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December 2005
library  >  Application Notes  >  Tony Kordyban

Everything You Know is Wrong -- PART XVI

answers to those doggone thermal design questions
by tony kordyban


dear thermal guru,


your no-nonsense writing about cooling electronics has finally convinced me (and more to the point, my boss) that we need some cfd software. i have looked at the web sites and the sales brochures and even some demos of all the vendors for electronics cooling cfd tools. each one has a good story to tell about why theirs is the one to buy. i think i can trust you to cut through the hype and give us the straight dope -- which cfd package is the best?


bubbles from townsville


dear bubbles,


you trust me to cut through the hype after you've seen the way i shamelessly promote my book in this forum? i'll give you an honest answer, but just remember, what you've asked me for is my opinion, which is a lot different from asking me for the truth.


for the benefit of those who haven't had the pleasure of seeing all the lovely color brochures that you have, i'll explain what cfd is. computational fluid dynamics. that's what the letters stand for, but that doesn't give you much of an idea why it might be useful for predicting electronic component temperatures. cfd is the technology behind computer codes that solve fluid flow problems. you specify a flow geometry and some boundary conditions, and the code will solve for the fluid pressure, velocity, and temperature at every point in the domain. cfd was originally developed to solve general purpose, high-budget flow problems, ranging from flow around an airplane wing to blood pumping through an artificial heart.


most electronic assemblies use some kind of fluid (like air) to get the heat out. a cfd tool can predict that flow through an electronic assembly, which is where its usefulness to us comes in. the same code can calculate temperatures for the solid parts of the domain (that is, the components and circuit boards) by treating them as fluid that moves very, very, very slowly. general purpose cfd tools can be used to analyze electronics, but they are very big, very expensive, and have lots of features that don't apply to air flow through electronics, but still require you to know how to turn them off. some would say that the electronics cooling cfd packages are "stripped down" versions of the general purpose tools. i agree, in the sense that a finely tailored suit of clothes can be considered a "stripped down" version of a circus tent. it has had a lot removed to make it fit a lot better. packages like flotherm, icepak, and coolit won't help you design the next space shuttle, but they might help you design the next computer that goes into it.


i also want to distinguish cfd tools from other computer tools that deal with heat transfer in electronic assemblies, but are not based on cfd technology. there are finite element programs that are very good at solving solid conduction problems, in addition to stress and vibration but don't deal directly with the fluid flow. there are resistance network solvers, like macroflow, which are excellent at solving flow problems that can be defined as a network of simple resistances, like a bunch of different pipes or ducts connected together. you didn't ask me about those, so i won't say much about them, except that they aren't really competitors of cfd. they solve specific types of problems better, or at least faster, than cfd, but they can't really solve the fluid flow problems that cfd is best at. you may want to have all three types of tools if you are really serious about thermal analysis.


so back to your original question. which one is the best? the honest answer is: i can't tell you. there are three reasons, ranging in nature from corporate regulations to epistemology:


1. my employer expressly forbids me (and all their other employees) from saying good or bad things about the products of other companies. they hate getting sued, and i hate getting fired.


2. i don't actually know which one is the best, because i haven't tried all of them. i have used one for many years, and i played with one for a couple of months on a demo basis. even if i had an opinion (which reason 1 forbids me from having), it would not be an informed one.


3. i think the question may be practically impossible to answer, not just for me, but for anybody.


you are asking me a question very similar to, "who is the best spouse for me?" because taking on a cfd tool is very much like getting married. the problem is two-fold: such a judgment would be very personal, plus it takes a long time to form one. it will take years for you to really get to know each other, what you each bring to the relationship. who will know your potential spouse well enough to give you an informed opinion, without being married to that person? he or she couldn't be married to all your potential spouses at the same time, either. and by the time you may find out that you don't have a good match, years will have passed, and your needs will have changed, and the cfd tool that might have been best for you at the beginning, may not be best for you anymore. the truth is that there are many people who would make an excellent spouse for you. but you will never find out which one would have been the best, because you don't get several lifetimes to try them all out and compare them one against the other.


another similarity is that even the best spouse will cause you lots of aggravation at time, along with the many benefits and joys.


the brochures won't tell you this. but it take time, lots of practice with a cfd tool, solving real problems day after day, before you learn how to get good, reliable results from it. thermal modeling is an art, as well as a science. give the same problem to ten engineers, and they will come up with ten unique ways to solve it using the same cfd tool. and the results they get may vary quite a bit. some vendors make claims that their package is easiest to learn, generates grid the fastest, or has the fanciest way of displaying results. it doesn't matter whether you can learn the user interface in three hours, three days or three weeks. you won't know whether you have a good engineering tool for at least a year after you buy it. and if you are only using it a few hours a week, you may never learn all the things it can and can't do.


so unless someone has used each of the cfd packages for a minimum of a year apiece, on a daily basis to solve real electronics cooling problems, i wouldn't trust him to give you a realistic answer about which one is the best. and if that person exists, his knowledge of at least one tool is already a few years out of date. presumably the software has been revised over time, maybe for the better. that's why i think there is no real way to get an answer to your question. nobody knows the answer. perhaps nobody can know.


i suppose you ask the question because you don't want to buy an expensive one-year license and then find you bought a lemon -- a horrible piece of software that just doesn't work. based on what i have seen of all the packages, they are all horrible, but they all work. cfd is not a simple subject. i am amazed that they work at all, let alone as well as they do. so no matter which one you buy, there will be bugs, there will be incomprehensible menus, there will be gridding headaches, and there will be convergence problems. that is the nature of cfd. but they are all capable of solving a good portion of electronics cooling problems, with reasonable accuracy, if you stick with them. i don't think they could stay in business if the software just plain didn't work.


so i think you will have to decide which cfd package to get based on something else besides which one is "the best". will you get good technical support? i promise that no matter which one you buy, you will be on the phone with their support people a lot. maybe you need the absolute lowest price, or you won't be able to get any cfd at all. cheap cfd is better than none. do you also do the mechanical design? maybe an interface to your mechanical cad is important (my opinion is that it usually isn't, but that's just me.) is the vendor financially committed to the product? it would be frustrating if after you get really good with it in three years, the product is discontinued and you have to switch to another one. did the sales rep give you a cool t-shirt, coffee mug, or flashlight? one or another of these things not embedded in the software itself will matter to you. you might as well base your choice on them as on somebody's partially informed opinion.


so, just as in choosing your spouse, try to get to know as much as you can before the wedding, don't worry about getting the absolute best, and pick someone that is at least "good enough" before you make your commitment. isn't that what most of the decisions in engineering, and in life, are like? 


hi tony,


honest? boy, you sure weaseled out of that cfd question. here's something simpler. what type of thermocouple wire is the best in electronics cooling testing?


mike and the bots, s.o.l.


dear mike,


that one is a little easier. my employer won't come down on me, i hope, for giving out this preference.


the answer is type j. my thermocouple meter is calibrated for type j wire. i find that the measurements are somewhat less accurate if i use the other types of wire. of course, the closer the object you want to measure is to room temperature, the more accurate they all become.







about tony kordyban


tony kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last 20 years. maybe that doesn't make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways not to cool electronics.


he does have some book-learnin', with a b.s. in mechanical engineering from the university of detroit and a master’s in mechanical engineering from stanford. in those 20 years tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney. he has run into so much nonsense in the field that he has found it easier to just assume "everything you know is wrong" (from the comedy album by firesign theatre), and to question everything against the basic principles of heat transfer theory.



tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down. these have been published recently by the asme press in a book called, "hot air rises and heat sinks: everything you know about cooling electronics is wrong." it is available at https://www.amazon.com/hot-air-rises-heat-sinks/dp/0791800741. this advice column is an extension of that educational effort. 

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