Engineering Heat Transfer

William S. Janna

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781000055030
• Edition: 3
• Topic: Ciencias físicas
• Subtopic: Termodinámica

Most heat transfer texts include the same material: conduction, convection, and radiation. How the material is presented, how well the author writes the explanatory and descriptive material, and the number and quality of practice problems is what makes the difference. Even more important, however, is how students receive the text. Engineering Heat Transfer, Third Edition provides a solid foundation in the principles of heat transfer, while strongly emphasizing practical applications and keeping mathematics to a minimum.
New in the Third Edition:

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• Coverage of the emerging areas of microscale, nanoscale, and biomedical heat transfer
• Simplification of derivations of Navier Stokes in fluid mechanics
• Moved boundary flow layer problems to the flow past immersed bodies chapter
• Revised and additional problems, revised and new examples
• PDF files of the Solutions Manual available on a chapter-by-chapter basis

The text covers practical applications in a way that de-emphasizes mathematical techniques, but preserves physical interpretation of heat transfer fundamentals and modeling of heat transfer phenomena. For example, in the analysis of fins, actual finned cylinders were cut apart, fin dimensions were measures, and presented for analysis in example problems and in practice problems. The chapter introducing convection heat transfer describes and presents the traditional coffee pot problem practice problems. The chapter on convection heat transfer in a closed conduit gives equations to model the flow inside an internally finned duct. The end-of-chapter problems proceed from short and simple confidence builders to difficult and lengthy problems that exercise hard core problems solving ability.
Now in its third edition, this text continues to fulfill the author's original goal: to write a readable, user-friendly text that provides practical examples without overwhelming the student. Using drawings, sketches, and graphs, this textbook does just that.
PDF files of the Solutions Manual are available upon qualifying course adoptions.

1

Fundamental Concepts

1.1 Introduction

Heat transfer is the term applied to a study in which the details or mechanisms of the transfer of energy in the form of heat are identified and modeled. There are many examples of heat transfer. Familiar domestic examples include broiling a turkey, toasting bread, and heating water. Industrial examples include curing rubber, heat treating steel forgings, and dissipating waste heat from a power plant. The analysis of such problems is the topic of study in this text.

1.2 Mechanisms of Heat Transfer

Heat transfer is energy in transit, which occurs as a result of a temperature gradient or difference. This temperature difference is thought of as a driving force that causes heat to flow. Heat transfer occurs by three basic mechanisms or modes—conduction, convection, and radiation.

Conduction is the transmission of heat through a substance without perceptible motion of the substance itself. Heat can be conducted through gases, liquids, and solids. In the case of fluids in general, conduction is the primary mode of heat transfer when the fluid has zero bulk velocity. In opaque solids, conduction is the only mode by which heat can be transferred. The kinetic energy of the molecules of a gas is associated with the property we call temperature. In a high-temperature region, gas molecules have higher velocities than those in a low-temperature region. The random motion of the molecules results in collisions and an exchange of momentum and energy. When this random motion exists and a temperature gradient is present in the gas, molecules in the high-temperature region transfer some of their energy, through collisions, to molecules in the low-temperature region. We identify this transport of energy as heat transfer via the diffusive or conductive mode.

Conduction of heat in liquids is the same as for gases—random collisions of high-energy molecules with low-energy molecules causing a transfer of heat. The situation with liquids is more complex, however, because the molecules are more closely spaced. Therefore, molecular force fields can have an effect on the energy exchange between molecules because molecular force fields can influence the random motion of the molecules.

Conduction of heat in solids is thought to be due to motion of free electrons, lattice waves, magnetic excitations, and electromagnetic radiation. The motion of free electrons occurs only in substances that are considered to be good electrical conductors. The theory is that heat can be transported by electrons (known as the electron gas), which are free to move through the lattice structure of the conductor in the same way that electricity is conducted. This is usually the case for metals.

The molecular energy of vibration in a substance is transmitted between adjacent molecules or atoms from a region of high to low temperature. This phenomenon occurs from lattice waves that can be considered as energy being transmitted by a gas composed of an integral number of quanta, known as phonons. Phonon motion is thought of as diffusing through the lattice in the same way as the electron gas does. The lattice-wave mechanism is usually not a significant factor in conduction of heat through metals. It is significant for nonmetals.

Magnetic dipoles of adjacent atoms in some cases provide effects between magnetic moments that may aid the conduction of heat in the solid. Electromagnetic radiation in translucent materials may have an effect on the conduction of heat. This is the case when the material has little capacity for absorbing energy.

The concern here is not so much in describing the molecular or microscopic activity associated with conduction, but in being able to describe mathematically the macroscopic effect of heat transfer via the conduction mode. An example of conductio...