Experimental study of flow characteristics in the near field of thermally stratified co-axial free jet

Abstract

Experimental investigations of the mean flow characteristics in velocity field as well as thermal field of thermally stratified co-axial free jets have been carried out for different boundary conditions. Non-isothermal as well as thermally stratified or non isothermal co-axial jet flows are developed by issuing two jets with different unidirectional velocities from a concentric compound nozzle. The hot central jet comes out from a 29 mm central nozzle while the annular ambient jet is emitted . through the annular space between the outer and the central nozzle. Two outer nozzles having inner diameters of 80 and 63 mm are used that results in the area ratio of annular to central jet as 6.6 and 3.57 respectively. Four outer to inner jet velocity ratios ranging from 0.0 to 0.75 have been considered for each nozzle configuration. The temperature of the central jet is varied to establish a temperature gradient between the central jet and the annular jet by using an electric heater placed upstream of the central nozzle. Three different values of the ratio of annular jet temperature to that of the central jet have been considered as 1.0, 0.974 and 0.925 on the basis of absolute temperature scale. Measurements of mean velocity and mean temperature are made in the mixing zone of these two jets with the help of a pitot static tube with embedded thermocouple for two different Reynolds numbers of 3.72x104 and 4.80x104 based on the exit velocity and diameter of the inner jet. For the nozzle configurations used in the present study, the effect of area ratio on the developed region of co-axial jets is found to be more prominent than that on the developing region. Mixing as well as momentum exchange in co-axial jets occur more readily at lower velocity and lower area ratios than those of the higher ones. Thermal diffusion from the hot central jet is found to occur more rapidly at lower velocity ratios. With the increase of velocity ratio, the thermal potential core is, noted to be transversely shrunk but longitudinally elongated. The attainment of self preservation state of dynamical velocity field is observed to depend much on velocity and area ratios rather than temperature ratio. On the other hand, the self-preservation state of thermal field is observed to be independent of dynamical conditions.