Why is the first law of thermodynamics useful in developing these equations?

[lee123456789]

Homework Statement

dont know what this means

Relevant Equations

in the post

why it is first law of thermodynamics useful in developing the non-flow energy equation and steady-flow energy equation.

First Law Equation (FLE) =
1

1.1

Non-Flow Equation =
2

Steady-Flow Equation =
3

Specific Entrapy of Steady-Flow

4
Mass Flow
5

I understand (i think) but can't explain.
Im understand it (i think) but finding hard to explain can somebody help. is what i put fitting the question?, how do i make it better? can somebody help it

FLE can be expressed as 1.1.
The first law of thermodynamics (1) contains (kinetic energy change) & PE (potential energy change). Theses contain contain two equation components that if u minus second half from first half will equal U,KE or PE. This allows (1) RHS to be broken down and reranged into two individual equation components forming (1.1).

Non-Flow
The system is closed so is not effected by external factors so addaction components need not be included into the eqaution. FLE (1) under non-flow conditions Δ𝐾 and Δ𝑃E are negligible, so equal ( 0). Under theses conditon Δ𝐾 and Δ𝑃E are omitted from the equation forming equation (2)

Steady-Flow
for steady flow systems, the system is open so if effected by air flow so the equation is required to change to accommodate this factor. Mass flow change of area, speed or density cause mass flow change change. There is 2 mass flows, mass flow in and out. FLE (1.1) m for both equation components will be label m1 or m2. m effect all conditions within both equation components so can be brought out sided the brackets.

(U) specific internal energy is effected by specific entraphy and pressure & specific pressure. rearrange the equation allows you to determine specific entraphy of the system in and out. Specific entraphy is important for steady-flow and allows you to devople a key component of the eqaution.

I don't know what Wx (work) is and how to explain it?

 

[Chestermiller]

In the development of the steady flow equation, the starting point is the first law equation. The terms involving KE and PE represent KE and PE flowing into and out of the control volume. The terms involving specific enthalpy are partly related to the internal energy of the streams flowing into and out of the control volume. But, in addition to this, the most important point about SFEE is that the work done by the system on the surroundings is subdivided into 2 parts: (1) work associated with pushing fluid into and out of the control volume and the remainder of the work called "shaft work" (since a rotating shaft of a turbine, compressor, or pump are typically involved) ##\dot{W_x}##. The part associated with pushing fluid into and out of the control volume is separated, and included with the specific internal energy flowing into and out of the control volume, to form the specific enthalpy of these streams. This is a very convenient way of work with an open system. In the steady flow equation, the internal energy, kinetic energy, and potential energy of the fluid contained within the control volume are not changing with time, so that part of the first law doesn't play a role in the SFEE.

 

[lee123456789]

In the development of the steady flow equation, the starting point is the first law equation. The terms involving KE and PE represent KE and PE flowing into and out of the control volume. The terms involving specific enthalpy are partly related to the internal energy of the streams flowing into and out of the control volume. But, in addition to this, the most important point about SFEE is that the work done by the system on the surroundings is subdivided into 2 parts: (1) work associated with pushing fluid into and out of the control volume and the remainder of the work called "shaft work" (since a rotating shaft of a turbine, compressor, or pump are typically involved) ##\dot{W_x}##. The part associated with pushing fluid into and out of the control volume is separated, and included with the specific internal energy flowing into and out of the control volume, to form the specific enthalpy of these streams. This is a very convenient way of work with an open system. In the steady flow equation, the internal energy, kinetic energy, and potential energy of the fluid contained within the control volume are not changing with time, so that part of the first law doesn't play a role in the SFEE.

hi

thanks
can i ask
does what i put make any sense to the question they ask me. I was looking at as math way at how to devople the equation. its my HND question

 

[Chestermiller]

To see the development of the open system version of the first law done correctly, see

Fundamentals of Engineering Thermodynamics by Moran et al (available as a PDF on line) and

Introduction to Chemical Engineering Thermodynamics by Smith and Van Ness.