Is there a way to model a pipe under vacuum pressure using Simscape?

Is there a way to model fluid flow in a pipe under vaccum pressure using simscape?

[Yifeng Tang]

Hey Delika,

Could you please elaborate on your question? What do you mean by "vacuum"? Is there still some air left in the "vacuum"? Will the liquid evaporate to fill the vacuum? A simple diagram would be very helpful.

[Delika Weragoda]

Hi Yifeng, So basically what I'm trying to do is model a heat pipe, which is a two phase pipe. And since it's a vacuum sealed container I was wondering if this pressure can be defined as a boundary condition using simscape blocks. So yeah, there will be liquid evaporation and condensation and the same time, but at either ends of the pipe. Hope that helps. Cheers Delika

Answer 1

[Yifeng Tang]

Hey Delika,

Thanks for the clarification.

It seems to me that it's possible to model the evaporation and condensing effects at two ends of a heat pipe, using blocks from the two-phase (2P) domain. It may not be easy though.

Couple blocks that I think may be useful for you:

Receiver Accumulator (2P) https://www.mathworks.com/help/releases/R2020a/physmod/hydro/ref/receiveraccumulator2p.html This one will allow you to model the two ends of the heat pipe. I suggest this instead of a simple chamber, because you need a way to allow only the liquid phase to travel away from the cold end through wicking, and gas phase away from the hot end (by pressure?). This block gives you the option to do that. The thermal port will allow you to heat or cool the fluids.

Pipe (2P) https://www.mathworks.com/help/releases/R2020a/physmod/simscape/ref/pipe2p.html You need pipes to tranport fluid (vapor & liquid) between the two ends.

What I am not sure or clear about is how to model the wicking of liquid from the cold end to the hot end. Any correlation you are aware of for the capilliary effect that's driving the flow? You'll need to find a way to implement that on the liquid path. The vapor path, I think, will be driven by the pressure difference because the hot end will have a higher vapor pressure.

Your model could conceptually look like this:

Hope this helps.

[Delika Weragoda]

Hi Yifeng,

Thanks alot for the detailed explanation.

Yeah, modeling the wick is tricky. Will have to look into that.

Much appreciated.

Cheers

[Delika Weragoda]

Hi Yifeng,

So I managed to model the heat pipe, but I seemed to have trouble with defining the internal pressure of the heat pipe. I've set my initial pressure to 0.01MPa (i.e. for each simscape block I've used) and the pressure differential (using a pressure source) to be around 0.001 MPa. But once I run the model the internal pressure of the blocks rises upto about 10 - 20 MPa, which is quite unusual. Any thoughts on this is highly appreciated.

Many thanks.

[Yifeng Tang]

what is that pressure source supposed to do? pulling liquid from cold (low P) to hot (high P) end? Since this is a closed system, having such a pressure source is like an unlimited fluid power source. I'd guess the overall temperature of the system goes up, too. One thing to try: in the pressure source, change the "power added" parameter from default isentropic to none. This may help.

[Delika Weragoda]

Hi Yifeng,

The heat pipe is vacuum sealed, so the entire closed system is under negative pressure, hence the pressure source.

Yeah, I used the pressure source to set the differential pressure of the heat pipe, but I'm not sure as to why the pressure in the blocks rise up to around 10MPa.

I've tried using both the pressure source (to set the pressure differential) and the controlled reservoir to set the boundary condition (i.e. vacuum pressure as 0.01MPa) but it seems like both these blocks cannot be used in the same system.

Many thanks.

[Yifeng Tang]

I think I see the confusion here. The proper way to set the pressure inside the heat pipe is through initial conditions. Look for the "variables" tab in each block; that's where you set the initial condition for each block. I don't think you'll need any source connected to the "outside" to enforce a specific pressure inside the heat pipe, as the pressure source can lead to flow in or out of the sealed system. Once you set a reasonable initial condition, the total mass inside the heat pipe will be conserved.

[Delika Weragoda]

Yeah, I've set the initial conditions of the blocks to the initial pressure, but it still gives a considerable amount of pressure reading inside. That is why I could not understand as to why there's such a large reading and decided to use another block to set the vacuum pressure, which actually works (but only with the constant volume chamber but not with the receiver accumulator block, which is confusing again). I'll check if I've made a mistake in the model and see.

Thanks alot Yifeng.