Jordanian-German Winter Academy 2006

1

• Jordanian-German Winter Academy 2006
• NATURAL CONVECTION
• Prepared by
• FAHED ABU-DHAIM
• Ph.D student
• UNIVERSITY OF JORDAN
• MECHANICAL ENGINEERING DEPARTMENT

2
(No Transcript)
3
(No Transcript)
4
Natural Convection Heat Transfer

• Examples
• Electronic devices (computer boards, T.V, etc).
• Baseboard heaters.
• Heat transfer from pipes and transmission lines
  
• Steam radiators-central heating systems to heat a
  room, heating elements.
• Refrigeration coils (condenser and evaporator).
• Heat transfer from bodies of human or animals.

5
Natural Convection Heat Transfer

• WHAT DRIVES THE NATURAL CONVECTION FLOW?

• In natural convection , or free convection , the
  fluid flows naturally (by it self) , not forced
  motion.
• It is driven by the effect of the buoyancy.
• It is observed as a result of the fluid motion
  due to density change arising from the heating
  processes.
• The motion of the fluid results from the buoyancy
  forces imposed on the fluid when its density is
  changed.

6
Natural Convection Heat Transfer

• The buoyancy forces are present because the fluid
  is acted upon by gravity, which is an external
  force field.
• As a conclusion whenever a fluid is heated or
  cooled in a gravitatational field, there is a
  possibility of natural convection.

7
Example
8
Natural Convection Heat Transfer

• IMPORTANCE OF NATURAL CONVECTION
• Convective heat transfer coefficient h is very
  small in multimode heat transfer systems.
• Natural convection resistance is large and thus
  natural convection affects system design.
• Natural convection is preferred when large heat
  rates to be avoided.
• Natural convection mode is economically
  attractive (no need for a pump or blower).

9
Natural Convection Heat Transfer

• Natural convection boundary layers

10
The Governing Equations
11
Similarity Solution
12
Continued, Similarity Solution
13
Laminar, free convection boundary layer
conditions on an isothermal, vertical surface

• a) Velocity profile b) Temperature profile

14
Effect of turbulence on Natural Convection Heat
Transfer
15
Empirical Correlations

• Vertical Plates
• laminar flow
• Turbulent flow

16
Horizontal Plates
17

• Upper Surface of Heated Plate or Lower Surface of
  Cooled Plate
• Lower Surface of Heated Plate or Upper Surface of
  Cooled Plate

18
Heated Horizontal Cylinder
19
Spheres
20
Numerical Study of Natural Convection in inclined
Rectangular Glazing Cavities

• INTRODUCTION
• Natural convection in confined rectangular
  cavities has received much attention in recent
  years.
• Such type of flow has a wide range of
  applications, for example, multi-pane windows,
  solar collectors.
• Especially recently, sloped windows and skylights
  have been more and more frequently applied in
  buildings.
• This study is useful for air conditioning design
  loads (in summer or winter) .

21

• ABSTRACT
• In this study, numerical method is applied to
  predict the heat transfer in natural convective
  flow in inclined rectangular glazing cavities.
• The inclination orientation changes from vertical
  to horizontal position.
• Then 3-D modeling is applied and found to predict
  well the average heat transfer quantities.

22

• A lot of experimental work has been performed and
  it is found that heat transfer in the inclined
  cavities is directly related to the flow mode
  transition.
• Most of these experimental researches only
  studied cavities with small to medium aspect
  ratios, with the maximum aspect ratio 15.5

23
(No Transcript)
24

• MATHEMATICAL FORMULATION
• The Boussinesq approximation is applied with
  constant fluid properties, and negligible viscous
  dissipation and internal heat sources.
• The derived incompressible three-dimensional
  Navier-Stokes equations for a cavity with the
  gravity force pointing in any direction in the
  x-y plane are given below

25
(No Transcript)
26

• Results
• Two-Dimensional Model with Ideal Boundary
  Conditions
• The average Nusselt number results are plotted
  versus tilt angle, and compared with the
  numerical results.
• Very good agreement is shown, which can be a
  proof that the current 2-D numerical method is
  correct in the aspects of mathematical model and
  the numerical manipulation.

27
The average Nusselt number results are plotted
versus tilt angle
28

• THREE-DIMENSIONAL NUMERICAL METHOD
• The finite volume is used for 3-D numerical
  simulation .
• All the three-dimensional calculations are
  initialized with a random velocity field and a
  uniform mean-temperature field.
• Then the steady state governing equations are
  solved.

29
Results

• The heat transfer results are shown below

30
The average Nusselt no versus tilt angle for a
3-D cavity with aspect ratio 20 and Rayleigh no
9320.
31
CONCLUSIONS

• A two-dimensional finite element model is used
  firstly and found not able to predict correct
  heat transfer results.
• Only three-dimensional modeling is an effective
  way to predict heat transfer results.

32
Thanks