  This paper discusses a modified penetration theory where the time scale for
the momentum wall layer is based on the onset of ejections from the wall but
the time scale for the thermal layer is based on the unsteady state diffusion
of heat from the wall into the streamwise flow, not the renewal of heat from
eddies penetrating the wall layer. A method for the determination of the
thickness of the thermal wall layer has been developed. The theory correlates
adequately experimental data for both the thickness of the thermal wall layer
and the Nusselt number.
  Key words: Heat transfer, penetration theory, turbulence, time scales.
