Electric Fan Design for Automotive Applications

Electric Fan Design for Automotive Applications


So we’re here with Thanos Alexekis today who’s the Lead Engineer in Avid’s thermofluids and eMachines engineering team and we’re going to talk about electric fans and that’s one of your particular areas of expertise you’ve started at Avid doing fan blade design Thanos so thanks for agreeing to do this today. When you’re doing a fan design is there a sort of a particular process that you would normally follow for a new fan design? Yes so we will start from the blade that’s what delivers the airflow [Yeah] and obviously some appreciation of the cooling system [Yeah] is required just so you can understand the requirement and then once you have a blade design then that informs the motor design [Yeah] and then that informs the electronics and software design [Okay] and obviously that goes hand-in-hand with your space claim limitations [Yeah] and then other things like weight and noise that’s very important certainly with hybrid and electric vehicles [Okay] because you don’t have a big diesel engine to listen to so you have like almost like a silent very different sound profile motor you know traction motor [Right] so if your fans are blasting then that immediately becomes a notable Noticeable noise yeah yeah [So] You don’t want noisy fans [No no] You cannot avoid them completely [Okay] because you are obstructing airflow [Yeah] and try to force it through a ring [Yeah] and then that’s being then blown against some sort of restriction [Okay yeah] so there will be noise but there are ways you know to go about it but it is an important consideration, noise. Okay and just to explore that a little bit more then what are the kind of things that you can do in terms of improving the noise performance of a fan blade design? There are things like the annular ring which is almost like the equivalent of the winglets you see at the the wings of an aeroplane [Okay yeah] the tips Then there is the non radial blade stacking [Yeah] so backwards or forwards swept blades [Yeah] that helps and then and there are other things like serrated trailing edges on the fan blade [Yeah] these are less common or asymmetrical blade spacing [Ah okay] but all you know all these things you know they provide a noise reduction benefit. Is there a trade-off? So if you if you’re aiming for the quietest fan is there a trade-off in other areas of its performance? Yeah there is a risk that you know the airflow delivery will suffer [Okay] or there may be, you know, structural issues you know if you have too aggressive 3D shapes on the blade [Yeah] you may introduce a weak spot [Right] structurally but then incidentally for example when we talk about annular rings [Yeah] they do provide a strengthening enhancement [Yeah] in addition to the noise improvement [Okay] for example So yeah there are compromises and trade-offs there are some trends for example larger-diameter, slower spinning fans are usually better for noise [Okay yeah] or the opposite, you trying to pass a lot of flow through a very small fan it will probably be noisy [Yeah] But then you know the how many fans- how big a fan has system level considerations Yeah it’s pretty rare that you’d have- in fact it seems always to be pretty rare that you’d even have the space that you need for a basic system let alone [Yeah] the space claim to really add surface area for lots of extra cooling [Yeah] That’s typically a big challenge isn’t it? Do you see any sort of themes in terms of the requests that you get from customers for new fan designs? What are the trends? Again, noise [Okay] so low noise and I think now the customers typically prescribe a speed which is usually slow it’s like [Yeah] and I think that’s to mitigate against noise [Right] but then that has an implication on the motor design [Yeah] and the electronics as well Because you might be trying to deliver higher torque at a lower speed [Yeah yeah] which means typically more current, more copper [Yeah yeah] a bigger more expensive motor So- but yeah that’s very typical, so there is a firmer noise requirement or specification and then there is a speed requirement that goes hand-in-hand with that [Right] But that’s good because it sort of like it frames the requirement straight away [Yeah] In your experience and I know you’ve designed a good number of fans now What are the main challenges that you’ve come across? So the trade-off between noise and performance [Okay so that would be the main challenge] How do you go about once you’ve done a design on something like a fan how do you go about making sure that it’s going to deliver consistent performance? So you’ve got the initial design what do you have to do before you launch that as a production product? A good design process I think is central to front-loading and ensuring that the fan will deliver on and on consistently testing, loads of testing [Yeah] wind tunnel testing and as much simulation as you can afford within [Yeah] a given timescale So there’s a- you’re doing product validation testing on the fan Is there like different manufacturers do they have different requirements for product testing or do you always end up doing basically the same bits of work? They are usually variations of the same thing so and typically those tests refer to some ISO Standard for example [Yep] so every now and then there will be a specific customer driven requirement that’s not in any standard documents just what the customer has specified internally and requires of the suppliers so you know there’s nothing particular there that you know stands out and interestingly in the product validation and verification stage more importantly the one that you do towards the end [Yeah] you know the functionality for example the delivery of flow [Yeah] that’s already established [Okay yeah] so what you need to make sure is that after you’ve done test 1, 2, 3 and 4 and you’ve passed [Yeah] then you do a functional test it That it still delivers the same- [Exactly] [Yeah yeah] there’s no detrimental effect on airflow on airflow because of something in the [Yeah] I guess- I mean do you often see problems in testing do you kind of encounter challenges when you’re doing those validation tests? There are some challenges at the earlier design verification tests [Yeah] mainly around validating blade performance [Yeah] so the material of the blade, of the prototype blades is important [Yeah] and so for example you know polyamide-based rapid prototype blades they may not be as structurally sound because of the material properties or the you know the finishing- like the surface roughness of the blade is not what it would be for like a the same as a PA 66 injection mould [Yeah] plastic lid for example and that has a detriment on the flow performance [Right] so you go in the wind tunnel and you don’t get what you thought and you don’t know where it comes from you know is it because you know the blades are you know deflecting or is because the surface finish on the blade is inhibiting your you know lift performance things like that so then you know can you make a more robust blade from metal? [Yeah] yep is it too heavy you know how do you- [Yeah, how does that affect] [affect the performance?] Exactly yeah So these are the things that tend to to make a difference early on but as soon as you go through that successfully [Yeah] then in terms of airflow delivery that side of performance validation is easy to manage across the development [Right okay] And what would be, you know if what would be your sort of key tips and advice for a vehicle system engineer who was looking at specifying a fan Are there other any kind of mistakes that you see people commonly make? Or- Yeah so we have seen a few things we have seen so the airflow requirement the number so sometimes it seems a bit odd or off or a bit too aggressive [Yeah] and and we have been suspecting for quite a while that you know that people pay more attention on the data sheet instead of the fan curve [Okay] and the data sheet will probably mention like the maximum flow [Yeah] number but that in most cases will correspond to like a free flow condition so no restriction Right which you don’t have on a vehicle [Yeah exactly] So that’s one common theme [So be careful about your airflow] [and restriction] Exactly yes so yeah so fan performance needs to be discussed and addressed using the fan curve [Yeah okay] Now in- when it comes to electric fans we sometimes see that maybe the customer is referring to kilowatts of heat rejection versus kilowatts of fan motor [Yeah] so you know, knowing which one’s what [Knowing your kilowatts] Knowing your kilowatts yep knowing your kilowatts so that’s my heat rejection and that’s how many kilowatts of fan motor I need [Yeah] and they don’t [Yeah] So distinguishing between those and knowing your kilowatts The last thing that I would consider as as an important design point is you know when we’re talking about using the fan curve instead of a single number on the data sheet [Yeah] not all of that curve that you see on the paper not all of that is effective so you- it doesn’t mean that you can be anywhere on that curve and that’s your fan sorted and working great Ah okay is that because fan blades tend to have quite a narrow efficiency window so you’ve got like one point where it’s efficient and then it falls off quite quickly [Yeah] outside of that [Yeah] Yeah so there is what we call the the unstable region of the fan towards the higher pressure lower flow rates then there is like the stall region [Yeah] so so it’s very important to have a system restriction curve and then map that against the fan performance curve [Yeah] and leave that intersection point it’s too far in your left [Yeah] then you know give someone a call and- Yeah because even so- because you might think oh, actually if I just fit a huge fan it’ll be fine because it’s going to have more airflow than I need and that’s great great but actually you could end up in a situation where you’re not getting any airflow because you’re running the fan so slowly it doesn’t actually generate the pressure to to go through the system so over- fanning your system can be as bad a thing as under-under providing for it as well or a wildly- a very poor efficiency for the fan [Yeah exactly] and of course they consume- I mean fans generally are not massively efficient are they? So axial fans? So you’re looking at a lot of power consumption on an EV but then I guess it depends on the kind of EV if you’ve got RAM Air or no RAM Air [Yeah] It can be a really high power consumption figure for the, you know parasitic loss on the powertrain Talking about RAM Air another interesting design challenge and maybe because you know we come from a heavy-duty commercial vehicle background where there’s no RAM Air [Yeah] is you know, how do you manage a freewheeling fan under excessive RAM Air? [Okay yeah] Do you make it run as a wind turbine? If you do that and your software your hardware needs to be tuned to sustain that I think it’s called fourth quadrant regenerative mode [Yeah so if you basically] the fan is being spun by airflow so this something called windmilling [Yep] So then if you just let that spin up you could go too fast and damage stuff but with the kind of motor that we have you you’re generating- effectively you could be generating [Yeah] power, but then that creates drag [Yeah] so what do- what’s the strategy to control the fan speeds prevent damage to the system but also not to minimize drag on the vehicle Yeah and you know if there is any amount of DC current generated back into the system you know how that- does that need any management? [Yeah] Is it you know circling around the system? It’s this type of consideration the other thing is, what the team was describing to me recently is that grad is also following a curve [Yeah] so depending on your RAM Air and your fan speed, you know it doesn’t necessarily mean you’ll always have a lot of drag you may not have a lot of drag because subject to curve as well [Yeah] so it’s So in theory you could move the fan into an operating speed relative to the vehicle or the RAM Air, it’s velocity and effectively cancel out the drag [Yeah] of the fan blade just by holding it at the right operating point and is there anything that you think that a vehicle designer or a system designer could do at a vehicle system level to basically make life easier for the fans to improve the fan condition? What are the key things that they should try and take into account? So system restriction on the sort of like on the direction of the flow on the direction of flow so usually axially so you know the proximity of components upstream and downstream of the fans [Yeah] and with the grills and the density of the radiators and how close these are and it will be different for different fan designs [Yeah] so that’s key and that then will affect the noise performance and the fan size and the fan efficiency The other consideration is how uniform the flow is across the radiator pack [Yeah] and whether that’s important or not for example you know, do you want many- an array of many fans or an array of not so many fans? [Yeah] With passenger automotive, you know crash and where the fan is and how many fans you have is important, you know for the crumple zone and what ends up being passed on to the passenger basically in the event of a crash So these are interesting things that come up [Yeah] and windmilling is interesting so it’s we have been- Especially some of the- because you’re on a couple of programs where the vehicles are going like [400 kilometers per hour] Kinda slightly fast [Yeah yeah] Yeah so not a massive issue on a bus- in fact not an issue at all on a bus because the cooling pack is normally at the back so there’s no RAM Air, but yeah some really exotic EV / HEV programs with very very high vehicle velocities so yeah 400 kilometers an hour is that’s moving Yeah and the aerodynamics of the whole vehicle and your floor regions, you know around and through the vehicle and the fans and the cooling system, this is really this all becomes very challenging and interesting and hard to get your head around in such a fast moving vehicle [Yeah] we have seen, you know weird flow and pressure figures [Yeah] for this type of scenario [You some strange phenomena] [aerodynamic phenomena building up] So understanding your Pascal’s and and when cubic meters per second and where those happen and how and why [Yeah] and the system restrictions that’s really important certainly in a very- in such a high speed scenario. The other thing is in the spirit of being creative we have seen people putting fan systems at the rear of the vehicle [Yeah] and after a certain speed after a certain vehicle speed that’s also problematic because you create like a like a zone of no air to pull air from the back Oh okay right so negative pressure of a low pressure zone [Yeah] So there are things like that they don’t come up very often but they do come up [Yeah] so There’s always a lot of trade offs at a vehicle- particularly passenger vehicle level with vehicle styling, vehicle aerodynamics and then thermal system [Yeah] it’s like those are the key kind of headbutt topics of- like the stylist wants one thing the cooling guy wants another thing and the aerodynamics guy wants something completely different [Yeah] and trying to make all of those things overlay and actually make something that works especially at 400 kilometers an hour is is a really big challenge yeah. Okay great Great well thanks very much for that. I really appreciate you taking out the time to do this today [Thank you] Great To find out more about Avid contact us today. If you have enjoyed this video don’t forget to like it and subscribe to our channel for more updates.

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