Experimental Thermal and Fluid Science最新文献

The objective of this study is to experimentally and theoretically explore the impact of particle volume fractions and nozzle thread structure on the coaxial air blast atomization of particulate gel suspension jets through the high-speed flow visualization technology. The four breakup modes of particulate gel suspension jets are confirmed, namely oscillation mode, membrane-type breakup, fiber-type breakup, and superpulsating submode. However, the fiber-type breakup of jets disappears as the particle concentration reaches 40%, primarily attributed to the numerous particles sharply promoting the instability on the free jet surface. The experimental statistical results demonstrate that an increase in particle concentration and the adoption of the threaded nozzle both result in an expansion of the jet spray angle. The breakup length exhibits a reduction trend with the increase of particle concentration. However, it remains nearly the same value as that in the case of volume fraction 20% when the particle concentration continues to increase, contributed by the surge of viscous dissipation counteracting the enhanced instability on the jet interface caused by a large number of particles. The breakup length of a pure gel jet is predicted through the linear instability analysis, which is further modified by the viscosity model of particle suspensions to derive the breakup lengths of particulate gel suspension jets with different concentrations. The theoretical predictions align well with the statistical results in the experiment. The research is poised to have potential implications for numerous industrial processes and engineering applications including gel propellants.

The development of dilute particle heat exchangers and reactors for advanced energy systems requires an understanding of the multi-mode heat transfer from a heated wall to falling particles. This study presents experimental results of the overall heat transfer coefficient for a free-falling, dilute flow of particles with solid volume fraction from 0.0005 to 0.006 corresponding to feed rates from 3.7 kg s⁻¹ m⁻² to 44 kg s⁻¹ m⁻² in a vertical, heated tube containing quiescent air at atmospheric pressure. Tube wall temperatures are varied between 300°C to 900°C while keeping the particle inlet temperature constant. The experimental results show that the overall heat transfer coefficient is a strong function of particle feed rate and surface temperature. Good agreement was obtained with prior studies conducted at comparable temperatures but lower particle feed rates (<4 kg m⁻² s⁻¹). The established correlations for particle-to-wall radiation and particle-to-gas convection were used to estimate the wall-to-gas convective contribution from the measured overall heat transfer coefficient. The experimental results indicated a 4 to 6 times improvement in the wall convection in the solid–gas mixture compared to that expected from natural convection in a single-phase gas. The data presented here are applicable to characterize heat transfer in dilute particle heat exchangers, furnaces, and solar receivers.

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

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 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 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.