Heat Transfer

Heat is a form of energy that derives its origins at the molecular scale. Those molecules of a substance vibrate at their positions either fixed or not when energy is supplied to them. When they vibrate, they often transfer their energy to the surrounding molecules, allowing them to vibrate.

Heat energy can transfer from one body to another, or from one body location to another. The study of the methods and techniques adopted to transfer heat energy is called 'Heat Transfer.' In order to enable heat transfer between 2 bodies, there must be a temperature difference between them. That ensures that these two bodies must be at two different temperatures, one higher than another, that allow heat to flow from one body to another.

For example, when an ice cube is kept in a glass of water of normal temperature, the water transfers some of the heat to the ice cube causing it to melt. 

What are Different Modes of Heat Transfer

In our daily life, it has been observed that when a pan is boiled full of water on a flame, its temperature increases. However, it slowly cools down when the flame is turned off.

This happens due to the phenomenon of heat transfer occurring between the pan full of water and flame. It has been confirmed that heat transfer takes place from hotter objects to colder objects.

When objects are falling at different temperatures or if there is an object at a different temperature from the surroundings, then the transfer of heat takes place such that both the object and the surroundings reach an equilibrium temperature.

There exist three modes of heat transfer. Examples of modes of heat transfer of those are given below.

1. Conduction

2. Convection

3. Radiation

Modes of Heat Transfer with Example

Conduction of Heat

Conduction of Heat is a process where heat is transferred from the hotter part of the body to the colder part without involving any actual movement of the body molecules. Here, the heat transfer occurs from one to another molecule as a result of the molecules' vibratory motion. Transfer of heat happens through the process of conduction occurring in substances which are in direct contact with each other. Generally, it takes place in solids.

Some modes of heat transfer examples are when frying vegetables in a pan, Heat transfer takes place from the flame to the pan and next to the vegetables.

Based on the heat conductivity, substances can be divided as conductors and insulators. Substances that conduct heat quickly are known as conductors, and the one which does not conduct heat are called insulators. Materials that are bad conductors of heat are mostly bad conductors of electricity. 

Suppose, when we put our hand on a hot kettle, we instantly feel the sensation of the heat oozing through it. We end up removing our hands.

In sum, when a Substance is heated, the molecules inside it move more quickly. In this process, they bump into the things beside them and transfer their energy. This process is termed conduction. 

The heat transfer coefficient is the quantitative characteristic of convective heat transfer between a fluid medium and the surface flowed over by the fluid. 

In studies of chemical and mechanical engineering, the heat transfer coefficient is used for calculating the heat transfer that happens between a fluid and a solid, between fluids that are separated by a solid, or between two solids. It is always the inverse of thermal insulance. The heat transfer coefficient has the SI units of W / ( m2K ). It is calculated with regard to the following way:

h = ∆ Q / (A * ∆T * ∆t)

here h is the heat transfer coefficient, ∆ Q is the heat input of the system or the heat that has been lost, A is the surface area where the heat is transferred, ∆ T is the temperature difference in between solid surface and the surrounding fluid area, and ∆t is the change in time which comprises the time period in which the heat transfer has occurred constantly.

Depending on the ways in which heat is transferred, the heat transfer coefficient is calculated in different ways possible. Most solid substances have known thermal conductivity which can be used on a basis for calculating the heat transfer coefficient. The rate of heat transfer through emitted radiation is determined by the Stefan Boltzmann law of radiation: Qt=eAT4 Q t = e A T 4 , where = 5.67 10 8 J/s m2 K4 is the Stefan Boltzmann constant, A is the surface area of the object, and T is its absolute temperature in kelvin. In English units h is expressed in units of Btu/(hft2R), and in SI units it is W/(m2K). The algebraic sign of Newton’s law of cooling is positive for T > Ts (heat transfer into the object) and negative when T < Ts (heat transfer out of the object). Heisler charts are used for determining the temperature distribution, and the heat flow when conduction and convection resistance are almost equal or has to be Bi = 1.

Convection of Heat

This is a process where heat is transferred in both liquid and gasses from a region of higher temperature to that of lower temperature. Convection heat transfer happens partly because of either the actual movement of molecules or due to the mass transfer.

One of the modes of heat transfer examples is Heating milk in a pan.

Radiation of Heat

Radiation of heat is the process where heat is transferred from one to another body without involving the medium molecules. This type of heat transfer does not depend on the medium.

One of the modes of heat transfer examples is in an oven, the substances are heated directly without a heating medium.

Factors Affecting Heat Transfer

Now, let us discuss the factors or rate of heat transfer on which it depends. The rate of heat transfer generally depends on the following factors.

ΔQΔt ∝ A(T1–T2)x

The resultant heat transfer equation will be, ΔQΔt = K A(T1–T2)x 

where, K is the coefficient of heat transfer. 

Here, if the heat flow is positive, then we can infer T1 > T2. So heat flows from higher to lower temperature. We can observe that an analogy with electricity can be drawn, and here the temperature plays a role in the rate of heat transfer, and a potential difference is like current while the rest of expression is like an Electric Resistance. Now that we have drawn an analogy, there must also be a series and a parallel connection here, as described below.

1. Heat Transfer in Series

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Let the temperature of the junction is T. Thus, for the first rod,

⇒ ΔQΔt = K1 A1(T1–T)L1 --- (1)

For the second rod,

⇒ ΔQΔt = K2 A2(T–T2)L2 —- (2)

Because the temperature of conjunction remains constant, the rate of heat transfer in the equations (1) and (2) must be the same. By using the equation, we can find the value of temperature (T).

2. Heat Transfer in Parallel

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For the first rod,

ΔQΔt = K1A1(T1–T2)L -- (3)

For the second rod,

ΔQΔt = K2 A2(T1–T2)L -- (4)

So, net heat flow is the summation of equations (3) and (4). Assume that the depth of the lake is h, and the outside temperature is T. How much time does it take to freeze the entire lake? The thermal conductivity is K, and the latent heat of ice is L.

The rate of heat transfer at this point is, ⇒ dQdt = KATx

⇒ dQ = KATx dt -- (5)

Now, this heat is taken out and a dx layer of ice is formed.

dm = ρA.dx -- (6)

Also,

dQ = dm.L

Substituting values from (5) and (6) we get,

KATx dt = ρA.dx.L

⇒ ∫t0 dt = ρLKT∫h0x.dx

Integrating with the limits we get,

t = ρLh22KT