Geek Squared

Brent Cullimore

To say that Pythagoras, who lived over 2500 years ago, would be astounded by what I just did in his honor says nothing. I mean, let’s face it: he’d be astounded by kale chips, boomerangs, and tubas.

So I didn’t do it for him. I didn’t do it for any valid reason, really.

I’ve always been impressed that a mathematical theorem can be demonstrated visually in a few seconds, providing such a strong intuitive grasp. Plus, there are bubbles and waves, which always attract my attention.

Amazingly, it took me a couple of years to realize that I could simulate this using FloCAD Compartments. Then a couple of more minutes to wonder whether I should ... before I dismissed that silly notion.

You can’t see the liquid, just the water/air interface. So I tilted the view a bit so the interface looked like a surface rather than a line.

I just used a 5x5x1 cm block initially on top of 4x4x1 cm and 3x3x1 cm blocks. Small compared to what was in the video, but since there is no information about the hole or slot sizes, it’s all a guesswork homage anyway. I had to adjust the flow resistances until the timing was about right.

Of course, Thermal Desktop wants to solve for temperatures even if you don’t care, so you can see some slight temperature changes as slight compression and expansion occur. But this is an easy model to make. I left the grid visible so you could visually see the volume sizes too.

The only hard part is the chug-chug office-watercooler action of the holes, which you can see is somewhat violent and chaotic in the original video. If it had been a vertical duct, I could have applied slip flow and been done with it. But in slot with liquid falling and bubbles rising against that liquid flow … yikes. Thankfully, some “two holes in parallel” methods had been developed for this boat fuel tank model already.

Now, if anyone out there wants to build an infinite number of N-dimensional Compartment models to demonstrate Fermat's Last Theorem, you will have out-geeked me.

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.