scholarly journals Effects of Mean Inflow Velocity and Droplet Diameter on the Propagation of Turbulent V-Shaped Flames in Droplet-Laden Mixtures

Fluids ◽  
2020 ◽  
Vol 6 (1) ◽  
pp. 1
Author(s):  
Gulcan Ozel Erol ◽  
Nilanjan Chakraborty
Keyword(s):  
Burning Velocity ◽  
Droplet Diameter ◽  
Gaseous Mixture ◽  
Mean Flow ◽  
Mean Values ◽  
Inflow Velocity ◽  
The Mean ◽  
Turbulent Burning Velocity ◽  
Mean Flow Velocity ◽  
Flame Brush

Three-dimensional carrier phase Direct Numerical Simulations of V-shaped n-heptane spray flames have been performed for different initially mono-sized droplet diameters to investigate the influence of mean flow velocity on the burning rate and flame structure at different axial locations from the flame holder. The fuel is supplied as liquid droplets through the inlet and an overall (i.e., liquid + gaseous) equivalence ratio of unity is retained in the unburned gas. Additionally, turbulent premixed stoichiometric V-shaped n-heptane flames under the same turbulent flow conditions have been simulated to distinguish the differences in combustion behaviour of the pure gaseous phase premixed combustion in comparison to the corresponding behaviour in the presence of liquid n-heptane droplets. It has been found that reacting gaseous mixture burns predominantly under fuel-lean mode and the availability of having fuel-lean mixture increases with increasing mean flow velocity. The extent of flame wrinkling for droplet cases has been found to be greater than the corresponding gaseous premixed flames due to flame-droplet-interaction, which is manifested by dimples on the flame surface, and this trend strengthens with increasing droplet diameter. As the residence time of the droplets within the flame decreases with increasing mean inflow velocity, the droplets can survive for larger axial distances before the completion of their evaporation for the cases with higher mean inflow velocity and this leads to greater extents of flame-droplet interaction and droplet-induced flame wrinkling. Mean inflow velocity, droplet diameter and the axial distance affect the flame brush thickness. The flame brush thickens with increasing droplet diameter for the cases with higher mean inflow velocity due to the predominance of fuel-lean gaseous mixture within the flame. However, an opposite behaviour has been observed for the cases with lower mean inflow velocity where the smaller extent of flame wrinkling due to smaller values of integral length scale to flame thickness ratio arising from higher likelihood of fuel-lean combustion for larger droplets dominates over the thickening of the flame front. It has been found that the major part of the heat release arises due to premixed mode of combustion for all cases but the contribution of non-premixed mode of combustion to the total heat release has been found to increase with increasing mean inflow velocity and droplet diameter. The increase in the mean inflow velocity yields an increase in the mean values of consumption and density-weighted displacement speed for the droplet cases but leads to a decrease in turbulent burning velocity. By contrast, an increase in droplet diameter gives rise to decreases in turbulent burning velocity, and the mean values of consumption and density-weighted displacement speeds. Detailed physical explanations have been provided to explain the observed mean inflow velocity and droplet diameter dependences of the flame propagation behaviour.

Experimental Investigation of the Acoustic Reflection Coefficient of a Modeled Gas Turbine Impingement Cooling Section

2012 ◽  
Author(s):  
Christoph Jörg ◽  
Michael Wagner ◽  
Thomas Sattelmayer
Keyword(s):  
Reflection Coefficient ◽  
Flow Velocity ◽  
Gas Turbines ◽  
Acoustic Energy ◽  
Quality Data ◽  
Mean Flow ◽  
Impingement Cooling ◽  
Acoustic Reflection ◽  
The Mean ◽  
Mean Flow Velocity

The thermoacoustic stability of gas turbines depends on a balance of acoustic energy inside the engine. While the flames produce acoustic energy, other areas like the impingement cooling system contribute to damping. In this paper, we investigate the damping potential of an annular impingement sleeve geometry embedded into a realistic environment. A cold flow test rig was designed to represent real engine conditions in terms of geometry, and flow situation. High quality data was delivered by six piezoelectric dynamic pressure sensors. Experiments were carried out for different mean flow velocities through the cooling holes. The acoustic reflection coefficient of the impingement sleeve was evaluated at a downstream reference location. Further parameters investigated were the number of cooling holes, and the geometry of the chamber surrounding the impingement sleeve. Experimental results show that the determining parameter for the reflection coefficient is the mean flow velocity through the impingement holes. An increase of the mean flow velocity leads to significantly increased damping, and to low values of the reflection coefficient.

The nature of saltation and of ‘bed-load’ transport in water

1973 ◽  
Vol 332 (1591) ◽  
pp. 473-504 ◽  
Keyword(s):  
Flow Velocity ◽  
Bed Load ◽  
Flow Depth ◽  
Mean Flow ◽  
Stream Power ◽  
Fluid Turbulence ◽  
Load Transport ◽  
Bed Load Transport ◽  
The Mean ◽  
Mean Flow Velocity

Owing to observational difficulties the distinction between a ‘suspended’ load of solids transported by a stream and a ‘ bed-load ’ has long remained undefined. Recently, however, certain critical experiments have thrown much light on the nature of bed-load transport. In particular, it has been shown that bed-load transport, by saltation, occurs in the absence of fluid turbulence and must therefore be due to a separate dynamic process from that of transport in suspension by the internal eddy motion of a turbulent fluid. It has been further shown that the forward motion of saltating solids is opposed by a frictional force of the same order as the immersed weight of the solids, the friction coefficient approximating to that given by the angle of slip. The maintenance of steady motion therefore requires a predictable rate of energy dissipation by the transporting fluid. The fluid thrust necessary to maintain the motion is shown to be exerted by virtue of a mean slip velocity which is predictable in the same way as, and approxim ates to, the terminal fall velocity of the solid. The mean thrust, and therefore the transport rate of saltating solids, are therefore predictable in terms of the fluid velocity close to the bed, at a distance from it, within the saltation zone, of a ‘centre of fluid thrust’ analogous to the ‘centre of pressure’. This velocity, which is not directly measurable in water streams, can be got from a knowledge of stream depth and mean flow velocity. Thus a basic energy equation is obtained relating the rate of transporting work done to available fluid transporting power. This is shown to be applicable to the transport both of wind-blown sand, and of water-driven solids of all sizes and larger than that of medium sand. Though the mean flow velocity is itself unpredictable, the total stream power, which is the product of this quantity times the bed shear stress, is readily measurable. But since the mean flow velocity is an increasing function of flow depth, the transport of solids expressed in terms of total stream power must decrease with increasing flow depth/grain size ratio. This considerable variation with flow depth has not been previously recognised. It explains the gross inconsistencies found in the existing experimental data. The theoretical variation is shown to approximate very closely to that found in recent critical experiments in which transport rates were measured at different constant flow depths. The theory, which is largely confirmed by these and other earlier experiments, indicates that suspension by fluid turbulence of mineral solids larger than those of medium sands does not become appreciable until the bed shear stress is increased to a value exceeding 12 times its threshold value for the bed material considered. This range of unsuspended transport decreases rapidly, however, as the grain size is reduced till, at a certain critical size, suspension should occur at the threshold of bed movement.

scholarly journals Three-dimensional spatial structure of the macro-pores and flow simulation in anthracite coal based on X-ray μ-CT scanning data

2020 ◽  
Vol 17 (5) ◽  
pp. 1221-1236
Author(s):  
Hui-Huang Fang ◽  
Shu-Xun Sang ◽  
Shi-Qi Liu
Keyword(s):  
Spatial Structure ◽  
Flow Velocity ◽  
Single Channel ◽  
Three Dimensional ◽  
Bedding Plane ◽  
Mean Flow ◽  
Equivalent Radius ◽  
Flow Pressure ◽  
The Mean ◽  
Mean Flow Velocity

Abstract The three-dimensional (3D) structures of pores directly affect the CH4 flow. Therefore, it is very important to analyze the 3D spatial structure of pores and to simulate the CH4 flow with the connected pores as the carrier. The result shows that the equivalent radius of pores and throats are 1–16 μm and 1.03–8.9 μm, respectively, and the throat length is 3.28–231.25 μm. The coordination number of pores concentrates around three, and the intersection point between the connectivity function and the X-axis is 3–4 μm, which indicate the macro-pores have good connectivity. During the single-channel flow, the pressure decreases along the direction of CH4 flow, and the flow velocity of CH4 decreases from the pore center to the wall. Under the dual-channel and the multi-channel flows, the pressure also decreases along the CH4 flow direction, while the velocity increases. The mean flow pressure gradually decreases with the increase of the distance from the inlet slice. The change of mean flow pressure is relatively stable in the direction horizontal to the bedding plane, while it is relatively large in the direction perpendicular to the bedding plane. The mean flow velocity in the direction horizontal to the bedding plane (Y-axis) is the largest, followed by that in the direction horizontal to the bedding plane (X-axis), and the mean flow velocity in the direction perpendicular to the bedding plane is the smallest.

Alteration of intraaneurysmal hemodynamics by placement of a self-expandable stent

2009 ◽  
Vol 111 (1) ◽  
pp. 22-27 ◽  
Author(s):  
Satoshi Tateshima ◽  
Kazuo Tanishita ◽  
Yasuhiro Hakata ◽  
Shin-ya Tanoue ◽  
Fernando Viñuela
Keyword(s):  
Flow Velocity ◽  
Vortex Flow ◽  
Patient Specific ◽  
Mean Flow ◽  
Single Vortex ◽  
Clinical Images ◽  
The Mean ◽  
Fast Flow ◽  
Mean Flow Velocity ◽  
Flow Stream

Object Development of a flexible self-expanding stent system and stent-assisted coiling technique facilitates endovascular treatment of wide-necked brain aneurysms. The hemodynamic effect of self-expandable stent placement across the neck of a brain aneurysm has not been well documented in patient-specific aneurysm models. Methods Three patient-specific silicone aneurysm models based on clinical images were used in this study. Model 1 was constructed from a wide-necked internal carotid artery–ophthalmic artery aneurysm, and Models 2 and 3 were constructed from small wide-necked middle cerebral artery aneurysms. Neuroform stents were placed in the in vitro aneurysm models, and flow structures were compared before and after the stent placements. Flow velocity fields were acquired with particle imaging velocimetry. Results In Model 1, a clockwise, single-vortex flow pattern was observed in the aneurysm dome before stenting was performed. There were multiple vortices, and a very small fast flow stream was newly formed in the aneurysm dome after stenting. The mean intraaneurysmal flow velocity was reduced by ~ 23–40%. In Model 2, there was a clockwise vortex flow in the aneurysm dome and another small counterclockwise vortex in the tip of the aneurysm dome before stenting. The small vortex area disappeared after stenting, and the mean flow velocity in the aneurysm dome was reduced by 43–64%. In Model 3, a large, counterclockwise, single vortex was seen in the aneurysm dome before stenting. Multiple small vortices appeared in the aneurysm dome after stenting, and the mean flow velocity became slower by 22–51%. Conclusions The flexible self-expandable stents significantly altered flow velocity and also flow structure in these aneurysms. Overall flow alterations by the stent appeared favorable for the long-term durability of aneurysm embolization. The possibility that the placement of a low-profile self-expandable stent might induce unfavorable flow patterns such as a fast flow stream in the aneurysm dome cannot be excluded.

Effects of Pipeline Elevation Changes on Optimum Expelling Procedures for Gas Pipelines

2016 ◽  
Author(s):  
K. K. Botros ◽  
C. Edwards ◽  
B. Watson ◽  
T. Thrall
Keyword(s):  
Mean Flow ◽  
Pipeline Section ◽  
Buoyancy Driven Flows ◽  
Elevation Changes ◽  
The Mean ◽  
Normal Spacing ◽  
Air Ingress ◽  
Combustible Gases ◽  
The Cost ◽  
Mean Flow Velocity

Expeller performance has been evaluated in terms of the capability to create suction pressure at the throat. This formulation has been used to assess the effectiveness of evacuating combustible gases from an isolated, depressurized, pipeline section involving mainline block valves up to two times normal spacing with an intermediate vent stack. Additionally, the effects of elevation changes that promote buoyancy driven flows are accounted for in time as the interface between air and gas travels along the pipeline section during expelling. Two expelling strategies were introduced and assessed. These are simultaneous expelling, in which gas is expelled from the pipeline section from both ends, and sequential expelling, in which an intermediate vent stack is used to expel gas from the upstream and downstream segments. The effects of elevation changes and the location of the intermediate vent stack determine the best strategy for expelling so as to maximize the purge velocity in the section of a pipeline to be purged, while maintaining the mean flow velocity in the pipe above the minimum purge velocity to prevent air-gas stratification. It was found that for a ‘Flat-’ or a ‘Cusp-type’ elevation profile it is advantageous to follow a sequential expelling procedure using one expeller at the intermediate vent stack location. In the case of a ‘Vee-type’ elevation profile, a simultaneous expelling procedure is a better option in terms of expelling time, at the cost of needing to deploy two expellers to different sites quite far apart. Air ingress location depends on the expelling strategy and elevation profile.

An Experimental Study on Properties of Local Burning Velocity for Hydrogen Added Hydrocarbon Premixed Turbulent Flames

2011 ◽  
Author(s):  
Masaya Nakahara ◽  
Koichi Murakami ◽  
Jun Hashimoto ◽  
Atsushi Ishihara
Keyword(s):  
Burning Velocity ◽  
Quantitative Relationship ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Laminar Flames ◽  
Mean Values ◽  
Laser Tomography ◽  
Turbulence Condition ◽  
Turbulent Burning Velocity ◽  
Local Burning

This study is performed to investigate directly the local flame properties of turbulent propagating flames at the same weak turbulence condition (u′/SL0 = 1.4), in order to clarify basically the influence of the addition of hydrogen to methane or propane mixtures on its local burning velocity. The mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δH are prepared. A two-dimensional sequential laser tomography technique is used to obtain the relationship between the flame shape and the flame displacement. The local flame displacement velocity SF is quantitatively obtained as the key parameters of the turbulent combustion. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effects of positive stretch and curvature on burning velocity. It was found that the trends of the mean values of measured SF with respect to δH, the total equivalence ratio Φ and fuel types corresponded well its turbulent burning velocity. The trend of the obtained Ma could explain the local burning velocity of turbulent flames only qualitatively. Based on the Ma, the local burning velocity at the part of turbulent flames with positive stretch and curvature, SLt, is estimated quantitatively. As a result, a quantitative relationship between the estimated SLt and the SF at positive stretch and curvature of turbulent flames could be observed for mixtures with increasing the Lewis number.

Dissipation in Small Scale Gaseous Flows

2003 ◽  
Vol 125 (5) ◽  
pp. 944-947 ◽  
Author(s):  
Nicolas G. Hadjiconstantinou
Keyword(s):  
Slip Flow ◽  
Small Scale ◽  
Mean Flow ◽  
Slip Effects ◽  
Gaseous Flows ◽  
Constant Wall Heat Flux ◽  
Gravity Driven ◽  
The Mean ◽  
Slip Flow Regime ◽  
Mean Flow Velocity

We discuss the effect of shear work at solid boundaries in small scale gaseous flows where slip effects are present. The effect of shear work at the boundary on convective heat transfer is illustrated through solution of the constant-wall-heat-flux problem in the slip-flow regime. We also present predictions for the dissipation in terms of the mean flow velocity in pressure-driven and gravity-driven Poiseuille flows for arbitrary Knudsen numbers. All results are verified using direct Monte Carlo solutions of the Boltzmann equation.

A Machine Learning Approach for the Mean Flow Velocity Prediction in Alluvial Channels

2015 ◽  
Vol 29 (12) ◽  
pp. 4379-4395 ◽  
Author(s):  
Vasileios Kitsikoudis ◽  
Epaminondas Sidiropoulos ◽  
Lazaros Iliadis ◽  
Vlassios Hrissanthou
Keyword(s):  
Machine Learning ◽  
Flow Velocity ◽  
Learning Approach ◽  
Alluvial Channels ◽  
Mean Flow ◽  
Machine Learning Approach ◽  
Velocity Prediction ◽  
The Mean ◽  
Mean Flow Velocity
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