  This article shows the importance of flow compressibility on the heat
transfer in confined impinging jets, and how it is driven by both the Mach
number and the wall heat-flux. Hence, we present a collection of cases at
several Mach numbers with different heat-flux values applied at the impingement
wall. The wall temperature scales linearly with the imposed heat-flux and the
adiabatic wall temperature is found to be purely governed by the flow
compression. Especially for high heat-flux values, the non-constant wall
temperature induces considerable differences in the thermal conductivity of the
fluid. This phenomenon has to date not been discussed and it strongly modulates
the Nusselt number. In contrast, the heat transfer coefficient is independent
of the varying thermal properties of the fluid and the wall heat-flux.
Furthermore, we introduce the impingement efficiency, which highlights the
areas of the wall where the temperature is influenced by compressibility
effects. This parameter shows how the contribution of the flow compression to
raising the wall temperature becomes more dominant as the heat-flux decreases.
Thus, knowing the adiabatic wall temperature is indispensable for obtaining the
correct heat transfer coefficient when low heat-flux values are used, even at
low Mach numbers. Lastly, a detailed analysis of the dilatation field also
shows how the compressibility effects only affect the heat transfer in the
vicinity of the stagnation point. These compressibility effects decay rapidly
further away from the flow impingement, and the density changes along the
developing boundary layer are caused instead by variable inertia effects.
