Well, technically [rɛindaːl] >, but "Rhine Dahl", or even, "Rine Doll" is pretty close.
Although there’s a joke that it’s now pronounced "AES" since Rijndael was selected to be the standard’s algorithm >. There’s even a wave file > of it being pronounced as such, which serves both as the joke and the definitive guide to pronouncing Rijndael (he has to say "rhine dahl" in order to get the joke out).e