What is the difference between constant 'TP' and 'HP' in equilibrate function?

[Raja]

Hi all,

I am new to the combustion and also new to Cantera. I am trying to model constant pressure combustion. I am interested to get heat released by the fuel after combustion takes place. My Fuel is a mixture of gases. Since i am interested in final heat release i using equlibrate function to get the enthalpy of the product mixture. The heat release is calculated by (h_react - h_product)/massfraction_fuel to get per kg of fuel. I found two options for constant to mention in the equilibrate function. Namely 'TP' and 'HP'. What is mean by that. When i tried both giving different values and different products composition. I couldn't find any where in the fourm in my extensive search. Can any please explain what exactly it means and if you refer me some article related to this if any would be great help to me

Thanks,

Raja

[John Leylegian]

Raja,

In short, 'TP' holds temperature and pressure constant, while 'HP' holds enthalpy and pressure constant. The one to use will depend on your application.

The problem with your equation for heat release is that it ignores chemical enthalpy, i.e., the enthalpy contained in the chemical bonds. If you want an adiabatic reaction, you should use 'HP.' If you want an isothermal reaction, use 'TP.'

I hope this takes you in the correct direction.

Regards,

John Leylegian

[Raja Shankar]

Hi Mr. John,

Thanks for the reply. It is useful. But i couldn't get what you mentioned about the chemical enthalpy. As I understand, this is taken care by what products formed and what are the reactants. So the difference gives the heat release. I read about bond enthalpy and heat of reaction. Calculating heating heat of reaction using bond energy is same as the way i do as i understand.

I have one more doubt about setting equivalence ratio. I donot want to create new thread for this. In the documentation, I saw that when give phi value it considers the oxidation of C to CO2, H to H2O and S to SO2 and other elements are assumed not to participate in oxidation (that is, N ends up as N2). If i have other elements for example fuel species contains  Mg, Al in the gaseous form will it be considered for the oxidation  other than C, H and S.

Thanks and regards,

Raja

[John Leylegian]

Raja,

My apologies, I was in a rush when I sent my initial answer. If you perform an adiabatic calculation, i.e., using 'HP,' you will calculate a heat release of zero, since the enthalpy will not change. However, if you perform an isothermal calculation, you will be able to determine the heat release.

If you want to include other elements in your reactant stream, you will have to determine the fully oxidized products for that element, considering the combination of fuel and oxidizer element. In my opinion, the easiest way would be to determine which compounds result in the greatest heat release per unit mass of the initial fuel element. You should be able to determine that information from thermodynamic data for these compounds, which could be found in the JANAF tables available from the NIST website.

Best,

-----------------------

John C. Leylegian, PhD (he/him/his)

Associate Professor of Mechanical Engineering

Riverdale, NY 10471

Phone: 718.862.7279

Fax: 718.862.7163

john.le...@manhattan.edu

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[Raja Shankar]

Prof. John,

Thank you for your reply. I am using isothermal calculation only. To calculate stoichiometry air-to-fuel ratio, I will try what you have suggested. 

I have a doubt. I have tried both method for calculating the heat released. In isothermal process, the T and P are kept constant so the enthalpy is changing during the process. By adiabatic constant pressure process, the temperature of the products are raised and to calculate heat release i set the products mixture to my reference/initial T and P. But this gives lesser value than the isothermal process. When i checked the products for both cases, they have different species with different mole concentrations.  For example, the isothermal process has ex. more CO2 molar  fraction but the adiabatic process contains more CO than CO2. From my reading, I understand that temperature influence the products (for ex at high temp CO2 disassociates). This confuses me because i feel that the HP takes care of the high temp effect on products. So I am not able get what I am missing here to understand. The calculation I carryout is for constant pressure combustion(similar to jet engine combustor). I want to calculate the heat addition to the incoming air with different air-to-fuel ratio. Your suggestion will be very useful. 

Thanks and regards,

Raja

[John Leylegian]

Raja,

All of what you said is accurate. If you run an adiabatic calculation, some of the heat will be reabsorbed through dissociation, thereby lowering the temperature. If you do not get total conversion to CO2 and H2O (and whatever other products you are assuming based on fuel formulation), you will not get the same heat release as you would for an isothermal calculation. Furthermore, NOx chemistry is endothermic, which will lower the temperature a little more as well.

Good luck,

-----------------------

John C. Leylegian, PhD (he/him/his)

Associate Professor of Mechanical Engineering

Riverdale, NY 10471

Phone: 718.862.7279

Fax: 718.862.7163

john.le...@manhattan.edu

www.manhattan.edu

https://manhattan.edu/campus-directory/john.leylegian

LinkedIn Profile

Quick Links to Online Forms

Link to my Calendar 

"Perfer et obdura, dolor hic tibi proderit olim" - Publius Ouidius Naso

To view this discussion on the web visit https://groups.google.com/d/msgid/cantera-users/56657113-b105-4fb7-b85d-8d1c50937aban%40googlegroups.com.

[Raja Shankar]

Thank you professor.

Regards,

Raja

[K G-A]

I am simulating a gas turbine. None of the equilibrate() combinations below adequately simulates the combustion in the combustor. since temperature, entropy, internal energy and volume are not constant. The equilibrate() holds the pair in the bracket constant.

gas1.TP 
gas1.TD 
gas1.HP 
gas1.UV 
gas1.SP 
gas1.SV

If I equilibrate (HP) since the combustion in a gas turbine is at constant enthalpy and constant pressure to get the adiabetic flame temperature and the equilibrate again at constant pressure and constant temperature at the adiabetic temperature to obtain the new enthalpy. Will this be a good representation of the combustion in a gas turbine combustor?

Regards,

Kofi.