Re: How's most of the heat transfer to the combustion chamber wall take place?

Greetings:

Your questions revolve around the basic concepts of the science and laws of 
THERMODYNAMICS and the transfer of heat.

Fundamentally the maximum amount of power that can be obtained from an 
engine is related to the maximum temperature in the engines operation minus 
the minimum temperature in the engines operation. 
Equivalent units of power are: 
1 horsepower = 745.7 joules/second (watts) = 33,000 ft lbs/minute.

There are three ways heat can be transferred in an engine; by conduction, 
by convection and by radiation. In an engine operating in the earth’s 
atmosphere, all three of these methods of heat transfer must be taken into 
account.

Conduction: If one end of a metal rod is placed in a flame while the other 
end is held in your hand, you will feel the rod becoming hotter and hotter 
even though your hand is not in direct contact with the flame. Heat is said 
to flow along or through the material by conduction.

Convection: The transfer of heat from one place to another by the actual 
motion of material is called convection. Examples are water cooling or air 
cooling of a hot engine.

Radiation: When we hold a hot body near our hand in still air or a vacuum, 
we feel the heat by radiation. In the design of satellites operating in the 
vacuum of space, convection and conduction cannot be used to remove heat 
from the spacecraft, only radiation can be used to dissipate waste heat. 

A jet engine operates at very high temperatures of several thousand degrees 
Kelvin and all three methods of heat transfer must be taken into account 
for safe, reliable operation. 

The rate of heat transfer by conduction is determined by the thermal 
properties of the material that the engine is made of and the difference 
between the maximum temperature inside the engine and the temperature of 
the air entering (and perhaps leaving) the engine (Tmax – Tair).

The rate of heat tranfer by radiation is determined by the absorption and 
reflective properties of the materials involved and the difference of the 
forth power of the maximum and minumum temperatures (Tmax^4-Tmin^4). 

The rate of heat transfer by convection in a jet engine is called forced 
convection because of the turbines and fans forcing the air through and 
around the engine . The mathematical theory for convection is quite 
involved and there is no simple relationship between the differences in 
temperature as in conduction and radiation. This comes about because the 
heat lost or gained by a surface at one temperature in contact with a fluid 
(water vapor, fuel vapor, air etc.) at another temperature, depends on many 
factors including geometrical shapes, the velocity, density, pressure, heat 
capacity, and thermal conductivity of the fluid and whether the fluid is in 
laminar or turbulent flow.

All of the heat transfer methods and thermal characteristics of the 
materials used must be considered in the design of a jet engine. Conduction 
and radiation do not remove heat quickly enough to prevent a melt down of 
the engine structure so that controlling convection is the key to jet 
engine design. To increase the efficiency of many rocket engines and high 
performance jet engines often the cool fuel from the tanks is circulated 
through the interior of the exhaust nozzle material before entering the 
combustion chamber to help cool the interior of the nozzle material by 
convection. Also, the higher temperature fuel entering the engine requires 
less energy for combustion, increasing the engines efficiency while the 
convective cooling of the engine material reduces the possibility for a 
melt down. When powerful after burners are used in many combat aircraft, 
they can only be used for a limited period of time before damage to the 
engine structure occurs, here even convective cooling is not good enough 
for sustained operation.

Each year jet engines become more powerful and more efficient. Today twin 
engine aircraft such as the Boeing 777 and 767 are more efficient while 
carrying payloads comparable to older four engine aircraft such as the  
Boeing 747. 

For more detailed information the University of South Hampton in the UK has 
an excellent web site on jet engine design and operation at : 
 http://
www.soton.ac.uk/~genesis/Level2/Tech/Jetprinc.htm

Best regards, Your Mad Scientist
Adrian