 So, we're talking about combustion chemical reactions and we're looking at that as being a large source of energy for today's economy. We saw 83% of energy that we use comes from fossil fuel based energy and consequently it would involve combustion. So to begin with let's review combustion or take a look at what combustion is. So when we have combustion what is happening it's a rapid oxidation reaction so it takes place very quickly and in the process it generates heat or both light and heat and that's why when we look at a flame we can actually physically see it although as the temperature drops down higher up in the flame you no longer are able to see it however it is still very very hot and it's still a buoyancy driven flow. The other thing that goes on is we have a chemical reaction whereby the chemical bonds within the fuel are being converted into heat and so it's what we would call an exothermic reaction. Now in order to understand combustion which is what we will be looking at here in order to understand combustion it requires a combined knowledge first of all of this course thermodynamics. It also involves heat transfer because as you have the heat release heat is being transferred through the fluid and the fuel itself and mass transfer. Mass transfer is the process by which the reactants are allowed to come into contact with one another so it can either be just straight diffusion or it can be turbulent diffusion laminar diffusion turbulent diffusion where the fluid flow actually provides a bit of the mixing drive for the mass transfer to take place and so we could refer to this as being diffusion of species and if it's just simple diffusion then that would be you could call it laminar mixing and we can also have turbulent flow mixing and when we looked at the combustion taking place within the engine that I showed you in the previous clip or segment that was definitely turbulent combustion going on there we had very very large vertical structures they're causing the oil once it had been vaporized to mix and react with the oxygen that would be present within the air and the final thing that we would need to understand is what they sometimes refer to as being chemical kinetics and that enables us to figure out how quickly the reactions are going to take place and the reactions can get rather complex so essentially what we're looking at is mixed between thermodynamics which will cover here heat transfer and mass transfer which could extend into fluid mechanics and then chemical kinetics so those are the three areas we will primarily focus on the thermodynamic aspects but in order to develop a full and comprehensive understanding of combustion you really need to study all three of those simultaneously in order to make proper gains so what we will be doing we will be simplifying the reactions and by that what I mean is that we usually will consider the reaction as being a single step process so let's take one of the simplest oxidation reactions that exists which is the oxidation of hydrogen with oxygen and when that occurs we get two moles of water and let's assume it's water vapor doesn't really matter but in reality this is what we would study if we were taking a class in chemistry actually it's what we're going to be doing here in thermodynamics because it simplifies things but in reality this is actually more complex in reality what happens is we have the H2 they're actually a number of reactions so you start to get the picture that it gets more and more complex we have intermediate reactions going on and an actual fact for a complete picture of what's going on here of the oxidation reaction of hydrogen we would require 20 or more separate reactions so you can see it does start to get complex and that gets into the chemical kinetic side what we are going to do we're going to stay back here with just a single step reaction it is a simplification however it enables us to do our analysis and and then look at the thermodynamic aspects of the heat release that is occurring with this type of reaction that's primarily what we'll be doing in this chapter