Experimental Thermal and Fluid Science最新文献

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.

The current study experimentally investigates a passive control method for the flow field by placing spoilers symmetrically on the leading edge of a 5:1 rectangular cylinder. The Reynolds number (Re) is based on the inflow velocity and the height of the model. The length of the spoiler is equal to the span length of the model, and the width and angle are defined as w and α. At $\text{Re = 1.07 2.50}\times {\text{10}}^{\text{4}}$, the surface pressure distribution of the model is obtained to initially investigate the effects of α and w on the aerodynamic characteristics. Based on the aerodynamic results, some cases are selected to reveal the control mechanism using the particle image velocimetry (PIV) technique. The proper orthogonal decomposition (POD) is adopted to analyze the POD modes and instantaneous flow. The results show that the spoiler with a certain α can suppress the aerodynamic forces of the model. Spoilers with a relative angle of 247.5° significantly reduce ${\text{C}}_{\text{L}}^{\prime }$ by 75 % and slightly reduce $\overline{{\text{C}}_{\text{D}}}$ by 5.5 %. Also, its TKE and RSS values are reduced by 56 % and 57 %, respectively. The PIV visualization shows that the spoiler affects the flow separation at the leading edge. Then, the rolling and interactions of shear layers are suppressed, making them tend to be parallel. Besides, spoilers with a relative angle of 67.5° almost eliminate the flow separation.

In this study, laminar flow and forced convective heat transfer of water-based ferrofluids flowing through a uniformly heated pipe are experimentally investigated under the presence of phase-shifted oscillating magnetic fields. To investigate the effect of phase shift on heat transfer, electromagnets are positioned along the tube, and oscillating magnetic fields are applied with various phase shift angles between opposing magnetic poles. Experiments are conducted for different Reynolds numbers (400 to 1000), magnetic field frequencies (0 Hz, 1 Hz, and 5 Hz), phase shift angles (0°, 90°, and 180°), and nanoparticle volume fractions (0.5 % and 1 %). For each parameter set, local and average Nusselt numbers, as well as pressure drop values, are determined, and the effect of applied magnetic fields on the heat transfer rate is extensively discussed. Results showed that, applying an external magnetic field resulted in significant enhancements in the forced convective heat transfer of ferrofluid. Under an oscillating magnetic field with 0° phase shift, maximum of 40 % and 20.6 % enhancements were observed in local and average Nusselt numbers respectively under the investigated parameters. Furthermore, applying oscillating magnetic fields with a phase shift between opposing poles caused significant fluctuations in the fluid, led to remarkable improvements in convective heat transfer rates. For 90° and 180° phase shifts, enhancements in local and average Nusselt numbers were observed to increase up to 73 % and 36 %, respectively.

Based on the shape of the interface elongated bubble/liquid slugs and the liquid slugs’ aeration, the horizontal intermittent flow can be divided into three sub-regimes including plug (PG), Less Aerated Slug (LAS) and Highly Aerated Slug (HAS) flows. These flow sub-regimes were observed from experiments performed using air–water mixture and small pipe diameters. This paper presents an analysis of the results obtained with the aim of constituting the state-of-the-art of this sub-regimes classification.

The critical review, of the current state of knowledge, has led to the conclusion that the subdivision of intermittent flow into sub-regimes may provide a better means of apprehending, understanding and advancing in the modelling of slug parameters, Interfacial Area Concentration, Pipeline Integrity Management, intermittent flow behavior across singularities, as well as for the development of more realistic mechanistic models. The acquired knowledge can be beneficial for petroleum and gas, nuclear and chemical engineering industries among others.

Finally, based on the presented state-of-the art, some recommendations are given for future works using this approach. These reflection paths will allow improving our comprehension on intermittent flow, promoting the development of more robust models.

This paper experimentally characterizes unsteady effects and flow fields around the Ahmed Body, by analyzing global forces and detailed wake effects. The results are compared to those obtained under steady conditions, with varying wind tunnel velocities and different yaw angles between the model and the free stream. Unsteady fields are generated by means of oscillating blades positioned at the inlet of the test section, whose amplitudes and frequencies can be easily controlled. Specifically, low frequencies, around a few Hertz, as those in the typical range generating load oscillations on vehicles, are considered. The results in terms of force coefficients, obtained by a dynamometric balance, and velocity fields, obtained by Particle Image Velocimetry, are processed in order to derive time-average statistics and also phase-average statistics, as related to forcing blade instantaneous positioning. This type of analysis can be performed thanks to the high temporal resolution of measurement systems, around 100 Hz for the force measurements and around 4000 Hz for the velocity measurements. Results in steady conditions well compare with previous results in references, both as functions of wind tunnel velocity and yaw angles. In unsteady conditions, whatever amplitude is considered, time-average drag and lift coefficients and their dependence on yaw angle are consistently lower compared to the steady case. Phase-averaged coefficients in unsteady conditions can oscillate by around 20 % in comparison to time-average values and these fluctuations are strongly dependent on yaw angle and amplitude of oscillations, thus suggesting that they both contribute to instantaneous loads. Present investigations are related to improvements in set-up of control systems in assisted-driving (self-driving) vehicles.

The treatment of oily wastewater has become a serious problem in the late stage of oilfield development. At the same time, it is of great significance to the improvement of ecological environment. As the key process of oil-bearing wastewater treatment, the study on the binding and adhesion law of oil droplets and bubbles in flotation and its related mechanism can provide reference for its application. Using a double high-speed camera acquisition system, the floating process and collision adhesion law of oil droplets and bubbles in a vertical transparent circular tube were experimentally studied, and the collision adhesion process of oil droplets (0.721 ∼ 3.759 mm) and bubbles (0.797 ∼ 2.886 mm) in different diameters were analysed. It is concluded that the oil droplet and the bubble collide with each other, and then the bubble slides along the surface of the oil droplet, and a neck shape appears at the end of the contact site, and then the neck increases with the diffusion of the oil droplet to form an oil-bubble adhesion body. And the process of elastic drags and contraction separation of the mixture is also demonstrated. It is found that the combination of oil droplets and bubbles with different diameters will have two types of collision adhesion modes, which oil droplet type and oil film type, respectively. Therefore, the diameter ratio of oil droplets and bubbles is a key factor, and when the diameter ratio is greater than 0.75, the adhesion mode of the adhesive body changes from unstable oil droplet type to more stable oil film type.