[Tobi]

Hi all,

I am a bit confused about the NASA Polynomials and the units they have. I am right with the following stuff:

• cp -> kJ/molK
• H -> kJ/mol
• S -> kJ/molK

or is it kcal/mol or only J/mol? Thanks in advance.


PS: I think it depend on the unit of the individuel gas constant R. If I use it in J/molK the units should be like that.

[wangmianzhi]

I think you are right about the R.
If I remember correctely, they are dimensionless, e.g. h/RT, s/R, cp/R.

[jennap223]

I believe you are correct in treating it that way. That is how I have done it, using R =8.314 J/mol-K to get Cp in those same units and dividing by the molar mass such that it is J/kg-K

A good sources for NASA Polynomial explanation is:
http://combustion.berkeley.edu/gri_m...nasa_plnm.html

and a list of polynomials themselves
https://combustion.llnl.gov/content/...anes_therm.txt

[Tobi]

Dear all,

thanks for the replays and the feedback. The sites of grimech and the nasa polynomials I know

Thus you showed the NASA site, can you just clear my mind about that line:

 H in the above equation is defined as 
    H(T) = Delta Hf(298) + [ H(T) - H(298) ]  so that, in general, H(T) is not equal to Delta Hf(T) and one needs to have the data for the reference elements to calculate Delta Hf(T).

Till now I did not get the point of the defintion of H
For example: If I calculate H at 298 I get H(298) = deltaHf

[wangmianzhi]

It means if you want Delta Hf(500) of water, You'll need not only H(T) of water, but also H(T) of H2 and O2 as well.
The fact that Delta Hf(298) = H(298) for any species is because H(298) = 0 for any species like H2, O2 (the most stable single-element species at reference state).

Originally Posted by Tobi

Dear all,

thanks for the replays and the feedback. The sites of grimech and the nasa polynomials I know

Thus you showed the NASA site, can you just clear my mind about that line:

 H in the above equation is defined as 
    H(T) = Delta Hf(298) + [ H(T) - H(298) ]  so that, in general, H(T) is not equal to Delta Hf(T) and one needs to have the data for the reference elements to calculate Delta Hf(T).

Till now I did not get the point of the defintion of H
For example: If I calculate H at 298 I get H(298) = deltaHf

[Tobi]

Hello Wang,

I have a question for the deltaH calculation. If I made some comparison I always have problems with deltaH (I did not get the point). For example we can check some data here:

http://combustion.berkeley.edu/gri-m...cies/c3h8.html

If I calculate cp(T), S(T) and "H(T)" I get this one:

T: 200, 11.6896  58.7521  -26285.9
T: 300, 17.4924  64.6154  -24820.2
T: 400, 22.5544  70.3576  -22812.1
T: 500, 26.9572  75.8749  -20331.3
T: 600, 30.7775  81.1358  -17440.1
T: 700, 34.0862  86.1348  -14192.9
T: 800, 36.9492  90.8779  -10637.7
T: 900, 39.4266  95.3764  -6815.89
T: 1000, 41.5734  99.6442  -2763.35
T: 1100, 43.4391  103.696  1489.42
T: 1200, 45.0678  107.547  5916.56
T: 1300, 46.4983  111.212  10496.4
T: 1400, 47.7639  114.705  15210.7
T: 1500, 48.8926  118.04  20044.6
T: 1600, 49.907  121.228  24985.4
T: 1700, 50.8242  124.282  30022.8
T: 1800, 51.6561  127.211  35147.4
T: 1900, 52.4093  130.024  40351.4
T: 2000, 53.0877  132.73  45626.8
T: 2100, 53.6953  135.335  50966.5
T: 2200, 54.236  137.846  56363.6
T: 2300, 54.714  140.268  61811.6
T: 2400, 55.1335  142.605  67304.5
T: 2500, 55.4988  144.863  72836.5
T: 2600, 55.8144  147.046  78402.6
T: 2700, 56.0847  149.158  83997.9
T: 2800, 56.3145  151.202  89618.2
T: 2900, 56.5086  153.182  95259.6
T: 3000, 56.6718  155.1  100919
T: 3100, 56.8092  156.961  106593
T: 3200, 56.9258  158.766  112280

It can be seen that cp and S are very nice but for deltaH its total bad. I know why, because I calculate H and not deltaH. But I still do not get the point how to calculate deltaH.

[wangmianzhi]

Originally Posted by Tobi

Hello Wang,

I have a question for the deltaH calculation. If I made some comparison I always have problems with deltaH (I did not get the point). For example we can check some data here:

http://combustion.berkeley.edu/gri-m...cies/c3h8.html

If I calculate cp(T), S(T) and "H(T)" I get this one:

T: 200, 11.6896  58.7521  -26285.9
T: 300, 17.4924  64.6154  -24820.2
T: 400, 22.5544  70.3576  -22812.1
T: 500, 26.9572  75.8749  -20331.3
T: 600, 30.7775  81.1358  -17440.1
T: 700, 34.0862  86.1348  -14192.9
T: 800, 36.9492  90.8779  -10637.7
T: 900, 39.4266  95.3764  -6815.89
T: 1000, 41.5734  99.6442  -2763.35
T: 1100, 43.4391  103.696  1489.42
T: 1200, 45.0678  107.547  5916.56
T: 1300, 46.4983  111.212  10496.4
T: 1400, 47.7639  114.705  15210.7
T: 1500, 48.8926  118.04  20044.6
T: 1600, 49.907  121.228  24985.4
T: 1700, 50.8242  124.282  30022.8
T: 1800, 51.6561  127.211  35147.4
T: 1900, 52.4093  130.024  40351.4
T: 2000, 53.0877  132.73  45626.8
T: 2100, 53.6953  135.335  50966.5
T: 2200, 54.236  137.846  56363.6
T: 2300, 54.714  140.268  61811.6
T: 2400, 55.1335  142.605  67304.5
T: 2500, 55.4988  144.863  72836.5
T: 2600, 55.8144  147.046  78402.6
T: 2700, 56.0847  149.158  83997.9
T: 2800, 56.3145  151.202  89618.2
T: 2900, 56.5086  153.182  95259.6
T: 3000, 56.6718  155.1  100919
T: 3100, 56.8092  156.961  106593
T: 3200, 56.9258  158.766  112280

It can be seen that cp and S are very nice but for deltaH its total bad. I know why, because I calculate H and not deltaH. But I still do not get the point how to calculate deltaH.

The subscript "f" in the "Delta h_f" means formation from the most stable single-element specie (i.e. 3*C and 4*H2 for propane).
In this case:
Delta h_f (T)_propane = h(T)_propane - 3h(T)_C -4h(T)_H2

[Tobi]

Thank you very much.
Now I got it (:

[Tobi]

Just to get it 100% clear:

h_f(T)_C2H3CHO = h(T)_C2H3CHO -  2h(T)_C - 4h(T)_H2 - h(T)_O

or would be O -> 1/2 O2 as:

h_f(T)_C2H3CHO = h(T)_C2H3CHO -  2h(T)_C - 4h(T)_H2 - 0.5h(T)_O2

Or another example for butyl (C4H9):

Delta h_f (T)_butyl = h(T)_butyl - 4h(T)_C -4.5h(T)_H2

As far as I understand you:

the most stable single-element species at reference state

its always H2, O2, C, N2, HE and never H, O, C2, N...
I am right?

[Tobi]

Hey,

today I investigated into that problem again and with your statement I figured it out but I have still a problem. For example my calculated results are similar to that one postulated in the above link. I pointed out that its is like that:

Hf(NO2) = H(NO2) - H(O2)

Hf(OH) = H(OH) - 0.5 H(O2) - 0.5 H(H2)

Hf(O2) = H(O2) - H(O2) = 0  (also for N2, H2, AR)

Here one example: NNH

Hf(NNH) = H(NNH) - H(N2) - 0.5 H(H2)

b) T = 200 :::: dHf(NNH) = 59.8241
b) T = 300 :::: dHf(NNH) = 59.6058
b) T = 400 :::: dHf(NNH) = 59.4136
b) T = 500 :::: dHf(NNH) = 59.2674
b) T = 600 :::: dHf(NNH) = 59.1694
b) T = 700 :::: dHf(NNH) = 59.1105
b) T = 800 :::: dHf(NNH) = 59.0774
b) T = 900 :::: dHf(NNH) = 59.0592
b) T = 1000 :::: dHf(NNH) = 59.0538
b) T = 1100 :::: dHf(NNH) = 59.0617
b) T = 1200 :::: dHf(NNH) = 59.0763
b) T = 1300 :::: dHf(NNH) = 59.0964
b) T = 1400 :::: dHf(NNH) = 59.1212
b) T = 1500 :::: dHf(NNH) = 59.1495
b) T = 1600 :::: dHf(NNH) = 59.1806
b) T = 1700 :::: dHf(NNH) = 59.2138
b) T = 1800 :::: dHf(NNH) = 59.2485
b) T = 1900 :::: dHf(NNH) = 59.2839
b) T = 2000 :::: dHf(NNH) = 59.3198
b) T = 2100 :::: dHf(NNH) = 59.3555
b) T = 2200 :::: dHf(NNH) = 59.3909
b) T = 2300 :::: dHf(NNH) = 59.4254
b) T = 2400 :::: dHf(NNH) = 59.459
b) T = 2500 :::: dHf(NNH) = 59.4913
b) T = 2600 :::: dHf(NNH) = 59.5222
b) T = 2700 :::: dHf(NNH) = 59.5514
b) T = 2800 :::: dHf(NNH) = 59.579
b) T = 2900 :::: dHf(NNH) = 59.6046
b) T = 3000 :::: dHf(NNH) = 59.6283

ALso with H20, NO2 and so forth the results are excellent. But if I take a species where "C" is included it is not working anymore. This can not be correct:

Hf(C3H8) = H(C3H8) - 3*H(C) - 4*H(H2)

because if I calculate Hf(C) it is H(C)

Hf(C) = H(C)

If we compare this one http://combustion.berkeley.edu/gri-m...ecies/ch3.html with my calculated one:

b) T = 200 :::: dHf(CH3) = 35.2246
b) T = 300 :::: dHf(CH3) = 35.0925
b) T = 400 :::: dHf(CH3) = 35.0072
b) T = 500 :::: dHf(CH3) = 34.9932
b) T = 600 :::: dHf(CH3) = 35.057
b) T = 700 :::: dHf(CH3) = 35.193
b) T = 800 :::: dHf(CH3) = 35.3905
b) T = 900 :::: dHf(CH3) = 35.6399
b) T = 1000 :::: dHf(CH3) = 35.9392
b) T = 1100 :::: dHf(CH3) = 36.2862
b) T = 1200 :::: dHf(CH3) = 36.6695
b) T = 1300 :::: dHf(CH3) = 37.0838
b) T = 1400 :::: dHf(CH3) = 37.5241
b) T = 1500 :::: dHf(CH3) = 37.9862
b) T = 1600 :::: dHf(CH3) = 38.4664
b) T = 1700 :::: dHf(CH3) = 38.9615
b) T = 1800 :::: dHf(CH3) = 39.4685
b) T = 1900 :::: dHf(CH3) = 39.9851
b) T = 2000 :::: dHf(CH3) = 40.5092
b) T = 2100 :::: dHf(CH3) = 41.0392
b) T = 2200 :::: dHf(CH3) = 41.5736
b) T = 2300 :::: dHf(CH3) = 42.1113
b) T = 2400 :::: dHf(CH3) = 42.6515
b) T = 2500 :::: dHf(CH3) = 43.1932
b) T = 2600 :::: dHf(CH3) = 43.7361
b) T = 2700 :::: dHf(CH3) = 44.2796
b) T = 2800 :::: dHf(CH3) = 44.8234
b) T = 2900 :::: dHf(CH3) = 45.3671
b) T = 3000 :::: dHf(CH3) = 45.9105

There is something wrong. Its with each species where "C" is included. I use exact the same NASA Polynomials. It would be very nice if you could clear my doubts.

Kind regards,
Tobi

[Tobi]

It is already late but I think I am wrong in one case:

Hf(CH3) = H(CH3) - H(C)_solid - 1.5 * H(H2)

I will check it out because in other NASA Polynomials I found that I have two entrys for C. One for the gas and one for solid. I think H(C) is always solid state due to the fact of stable phase.

[AR Bangash]

Hi Tobi,

Would you share how did you solve the problem with carbon?

Regards