High Jet Flow Research Articles

On the role of aortic valve architecture for physiological hemodynamics and valve replacement, Part I: Flow configuration and vortex dynamics

Aortic valve replacement has become an increasing concern due to the rising prevalence of aortic stenosis in an ageing population. Existing replacement options have limitations, necessitating the development of improved prosthetic aortic valves. In this study, flow characteristics during systole in a stenotic aortic valve case are compared with those downstream of two newly designed surgical bioprosthetic aortic valves (BioAVs). To do so, advanced three-dimensional fluid–structure interaction simulations are conducted and dedicated analysis methods to investigate jet flow configuration and vortex dynamics are developed. Our findings reveal that the stenotic case maintains a high jet flow eccentricity due to a fixed orifice geometry, resulting in flow separation and increased vortex stretching and tilting in the commissural low-flow regions. One BioAV design introduces non-axisymmetric leaflet motion, which reduces the maximum jet velocity and forms more vortical structures. The other BioAV design produces a fixed symmetric triangular jet shape due to non-moving leaflets and exhibits favourable vorticity attenuation, revealed by negative temporally and spatially averaged projected vortex stretching values, and significantly reduced drag. Therefore, this study highlights the benefits of custom-designed aortic valves in the context of their replacement through comprehensive and novel flow analyses. The results emphasise the importance of analysing jet flow, vortical structures, momentum balance and vorticity transport for thoroughly evaluating aortic valve performance.

Turbofan and turbojet engines: Working process and future development

This article is based on the fundamental thermodynamics theory to analyze the difference between the turbojet and turbofan engines. The result of this article is that the turbofan engine is suitable for commercial use since it consumes less fuel and has a higher bypass ratio than the turbojet engine. But low bypass ratio engines can serve the military since they have a smaller volume and higher speed, suitable for air combat. The turbojet engine can travel at a supersonic speed since it has extremely high jet flow from the nozzle and much higher fuel consumption, so it is for military use. This article also analyses two cases: i) How the number of fan blades and outlet guide vans affects the noise of the engine produced. ii) When airplanes fly through volcanic ash, how does the ash damage the airplane and the engine? The engineer found that 55 outlet guide vans are better than 37. To prevent airplanes from flying through volcanic ash, satellite weather photos start to show the volcanic ash area.

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

MELCOR Analysis of a SPARC Experiment for Spray-PAR Interaction during a Hydrogen Release

A series of experiments were performed in the SPARC (spray-aerosol-recombiner-combustion) test facility to simulate a hydrogen mitigation system with the actuation of a PAR (passive auto-catalytic re-combiner) and spray system. In this study, the SPARC-SPRAY-PAR (SSP1) experiment is chosen to benchmark the MELCOR (a lumped-parameter code for severe accident analysis) predictions against test data. For this purpose, firstly we prepared the base input model of the SPARC test vessel, and tested it by a simple verification problem with well-defined boundary conditions. The implementation of a currently used PAR correlation in MELCOR is shown to be appropriate for the simulation of a PAR actuation experiment. In an SSP1 experiment, the PAR is reacting with hydrogen, and the spray actuation starts as soon as hydrogen injection is complete. The MELCOR simulation well predicts the pressure behavior and the gas flow affected by operating both a PAR and spray system. However, the local hydrogen concentration measurement near the inlet nozzle is much higher than the volume average-value by MELCOR, since high jet flow from the nozzle is dispersed in the corresponding cell volume. The experimental reproduction of the phenomena we expect, or, conversely, the identification of phenomena we do not understand, will continue to support the verification of analytical models using experimental data and to analyze the impact of spray on PAR operations in severe accident conditions.

DEVELOPMENT OF SQUEEZE FLOW IN MECHANICAL HEART VALVE-A PARTICLE IMAGE VELOCIMETRY INVESTIGATION

Background and Objectives: MHVcavitation has been widely investigated by capturing the formation and collapse of vaporous bubbles caused by sharp pressure drop. This momentarily pressure drop occurs when the valve occluder approaches the orifice wall. With the reducing flow channel between the occluder and the wall, fluid in-between tends to be squeezed out causing a high-speed flow. Limited experimental work has been into this squeeze flow phenomenon, which is likely to be the origin of cavitation. Methods: The experiment was carried out in pulsatile mock loop simulating physiological flow conditions. PIV was applied and an actual size transparent MHVmodel was used. To observe flow within a narrowing flow channel in the region of 0.1 to 0.5 mm, a microscope was incorporated into the PIV system.The system was time triggered by a mechanism to capture images when the occluder nearly closed. Results: Series of images of flow within the narrowing flow channel were captured. As the gap between the tip of the valve occluder and orifice wall becomes narrower, evidence of high speed jet flow becomes more apparent. When the gap narrowed down to around 0.1 mm, flow velocity of up to 3 m/s was noted with a corresponding high vorticity of 3800 is near the leading edge tip of the occluder. The sudden increase in high-speed jet flow is expected to be accompanied by a local pressure reduction. Conclusion: Squeeze flow arising from the closing of the occluder is evident from experimental findings. High jet flow can be detrimental due to the potential cavitation and momentarily high shear stress.

Behavior of an air jet interacting with a Helmholtz resonator

The internal spectrum of a Helmholtz resonator excited by an air jet was studied as a function of jet speed for a variety of jet parameters. Special attention was given to the relation between jet speed and amplitude of the Helmholtz mode, which shows a pattern of characteristic ‘‘domains’’ first reported by Khosropour and Millet [J. Acoust. Soc. Am. 88, 1211–1221 (1990)]. The effects of variation of air-jet length on major features of these domains were studied. The deduced behavior of the air jet was compared with previous models, in particular that of Fletcher and Thwaites [Acustica 42, 323–334 (1979)]. Good agreement with the predictions of this model was discovered except at high jet flow rates.

THE MAGNITUDE OF AIR ADMIXTURE WITH A JET DEVICE IN PAEDIATRIC ANAESTHESIA

The admixture of air in inspired gas with a paediatric insufflation system based on a gas jet has been investigated. During anaesthesia with spontaneous ventilation 1% of air per kg body weight was found in tracheal gas samples. A venturi effect was virtually absent even at high jet flows, which suggests that artificial ventilation prevents air admixture.