No idea if this of any interest to anyone but I've made the models to get CNC'd so I might as well share it. My MK4 has no load speed radiator fans as the resistors have both blown (very common). Instead of replacing the fans, many people fit a pair or resistors on a plate in the arch. I decided to go this route and do some thermal analysis first. All metals are 1060 allow with a high end thermal compound used with the heatsink and set to an emissivity of 0.88 (black anodised) with no airflow. Air temp is 298K. Will do a full writeup on these tomorrow. For context that heatsink is 200 x 130 x 25 mm and still leaves the resistors operating at 128 degrees Celsius, their absolute limit being 200
Sounds much higher than I'd expect? How have you calculated the electrical dissipation in each resistor?
It's only very rough at the moment, need some refining. Resistor are at 0.75 (rough value because yellow anodising) emissivity, 50 watts dissipation which is approximately correctly. I'm only modelling radiative heat transfer for simplicity. In reality there will be convective effects of course but I expect those to be minimal because the resistors are mounted upside to a plate in the wheel arch. One source I found said their resistor were at 190 Celsius, and that was just mounted on a 2mm Aluminium plate in the footwell. I know the heatsink model is roughly right because it calculates to about 0.9 K/W and the manufacturer spec is 0.7, so I'm on the right side of the error
50W rated resistors or 50W actual measured dissipation? When I first did this on the main fan of a Skoda Fabia I think I used a 1 Ohm resistor and subsequently measured the current and it was around 5 Amps, so that's only 25W electrical power dissipation (I²R).
They're 100 watt resistors and I'm assuming the fans take 7 amps each, so 50 watts per resistor, that something I found online, yet to test it myself.
