A jet impinging normally on a surface produces the highest local heat transfer coefficients obtainable. That is why it is used in glass tempering, turbine blade internal cooling, electronics spot cooling and drying processes.
Three zones
Stagnation zone. The jet strikes the surface, the velocity goes to zero and the boundary layer is very thin; heat transfer peaks here.
Wall jet. The flow spreads sideways, accelerates, then decelerates.
Secondary peak. At certain nozzle-to-surface spacings a second heat transfer peak appears just outside the stagnation point, because the boundary layer undergoes transition there. A design expecting a single-peaked distribution misses that zone.
Why it is hard to model
The impingement zone lies outside the assumptions standard turbulence models were built on: the flow is normal to the wall, normal stresses dominate, shear stress is small. Isotropic two-equation models systematically overproduce turbulent kinetic energy at the stagnation point and overpredict the heat transfer.
Practical remedies: model variants with a stagnation-point correction, transition-sensitive models [1], or Reynolds stress models that carry the anisotropy [2].
Mesh and spacing
In the stagnation zone y+ < 1 is mandatory; heat transfer is the temperature gradient at the wall. See the y+ map note.
The nozzle-to-surface spacing is a governing design parameter and the optimum lies in a narrow range: too close and the jet is not developed, too far and it spreads and loses its energy.
Arrays of jets
What holds for a single jet does not transfer directly to an array: the spent air of neighbouring jets creates a cross-flow that bends downstream jets sideways. Jets at the end of an array do markedly less work. How the spent air is exhausted matters as much as the jets themselves.
References
- Langtry, R. B. & Menter, F. R. (2009). Correlation-based transition modeling for unstructured parallelized computational fluid dynamics codes. AIAA Journal, 47(12), 2894–2906. DOI: 10.2514/1.42362
- Launder, B. E., Reece, G. J. & Rodi, W. (1975). Progress in the development of a Reynolds-stress turbulence closure. Journal of Fluid Mechanics, 68(3), 537–566.