Effect of coherent structures on convective heat transfer in a swirling impinging jet

The present paper reports on the detailed investigation of the flow dynamics and unsteady heat transfer in an impinging jet in regimes with high swirl and vortex breakdown. A combination of the time-resolved stereoscopic PIV, time-resolved PLIF and high-speed IR-thermometry methods is used. Two cases of distances between the jet nozzle and impingement surface are considered, H = d and H = 2d. The Reynolds number is fixed as Re = 5000. The temperature distribution in the flow has a maximum on the jet axis near the surface in the region of the central recirculation zone. The data are processed using the POD method to extract coherent flow structures and quantify temperature fluctuations on the impact surface. The helical vortex structure in the case of H = d influences heat transfer between the swirling jet and the surface, the temperature fluctuations on the surface reach 0.05 degrees.

Large-eddy simulations of a swirling flow in a model Francis turbine

We performed Large-eddy simulations of the flow in a model air Francis turbine in a range of low-load regimes with a swirler rotating at fixed frequency. All investigated regimes revealed the presence of coherent helical vortex structure in the draft tube: the precessing vortex core. We identified the frequency of this instability and obtained mean flow velocity fields to be utilized in further works.

Multi-Vortex Regulation for Efficient Fluid and Particle Manipulation in Ultra-Low Aspect Ratio Curved Microchannels

Inertial microfluidics enables fluid and particle manipulation for biomedical and clinical applications. Herein, we developed a simple semicircular microchannel with an ultra-low aspect ratio to interrogate the unique formations of the helical vortex and Dean vortex by introducing order micro-obstacles. The purposeful and powerful regulation of dimensional confinement in the microchannel achieved significantly improved fluid mixing effects and fluid and particle manipulation in a high-throughput, highly efficient and easy-to-use way. Together, the results offer insights into the geometry-induced multi-vortex mechanism, which may contribute to simple, passive, continuous operations for biochemical and clinical applications, such as the detection and isolation of circulating tumor cells for cancer diagnostics.

Numerical Study of Multivortex Regulation in Curved Microchannels with Ultra-Low-Aspect-Ratio

The field of inertial microfluidics has been significantly advanced in terms of application to fluid manipulation for biological analysis, materials synthesis, and chemical process control. Because of their superior benefits such as high-throughput, simplicity, and accurate manipulation, inertial microfluidics designs incorporating channel geometries generating Dean vortexes and helical vortexes have been studied extensively. However, existing technologies have not been studied by designing low-aspect-ratio microchannels to produce multi-vortexes. In this study, an inertial microfluidic device was developed, allowing the generation and regulation of the Dean vortex and helical vortex through the introduction of micro-obstacles in a semicircular microchannel with ultra-low aspect ratio. Multi-vortex formations in the vertical and horizontal planes of four dimension-confined curved channels were analyzed at different flow rates. Moreover, the regulation mechanisms of the multi-vortex were studied systematically by altering the micro-obstacle length and channel height. Through numerical simulation, the regulation of dimensional confinement in the microchannel is verified to induce the Dean vortex and helical vortex with different magnitudes and distributions. The results provide insights into the geometry-induced secondary flow mechanism, which can inspire simple and easily built planar 2D microchannel systems with low-aspect-ratio design with application in fluid manipulations for chemical engineering and bioengineering.

Effect of Ribs on Heat Transfer and Pressure Drop in a U-Bend Channel With Double-Layer, Dome-Shaped Turning Vanes

In this paper, the combined effects of ribs and double-layer, dome-shaped turning vanes on heat transfer and pressure drop are investigated in an idealized U-bend channel. Five kinds of ribs including transverse ribs, 45° ribs, 135° ribs, V-shaped ribs, and reverse V-shaped ribs combined with one kind of double-layer, dome-shaped turning vanes are applied. Baseline results are compared with the above composite cooling structures. Numerical simulations are performed by solving 3D, steady Reynolds-averaged Navier-Stokes (RANS) equations with k-ω turbulence model. The channel aspect ratio is 1:2 and its hydraulic diameter is 93.13 mm, respectively. Based on the cooling air inlet velocity and the channel inlet hydraulic diameter, the inlet Reynolds numbers are ranging from 100,000 to 440,000. The detailed three-dimensional fluid flow, pressure and heat transfer distributions are presented. Moreover, the thermal performances of the U-bend channel are also evaluated and compared with different cases. The results revealed that combined with the double-layer, dome-shaped turning vanes, the transverse ribs case has the best thermal performance at the tip wall, and the reverse V-shaped ribs case is the best for the leading wall. The pressure drop of the channel with double-layer, dome-shaped turning vanes without any rib turbulator is the lowest, and that of the channel with inclined ribs is significantly higher than that of the channel with transverse ribs. The superposition of the secondary flow induced by the ribs and the Dean vortex induced by the 180° sharp turn has a marked impact on the flow and heat transfer in the channel. In the double-layer, dome-shaped turning vanes channel, the mass flow distribution of the coolant also affects the heat transfer on the tip wall of the channel, and the ribs can adjust the mass flow distribution. The helical vortex superposed by the mainstream flow and the secondary flow induced by the ribs represents typical flow phenomenon in ribbed channels. The flow and development of the helical vortex are the main factors affecting the heat transfer on the leading/trailing walls.