Forced Convection Heat Transfer


In this article “Forced convection heat transfer” term and forced convection heat transfer related facts will be prate in a brief manner. Forced convection heat transfer uses in pump, ceiling fan, suction device.

Forced convection heat transfer is a term that is a classification of transport or forced convection heat transfer is a mechanism which helps to produce motion of a flowing fluid by applying force from externally. Almost in everywhere forced convection heat transfer is used such as steam turbine, central heating and many more.

What is forced convection heat transfer?

Alongside thermal conduction, natural convection and thermal convection is are the classification of transferring the heat and also allow to sufficient quantity of heat energy should be transform without any hassle.

Forced convection heat transfer is actually a very special classification of heat transfer. By the help of forced convection heat transfer a fluid is impression to move from one are to another area by applying force from outer side. In this case the amount of heat transfer is an increase for this it’s another term is Heat Rises.

Forced convection heat transfer equation:

When potentially mixed convection is analyze in that case the physical parameter known as Archimedes Number.

In the Archimedes Number two conditions are involves forced convection and free of relative strength. Forced convection heat transfer equation is given below,

Ar = Gr/Re2

Where,

Ar = Archimedes Number

Gr = Grashof Number

Re = Reynolds Number

By the help of Grashof number buoyancy force is express and by the help of Reynolds number inertia force is express. So, from the forced convection heat transfer equation it is clear that Archimedes Number also means ration of Grashof number and square of Reynolds number.

When the value of Ar < < 1 then its represent forced convection heat transfer equation.

The another physical parameter to express forced convection heat transfer is Peclet number. Peclet number is ratio of movement by current means advection and movement from higher to lower concentration means diffusion.

Pe = UL/α

When the value of Pe > > 1 means advection dominates diffusion.

When the value of Pe < < 1 means diffusion dominates advection.

forced convection heat transfer
Image – Forced convection by a fan in a snow machine;
Image Credit – Wikipedia

Forced convection heat transfer coefficient:

The equation of the forced convection heat transfer coefficient is discuss below,

Forced convection heat transfer coefficient in internal flow and laminar flow:-

Tate and Sieder give a concept of correlation to account for laminar flor in entrance effect.

A forced convection heat transfer coefficient in internal flow and laminar flow can be express as,

NuD = 1.86 (Re . Pr)1/3 (D/L)1/3bw})0.14

Where,

D = Internal diameter

μb = Fluid viscosity of bulk mean temperature

μw= Fluid viscosity at the wall temperature of the pipe

NuD= Nusselt number

Re =  Reynolds Number

Pr = Prandtl number

L = Length of the tube

When laminar flow is fully developed in that case Nusselt number stays at constant and value of the Nusselt number will be 3.66. In that case the forced convection heat transfer coefficient in internal flow and laminar flow can be express as,

Nu_D = 3.66 + (0.065 x Re x Pr x D/L)/(1 + 0.04 x (Re x Pr x D/L)2/3

Forced convection heat transfer coefficient in internal flow and turbulent flow:-

When in a circular tube fluid is flowing in that case Reynolds number stays at the range between 10,000 to 12,000 and Prandtl number stays at the range between 0.7 to 120. Forced convection heat transfer coefficient in internal flow and turbulent flow can be written as,

hd/k = (0.023 jd/μ)0.8 (μ cp/k)n

Where,

d = Hydraulic diameter

μ = Fluid viscosity

k = Thermal conductivity for the bulk fluid

cp = Isobaric heat capacity for the fluid substance

j = Mass flux

n = 0.4 for hotter wall than the bulk fluid and 0.33 for cooler wall than the bulk fluid

How forced convection does affect heat transfer?

The most advantage of the forced convection heat transfer than the free convection heat transfer is to able increasing more amount of heat transfer.

By the help of forced convection heat transfer the amount of heat transfer can be increases by the help of force exerted by outer side. The relations between the forced convection heat transfer and heat transfers is directly proportional. Increasing the forced convection the heat transfer of the system source also increases.

What affects Convective heat transfer coefficient?

The convective heat transfer coefficient is depending on some factors. They are listed below,

Fluid velocity:-

Fluid velocity or flow velocity is a vector field. By the help of fluid velocity motion of a flowing fluid can be determine in the mathematical form. The total amount of length of the fluid velocity is determined as fluid speed. Flow velocity in fluids is the vector field that provides the velocity of fluids at a certain time and position.

The formula of the fluid velocity is given below,

Q = vA

Where,

Q = Volumetric flow rate of the liquid substance

V = Velocity of the liquid substance

A = Cross sectional area of the open system

Density of the fluid:-

From the law of conversion of mass we get a clear concept about the density of fluid. The conversion of mass flow rates states that, the amount of the mass of a particular object cannot not be created or destroyed. The mass of a body is measured by lever balance.

Density of fluid can be defined as the, an object which is containing mass is constant at standard temperature and pressure.

The formula of the density of the fluid is given below,

ρ = m/v

Where,

ρ = Density of the fluid

m = Mass of the fluid

v = Volume of the fluid

The S.I. unit of the density of the fluid is kilogram per cubic meter

Image – A graduated cylinder containing various non – miscible various  colored liquids with different densities;
Image Credit – Wikipedia

Thermal conductivity:-

Thermal conductivity states that the rate at which heat is transferred through a given material is proportional to the negative value of the temperature gradient. And it is also proportional to the area through which the heat flows, but inversely proportional to the distance between the two isothermal planes.

The formula of the thermal conductivity is given below,

K = Qd/AΔT

Where,

K = Thermal conductivity and unit is

Q = The quantity of heat transfer unit is Joules/second or Watts

d = Distance by the two planes of the isothermal unit is  

A = Area of the surface unit is square meters

ΔT = Temperature difference unit is Kelvin

Image – Thermal conductivity can be defined in terms of the heat flow q across a temperature difference;
Image Credit – Wikipedia

Dynamic viscosity of the fluid:-

Dynamic viscosity of the fluid can be deriving as the ration between the shear stress to the shear strain. The unit of the dynamic viscosity of the fluid is Pascal. By the help of dynamic we can easily understand which product how much thick and how can the fluid can flow in a motion means by the help of viscosity we can recognize the behaviour of the fluid.

The formula of the dynamic viscosity of the fluid is given below,

η = T/γ

Where,

η = Dynamic viscosity of the fluid

T = Shearing stress

γ= Shear rate

Image – A simulation of liquids with different viscosities. The liquid on the left has lower viscosity than the liquid on the right;
Image Credit – Wikipedia

Specific heat:-

Specific heat can be deriving as; the amount of heat is needed to raise the temperature of one gram of a matter by one Celsius degree. The units of the specific heat are such as calories or joules per gram per Celsius degree.

Specific heat is also known as massic heat capacity. As an example, the specific heat of water is 1 calorie (or 4.186 joules) per gram per Celsius degree.

The formula of the specific heat of the fluid is given below,

Q = mcΔ T

Where,

Q = Heat energy

m = Mass of the fluid

c = Specific heat capacity

ΔT= Change in temperature

Image – Graph of temperature of phases of water heated from −100 °C to 200 °C – the dashed line example shows that melting and heating 1 kg of ice at −50 °C to water at 40 °C needs 600 kJ;
Image Credit – Wikipedia

How to find convective heat transfer coefficient for air?

Common units which are used to measure the Convective heat transfer coefficient for air is listed below,

  1. 1 W/(m2K) = 0.85984 kcal/(h m20 C) = 0.1761 Btu/(ft2 h 0 F)
  2. 1 kcal/(h m20 C) = 1.163 W/(m2K) = 0.205 Btu/(ft2 h 0 F)
  3. Btu/hr – ft2 -0 F = 5.678 W/(m2K) = 4.882 kcal/(h m20 C)

Forced convection heat transfer through a pipe:

When in a circular tube fluid is flowing in that case Reynolds number stays at the range between 10,000 to 12,000 and Prandtl number stays at the range between 0.7 to 120.

Forced convection heat transfer coefficient in internal flow and turbulent flow can be written as,

hd/k = 0.023 (jd/μ)0.8 (μ cp/k)n

Where,

d = Hydraulic diameter

μ= Fluid viscosity

k = Thermal conductivity for the bulk fluid

cp = Isobaric heat capacity for the fluid substance

j = Mass flux

n = 0.4 for hotter wall than the bulk fluid and 0.33 for cooler wall than the bulk fluid

The properties of the flowing fluid are need for the application in the method of the equation and can be calculated at bulk temperature for this reason iteration can be avoided.

Application of forced convection heat transfer:

The Application of forced convection heat transfer is listed below,

  1. Heat removal
  2. Heat sink simulation
  3. Thermal optimization
  4. Heat sensitivity studies
  5. Electric fan simulation
  6. Computer case cooling
  7. Cooling system design
  8. Heating system design
  9. Fan cooled central possessing unit
  10. Water cooled central possessing unit
  11. Printed circuit board simulation

Forced convection heat transfer examples:

Examples of the forced convection heat transfer is listed below,

  1. Air conditioning system
  2. Convection oven
  3. Pump
  4. Suction device
  5. Ceiling fan
  6. Hot air balloon
  7. Refrigerator
  8. Car radiators

Difference between free and forced convection heat transfer:

The major difference points between free and forced convection heat transfer is given below,

ParameterFree convection heat transferForced convection heat transfer
DefinitionFree convection heat transfer is appearing for density difference between the higher temperature fluid and lower temperature fluid.By the help of forced convection heat transfer a fluid is impression to move from one are to another area by applying force from outer side
Application1. Heat exchanger
2. Gas turbine blades
3. Solar water heater
4. Nuclear reactor design
5. Aircraft cabin insulation
1. Air conditioning system
2. Pump
3. Suction device
4. Ceiling fan  
Heat transfer rateHeat transfer rate for free convection heat transfer lowHeat transfer rate for forced convection heat transfer high
External equipmentNot neededNeeded
Motion of particlesSlowMove faster
Equipment sizeThe size of equipment used in the free convection heat transfer is larger.The size of equipment used in the forced convection heat transfer is smaller.
Flow of moleculesNot controlledControlled
Heat transfer coefficientLessHigh
Movement of the moleculesFor the reason of temperature and density difference free convection heat transfer work.For the reason of exerted force apply forced convection heat transfer work.

How does forced convection heat transfer work?

Forced convection heat transfer work when the area of the gaseous substance or liquid substance is facing higher temperature or lower temperature comparative to greater than their neighbouring temperature and causes a difference between the system temperature and neighbouring temperature.

The temperature gap causes the spaces to move as the higher temperature less dense space rise, and the lower temperature more dense space sink.

Indrani Banerjee

Hi..I am Indrani Banerjee. I completed my bachelor's degree in mechanical engineering. I am an enthusiastic person and I am a person who is positive about every aspect of life. I like to read Books and listen to music. Let's connect through LinkedIn-https://www.linkedin.com/in/indrani-banerjee-2487b4214

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