Webinar: When Heat Sinks Aren't Enough, Go beyond traditional heat sinks
Hello everyone, thank you for joining us today. Today we're going to talk about when heat sinks aren't enough and the concept here is that you're running into limitations with very routine metallic based heat sinks and you need some improvement and we're going to look at a bunch of two-phase technologies to improve the performance and enhance your heat sink capability. So again, thanks for joining us, my name is Bryan Muzyka i'll be kind of running through the slides with you today and also do want to encourage please type in questions in the in the question box. We'll make sure to go through all of those after the webinar concludes and get you all responses with a lot of detail on those so please ask as many questions as come to mind during this, there's a lot to cover and we'll get into kind of some of the basics but yeah, certainly not enough time here today to go through in significant detail so please feel free to to ask questions as we go. So for those of you who haven't joined us in the past we do want to just kind of touch base on the act we are a thermal management solutions company and we handled the entire gambit of thermal solutions from passive two-phase heat transfer technologies kind of at the component and sub-assembly level all the way to full solutions fairly complex active systems looking at pump single phase pump two-phase as well as vapor compression cycles so we do have a lot of tools in our toolbox and we work very closely with our customers to make sure that the optimal solution is is determined and over the course of our history showing there we have won several awards i'll just maybe go through the most recent past couple of years we've won three different military and aerospace electronics innovators awards so those were for our ICE-lok product line our Pumped two-phase product line and we're in our space copper water heat pipe product line so we're very proud of those awards and last year we also won the AHR product of the year award in the green building category.
For some of the work we do in energy recovery, for large HVAC systems and overall we are slightly over 220 employees and over 200, 000 square feet in two facilities here in Pennsylvania, one in Lancaster Pennsylvania and one in york Pennsylvania. So the agenda for today we're going to look at as i mentioned two-phase technologies that are used to enhance performance of your heat sink or of your system so the first one we'll dive into here is heat pipes then we'll go into kind of more of an application with HiK plates. i'll talk about vapor chambers and then we'll talk about phase change material heat sinks as well as pulsating heat pipes so a lot to cover here we'll go through each very quickly but as I mentioned please type in as many questions as come to mind as we go through these slides. so heat pipe technology I know a lot of those who are joining us today um have joined past webinars and are probably familiar with heat pipes so I won't go through in a lot of detail the operating principles but effectively they are a two-phase liquid and vapor heat transfer device a closed-loop system where we run a vacuum on the pipe wherever heat is input it will create a boiling effect use the lean heat of vaporization to enhance your heat transfer coefficient and that boiling will create an internal pressure and temperature gradient which will push the fluid to wherever it's colder in the system and wherever it's colder and coupled with the heat sink the fluid will give up its latent heat condense back to a liquid and then be transferred back from the condenser to the evaporator either by gravity or some type of internal wick structure that creates a capillary pressure so overall you have a very low delta t from evaporator to condenser which gives you that kind of really good thermal performance as you integrate these into your system so most of the customers that come to us are really looking for design help with heat pipes so a lot of the questions we get are how how can we bend them how can we flatten them how can we retrofit fit them into current designs so just looking at those areas we'll give some general guidelines here the first is bending so we do recommend a centerline bend radius of 3x the outside diameter so you can see the figure to the right there is kind of showing you the centerline bend radius and if we keep that 3x the OD we should be in good shape to not degrate performance on the flattening side again this is kind of a general guideline rule of thumb but usually, we recommend keeping it above two-thirds the outside diameter so you can go flatter in certain instances as you change your profile you calculate your power capabilities based on the hydraulic diameter instead of the you know circular diameter of the pipe so that will limit performance a little bit but sticking above two thirds of the outer diameter still can move pretty significant power and then the last question we typically get is is on how to integrate these into your system how do we get a good heat transfer path into the heat pipe so that we can take advantage of the fluid effects and the vaporization and there we typically recommend solders as the best mechanical and thermal performing bonding technique usually it's a lead-tin solder but there are also rojas compliance solders that can be used as well outside of that we can also use epoxy which has some advantages as far as ease of manufacturing but not quite as good mechanically or thermally as a solder joint would be and then along the bottom there you can kind of see just a general thermal resistance network and the goal here with with any thermal solution is to keep the device temperature as low and close to the safe operating to improve the life of your electronics and so analyzing each resistance separately and optimizing each will provide you the best performance but the heat pipe resistance itself there is somewhat of a short circuit to that resistance network it'll provide a very low delta t to move heat fairly long distances so it's a really nice way to improve your overall thermal resistance network with a pretty easy straightforward design and now jumping into the design route a little more we do have some tools that are free to the public that you can use to help with your heat pipe design the first one we want to talk about here is the heat pipe calculator which is on our website under the resources tab and this gives a pretty basic diagram of a heat pipe it talks about kind of how to set it up your evaporator length being the length that's coupled with your heat input and the condenser length is basically the length that is coupled to your heat sink or your heat rejection area and then anything in between there would be called the adiabatic section and we kind of just use the total length of the heat pipe to run the calculations here so you can see the inputs required on our tool there to the bottom right and the other input that i haven't mentioned yet is the height against gravity and that is basically the height differential between your evaporator and your condenser so if you do have a system that has bends or more complex geometry just taking the overall height difference from your evaporator to the condenser or the height difference of the longest region where the working fluid would need to pump against gravity is basically what we're trying to calculate there and so here's just kind of a basic design looking at one example so this is an example where you have about an inch and a quarter of heat input area um the the pipe goes up is bends at a 90 degree and then couples the heatsink with the two inch area and has a about four inch of total height change now you can see in that one the the height change is actually favorable for heat pipe performance because the evaporator is below the condenser so when we input that we actually put a -4 as the height against gravity and then we can run our calculations so the plot you see there is the results and if if you go above to the title bar of the plot the red areas the red figures are the um inputs that you put into your system so you can see we use a minus four instead of a four operating height against gravity but overall you can move some pretty good heat here with a seven inch long pipe in these conditions and then from there you kind of have the option to use a single pipe that can move the entire heat load or use two smaller pipes that can move the heat load so for this example we have between 60 and 70 degrees wanting to move about 70 watts and you can see kind of your options there so in this case you had you could use one six millimeter pipes or two eighth inch pipes or something larger than eight inch pipe so that gives a kind of an example of heat pipes and how they're used um so the second module here is on high k plates which is the application of heat pipes into aluminum frames so again the idea here is to embed heat pipes completely within an aluminum frame high k is an acronym for high thermal connectivity so by doing this you can actually increase your thermal conductivity of aluminum which is the most typical material because of its easy to machine low cost and a lot of the different mechanical properties associated with aluminum but you can increase your your thermal conductivity or k value of like 160 to 180 watts per meter k upward to 500 to 1200 watts per meter k and that value there is real-world examples where we've actually built high k plates tested them to real customer inputs and then went back to our models and put a bulk thermal conductivity until we matched our test results so those are very real-world solutions we're not strategically putting our heat input or heat pipes in in conditions where it would show favorable results those are very real world and we usually tell customers if you put 600 watts per meter k that's something we can achieve the other benefits outside of thermal performance is strength and weight by integrating heat pipes into aluminum with a soldered assembly you're not affecting the weight very much and you are um staying at very similar strength and we've done a lot of programs with defense primes and things like that to verify those claims and in many conditions we can because the heat pipes are doing all your thermal work we can actually reduce the weight of the system by taking out some of the area that's not needed for thermal performance anymore as long as you're maintaining the structural strength integrity required of the system you could a lot of times reduce the overall weight of your system and then here's one kind of example case study this is a very large plate it's roughly three and a half by two feet and you can see there is several hot spots from the initial design requirements the goal here was to get it out to those liquid-cooled rails so that's the blue rail temperature you can see there and you can see in the couple hotspots we'll call them three main hotspots there was the two on the top have a very small thermal path but they had such a high heat flux that they were still creating kind of that failure mechanism up there so if you look to the heat pipe layouts on the the two right figures you can see that it was it was a matter of not only getting the heat to the edge but routing the heat pipe along the edge to lower that heat flux into the liquid coolant and drive down those temperatures and then on the bottom figure uh or bottom hot spot it was really all about the length the conduction path so there we just wanted to get as many heat pipes in there and bring it out to the rail as efficiently as possible so the overall design here was dropping the component temperatures of those three main hotspots by about 20 degrees so significant thermal performance improvements and again no real impact on weight or strength of the system and here's one example of actually reducing weight using a high k design so in a standard heatsink your conduction is mainly due to the the thickness of the material and then your convection is on kind of the the fin area so you need large enough um volume to lower your volumetric resistance enough to get the convection benefits and then the spreading within the aluminum by increasing your thermal conductivity significantly you can actually thin down your base and you can also get better fin efficiency by operating most of the fin length at similar temperatures so you can achieve similar performance in a smaller form factor using high k heat sinks versus straight metal heat sinks and here's an example that you can see the the final design was over three pounds lighter and had a much thinner base and also got better performance out of their fins and this is the test results of of the case study you just saw so we've again put in the same inputs in both areas we ran the system and we designed it to be very similar performing we actually match the the performance there in both cases but you did get a much more lightweight design with the high k heat sink and then looking at just some different form factors with high k plate technology the uh we talked about high k heat sinks that's where you're basically embedding heat pipes into the base of the fan assembly you can see an example there in the top left with a folded fit and cover which is used and the heat pipes are embedded right under those folded fins so again better fin efficiencies better thermal conductivity to drive down not only your conduction gradients but also your convection gradients in those designs and then two on kind of the embedded computing side are HiK board frames and HiK chassis so the board frames is fairly routine done a lot in especially military embedded computing systems where they have kind of standardized form factors 3u 6u card frames and we can pretty quickly embed heat pipes and route them to the edge which is then coupled into the chassis so again the requirements we we basically need to know where the heat inputs are where those critical electronics are and then usually the design methodology is routing it to um avoid any through-holes and things like that and getting it out to the edge as efficiently as possible and then finally on the chassis side there's a lot of uh different options here if you have an air-cooled chassis again you can increase the fin efficiency if you have a liquid-cooled chassis a lot of times you can increase the reliability of the system by integrating where the liquid cooling is seeing so the bottom right is a good example of that this is one where originally the customer had liquid flowing through each of those vertical channels but by integrating heat pipes taking advantage of the thermal conductivity gains they actually were able to move the liquid loop to the top and bottom of the chassis and still keep the temperature under safe operating conditions so there you're adding an additional reliability factor if anything were to happen leaks in the system or things like that you're not damaging your electronics instantly so the next technology we'll talk about is vapor chambers this is another heat spreading technology similar to um heat pipe operation used using a two phase heat transfer but it does have some unique advantages in terms of thermal performance and heat flux capabilities so again the the operating principles are are very similar it's it's mainly a planar heat pipe so it's it's a heat pipe that's using the vaporization of the working fluid the spreading to cooler areas and then the condensing and wicking back to the evaporator zone but you can do this in a in a two-dimensional form factor so you can get even better spreading across your area but the really big benefit to to vapor chambers is the high heat flux capabilities even compared to heat pipes which can operate in the you know 10 25 maybe 50 watts per centimeter squared range uh vapor chambers can increase that dramatically to roughly 100 watts per centimeter squared for fairly straightforward designs and then even up to we've demonstrated over a thousand watts per centimeter squared for some advanced wicks so a very high heat flux capability um and a lot of thermal benefits the the penalty to vapor chambers compared to high k plates is the ruggedization so high k plates are a more rugged technology because you're fully embedding them the aluminum adds strength to the system compared to a planar vapor chamber which has as a name suggests vapor space in the middle so it has some structural posts and integrity there but not as much as embedding into a rugged aluminum plate and again here's some examples and some specifications you you would normally see in a vapor chamber design the the thickness we typically recommend upward of three millimeters the wick structure would be customally designed as i mentioned if you're below 100 watts per centimeter square you can go with a fairly straightforward continuous powder finish if you go into really high heat flux that's where we get into some of the intricate press designs for for wicks that give you higher heat flux capabilities um and then operating conditions fairly similar to to heat pipes minimum temperature we'd say um any anywhere below zero since water is usually the working fluid you're not getting performance but you can survive and then maximum temperatures around 105 degrees c so not quite as high as a heat pipe or a high case solution because the pressure containment is a little more challenging in vapor chambers but they're they're usually pretty suitable for the high end of electronics temperature range which typically our design point is in like the 70 to 85 degrees c range so vapor chambers again high heat plus capabilities and very strong spreading similar to other two phase liquid vapor heat transfer devices shifting a little bit we're going to talk next about phase change material heat sinks so as the previous three modules talked about liquid to vapor phase change this is going to be talking about solid to liquid phase change and when that might be applicable in your system so i guess first what is phase change material it is any material going from solid to liquid that we can take advantage of the latent heat of that phase transition to store thermal energy so if you look at the figure to the top right you can keep see kind of a a profile here where you're using the specific heat of a material to a certain point but once you hit that solid to liquid phase transition you transfer into the lean heat zone and the lane heat is typically several orders of magnitude above the specific heat so that allows you to hold temperature with a constant heat input and absorb energy for a specific period of time and then once you're fully melted you would go back to the specific heat of the now liquid of the of the phase change material but in real application what you're attempting to do is you're taking advantage of that phase transition you're storing energy and then it's used in either single time use or pulsed operation electronics and then you're refreezing during the off cycle and that allows you to kind of take advantage and take advantage of the repeatability of phase change material so the bottom right finger gives you kind of an idea of that so you have these peak pulse input loads and by utilizing a phase change material you can absorb that energy and then slowly dampen it off so that in this case this one was using dumping heat to a vapor compression loop but you only need to size that vapor compression loop or heat sink or whatever your rejection area is you only need to size that to average load based on the duty cycle and power of your your peak pulses so a very nice nice and simple um design and it can be used in a lot of different applications um a lot of the first questions we get on face change material is what would you recommend in terms of the actual material and most applications we've designed and worked with call for paraffin waxes the main reasons are that it's chemically compatible with metal so there's no corrosion during long-term operation when integrating into metal heat sinks again metal is preferred for the ruggedization and the thermal conductivity they can be there's a lot of different options for paraffins and they have a wide operating range so every couple degrees there's a different paraffin wax that could be design points so you can set set your pcm for a lot of different design points fairly easily without affecting your overall capabilities and they also have a proven high cycle count compared to some of the others that don't have a lot of data on paraffin waxes do have a fairly good data set and have shown you know thousands of cycles from solid to liquid and then the biggest considerations um for designing with phase change material is the the lane heat number so how much energy can you store that'll kind of determine how you size these um but the other big area is that most materials especially paraffin wax have low thermal conductivity so how do you design the remainder of the enclosure that you can properly melt your pcm without causing thermal run away so we'll go into one um one example here and this one was um an application for a spacecraft i was looking at over 300 watts at a 15 duty cycle and the goal here was to reduce as much as possible the radiator panel size so first we determined if pcm is the best solution we looked at the operating temperature ranges looked at the size of the pcm compared to the gains we would get by reducing the radiator panel size and then we worked with locating the the pcm strategically in the system to absorb the thermal load at the critical juncture and create that mass savings on the radiator side so in this application there's a little more complexity that will then we'll get into today but it did use a series of heat pipes as well to transfer heat to the radiators and along the radiators and then we coupled the the pcm in between the high density electronics and the aluminum ammonia cchps and here's kind of the the results here these were thermal desktop numbers but you can see um without the pcm you you could size your radiator and this again would need to be sized for peak load and then we explored many different pcm options and we're able to keep driving down that radiator area required for full rejection so all these are showing kind of similar thermal performance results but again reducing the radiator area by selecting the appropriate pcm and integrating it in the proper position within the thermal resistance network and just kind of quickly summarizing phase change material again they're used to absorb energy so they work kind of like a thermal battery or thermal capacitor to absorb transient loads impulse operating applications they can benefit the end user by simplifying the thermal solution so a lot of times they're used to avoid complex liquid loops or things like that if you could just put phase change material as a way to absorb melt re-freeze it's a fairly straightforward simple thermal design it can reduce mass and volume that was the case study we went into from a space radiation application but similar things would be for convection or liquid heat sinks as well you can reduce the onus on your ultimate heatsink based on the duty cycle and power of the electronics and the the other area we didn't talk too much but is a nice benefit and where pcm has been used in the past is adding operational safety so in many cases if you have a liquid cooled design you do need to protect the system against pump failure or leaks in the system and to do that you neither either need to have a very responsive electronics backlink where if the pump fails they are immediately shut down or the the option that we've seen in some of these systems that have you know extremely high cost electronics in it is to create a pcm that goes in between the electronics and the liquid loop and when the liquid loop is running the pcm stays frozen the entire time but as soon as there's any issue with the liquid loop that pcm can absorb the energy of the electronics and allow the user to safely shut down buying them time for the system feedback to work and the electronics to safely shut down so that can save a lot of hassle in your system and add an additional layer of safety and then the the final bullet point here is that the internal design is is critical so i mentioned the low thermal conductivity of phase change material but it really is a very important design aspect to have good conduction into the pcm so a lot of times that's done with fins or some type of foam material or keeping the layers really thin so there's a lot of different options but making sure that you melt efficiently and don't create kind of a bottleneck at your melt front is really important okay so final module here we have is on pulsating heat pipes so we'll introduce you the concepts of pulsing heat pipes and talk a little bit about the uh the benefits so we really just have one slide here i want to kind of introduce the technology i don't think we've done a webinar topic on pulsating heat pipes in the past but this is an emerg emerging passive heat spreading technology and you can see kind of the figure at the top right is a fairly basic design of a pulsating heat pipe where you have kind of a serpentine channel design and it's a full loop and then you create you charge it to have a fair amount of working fluid compared to the void volume and as you introduce heat it uh pulses the fluid along uh creating vapor regions and liquid regions so operates kind of similar to a heat pipe but it doesn't have a wick structure and doesn't rely on the capillary action it relies on the um pulsating nature of of the design to move the fluid along the benefits here so while uh you can't charge it with with water because of the freezing concerns um so you're never going to get really similar performance at the high end electronics temperature so if you're operating the 70 85 c's a high case solution is probably your your better choice but for some of the lower operating temperatures you can achieve thinner designs very low mass designs it does have very good geometric flexibility and you can do a lot of unique things with 3d printing to create orientation independence and very unique geometry for pulsating heat pipes so yeah there's a lot of good applications for it but there there is kind of a need to be very cognizant of your design requirements and figure out if this is the right solution or if a high k approach is the right solution but overall it does have a lot of capabilities and if the boundary conditions are are adequate for it it'll give you good performance at a very low mass low volume solution and before we wrap up today did want to share the online resources i think this presentation will be distributed to anyone who registers they'll have some some of these links in there some of the design areas we talked about like the heat pipe calculator and also a webinar of libraries and or webinars and white papers that have been done in the past that might be useful and just as we wrap up here again we'll leave it open for you to type questions in for for another minute or so and as i mentioned we aren't doing live Q&A here but we will make sure to get you very detailed responses as we go through all the questions that have been submitted so I really appreciate everyone joining today I hope there are a lot of good questions and we'll certainly get back to you in a very short order here but again please keep an eye out for ACT as we put more and more content out there and we look forward to working with you on any of your heat pipe, Phase Change material or active solutions in the future. Thanks a lot take care!