Mechanism Of Flow Reversal Research Articles

Experimental Observation of Flow Reversal in Thin Liquid Film Flow Falling on an Inclined Plate

A customized particle image velocimetry/planar laser induced fluorescence (PIV/PLIF) experimental method, aiming to capture transient hydrodynamics of solitary waves of inertia-dominated falling liquid films, is presented in this paper. A novel PIV/PLIF technique, which only uses one camera to capture simultaneously both particle image and fluorescence, and meanwhile a post-processing imaging method is also developed, which is able to simultaneously measure both internal velocity field in film and its topology. To validate the fidelity of the novel PIV/PLIF technique, a comparison between experimental results of streamwise velocity profile and film thickness and that of the Nusselt’s prediction at low Reynold number is carried out, and in addition, integral continuity is checked for transient wavy film, both of which shows that they are in good agreement. Based on experimental velocity fields and film topology, pressure distribution inside film is derived with the Poisson equation. Considering characteristics of traveling waves, the experimental results are presented respectively in spatial mode and temporal mode. In spatial mode, capillary wave dynamics are demonstrated out of velocity field, film topology and pressure distribution, which reveals that flow reversal occurs at capillary troughs. In temporal mode, the mechanism of flow reversal at capillary troughs is scrutinized on the basis of high-frequency velocity sampling and the derived pressure gradient. It is shown that flow reversal at capillary troughs is triggered due to occurrence of positive pressure gradient at the back side of the capillary wave crest, rather than the trough upstream as stated by the previous researchers. By elucidating the dynamics of flow reversal, mechanisms for the upper limit of Reynold number with respect to flow reversal underneath capillary wave trough were proposed, which might be the gradually saturated deceleration from the capillary curvature and shorten deceleration duration determined by the wave speed and capillary wave length. Our results should be of interest for optimization of the mass transport model of falling liquid films and shed light on the revealing of flow reversal mechanism.

Comparison study between lateral flow immunochromatographic assay and hemoglobin H staining for the screening of alpha thalassaemia

This paper presents a numerical study of the heat transfer improvement around a bubble rising near a wall in a shear flow. The Navier Stokes equations and energy equation are solved in a frame of reference following the bubble to examine the dependence of the wall to liquid heat transfer on flow patterns around the bubble. The front tracking method is used to track the interface of non-condensable gas bubbles, and the finite volume method is used to solve for temperature and momentum. Our simulations show an enhancement and a diminishment of heat transfer in the downstream and upstream regions of the bubble, respectively. The enhancement and reduction of heat transfer are attributed to the reversal of flow that occurs near the wall due to the presence of a bubble. The heat transfer rate non-monotonically varies with the dimensionless shear rate γ̇‾=γ̇dg. In the range of 0.3⩽γ̇‾≤2.25, the heat transfer rate first increases, attains a maximum value and then decreases. The optimum γ̇‾ increases as the Archimedes number is increased. The heat transfer rate is found to decrease as the Laplace number is increased. The flow reversal near the wall is characterized by two parameters, namely the ‘reversal height’ and ‘reversal width’. A control volume analysis is performed for a simplified 2D version of the problem in order to relate heat transfer enhancement to the reversal height and width. The functional dependence of heat transfer enhancement on reversal height and width from our 3D simulations match the result of the control volume analysis. The analytical solution of an inviscid shear flow over a cylinder is used for qualitative insights into flow reversal mechanism. Our simulations of transient heat transfer showed that the fractional improvement in heat transfer compared to a single phase flow attains a constant value after a certain amount of time. Finally, we report the results of heat transfer enhancement due of an array of bubbles rising near a wall.

Mechanism of flow reversal during solidification of an anomalous liquid

In a wide variety of fluidic systems involving thermal and compositional gradients, local density changes lead to the onset of natural convection that influences the process itself, for example, during phase-change phenomena and magmatic flows. Accurate knowledge of the flow characteristics is essential to quantify the impact of the flow of the processes. In this work, the first-ever demonstration of flow reversal during bottom-up solidification of water using full-field thermal and flow measurements and its direct impact on the solidifying interface is presented. Based on prior optical interferometric measurements of full-field temperature distribution in water during solidification, we use the particle image velocimetry technique to quantify and reveal the changing natural convection pattern arising solely due to the density anomaly of water between 0 °C and 4 °C. The independently captured thermal and flow fields show striking similarities and clearly elucidate the plausible mechanism explaining the formation of a curved interface at the stagnation point and the subsequent reversal of flow direction due to a changed interface morphology. A control volume analysis is further presented to estimate the energy invested in the formation of a perturbation and the resulting flip in the flow direction caused by this perturbation.

Impact of a single bubble rising near a wall on the wall-to-liquid heat flux

This paper presents a numerical study of the heat transfer improvement around a bubble rising near a wall in a shear flow. The Navier Stokes equations and energy equation are solved in a frame of reference following the bubble to examine the dependence of the wall to liquid heat transfer on flow patterns around the bubble. The front tracking method is used to track the interface of non-condensable gas bubbles, and the finite volume method is used to solve for temperature and momentum. Our simulations show an enhancement and a diminishment of heat transfer in the downstream and upstream regions of the bubble, respectively. The enhancement and reduction of heat transfer are attributed to the reversal of flow that occurs near the wall due to the presence of a bubble. The heat transfer rate non-monotonically varies with the dimensionless shear rate γ̇‾=γ̇dg. In the range of 0.3⩽γ̇‾≤2.25, the heat transfer rate first increases, attains a maximum value and then decreases. The optimum γ̇‾ increases as the Archimedes number is increased. The heat transfer rate is found to decrease as the Laplace number is increased. The flow reversal near the wall is characterized by two parameters, namely the ‘reversal height’ and ‘reversal width’. A control volume analysis is performed for a simplified 2D version of the problem in order to relate heat transfer enhancement to the reversal height and width. The functional dependence of heat transfer enhancement on reversal height and width from our 3D simulations match the result of the control volume analysis. The analytical solution of an inviscid shear flow over a cylinder is used for qualitative insights into flow reversal mechanism. Our simulations of transient heat transfer showed that the fractional improvement in heat transfer compared to a single phase flow attains a constant value after a certain amount of time. Finally, we report the results of heat transfer enhancement due of an array of bubbles rising near a wall.

Flow reversal bypass surgery: a treatment option for giant serpentine and dolichoectatic aneurysms-internal maxillary artery bypass with an interposed radial artery graft followed by parent artery occlusion.

Giant serpentine and dolichoectatic aneurysms (GSDAs) are a small subgroup of intracranial aneurysms that often have poor clinical outcomes and are hardly to be treated by either conservative or endovascular management. We assessed seven patients who underwent internal maxillary artery bypass using radial artery graft interposition followed by parent artery occlusion to treat GSDAs. Intraoperative Doppler ultrasonography was used to assess the patency of the graft. CT angiogram or DSA were used postoperatively. The patients received follow-up after discharge and their neurological outcomes were measured using the Glasgow Outcome Score. Postoperative angiogram demonstrated that all the grafted conduits were patent, and the aneurysms were absent in two and collapsed or shrank in five of the patients. The rate of normal life or independent daily activities after internal maxillary artery bypass in the anterior and posterior GSDA patients was 66.7 and 75%, respectively. One patient experienced postoperative frontal ischemia and was managed conservatively. His symptoms had completely resolved at discharge. No patient encountered perforator's occlusion in the series. During 2.6years follow-up, six patients experienced favorable outcomes. One patient died in the third year after discharge attribute to unrelated disease. Based on the mechanism of flow reversal, internal maxillary artery bypass with radial artery graft followed by patent artery occlusion is an effective, minimally invasive treatment modality for select patients with GSDAs.

Analysis of the Flow Reversal in Natural-Circulation Boiler

Flow reversal phenomena in natural-circulation boilers are studied analytically according to both the homogeneous-flow model and the slip-flow model. In the latter model Thom's pressure drop correlation is used. The pressure drop between the upper steam-water drum and the lower header is assumed not to be affected by inception of reverse flow in an evaporator tube which will be analyzed later.The mechanism of flow reversal is examined through analysis of the static flow characteristics of the evaporator tube in the non-dimensional form. Some important dimensionless parameters are introduced and their effects on inception of reverse flow are studied.It was found that the criterion of flow reversal is correlated practically with only three dimensionless parameters. Finally, an approximate method to predict flow reversal is put forward.