physical mechanism of convection

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.



11 May 2014

Chapter 3:



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



11 May 2014

Chapter 3:



Tue 8:54:24 AM



11 May 2014

Chapter 3:



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,



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.



11 May 2014

Chapter 1:

Introduction

CLASSIFICATION OF FLUID FLOWS

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• Viscous versus Inviscid Flow

• Internal versus External Flow

• Compressible versus IncompressibleFlow

• Laminar versus Turbulent Flow



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



11 May 2014

Chapter 1:

Introduction

VELOCITY BOUNDARY LAYER

Tue 8:58:10 AM



11 May 2014

Chapter 1:

Introduction

Surface Shear Stress

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11 May 2014

Chapter 1:

Introduction

THERMAL BOUNDARY LAYER

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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



11 May 2014

Chapter 1:

Introduction

LAMINAR AND TURBULENT FLOWS

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11 May 2014

Chapter 1:

Introduction

CONVECTION COEFFICIENTS

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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.



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



11 May 2014

Chapter 1:

Introduction

PARALLEL FLOW OVER FLAT PLATES

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11 May 2014

Chapter 1:

Introduction


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11 May 2014

Chapter 1:

Introduction

Example

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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.



11 May 2014

Chapter 1:

Introduction

FLOW ACROSS CYLINDERS AND SPHERES

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11 May 2014

Chapter 1:

Introduction


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11 May 2014

Chapter 1:

Introduction

Exxample

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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.



11 May 2014

Chapter 1:

Introduction

INTERNAL FORCEDCONVECTION

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11 May 2014

Chapter 1:

Introduction

THE ENTRANCE REGION

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11 May 2014

Chapter 1:

Introduction


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11 May 2014

Chapter 1:

Introduction

Example

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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



11 May 2014

Chapter 1:

Introduction

Laminar Flow in Pipes

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11 May 2014

Chapter 1:

Introduction

Turbulent Flow in Pipes

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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



11 May 2014

Chapter 1:

Introduction

Turbulent Flow in Pipes

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11 May 2014

Chapter 1:

Introduction

Tue 9:03:41 AM

physical mechanism of convection

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