Experimental Thermal and Fluid Science最新文献

The study of single bubble nucleation, growth and detachment processes has being carried out with remarkable relevance in the last fifty years. The complexity of the associated phenomena are still a challenge for the researchers in the field, yielding a lot of works trying to explain the enrolled mechanisms. These studies include experimental, numerical and even theoretical approaches. In particular but not exclusively for numerical approaches, it is of essential importance having a solid and detailed experimental description of the full event. It is in this context that this work emerges, proposing a methodology for characterizing the single bubble nucleation, growth and detachment process. This methodology is based in the determination of a “typical bubble” that represents the whole phenomena, providing the reconstruction of a main bubble life-cycle with its uncertainty (both in space and time). This reconstruction allows the determination of several important characteristics, such as bubble volume, apparent contact angle, dry radius, and the forces acting on the bubble. This work analyzes a set of bubbles growing from a cavity of $91\mu m$ diameter, under saturated conditions at 0,583 bar and superheating of 25,8 °C. All the outputs were obtained considering a carefully uncertainty determination and propagation from both systematic and random sources, which are not negligible as it is demonstrated in this work. The importance of considering a sufficient number of bubbles to characterize the phenomenon was also addressed, considering an analysis of uncertainties for different set of bubbles. The methodology was also confronted to previous approaches reported elsewhere, that ensured its performance, robustness, and validation.

The flow over two circular cylinders, arranged in a tandem setup, is controlled with the help of dielectric-barrier-discharge (DBD) plasma actuators mounted on the upstream cylinder at a Reynolds number (Re) of 4700. The plasma actuators are mounted at $\pm 80^{\circ }$ from the forward stagnation point of the upstream cylinder. Three tandem configurations are tested, where the distance, $L$, by which the cylinder centers are separated are fixed at 3, 4, and 5 cylinder diameters (D). For each configuration, the plasma actuators are operated at two distinct blowing ratios (BR) of 0.8 and 1.4, which are named as the low-power and high-power forcing cases, respectively. Results include static-pressure measurements on the downstream cylinder and wake surveys using Particle Image Velocimetry (PIV). High-power forcing changes the flow pattern in the $L=3D$ upstream wake from reattached to co-shedding flow, enabling alternating vortex shedding to occur between the tandem cylinders. High-power forcing also significantly weakens vortex shedding from the upstream cylinder for $L=4D$ and $L=5D$. This weakening is manifested through 39.27% and 35.32% reductions in the total area of vorticity contours for $L=4D$ and $L=5D$, respectively. However, the effect of this cancellation is most prominent on the downstream cylinder when the separation distance is $L/D=3$. During forcing with BR = 1.4, the static pressure on the downstream cylinder resembles that of a flow over a regular cylinder for all the cases tested. Hence, at this blowing ratio, the wake signature of the upstream cylinder is severely diminished, by delaying the shear-layer separation point. During forcing with BR = 0.8, no significant effect on the downstream cylinder is observed.

This study systematically investigates the behavior of lifted non-premixed jet flame under the influence of various electric field waveforms. In our experimental setup, high voltage is applied to the fuel nozzle, which acts as an electrode, while the flame serves as a floating electrode. The primary areas of interest are the responses of flame lift-off height, flame lift-off velocity, ion current and flow structure to electric fields. Results reveal that the flame lift-off height is predominantly influenced by the charging voltage, with the sequence of effectiveness being AC pulse >AC sine >DC pulse = DC. Furthermore, an increase in voltage frequency significantly enhances the flame lift-off velocity. A simplified model, based on charge transfer in corona wind, has been utilized to derive the electric force acting on the flame. This model establishes a scaling relation that correlates flame lift-off velocity with charging voltage, frequency, waveform and duty cycle. The ion current response in our system exhibits characteristics similar to those of an electrical RC circuit, where the charging voltage has a more significant impact on charge transfer and, subsequently, ion current enhancement compared to frequency. Elevated ion current values correspond to increased flame displacement speeds and reduced flame lift-off heights. The alternation of electric fields introduces a greater degree of turbulence within flames. Near the nozzle exit, a vortex ring with a consistent rotation direction is formed. This vortex ring, driven by the induced ionic wind, facilitates flame propagation and enhances flame stabilization.

The effects of the combined magneto-thermal conditions on blood viscosity and the underlying mechanisms of the key factors were elucidated. An integrated magneto-thermal experimental apparatus was devised to measure the viscosity of blood samples sourced from healthy blood donors subject to various non-thermal alternating magnetic fields (AMF), temperature, and magneto-thermal conditions. Mechanisms governing the influence of different factors on blood viscosity were further probed by assessing the viscoelastic modulus of red blood cells (RBCs) and observing RBC morphology. Results show that the hierarchy of significance of three main factors in magnetic hyperthermia on blood viscosity is as follows: temperature > AMF intensity > AMF frequency. Both heightened AMF field strength and elevated temperature led to a nonlinear decrease in blood viscosity, particularly in blood samples with higher hematocrit levels. This is associated with enhanced rheological characteristics of RBCs at increased temperatures and alterations in the repulsive forces between RBCs owing to changes in the cell surface charge induced by AMF. These changes ultimately reduce the flow resistance. However, when the temperature exceeded 46 °C, the deterioration of spectrin on the RBC membrane, coupled with the formation of spicules on the cell membrane surface and subsequent RBC rupture, contributed to increased blood viscosity at higher temperatures.

The simulation of ice crystal icing at high altitudes in the icing wind tunnel holds significant value for the study of icing on the aero-engine, pitot probe and other components of aircraft. Concurrently, the measurement of ice crystals in the icing wind tunnel is an aspect that warrants attention. This study introduces the Holographic Airborne for Cloud Particle Imager (HACPI) based on in-line holography, and conducts a series of experiments in the icing wind tunnel at AVIC Chengdu CAIC Electronics Co., LTD. The icing wind tunnel is equipped with an ice crystal generation system (ICGS) that operates on a grinding-based method. The morphology, size distribution and concentration of the ice crystals are simultaneously measured under airspeeds of Mach 0.55 and Mach 0.62 with HACPI, the results indicate that the morphology of ice crystals generated by the ICGS exhibits randomness. It is observed that with an increase in particle size, the irregularity also increases, and the maximum size of the ice crystals produced can approach 1000 µm. Under two typical test conditions, the MVD are 202.4 µm and 247.7 µm, respectively, while the MMD are 103.7 µm and 166.9 µm. In addition, the IWC are 4.47 g/m³ and 6.67 g/m³. A close relationship is observed between the particle size distribution and concentration of the ice crystals and the operating frequency of the ICGS components. Furthermore, this work demonstrates the significant potential of digital holography in measuring ice crystals in icing wind tunnels.

The present research aims at an experimental study to investigate the effect of velocity ratio (R) on the evolution of elevated round jet issued from a stack having an aspect ratio (AR) of 9.0 in a uniform crossflow at a Reynolds number of 2000. The experiments are conducted in a low-speed, recirculating water tunnel. Laser Doppler Velocimetry (LDV) is employed to characterize the water tunnel. Particle Image Velocimetry (PIV) is used to measure the flow field, and the dye visualization technique is utilized to capture the accompanying instantaneous flow structures. Depending on the velocity ratio (R = 0.16 - 1.5), distinct jet shear layer (JSL) structures are observed in the symmetric plane (XY), confirmed by both the flow visualization and PIV data. These structures include clockwise vortices, anticlockwise vortices, swing-induced mushroom vortices, and jet-like vortices. For $R<0.5$, entrainment of a substantial amount of jet fluid into the stack-wake region (downwash) has been observed. At R=0.5, the streak image captured in the wall parallel plane reveals the ring-like vortices that appear to wrap around the jet core while upright vortices are detected downstream on the lee side of the jet. Additionally, variations in average velocity and turbulent fluctuations in the near field, contingent on different velocity ratios, are discussed.

The liquid–solid particle flow within a horizontal pipeline is a typical particle-laden flow. The interplay between loaded particles and carrier phase engenders significant complexities in the dynamic behavior of such flows. We investigated the dynamics of a liquid–solid particle flow featuring dilute, slightly buoyant, hundred-micron-sized spherical particles fully suspended in the turbulent flow in a pipe, where Re is 7038. Cases with solid volume fractions of 0, 2.67 × 10^-4, 5.33 × 10^-4, 8.00 × 10^-4, 1.07 × 10^-3 and 1.33 × 10^-3 were considered in this study. Experimental measurement techniques were utilized to acquire images of particles in the two-phase flow across the entire flow field via optical field feedback. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) were employed to simultaneously obtain liquid-phase velocity fields and particle trajectories. This approach allowed for a comprehensive depiction of the velocity field of each phase. Subsequently, a detailed explanation and analysis of the flow characteristics of the liquid–solid particle flow were provided based on the distribution of macroscopic velocity fields, the microscopic vorticity field, particle velocity vectors, and velocity slip. As a result, it was found that with the addition of solid particles and an increase in volume concentration, the drag effect of the solid phase and the trend of accumulation near the lower wall caused a decrease in the liquid phase velocity profile and deformation of the parabolic shape. In the liquid–solid particle flow with a volume concentration of 1.33 × 10^-3, compared to the single-liquid phase flow, the standard deviation of velocity in the central region increased from 0.0097 m/s to 0.0159 m/s, and in the near-wall region, it increased from 0.0257 m/s to 0.0347 m/s, representing increases of 1.54 times and 1.26 times respectively. The proportion of medium vortices increased from 17 % to 30 %, nearly doubling. This study actively explores the concurrent measurement and flow characteristics of liquid–solid particle flow.

Based on seepage flow through porous media, the seeping gas film cooling method is an effective means to protect large areas of optical windows and other hot components of hypersonic vehicles. Here, the pressure-sensitive paint (PSP) technique was applied and experiments were conducted on seeping gas film effectiveness downstream of a porous injector. A Mach 3 wind tunnel was used to explore the influence of different cooling gas blowing ratios, gas types, and incoming boundary-layer conditions on film effectiveness. Results show film effectiveness decreases monotonically along the downstream wall; however, with blowing ratio increases the film effectiveness increases linearly. Once the blowing ratio exceeds 0.4% film effectiveness ascends nonlinearly, indicating that the contribution to film effectiveness per unit mass flow-rate promotion gradually decreases. In contrast, the non-uniformity of the film coverage on the downstream wall in the spanwise direction becomes more significant at a higher blowing ratio. For the same blowing ratio, helium, with a low molecular weight has a higher film effectiveness compared to nitrogen and carbon dioxide. With blowing ratio less than 0.4 %, the film effectiveness downstream of the porous injector under laminar flow conditions is almost twice that of turbulent flow. However, with blowing ratio above 0.5 %, the growth rate of the film effectiveness decreases dramatically to 50 %. Preliminary analysis suggests that this is caused by the complete transition of the laminar boundary layer to turbulent flow after passing through the porous wall under high blowing ratios, where the mixing effect of turbulence is fully manifest.

Understanding the passive acoustic signals of gas bubbles has become increasingly imporant due to their growing relevance in ambient marine acoustics and medical areas, as well as the recent search for dark matter by Bubble Chamber Detectors. Here the acoustic signatures of 2.24, 1.83, 1.75, 1.66 and 1.43 mm radii bubbles are reported experimentaly. They were generated by two different air injectors in a quiescent liquid, a standard plastic syringe tube and 97 degrees-rotated metallic needles. The evolution of the sound pulse over time is presented alongside simultaneous images of the bubbles detaching from the injector and moving freely in the liquid. The beat-wave phenomenon is the standard interference pattern between two sounds of slightly different frequencies, generating a beat-period envelope of “rosary chain” form. In this study, beats are observed when the bubble exhibits changes in its shape through Cassini-oval, trapezoid, ‘guitar pick’, ellipsoid and oblate shapes, which is in agreement with earlier results on bubble fragmentation in a locally sheared flow. Finally, theoretical curve fits of freely-oscillating, lightly-damped bubble sound emissions confirm that the beats are due to changes in the bubble shapes.