Thermal and fluid dynamics in Jet Flow

The fluid dynamics of a free submerged jet in the literature has been reviewed. Different conclusions for what concerns the lengths of the potential core region and the line source of the jet have been found among different researches. The submerged slot jet of air has been the subject of several works of the present authors in order to establish its evolution with the distance from the slot exit. Hot anemometry was used to measure velocity and turbulence distributions on the exit as well as at several distances from it. The shadowgraph visualization technique was also employed to confirm the hot anemometry measurements. The existence of the undisturbed region of flow, where the fluid dynamics characteristics are equal to those on the exit, was found in a preliminary experiment with a first small wind tunnel at a turbulent Reynolds number, based on the hydraulic diameter, equal to Re=12,100 in the measurements and Re=10,600 in the visualizations. In a slot of height H=4.25 mm the dimensionless length of the undisturbed region was found equal to about Lu/H=1.6-2. It was confirmed later in systematic experiments with the same wind tunnel and slot. The length of the undisturbed region was shown to decrease with the increase of the Reynolds number from the laminar to the turbulent regime. The velocity and turbulence measurements, carried on in the Reynolds number range from Re=13,500 to Re=22,100, confirmed the dimensionless length of the undisturbed region was in the range Lu/H=1.6–2. The length of the undisturbed region of flow was evaluated also with the shadowgraph visualization technique in the Reynolds range from Re=1,700 until Re=22,100. The length of the undisturbed region was found equal to Lu/H>3 for the laminar Reynolds number of Re=1,700, decreasing to Lu/H=1 for the turbulent Reynolds number of Re=22,100. The velocity and turbulence measurements were performed in a second small wind tunnel with a slot of height 10 mm in the range from Re=7,600 to Re=37,300. At Re=7,600 the undisturbed length was equal to Lu/H=3–4, at Re=23,300 it was equal to Lu/H=1–2 and it was zero at Re=37,300. The flow visualizations with the shadowgraph technique were performed in the same wind tunnel and slot in the range from Re=7,100 to Re=33,400 confirming the dimensionless length of the undisturbed region of flow was decreasing from Lu/H=3.8 for Re=7,100, to Lu/H=0 for Re =33,400. The flow visualizations were also performed in the first wind tunnel with a slot of height H=17 mm. The length of the undisturbed region was decreasing from Lu/H=2.35 for Re=8,400, to Lu/H=1 for Re=31,200 and to Lu/H=0 for Re=52,900. The introduction of a metallic grid inside the wind tunnel, between the converging duct and the slot, and on the slot exit, showed a shorter length of the undisturbed region due to the higher turbulence. Hot anemometry was used also to measure velocity and turbulence evolution in a free jet up to an impinged cylinder, set at the two distances from the slot exit equal to L/H=4 and 6, for the Reynolds number of Re=23,300, showing the undisturbed region was equal to Lu/H=1 when the cylinder is set at L/H=4 and Lu/H=2 when the cylinder is set at L/H=6. The experimental results of the present authors on the length of the undisturbed region of flow, obtained in the turbulent as well as in the laminar regime with hot anemometer and shadowgraph visualizations, have been finally collected together with previous results of the literature, obtained with shadow, Schlieren and PIV visualization in laminar regime, with an interesting good agreement. The local heat transfer, or Nusselt number, experimental data of the present authors are reviewed in the present paper at the light of the evolution of a submerged jet, i.e., to verify the influence of the length of the undisturbed and the potential core regions. Preliminary experiments of heat transfer with a first small wind tunnel without the three metallic screens, i.e., at relatively higher level of turbulence, was used to cool a cylinder of diameter D=10 mm with a slot of height H=5 mm. The experiments showed a qualitative increase of the average and local Nusselt number at Re=17,000. Heat transfer of air jets with the same wind tunnel, slot and circular cylinder was investigated more systematically for Reynolds number ranging from Re=4,000 to Re=20,000. The influence of the distance between the exit of the slot jet and the cylinder, X, was investigated in the range of X/H=2–10. The local Nusselt number was shown to be a function of the X/H ratio, of the angle from the impinging point and of the Reynolds number. The local heat transfer was investigated with the same wind tunnel at reduced turbulence. Slot jets of different heights were used to cool a circular cylinder heated by electric current. The slot jet with the highest ratio between cylinder diameter and slot height, i.e., D/H, realized the highest mean Nusselt number at the same Reynolds number, defined with the hydraulic diameter. The distance from the slot exit which realized the highest mean Nusselt number is equal to X/H=8 for D/H=4. The similar distance observed in a slot jet with D/H=2 is X/H=6. An empirical expression was proposed to correlate mean Nusselt numbers for the three slot jets experimented. The local heat transfer along the cylinder perimeter was measured in an upgraded wind tunnel with lower turbulence level for a large range of Reynolds numbers, i.e., from Re=7,400 to Re=37,300. The local Nusselt number measured at different distances from the slot exit, along the perimeter of the cylinder presented interesting features from the point of view of the presence of the undisturbed region. Similar considerations were done about the mean Nusselt number.