tailieunhanh - Ebook Handbook of heat transfer (3rd edition): Part 2
(BQ) Part 2 book "Handbook of heat transfer" has contents: Microscale transport phenomena, heat transfer in porous mediac, nonnewtonian fluids, techniques to enhance heat transfer, heat pipes, heat transfer in packed and fluidized beds, condensation,.and other contents. | CHAPTER 8 MICROSCALE TRANSPORT PHENOMENA A. Majumdar University of California at Berkeley INTRODUCTION Heat transfer involves the transport of energy from one place to another by energy carders, In gas phase, gas molecules carry energy either by random molecular motion (diffusion) or by an overall drift of the molecules in a certain direction (advection). In liquids, too, energy can be transported by diffusion and advection of molecules, In solids, however, energy is transported by phonons, electrons, or photons. A phonon is a quantum of crystal vibrational energy and dominates heat conduction in insulators and semiconductors Electrons dominate energy transport in metals. Photons are quanta of electromagnetic energy and can transport energy in solids as well as interact with photons and phonons to render radiative properties of solids, The only mode of energy transport in vacuum is by photons The study of heat transfer at macroscales by all these energy carriers is well established and is based on continuum theories. For example, energy diffusion in gases, liquids, and solids is usually studied by the Fourier law of heat conduction. Although the physics of all the energy carders are vastly different, what seems remarkable is the applicability of the Fourier law of heat conduction for any energy carriermmolecules, phonons, electrons, and even photons under the optically thick limit. So, there must be some universality in the transport of all these energy carriers at the macroscopic or continuum scales. The question that one could ask is the following. Does this universality break down at some length scale or is it universal over all length scales? This chapter will attempt to answer this question. The second fact that is implicit in macroscopic or continuum laws is the idea of local thermodynamic equilibrium. For example, when we write the Fourier law of heat conduction, it is inherently assumed that one can define a temperature at any point in space. This is a .
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