Limitations of Cp 1

[zaconova]

Hey guys, I was wondering if you guys could help me better understand when I can use Cp.

I know you may only use Cp under "constant pressure," like to characterize the heating of water in a container open to the atmosphere.

However, lets say I blow up dynamite in a field. Since there is transient pressure associated with the explosion, can I still use Cp to model the thermal output of the dynamite since it is only subjected to the atmosphere?

In other words, does the "constant pressure" of Cp demand constant internal pressure of the control volume, or constant pressure acted upon the control volume by the surroundings (in this case, the atmosphere)?

Thanks in advance,
Zach

 

[GregLocock]

I suggest you look at how they model combustion in an internal combustion engine, specifically under pre-ignition.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[NRP99]

Control Mass - Closed system with or without moving boundary. No mass transfer in and out of system. Energy can transfer in and out.
Control Volume - Open systems with mass and energy transfer in and out of system.

Constant volume process - No expansion or compression e.g. pressure built-up of air inside closed container when heated without volume change (no work done).
Constant pressure process - Volume increase (expansion) or decrease (compression) e.g. Movement of piston in piston with cylinder system when heated/cooled. (work done in terms of piston movement)

C[sub]p[/sub] and C[sub]v[/sub] can both be used for any process since both are properties of substance. Specific heat at constant pressure-C[sub]p[/sub] represents the change in enthalpy(=internal energy +work done) of substance per unit mass per degree Celsius. Similarly, specific heat at constant volume C[sub]v[/sub] represents the change in internal energy of the system. Both internal energy and enthalpy can be changed by means of transferring in or transferring out energy from the system.

For blowing dynamite in open atmosphere, what's your system? The open space region of atmosphere (open system)? Then latent heat energy stored in the dynamite (chemical energy of TNT or black powder) is converted to heat energy and other forms of energy. The air in your system is now heated and expanded so yes as far as I know you can use C[sub]p[/sub] to consider the change in enthalpy of air. But at the same time there is change in internal energy of air. So you can use C[sub]v[/sub] to calculate the change in internal energy of air. I am doubtful about this explosion process explanation though since its long back when I studied thermodynamics.

By the way this is not good example to study C[sub]p[/sub] and C[sub]v[/sub]. This is very complicated example to understand ( for me and explain it to you of course). Try with simple one first.