9.82. Heat transfer to gases is treated in much the same way as for ordinary fluids such as water, and convection heat-transfer coefficients can be predicted by means of the correlation in equation (9.36). For many gases, including helium, carbon dioxide, and air, Pr is approximately 0.70, and so equation (9.36) reduces to

Nu — 0.020Re0 8.

The marked differences in physical (especially thermal) properties result, however, in some important general differences in heat-transfer behavior between gases and liquids. For example, the thermal conductivity of helium gas at atmospheric pressure is only about a third that of water although it is enhanced by an increase in pressure. In considering the transport of heat away from the heated surface by the fluid, the specific heat of the latter is important. Here again, an increase in the gas pressure is advantageous; there is a density increase accompanied by a corresponding increase in the heat capacity per unit volume.

9.83. In so-called high-speed flow, where the gas velocity is greater than about 0.2 that of sound (§9.129), certain special effects occur that influence the temperature driving force for convection heat transfer. For example, “aerodynamic heating” results from the frictional effects in the boundary layer, and kinetic energy considerations (stagnation effects) lead to a dif­ference between flowing and “at rest” conditions. For such matters, heat — transfer texts should be consulted [7].