Tuesday, 19 July 2016

FLOW WORK OR FLOW ENERGY IN THERMODYNAMICS

FLOW WORK OR FLOW ENERGY IN THERMODYNAMICS

We have seen the basic concepts of work and heat transfer in the field of thermal engineering and simultaneously we have also seen the sign conventions used in thermodynamics for heat and work transfer. We were discussing displacement work or P.dV work and also paddle wheel work in thermodynamics in our previous posts.

Today we will focus here about the flow work in thermodynamics. Flow work is also termed as flow energy in the field of thermodynamics.

So let us see here first, what is flow work?

Flow work is basically defined as the work which will be needed in order to maintain a continuous flow through a control volume. Flow work could be seen in a control volume system.

We will see here the flow work in detailed with the help of this post. Before going ahead, we must have to understand about the control volume system.

Control volume system is basically a system where mass will enter and leave continuously the system such as air compressors, heat exchangers, turbines etc. Air compressor is one of the best examples of control volume systems. In an air compressor, atmospheric air will be entered in to the compressor i.e. control volume system at low pressure and will exit the system at high pressure.

Now let us again come to the flow work fundamentals. Following figure indicates the basics of a typical control volume system. Let us assume mass is continuously entering from left side of control volume and leaving continuously the system at right end as shown in figure.
Control volume system and flow work
Let us concentrate over a layer of fluid at a section which is entering to the control volume system. Now as we are aware that in order to maintain a continuous flow through a control volume, layer of fluid will have to move further and hence layer of fluid will have to do some work to push the adjacent layer of fluid in forward direction.

Therefore the work which will be required by a layer of fluid at a section for pushing the adjacent or neighbor layer of fluid in forward direction in order to maintain a continuous flow through a control volume will be termed as flow work in thermodynamics.

We have considered here a section of fluid or an element of fluid with mass m, volume V and cross sectional area A as shown in above figure. Now if we concentrate here, we will come to know that fluid left side of considered fluid section will exert the pressure force over the considered section of fluid to push it through the control volume system in order to maintain a continuous flow through a control volume.

So let us see here the work done by the fluid, left side of considered section of fluid, over the considered section of fluid in order to push it for maintaining the continuous flow through a control volume.
Work, W = F. L
W = P. A. L
W = P.V

Therefore this is the work which will be required by a section of fluid for pushing the adjacent or neighboring section of fluid in forward direction for maintaining the continuous flow through a control volume and this work will be termed as flow work in the field of thermal engineering.

We must note it here that work which will be required for pushing a fluid section or element in to the control volume system will be similar to the work required for pushing the fluid section out from the control volume system.

We will see “Shaft work transfer” in our next post in thermodynamics.
Do you have suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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