Axisymmetric Computational Fluid Dynamics Simulation Research Articles

Global stability of supersonic flow over a hollow cylinder/flare

Supersonic flow over a hollow cylinder/flare with a free-stream Mach number of 2.25 is numerically investigated in this study. Axisymmetric computational fluid dynamics simulations and global stability analysis (GSA) are performed for a wide range of cylinder radii and flare deflection angles. The onset of incipient and secondary separation is delayed as the cylinder radius is decreased due to the axisymmetric effects. The GSA reveals that a decrease in cylinder radius also postpones the emergence of global instability. The GSA results agree well with the results of direct numerical simulations for a supercritical case in the linear stage. The saturated flow exhibits pairs of unsteady streamwise streaks downstream of reattachment. The criterion of the global stability boundary established for supersonic flow over a compression corner (Hao et al., J. Fluid Mech, vol. 919, 2021, A4) is extended to its axisymmetric counterpart.

Numerical modelling of oxy-coal combustion to access the influence of swirl strength, combustion environment and gasification reactions on the flow and combustion behaviour

A simplified 2D axisymmetric computational fluid dynamics simulation of oxy-coal combustion is performed to address the influence of swirl strength and the combustion environment on the flow field and combustion characteristics. A quantitative analysis of the internal recirculation zone length and flame length under various operating conditions is studied by employing developed numerical models. The influence of char-CO2 and char-H2O gasification reactions on the species consumption and temperature profile is also studied through numerical modelling. The swirl strength considerably influences both the flow field and temperature profile inside the furnace. At higher swirl strength, radially dispersed shorter flames are obtained due to spreading of the secondary stream. Results showed that the flame shifted towards the burner at higher oxygen content due to rapid devolatilisation at higher flame temperature. The ignition of pulverised coal particles is enhanced at higher oxygen concentration due to increased stoichiometry near the burner. Oxy-35% O2 has 300 K higher flame temperature than oxy-21% O2. Endothermic gasification reactions significantly influence the temperature profile in the furnace. CO2-char gasification reaction has a more pronounced influence than the H2O-char gasification reaction. The consideration of gasification reactions has resulted in an approximately 5% reduction in peak temperature.

INVESTIGATING THE USE OF PHASE-CHANGE MATERIALS FOR TEMPERATURE CONTROL DURING FAST FILLING OF HYDROGEN CYLINDERS

This paper explores the use of phase-change materials in the process of fast filling of hydrogen cylinders in order to limit the rise in the gas temperature by enhancing heat transfer from the gas. It is necessary to limit the temperature rise because the structural performance of the cylinder materials can be degraded at higher temperatures. Initially, two computational approaches for modeling the fast filling of hydrogen cylinders are presented and validated; the first is an axisymmetric computational fluid dynamics simulation and the second is a single-zone approach with one-dimensional conjugate heat transfer through the cylinder walls. The models are applied to study fast filling of a hydrogen-powered passenger car. The predictions show that the minimum safe fill time for Type III cylinders with aluminum liners is generally shorter than for Type IV cylinders with plastic liners, for given ambient and precooling temperatures. Alternatively, Type III cylinders require less precooling for a given fill time. Introduction of a phase-change material heat sink is assessed as a means of reducing the fill time for Type IV cylinders. Paraffin-based phase-change materials are considered. The predictions show that the use of pure paraffin wax does not help in reducing the gas temperature due to its low thermal conductivity, however materials with improved thermal conductivity, for example, mixtures of paraffin wax and graphite, can facilitate reduced fill times. Without use of phase-change material it is not possible to reduce the fill time of Type IV cylinders below three minutes unless the gas supply is precooled. While the fill time can be reduced by precooling the gas supply, the phase-change material reduces the degree of precooling required for a given fill time by 10–20 K, and reduces the minimum theoretical power consumption of the cooler by 50–100%, depending on the ambient temperature.

A two-dimensional numerical study of peristaltic contractions in obstructed ureter flows

The flow of urine from the kidneys to the bladder is accomplished via peristaltic contractions in the ureters. The peristalsis of urine through the ureter can sometimes be accompanied, more specifically, obstructed to a certain degree, by entities such as kidney stones. In this paper, 2D axisymmetric computational fluid dynamics simulations are carried out using the commercial code ANSYS FLUENT, to model the peristaltic movement of the ureter with and without stone. The peristaltic movement was assumed to be a sinusoidal wave on the boundary of the ureter with a specific physiological velocity. While the first part of the study considers flow in the ureter with prescribed peristaltic contractions in absence of any obstruction, the second part compares the effect of varying obstructions (0, 5, 15, and 35%) in terms of spherical stones of different sizes. Pressure contours, velocity vectors, and profiles of pressure gradient magnitudes and wall shear stresses are presented along one bolus of the ureter, during contraction and expansion of the ureteral wall, in order to understand backflow, trapping and reflux phenomena, as well as monitor the health of the ureteral wall in the presence of any obstruction. The 35% ureteral obstruction case resulted in a significant backflow at the inlet in comparison to the other cases, and also a wall shear stress that was up to 20x larger than the case without any obstruction.

Earth Reentry Flow over a Phenolic Aeroshell in the X2 Expansion Tube

The effects of phenolic decomposition on shock layer radiation in air were investigated in the X2 expansion tube. By subjecting a carbon phenolic aeroshell to a flow with a velocity of , decomposing resin on the model surface was allowed to react with the boundary-layer gases. Emission spectroscopy was used to measure radiation emitted by the gas in parts of the ultraviolet, visible, and near-infrared spectrum and then compared with control measurements taken with a steel model. With the composite model in place, measured spectral radiance was seen to increase for CN violet band radiation but remained unchanged for atomic emissions in the visible and near-infrared spectrum. Recorded data were then compared with spectra produced by two-dimensional axisymmetric computational fluid dynamics simulations parsed to the NEQAIR radiation solver. Both the applied chemistry models overpredicted CN violet band radiation, but excellent comparison was found for the band. Good comparisons were also achieved for the O triplet (777 nm) and atomic N line intensities, between 740 and 750 nm. However, peak intensities were underestimated by the numerical simulations in the visible spectrum. Background radiation and broadening of the spectral wings were not accurately accounted for by simulations.

An Axisymmetric Computational Fluid Dynamics Approach to the Analysis of the Working Process of a Solar Stirling Engine

The use of computational fluid dynamics (CFD) models significantly extends the capabilities for the detailed analysis of the complex heat transfer and gas dynamic processes that occur in the internal gas circuit of a Stirling engine by more accurately predicting the engine’s performance. This accurate data on operational characteristics of the engine can then contribute to more precise calculations of the dimensions of a parabolic concentrator in a dish/Stirling engine installation. In this paper a successful axisymmetric CFD simulation of a solar “V”-type Stirling engine is described for the first time. The standard κ-ε turbulence model, with a moving mesh to reflect the reciprocating motion of the pistons, has been employed for the analysis of the engine’s working process. The gas temperature and pressure distributions and velocity fields in the internal gas circuit of the machine have been obtained and the pressure-volume diagrams have been calculated. Comparison of the numerical results produced from the axisymmetric CFD simulation of the engine’s working process with those computed with the use of second-order mathematical analysis shows that there are considerable differences. In particular, analysis of the data obtained indicates that the gas temperature in the compression space depends on the location in the cylinder for the given moment in the cycle and it may differ substantially from being harmonic in time.