I have a problem with understanding basic concept about enthalpy.
I know there is an enthalpy (heat) of formation which tells how much energy is going to be released or consumed during chemical reaction. I found information that specific heat of formation for one of the hydrocarbon jet fuel is equal to -200 BTU/lbm. How to interpret that? If all the elemental 'building blocks' of the fuel undergoes reaction of mixing of whatever there will be 200 BTU/lbm heat released?
There must be second 'meaning' of enthalpy because I also found example problem of burning jet fuel with air, where in the given data is information that fuel enthalpy is equal to +189 BTU/lbm (there was no information if that's heat of formation). What does it mean that fuel has enthalpy? It sits in the tank, and before burning process is no chemical reaction
How to interpret both cases? What's the difference here. Generally we talk about enthalpy of fluid. Like compressor inlet flow enthalpy is equal to XXX, compressor exit flow enthalpy = YYY. There is no chemical reaction between inlet/outlet plane so ...
Could you give me a 'dummy' type explanation?
Thank you, Lukasz from Poland
Let me paste one of my comments here:
This is how book describes combustion in jet engine:
*specific enthalpy of mix air and fuel
hIn = ( mAir * hAir + mFuel * hFuel ) / ( mAir + mFuel)
*specific enthalpy at the end of combustion process
hOut = hIn + (LHV * (mFuel/mAir) * efficiency)
Looks like that I can't understand what hFuel, and hAir mean in that case.