Combined Convection and Radiation

12 Key excerpts on "Combined Convection and Radiation"

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  eBook - ePub

  Thermal Radiation

  An Introduction

  • John R. Howell, M. Pinar Mengüc, Kyle J. Daun(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  In the preceding chapters, an enclosure theory was formulated to find radiative exchange among surfaces. The local net-radiation loss at a surface was balanced by energy supplied by “some other means” that were not explicitly described. This chapter considers this energy balance at the surface as affected either by conduction from within the volume interior to the surface (such as from within a wall of an enclosure) or by convection or conduction at the surface from a surrounding medium. At each location along the surface, the radiation, convection, and conduction need to be combined to set the required boundary conditions. The solution of the energy equation subject to these boundary conditions supplies the surface temperature and energy flux distributions. The radiation analysis has the same restrictions as before: we assume the surfaces are opaque, and the medium between the radiating surfaces is transparent. The medium between the radiating surfaces may be conducting or convecting energy, but it does not interact with radiation.

  One example of combined-mode energy transfer is a space radiator that is part of a vapor-cycle power plant operating in outer space. Waste energy must be rejected by radiation transfer to outer space. In a space radiator (see Figure 7.1a ), the vapor of the working fluid in a thermodynamic cycle is condensed, releasing its latent heat. This energy is conducted through the condenser wall and into fins that radiate the energy into outer space. The temperature distribution in the fins and their radiating efficiency depend on combined radiation and conduction energy transfer.

  FIGURE 7.1 Energy transfer devices involving combined radiation, conduction, and convection: (a) space radiator or absorber plate of flat-plate solar collector, (b) steel-strip cooler, and (c) nuclear rocket.

  A fin-tube geometry is commonly used for the absorber in a flat-plate solar collector. Solar energy is incident on the absorber plate through one or more transparent cover glasses that reduce convective losses to the atmosphere. A fluid is heated as it flows through tubes attached to the absorber plate. The collector design requires analysis of radiation, conduction, and convection.

  In one type of steel-strip cooler in a steel mill (Figure 7.1b ), a sheet of hot metal radiates energy to a bank of cold tubes as it moves past them. At the same time, cooling gas is blown across the sheet. A combined radiation and convection analysis is needed to find the temperature distribution along the steel strip. In a concept for a nuclear rocket engine, illustrated by Figure 7.1c , transparent hydrogen gas is heated by flowing through a high-temperature nuclear reactor. The hot gas then passes out through the rocket nozzle. The interior surface of the rocket nozzle receives radiant energy from the exit face of the reactor core and by convection from the flowing gas. Cooling the nozzle is done by conducting this energy through the nozzle wall and removing it to a flowing coolant. (NASA has conducted tests on such an engine; see Robbins and Finger 1991

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  Thermal Radiation Heat Transfer

  • John R. Howell, M. Pinar Mengüc, Kyle Daun, Robert Siegel(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  7 Radiation Combined with Conduction and Convection at Boundaries

  Ernst Rudolph George (ERG) Eckert (1904–2004)

  researched radiation from solids and gases, and published measurements of directional emissivity from various materials as well as directional reflectivity of blackbody radiation. He also developed optical methods for obtaining configuration factors. In 1937, he turned to measurement of the emissivity of CO

  2

  -N

  2

  mixtures as well as water vapor at various temperatures and partial pressures.

  7.1 Introduction

  In the preceding chapters, enclosure theory was formulated for radiative exchange between surfaces. The local net radiation loss at a surface was balanced by energy supplied by “some other means” that were not explicitly described. This chapter is concerned with this energy at the surface either by conduction from within the volume interior to the surface (such as from within a wall of an enclosure) or by convection or conduction at the surface from a surrounding medium. At each location along the surface, the radiation, convection, and conduction combine to form a thermal boundary condition. The solution to the energy equations subject to this condition provides the surface temperature and heat flux distributions. The analysis has the same restrictions as in the previous theory: The surfaces are opaque , and the medium between the radiating surfaces is perfectly transparent . The medium between the radiating surfaces may be conducting or convecting energy, but it does not interact with radiation passing through it.

  One example of combined-mode energy transfer is a vapor-cycle power plant operating in outer space. Waste heat must be rejected by radiation. In the space radiator in Figure 7.1a

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  Introduction to Food Process Engineering

  • Albert Ibarz, Gustavo V. Barbosa-Canovas(Authors)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  18.7 SIMULTANEOUS HEAT TRANSFER BY CONVECTION AND RADIATION Heat transfer, in practice, occurs by more than one mechanism at the same time. Thus, in the case of heat transfer from a hot surface to the exterior, convection and radiation perform such transmis-sion simultaneously. Consider a hot surface at a temperature T S , which is surrounded by a fluid at a temperature T G , T W being the temperature of the walls. The heat transfer mechanisms are radiation and convection, so the heat flow transferred from the hot surface will be the sum of heat transferred by radiation plus the heat transferred by convection: dotnosp dotnosp dotnosp Q Q Q TOTAL R C = + dotnosp Q h A T T R R S W = -( ) dotnosp Q h A T T C C S G = -( ) Hence, dotnosp Q h A T T h A T T TOTAL R S W C S G = -( ) + -( ) In the case where the temperature of the fluid T G is the same as the temperature of the wall, dotnosp Q h h A T T TOTAL R C S W = + ( ) -( ) (18.35) The values of the coefficients h R and h C should have been calculated previously. The individual coefficient of heat transfer by convection h C is calculated from graphs or equations obtained in an empirical way and based on a dimensional analysis. The coefficient h R can be obtained from graphs or equations, as indicated in the previous section. TABLE 18.2 Values of ( h C + h R ) for Steel Pipes toward Their Surroundings a d 0 (in.) ( T S − T G ) (°F) 30 50 100 150 200 250 300 350 400 450 500 550 600 650 700 1 2.16 2.26 2.50 2.73 3.00 3.29 3.60 3.95 4.34 4.73 5.16 5.60 6.05 6.51 6.99 3 1.97 2.05 2.25 2.47 2.73 3.00 3.31 3.69 1.03 4.73 4.85 5.26 5.71 6.19 6.08 5 — 1.95 2.15 2.36 2.61 2.90 3.20 3.54 3.90 10 1.80 1.87 2.07 2.29 2.54 2.80 3.12 3.47 3.84 Source: Perry, R.H. and Chilton, C.H., Chemical Engineer’s Handbook , New York, McGraw-Hill, 1973. a Units of ( h C + h R ) Btu/(h ft 2 °F).

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  Elements of Heat Transfer

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

    (Publisher)

  • 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. 1.1.1 Driving Potential We know that, in an electric circuit the current flow through a wire depends upon the potential difference across the two ends of the wire, that is, Current flow = Potential difference Resistance Analogous to this, we can state that in a heat transfer process, the heat flow is given by Heat flow = Thermal potential difference Thermal resistance From the above observations, we may define in a simple, but general, manner that, “ heat transfer is the process of energy transfer due to temperature dif-ference .” Whenever there is a temperature difference in a medium or between media, heat transfer occurs. Examine the heat transfer processes illustrated in Figure 1.1. From the process shown in Figure 1.1, we can infer that, • Heat transfer due to a temperature gradient in a stationary solid or fluid medium is referred to as conduction . • Heat transfer between a solid surface and a moving fluid when they are at different temperatures is referred to as convection . Dimensions and Units 3 T 2 ˙ q T s ˙ q ˙ q 2 (a) (c) (b) T 1 T 2 Flow, T ∞ Solid or fluid Solid surface Surface 1 T 1 ˙ q 1 Surface 2 T 1 > T 2 T s > T ∞ Figure 1.1 Conduction, convection, and radiation heat transfer modes: (a) conduction through a solid or a stationary fluid, (b) convection from a solid surface to a moving fluid, (c) net radiation exchange between two surfaces. Table 1.1 Common system of units Quantity Unit SI CGS FPS MKS Mass kilogram kg g lb kg Length meter m cm ft m Time second s s s s Temperature kelvin K ◦ C ◦ F ◦ C • The third mode of heat transfer shown in Figure 1.1(c) is termed thermal radiation .

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  General Physics

  • Nelson Bolívar(Author)
  • 2020(Publication Date)
  • Arcler Press

    (Publisher)

  Convection is how the heat travels through the fluids-gases, and liquids. Hot fluids rise, whereas cold fluids generally sink down (Tan and Howell, 1991; Dehghan and Behnia, 1996; Mezrhab et al., 2006). This up and down motion is known as convection current. Convection current distributes the heat in a circular, up, and down pattern. Radiation is how the heat travels through the empty space. Radiation does not need molecules to travel through. Any time one feels the heat without touching it, he/she is experiencing radiation (Balaji and Venkateshan, 1995; Reddy and Kumar, 2008) (Figure 8.15). General Physics 240 Figure 8.15. Heat transfer because of the combination of convection, conduc-tion, and radiation. Source: https://fl-pda.org/independent/courses/elementary/science/ section4/4f12.htm. Heat Transfer by Conduction, Convection, and Radiation 241 REFERENCES 1. Arpaci, V. S., & Arpaci, V. S., (1966). Conduction Heat Transfer (Vol. 237, pp. 1–40). Reading, MA: Addison-Wesley. 2. Aziz, A., & Na, T. Y., (1984). Perturbation Methods in Heat Transfer (Vol. 1, No. 1, p. 212). Washington, DC, Hemisphere Publishing Corp. 3. Balaji, C., & Venkateshan, S. P., (1995). Combined conduction, convection and radiation in a slot. International Journal of Heat and Fluid Flow , 16 (2), 139–144. 4. Bég, O. A., Uddin, M., Rashidi, M. M., & Kavyani, N., (2014). Double-diffusive radiative magnetic mixed convective slip flow with Biot and Richardson number effects. Journal of Engineering Thermophysics , 23 (2), 79–97. 5. Bejan, A., & Kraus, A. D., (2003). Heat Transfer Handbook (Vol. 1, pp. 1–39). John Wiley & Sons. 6. Bejan, A., (2013). Convection Heat Transfer (Vol. 1, pp. 1–24). John Wiley & Sons. 7. Berber, S., Kwon, Y. K., & Tománek, D., (2000). Unusually high thermal conductivity of carbon nanotubes. Physical Review Letters , 8, 1–11. 8. Biot, M. A., (1970). Variational Principles in Heat Transfer: A Unified Lagrangian Analysis of Dissipative Phenomena (Vol. 1, pp. 1–28).

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  Physics

  • John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Concept Summary 13.1 Convection Convection is the process in which heat is carried from place to place by the bulk movement of a fluid. During natural convection, the warmer, less dense part of a fluid is pushed upward by the buoyant force provided by the surrounding cooler and denser part. Forced convection occurs when an external device, such as a fan or a pump, causes the fluid to move. 13.2 Conduction Conduction is the process whereby heat is transferred directly through a material, with any bulk motion of 410 CHAPTER 13 The Transfer of Heat the material playing no role in the transfer. Materials that con- duct heat well, such as most metals, are known as thermal con- ductors. Materials that conduct heat poorly, such as wood, glass, and most plastics, are referred to as thermal insulators. The heat Q conducted during a time t through a bar of length L and cross- sectional area A is given by Equation 13.1, where ΔT is the temper- ature difference between the ends of the bar and k is the thermal conductivity of the material. Q = (kA ΔT)t _ L (13.1) 13.3 Radiation Radiation is the process in which energy is trans- ferred by means of electromagnetic waves. All objects, regardless of their temperature, simultaneously absorb and emit electromagnetic waves. Objects that are good absorbers of radiant energy are also good emitters, and objects that are poor absorbers are also poor emitters. An object that absorbs all the radiation incident upon it is called a perfect blackbody. A perfect blackbody, being a perfect absorber, is also a perfect emitter. The radiant energy Q emitted during a time t by an object whose surface area is A and whose Kelvin temperature is T is given by the Stefan–Boltzmann law of radiation (see Equation 13.2), where σ = 5.67 × 10 −8 J/(s · m 2 · K 4 ) is the Stefan–Boltzmann constant and e is the emissivity, a dimensionless number characterizing the surface of the object.

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  Physics

  • John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  Forced convection occurs when an external device, such as a fan or a pump, causes the fluid to move. 13.2 Conduction Conduction is the process whereby heat is transferred directly through a ma- terial, with any bulk motion of the material playing no role in the transfer. Materials that conduct heat well, such as most metals, are known as thermal conductors. Materials that conduct heat poorly, such as wood, glass, and most plastics, are referred to as thermal insulators. The heat Q conducted during a time t through a bar of length L and cross-sectional area A is given by Equa- tion 13.1, where DT is the temperature difference between the ends of the bar and k is the thermal conductivity of the material. 13.3 Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. All objects, regardless of their temperature, simultaneously absorb and emit electromagnetic waves. Objects that are good absorbers of radiant energy are also good emitters, and objects that are poor absorbers are also poor emitters. An object that absorbs all the radiation incident upon it is called a perfect blackbody. A perfect blackbody, being a perfect absorber, is also a perfect emitter. The radiant energy Q emitted during a time t by an object whose surface area is A and whose Kelvin temperature is T is given by the Stefan–Boltzmann law of radiation (see Equation 13.2), where s 5 5.67 3 10 28 J/(s ? m 2 ? K 4 ) is the Stefan–Boltzmann constant and e is the emissivity, a dimensionless number characterizing the surface of the object. The emissivity lies between 0 and 1, being zero for a nonemitting surface and one for a perfect blackbody. The net radiant power is the power an object emits minus the power it absorbs. The net radiant power P net emitted by an object with a temperature T located in an environment with a temperature T 0 is given by Equation 13.3. Q 5 (kA DT ) t L (13.1) Q 5 es T 4 At (13.2) P net 5 es A(T 4 2 T 0 4 ) (13.3)

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  Solar Energy

  Renewable Energy and the Environment

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

    (Publisher)

  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. In convection heat transfer, the heat is moved through the bulk transfer of a nonuniform temperature fluid. This type of heat transfer can occur in a flow of air over a lagoon or a waste-water treatment system. Radiation heat transfer is energy emitted by matter in the form of photons or electromagnetic waves. Radiation can take place through space without the presence of matter. In fact, radiation heat transfer is highest in a vacuum environment. Radiation can be important even in situations in which there is an intervening medium; a familiar example is the heat transfer from a glowing piece of metal or from a fire. 3.2 CONDUCTION HEAT TRANSFER To examine conduction heat transfer, it is necessary to relate the heat transfer to mechanical, ther-mal, or geometrical properties. Consider steady-state heat transfer through a wall of thickness ∆ x that is placed between two reservoirs: hot ( T H ) and cold ( T C ), respectively. Figure 3.1 shows the process pictorially. As shown by the figure, heat transfer rate,  Q , is a function of the hot and cold temperatures, the slab geometry, and the following properties:  Q = f ( T H , T C , geometry, properties) (3.1) 56 Solar Energy: Renewable Energy and the Environment It is also possible to express the heat transfer rate,  Q . (w), based on the hot and cold temperature difference, T H – T C , where the heat transfer rate is zero when there is no temperature difference.

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  Industrial Energy Systems Handbook

  • A.E. Williams(Author)
  • 2023(Publication Date)
  • River Publishers

    (Publisher)

  Chapter 10 Heat Exchange Systems Albert Williams Institute of Energy Professionals Africa, South Africa

  The transfer of thermal energy or heat is driven by a temperature difference. The rate at which heat moves from a high temperature body to a body at a lower temperature is determined by the difference in temperatures and the materials through which the heat transfer takes place.

  10.1 Concepts of Conduction, Convection and Radiation

  There are three fundamental processes by which heat transfer takes place. These are:

  • conduction,
  • convection and
  • radiation.

  All heat transfer occurs by at least one of these processes, but typically, heat transfer occurs through a combination of these processes. All heat transfer processes are driven by temperature differences and are dependent on the materials or substances involved. Figure 10.1 and Figure 10.2 demonstrates the process graphically.

  Figure 10.1 Heat transfer processes.

  Figure 10.2 Heat transfer processes examples.

  10.1.1 Conduction

  Conduction is the heat transfer between two objects that are in contact with each other. The hotter object will heat the cooler object. The molecules of the object with a higher temperature have more energy, and they bump into the molecules of the cooler object. In this way they transfer some of their energy, causing the temperature to increase. Conduction can also be described as heat flow through materials. In buildings, conduction happens through walls, ceilings/roofs and floors. The heat flow through materials can be described using the terms discussed next and Figure 10.3 .

  Figure 10.3 Heat conduction through a certain material.

  Conductivity (k): This is the amount of heat transmitted in time through the material. It is defined using the following equation (Fourier’s Law):

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  Extraction Techniques for Food Processing

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

    (Publisher)

  (11) covers the situation; for example for a loaf in an oven receiving radiation from the walls around it, or a meat carcass radiating heat to the walls of a freezing chamber. In order to be able to compare the various forms of heat transfer, it is necessary to see whether an equation can be written for radiant heat transfer similar to the general heat transfer eqn. (4). This means that for radiant heat transfer: where h r is the radiation heat-transfer coefficient, T 1 is the temperature of the body and T 2 is the temperature of the surroundings. (The T would normally be the absolute temperature for the radiation, but the absolute temperature difference is equal to the Celsius temperature difference, because 273 is added and subtracted and so ( T 1 - T 2 ) = ( T 1 - T 2 ) = ΔT Equating eqn. (11) and eqn. (12) Therefore This ebook is exclusively for this university only. Cannot be resold/distributed. = 2T m 2 + (T 1 - T 2 ) 2 /2 Therefore Now, if ( T 1 - T 2 ) ≪ T 1 or T 2 , that is if the difference between the temperatures is small compared with the numerical values of the absolute temperatures, we can write: and so Convection Heat Transfer Convection heat transfer is the transfer of energy by the mass movement of groups of molecules. It is restricted to liquids and gases, as mass molecular movement does not occur at an appreciable speed in solids. It cannot be mathematically predicted as easily as can transfer by conduction or radiation and so its study is largely based on experimental results rather than on theory. The most satisfactory convection heat transfer formulae are relationships between dimensionless groups of physical quantities. Furthermore, since the This ebook is exclusively for this university only. Cannot be resold/distributed. laws of molecular transport govern both heat flow and viscosity, convection heat transfer and fluid friction are closely related to each other.

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  Weather

  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.

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  Food Plant Engineering Systems

  • Theunis Christoffel Robberts(Author)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  137 © 2010 Taylor & Francis Group, LLC Chapter 8 Heating Systems for Processing Plants 8.1 Heat Transfer Heat transfer is the movement of thermal energy from one point to another. The driving force behind heat transfer is the difference between the temperatures. Heat always flows from a high temperature to a lower temperature. Heat transfer is an equilibrium process that can take a considerable time to reach equilibrium. Heat flow can be considered just like fluid flow, since we consider the rate of flow and the obstacles to flow. Thermal energy in a system manifests itself by the random motion of molecular particles within the system. The temperature of the system is just a physical manifestation of the particle motion. Heat transfer is thus concerned with the transfer of the molecular motion from one region to another. Heat transfer occurs through conduction, convection, and radiation. In con-duction, the energy is passed from one body or particle to an adjacent particle or body without any bulk movement of material. In convection, some bulk movement occurs and high-energy particles move to regions with lower energy. Convection takes place in liquids and in gases. Radiation is transfer of energy from a radiating source through space that can be void of matter. 8.2 Conductive Heat Transfer Heat is transferred through solids by molecular excitation. Molecules that possess more energy vibrate faster than other molecules. When they collide with molecules 138 ◾ Food Plant Engineering Systems © 2010 Taylor & Francis Group, LLC that possess less energy, they transfer some of their energy to these molecules. In this way heat is transferred from the high-energy “hot” part of the system to the low energy “colder” part of the system. The heat transfer is because of the temperature difference. Some materials, like metals, consist of atoms with large numbers of valence electrons that can vibrate.

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