(A) study on characteristics of entrainment and velocity fluctuation in swirling jets

Abstract

Entrainment characteristics of swirling free jets were investigated with a flow visualization technique and the direct measurement of the velocity of entrained surrounding air. As a result of the flow visualization experiments, it was clearly shown that the entrainment rate increases with the swirl degree. Synchronization of the hot-wire anemometry and the schlieren photography showed that the precessing vortex core(PVC) which often occurs in strongly swirling jets induces corresponding large scale periodic eddy motion on the flow boundary at the near-exit region, consequently leading to significant enhancement in entrainment rate. A new method for measuring the entrainment of swirling jets was developed, and turned out to be reasonably accurate as demonstrated by the fact that the present results agree well with previous ones for the non-swirling jet. In the weakly swirling jet($S <0.6$), the entrainment rate is independent of Reynolds number and varies more or less nonlinearly at the near-exit region, whereas in the strongly swirling jet (($S>0.6$), it increases with Reynolds number because of the corresponding increase in frequency of the PVC. From comparison of the entrainment rates of two typical swirling jets with and without the PVC, the PVC was identified as having a very significant effect on the entrainment characteristics of the swirling jet. For the isothermal swirling free jets with and without the PVC motion, the instantaneous velocity was measured using a laser Doppler velocimeter. Several kinds of analyses have been carried out using the instantaneous velocity signals; time-mean and r.m.s values of velocity fluctuations, spectral analysis, and probability density function. In case of the strongly swirling jet in which the precessing vortex core exists, r.m.s velocity fluctuations measured display a servere nonisotropy and very high at the near-exit region, whereas at downstream region they are nearly isotropic and very similar to those in a weak...