Experimental Thermal and Fluid Science最新文献

The flow field of the swirl-stabilized combustor plays a significant role in fuel atomization and flame stability. The experimental investigation of the non-reacting flow field downstream of a swirl cup with no confinement is carried out by means of particle image velocimetry measurements. The statistical uncertainty is calculated to evaluate the turbulence convergence and projection errors. The flow fields provide a compelling picture of the basic characteristics of the swirl flow, while the root mean square velocity analysis illustrates the upward and downward fluctuations of the emanating jet. The proper orthogonal decomposition (POD) modes reveal the most pronounced features of the flow, namely the central recirculation zone and the precessing vortex core (PVC) at its boundaries, as well as a significant feature that occurs several times in the modes, i.e., the entrainment of the surrounding atmosphere as an alternative to the corner recirculation zone. Furthermore, the dynamic mode decomposition (DMD) modes in the low-frequency region characterize the slow change ($S_t=0.0026$) that occurs when the emanating jet is shifted upward as well as the PVC oscillations ($S_t=0.113$) in the flow. The DMD modes in the high-frequency then characterize the high-frequency oscillations induced by vortex shedding in the swirl flow. The research is helping to provide a clear picture of the flow downstream of the swirl cup without any confinement.

This study experimentally explores the propagation mechanisms of acetylene/air detonation waves within a channel intermittently constrained on one side, utilizing soot foil and high-speed schlieren photography to capture the cellular structure and shock-flame evolution. The experiments revealed that the detonation waves traverse the semi-enclosed channel with various discrete wall configurations on the side in three distinct propagation modes: (I) periodic detonation failure and re-initiation; (II) single extinction and re-initiation; (III) non-extinction. Mode I occurs exclusively when the open area ratio exceeds 0.85, while detonation tends to favour Mode II when the gaps between discrete walls exceed three times the cell size; otherwise, it tends towards Modes III. The re-initiation mechanism of detonation involves curvature shocks inducing local explosions of reactive mixtures through multiple Mach reflections off the discrete walls. The self-sustained propagation mechanism of the detonation wave is maintained by the interaction of strong transverse shocks reflected from discrete walls with the inherent transverse waves within the detonation structure, sustaining the instability of the cellular detonation.

Microchannel heat transfer plays an important role in microelectronics technology for heat dissipation, due to its high efficiency and low heat transfer temperature difference and flow resistance. To underpin the fundamental understanding of this technology, the natural evaporation process of absolute ethanol in a capillary tube at inclination angles ranging from 0° to 90° was investigated experimentally by exploring a spectrum of properties, such as Marangoni flow patterns, evaporation rate, heat flux, and temperature distribution. We found that the morphology of the meniscus is similar under different inclination angles, but the liquid and the tube wall slip to varying degrees due to the pressure difference at the liquid–vapor interface during evaporation. Therefore, the force distribution of the meniscus interface is different, and the resultant force is F_max(60°) > F_max(0°) > F_max(30°) > F_max(90°). We found that the morphology of the meniscus is independent of the inclination angle when absolute ethanol evaporates naturally. And the evaporation rate, heat flux and temperature distribution of meniscus at the initial stage of evaporation follow the law of resultant force distribution. That is, when the inclination angle is 60°, the evaporation rate and heat flux reach the maximum, i.e. 1.64 μm/s and 10.96 W/cm², respectively, and the temperature between the center of the meniscus and the wedge region reaches 1.5 ℃. We used μ-PIV to observe the Marangoni vortex morphology of the vertical section of the meniscus, and found that there are different degrees of deformation at different inclination angles. When the inclination angle is 90°, the Marangoni vortex structure is destroyed.

A flow condensation experiment was performed in a 300 mm long mini channel with a diamond pin fin array. The working fluid is R134a and four pin fin arrays were tested, including different channel widths of 1.0, 1.2, and 1.4 mm, as well as fin angles of 60°and 90°. The experimental system used in previous studies was adopted to obtain the local heat transfer coefficient. The measurements were done within the saturation pressure range of 600–1500 kPa with mass flux ranging from 160 to 450 kg/m²s. The experimental results indicated that the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas it decreases with an increase in saturation pressure. The influence of heat flux and pin fin array structure on heat transfer coefficient was more significant in the high vapor quality region relative to that of the low vapor quality region. Higher fin density and larger fin angles contribute to improved condensation. With a diamond fin angle of 60°, the heat transfer coefficient of the pin fin array with a fin density of 0.22 is 24 %∼56 % higher than that of the pin fin array with a fin density of 0.16. For the pin fin array with the same fin density, the heat transfer coefficient at fin angle 90° is 1.1–1.4 times that at fin angle 60°.Additionally, the performance evaluation criteria named Penalty Factor was applied to evaluate the performance of the pin fin array, and SG60_3 outperforms the other channel, corresponding to a fin angle of 60° and channel widths of 1.4 mm. The Penalty Factor value of SG60_3 is 70 %∼80 % of that of the other three pin fin array. The existing correlations fail to give a reasonable prediction for the heat transfer coefficient of the present experimental data. Therefore, a new correlation accounting for the effects of geometric sizes of pin fin array and heat flux was developed with the maximum mean absolute deviation of 7.48 % on four test channels. The present study can provide valuable knowledge on the design optimization of mini channel condensers with pin fin array.

Electric field-assisted separation is considered one of the most effective ways of dehydrating water-in-oil emulsions. In the uniform electric field usually used in electrodehydrators, electrocoalescence leads to droplet enlargement, thus accelerating their gravitational settling. Meanwhile, using a nonuniform field is expected to provide an additional tool for phase spatial separation due to dielectrophoresis while keeping the conditions favorable for electrocoalescence. This study aims to investigate experimentally the dynamics of water-in-oil emulsion in a nonuniform electric field and the efficiency of its separation due to the dielectrophoretic effect. A high-frequency field and emulsions with zero-density contrast were used allowing us to study the action of the dielectrophoretic force in the absence of electrokinetic phenomena and gravitational settling. We found that droplets always move towards the electric field strength gradient, eventually accumulating at the internal electrode. We demonstrate that the separation efficiency increases as the average droplet size, the voltage, the dispersion medium permittivity, and the initial droplet concentration increase. In the latter case the separation enhancement is due primarily to droplet coalescence, the rate of which increases appreciably with increasing concentration. We demonstrate that all the experimental results can be combined into unified dependence based on a simple physical model.

The noise mitigation effect of bio-inspired geometries has attracted growing attention from both research and industry, such as the vibrissa-shaped cylinder derived from the harbor seal. Experiments were conducted to investigate the far-field noise and the near-field wake of the flow past a vibrissa cylinder, a circular cylinder, and an elliptical cylinder at $Re=3.6\times 10^4$, in the subcritical flow regime. The frequency characteristic of the far-field acoustic pressure and the near-field velocities are analyzed. The mean and fluctuating velocities, dominant flow modes from proper orthogonal decomposition in both vertical and horizontal planes as well as the time-frequency behavior of the dominant flow structures from wavelet transform are also presented to better understand the wake dynamics and the direct relation of these flow structures with the far-field noise. The vibrissa cylinder reduces the overall sound pressure level by 13.2 dB and 8.3 dB compared with the circular and the elliptical cylinders, respectively, with a remarkable attenuation of the tonal peak associated with vortex shedding. From the detailed velocity measurements in multiple wake planes, it is clearly observed that vortex shedding of the vibrissa cylinder is weaker in strength and significantly less coherent in the spanwise direction than the other two cylinder cases, accompanied by more transient changes. The results also reveal the distinct flow behaviors behind the nodal and saddle planes of the vibrissa cylinder, further contributing to this three-dimensional vortex shedding. Consequently, the power spectral density of the tonal peaks associated with the vortex shedding in both near-field velocities and far-field acoustic pressure are attenuated, leading to a lower noise level. Understanding the detailed flow dynamics of the vibrissa cylinder will provide useful insights into more efficient bio-inspired cylinder designs in noise mitigation and wake control.

Despite recent advances in 3D particle image velocimetry (PIV), challenges remain in measuring small-scale 3D flows, in particular flows with large dynamic range. This study presents a scanning 3D-PIV system tailored for oscillatory flows, capable of resolving transverse flows less than a percent of the axial flow amplitude. The system was applied to visualize transverse flows in millimetric straight, toroidal, and twisted ducts. Two PIV analysis techniques, stroboscopic and semi-Lagrangian PIV, enable the quantification of net motion as well as time-resolved axial and transverse velocities. The experimental results closely align with computational fluid dynamics (CFD) simulations performed in a digitized representation of the experimental model. The proposed method allows the examination of periodic flows in systems down to microscopic scale and is particularly well-suited for applications that cannot be scaled up due to their complex, multi-physics nature.

One of the important strategies for the scale-up of microreactors is sizing-up, which is conducted by increasing the hydrodynamic diameter of microreactors. However, the interphase mass transfer deteriorates seriously in the sizing-up. This work aimed to probe the possibility of adding an inert gas phase to offset the adverse effect of microreactor sizing-up on the mass transfer between two immiscible liquid phases. Using a high-speed camera, four flow patterns were observed in three capillaries with their diameters ranging from 0.8 to 3.0 mm. Empirical equations were given to describe the flow-pattern transitions. The influencing mechanism of the capillary diameter on the liquid–liquid mass transfer was analyzed by taking the effect of adding the inert gas phase into account. Finally, the evaluation of the energy consumption suggested that adding an inert gas phase to agitate the flow could utilize the input energy more efficiently to intensify the liquid–liquid mass transfer in the microchannel with a larger hydrodynamic diameter. Therefore, the method of inert gas agitation is a meritorious assistive technology in the sizing-up of microreactors.

Pintle injectors have garnered significant research attention in recent years, particularly for their applicability in reusable launch vehicles, owing to their wide thrust control range and excellent combustion stability. While research has explored the characteristics of pintle injectors in the context of developing these components for actual engine applications, studies focusing on the effects of design parameters on injector performance have been limited. This study investigated the effects of slit geometric parameters, specifically the blockage factor ($BF$) and slit area ratio ($\gamma$), on the spray characteristics of double-slit pintle injectors. Cold-flow tests were conducted using planar pintle injectors with water and ethanol as simulants. The spray angle and Sauter mean diameter (SMD) were measured using the shadowgraph technique, and the distribution of mass flow rate and mixture ratio was analyzed using a mechanical patternator. The experimental results revealed that two distinct streams were injected at different angles from each row of slits, resulting in a division of spray shape, SMD, and mass flow distribution into three regions based on the two streams. These spray angles, termed primary and secondary spray angles, were quantified as functions of the local momentum ratio, determined by $BF$ and $\gamma$. To correlate the spray characteristics with combustion performance, mixing quality and a representative droplet size metric, the integral Sauter mean diameter ($I{D}_{32}$), were presented. The study found that higher values of $\mathrm{BF}$ and $\gamma$ corresponded to improved mixing quality.

Heat transfer coefficient (HTC) is one of the most important parameters for modeling forced flow condensation inside tubes. This manuscript presents an extensive review of HTC measurement techniques, experimental databases, and prediction methods for in-tube flow condensation to evidence the latest literature achievements and identify new research opportunities. HTC measurement techniques were reviewed, classified, and the most used techniques were identified along with their main characteristics. Experimental databases from the literature were grouped for analysis, totaling 15,021 data points for channel diameters ranging from 0.067 to 20.8 mm, 82 working fluids, horizontal and vertical flow directions, and 4 different tube wall materials for smooth tubes. The measurement techniques and uncertainties of individual databases were identified and discussed. Recently identified trends are the increasing interest in low GWP refrigerants, new fluid mixtures, and experiments for small-diameter channels. Many of these experimental conditions were not incorporated or tested on previous correlations, representing an extrapolation when doing so. A total of 34 prediction methods, proposed from 1958 to 2024, were evaluated and compared to this broad database to verify their prediction errors and physical fundamentals. The best predictions obtained a mean absolute percentage error of 23.4 %, showing that further work for minimizing the experimental uncertainties is still needed. In addition, HTC values higher than 10 kW/m²K are commonly observed in recent experiments. One of the challenges identified for new measuring techniques is the measurement of such high values of HTC while keeping low uncertainty levels. The experimental database collected in this work is available for download in the supplementary material.