Effect of oscillatory motion on heat transfer at vertical flat surfaces

Abstract

The effect of oscillations on heat transfer at vertical surfaces is investigated and a model is developed that predicted both the transient and time average heat transfer rates. The transient behavior of the heat transfer indicates the presence of an oscillatory component superimposed on a larger steady one that does not reach zero during flow reversal. This was explained in terms of the interaction between a “quasi-steady oscillatory” mechanism near the leading edge, and a “pseudo-steady diffusive” far from it. The analysis further revealed that the time average heat transfer rate can be adequately estimated using a mixed “forced-natural” convections correlation, with the forced convection component estimated based on the time average oscillatory Reynolds number Re v = awL/ ν. The agreement between the model predictions and the experimental measurements makes it applicable for predicting heat transfer characteristics and velocity fluctuations near heated vertical surfaces in presence of oscillatory motion. The model is also applicable for predicting heat transfer rates under conditions where oscillatory motion is used to achieve specificity in temperature control without affecting process residence time, such as in biomedical and biochemical applications. The modest heat transfer enhancement (<2) due to oscillatory motion is attributed to the small convective term in the energy equation, which is consistent with previous investigations where increasing the axial temperature gradient in presence of oscillatory motion was shown to achieve much higher heat transfer enhancement.