Page 264 - Fluid mechanics, heat transfer, and mass transfer

9

CONVECTIVE HEAT TRANSFER BASICS

9.1 Convective Heat Transfer 245 different temperatures. Convection currents are in-
9.1.1 Heat Transfer Coefficients 246 duced by temperature differences only without any
9.1.2 Condensation 251 external energy inputs such as pump energy. Local
9.1.3 Boiling and Vaporization 254 velocities are generally low, giving rise to near
9.1.4 Heat Transfer Coefficients: Magnitudes laminar flow conditions.
and Data 259
& Forced Convection: Fluid is forced over the surface
9.1.5 Heat Transfer Fluids 262
by external means such as a fan, pump, mixer, or
9.2 Annexure (Heat Transfer Coefficients for Refinery
wind, in addition to natural temperature difference-
Services) 269
induced convection currents, to increase localized
velocities to increase turbulence and hence heat
transfer coefficients.
. Give examples for natural convection processes.
& Air circulation in the atmosphere and water circula-
tion in oceans and lakes.
9.1 CONVECTIVE HEAT TRANSFER
& Storms, including dust storms, are propelled by
. What is convection? Explain. density differences in the atmosphere caused by
& Convection heat transfer can be defined as transport temperature differences.
of heat from one point to another in a fluid as a result & Example of a burning candle: Air next to the flame is
of macroscopic motions of the fluid, the heat being hot and rises up while air away from the flame is
carried as internal energy. cooler and moves downward to fill the region from
& Convection between a solid surface and adjacent which hot air moved up.
fluid that is in motion involves combined effects of & Stacksfromwhichhotcombustiongasesmoveupward.
conduction and fluid motion.
& Light gases like hydrogen rising in open environment
& Density differences between adjacent layers of fluids
when leaked from a container due to density differ-
induce relative motion between fluid layers. ence between air and hydrogen.
& The density differences may be due to temperature
. ‘‘For large pipe diameters and large DT between pipe
differences between two layers of the same fluid or wall and bulk of the fluid, natural convection effects are
two different fluids with different densities at the more.’’ True/False?
same temperature.
& True. In the case of small pipe diameters, scope for
& External means of energy input could also induce
natural convection currents will be very poor.
relative motion.
. What is conjugate heat transfer?
. What is the difference between natural and forced
& Conjugate heat transfer refers to the ability to com-
convection?
pute conduction heat transfer through solids coupled
& Natural or Free Convection: Iffluid motion is caused
by convective heat transfer in a fluid.
by buoyancy forces that are induced by density
& Either the solid zone or fluid zone or both may
differences due to variation in fluid temperature or
contain heat sources, for example, coolant flowing
due to variation in densities of two different fluids
over fuel rods that generate heat.
adjacent to each other or solid–fluid interfaces are at
Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.
245