Hysteresis in annular impinging jets

Annular impinging air jets were investigated experimentally using the techniques of flow visualization, measurement of the stagnation pressure, and of the mass transfer by the naphthalene sublimation method. The bistability and hysteresis lead to two different alternative flow field patterns (A or B) under identical boundary conditions. The patterns differ in the size of the separated-flow recirculation regions: the pattern A is characterized by recirculation region (bubble) of separated flow quite small and located immediately under the nozzle centerbody. The heat/mass transfer distribution on the impingement wall is bell-shaped, with the maximum in the central stagnation point. On the other hand, the flowfield of the pattern B exhibits a large recirculation region of the separated flow, reaching up to the impingement wall, on which there is a stagnation circle. The entire space between the inner lip of the nozzle exit and the stagnation circle is filled with the recirculating fluid. The heat/mass transfer rate distributions reach the maximum on the stagnation circle. The bistability and hysteresis were found only at small Reynolds numbers, and at small annular nozzle slot widths. An extension of the nozzle centerbody promotes the tendency to the hysteretic behavior. An enhancement effect for the area-average heat/mass transfer coefficients for the pattern B over A has been quantified. The highest achieved increase was quite significant 32%.