Heat Transfer 1 | Basics In This Blog we will learn about Basics of heat transfer.

Content of this blog

Interview Questions from this Heat transfer basics

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Thanks for reading - Heat Transfer 1 | Basics

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Content of this blog

- Definitions
- Basics of Heat Transfer
- Modes of Heat Transfer
- Practical example

Interview Questions from this Heat transfer basics

- What is definition of Heat transfer?
- What is definition of Temperature?
- What is Definition of Heat?
- Which data predicted from Heat transfer and Thermodynamics?

### 1. Definitions

Following definitions must known before study of heat transfer**A. Heat Transfer**- Heat Transfer is energy in transit due to temperature difference.

**B. Temperature**

- Measure of the amount of energy possessed by the molecules of a substance
- Measure of how hot or cold a substance is
- Predict the direction of heat transfer
- Symbol is T and its unit is Celsius and Kelvin

**c. Heat**

- Energy in transit
- The transfer of energy as heat occurs at the molecular level as a result of a temperature difference
- The usual symbol for heat is Q.
- Common unit of heat is Joule and calorie in the SI system.
- Thermal energy is related to the temperature of matter.

- Greater its thermal energy.

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2. Basics of Heat Transfer

Heat
transfer is a study of:

- Exchange of thermal energy through a body or between bodies. This occurs there is a temperature difference.
- When two bodies are at different temperatures. Thermal energy transfers from the one with higher temperature to the one with lower temperature
- Heat always transfers from hot to cold. Heat always moves from a warmer place to a cooler place.
- Hot objects in a cooler room will cool to room temperature. Cold objects in a warmer room will heat up to room temperature.
- If a cup of coffee and a ice cube were left on the table in this room what would happen to them? Why?

The cup of coffee
will cool until it reaches room temperature.

The ice cube will
melt and then the liquid will warm to room temperature.

Thermodynamics tells us:

• How much heat is transferred (δQ)

• How much work is done (δW)

• Final state of the system

Heat transfer tells us:

• How (with what modes)
δQ is transferred

• At what rate δQ
is transferred

• Temperature distribution inside the body

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### 3. Modes of Heat transfer

Watch video in Hindi in YouTube

There are three modes of heat
transfer:

Conduction

Convection

Radiation

•
Any energy exchange between bodies occurs through one of
these modes or a combination of them.

•
An energy transfer across a system boundary due to a
temperature difference

**Conduction**

- Transfer of heat through solids or stationery fluids by the mechanism of inter-molecular interactions. It needs a matter It does not require any bulk motion of matter.

- When you touch a hot object, the heat you feel is transferred through your skin by conduction.

- When you heat a metal strip at one end, the heat travels to the other end.As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat.

- The outer electron of metal atoms drifts, and is free to move.When the metal is heated, this ‘sea of electrons’ gain kinetic energy and transfer it throughout the metal.

- Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

It is a thermodynamic property of a material.b Unit is W/m K

Thermal Conductivity of Selected Materials at Room
Temperature.

•

•
Copper 401

•
Silver 429

•
Gold 317

•
Aluminum
237

•
Steel 60.5

•
Limestone
2.15

•
Bakelite
1.4

•
Water 0.613

•
Air 0.0263

An energy transfer across a system boundary due to a temperature difference

**Convection**

Convection uses the motion of fluids to transfer heat

- By the combined mechanisms of Intermolecular interactions Bulk transport.
- Convection needs fluid matter.
- In a typical convective heat transfer
- Hot surface heats the surrounding fluid
- This is then carried away by fluid movement such as wind.
- The warm fluid is replaced by cooler fluid
- Which can draw more heat away from the surface?
- Since the heated fluid is constantly replaced by cooler fluid
- Rate of heat transfer is enhanced.

Free or natural convection induced by buoyancy forces

*Natural convection*

The fluid movement is created by the warm fluid itself.

The density of fluid decrease as it is heated

Hot fluids are lighter than cool fluids

Warm fluid surrounding a hot object rises

It is replaced by cooler fluid.

The result is a circulation of air above the warm surface

*Forced convection*

Induced by external means

May occur with phase change boiling condensation

*What happens to the particles in a liquid or a gas when you heat them?*

The particles spread out and become less dense.

This effects fluid movement

Cooler, dense fluids sink through warmer less dense fluids.

In effect, warmer liquids and gases rise up.

**Radiation**

- Heat transfer involves the transfer of heat by electromagnetic radiation
- That arises due to the temperature of the body.
- Radiation does not need matter.
- There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection.

### Practical example

Four containers were placed equidistant from a heater. Which container would have the

Warmest water after ten minutes?

Black container would be the warmest after ten
minutes because its surface absorbs heat radiation the best. The shiny metal
container would be the coolest because it is the poorest at absorbing heat
radiation.

•
Conduction rate equation is described by the Fourier Law:

q

_{conduction}= - k A dT/dx
q
= heat flow:

k =
thermal conductivity, (W/m K)

A =
Cross-sectional area through which the heat is conducting

dT/dx
= dT is the change in
temperature

- By adding more insulation to a wall always decreases heat transfer.
- The thicker the insulation, the lower the heat transfer rate.
- This is expected, since the heat transfer area A is constant
- Adding insulation always increases the thermal resistance of the wall without affecting the convection resistance.
- Adding insulation to a cylindrical piece or a spherical shell, however, is a different matter
- The additional insulation increases the conduction resistance of the insulation layer
- But decreases the convection resistance of the surface because of the increase in the outer surface area for convection.
- The heat transfer from the pipe may increase or decrease, depending on which effect dominates.
- The value of r2 at which heat transfer rate reaches maximum is determined from the requirement that (zero slope).
- Performing the differentiation and solving for r2 yields the critical radius of insulation for a cylindrical body to be rc = k/h
- Note that the critical radius of insulation depends on The thermal conductivity of the insulation k
- The external convection heat transfer coefficient h .

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