VASIMR

ladajo · Post by **ladajo** » Wed Jul 14, 2010 12:49 am

It is a process (cycle), that is what makes it solvable.

You have attempted to apply the Ideal Carnot Cycle efficiency equation, but this does not apply to an Ideal Otto Cycle. You also did not identify the process points from the Carnot PV diagram to an ICE, which you will find does not recognize the full process of Otto (ICE), and does not translate fully.

The principles are similar in concept, but the Otto cycle is different, that is why it exists. As someone else stated above, Carnot does not capture Otto, it is to simplistic.

This page may help you understand;

http://web.mit.edu/16.unified/www/FALL/ ... ode26.html

MSimon · Post by **MSimon** » Wed Jul 14, 2010 1:27 am

You might want to check section 3.5.1

There seems to be a fundamental mistake right at the top.

==========================

Never mind. I got the sign wrong.

ladajo · Post by **ladajo** » Wed Jul 14, 2010 1:47 am

Did the page help? You can go through the whole semester's notes if you wish, but I think it is captured well enough here, and you have the background not to need the fundamentals discussed in the previous sections.

MSimon · Post by **MSimon** » Wed Jul 14, 2010 4:14 am

ladajo wrote:Did the page help? You can go through the whole semester's notes if you wish, but I think it is captured well enough here, and you have the background not to need the fundamentals discussed in the previous sections.

I have yet to see anything that convinces me that an IC engine is not a heat engine but a "chemical" engine. i.e. a process on the order of photo synthesis or fuel cell.

All the analysis is based on heat engine analysis.

ladajo · Post by **ladajo** » Wed Jul 14, 2010 5:56 am

And that is the rub.

IC is not Carnot, it is Otto. Some argue that ICE are not a heat engine because it is not a bounded system nor reversable process.
Semantics to be sure.

I still think the classic definition of a heat engine does not fit, however I do concede that it can be plotted on a PV curve as a cycle of sorts. It all lays in how you establish (or not) the system analysis boundries.

I think the rub lays in the definitions of heat and work inrelation to energy balance.
Either way, the world has not ended.

93143 · Post by **93143** » Wed Jul 14, 2010 8:23 am

An Otto cycle cannot possibly be more efficient than a Carnot cycle operating between the same two temperatures, regardless of parameter adjustments. No heat engine cycle can. This is a theoretical absolute. All the tweaks that WizWom talks about are just tweaks to get the efficiency closer to that of a theoretical Otto cycle (except for the drag coefficient of the car, which is a complete red herring as it has nothing whatsoever to do with the power cycle).

Also, what has heat transfer got to do with anything? The "hot reservoir" and "cold reservoir" are states, not places. No one ever said a heat engine had to involve heat transfer across a boundary as an essential operating principle.

MSimon · Post by **MSimon** » Wed Jul 14, 2010 8:46 am

93143,

Thanks. I was running out of steam. Heh.

Giorgio · Post by **Giorgio** » Wed Jul 14, 2010 9:28 am

Like I said before, the main problem in this thread has been (and is) the use and the meaning of terms and the correct understanding of the type of processes where "Carnot Cycle" and "Carnot Cycle Efficiency" have a meaning.

IC engines are Thermo-Chemical-Mechanical apparatus.

Being "Thermo" they have a "Carnot Cycle" and they have to deal with "Carnot Cycle Efficiency" for that part of the system.
Being Chemical they might or might not have a "Carnot Cycle", thus "Carnot Cycle Efficiency" might or might not apply according the type of chemical reaction occurring.
Being "Mechanical" they can ignore "Carnot Cycle Efficiency" for the mechanical part.

Being an APPARATUS composed by different subsystems, its "Engine Efficiency" is calculated in a much more complicated way.
The fact that we can estimate the efficiency of a good part of this apparatus with a "Carnot Cycle Efficiency" calculation does not mean that it is the way it should be calculated or that it is the limit efficiency of the apparatus.

This is also the reason why chemical process in fuel cells (and human beings) are not limited by "Carnot Cycle Efficiency".
These chemical process are still subject to thermodynamic laws (Gibbs and Helmholtz functions) but in these cases "Carnot Cycle Efficency" simply does not apply, because there is no thermal cycle.

ladajo · Post by **ladajo** » Wed Jul 14, 2010 12:42 pm

No one ever said a heat engine had to involve heat transfer across a boundary as an essential operating principle.

MIT does.

Georgio - thanks, you summarized well. These were the thoughts rolling around my head last night and this morning after my last post. It was what I was starting to get at above, but did not complete well.
I too was starting to run out of steam...

WizWom · Post by **WizWom** » Wed Jul 14, 2010 1:33 pm

We got on this digression because the powering of a VASIMR (or any ion drive) has to be done with something that generates heat, and the heat can only be removed in space by sending it out in the exhaust or radiation, since there is no contact for convection or conduction.

A minuscule amount of thrust could be generated by designing the radiator to radiate only in one direction.

ladajo · Post by **ladajo** » Wed Jul 14, 2010 1:51 pm

How does ISS remove internal heat? Honest question, I know it is an issue, but have not stopped to see how they actually sched heat from inside. The average person gives off about 100W as I recall, and each operating piece of equipment adds in.
I know from personal experience that it gets REALLY FREAKIN HOT REALLY FREAKIN FAST in a submarine when you turn off the AC units. Not fond memories at all.

WizWom · Post by **WizWom** » Wed Jul 14, 2010 2:09 pm

ladajo wrote:How does ISS remove internal heat? Honest question, I know it is an issue, but have not stopped to see how they actually sched heat from inside. The average person gives off about 100W as I recall, and each operating piece of equipment adds in.
I know from personal experience that it gets REALLY FREAKIN HOT REALLY FREAKIN FAST in a submarine when you turn off the AC units. Not fond memories at all.

http://science.nasa.gov/science-news/sc ... st21mar_1/
That page is old, and speaks of 14 panels, with no specs.

http://www.lockheedmartin.com/products/ ... index.html
Speaks of 6 HRS assemplies they supplied, with 8 panels each, rejecting 11.8 kW each, for a total of 70.8 kW radiation.
Also 4 PVR assemblies, each rejecting 9 kW, for another 36 kW
Total then is 106.8 kW heat rejection.
HRS radiators were deployed in 2007.

zapkitty · Post by **zapkitty** » Wed Jul 14, 2010 2:14 pm

ladajo wrote:How does ISS remove internal heat? Honest question, I know it is an issue, but have not stopped to see how they actually sched heat from inside. The average person gives off about 100W as I recall, and each operating piece of equipment adds in.
I know from personal experience that it gets REALLY FREAKIN HOT REALLY FREAKIN FAST in a submarine when you turn off the AC units. Not fond memories at all.

Heat rejection is via some big radiators.

http://www.lockheedmartin.com/products/ ... index.html

The short version: two arrays of three units each provide about a total of 70 kW heat rejection for the hab modules and four additional similar units , one for each pair of solar arrays, provide a total of about 36 kW of additional cooling for the solar arrays and their big battery packs.

The radiators use ammonia for their working fluid.

Water is used for the hab module interior loops and the water carries the hab heat to the exterior ammonia loop via heat exchangers.

ladajo · Post by **ladajo** » Wed Jul 14, 2010 3:54 pm

Thanks guys,

GIThruster · Post by **GIThruster** » Wed Jul 14, 2010 6:39 pm

Edited since others made my point.