dr. emad m. saad - fayoum transfer mechanisms.pdf · dr. emad m. saad mechanical engineering dept....

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Fayoum University

Faculty of Engineering

Mechanical Engineering Dept.

Dr. Emad M. Saad Mechanical Engineering Dept.

Faculty of Engineering

Fayoum University

Heat Transfer Mechanisms

Lecture (9)

on

By

2015 - 2016

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms - Conduction

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

For Plane wall

For Cylinder

For Sphere

To find the general form of one-dimensional heat transfer by conduction through plane wall and temperature distribution

Heat Transfer Mechanisms - Conduction

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms - Conduction

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Internal Forced Convection

External Forced Convection

Convection Mechanism

Forced Convection Natural or Free Convection

Heat Transfer Mechanisms - Convection

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Forced Convection

Natural or Free Convection

Heat Transfer Mechanisms - Convection

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Natural or Free Convection

Heat Transfer Mechanisms - Convection

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms - Convection

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms - Radiation

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Heat Transfer Mechanisms - Radiation

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Thermal Resistance Concept

Conduction Thermal Resistance

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Thermal Resistance Concept

Convection Thermal Resistance

Radiation Thermal Resistance

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Thermal Resistance Concept

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Thermal Resistance Concept

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Lecture (1) – Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Example 1: Heat transfer by Conduction

Solved Examples

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Lecture (1) – Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Example 2: Heat transfer by Convection

Solved Examples

A 2-m-long, 0.3-cm-diameter electrical wire extends across a room

at 15°C, as shown in the following figure. Heat is generated in the

wire as a result of resistance heating, and the surface temperature

of the wire is measured to be 152°C in steady operation. Also, the

voltage drop and electric current through the wire are measured to

be 60 V and 1.5 A, respectively. Disregarding any heat transfer by

radiation, determine the convection heat transfer coefficient for

heat transfer between the outer surface of the wire and the air in

the room.

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Lecture (1) – Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Example 2: Heat transfer by Convection

Solved Examples

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Lecture (1) – Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

Example 3: Heat transfer by Radiation

Solved Examples

Consider a person standing in a room maintained at 22°C at all times. The

inner surfaces of the walls, floors, and the ceiling of the house are observed to

be at an average temperature of 10°C in winter and 25°C in summer.

Determine the rate of radiation heat transfer between this person and the

surrounding surfaces if the exposed surface area and the average outer surface

temperature of the person are 1.4 m2 and 30°C, respectively. Take the

emissivity of a person is 0.95

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Mechanical Engineering (2) – 2nd year – Electrical Power Dept.

A transistor with a height of 0.4 cm and a diameter of 0.6 cm is mounted on a circuit

board. The transistor is cooled by air flowing over it with an average heat transfer

coefficient of 30 W/m2·°C. If the air temperature is 55°C and the transistor case

temperature is not to exceed 70°C, determine the amount of power this transistor can

dissipate safely. Disregard any heat transfer from the transistor base.

Quiz

Homework

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