What Is Radiation Heat Transfer

[caption id="attachment_208" align="alignright" width="270" caption="radiation heat transfer"][/caption]

Radiation heat transfer is basically the energy transfer via electromagnetic waves. Before going into details about radiation heat transfer, it is essential to understand the radiations first.

What Are Radiations?

Radiations are defined as the electromagnetic waves having wavelength of 0.1 to 100 microns, which doesn’t require any medium to travel. Radiation waves are classified into two types; ionizing radiations and non ionizing radiations. Ionizing radiations have the tendency (because of having sufficient energy) to ionize an atom. Non-ionizing radiations cannot ionize an atom (heat waves, radio waves and light waves are the examples of non-ionizing radiations).  Hence, in physics on nuclear engineering, we deal with the ionizing radiations, while here; non-ionizing radiations are of our main interest.

 Image: Salvatore Vuono

What Is Radiation Heat Transfer?

According to the quantum theory, radiations consist of energy packets (named as photons), that has no rest mass and move at the velocity of light. So, radiation heat transfer basically deals with the exchange or transfer of that energy between the bodies.  Every object, having temperature greater then absolute zero (0 K) emits radiations. Mostly, the solids are considered as the radiation emitters, because the energy emitted by the fluid particles is usually absorbed by the nearby molecules, and thus this energy cannot reach the surface. These emissions are directly proportional to the temperature of the body; the higher the temperature, higher will be the radiations emissions.

Hence:

Ever object, above absolute zero temperature emits energy carrying electromagnetic radiations. When these radiations fall on the other object, some energy is transferred from the radiation waves to the object. This transferred energy is known as the radiation heat transfer.

Emissivity:

Emissivity is the tendency of an object to release electromagnetic radiations per unit area and per unit time. In order to calculate the radiation emissive power, we assume an ideal surface, which can absorb and radiate all wavelength radiations. This ideal surface is named as the black body, or the ideal radiator. However, the heat flux of the real surface is less than that of the black body. According to Stefan Boltzmann’s law:

E = ԑσ T_s⁴

Where:

E             =             Emissive power of real surface;  (W/m²)

ԑ             =             Radiative property of the surface.

σ             =             Stefan Boltzmann constant;          (5.67 x 10^-8 W/m².K⁴)

T_s            =             Absolute temperature;                   (K)

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What Is Radiation Heat Transfer

Radiation heat transfer is basically the energy transfer via electromagnetic waves. Before going into details about radiation heat transfer, it is essential to understand the radiations first.

What Are Radiations?

Radiations are defined as the electromagnetic waves having wavelength of 0.1 to 100 microns, which doesn’t require any medium to travel. Radiation waves are classified into two types; ionizing radiations and non ionizing radiations. Ionizing radiations have the tendency (because of having sufficient energy) to ionize an atom. Non-ionizing radiations cannot ionize an atom (heat waves, radio waves and light waves are the examples of non-ionizing radiations).  Hence, in physics on nuclear engineering, we deal with the ionizing radiations, while here; non-ionizing radiations are of our main interest.

What Is Radiation Heat Transfer?

According to the quantum theory, radiations consist of energy packets (named as photons), that has no rest mass and move at the velocity of light. So, radiation heat transfer basically deals with the exchange or transfer of that energy between the bodies.  Every object, having temperature greater then absolute zero (0 K) emits radiations. Mostly, the solids are considered as the radiation emitters, because the energy emitted by the fluid particles is usually absorbed by the nearby molecules, and thus this energy cannot reach the surface. These emissions are directly proportional to the temperature of the body; the higher the temperature, higher will be the radiations emissions. 

Hence:

Ever object, above absolute zero temperature emits energy carrying electromagnetic radiations. When these radiations fall on the other object, some energy is transferred from the radiation waves to the object. This transferred energy is known as the radiation heat transfer. 

Emissivity:

Emissivity is the tendency of an object to release electromagnetic radiations per unit area and per unit time. In order to calculate the radiation emissive power, we assume an ideal surface, which can absorb and radiate all wavelength radiations. This ideal surface is named as the black body, or the ideal radiator. However, the heat flux of the real surface is less than that of the black body. According to Stefan Boltzmann’s law:

E = ԑσ T_s⁴

Where:

E             =             Emissive power of real surface;  (W/m²)

ԑ             =             Radiative property of the surface.

                σ             =             Stefan Boltzmann constant;          (5.67 x 10^-8 W/m².K⁴)

T_s               =             Absolute temperature;                  (K)

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What Is Convection?

 

[caption id="attachment_210" align="alignright" width="300" caption="convection heat transfer"][/caption]

What Is Convection:

Convection is the mechanism of heat transfer occurs as a result of movement of fluid on a macroscopic scale. I.e. heat transfer due to the mixing of elements in fluid or the heat transferred from a solid surface to the moving fluid.

There are several factors, on which heat transfer by convection depends on, such as fluid thermal conductivity, fluid density, fluid velocity, solid surface roughness, temperature difference between fluid and solid surface, moving fluid turbulence, etc. however, as a general rule, it has been experimentally proven that the higher the fluid velocity, the higher is the convective heat transfer coefficient [some times called as film conductance, because of its relation to the conduction process].

Difference Between Conduction And Convection:

It generally doesn’t make sense trying to differentiate between the conduction and convection; as it is the same energy, which is transferred by the combined action of conductivity and the movement of the fluid. Initially, the energy is delivered from solid to the fluid at the solid-fluid interface by conduction then the fluid stream absorbs and transfers energy as convection.

Classification Of Convective Heat Transfer Coefficient:

Convective heat transfer is classified as:

• Forced convection

In forced convection, the fluid is forced to flow by external means, such as fans, stirrers, etc. generally, the magnitude or rate of heat transfer in force convection is greater then that of natural convection. In this mode of heat transfer, the heat transfer coefficient, h, mainly depends on the fluid velocity.

• Free convection

Free convection is also called as natural convection, i.e. fluid flows naturally because of the gravitational and buoyancy forces.

Newton’s Cooling Law For Heat Convection:

Newton’s law of cooling is considered as the basic law for convection; which is stated as:

“The heat transfer per unit area by convection is directly proportional to the temperature difference between solid and fluid which, using proportionality constant called the heat transfer coefficient, i.e.

\[Q=hA(T_{fluid}-T_{solid})\]

Where,

h = Convective heat transfer coefficient; W/m².^oC

 

Dimensionless Numbers Used For Convection Heat Transfer Analysis:

• Reynolds Number

Reynolds number is related to the flow of fluids; specially the transition of flow from laminar flow to turbulent flow conditions. This dimensionless number is used to describe whether the flow is laminar or turbulent; hence this is the main step for the convection heat transfer analysis.

\[Re=\ \frac{\rho DV}{\mu }\]

Where,

ρ = density of fluid

V = average fluid velocity

D = tube diameter [internal]

µ = dynamic viscosity of fluid

• Nusselt Number:

This is actually the empirical correlation of the tube size along with the flow conditions.

\[Nu=\ \frac{hL}{k_f}\]

Where,

h = connective heat transfer coefficient.

L = characteristic length of the tube

k_f = thermal conductivity of fluid

• Prandtl Number

It is the ratio of the kinematic viscosity (υ) to the thermal diffusivity (α). It represents the thermo-physical property of fluid, and is independent of flow conditions.

\[Pr=\frac{\upsilon }{\alpha }=\frac{{cp}_{\upsilon }}{k_f}\]

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Reference books:

• Kirk Othmar, “ Encyclopedia Of Chemical Technology”, vol. 12, 4^th ed. , “Heat Exchange Technology”.


• J.P. Holman, “Heat Transfer”, 10^th edition.


• Eduardo Cao, “Heat transfer In Process Engineering”, chap. 4

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What Is Convection?

Convection is the mechanism of heat transfer occurs as a result of movement of fluid on a macroscopic scale. I.e. heat transfer due to the mixing of elements in fluid or the heat transferred from a solid surface to the moving fluid.

There are several factors, on which heat transfer by convection depends on, such as fluid thermal conductivity, fluid density, fluid velocity, solid surface roughness, temperature difference between fluid and solid surface, moving fluid turbulence, etc. however, as a general rule, it has been experimentally proven that the higher the fluid velocity, the higher is the convective heat transfer coefficient (some times called as film conductance, because of its relation to the conduction process).

Difference Between Conduction And Convection:

It generally doesn’t make sense trying to differentiate between the conduction and convection; as it is the same energy, which is transferred by the combined action of conductivity and the movement of the fluid. Initially, the energy is delivered from solid to the fluid at the solid-fluid interface by conduction then the fluid stream absorbs and transfers energy as convection.

Classification Of Convective Heat Transfer:

Convective heat transfer is classified as:

• Forced convection

In forced convection, the fluid is forced to flow by external means, such as fans, stirrers, etc. generally, the magnitude or rate of heat transfer in force convection is greater then that of natural convection. In this mode of heat transfer, the heat transfer coefficient, h, mainly depends on the fluid velocity.

• Free convection

Free convection is also called as natural convection, i.e. fluid flows naturally because of the gravitational and buoyancy forces.

Newton’s Cooling Law For Heat Convection:

Newton’s law of cooling is considered as the basic law for convection; which is stated as:

“The heat transfer per unit area by convection is directly proportional to the temperature difference between solid and fluid which, using proportionality constant called the heat transfer coefficient, i.e.

           

Q = hA (T_fluid – T_solid )

 Where,

            h = Convective heat transfer coefficient; W/m².^oC

Dimensionless Numbers Used For Convection Heat Transfer Analysis:

• ·       Reynolds Number

Reynolds number is related to the flow of fluids; specially the transition of flow from laminar flow to turbulent flow conditions. This dimensionless number is used to describe whether the flow is laminar or turbulent; hence this is the main step for the convection heat transfer analysis.

                                                            Re = ρVD

                                                                      µ

Where,

            ρ = density of fluid

            V = average fluid velocity

            D = tube diameter (internal)

             µ = dynamic viscosity of fluid

•  Nusselt Number:

This is actually the empirical correlation of the tube size along with the flow conditions.

                                                            Nu = hL

                                                                     k

Where,

            h = connective heat transfer coefficient.

            L = characteristic length of the tube

            k = thermal conductivity of fluid

• Prandtl Number

It is the ratio of the kinematic viscosity (υ) to the thermal diffusivity (α). It represents the thermophysical property of fluid, and is independent of flow conditions.

                                                            Pr = υ = c_p υ

                                                                   α      k_f

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Reference books:

• Kirk Othmar, “ Encyclopedia Of Chemical Technology”, vol. 12, 4^th ed. , “Heat Exchange Technology.

• J.P. Holman, “Heat Transfer”, 10^th edition.

• Eduardo Cao, “Heat transfer In Process Engineering”, chap. 4

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What is conduction?

The heat transferscience and its basic concepts have already been discussed in details. This post is about the Conduction phenomena in detail.

What Is Conduction?

Conduction is the phenomena of transfer of energy due to the temperature gradient. On the molecular level, conduction definition can be described as the transfer of kinetic energy between the molecules;due to the elastic and inelastic collisions between the molecules.

The term conduction is basically used for the heat transfer between the solids. In liquids and gases pure conduction can not exist.

Fourier’s Law Of Heat Conduction:

 Fourier law is used as the general equation of conduction. Fourier law states that:

“The rate of heat transfer per unit area is directly proportional to the normal temperature gradient.”

\[Q=\ -kA\ \frac{dT}{dx}\]

or

\[q_x=-k\frac{dT}{dx}\]

The negative sign of the equation shows the negative temperature gradient, which ensures that the thermal energy flows in the direction of decreasing temperature.

Where:

Q= rate of heat transfer

A= heat transfer area

k  = thermal conductivity of material; W/m.K

q = heat flux ; W/m²

The above two equations are the equations for heat conduction in single direction. As per to the Cartesian coordinates system,the above equations can be simplified as the most general equation of conduction is:

\[q=-k\nabla T\]

Thermal Conductivity Units (k):

Thermal conductivity units in SI system    :   W/m.^oC

Thermal conductivity units in FPS system :   Btu/hr·ft⋅F

Where:

1W/(m. ^oC) = 0.5778 Btu/hr·ft⋅F

One Dimensional Steady State Conduction:

The term steady state conduction describes that the temperatures at any point are independent of the time factor. The one dimensional conduction refers to the fact that the temperature gradients exist along in the single direction only.

• Plane wall:

The heat transfer rate through a plane wall (made up of single material) is :

\[Q_x=-\frac{kA}{\triangle x}(T_2-T_1)\]

            Or

\[Q_x=\frac{T_1-T_2}{R_th}\]

            Where R_th is the resistance to the heat transfer, which is equal to the Δx/ kA

• Composite wall:

The heat transfer rate through a composite wall made up of more then 1 material is :

\[Q_x=\frac{{\triangle T}_{overall}}{\Sigma R_{th}}\]

• Cylinders:

The heat transfer rate along the cylinder are:

\[Q=\frac{2\pi Lk(T_i-T_o)}{{\rm ln}⁡(\frac{r_0}{r_i})}\]

• Spheres

Spherical systems are also considered as the one dimensional systems. The heat transfer rate along the sphere is:

\[Q=\frac{4\pi k(T_i-T_o)}{\frac{1}{r_i}-\frac{1}{r_0}}\]

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What is conduction?

The heat transfer science and its basic concepts have already been discussed in details. This post is about the Conduction phenomena in detail.

What Is Conduction?

Conduction is the phenomena of transfer of energy due to the temperature gradient. On the molecular level, conduction definition can be described as the transfer of kinetic energy between the molecules; due to the elastic and inelastic collisions between the molecules.

The term conduction is basically used for the heat transfer between the solids. In liquids and gases pure conduction can not exist.

Fourier’s Law Of Heat Conduction:

 Fourier law is used as the general equation of conduction. Fourier law states that:

“The rate of heat transfer per unit area is directly proportional to the normal temperature gradient.”

Q = -kA dT

           dx

Or

q_x = -k dT

            dx

The negative sign of the equation shows the negative temperature gradient, which ensures that the thermal energy flows in the direction of decreasing temperature.

Where:

            Q = rate of heat transfer

            A = heat transfer area

            k  = thermal conductivity of material; W/m.K

            q  = heat flux ; W/m²

The above two equations are the equations for heat conduction in single direction. As per to the Cartesian coordinates system, the above equations can be simplified as the most general equation of conduction is:

q = - k ▽T

Thermal Conductivity Units (k):

Thermal conductivity units in SI system    :   W/m. ^oC

Thermal conductivity units in FPS system :   Btu/hr·ft⋅F

Where:

1 W/(m. ^oC) = 0.5778 Btu/hr·ft⋅F

One Dimensional Steady State Conduction:

The term steady state conduction describes that the temperatures at any point are independent of the time factor. The one dimensional conduction refers to the fact that the temperature gradients exist along in the single direction only.

• Plane wall:

The heat transfer rate through a plane wall (made up of single material) is :

Q_x = - kA ( T₂ – T₁)

               Δx

            Or

Q_x =  T₁ – T₂

           R_th

            Where R_th is the resistance to the heat transfer, which is equal to the Δx/ kA

• Composite wall:

The heat transfer rate through a composite wall made up of more then 1 material is :

Q_x  =  ΔT_overall

             ∑R_th           

• Cylinders:

The heat transfer rate along the cylinder are:

Q=  2πLk (T_i – T_o)

ln(r_o/ r_i)

• Spheres

Spherical systems are also considered as the one dimensional systems. The heat transfer rate along the sphere is:

                                                         Q = 4πk (T_i – T_o)

                                                           1/r_i  - 1/r_o

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What Is Heat Transfer

[caption id="attachment_198" align="alignright" width="300" caption="what is heat transfer"][/caption]

Heat transfer is considered as one of the most basic discipline of chemical engineering & technology. It generally concerns with generation, consumption and conversion of heat energy in the system. Heat transfer; it self plays a very important role in process industries, and it is always better for the process industries, to optimize their heat transfer processes [may include furnaces, evaporators, distillation units, dryers, reaction vessels etc] by the selection of proper heat exchangers, preventing heat losses and controlling the heat flow rate.

Heat transfer under normal conditions always flows from a hotter region to a cooler region, i.e. heat transfer follows the temperature gradient between the two systems [or system and surrounding], until a thermal equilibrium is maintained between the two systems.  However, according to the “Clausius statement of second law of thermodynamics”, if the work is done on the system, the heat can flow from a colder region to a hotter region. This Clausius statement is the basic principle behind the working of refrigerators.

Now how to calculate heat transfer :

\[Q=UAΔT\]

Where,

Q = transfer of heat per unit time
A = heat transfer area
ΔT = temperature difference between two systems

This is the most basic expression to define heat transfer, there are several other heat transfer formulas derived from this expression. Picture at the right side is taken from images by dan.

There are three forms of heat transfer:

1) Conduction

Conduction is generally considered as the heat transfer phenomena for the solids, however conduction can also occur in fluids too [in microscopic level, like diffusion phenomena]. In solids, Conduction is the result of transfer of vibration energy from one molecule to other, while in fluids, it occurs in addition as a result of transfer of kinetic energy. Conduction follows the Fourier Law of Heat Conduction:

\[Q=-kAΔT\]

Where,

Q = transfer of heat per unit time
k = conductive heat transfer coefficient
A  = heat transfer area
ΔT = temperature difference between two systems

  The thermal conductivity units in SI system is W/mK.

2) Convection

Convective heat transfer occurs when the heat is transferred from a solid surface to a moving fluid owing to the temperature difference between the solid and the fluid. Convection follows the Newton’s Cooling Law of Heat Convection:

\[Q=hA(T-T_{'})\]

Where,

Q = transfer of heat per unit time
h = convection heat transfer coefficient
A  = heat transfer area
T = temperature of fluid
T'= temperature of solid

3) Radiation

All materials radiate thermal energy in the form of electromagnetic waves. So radiation is the transfer of heat by the emission of electromagnetic waves. When they fall on the body, they may partially be reflected, transmitted or absorbed. Radiation is that fraction which falls and absorbed by the body.

Read More

What Is Heat Transfer

Heat transfer is considered as one of the most basic discipline of chemical engineering & technology. It generally concerns with generation, consumption and conversion of heat energy in the system. Heat transfer; it self plays a very important role in process industries, and it is always better for the process industries, to optimize their heat transfer processes (may include furnaces, evaporators, distillation units, dryers, reaction vessels etc) by the selection of proper heat exchangers, preventing heat losses and controlling the heat flow rate.

Heat transfer under normal conditions always flows from a hotter region to a cooler region, i.e. heat transfer follows the temperature gradient between the two systems (or system and surrounding), until a thermal equilibrium is maintained between the two systems.  However, according to the “Clausius statement of second law of thermodynamics”, if the work is done on the system, the heat can flow from a colder region to a hotter region. This Clausius statement is the basic principle behind the working of refrigerators.

Now how to calculate heat transfer :
                                                           Q = UAΔT

Where,
  Q = transfer of heat per unit time
  A = heat transfer area
  ΔT = temperature difference between two systems

This is the most basic expression to define heat transfer, there are several other heat transfer formulas derived from this expression.

There are three forms of heat transfer:

1) Conduction

Conduction is generally considered as the heat transfer phenomena for the solids, however conduction can also occur in fluids too(in microscopic level, like diffusion phenomena). In solids, Conduction is the result of transfer of vibration energy from one molecule to other, while in fluids, it occurs in addition as a result of transfer of kinetic energy.

Conduction follows the Fourier Law of Heat Conduction:

                                                          Q = -kAΔT

Where,
  Q = transfer of heat per unit time
   k = conductive heat transfer coefficient
  A  = heat transfer area
 ΔT = temperature difference between two systems

  The thermal conductivity units in SI system is W/mK.

2) Convection

Convective heat transfer occurs when the heat is transferred from a solid surface to a moving fluid owing to the temperature difference between the solid and the fluid.

Convection follows the Newton’s Cooling Law of Heat Convection:

                                                        Q = hA(T – T')
Where,
  Q = transfer of heat per unit time
   h = convection heat transfer coefficient
  A  = heat transfer area
  T = temperature of fluid
  T'= temperature of solid

3) Radiation

All materials radiate thermal energy in the form of electromagnetic waves. So radiation is the transfer of heat by the emission of electromagnetic waves. When they fall on the body, they may partially be reflected, transmitted or absorbed. Radiation is that fraction which falls and absorbed by the body.

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what is heat exchanger

Heat transfer is considered as the basics of chemical engineering, and most important heat transfer equipment is the heat exchanger. A chemical engineer knows it very well that what is heat exchanger. Heat exchanger is the equipment used for effective transfer of heat between two fluids. These fluids are separated by means of solid wall, to avoid their mixing. The heat exchangers are the widely used equipments in chemical and process industries, So why a chemical engineer must have firm grip on heat exchangers designs, and their working. Most common application areas of them are boilers, condensers, intercoolers, preheaters , etc.

The heat exchanger performance is calculated by

                                                                                                 n = Q / Qmax 

  

where,
           n        =   performance or efficiency

           Q       =   actual rate of heat transfer

           Qmax    =   maximum rate of heat transfer

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what is heat exchanger

Heat transfer is considered as the basics of chemical engineering, and most important heat transfer equipment is the heat exchanger. A chemical engineer knows it very well that what is heat exchanger. A heat exchanger is the equipments used for effective transfer of heat between two fluids. These fluids are separated by means of solid wall, to avoid their mixing. The heat exchangers are the widely used equipments in chemical and process industries, So why a chemical engineer must have firm grip on heat exchangers designs, and their working. Most common application areas of them are boilers, condensers, intercoolers, preheaters , etc.

The heat exchanger performance is calculated by

                                                                     n = Q / Qmax 

  

where,
           n        =   performance or efficiency

           Q       =   actual rate of heat transfer

           Qmax    =   maximum rate of heat transfer

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