Effects of Back-Pressure in a Lean Blowout Research Combustor

1993 ◽  
Vol 115 (3) ◽  
pp. 486-498 ◽  
Author(s):  
G. J. Sturgess ◽  
S. P. Heneghan ◽  
M. D. Vangsness ◽  
D. R. Ballal ◽  
A. L. Lesmerises ◽  
...  
Keyword(s):  
Temperature Profile ◽  
Recirculation Zone ◽  
Shear Layers ◽  
Experimental Information ◽  
Back Pressure ◽  
Gaseous Fuel ◽  
Lean Blowout ◽  
Recirculation Zones ◽  
Flame Behavior

Experimental information is presented on the effects of back-pressure on flame-holding in a gaseous fuel research combustor. Data for wall temperatures and static pressures are used to infer behavior of the major recirculation zones, as a supplement to some velocity and temperature profile measurements using LDV and CARS systems. Observations of flame behavior are also included. Lean blowout is improved by exit blockage, with strongest sensitivity at high combustor loadings. It is concluded that exit blockage exerts its influence through effects on the jet and recirculation zone shear layers.

Effects of Back-Pressure in a Lean Blowout Research Combustor

1992 ◽  
Author(s):  
G. J. Sturgess ◽  
S. P. Heneghan ◽  
M. D. Vangsness ◽  
D. R. Ballal ◽  
A. L. Lesmerises ◽  
...  
Keyword(s):  
Temperature Profile ◽  
Recirculation Zone ◽  
Shear Layers ◽  
Experimental Information ◽  
Back Pressure ◽  
Gaseous Fuel ◽  
Lean Blowout ◽  
Recirculation Zones ◽  
Flame Behavior

Experimental information is presented on the effects of back-pressure on flame-holding in a gaseous fuel research combustor. Data for wall temperatures and static pressures are used to infer behavior of the major recirculation zones, as a supplement to some velocity and temperature profile measurements using LDV and CARS systems. Observations of flame behavior are also included. Lean blowout is improved by exit blockage, with strongest sensitivity at high combustor loadings. It is concluded that exit blockage exerts its influence through effects on the jet and recirculation zone shear layers.

Flow recirculation zone length and shear rate are differentially affected by stenosis severity in human coronary arteries

2013 ◽  
Vol 304 (4) ◽  
pp. H559-H566 ◽  
Author(s):  
Ashkan Javadzadegan ◽  
Andy S. C. Yong ◽  
Michael Chang ◽  
Austin C. C. Ng ◽  
John Yiannikas ◽  
...  
Keyword(s):  
Shear Rate ◽  
Coronary Arteries ◽  
Recirculation Zone ◽  
Fluid Dynamic ◽  
Zone Length ◽  
Flow Recirculation ◽  
Stenosis Severity ◽  
Recirculation Zones ◽  
The Relationship

Flow recirculation zones and shear rate are associated with distinct pathogenic biological pathways relevant to thrombosis and atherogenesis. The interaction between stenosis severity and lesion eccentricity in determining the length of flow recirculation zones and peak shear rate in human coronary arteries in vivo is unclear. Computational fluid dynamic simulations were performed under resting and hyperemic conditions on computer-generated models and three-dimensional (3-D) reconstructions of coronary arteriograms of 25 patients. Boundary conditions for 3-D reconstructions simulations were obtained by direct measurements using a pressure-temperature sensor guidewire. In the computer-generated models, stenosis severity and lesion eccentricity were strongly associated with recirculation zone length and maximum shear rate. In the 3-D reconstructions, eccentricity increased recirculation zone length and shear rate when lesions of the same stenosis severity were compared. However, across the whole population of coronary lesions, eccentricity did not correlate with recirculation zone length or shear rate ( P = not signficant for both), whereas stenosis severity correlated strongly with both parameters ( r = 0.97, P < 0.001, and r = 0.96, P < 0.001, respectively). Nonlinear regression analyses demonstrated that the relationship between stenosis severity and peak shear was exponential, whereas the relationship between stenosis severity and recirculation zone length was sigmoidal, with an apparent threshold effect, demonstrating a steep increase in recirculation zone length between 40% and 60% diameter stenosis. Increasing stenosis severity and lesion eccentricity can both increase flow recirculation and shear rate in human coronary arteries. Flow recirculation is much more sensitive to mild changes in the severity of intermediate stenoses than is peak shear.

Numerical Investigation of Turbulence in Abdominal Aortic Aneurysms

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Digvijay S. Rawat ◽  
Mathieu Pourquie ◽  
Christian Poelma
Keyword(s):  
Vortex Ring ◽  
Recirculation Zone ◽  
Abdominal Aortic Aneurysms ◽  
Aortic Aneurysms ◽  
Transitional Flow ◽  
Recirculation Region ◽  
Abdominal Aortic ◽  
Recirculation Zones ◽  
Primary Focus ◽  
Small Change

Computational fluid dynamics (CFD) is a powerful method to investigate aneurysms. The primary focus of most investigations has been to compute various hemodynamic parameters to assess the risk posed by an aneurysm. Despite the occurrence of transitional flow in aneurysms, turbulence has not received much attention. In this article, we investigate turbulence in the context of abdominal aortic aneurysms (AAA). Since the clinical practice is to diagnose an AAA on the basis of its size, hypothetical axisymmetric geometries of various sizes are constructed. In general, just after the peak systole, a vortex ring is shed from the expansion region of an AAA. As the ring advects downstream, an azimuthal instability sets in and grows in amplitude thereby destabilizing the ring. The eventual breakdown of the vortex ring into smaller vortices leads to turbulent fluctuations. A residence time study is also done to identify blood recirculation zones, as a recirculation region can lead to degradation of the arterial wall. In some of the geometries simulated, the enhanced local mixing due to turbulence does not allow a recirculation zone to form, whereas in other geometries, turbulence had no effect on them. The location and consequence of a recirculation zone suggest that it could develop into an intraluminal thrombus (ILT). Finally, the possible impact of turbulence on the oscillatory shear index (OSI), a hemodynamic parameter, is explored. To conclude, this study highlights how a small change in the geometric aspects of an AAA can lead to a vastly different flow field.

Recirculation Flow of Nanofluid Through Periodic Nanotube with Hexagonal Cross-Section

2020 ◽  
Vol 12 (2) ◽  
pp. 147-155
Author(s):  
Md. Nazmul Hasan ◽  
Asif Mahmud ◽  
Md. Shakhaoath Khan ◽  
Md. Azmol Huda ◽  
Nazmul Islam
Keyword(s):  
Cross Section ◽  
Recirculation Zone ◽  
Geometric Parameters ◽  
Governing Equations ◽  
Throat Radius ◽  
Recirculation Flow ◽  
Hexagonal Cross Section ◽  
Recirculation Zones ◽  
Flow Through ◽  
Fortran 90

In this paper, we have investigated the recirculation flow of a nanofluid in the developed flow zones of an infinite periodic nanotube with a hexagonal cross-section. A numerical analysis is commenced to identify the existence of single and double recirculation zones for various geometric parameters. The boundary element method (BEM) has been formulated for an unbound interim nanotube for solving the governing equations. We have developed the codes for the BEM method in FORTRAN 90, and the graphs have been plotted in MATLAB 2016. We have various geometric parameters to inaugurate circumstances for the onset of recirculation. We have found recirculation flow through this type of periodic tube for a set of geometric parameters such as amplitude, wavelength and throat radius etc. Firstly recirculation flow in the expansion region of the nanotube is prophesied to arrive beyond a critical amplitude and second order recirculation zone is also predicted for still higher amplitudes. The recirculation flow has great importance in the application, for example, it can be used in the particle separation process.

The Influence of Premixed Combustion Flame Stabilizer Geometry on Flame Stability and Emissions

1981 ◽  
Vol 103 (4) ◽  
pp. 749-758 ◽  
Author(s):  
N. A. Al Dabbagh ◽  
G. E. Andrews
Keyword(s):  
Gas Turbines ◽  
Recirculation Zone ◽  
Combustion Efficiency ◽  
Premixed Combustion ◽  
Major Influence ◽  
Flame Stability ◽  
Zone Size ◽  
Recirculation Zones ◽  
Combustion Systems ◽  
Ultimate Objective

Premixed combustion systems for gas turbines offer the possibility of low-pollution, high-combustion efficiency and good temperature distribution. They form a basis by which other well-mixed combustion systems may be assessed. The ultimate objective of this work is the development of nonpremixed rapid mixing combustion systems. Different geometries of baffle flame stabilizers are tested to study the influence of recirculation zone size and number of recirculation zones on flame stability, combustion efficiency and NOx. The results show that the flame stabilizer geometry has a major influence on combustion efficiency and flame stability but a lesser influence on NOx. Optimum equivalence ratios are identified for good combustion efficiency and low NOx at simulated low and high-power engine conditions.

An Experimental Study of Effects of Confinement Ratio on Swirl Stabilized Flame Macrostructures

2017 ◽  
Author(s):  
Yiheng Tong ◽  
Mao Li ◽  
Marcus Thern ◽  
Jens Klingmann
Keyword(s):  
Flame Front ◽  
Gas Turbines ◽  
Recirculation Zone ◽  
Operating Conditions ◽  
Combustion Stability ◽  
Reconstruction Method ◽  
Lean Blowout ◽  
Image Reconstruction Method ◽  
Root Position ◽  
Industrial Gas Turbines

Swirl stabilized premixed flames are common in industrial gas turbines. The flame shape in the combustor is highly related to the combustion stability and the performance of the gas turbine. In the current paper, the effects of confinement on the time averaged flame structures or flame macrostructures are studied experimentally. Experiments are carried out with swirl number S = 0.66 in two cylindrical confinements with diameters of d1 = 39 mm and d2 = 64 mm and confinement ratio c1 = 0.148 and c2 = 0.0567. All the experiments were carried out in atmospheric. CH∗ chemiluminescence from the flame was recorded to visualize the flame behavior. An inverse Abel image reconstruction method was employed to better distinguish the flame macrostructures. Different mechanisms forming the time averaged M shape flames are proposed and analyzed. It is found that the confinement wall plays an important role in determining the flame macrostructures. The flow structures including the inner and outer recirculation zones formed in the confinement are revealed to be the main reasons that affects different flame macrostructures. Meanwhile, the alternation of flame shapes determines the flame stability characteristics. A smaller confinement diameter forced the flame front to bend upstream into the outer recirculation zone hence forming a M shape flame. A strong noise caused by the interaction of the flame front in the outer recirculation zone with the combustor wall was observed. Another unsteady behavior of the flame in the bigger combustor, which was caused by the alternation of the flame root position inside and outside the premixing tube, is also presented. The V shape flame in the two combustors radiated weaker chemiluminescence but the main heat release zone was elongated than the M shape flame. Other operating conditions, i.e. total mass flow rate of the air flow and the equivalence ratio also affect the flame macrostructures. The flame blowout limits were also altered under different test conditions. The bigger confinement has better performance in stabilizing the flame by having lower lean blowout limits.

scholarly journals Estimation on migration characteristics of leachate using analysis of hydraulic conductivity at bioreactor landfill

2019 ◽  
Vol 38 (1) ◽  
pp. 59-68
Author(s):  
Hyeong-Jin Choi ◽  
Yong Choi ◽  
Seung-Whee Rhee
Keyword(s):  
Hydraulic Conductivity ◽  
Moisture Content ◽  
Bulk Density ◽  
Recirculation Zone ◽  
Migration Time ◽  
Bioreactor Landfill ◽  
Leachate Recirculation ◽  
Factors Affecting ◽  
Recirculation Zones ◽  
The Relationship

In bioreactor landfill, moisture content is one of the most important factors affecting the migration of leachate. The migration characteristics of leachate in the Sudokwon landfill site was estimated by examining relationships among self-loading weight of waste (waste weight load), bulk density, hydraulic conductivity and moisture content of the landfilled waste. Experimental zones in the Sudokwon landfill were divided into leachate recirculation zones (3-C and 4-C zone) and reference zone (3-D zone). The volume of the leachate recirculation was 207 m3 d-1 for 12 months in the 3-C zone and 190 m3 d-1 for 3 months in the 4-C zone. A logarithmic graph can describe the relationship between waste weight load and bulk density by moisture content. However, both the relationship between bulk density and hydraulic conductivity and the relationship between hydraulic conductivity and waste weight load can be expressed by an exponential graph. Through these relationships, generalised equations for hydraulic conductivity were established using moisture content and waste weight load. The hydraulic conductivity calculated from the generalised equation in the leachate recirculation zone was estimated to be 6.27 × 10-4 cm s-1 in the 3-C zone and 4.43 × 10-4 cm s-1 in the 4-C zone. The migration time of leachate in the leachate recirculation zone was estimated to be 64.2 days in the 3-C zone and 94.5 days in the 4-C zone, respectively.

The local and global stability of confined planar wakes at intermediate Reynolds number

2011 ◽  
Vol 686 ◽  
pp. 218-238 ◽  
Author(s):  
M. P. Juniper ◽  
O. Tammisola ◽  
F. Lundell
Keyword(s):  
Recirculation Zone ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Flat Plates ◽  
Apparent Contradiction ◽  
Wake Flows ◽  
Flow Parameters ◽  
High Reynolds ◽  
Global And Local

AbstractAt high Reynolds numbers, wake flows become more globally unstable when they are confined within a duct or between two flat plates. At Reynolds numbers around 100, however, global analyses suggest that such flows become more stable when confined, while local analyses suggest that they become more unstable. The aim of this paper is to resolve this apparent contradiction by examining a set of obstacle-free wakes. In this theoretical and numerical study, we combine global and local stability analyses of planar wake flows at $\mathit{Re}= 100$ to determine the effect of confinement. We find that confinement acts in three ways: it modifies the length of the recirculation zone if one exists, it brings the boundary layers closer to the shear layers, and it can make the flow more locally absolutely unstable. Depending on the flow parameters, these effects work with or against each other to destabilize or stabilize the flow. In wake flows at $\mathit{Re}= 100$ with free-slip boundaries, flows are most globally unstable when the outer flows are 50 % wider than the half-width of the inner flow because the first and third effects work together. In wake flows at $\mathit{Re}= 100$ with no-slip boundaries, confinement has little overall effect when the flows are weakly confined because the first two effects work against the third. Confinement has a strong stabilizing effect, however, when the flows are strongly confined because all three effects work together. By combining local and global analyses, we have been able to isolate these three effects and resolve the apparent contradictions in previous work.

On Swirl Stabilized Flame Characteristics Near the Weak Extinction Limit

2010 ◽  
Author(s):  
Svetoslav Marinov ◽  
Matthias Kern ◽  
Klaus Merkle ◽  
Nikolaos Zarzalis ◽  
Antonio Peschiulli ◽  
...  
Keyword(s):  
Combustion Process ◽  
Flame Stabilization ◽  
The Other ◽  
Gaseous Fuel ◽  
Reacting Flow ◽  
Atmospheric Conditions ◽  
Stable Regime ◽  
Operation Conditions ◽  
Extinction Limit ◽  
Recirculation Zones

One of the most promising methods for reducing NOx emissions of jet engines is the lean combustion process. In order to realize this concept the percentage of air flowing through the combustor dome has to be drastically increased. This requirement leads to nozzles with high effective area and to high mean velocities in the primary zone of the combustor chamber. The investigation of the lean blow out limit for those nozzles is of main interest for the design of lean combustor technology. It is reported on investigation of a kerosene-fueled, swirl stabilized flame at atmospheric conditions. Two lean operation conditions are investigated, one in stable regime and the other very close to the weak extinction limit. It has been determined, that the flame shape changes when shifted from the stable regime to the other one close to the weak extinction limit (also referred to further as LBO — lean blowout). Since all field measurement schemes are similar, the gained data can be associated and conclusions regarding the flame stabilization at lean conditions can be drawn. The velocity data yields information about the topology of both isothermal and reacting flow fields in the combustion chamber. The internal recirculation and the corner recirculation zones can be well distinguished, because it can be measured directly in the nozzle exit plane. The comparison of the experimental data at stable and near LBO conditions shows the importance of inner and outer recirculation zones for the stabilization process. Furthermore, a comparison with a gaseous fuel nozzle will exhibit the differences between liquid and gaseous fuel combustion.

Flow Characteristics of a Servco Fume Cupboard

2013 ◽  
Vol 393 ◽  
pp. 753-758 ◽  
Author(s):  
Mohd Amal Asrol Omar ◽  
Wirachman Wisnoe ◽  
Azman Bakri
Keyword(s):  
Cross Sections ◽  
Recirculation Zone ◽  
Flow Characteristic ◽  
Cfd Simulation ◽  
Flow Characteristics ◽  
Shape Design ◽  
Breathing Zone ◽  
Working Chamber ◽  
Recirculation Zones ◽  
Computational Fluid Dynamics Cfd

A fume cupboard is equipment used to carry out chemical reaction process in its working chamber. A suction fan takes air or gas from the working chamber and releases it outside. When the air or gas is flowing from the inlet to the outlet, some recirculation zones may be formed depending on the internal shape design of the fume cupboard. This recirculation zone may create back flow that can be the cause of leakage. Leakage happens when airborne contaminants escape through inlet of the fume cupboard to the user breathing zone and the surrounding air in the room. To have a good fume cupboard, the recirculation zone needs to be minimised. In this paper, the flow characteristic of a Servco fume cupboard will be presented as a result of computational fluid dynamics (CFD) simulation using κ-ω turbulence model. The results are presented in terms of velocity components at different cross sections of the fume cupboard.

Sign in / Sign up

Export Citation Format

Share Document