External Flow

Impinging Jet

A single or array of jets impinging normally to a surface may be used to enhance convective heat transfer. Applications include deicing systems on aircraft, cooling heated components in gas turbine engines, and annealing sheet metals.

Hydrodynamic and Geometric Considerations

Round Nozzles

For a single round nozzle, the relative nozzle area is A_r=\frac {D^2}{4r^2} and the average Nusselt number is

\frac {\overline{\mathrm{Nu}}}{\mathrm{Pr}^{0.42}}=G\left(A_r,\frac HD\right)\left[2\mathrm{Re}^{1/2}\left(1+0.005\mathrm{Re}^{0.55}\right)^{1/2}\right] \tag{1}

where G is the function below.

G\left(A_r,\frac HD\right)=\frac {2A_r^{1/2}\left(1-2.2A_r^{1/2}\right)}{1+0.2\left(\frac HD-6\right)A_r^{1/2}} \tag{2}

Eqn. 1 is valid for

\begin{gathered} 2\times10^3\lesssim\mathrm{Re}_D\lesssim4\times10^5 \\ 2\lesssim\frac HD\lesssim12 \\ 4\times10^{-3}\lesssim A_r\lesssim4\times10^{-2} \end{gathered}