Experimental Thermal and Fluid Science最新文献

The thermal diffusivity of magnetocaloric materials has a transition point at a given temperature that depends on the intensity of the applied magnetic field. Consequently, a fine temperature resolution on the material sample is needed to obtain an accurate determination of the thermal diffusivity variation with temperature. The coupling between the external and the internal fields has to be carefully mastered since the pertinent operating condition is fixed by the actual internal field which is not directly measurable and may be heavily affected by any element in contact with the sample. Therefore, contactless methods such as Photo-Thermal Radiometry (PTR) are privileged. The latter is based on a radiative excitation of the front face of a thin sample and the detection of the thermal effect on the opposite face. However, the powerful radiative source may significantly increase the sample temperature which is not suitable for caloric materials. In this work, a low power modulated PTR method is proposed to characterize second order magnetocaloric materials under magnetic field. It was compared to high energy thermo-flash PTR and validated on common materials such as steel and stainless steel, and then applied to gadolinium which is the reference magnetocaloric material for magnetic refrigeration and heat pumping study. The thermal diffusivity of gadolinium samples is measured in the 285.1 K to 305.1  K temperature range, including the magnetic transition temperature without and under an external magnetic flux density of 0.5 T in the 13 mm air gap of the permanent magnet magnetic circuit. The low power probe beam ensures a temperature stability with a negligible sample temperature fluctuation less than 0.05 K on the incident sample surface and less than 0.03 K on the measurement surface. The experimental results without magnetic field align with those using other methods including the magnetic transition temperatures determination. This low-power optical method proved its efficiency to characterize highly temperature dependent materials such as magnetocaloric materials sensitive to magnetic field. The data obtained partly fills the lack of information in the literature on excited gadolinium.

While boundary-driven acoustic streaming in fluids surrounded by flat walls has been extensively studied in the literature, theoretical studies on boundary-driven acoustic streaming generated by curved walls have recently emerged. This paper aims to present a quantitative analysis of acoustic streaming fields driven by forces induced by both flat and curved walls. A semi-circular channel made of stainless steel was designed to serve as a model channel with both flat and curved boundaries. A multi-layered glass-steel-glass device, actuated by a piezoelectric transducer, was assembled for experimental characterization of boundary-driven acoustic streaming in such scenarios. First, the various standing acoustic modes in the semi-circular channel were measured through the acoustophoretic patterning of 20 µm polystyrene particles. Next, the acoustic radiation force fields and boundary-driven acoustic streaming patterns under various resonant acoustic modes were characterized through micro-particle-image-velocimetry measurements of the motion of 20 µm and 1 µm polystyrene particles, respectively. Finally, the experimental results were explained using efficient finite element simulations of acoustofluidics and acoustophoresis in a semi-circular reduced-fluid model, with a focus on analyzing the streaming velocities driven by the flat and curved walls. Both experimental and numerical results demonstrated that the ratio of streaming velocities induced by the flat wall and the curved wall in this semi-circular channel depends on the resonant acoustic modes. This research highlights the diverse boundary-driven acoustic streaming patterns that arise in irregular channels and provides a theoretical foundation for choosing strategies for shape optimization to suppress acoustic streaming in acoustofluidic devices.

Plughole vortex dynamics and the corresponding air entrainment is of paramount importance in various process industries. The present study experimentally investigates plughole vortex induced air entrainment during the drainage of water through two closely spaced outlets from a tank. The discharge from the two outlet tubes has been modulated and the different stages of air entrainment including formation of air lamella, periodic slug bubble pinch-off, and transition into the stable annular flow are observed. The transition between the slug bubble pinch-off and the stable annular flow has been modelled by balancing the local dynamic pressure of the fluid and the Laplacian pressure jump across the gas-liquid interface. The balance yields the ratio of critical height, $H_c$ and radius of the stable air neck, $r_m$ as $H_c/r_m\approx 30.68{\mathrm{Bo}}^{-0.904}{\left(f\left(D,r_e\right)\right)}^{-0.210}$, with the Bond number, $\mathrm{Bo}=\Delta \rho g{D}^2/\sigma$ and the geometric parameter, $f\left(D,r_e\right)={\left(1-\frac{{2r}_e}{D}\right)}^2$.

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.