physical mechanism of convection
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Slide Nr. 0 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 3:
FUNDAMENTALS OF CONVECTION
PHYSICAL MECHANISM OF CONVECTION
Tue 8:54:24 AM
Heat transfer through afluid is by convection inthe presence of bulk fluidmotion and by conductionin the absence of it.
Slide Nr. 1 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 3:
FUNDAMENTALS OF CONVECTION
PHYSICAL MECHANISM OF CONVECTION
Tue 8:54:24 AM
Experience shows that convectionheat transfer strongly depends on thefluid properties dynamic viscosity m,thermal conductivity k, density r andspecific heat Cp, as well as the fluidvelocity v. It also depends on thegeometry and the roughness of thesolid surface, in addition to the typeof fluid flow (such as beingstreamlined or turbulent).
Thus, we expect the convectionheat transfer relations to be rathercomplex
Slide Nr. 2 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 3:
FUNDAMENTALS OF CONVECTION
PHYSICAL MECHANISM OF CONVECTION
Tue 8:54:24 AM
Slide Nr. 3 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 3:
FUNDAMENTALS OF CONVECTION
PHYSICAL MECHANISM OF CONVECTION
Tue 8:54:24 AM
An implication of the no-slip and theno-temperature jump conditions isthat heat transfer from the solidsurface to the fluid layer adjacent tothe surface is by pure conduction,
Slide Nr. 4 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Nusselt Number
Tue 8:58:10 AM
A Nusselt number of Nu = 1 for a fluidlayer represents heat transfer acrossthe layer by pure conduction.
Slide Nr. 5 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
CLASSIFICATION OF FLUID FLOWS
Tue 8:58:10 AM
• Viscous versus Inviscid Flow
• Internal versus External Flow
• Compressible versus IncompressibleFlow
• Laminar versus Turbulent Flow
Slide Nr. 6 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
CLASSIFICATION OF FLUID FLOWS
Tue 8:58:10 AM
• Natural (or Unforced) versus Forced Flow
• Steady versus Unsteady (Transient) Flow
• One-, Two-, and Three-Dimensional Flows
Slide Nr. 7 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
VELOCITY BOUNDARY LAYER
Tue 8:58:10 AM
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Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Surface Shear Stress
Tue 8:58:10 AM
Slide Nr. 9 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
THERMAL BOUNDARY LAYER
Tue 8:58:10 AM
Slide Nr. 10 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Prandtl Number
Tue 8:58:10 AM
The relative thickness of the velocity and the thermal boundarylayers is best described by the dimensionless parameter Prandtlnumber, defined as
Pr about 1 for gases
Pr << 1 for liquid metals
Pr >> 1 for oil
Slide Nr. 11 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
LAMINAR AND TURBULENT FLOWS
Tue 8:58:10 AM
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Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
CONVECTION COEFFICIENTS
Tue 8:58:10 AM
where m and n are constant exponents (usuallybetween 0 and 1), and the value of the constant Cdepends on geometry.
Sometimes more complex relations are used forbetter accuracy.
Slide Nr. 13 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
EXTERNAL FORCED CONVECTION
Tue 8:58:10 AM
the fluid properties are usuallyevaluated at the so-called filmtemperature, defined as
Slide Nr. 14 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
PARALLEL FLOW OVER FLAT PLATES
Tue 8:58:10 AM
Slide Nr. 15 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
PARALLEL FLOW OVER FLAT PLATES
Tue 8:58:10 AM
Slide Nr. 16 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
PARALLEL FLOW OVER FLAT PLATES
Tue 8:58:10 AM
Slide Nr. 17 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Example
Tue 8:58:10 AM
Engine oil at 60°C flows over the upper surface of a 5-m-long flat plate whosetemperature is 20°C with a velocity of 2 m/s. Determine the rate of heat transfer per unitwidth of the entire plate.
Slide Nr. 18 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
FLOW ACROSS CYLINDERS AND SPHERES
Tue 8:58:10 AM
Slide Nr. 19 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
FLOW ACROSS CYLINDERS AND SPHERES
Tue 8:58:10 AM
Slide Nr. 20 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
FLOW ACROSS CYLINDERS AND SPHERES
Tue 8:58:10 AM
Slide Nr. 21 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Exxample
Tue 8:58:10 AM
A long 10-cm-diameter steam pipe whose external surface temperature is 110°Cpasses through some open area that is not protected against the winds.Determine the rate of heat loss from the pipe per unit of its length when the airis at 1 atm pressure and 10°C and the wind is blowing across thepipe at avelocity of 8 m/s.
Slide Nr. 22 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
INTERNAL FORCEDCONVECTION
Tue 8:58:10 AM
Slide Nr. 23 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
THE ENTRANCE REGION
Tue 8:58:10 AM
Slide Nr. 24 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
INTERNAL FORCEDCONVECTION
Tue 8:58:10 AM
Slide Nr. 25 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
INTERNAL FORCEDCONVECTION
Tue 8:58:10 AM
Slide Nr. 26 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
INTERNAL FORCEDCONVECTION
Tue 8:58:10 AM
Slide Nr. 27 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
INTERNAL FORCEDCONVECTION
Tue 8:58:10 AM
Slide Nr. 28 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Example
Tue 8:58:10 AM
Water enters a 2.5-cm-internal-diameter thin copper tube of a heat exchangerat 15°C at a rate of 0.3 kg/s, and is heated by steam condensing outside at120°C. If the average heat transfer coefficient is 800 W/m2 C, determine thelength of the tube required in order to heat the water to 115°C
Slide Nr. 29 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Laminar Flow in Pipes
Tue 8:58:10 AM
Slide Nr. 30 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Laminar Flow in Pipes
Tue 8:58:10 AM
Slide Nr. 31 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Turbulent Flow in Pipes
Tue 8:58:10 AM
where n = 0.4 for heating and 0.3 for cooling of the fluid flowing throughthe tube.
The fluid properties are evaluated at the bulk mean fluid temperature
Slide Nr. 32 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Turbulent Flow in Pipes
Tue 8:58:10 AM
Slide Nr. 33 of 33 Slides
Dr. Eng. Sameh Shaaban
11 May 2014
Chapter 1:
Introduction
Tue 9:03:41 AM
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