HEAT TRANSFER AND MASS TRANSFER

 INTRODUCTION :-

HEAT is defined as energy transfer by virtue of temperature difference or gradient. Heat transfer is a vector quantity, flowing in the direction of decreasing temperature with a negative temperature gradient.

when a small amount of perfume vapour is sprayed into a room of a air, the mass transfer process causes the perfume vapour to diffuse throughout the room until its concentration is uniform , indicating an equilibrium condition. 

There are three distinct modes in which heat transmission can take place: 

• conduction
• radiation 
• convection 

only conduction and radiation should be classified as heat transfer processes , because only these two model depend on the existance of a temperature difference.

convection refers to the mass motion of the fluid , and the convective heat transfer a solid wall and a fluid depends not only on the temperature difference but also on the mass transfer of the fluid .

THERMODYNAMIC AND HEAT TRANSFER :-

The science of heat transfer is calculation of the rate at which heat flow within a medium across an interface or form one surface to another.

Thermodynamic deals with systems in equilibrium and calculate the energy transfers to change a system from one equilibrium state to another.

HEAT:-

• thermodynamics analysis (Q in joule )
• heat transfer (Q in joule/sec)

Conduction:-

1 molecular lattice vibration (30%)

2 free electron heat transfer (70%)

 

some example of conduction :-

silver (k =410 w/mk)
copper (k= 385w/mk)
aluminium (k=200w/mk)
gold (k= 319w/mk)
steel (k= 17 to 45 w/mk)
K(PURE METAL)> K(ITS ALLOY) 
DIAMOND (K =2300W/MK)

Insulator have very low thermal conductivity.

 HEAT CONDUCTION IN GASES:-

From kinetic theory of gases :-

VRMS  ∝ T^1/2

where :-

v = RMS velocity of gas molecule 

T = Temperature 

but in general gas are very bad conductor of heat 

liquid are better conductor of heat than gases KWATER = 0.63 W /mk  among liquid mercuary ( a liquid metal) has highest thermal conductivity. K HG= 8.43 w/mk

HG is a thermodynamics fluid because (1) high K (2) Lower vapour pressure (3) higher thermal expansion

HEAT CONDUCTION:-

IT is mode of heat transfer which generally occurs in solid due to temperature difference associated with molecular lattice vibration energy transfer and also by free electron transfer .

FOURIER LAW OF CONDUCTION:-

where:-