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# Heat Transfer

The theory of conservation of energy tells us energy cannot be created nor destroyed. It can only be transformed into different forms. Thermal energy is heat energy in the form of heat, which is transferred when a temperature difference is present. Thermal energy can only be transferred from a higher-temperature body to a lower-temperature body until both bodies reach thermal equilibrium.Thermal equilibrium is…

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# Heat Transfer

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The theory of conservation of energy tells us energy cannot be created nor destroyed. It can only be transformed into different forms. Thermal energy is heat energy in the form of heat, which is transferred when a temperature difference is present. Thermal energy can only be transferred from a higher-temperature body to a lower-temperature body until both bodies reach thermal equilibrium.

## Thermal equilibrium

Thermal equilibrium is a state where two substances of matter have reached the same temperature and hence there is no longer transfer of thermal energy between them. It can also be described as an isothermal condition. In order for it to be counted as an isothermal condition, the two bodies need to be in contact with each other.

If a block of ice with a temperature of -5°C is positioned on a table with a temperature of 25°C, will thermal equilibrium be reached?

Solution

The thermal energy of the table will transfer to the ice due to the temperature difference (ΔΤ) of 20°C, until these two objects reach thermal equilibrium. So thermal equilibrium will be reached when these two objects reach the same temperature.

### The direction of heat transfer

The temperature of the two regions under the present temperature gradient will provide the direction of the heat transfer. The direction of thermal energy will always be from the higher temperature body to the lower temperature body, when no external heat additions are in the system. In the above example, the direction of heat transfer is from the table to the ice as the table has a higher temperature.

## Methods of heat transfer

There are three main methods by which heat transfer can occur. These include convection, conduction, and radiation.

Also known as convective heat transfer, this entails the transfer of heat through the movement of a fluid that might be in the form of gas or liquid. When a fluid moves, thermal energy is being transferred from the motion of particles in the fluid.

Moving hot fluid inside a pipeline will cause the pipe to heat up as thermal energy will transfer from the hot fluid to the pipeline walls, causing the material to heat up.

This is a method of heat transfer that occurs due to collisions between particles and subatomic particles within a body. When subatomic particles collide with each other, there is a transfer of kinetic energy known as internal energy, which creates heat.

When a hot pan is removed from the stove and positioned on a table, the thermal energy from the hot pan will transfer locally to the surface of the table by the collision of particles between the two surfaces.

Radiation is a method of heat transfer in the form of light or electromagnetic waves. This type of energy is emitted by certain matters around us.

An example of radiation heat transfer is the heating of a room from the thermal energy emitted by a fireplace. In this example, heat transfer through conduction also occurs in the room, but at a lower rate.

## Heat transfer coefficient

This is a physical property of an object, used to express how well heat is being transferred. If the heat transfer coefficient (h) is large, then heat will be transferred from one object to another more easily and quickly, and vice versa. The heat transfer coefficient depends on two parameters - thermal conductivity, and the thickness of the materials in which heat transfer occurs.

Thermal conductivity is a measure of the ability of a material to conduct heat. If thermal conductivity is high, the heat transfer coefficient will also be higher. And also, if the material is quite thick, the resulting heat transfer coefficient will be lower.

## The equation for heat transfer

The rate at which heat transfer occurs can be described by the following equation. Keep in mind:

1. h is the heat transfer coefficient.
2. Q is the heat transferred measured in Watts.
3. ΔΤ is the temperature difference between the two objects of interest (it is measured in Kelvin).
4. A is the surface area of the conduction zone in m2.
5. T1 and T2 are the temperatures of the surface and surroundings respectively.

$Q = h \cdot A \cdot \Delta T = h \cdot A \cdot (T_2 - T_1)$

A pan with a diameter of 20cm and a depth of 5cm is being heated. Energy is transferred by convection from the stove to the bottom of the pan at a rate of 600 W. The resulting temperature on the outer bottom of the pan is 380 K. Find the heat transfer coefficient if the temperature at the inner bottom surface is 300 K.

Solution

We first need to find the surface area in SI units. We do that by converting 20 cm into meters and then converting diameter into radius to find the area as shown below.

$$D = 20 \space cm = 20 \cdot 10^{-2} \space m$$

$$R = \frac{20 \cdot 10^{-2} \space m}{2} = 0.1 \space mA = \pi \cdot r^2 = \pi \cdot 0.1^2 m^2 = 0.0314 \space m^2$$

Find the heat transfer coefficient by rearranging the heat transfer rate equation.

$$Q = h \cdot A \cdot (T_2 - T_1) \rightarrow h = \frac{Q}{A \cdot (T_2-T_1)}$$

Finally, we substitute the given values in the final equation to find the heat transfer coefficient.

$$h = \frac{Q}{A \cdot (T_2-T_1} = \frac{600}{0.0314 \cdot (380-300)} [W/m^2K] = 238.85 [W/m^2K]$$

## Heat Transfer - Key takeaways

• Thermal energy transfer is the transfer of thermal energy between two bodies due to temperature differences.

• Thermal energy can be transferred by three main methods.

• Convection, conduction, and radiation are the methods of thermal energy transfer.

• The heat transfer coefficient is a characteristic of the thermal conductivity of a material, which measures how well heat is being transferred through the material.

It is the thermal energy transferred in the form of  electromagnetic radiation or light.

Heat is transferred by three methods, convection , radiation and conduction.

The moving molecules between a fluid and a body are colliding, transferring heat from one body to the other.

Thermal energy is emitted in the form of electromagnetic waves by a heated body and travels and travels until it is absorbed.

It is the thermal energy being transferred between bodies due to a present temperature difference (ΔΤ).

## Heat Transfer Quiz - Teste dein Wissen

Question

What is thermal energy transfer?

It is the energy being transferred in the form of heat between bodies.

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Question

Which of the following is not a method for thermal energy transfer?

Absorption

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Question

What is convective heat transfer?

It is the transfer of thermal energy through a fluid due to movement of the particles.

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Question

It is thermal energy emitted from heated bodies as EM radiation.

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Question

What is conductive heat transfer?

It is the transfer of thermal energy through collisions of subatomic particles of a body.

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Question

What is the heat transfer coefficient?

It is a parameter that defines how well a medium can transfer thermal energy.

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Question

What is the formula for heat transfer?

It is defined as Q= hA(T2-T1).

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Question

How do we find the heat transfer coefficient?

By rearranging the above equation h = Q/A(T2-T1).

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Question

Which of the following parameters does not affect the heat transfer coefficient

Thermal resistance

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Question

Which of the following parameters does not affect heat flow?

Volume

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Question

Can thermal energy be created from nothing? Yes or no.

No.

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Question

State the conservation of energy theory.

Energy cannot be created nor destroyed, it can only change form.

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Question

Is thermal conductivity the same as heat transfer coefficient?

No, the heat transfer coefficient depends on the thermal conductivity.

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Question

Can the following example be described as convective heat transfer? Yes or no.

'A solid piece of ice is positioned in one’s hand. After a while, the ice melts into liquid.'

No.

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Question

Is it possible to have convective, conductive and radiative heat transfer simultaneously? Yes or no.

Yes.

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