Surface Radiation Effects on Flame Spread Over Thermally Thick Fuels in an Opposing Flow

1994 ◽  
Vol 116 (3) ◽  
pp. 646-651 ◽  
Author(s):  
J. West ◽  
S. Bhattacharjee ◽  
R. A. Altenkirch
Keyword(s):  
Flow Velocity ◽  
Gas Phase ◽  
Flame Spread ◽  
Radiation Effects ◽  
Solid Phase ◽  
Surface Radiation ◽  
Low Flow ◽  
Fuel Surface ◽  
Wide Range ◽  
Flow Velocities

A computational model of flame spread over a thermally thick solid fuel in an opposing-flow environment is presented. Unlike thermally thin fuels, for which the effect of fuel surface radiation is negligible for high levels of opposing flow, fuel surface radiation is important for thermally thick fuels for all flow levels. This result is shown to derive from the fact that the ratio of the rate of heat transfer by re-radiation from the surface to that by conduction from the gas to the solid is proportional to the length over which heat can be conducted forward of the flame to sustain spreading. For thin fuels, this length decreases with increasing flow velocity such that while radiation is important at low flow velocities it is not at the higher velocities. For thick fuels at low flow velocities, the conduction length is determined by gas-phase processes and decreases with increasing flow velocity. But at higher flow velocities, the conduction length is determined by solid-phase processes and is rather independent of the gas-phase flow. The result is that over a wide range of flow velocities, the conduction length of importance does not change substantially as it switches from one phase to another so that the ratio of radiation to conduction is of unit order throughout that wide range of flow.

scholarly journals Optical Micro-Wire Flow-Velocity Sensor

Sensors ◽  
2021 ◽  
Vol 21 (12) ◽  
pp. 4025
Author(s):  
Matej Njegovec ◽  
Simon Pevec ◽  
Denis Donlagic
Keyword(s):  
Response Time ◽  
Flow Velocity ◽  
Time Constant ◽  
Gas Flow ◽  
Low Flow ◽  
Velocity Sensor ◽  
Short Response Time ◽  
Wide Range ◽  
Cost Efficient ◽  
Flow Velocities

This paper presents a short response time, all-silica, gas-flow-velocity sensor. The active section of the sensor consists of a 16 µm diameter, highly optically absorbing micro-wire, which is heated remotely by a 980 nm light source. The heated microwire forms a Fabry–Perot interferometer whose temperature is observed at standard telecom wavelengths (1550 nm). The short response time of the sensor allows for different interrogation approaches. Direct measurement of the sensor’s thermal time constant allowed for flow-velocity measurements independent of the absolute heating power delivered to the sensor. This measurement approach also resulted in a simple and cost-efficient interrogation system, which utilized only a few telecom components. The sensor’s short response time, furthermore, allowed for dynamic flow sensing (including turbulence detection). The sensor’s bandwidth was measured experimentally and proved to be in the range of around 22 Hz at low flow velocities. Using time constant measurement, we achieved a flow-velocity resolution up to 0.006 m/s at lower flow velocities, while the resolution in the constant power configuration was better than 0.003 m/s at low flow velocities. The sensing system is constructed around standard telecommunication optoelectronic components, and thus suitable for a wide range of applications.

scholarly journals Relative impacts of the invasive Pacific oyster, Crassostrea gigas, over the native blue mussel, Mytilus edulis, are mediated by flow velocity and food concentration

NeoBiota ◽  
2019 ◽  
Vol 45 ◽  
pp. 19-37 ◽  
Author(s):  
Patrick W.S. Joyce ◽  
Louise Kregting ◽  
Jaimie T.A. Dick
Keyword(s):  
Flow Velocity ◽  
Pacific Oyster ◽  
Food Concentration ◽  
Ecological Impacts ◽  
Low Flow ◽  
Relative Impact ◽  
Clearance Rates ◽  
Mussel Beds ◽  
Food Concentrations ◽  
Flow Velocities

The ecological impacts of invasive species can be severe, but are generally viewed as highly unpredictable. Recent methods combining per capita feeding rates, population abundances and environmental contexts have shown great utility in predicting invader impacts. Here, clearance rates of the invasive Pacific oyster, Crassostreagigas, and native mussel, Mytilusedulis, were investigated in a laboratory experiment where oscillatory water flow and algal food concentrations were manipulated. Invasive oysters had lower clearance rates than native mussels in all experimental groups and did not differ among flow velocities or food concentrations. Native mussel clearance rates were higher at 5 cm s-1 compared to 0 and 15 cm s-1 flow velocities and increased with increasing food concentration. The Relative Impact Potential (RIP) metric was used to assess (i) the influence of flow velocity and food concentration on potential impacts of C.gigas on plankton resources and, (ii) the impacts of coexisting reefs, containing both species, on resources compared to monospecific native mussel beds. Greatest Relative Impact Potential of invasive oysters was seen at the lowest flow velocity, but became reduced with increasing flow velocity and food concentration. Relative Impact Potentials of coexisting reefs were generally greater than monospecific native mussel beds, with greatest impacts predicted at lowest flow velocity. We suggest that the greatest ecological impacts and competition potential of C.gigas will occur in areas with low flow velocity, but that increased flow will mediate co-existence between the two species.

scholarly journals Experimental Study on Hydroelectric Energy Harvester Based on a Hybrid Qiqi and Turbine Structure

Energies ◽  
2021 ◽  
Vol 14 (22) ◽  
pp. 7601
Author(s):  
Bin Bao ◽  
Quan Wang ◽  
Yufei Wu ◽  
Pengda Li
Keyword(s):  
Energy Harvesting ◽  
Flow Velocity ◽  
Water Flow ◽  
Energy Harvester ◽  
Hybrid Structure ◽  
High Flow ◽  
Low Flow ◽  
Flow Rates ◽  
Hydroelectric Energy ◽  
Wide Range

The Qiqi structure design can automatically upset and spill its content once it arrives at limit capacity under vertical water flow excitation. Considering this function, the Qiqi structure has been utilized for small hydroelectric energy harvesting lately. To investigate the tradeoff between the Qiqi structure and the turbine structure for small hydroelectric energy harvesting, an energy harvester based on a hybrid Qiqi and turbine structure is proposed for vertical water flow hydroelectric applications. The hybrid structure is composed of a rectangular Qiqi structure, with two blades inserted on both sides. Self-tipping function of the hybrid Qiqi structure and working principle of the structure is investigated in detail. The proposed structure has both the advantages of low flow velocity energy harvesting of the Qiqi structure and high flow velocity energy harvesting of the turbine structure. A hydroelectric energy harvesting application using the hybrid structure is given to demonstrate that the hybrid structure had a higher rotational speed than the Qiqi structure under vertical low water flow excitation and was able to work at relatively high flow rates. Thus, the investigated hybrid structure can help small rotational hydropower achieve better energy harvesting performance and work at wide-range flow rates under vertical ultra-low water flow applications. At 600 mL/min, 902 μJ of electrical energy was charged by the investigated structure, which is six times higher than that using the Qiqi structure alone.

Interaction of neutrophils and endothelium in isolated coronary venules and arterioles

1995 ◽  
Vol 268 (1) ◽  
pp. H490-H498 ◽  
Author(s):  
Y. Yuan ◽  
R. A. Mier ◽  
W. M. Chilian ◽  
D. C. Zawieja ◽  
H. J. Granger
Keyword(s):  
Flow Velocity ◽  
Shear Force ◽  
Low Flow ◽  
Microvascular Endothelium ◽  
Neutrophil Adherence ◽  
Flow Velocities

This study reports measurements of porcine neutrophil dynamics in isolated microvessels. Porcine coronary venules and arterioles were isolated, cannulated, and perfused with fluorescently labeled neutrophils at a series of flow velocities. In venules (62.50 +/- 5.41 microns diam) under control conditions, rolling neutrophils were often observed at intraluminal flow velocities ranging from 600 to 6,000 microns/s, and the rolling fraction varied inversely as a function of flow velocity. There was no significant adherence under the control conditions at any of the various flow velocities. Pretreatment of the neutrophils with human recombinant complement 5a (C5a, 10(-8) M) increased adherence at low flow velocities but did not alter the rolling fraction. In contrast to venules, rolling neutrophils were not observed in arterioles (58.80 +/- 5.6 microns diam). Furthermore, neutrophils that were pretreated with C5a did not adhere to the arteriolar endothelium even at low flow velocities. We suggest that 1) isolated microvessels perfused with fluorescently labeled neutrophils are suitable models for the study of the interaction between neutrophils and the microvascular endothelium, 2) shear force plays an important role in neutrophil rolling in coronary venules but is not the major factor that prevents neutrophil rolling and adherence in arterioles, and 3) C5a causes neutrophil adherence in venules but not in arterioles, indicating that different mechanisms underlie the interaction between neutrophils and endothelium in venules and arterioles.

Experimental Investigation of the Acoustic Power Around Two Tandem Cylinders

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
S. L. Finnegan ◽  
C. Meskell ◽  
S. Ziada
Keyword(s):  
Flow Field ◽  
Flow Velocity ◽  
Vortex Shedding ◽  
Acoustic Resonance ◽  
Low Flow ◽  
Interaction Mechanisms ◽  
Tandem Cylinders ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Flow Velocities

Aeroacoustic resonance of bluff bodies exposed to cross flow can be problematic for many different engineering applications and knowledge of the location and interaction of acoustic sources is not well understood. Thus, an empirical investigation of the acoustically coupled flow around two tandem cylinders under two different resonant conditions is presented. It is assumed that the resonant acoustic field could be decoupled from the hydrodynamic flow field, resolved separately, and then recoupled to predict the flow/sound interaction mechanisms using Howe's theory of aerodynamic sound. Particle image velocimetry was employed to resolve the phase-averaged flow field characteristics around the cylinders at various phases in an acoustic wave cycle. It was found that the vortex shedding patterns of the two resonant conditions exhibit substantial differences. For the first condition, which occurred at low flow velocities where the natural vortex shedding frequency was below the acoustic resonance frequency, fully developed vortices formed in both the gap region between the cylinders and in the wake. These vortices were found to be in phase with each other. For the second resonant condition, which occurred at higher flow velocities where the natural vortex shedding frequency was above the acoustic resonant frequency, fully developed vortices only formed in the wake and shedding from the two cylinders were not in phase. These differences in the flow field resulted in substantial variation in the flow-acoustic interaction mechanisms between the two resonant conditions. Corresponding patterns of the net acoustic energy suggest that acoustic resonance at the lower flow velocity is due to a combination of shear layer instability in the gap and vortex shedding in the wake, while acoustic resonance at the higher flow velocity is driven by the vortex shedding in the wake of the two cylinders.

Pressure Drop of Accelerating Slug Flow in Microchannels: Modeling and Experiment

2010 ◽  
Author(s):  
Shahnawaz Molla ◽  
Dmitry Eskin ◽  
Farshid Mostowfi
Keyword(s):  
Pressure Drop ◽  
Pressure Gradient ◽  
Flow Velocity ◽  
Gas Phase ◽  
Void Fraction ◽  
Slug Flow ◽  
Gas Bubbles ◽  
Liquid Slug ◽  
Local Pressure ◽  
Wide Range

An investigation on the pressure drop of a gas-liquid slug flow through a long microchannel of rectangular cross-section is presented. A constant pressure gradient in the microchannel was observed in a flow where gas bubbles progressively expanded and the flow velocity increased due to significant pressure drop. In contrast to majority of the earlier studies of slug flow in microchannels, where void fraction was nearly constant throughout the channel, we investigated systems where the volume of the gas phase increased significantly due to large pressure drop (up to 2000 kPa) along the lengthy (∼1 m) channel. This expansion of the gas phase led to a significant increase in the void fraction, causing considerable increase in flow velocity. Local pressure was measured along the channel using a series of embedded membranes acting as pressure sensors. The axial pressure profile for a gas-liquid system, namely, Dodecane/Nitrogen was studied. Our investigation on pressure gradient showed linear trend over a wide range of void fractions (30–90%) and flow conditions in the two-phase flow. The lengths and the velocities of the liquid slugs and the gas bubbles were also studied along the microchannel by employing video imaging technique. Furthermore, a model describing the gas-liquid slug flow in long microchannels was developed. Excellent agreement between the developed model and the experimental data was obtained.

Effect of flow velocity on phosphate release from sediment

1994 ◽  
Vol 30 (10) ◽  
pp. 263-272 ◽  
Author(s):  
Yoshiyuki Nakamura
Keyword(s):  
Flow Velocity ◽  
Release Rate ◽  
Bulk Water ◽  
Low Flow ◽  
Limiting Factor ◽  
Phosphate Release ◽  
Release Flux ◽  
Diffusive Boundary ◽  
Do Concentration ◽  
Flow Velocities

A mathematical model of phosphate release rate from sediment, fp, is presented which determines the fp as a function of flow velocity over the sediment and dissolved oxygen concentration. Oxygen consumption in the sediment is expressed as the sum of chemical consumption due to ferrous iron oxygenation and the bacterial consumption which is assumed to be a first order reaction of oxygen. At very low flow velocities, transport through the diffusive boundary layer is the limiting factor of SOD, and phosphate release rate is expressed as a linear decreasing function of the velocity. When flow velocities are increased, both SOD and phosphate release rate become independent of velocity, since the reactions in the sediment are the rate limiting factor. The model suggests that phosphate release flux is a linear decreasing function of DO in the bulk water, while SOD is an increasing function of DO concentration. The critical DO concentration at which the phosphate release ceases is expressed in terms of the flow velocity. The prediction of SOD and ϕp by the present model is favourably compared with experiments by former researchers.

scholarly journals Three different glacier surges at a spot: What satellites observe and what not

2021 ◽  
Author(s):  
Frank Paul ◽  
Livia Piermattei ◽  
Désirée Treichler ◽  
Lin Gilbert ◽  
Luc Girod ◽  
...  
Keyword(s):  
Short Pulse ◽  
Linear Increase ◽  
Low Flow ◽  
Wide Range ◽  
Satellite Sensors ◽  
Glacier Surges ◽  
Radar Satellite ◽  
Velocity Changes ◽  
Steep Terrain ◽  
Flow Velocities

Abstract. In the Karakoram, dozens of glacier surges occurred in the past two decades, making the region one of its global hot spots. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over one or two years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same region but differ strongly in surge behaviour. A full suite of satellite sensors and digital elevation models (DEMs) from different sources are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km y−1, velocities up to 30 m d−1 and surface elevations increased by 200 m. In contrast, the neighbouring and three times smaller North Chongtar Glacier had a slow and near linear increase of advance rates (up to 500 m y−1), flow velocities below 1 m d−1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km y−1. It seems that, despite a similar climatic setting, different surge mechanisms are at play and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges and elevations from ICESat-2 ATL06 data fit neatly. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes.

Intrinsic Stacking Interactions of Natural and Artificial Nucleobases

2019 ◽  
Author(s):  
Drew P. Harding ◽  
Laura J. Kingsley ◽  
Glen Spraggon ◽  
Steven Wheeler
Keyword(s):  
Gas Phase ◽  
Electrostatic Interactions ◽  
Density Functional ◽  
Molecular Descriptors ◽  
Stacking Interactions ◽  
Interaction Energies ◽  
Functional Theory ◽  
Binding Partner ◽  
Heavy Atoms ◽  
Wide Range

The intrinsic (gas-phase) stacking energies of natural and artificial nucleobases were explored using density functional theory (DFT) and correlated ab initio methods. Ranking the stacking strength of natural nucleobase dimers revealed a preference in binding partner similar to that seen from experiments, namely G > C > A > T > U. Decomposition of these interaction energies using symmetry-adapted perturbation theory (SAPT) showed that these dispersion dominated interactions are modulated by electrostatics. Artificial nucleobases showed a similar stacking preference for natural nucleobases and were also modulated by electrostatic interactions. A robust predictive multivariate model was developed that quantitively predicts the maximum stacking interaction between natural and a wide range of artificial nucleobases using molecular descriptors based on computed electrostatic potentials (ESPs) and the number of heavy atoms. This model should find utility in designing artificial nucleobase analogs that exhibit stacking interactions comparable to those of natural nucleobases. Further analysis of the descriptors in this model unveil the origin of superior stacking abilities of certain nucleobases, including cytosine and guanine.

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