Energy dissipation in oscillatory flow within a baffled tube

Pressure fluctuations and rates of energy dissipation have been measured for sinusoidal oscillatory flow of light oil in a tube equipped with a series of wall baffles. A laboratory shell-and-tube heat exchanger was fitted with a single length of 1.0 m, 12 mm internal diameter tubing containing 55 orifice-type wall baffles. The frequency of oscillation was varied in the range 3 to 14 Hz, and the amplitudes (centre to peak) in the range 1 to 6 mm. The pressure/time characteristics show that the pressure variation leads the velocity variation by a phase angle which decreases as frequency is increased. The power dissipation measured for oscillatory flow agrees with the well-known quasi-steady model at large amplitudes, but at lower amplitudes and higher frequencies the predictions of quasi steady theory are exceeded by a significant degree. A new flow model is proposed, based on acoustic principles and the concept of eddy viscosity. The model contains two adjustable parameters; the mixing length (approximately equal to the orifice diameter), and a correction factor for the fluid inertia. This model is able to predict the dynamic pressure response and the overall power dissipation rate with reasonable accuracy