Thermodynamics Energy balance equation for Non - Steady Flow

In summary, this text discusses how the work done in an open system can be split into two parts: the work needed to force material into and out of the control volume, and all other work. This is an unconventional approach, and is done only in some particular development of the first law of thermodynamics.
  • #1
SWJ
5
0
Hi,
I am currently studying Thermodynamics and stumbled upon this equation and is slightly confused as to how this works. Hopefully someone can help me with the understanding.

thermo.png


According to this text here, if I am not wrong e can be h + ke + pe or u + ke + pe depending on where i am looking the system at. So does that mean the final equation can be equal to (m1u1 - m2u2) or (m1h1 - m2h2) depending on where i am looking the system at? Or i should just ask when would it be mh rather than mu?

Thank you very much.
 
Engineering news on Phys.org
  • #2
Most developments of the open system (control volume) version of the first law of thermodynamics separate the work into two parts:

1. The work needed to force material (in the inlet and exit streams) into and out of the control volume

2. All other work (which is usually referred to as "shaft work.")

In these typical developments, the work in the equations is the shaft work, which is usually signified by a subscript s. The work needed to force the material into and out of the control volume is usually included in the energy term, and the term would then involve enthalpy h rather than internal energy u.

In the particularly development that you have cited, this separation of the work into two parts is not done. So the work in your equations includes the work needed to force material into and out of the control volume. As a result, the energy term involves internal energy u, and not enthalpy h. This approach is very unconventional.
 
  • #3
Ah I see what u meant. Then are these "Shaft Work" included when say the flow is turning a turbine before exiting?
 
  • #4
SWJ said:
Ah I see what u meant. Then are these "Shaft Work" included when say the flow is turning a turbine before exiting?
Yes. Shaft work includes that.
 
  • #5
Alright! Thank you very much for the clarification now I have cleared up the confusion I had. :biggrin:
 

What is the thermodynamics energy balance equation for non-steady flow?

The thermodynamics energy balance equation for non-steady flow is also known as the first law of thermodynamics, and it states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

What is the significance of the thermodynamics energy balance equation for non-steady flow?

The energy balance equation is a fundamental principle in thermodynamics and is used to analyze and understand energy transfer and transformations in various systems. It helps to determine the efficiency of a system and can be applied to many real-world situations, such as power plants and refrigeration systems.

What are the assumptions made in the thermodynamics energy balance equation for non-steady flow?

The energy balance equation assumes that the system is closed, meaning that no matter or energy is exchanged with the surroundings. It also assumes that the energy transfer is only due to heat and work, and that there are no other forms of energy involved, such as chemical or nuclear energy.

How is the thermodynamics energy balance equation for non-steady flow different from the equation for steady flow?

The main difference between the two equations is that in steady flow, the system is in a steady state, meaning that the properties of the system do not change with time. In non-steady flow, however, the properties of the system are changing over time, and the equation takes this into account by including the change in internal energy term.

What are some applications of the thermodynamics energy balance equation for non-steady flow?

The energy balance equation has many practical applications, such as in the design and analysis of engines, power plants, and refrigeration systems. It can also be used to calculate the efficiency of various processes, such as combustion, and to predict the performance of different systems under varying conditions.

Similar threads

How to determine mdot for choked airflow through a valve, given T,P
    • Mechanical Engineering
Replies
3
Views
988
Engineering Steady flow energy equation in Thermodynamics
    • Engineering and Comp Sci Homework Help
Replies
1
Views
1K
Resonance vs. Conservation of Energy
    • Mechanical Engineering
Replies
8
Views
2K
A Steady state confined flow field: Is it cyclic?
    • Classical Physics
Replies
9
Views
951
How Does the Steady Flow Energy Equation Account for Heat Transfer?
    • Mechanical Engineering
Replies
2
Views
1K
I Enthalpy change in a flow process
    • Thermodynamics
Replies
2
Views
860
Solving the Energy Balance Problem
    • Mechanical Engineering
Replies
1
Views
1K
Heat Equation and Energy Balance
    • Mechanical Engineering
Replies
19
Views
2K
Bernoulli equation and parallel pipe branch
    • Mechanical Engineering
Replies
31
Views
2K
Thermo - Simple Energy balance equation
    • Mechanical Engineering
Replies
5
Views
2K

Hot Threads

Recent Insights

Back
Top