Heat Release vs. Apparent Heat Release // Comparison with GT Power
 

Hi all,

currently i am working on a file with GT Power inputs and compare resulting cylinder pressure. Next step followed by that is the burn rate, however, there is no variable named like that in CONVERGE. This may or may not lead to confusion if comparing to 1D GT Power or experimental data.

Allie wrote in another Thread:
Basically, in theory, we have:
"Integrated Heat Release (Burn Rate * LHV) - total Heat Transfer = Apparent Integrated Heat Release"

Integrated Heat Release is column 11 from thermo.out file, total Heat Transfer can be calculated as sum from column 17 of all bound*-wall.out files attatched to cylinder region. To get the Apparent Heat Release we use the Apparent Heat Release Calculator in Studio, that takes the pressure trace, gamma and geometric volume from cylinder region into account.

In a complete combustion the Integrated Heat Release is equal to the burnt fuel mass times lower Heating value: QB=mB * Hu

In GT Power we have variables called Burn Rate and Apparent Heat Release Rate.
It seems like they are equal to the Heat Release Rate divided my LHV and Apparent Heat Release Rate of CONVERGE.

Regards,
Tobias

Q1: Is there an overview of species properties that CONVERGE uses, especially the lower Heating value of the different species? Depending on where i look i find different values for the species.

Q2: If the Heat Release Rate is actually the Burn Rate, would it be possible and sensful to rename the column in the thermo.out file if Engine Simulation is used?

The LHV of the fuel is calculated using property data in therm.dat. If you do not want to calculate it from the therm.dat data, you can get the calculated LHV of the fuel as an output from Converge. To get the LHV as an output, you will run a simple test case:
- create a hidden.in file with the following line:
1 lhv_flag
- create a file called lhv.in with the following lines:
2.0 lhv_value LHV VALUE TO BE ACHEIVED IN J/kg
1 lhv_num_species NUMBER OF FUEL SPECIES TO BE MODIFIED FOR LHV
c8h18 1.0 massfrac RELATIVE FRACTION OF EACH FUEL SPECIES


Run this test case for a short time - a file called lhv_info.out file will contain the calculated LHV as the second line (LHV SURROGATE). As this is a new feature, it is still in trial mode and has not been moved to a regular file input, and you will need a recent version of v2.3.

The chemical heat release rate, as named in thermo.out, is a precise and well understood term for engine research. Calculated burn rates & apparent heat release rates use different methods and assumptions.

Thanks Dan, that worked pretty well. The values i had previously were a bit different lower and i already wondered why my calculated mFuel*LHV was less than integrated Heat Release. :D

And to be precise, with Burn Rate i obviously meant fuelmass times LHV to get an Energy value as well. So shouldnt be the chemical heat release rate equal to the rate how fuel is burned multiplied with its LHV?

If you are considering the burn rate to be the fraction of fuel burned and the fuel is reacting completely to products, than the chemical heat release would equal the energy content of the consumed fuel. There are several factors that can affect the energy balance in combustion, however: incomplete combustion, heat transfer, unburnt fuel.
The reason we call it chemical heat release is because it is precise and understandable, as I said above.
Energy calculated from a burn rate can be different than chemical heat release, for example when the mixture is rich and there is significant CO in the products.
Furthermore, there are different experimental methods to calculate apparent heat release, burn rate, etc... which may introduce errors that show up in comparison to chemical heat release.

If i run a simulation with this hidden.in and lhv.in file, will it then use the specified LHV value instead of the LHV surrogate?
Or would i need to modify the therm.dat file Low_a6 value of my species manually and the method described previously is just an info output?

Yes, this allows you to modify the LHV.

Thank you Dan.

I really like the hidden, secret features :D

Hey Dan, quick question from my side again.

I have a simulation where i slightly reduced the LHV of my PRF (92% IC8H18, 8% C7H16) from 4.467920e+007 to 4.256100e+007, which is -4.74%.

Compared to a simulation, where i didnt make any changes, this had a quite drastic impact on mean pressure and temperature (e.g. peak firing pressure dropped from 45bar to 31bar). Also, the rate at which the fuel was consumed was significantly reduced.

I will rerun this to double check the result, but i simply expected only a minimal reduce in power output.

Reducing the energy content by ~5% is showing to be quite significant, as the lower heat release translates to less temperature gain, and hence slower combustion.
But also, you may want to run a 2nd and even 3rd cycle of each case to before drawing conclusions.

Sure, at the moment i am running from two cycles from -520 to 900 and the values i mentioned were from 2nd cycle.