I've looked into it a bit. I found some useful info but it is mainly experimental data with submerged jets. The only conditions where nozzle plate spacing becomes less important seems to be when the reynolds number is below 800 which means very low water velocities. I'm basing this on some experiments carried out by Elison and webb mentioned in "Advances in Heat Transfer" volume 26. I would recommend it for anyone interested in jet impingement as it goes into lots of detail about nozzle plate spacing, modified impingement surface, wall roughness, jet splattering, jet pulsation, motion of the impingement surface, all from 180 different references and technical papers. Should keep anyone busy for a while
