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As Dino and CF said! I have an example that may (or may not) clarify a little more from an engineering perspective. This is an effortpost, my apologies!!

In engineering analysis we often do a 'control volume analysis'. This means we take a process (say, an expansion at constant entropy, as in an ideal turbine), and model it with a diagram that kind of treats the process as a black box.

Here's an example of a turbine in control volume analysis. The fluid is processed by the turbine inside the dashed border. The space inside that border is the control volume (CV), and the borders themselves are called control surfaces (CS). The whole thing looks nothing like a turbine but that's ok, we're trying to understand what's going in to the CV, what's coming out, and how it's all related to one another without caring too much about the geometry of the whole thing. On the left, a fluid enters the turbine (through the left CS). This fluid has thermodynamic properties, which I label p_in (pressure), v_in (specific volume, which is like inverse density), and T_in, temperature. It will also have an entropy content s_in, and enthalpy h_in, which I haven't labeled. The m_in with a dot on top is the flow rate (mass-per-second) of the fluid.

On the right, this fluid exits the turbine with the same properties, subscripted _out. Something happens to the fluid inside the turbine (i.e. inside the CV). In fact, typically our incoming properties are for a high-temperature, high-pressure (etc) fluid, and in the turbine we extract this energy so that the outgoing properties are low-temperature, low-pressure. As we process that fluid inside the CV, we extract energy. More precisely, we say that the fluid does work on the turbine blades inside the CV. Intuitively, the high-energy fluid "pushes" on the blades, turning a shaft the blades are attached to, and that shaft leads outside the CV. We label this whole action by the W_out (with a dot on top): that is, the work done by the fluid on the turbine. The work is not a property of the fluid or the turbine, it happens between the fluid and the turbine. In our simplified representation of the turbine, this looks like work done at the boundary of the turbine, or at the boundary of our system. As this work is done by the fluid, its properties (T, v, p etc) will change because, having done the work, it has lost energy.

Also, because the turbine is not perfectly thermally insulated from its surroundings, and the turbine interior is at a higher temperature than its surroundings, there may be some heat loss at the control surface. The heat loss represents an energy transfer from the fluid inside the turbine to its surroundings. Because the fluid inside the turbine lost this energy, its properties (T, v, p etc) will change. The properties of the surroundings will also change. But the heat loss is the "agent" that does this, and it happens at the boundary of the system.

(I'll just add as a side note that the flow rate out m_out(dot) must be the same as the flow-rate in because of a condition we call "continuity", or conservation of mass. This just means that assuming our turbine doesn't accumulate fluid over time, then the stuff coming out must equal the stuff coming in.)


So when you read an article that talks about 'heat content of the oceans', what it really is talking about is the amount of heat that's been absorbed by the oceans, and the increased energy that the oceans have because of this heat transfer. The energy content of a fluid may change... but it will do so by either heat transfer at its boundaries (e.g. the ocean surface, ocean floor), or by doing work or having work done on it. This is actually the expression of the first law of thermodynamics: the energy content of a system must be accounted for by (a) how much that content changes inside the system, and (b) how much energy is gained and lost at its boundaries. That gain and loss is expressed by work or heat.


Oooh! I had a control engineering class, but we mostly worked with 2-point and 3-point controllers, PID-controllers and fuzzy controllers. :roll:

Thank you very much for this great explanation! I think I got the concept now. :P
Automation engineer, lateral thinker, soldier, addicted to music, books and gaming.
Nothing to see here
Flatfingers wrote: 23.01.2017: "Show me the smoldering corpse of Perfectionist Josh"


Pamelrabo wrote:So, where is this whole irc stuff happening?

Can I have the address and all that for my phone app?
Dinosawer wrote:server:
channel: #limittheory
Hmm...a new recruit? The second today? :think:

Wait, no. Cross that out. They both wrote here before. :D
Automation engineer, lateral thinker, soldier, addicted to music, books and gaming.
Nothing to see here
Flatfingers wrote: 23.01.2017: "Show me the smoldering corpse of Perfectionist Josh"


<Talvieno> look what josh did
* Detviet has no imgur HALP
<Detviet> SILVER
<Talvieno> too late det
<Talvieno> it's already gone
<Talvieno> !muhaha
<Detviet> JEW IT NOW :V
<Saoirse> ψ(`∇´)ψ
<Detviet> !sad
<Detviet> !cry
<Saoirse> 。・゚゚・(>д<)・゚゚・。
<Detviet> !tears
<Detviet> !halp
<Detviet> !waah
<Silver> huh?
<Silver> :V
<Silver> C Taiya
<Silver> oh
<Silver> OH
<Detviet> :D
<Talvieno> xD
<Silver> not today head is dead
<Silver> ask tomorrow
<Talvieno> lol
<Talvieno> xD
<Detviet> Jew up, silver >.>
<Silver> today I try purgatory stuff
<Talvieno> that reminds me of the troll under the bridge in witcher 2
<Talvieno> "head is... dead. ask tomorrow"
* narwhalz0111 regrets waiting so long before joining IRC.
<Talvieno> "gotsta drink"
<narwhalz0111> You crazy bastards.
<Talvieno> lol narwhalz :P You missed a LOT xD but there's plenty more in the future
<Talvieno> (yeah, like I was going to be nice and say "it's okay, you didn't miss much")
<narwhalz0111> Don't know how the snippets from the IRC thread failed to convince me long ago.
<Detviet> :D
<Detviet> well
<Talvieno> "Man, those guys are totally nuts"
<Talvieno> "Better stay away from there, yup"
<Detviet> now YOU can make those snippets :D
Have a question? Send me a PM!


BFett wrote:
Thu Jun 15, 2017 9:14 pm
Our IRC is certainly an entertaining place to hang out at. I've had some very good discussions there as well.
These are both true statements. I can agree on them. :P
Automation engineer, lateral thinker, soldier, addicted to music, books and gaming.
Nothing to see here
Flatfingers wrote: 23.01.2017: "Show me the smoldering corpse of Perfectionist Josh"

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