I just installed a 480GB SanDisk "SSD Plus" in a mid-2011 iMac (with no external thermal sensor on the old drive). I've booted from an El Capitan installer flash drive, formatted the SSD, and am installing El Cap as I write this. No fan noise at all so far. I'm hesitant to clip wires on the power cable, so I'm really hoping my luck holds!
If the fans start roaring when booted from the new SSD/El Cap, then I guess I'll have to try P J Sch's technique...
The weird thing is that I feel like I've swapped drives in other 2011 iMacs (possibly early-2011?) that still used the sensor cable, and therefore require the OWC part that Dan keeps recommending above. I have, and was prepared to use, that part, but there's no place to plug it in on this model!
Learn something new everyday, eh? I hope so at least!