This page provides a limited notes on thermodyamics and heat transfer that may be useful to mechanical engineers.
Thermodynamic LawsZeroth Law of Thermodynamics...
When two objects are separately in thermodynamic equilibrium with a third they are in equilibrium with each other
Objects in thermodynamic equilibrium are at the same temperature
First Law of Thermodynamics...
This law expresses the general law of conservation of energy. and states that heat and work are mutually convertible
Heat In = Work Out over complete cycle
dQ = dU + dWSecond Law of Thermodynamics...
This law in its simplest states that heat can only flow from hot to cold and not vice versa. In terms of thermodynamic engine cycles the law states that the gross heat supplied to a system in a complete cycle must exceed the work done by the system. Therefore heat must be rejected. The thermal efficiency of an heat engine must be less than 100%.
Reversible Polytropic Process
p v n = constant
W = ( p 2 v 2 - p 1 v 1 ) / ( 1 - n ) .. (n not 0 )
For a perfect gas
W = R ( T 2 - T 1 ) / (1 -n )
Q = ( Cv + R /(1 - n) ) ( T 2 -T 1 )
T 2 / T 1 = ( p 2 / p 1 ) ( n-1 ) / n
For Adiabatic processes (Q = 0 ) n = γ = cp / cv
γ = 1.4 for Air, H 2, O 2, CO, NO, Hcl
γ = 1.3 for CO 2, SO 2, H 2O, H 2S, N 2O, NH 3, CL 2, CH 4, C 2H 2, C 2H 4
Heat Transfer takes place by Conduction, Convection and Radiation
Heat Transfer by Conduction
The heat has to pass through the surface layers on both sides of the wall
Table Showing Various values for k at 20 oC
Heat Transfer by Radiation
Heat Transfer by Convection
Convective heat transfer occurs between a moving fluid and a solid surface.The rate of convective heat transfer between a surface and a fluid is given by the Newtons Law of Cooling;
It is customary to express the convection coefficient (average or local), in a non-dimensional form called the Nusselt Number.Natural convection
Nu = C(Gr.Pr) n C and n are tabled below
Note: Convection heat transfer values are very specific to the geometry of the surface and the heat transfer conditions - These example equations are very general in nature and should not be used for serious calcs. The links below provide much safer equations..
Laminar flow over Plate Nu = 0.664(Re) 1/2(Pr) 1/3
Fully Developed pipe flow Nu = 0.0866(D/L)Re.Pr / (1+0.04[D / L(Re.Pr)] 2/3) + 3.66
Turbulent Flow Over Flat Plate Nu = 0.036Pr 1/3Re 0.8
Turbulent Flow In Pipe Nu = 0.023Pr 0.4Re 0.8
Typical Values of Heat Transfer Coefficient h = W.m -2K -1
Heat exchangers normally transfer energy from a hot fluid to a colder fluid.
The energy in = The energy out.
Typical Values for Overall Heat transfer U are