point of water. The unit for temperature, in the US, or imperial, realm is °F or degrees Fahrenheit . On the Fahrenheit temperature scale system, 32°F or degrees Fahrenheit represents the freezing point of water.
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