HEAT TRANSFER SYMBOLS AND FORMULAS-2

 

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CONTINUED FROM THE PREVIOUS SHEET ON HEAT TRANSFER SYMBOLS AND FORMUL

 

HEAT TRANSFER SYMBOLS AND FORMULAS

Heat transfer symbols and formulas help to learn the topic with high degree of clarity, full understanding and with lot of ease

25.

Stefan’s Boltzmann Law

Gives TOTAL emissive power of a black body

E total =σ T4   .        T is absolute temp

E total  is total emissive power

E total =σ T4   W/m2

WIEN’S DISPLACEMENT LAW

The wavelength for maximum emissive power decreases with increase in absolute temperature

λmaxT =289

λmaxT =289 μm

26. Reflectivity

ρ = Reflected radiations/incident radiation

No unit

2

Absorptivity

α = Absorbed radiations/incident radiation

 

 

No unit

2

Transmissivity

τ = Transmitted radiations/incident radiation

 

 

No unit

3

Emissivity

It is a ratio of emissive power of a grey body to the emissive power of a black body at the same temperature

Є =(Eg/Eb) at the same temperature

 

Є =(Eg/Eb)  at T=constant

No unit

3

Kirchhoff Law

ϵ = α, emissivity = absorptivity

 

ϵ =

 

3

Lambert’s Cosine Law

Iϴ = In cos

Where Iϴ is intensity in ϴ direction

In is intensity in the normal direction

 

Iϴ = In cosϴ

 

 

Shape factor

F12 = q.1-2/q.1 = (Radiations incident on body 2 from the radiations emitted from body 1)/( radiations emitted from body 1

F1

F12 = q.1-2/q.

No unit

3

Radiosity

Sum of reflected and emitted radiation

 

J = ρG + Є

W/m

3

Radiation Shield

A device too product from the radiations. UMBRELL

 

 

 

3

Hydrodynamic boundary layer

It has velocity variation.               DRAW A SKETCH

 

 

 

3

Thermal boundary layer

Thermal boundary layer has temperature and velocity variation

DRAW A SKETCH

 

 

 

3

Pool boiling

Heating Process of conversion of a stationary liquid into vapor at the saturation temperature

 

 

 

3

Forced convection boiling

Moving liquid on heating is converted into vapor at the saturation temperature. In this, liquid is moved with a pump. Gas is moved with a blower

 

 

 

40

Condensation

Cooling Process of conversion of vapors into liquid at the saturation temperature

 

 

 

41

Drop-wise condensation

When vapors changes into drops of liquid on cooling. It is on an oily surfac

 

 

 

4

Film-wise condensatio

On cooling, when vapors changes into a film of liquid. It is on a finished surface

 

 

 

43

Radiation densit

Radiation density = Eb /

Radiosity density = J /

 

 

 

DEFINITION AND FORMULA FOR OVERALL HEAT TRANSFER COEFFICIENT ‘U’

DEFINITIO

U accounts for convection + conduction +convection in a heat exchanger

EQUATION OF ‘U’ FOR A PLAIN WAL

1/UA =1/hiAi  + x/kA +1/hoAo

For a wall A=Ai=A

FORMULA OF ‘U’ FOR A PIP

1/UA =1/hiAi  + ln (r2/r1)/2πkL +1/hoA

CONDITIONS FOR LAMINAR AND TURBULENT FLO

Ite

Laminar flo

Turbulent flo

Free Convectio

Gr Pr  10

Gr Pr >10

Forced convection over a flat plat

R

Re> 7 lac

Pipe Forced convection

R

Re>400

Thickness of hydrodynamic layer =

Thermal boundary layer Thickness = δt

RELATION BETWEEN δth  and

δth = δ Pr–1/

black bod

α =1, ρ =0 and τ=

OPAQUE

α+ρ =1  and τ =

grey

α +ρ+τ =

white

α=0,ρ=1 and τ =

transparent

τ =1, ρ=0 and α =

ASSUMPTIONS FOR NUMERICAL PROB

Grey Body Is Taken As Opaque Body And Hemispherical Body.