Perturbed? Good!

Brent Cullimore

Being perturbed isn’t much fun if you are a human.

But models love getting perturbed. We don’t perturb them enough, in fact.

OK, perturb has negative connotations. And perturber just sounds wrong in so many ways!

Now your understand why we called the new perturbation feature in Version 5.8 of Thermal Desktop the Model Kicker instead. It sounds more violent, I know, but it was intended to be reminiscent of “kicking the tires before you buy a car.” You should “kick your math model before accepting its results.”

This all started a year or two ago when a friend and I were pondering the question of how we could use the increasing power of computers to produce a better model and not just a bigger one. How we could use Moore’s Law for good instead of evil, given that we’re way past the days when we struggled to afford enough nodes to get acceptable accuracy.

Full disclosure: my friend and I were sampling flights of craft beer at one of the many brew pubs here in Boulder Colorado. (Why we would be discussing Thermal Desktop in that situation can’t be explained. Would it surprise you to know that we were alone, and that the booths next to us had mysteriously cleared out?)

Surprisingly, we came up with some good ideas.

Miraculously, we remembered them.

I was especially perturbed (!) by this thought: we’re building such big models these days that we don’t always understand what is going on inside them.

Big models hide mistakes, and make it harder to run the model many times in order to evaluate various scenarios. You have to spend a lot of time staring at colored contours and plotted transient results in order to figure out what exactly is happening inside your model. You can easily find out where heat is flowing, but it is much harder to figure out exactly why it flows where it does when it can follow so many parallel routes.

How about diverting some of those fast new multi-core CPUs and screaming solution algorithms into helping build your intuition about how the system behaves?

The purpose of computing is insight, not numbers.” 
                                                        - Richard Hamming

Does this sound hard? It isn’t! It turns out the Model Kicker is really easy to set up, ridiculously fast to run, and very easy to intuit relative strengths of connection.

Here is a snapshot (at one time point) of a box full of batteries and electronics, isothermalized by four heat pipes. Can you tell how well the heat pipes are helping? Can you even tell whether they are hooked up correctly? You can certainly see what is hottest and what is coldest, but for all you know you forgot to put in the thermal contact in one spot, or you put it in twice, or you put in the wrong value. Or you forgot to merge the edges of the side walls. (Relax, we’ve all goofed up. We just don’t want other people to find those goof-ups before we do!)


 
Now take a look at what happens when you “kick” one heat pipe and measure not just temperature, but the strength of connection between that heat pipe and the rest of the model on a scale of 0.0 to 1.0. In other words, 0.0 means “thermally not connected” and 1.0 means “extremely well-connected thermally.”


 
If the heat pipe had experienced a connection problem (e.g., missing contact conductance), that issue would have jumped out of the screen at you.

Or you can perturb all four heat pipes at once to ask questions such as “Are the heat pipes doing that big cylinder any good?”


 
To learn how to use the Kicker, you can read about it in the TD User’s Manual. You can also view this class or ask us for the class notes.

You don’t have to use the Kicker to explore your model. You can also make your own perturbations in any inputs (such as the bond line conductance, or a convection coefficient scaling factor) and use the powerful TD postprocessing option to compare the temperature differences between two answers. You’ll be surprised what jumps out at you!

Now, go take a look at the last model you built and get perturbed!

 

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 will last 60 minutes and are designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts. 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.

May 31, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar provides an overview of LHP design and operation, from a basic understand of components to a review of important performance considerations and limitations.

Many topics will be covered, from start-up issues to the purpose of the evaporator bayonet to capillary flow regulators to load balancing in parallel LHP units. However, we will cover these topics only in enough depth that you will be able to understand the reasons for various modeling approaches that will be covered in later webinars. In other words, this webinar will survey the various ways in which LHPs require a specialized approach to design analysis and simulation.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of two-phase thermodynamics and heat transfer.
Please register for Part 1 here

June 5, 2018, 8-9am (PT), 11am-noon (ET)

This webinar explains how the toolbox approach of Thermal Desktop and FloCAD can be used to design and simulate LHPs at a system level, where the focus is on predicting conductance of nominally operating LHP, including thermostatic control (variable conductance).

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of Thermal Desktop and FloCAD operation as applied to two-phase systems. Basic familiarity with LHP components and operation (see Part 1).
Please register for Part 2 here

June 7, 2018, 8-9am (PT), 11am-noon (ET)

Modeling wick back-conduction in an LHP is critical to accurate prediction of the overall loop conductance and operating point. This prediction can't be separated from an understanding of what is happening in the wick core. This webinar presents time-honored methods of dealing with these complex topics in a relatively simple (if abstract) thermal/fluid network.Prerequisites:

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Basic understanding of Thermal Desktop and FloCAD operation as applied to LHP modeling (see Part 1 and Part 2).
Please register for Part 3 here

June 12, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar explains how Thermal Desktop and FloCAD can be applied to simulate complex and transient phenomena in LHPs, including condenser design, start-up, thermostatic control, and gravity assist (evaporator below condenser). The design of an actual LHP will be used to demonstrate concepts; the implications of attaching large masses to the evaporators (cooled electronics and support structures) will become clear as a result.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Familiarity with LHP modeling approaches in TD/FloCAD (see Part 1Part 2 and Part 3).
Please register for Part 4 here