Heat transfer to a resonant pulsating air stream in a pipe

Abstract

The effect on convective heat transfer of resonant, longitudinal oscillations superimposed on a turbulent mean flow in a pipe has been investigated relative to the equivalent steady flow. Theoretically it is shown that the effect of acoustic streaming velocities is negligible for the range of pulsation parameters, but that the oscillating velocity can generate changes in the time-mean flow diffusivity - the change in mean diffusivity can only be predicted if quasi-steady pulsations are assumed. Heat transfer coefficients for the mean flow are evaluated from the Energy equation, for fully established conditions, assuming quasi-steady oscillations. It is proposed that a frequency factor can be derived to relate experimental heat transfer to the quasi-steady predictions, and that the factor would be a function of Strouhal number only. Local heat transfer coefficients were measured for a constant heat flux supply to an oscillating air flow in a pipe. The pulsations were generated by a siren. It was shown that the centre-line velocity amplitudes could be predicted from inviscid flow theory using a mean velocity of sound. The range of the major parameters was: Diraensionless pulsation velocity 0.3 < B < 5 Strouhal number 0.5 < S < 10 Reynolds number 14,300 < Red < 31,250. For fully developed flow, the experimental results were related to the corresponding quasi-steady predictions by a function of Strouhal number. It was shown that the changes in heat transfer were due to changes in the mean diffusivity generated by the acoustic velocity. For a defined range of pulsation parameters, it is possible to predict local heat transfer coefficients under fully established conditions for a pulsating flow from the empirical frequency correction factor applied to the theoretical quasi-steady predictions.