convection heat transfer - sharifmech.sharif.ir/~saman/files/lecture_2_introduction.pdf ·...
TRANSCRIPT
Convection Heat TransferInstructor: M.H.Saidi
PhD, Professor
School of Mechanical Engineering
Sharif University of Technology
Fall 2011
1
Lecture #2
Convection Heat Transfer
2
Convection Heat Transfer
3
:Summary of first sessionvBasic concepts and applications
vFundamental governing equations: Mass conservation and Momentum principle
This session: First and second laws of Thermodynamics
Convection Heat Transfer FIRST LAW OF THERMODYNAMICS
The importance and usage of First law of
Thermodynamics
4
In case the natural flow is driven by the heat administered to the flowing fluid.
In all cases, however, the heat transfer part of the convection problem requires a solution for the temperature distribution through the flow,
especially in the close vicinity of the solid walls bathed by the heat-carrying fluid stream .
Convection Heat Transfer
5
First law of thermodynamics applied to a control volume in two-
dimensional flow
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
6
According to the energy flow diagrams sketched in the Figure , the groups of
the above terms are
FIRST LAW OF THERMODYNAMICS
. . . .''
.1 2 3 4 5{ } [ ] [ ] [ ] [ ]cvc vi i e e i
dE m h m h q x y q Wdt
= − + ∆ ∆ + −∑ ∑ ∑
The origin of the dissipation rate term {•}5 lies in the work
transfer affected by the normal and tangential stresses
Convection Heat Transfer
7
FIRST LAW OF THERMODYNAMICS
Represents the change in kinetic energy of the fluid packet; in the present
treatment, this change is considered negligible relative to the internal energy change d(re)/dt appearing in term {•}1
In the {•}5 expression, the eight terms have
been separated into two groups.
Assembling expressions (25) into the energy conservation statement and using constitutive relations, we obtain:
Convection Heat Transfer
8
In the special case where the flow can be modeled as incompressible and two-dimensional, the
viscous dissipation function reduces to
FIRST LAW OF THERMODYNAMICS
Heat flux vector
Viscous dissipation function
0 ; Due to mass conservation
Convection Heat Transfer
9
To express Eq. (26) in terms of enthalpy, we use the thermodynamics definition:
FIRST LAW OF THERMODYNAMICS
we can express the directional heat fluxes q“x and q“y in terms of the local temperature gradients(we invoke the Fourier law of heat conduction)
Convection Heat Transfer
10
FIRST LAW OF THERMODYNAMICS
0 ; Due to mass conservation
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
11
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
12
FIRST LAW OF THERMODYNAMICS
Summary of Thermodynamic relations
Convection Heat Transfer
13
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
14
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
15
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
16
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
17
Assignment #2
Derive the energy equation in the spherical coordinate system
Due date: One week after this session
Convection Heat Transfer
18
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
19
FIRST LAW OF THERMODYNAMICS
Convection Heat Transfer
20
SECOND LAW OF THERMODYNAMICS
Convection Heat Transfer
21
SECOND LAW OF THERMODYNAMICS
Convection Heat Transfer
22
SECOND LAW OF THERMODYNAMICS
Convection Heat Transfer
23
SECOND LAW OF THERMODYNAMICS
Convection Heat Transfer
24
RULES OF SCALE ANALYSIS
Convection Heat Transfer
25
RULES OF SCALE ANALYSIS
Convection Heat Transfer
26
:Next session
vRules of scale analysis
vChapter 2: Laminar Boundary Layer Flow