heat transfer by convection - libvolume6.xyzlibvolume6.xyz/mechanical/btech/semester6/heatand... ·...
Post on 17-Jun-2020
3 Views
Preview:
TRANSCRIPT
Dr. Şaziye Balku 1
HEAT TRANSFER BY CONVECTION
Dr. Şaziye Balku 2
CONDUCTION
Mechanism of heat transfer through a solid or fluid in the absence any fluid motion.
CONVECTION
Mechanism of heat transfer through a fluid in the presence of bulk fluid motion
• Natural (free) Convection
• Forced Convection
(depending on how the fluid motion is initiated)
Dr. Şaziye Balku 3
• Viscous-inviscid
• Internal flow-External flow
• Open-closed channel
• Compressible-Incompressible• Laminar-Turbulent• Natural- Forced• Steady- Unsteady• One-,two-,three-dimensional
CLASSIFICATION OF FLUID FLOWS
Dr. Şaziye Balku 4
VISCOSITYWhen two fluid layers move relative to each other, a friction force develops between them and the slower layer tries to slow down the faster layer.
internal resistance to flow• cohesive forces between the molecules in liquid• molecular collisions in gases.
Viscous flows: viscous effects are significant
Inviscid flow regions: viscous forces are negligibly small compared to inertial or pressure forces.
measure of stickness or resistance to deformation1. Kinematic viscosity2. Dynamic viscosity
Dr. Şaziye Balku 5
VISCOSITY DEPENDS ON
• TEMPERATURE
• PRESSURE
For liquids dependence of pressure is negligible
For gases kinematic viscosity depends on pressure since its relation to density
µ Dynamic viscosity (kg/m.s or poise)
ν Kinematic viscosity, m2/s or stroke
ρ
µν =
Dr. Şaziye Balku 6
Convection heat transfer
- Dynamic viscosity
- Thermal conductivity
- Density
- Specific heat
- Fluid velocity
- Geometry
- Roughness
- Type of fluid flow
Dr. Şaziye Balku 7
NEWTON’S LAW OF COOLING
)( ∞
•
−= TThAQ SSconv
h
(W)
Convection heat transfer coefficient (W/m2.0C)
The rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference
Dr. Şaziye Balku 8
GENERAL THERMAL ANALYSIS
)(∞
•
−= TThAQSSconv
)( iep TTCmQ −=••
Dr. Şaziye Balku 9
FORCED CONVECTION
Dr. Şaziye Balku 10
• LAMINAR FLOW
Smooth streamlines
Highly- ordered motion
(highly viscous fluids in small pipes)
• TURBULENT FLOW
Velocity fluctuations
Highly-disordered motion
• TRANSITIONAL FLOW
Dr. Şaziye Balku 11
REYNOLDS NUMBER
Flow Regime:
Geometry
Surface roughness
Flow velocity
Surface temperature
type of fluid
Ratio of the inertial forces to
viscous forces in the fluid
µ
ρυυ D
v
D mm ==Re
mυ
D
ρµ /=v
Mean flow velocity
Characteristic length of
the geometry
Kinematic viscosity
Dr. Şaziye Balku 12
• Critical Reynolds number (Recr)
for flow in a round pipeRe < 2300 ⇒ laminar
2300 ≤ Re ≤ 4000 ⇒ transitional
Re > 4000 ⇒ turbulent
• Note that these values are approximate.
• For a given application, Recr depends
upon
– Pipe roughness
– Vibrations
– Upstream fluctuations, disturbances
(valves, elbows, etc. that may disturb
the flow)
Definition of Reynolds number
Dr. Şaziye Balku 13
• For non-round pipes,
• the hydraulic diameter
Dh = 4Ac/P
Ac = cross-section area
P = wetted perimeter
HYDRAULIC DIAMETER
Dr. Şaziye Balku 14
Velocity
No-slip condition
Dr. Şaziye Balku 15
THERMAL BOUNDARY LAYER
Flow region over the surface in which the temperature variation in the direction normal to the surface
Velocity profile influences temperature profile
Dr. Şaziye Balku 16
VELOCITY
in rectangular),,( zyxυr
),,( zr θυr
One dimensional
flow in a circular pipe
A flow field is best characterized by the velocity distribution, and velocity may vary in three dimension
in cylinderical coordinates
In which direction does the velocity change in this figure???
Dr. Şaziye Balku 17
NUSSELT NUMBER(Dimensionless number)
k
hLNu c=
Thqconv
∆=•
L
Tkq
cond
∆=
•
Nuk
hL
LTk
Th
q
q
cond
conv ==∆
∆=
•
•
/
Dr. Şaziye Balku 18
PRANDTL NUMBER
• Boundary layer theory
k
C pµ=Pr
Pr<<1 heat diffuses very quickly in liquid metals, tbl thicker
Pr>>1 heat diffuses very slowly in oils relative to momentum, tblthinner than vbl
k
C
heatofydiffusivitmolecular
momentumofydiffusivitmolecularpµ
α
ν===Pr
Dr. Şaziye Balku 19
PARALLEL FLOW OVER FLAT
PLATES
5105Re ×==µ
ρυ cr
cr
x
53/15.0 105RePrRe664.0 ×<== LLk
hLNu
75
3/18.0
10Re105
60Pr6.0PrRe037.0
≤≤×
≤≤==
L
Lk
hLNu
laminar
turbulent
Dr. Şaziye Balku 20
NATURAL CONVECTION
Dr. Şaziye Balku 21
CONVECTIVE HEAT TRANSFER
COEFFICIENT
2
3)(
υ
β CsL
LTTgGr ∞−
=
PrLL GrRa =
n
L
C CRak
hLNu ==
Grashof number
Rayleigh number
Prandtl number
Nusselt number
Table 20-1
viscosity
Coefficient of volume expansion
top related