Theoretical Analysis of Flame Propagation in Meso and Microscale Channels

2003 ◽  
Author(s):  
Yiguang Ju
Keyword(s):  
Flow Velocity ◽  
Heat Loss ◽  
Flame Propagation ◽  
Critical Value ◽  
Flow Speed ◽  
External Heat ◽  
Channel Width ◽  
Recent Experimental Data ◽  
Quenching Distance ◽  
Large Flow

Extinction and flame propagation in a meso and microscale channels are investigated analytically. Emphasis was paid to the coupling of wall heat loss, wall preheating, external heat loss and chemical reaction. The results showed that, wall thermal properties, channel width and flow velocity have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease of channel width, flame reaches its first quenching limit, the so called critical quenching distance. However, with a further decrease of channel width, the results show that there exists a slow burning flame. With the increase of wall heat loss the speed of the slow burning flame slightly decreases and eventually reaches its second burning limit. With the change of the flow velocity, the results show that sub-limit flame can only exist at flow velocity larger than a critical value. At moderate flow velocity, flame speed increases with the increase of flow speed. At very large flow velocity, flame will be blown off. The above results are confirmed from the recent experimental data.

Effects of Shield Gas Flow on Meltpool Variability and Signature in Scanned Laser Melting

2020 ◽  
Author(s):  
David C. Deisenroth ◽  
Jorge Neira ◽  
Jordan Weaver ◽  
Ho Yeung
Keyword(s):  
Additive Manufacturing ◽  
Aspect Ratio ◽  
Flow Velocity ◽  
Process Monitoring ◽  
Gas Flow ◽  
Laser Energy ◽  
Flow Speed ◽  
Powder Bed ◽  
Flow Speeds ◽  
Gas Flow Velocity

Abstract In laser powder bed fusion metal additive manufacturing, insufficient shield gas flow allows accumulation of condensate and ejecta above the build plane and in the beam path. These process byproducts are associated with beam obstruction, attenuation, and thermal lensing, which then lead to lack of fusion and other defects. Furthermore, lack of gas flow can allow excessive amounts of ejecta to redeposit onto the build surface or powder bed, causing further part defects. The current investigation was a preliminary study on how gas flow velocity and direction affect laser delivery to a bare substrate of Nickel Alloy 625 (IN625) in the National Institute of Standards and Technology (NIST) Additive Manufacturing Metrology Testbed (AMMT). Melt tracks were formed under several gas flow speeds, gas flow directions, and energy densities. The tracks were then cross-sectioned and measured. The melt track aspect ratio and aspect ratio coefficient of variation (CV) were reported as a function of gas flow speed and direction. It was found that a mean gas flow velocity of 6.7 m/s from a nozzle 6.35 mm in diameter was sufficient to reduce meltpool aspect ratio CV to less than 15 %. Real-time inline hotspot area and its CV were evaluated as a process monitoring signature for identifying poor laser delivery due to inadequate gas flow. It was found that inline hotspot size could be used to distinguish between conduction mode and transition mode processes, but became diminishingly sensitive as applied laser energy density increased toward keyhole mode. Increased hotspot size CV (associated with inadequate gas flow) was associated with an increased meltpool aspect ratio CV. Finally, it was found that use of the inline hotspot CV showed a bias toward higher CV values when the laser was scanned nominally toward the gas flow, which indicates that this bias must be considered in order to use hotspot area CV as a process monitoring signature. This study concludes that gas flow speed and direction have important ramifications for both laser delivery and process monitoring.

scholarly journals Study on the Formation Law of the Freezing Temperature Field of Freezing Shaft Sinking under the Action of Large-Flow-Rate Groundwater

2019 ◽  
Vol 2019 ◽  
pp. 1-20 ◽  
Author(s):  
Bin Wang ◽  
Chuan-xin Rong ◽  
Jian Lin ◽  
Hua Cheng ◽  
Hai-bing Cai
Keyword(s):  
Temperature Field ◽  
Groundwater Flow ◽  
Flow Velocity ◽  
Flow Rate ◽  
Freezing Temperature ◽  
Freezing Process ◽  
Flow Rates ◽  
Porosity Reduction ◽  
Shaft Sinking ◽  
Large Flow

Taking into account moisture migration and heat change during the soil freezing process, as well as the influence of absolute porosity reduction on seepage during the freezing process, we construct a numerical model of hydrothermal coupling using laws of conservation of energy and mass. The model is verified by the results of large-scale laboratory tests. By applying the numerical calculation model to the formation of artificial shaft freezing temperature fields under the action of large-flow groundwater, we conclude that groundwater with flow rates of less than 5 m/d will not have a significant impact on the artificial freezing temperature field. The maximum flow rates that can be handled by single-row freezing pipes and double-row freezing pipes are 10 m/d and 20 m/d, respectively, during the process of freezing shaft sinking. By analyzing the variation of groundwater flow rate during freezing process, we find that the groundwater flow velocity can reach 5–7 times the initial flow velocity near the closure moment of the frozen wall. Finally, in light of the action characteristics of groundwater on the freezing temperature field, we make suggestions for optimal pipe and row spacing in freezing pipe arrangement.

Dynamics of capillary-driven liquid–liquid displacement in open microchannels

2014 ◽  
Vol 16 (44) ◽  
pp. 24473-24478 ◽  
Author(s):  
D. Yang ◽  
M. Krasowska ◽  
C. Priest ◽  
J. Ralston
Keyword(s):  
Flow Velocity ◽  
Channel Width ◽  
Liquid Front ◽  
Liquid Displacement

For capillary-driven liquid–liquid displacement in rectangular open microchannels, the square of the position of the liquid–liquid front increases linearly with time, whereas the flow velocity decreases with increasing channel width.

scholarly journals Thermal Performance of Solar Collectors with EPDM Absorber Plates

1988 ◽  
Vol 41 (4) ◽  
pp. 623
Author(s):  
MJ O'Keefe ◽  
JLA Francey
Keyword(s):  
Experimental Study ◽  
Thermal Resistance ◽  
Flat Plate ◽  
Heat Loss ◽  
Thermal Performance ◽  
Critical Value ◽  
Solar Collectors ◽  
Flow Rates ◽  
Mass Flow Rates ◽  
High Thermal Resistance

An experimental study of flat-plate solar collectors using ethylene, propylenediene monomer (EPDM) absorber plates is descn"bed. In spite of the high thermal resistance of this material the performance is found to compare well with metal absorbers and to be in agreement with the Hottel-Whillier-Bliss equation. There is, however, an observed increase in the heat loss coefficient for mass flow rates below a critical value.

Numerical Study on the Migration of Two Spheres in Upward Pipe Flow

2018 ◽  
Author(s):  
Lei Liu ◽  
Haining Lu ◽  
Jianmin Yang ◽  
Xinliang Tian ◽  
Tao Peng ◽  
...  
Keyword(s):  
Flow Velocity ◽  
Poiseuille Flow ◽  
Pipe Flow ◽  
Deep Sea ◽  
Numerical Study ◽  
Critical Value ◽  
Dynamic Responses ◽  
Phase Flow ◽  
Two Phase ◽  
Offshore Engineering

Migration of particles in pipe flow is commonly seen in offshore engineering, such as vertical transport of ores in deep sea mining. As the basis of the investigation on fluid-particle two-phase flow, the interaction of two spheres in upward pipe flow is studied by direct numerical simulations in this paper. The pipe flow is set as Poiseuille flow; the Reynolds number is no more than 1250. The dynamic responses of the spheres and the flow pattern are analyzed at different flow velocity. When compared to the sedimentation of two spheres in quiescent flow, the trailing sphere in Poiseuille flow will never surpass the leading one in Poiseuille flow. As the flow velocity increases in the pipe, the spheres are easier to separate after collision. When the flow velocity exceeds a critical value, the spheres will never collide.

scholarly journals Basal roughness of the East Antarctic Ice Sheet in relation to flow speed and basal thermal state

2020 ◽  
Vol 61 (81) ◽  
pp. 162-175
Author(s):  
Olaf Eisen ◽  
Anna Winter ◽  
Daniel Steinhage ◽  
Thomas Kleiner ◽  
Angelika Humbert
Keyword(s):  
Flow Velocity ◽  
Thermal State ◽  
Ice Sheet ◽  
Flow Speed ◽  
Surface Flow ◽  
Roughness Parameters ◽  
Antarctic Ice Sheet ◽  
Flow Models ◽  
Radio Echo Sounding ◽  
East Antarctic Ice Sheet

AbstractBasal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.

Fluid-Elastic Instabilities of Clamped-Clamped Aligned and Inclined Cylinders in Turbulent Axial Flow

2015 ◽  
Author(s):  
Jeroen De Ridder ◽  
Joris Degroote ◽  
Olivier Doaré
Keyword(s):  
Flow Velocity ◽  
Reactor Core ◽  
Axial Flow ◽  
Flow Speed ◽  
Experimental Results ◽  
Flexible Cylinder ◽  
Damping Force ◽  
Fluid Forces ◽  
Elastic Instabilities ◽  
Analytical Approaches

Fluid-elastic instabilities arise due to the coupling of structural motion and fluid flow. In the specific case of a clamped-clamped cylinder in axial flow, it will buckle at a sufficiently high flow velocity and start to flutter at even higher flow velocities. This dynamic behavior is of importance to nuclear reactor core design, undersea pipe lines and devices for energy harvesting. In this contribution, the fluid forces and the dynamics of a flexible clamped-clamped cylinder in turbulent axial flow are computed numerically. In contrast to present analytical approaches, this numerical model does not require to tune parameters for each specific case or to obtain coefficients from experiments. To provide insight in the way viscous fluid forces affect the dynamics of a cylinder in axial flow, fluid forces are computed on rigid inclined cylinders, mimicking the damping force experienced by the same cylinder moving perpendicular to the axial flow. The computations showed the existence of two different flow regimes. Each regime gave rise to a different lift force behavior, which will also influence the damping of the coupled system. Furthermore it is shown that the inlet turbulence has a non-negligible effect on these forces and thus on the dynamics of the cylinder. Next, the dynamics of a flexible cylinder clamped at both ends in axial water flow are computed by means of a methodology developed earlier. The results are successfully compared with dynamics measured in experiments available in literature. Computationally it was found that the cylinders natural frequency decreases with increasing flow velocity, until it loses stability by buckling. The threshold for buckling is in quantitative agreement with experimental results and weakly nonlinear theory. Above this threshold, the amplitude of the steady deformation increases with increasing flow speed. Eventually, a fluttering motion is predicted, in agreement with experimental results. It is also shown that even a small misalignment (1°–2°) between the flow and the structure can have a significant impact on the coupled dynamics.

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