"K A over L and MLI"

Brent Cullimore

More than once, I got into an argument with a boss in the 80s. And I almost never had a boss who was a thermal engineer during that decade. They never knew what to do with us thermal people. Systems? Power? Materials? Propulsion? Avionics? Been there, done that, probably had a disagreement with the boss as I rotated through.

I spent the most time as part of a propulsion group. But when that boss dismissed my whole tribe by saying “thermal is just K A over L and MLI,” he left a scar. Being a thermal engineer, I wasn’t supposed to understand fluids, and I certainly couldn’t be trusted to touch anything wet. I was just supposed to know how to calculate a conductance (KA/L) and how to wrap boxes with multilayer insulation (MLI).

The irony is that I had two offers from that corporation when I came out of grad school: one in the thermal department and another in the propulsion department. I chose the thermal group since they needed more fluids help, and since they were busy programming their own design and simulation tools. Apparently, had I accepted the propulsion offer, I would have magically acquired a different background too and been transformed into a trustworthy type. Allowed to turn valves and everything! But having accepted the thermal position, then having been placed inside a propulsion group anyway, I apparently had been lobotomized.

I was the Principal Investigator (PI) for a thermal R&D project, and I was given less than a fifth of the budget that the parallel propulsion R&D project received. Then I was asked to spend half of that tiny budget in travel costs, explaining how I used the remaining spare change to mess around with heat switches, evaporative coldplates, heatpipes, and capillary loops.

So when the propulsion project built a test facility whose liquid hydrogen (LH2) expenditures alone were more than my pre-travel R&D budget, I took notice. They were trying to keep LH2 in a cryostat for months or years, and had developed a complex system with a thermodynamic vent system (TVS): a throttle, an internal heat exchanger, and a vapor-cooled shield. But the darned thing took several days to come to equilibrium, burning through about a trailer full of LH2 per week while trying to test another set point on the TVS system throttle.

I offered to calculate the set point for them using my new-fangled FLUINT software. Maybe I was secretly hoping that they’d pass on their LH2 savings to my thermal R&D budget. Fortunately, the PI of the propulsion project, Dr. John Anderson, was a very kind and mentoring gentleman, who tolerated my questions and even let me turn the resulting model into an example problem for the newly minted SINDA/FLUINT User’s Manual.

Not that he trusted the answer that I came up with (even though it was later shown to be about what they found in test, having bought several trailers full of that expensive stuff). After all, I was still just a thermal type who had accepted the wrong offer.

So there was a lot of nostalgic dust kicked up this last month as I replaced the old “Sample Problem F” with a modern incarnation using all the latest tools. Also a lot of chagrin over having been such a young hothead with an axe to grind, but that’s off topic and requires apologies, so let's skip over it for now!

The original sample problem model was a “30 node wonder” with lots of hand calculations, because that is what we did in those days. We were using a VAX mainframe that charged us per CPU minute.

Of course, the new model uses thousands of nodes and is much more geometrically faithful, exploiting all of the wonderful CAD and FEA tools we have access to. It runs in a flash on a PC. It doesn’t produce much better results, but there are few if any hand calcs and assumptions left. It is easier to peer review.

It is also easier for someone else to inherit and update. Which is good, because I won’t be the one updating it 30 years from now to take advantage of new quantum computing.

Maybe by then they’ll have figured out how to classify thermal engineers who work a lot with fluids.

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.