Energy calculation through inlet and outlet face

Vishnu_bharathi · November 29, 2017, 13:31

I have an Inlet face where fluid at 500°C at the rate 1 Kg/s is given as input. Outlet boundary condition is 0 Pa relative pressure. Inlet and outlet face area are 2 m^2.

How do you calculate the Energy fed at Inlet face and Energy exited at oulet face in Joules? may be for each time step?

I am doing transient simulation with time step 20 sec. Total simulation time for 5 hr.

Opaque · November 29, 2017, 13:52

You have to define what kind of energy you are accounting for.

If you want to account for total energy (internal + flow work + kinetic energy + etc), I would use Total Enthalpy, then

Energy at inlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Inlet

Energy at outlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Outlet

Not sure how valuable those integrals are for open systems since I usually work with energy rates; therefore, no need for the time integral.

Vishnu_bharathi · November 29, 2017, 14:07

Quote:

Originally Posted by Opaque

You have to define what kind of energy you are accounting for.

If you want to account for total energy (internal + flow work + kinetic energy + etc), I would use Total Enthalpy, then

Energy at inlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Inlet

Energy at outlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Outlet

Not sure how valuable those integrals are for open systems since I usually work with energy rates; therefore, no need for the time integral.

Thanks for the quick response. I want to account total energy. I do not understand the Time integral part(0 --> 5hr). Till now I was using 20[s]*(massFlowInt(totalEnthalpy)@Inlet for 1 time step and for 5 hours I just times up by 900(total iterations) for Total Input energy for the whole simulation time of 5 hour(18000s). Is it correct?

Opaque · December 4, 2017, 16:23

Difficult to answer w/o every detail about your boundary conditions.

To be able to convert the time integral to a "number of time steps" (not iterations) times "Delta Energy Flow", you must be certain that every variable involved in the calculation of the Delta Energy Flow is constant in time. Are you certain of this ?

My first initial response would be No until a careful review of your setup is done.

November 29, 2017, 13:31	Energy calculation through inlet and outlet face	#1
Vishnu_bharathi Member VB Join Date: Jul 2016 Posts: 35 Rep Power: 9	I have an Inlet face where fluid at 500°C at the rate 1 Kg/s is given as input. Outlet boundary condition is 0 Pa relative pressure. Inlet and outlet face area are 2 m^2. How do you calculate the Energy fed at Inlet face and Energy exited at oulet face in Joules? may be for each time step? I am doing transient simulation with time step 20 sec. Total simulation time for 5 hr.

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November 29, 2017, 13:52		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	You have to define what kind of energy you are accounting for. If you want to account for total energy (internal + flow work + kinetic energy + etc), I would use Total Enthalpy, then Energy at inlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Inlet Energy at outlet = Time Integral (from 0 -> 5hr) of massFlowInt(Total Enthalpy)@Outlet Not sure how valuable those integrals are for open systems since I usually work with energy rates; therefore, no need for the time integral.

December 4, 2017, 16:23		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	Difficult to answer w/o every detail about your boundary conditions. To be able to convert the time integral to a "number of time steps" (not iterations) times "Delta Energy Flow", you must be certain that every variable involved in the calculation of the Delta Energy Flow is constant in time. Are you certain of this ? My first initial response would be No until a careful review of your setup is done.