جستجوی مقالات مرتبط با کلیدواژه "flow visualization" در نشریات گروه "مکانیک"

This study experimentally evaluated the mixing augmentation of twin tabs mounted along a diameter at the outlet of a convergent-divergent Mach 1.62 circular nozzle. The usefulness of the plain and grooved tabs is examined at various expansion levels prevailing at nozzle outlet. The tab's performance is assessed through pitot pressure distribution measured along and perpendicular to the jet centerline at different nozzle pressure ratios (NPRs). The shadowgraph technique visualized the shocks and expansion fans in uncontrolled and controlled jets. With the introduction of uncorrugated or plain tabs at the nozzle outlet operating under overexpanded conditions corresponding to NPR 4, the supersonic length (SL) was decreased only by 35.4%. On the other hand, the corrugated or grooved tabs under similar conditions decreased the SL substantially. Interestingly, the performance of grooved tabs was best at underexpanded conditions associated with NPR 6, where the SL was reduced by about 88%. The pressure profiles also established the superiority of tabs with grooved edges in mixing augmentation without introducing any significant asymmetry to the flow field. In addition, the Shadowgraph images also confirmed the weakening of shock strength and reduction of shock-cell length in the case of grooved tabs at the nozzle exit compared to the plain nozzle.

This paper investigates the air bubble size and its transition in a horizontal tube of 700 mm. The tube was assembled with a venturi-nozzle bubble generator. Air and water flow-rates vary in the present study. The data collection mainly used high-speed camera to capture the bubbles at different distances along the horizontal tube at water flow-rates (Qw) of 120-170 litre per min (LPM) and air flow-rates (Qa) of 2-10 LPM. MATLAB was used in image processing for evaluating the bubble size. The data interpretation used YW dimensionless parameter in representing the height of the bubbles’ vertical rise in the horizontal tube. The bubble size along the horizontal tube was characterized by the Weber number as well. The type of two-phase (water-air bubbles) flow along the horizontal tube from the venturi-nozzle bubble generator was determined using flow pattern map and Lockhart-Martinelli parameter. The bubble generator produced bubbles in the range of 0.8-3.1 mm at the inlet of horizontal tube. The bubble diameters increased as the bubbles moved horizontally from inlet to outlet of the horizontal tube and this finding was statistically significant. The vertical rise height of bubbles along the horizontal tube at different water and air flow-rates had been quantified and compared. The vertical rise height of bubbles increased axially from 41 % to 89 % from inlet to outlet of the horizontal tube. The bubbles’ vertical rise height increased when either the air flow-rate or water flow-rate is reduced. The mean Weber number increased along the horizontal tube due to an increase in bubble size. The decrease in water flow-rate caused a decrease in the mean Weber number. The Lockhart-Martinelli parameter of the water-air bubbles flow in the horizontal tube was within 0.58-2.94, indicating that it was a multiphase flow. The findings from this study give fundamental insight into bubble dynamics behaviour in its horizontal transition. This study focuses on the size and transition of air bubbles produced by venturi-nozzle bubble generator along a horizontal tube at different water and air flow-rates, unlike previous studies which only investigate the air bubbles inside or near bubble generator. These findings are very useful for practical industrial applications because the exact air bubble size before being used is known.

Importance of study of pulsating heat pipes (PHPs) behavior and limitations in conducting experimental studies, the necessity of numerical simulations is getting critical in this area. In present work, numerical simulations are carried out for pulsating heat pipes. Thermal performance of closed loop pulsating heat pipes is investigated at different operating conditions such as evaporator heating power and filling ratio. Water, ethanol, methanol and acetone are employed as working fluids. A two-dimensional single loop PHP is used for present study. Computational Fluid Dynamics (CFD) video technique is employed for flow visualization purpose. Perfect match was observed between the present CFD video clip and previous experimental video-based studies in terms of flow pattern and behavior. Present study shows how researchers can benefit from developments of numerical tools to test pulsating heat pipes behavior at different operating conditions or different working fluids without facing difficulties and limitations of applying laboratory thermal measurement equipment or high-speed cameras. The CFD video clip as result of numerical simulation was found very informative for flow visualization purpose. The simulated clip made it much easier to capture phenomena occur in a pulsating heat pipe. The thermal performance investigation at different operating conditions and working fluids was found very informative in terms of application and design purposes especially for experimental studies. By increasing heating power greater than 60 W, circulation velocity was increased for most cases. Phase contour videos are inserted at the bottom of the article.

Unstart/restart phenomena induced by backpressure in a general inlet with a freestream of M = 2.7 are investigated in an in-draft supersonic quiet wind tunnel. The boundary layers are turbulent on the forebody while are laminar on the lip wall, which could mimick real flight conditions. The high-speed Schlieren imaging system and the nanoparticle-based planar laser scattering (NPLS) method are used to visualize the inlet flowfield. The inlet wall pressure is measured by high-frequency pressure transducers. The backpressure is reproduced by downstream transverse jets other than mechanical throttlers, which is more suitable to mimic backpressure caused by combustion. The high spatio-temporal resolution full-view images of inlet flow features during the complete unstart/restart process are captured, which are seldom seen before. The formation and disappearance process of massive boundary layer separation at the entrance of the unstarted inlet is observed. The backpressure transmits upstream through the shock wave/boundary layer interaction (SWBLI) regions. The shock structures change the angles and merge upstream to balance the pressure rise. The Mach shock reflection configuration is observed in both unstart/restart process, accompanied by the boundary layer separation extending to the leading edge. The experiment also revealed notable hysteresis in the unstart/restart process.

The dynamics of a vertical two-dimensional air jet under acoustic excitations at a low Reynolds number are investigated experimentally. The perturbation is introduced by means of a loudspeaker located in a settling chamber before the nozzle exit. The experiments are operated at Strouhal number St ranging from 0 to 1 and for different pulsation amplitudes. A laser plan is used to visualize the flow and the hot-wire anemometry for more specific measures of the mean and fluctuation velocity. The discussion is focused on the influence of two parameters governing the flow: the Strouhal number St and pulsing amplitude. The main results show that the flow consisting of the vortex propagating downstream a nozzle exit is strongly affected by the excitation. Indeed, the introduction of an external perturbation introduces a more rapid degeneration of the potential core with the appearance of vortices near the nozzle as the pulsation amplitude increases. These vortices are amplified and become larger than the nozzle width which induces the enhancing of the entrainment and mixing effects of the shear layers. Another very important phenomenon is observed: the excitation has led to the formation of a switching from the asymmetric mode (sinuous mode) to the symmetric mode (varicose mode).

An experimental study of the laminar-turbulent transition between two coaxial rotating cylinders with the inner cylinder rotates and outer one stationary is presented in this paper. Special attention is given to the onset of various flow modes in tilted and partially filled system. The effect of the inclination angle was investigated on the different flow regimes occurring at fully and/ or partially filled space between two rotating cylinders. The flow behavior, the transitional phenomena and the features of various flow modes are discussed for different inclination angles, filling ratio and Taylor numbers. It is established that the different filling ratio and inclination angles of the Taylor-Couette system deeply affect the flow patterns. Furthermore, the most significant result concerns the relaminarization of the flow when the aspect ratio is decreased and inclination angle is increased for a given value of Taylor number.