Who are Thermal Engineers? You might be one!

Brent Cullimore

I admit that I have chafed in the past at being called a thermal engineer. I have degrees in Mechanical Engineering and in Thermosciences. I consider myself more of a fluid/thermodynamics type: I love bubbles, and I think it is a sin if you stir cream into your coffee too fast and miss all the cool eddies. One of my hopes is to visit the grave of Boltzmann, where his legendary equation describing the entropy of systems like tossed coins is inscribed above his headstone, S = k * log(W).

What do any of those passions have to do with “thermal?”

Maybe you work in electronics cooling and don’t see any problem with the term “thermal engineering” (aka, thermal control, thermal management). Or perhaps you work with power generation systems, aircraft fuel systems, or liquid propulsion systems. What do those have to do with thermal?

I guess we could call ourselves “energy engineers” or even “thermal/fluid engineers.” I certainly like the sound of “power engineers!”

I’ve teased people that we are the Wet Side of Mechanical Engineering, to distinguish ourselves from the Dry Side: people who are way more gifted with gizmos and gears than I’ll ever be. Sigma doesn’t mean stress to us, it means either surface tension or the Stefan-Boltzmann constant. And no, we can’t change these values to make your latest wild idea work!

Some of the confusion is historical. As far as I can tell, the original thermal engineers came out of the shops of the early satellite makers, where vacuum testing was (and still is) very expensive. The value of thermal analysis (which unfortunately is itself a confusing term) was clear in that industry: any mistakes discovered after launch couldn’t be fixed.

But even that industry didn’t know what to do with us. Were we mechanical engineers? We sure aren’t like normal mechanical engineers. Were we designers? Except for the occasional heat pipe or radiator, we sometimes didn’t have any hardware associated with us. Were we systems engineers? We certainly tended to focus on the entire system in order to calculate energy flows and balances. Were we analysts? Well yes ... except for all the testing and the aforementioned heat pipes and radiators. Sometimes thermal engineers were placed in systems engineering, sometimes with materials folks. Most often thermal engineers were located where there was more overlap: with propulsion or environmental control (aka “climate control” in other industries).

Meanwhile, in the early electronics industry, some mechanical “packaging engineers” were tasked with making sure that chips and batteries didn’t overheat after they were designed. (Hey, I did say it was the early years, and if you were around then you know that electrical engineers were making all the decisions and we were left to deal with the consequences of those decisions.)

In the early automotive industry, the pressure to start from scratch and get the design right in terms of months instead of years did not exist. So people working fuel systems, AC systems, cabin climate control, engine oil systems, automatic transmissions, and so forth didn’t do a lot of analysis. Also, they all worked largely independently of each other. But when the need to catch up with the minivan craze (as one sad example) in 18 months became the norm in the 80s, CAD/CAE tools were quickly expanded and adopted. Gradually, engineers working diverse systems began to have more and more in common with each other.

I don’t know how you answer when asked “What kind of engineer are you?” But if you found this blog and bothered to read this far, I’m guessing something in the paragraphs above rang true for you.

These days, I’m OK being called a “thermal engineer.” If you aren’t OK with that label, at least maybe you can see why it is a convenient shorthand.

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.