Thermodynamics Energy balance equation for Non - Steady Flow

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Discussion Overview

The discussion revolves around the energy balance equation in thermodynamics, specifically for non-steady flow systems. Participants explore the relationship between internal energy, enthalpy, and the work done in control volumes, as well as the conditions under which different forms of energy are used in the equations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the energy balance equation, questioning whether it can be represented as (m1u1 - m2u2) or (m1h1 - m2h2) depending on the perspective of the system.
  • Another participant explains that in typical developments of the first law of thermodynamics for open systems, work is divided into two parts: work to move material into and out of the control volume and shaft work, which is usually denoted by a subscript s.
  • This participant notes that in the context of the cited development, the work includes the energy needed to move material, leading to the use of internal energy u instead of enthalpy h, which they describe as unconventional.
  • A follow-up question is posed about whether shaft work is included when the flow is turning a turbine before exiting, to which another participant confirms that it is included.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of shaft work and the energy terms involved, but there is some uncertainty regarding the conventionality of the approach discussed and the conditions under which different energy forms are used in the equations.

Contextual Notes

The discussion highlights potential limitations in understanding the separation of work types and the implications for energy terms used in the equations, which may depend on specific system conditions and definitions.

SWJ
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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.

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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.
 
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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.
 
Ah I see what u meant. Then are these "Shaft Work" included when say the flow is turning a turbine before exiting?
 
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.
 
Alright! Thank you very much for the clarification now I have cleared up the confusion I had. :biggrin:
 

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