Thermodynamics Energy balance Equation

[jaredogden]

I am going over some thermodynamics right now and I am looking at the energy balance equation which is dE/dt = mdot*j(in) - mdot*j(out) + Qdot + Wdot(external)

I understand this means that the change in energy with respect to time equals the sum of the mass energy that goes into a system, minus the mass energy that exits the system plus the heat transfer, minus the work transfer from external things such as pdv work, shaft work, or electrical work.

My question for this equation is what exactly is the j in the equation? I have been trying to figure out what exactly j is and its units. If anyone has an explanation for this and can help it would be greatly appreciated.

Thanks ahead of time
Jared

(I tried using summation for the mdot*j in and out and I couldn't get it to work properly)

EDIT: I read that j is: the work, or energy, needed to push mass into, or out of, the control volume, in order to maintain continuous flow.

I think that explains it pretty well but it anyone has an even simpler way to break this down it might help even more. Thanks again

 

[Studiot]

I presume you are referring to this flow equation?

http://www.edforall.net/index.php/engineering-a-technology/aeronautical-engineering/78-thermodynamics/1630-steady-flow-energy-equation

 

[jaredogden]

Yes that is the flow equation I was referring to. That looks like a good web site, I'll have to take a more detailed look at it. Thanks

 

[Studiot]

but the end result is always that energy is conserved. So energy is very similar to a bank account in this sense.

I wish, I wish my bank account had this enviable property.



Good exposition, though.

go well

 

[firavia]

simply : j(out) and j(in) are the enthalpy of the fluid while entring and exiting the studied volume, the unit is kj/kg.
http://www.firavia.com

 

[jaredogden]

Thank you guys for your help. I understand it a lot better now and I'm definitely going to have to bookmark those pages. Now if I could just get some problems right.. I feel like I understand the lectures when my professor is talking in class, but then I start to do the homework or example problems and are completely lost.

Some of the example problems I have done and got lost on are located at my professors website thermofluids.net in the examples tab at the bottom and chapter 2 in case it interests you guys. If I find a specific problem that isn't worked out I'll have to post it to get help. Most of them are worked out and I understand where I made my mistakes, the problem for me is fixing those mistakes and being able so see things that I normally don't pick up on.