Combined Non-Luminous Flame Radiation and Surface Tension Effects During Methanol Droplet Combustion

2007 ◽  
Author(s):  
Vasudevan Raghavan ◽  
Daniel N. Pope ◽  
George Gogos
Keyword(s):  
Surface Tension ◽  
Reynolds Number ◽  
Relative Velocity ◽  
Droplet Diameter ◽  
Droplet Combustion ◽  
Linear Variation ◽  
Two Phase ◽  
Flame Radiation ◽  
Initial Droplet ◽  
Product Species

The effect of non-luminous thermal radiation on suspended (constant relative velocity) methanol droplet combustion in a low temperature (300 K) and low pressure (1 atm) environment is discussed in detail. Numerical results are obtained using a predictive, transient, two-phase, axisymmetric numerical model that includes surface tension effects. Radiation is modeled using the optically thin approximation with the product species CO2 and H2O as the radiating species. Results for combustion in a quiescent atmosphere (initial Reynolds number 0.01) and initial droplet diameters in the range of 0.43 mm to 3 mm are presented. The results show that the effect of flame radiation is negligible when the initial droplet diameter is less than approximately 1 mm and becomes increasingly important for droplets with initial diameters greater than approximately 1 mm, as reported in previous literature. The average evaporation constant decreases with the initial droplet diameter. Both radiation and surface tension have a significant effect on the predicted extinction diameters of initially larger droplets. The extinction diameter presents a non-linear variation with the initial droplet diameter for initially larger droplets and agreement with experiments is good.

Spray Flames Simulation Using a Two Phase Combustion Model With Single Droplet Combustion Mode

2012 ◽  
Author(s):  
F. Wang ◽  
Y. Huang
Keyword(s):  
Droplet Diameter ◽  
Navier Stokes ◽  
Combustion Model ◽  
Droplet Combustion ◽  
Single Droplet ◽  
Two Phase ◽  
Spray Flame ◽  
Spray Flames ◽  
The Individual ◽  
Individual Droplet

There are three combustion regimes of individual droplet combustion behavior: the fully enveloped flame, the partially enveloped flame, and the wake flame. From PLIF measurement results, single droplet combustion phenomenon happens in spray flame, as well as lean type gas turbine combustion chamber sometimes. The drag coefficient, evaporation rate, and combustion rate are different according to the burning modes. At present, in Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) method, the droplets are treated as point source because the grid scale is bigger than the droplet diameter. A two phase combustion model with the consideration of the individual droplet burning mode is proposed before. In this paper, this model is tested by spray flames here again. Furthermore, this model was used in a concept lean premixed pre-vaporized (LPP) combustion case too. In spray flame, the predicted results are close to the experimental data.

The Role of Surface Tension Effects During Methanol Droplet Combustion

2006 ◽  
Author(s):  
Vasudevan Raghavan ◽  
Daniel N. Pope ◽  
George Gogos
Keyword(s):  
Surface Tension ◽  
Reynolds Number ◽  
Liquid Phase ◽  
Water Absorption ◽  
Flow Pattern ◽  
Liquid Droplet ◽  
Reynolds Numbers ◽  
Droplet Combustion ◽  
Complex Flow ◽  
Suspended Droplet

A numerical investigation of methanol droplet combustion in a zero-gravity and low-pressure convective environment is presented. Simulations have been carried out using a predictive, transient and axisymmetric model, which includes droplet heating, liquid-phase circulation and water absorption. First, a suspended droplet (constant relative velocity) burning in an ambient of air at 300K is considered. A nearly quiescent environment (initial Reynolds number Re0=0.01) is used to impose a weak gas-phase convective flow, introducing a deviation from spherical symmetry. The resulting weak liquid-phase circulation is greatly enhanced due to surface tension effects, which create a complex, time-varying, multicellular flow pattern within the liquid droplet. The complex flow pattern, which, in the presence of surface tension, results in nearly perfect mixing, causes increased water absorption within the droplet, leading to larger extinction diameters. Surface tension effects are shown to be dominant in causing water absorption, even at initial Reynolds numbers as high as 5. Results for combustion in a nearly quiescent environment (Re0=0.01) with varying initial droplet diameters, (d0 = 0.16 to 1.72 mm), show that predictions of droplet extinction diameters, although they are still below the experimental data, do improve substantially when surface tension effects are included. Next, results for suspended droplets and for moving droplets burning in an ambient of air at 1200K, for a range of initial Reynolds numbers that are of interest in spray combustion (Re0=1-100) are presented. It is shown that, for moving droplets, due to the presence of an envelope flame at some stage during the droplet lifetime, surface tension is important over the entire range of Re0 considered; the extinction diameter decreases with increasing Re0. Extinction is not observed for a moving droplet when surface tension effects are neglected. For suspended droplets, when transition or envelope flame is present, which corresponds to Re0 less than approximately 15, surface tension is important; when an envelope flame is present (Re0 less than approximately 10), the extinction diameter increases with Re0. The variation of droplet lifetime with Re0 is much stronger for suspended droplets than for moving droplets. Depending on the Reynolds number, results on methanol droplet lifetimes and extinction diameters measured through suspended droplet experiments may not be applicable to moving droplets.

Effects of Initial Diameter on Burning of Nonane Droplets in a Non-Buoyant and Buoyant Ambience: Burning Rate, Soot Formation and Flame Structure

2001 ◽  
Author(s):  
J. H. Bae ◽  
C. T. Avedisian
Keyword(s):  
Burning Rate ◽  
Droplet Size ◽  
Soot Formation ◽  
Droplet Diameter ◽  
Flame Structure ◽  
Large Droplet ◽  
Droplet Combustion ◽  
Small Droplet ◽  
Burning Process ◽  
Initial Droplet

Abstract The results from nonane droplet combustion experiments conducted at 1g and μg are analyzed and compared in the following aspects: the burning rate, soot formation, flame structure. By varying the initial droplet diameter, we observe and discuss the effect of Do on droplet burning. The μg experiments were performed in a drop tower and a drag shield was used to create a low buoyant environment All experiments were fiber-supported and used the same experimental instruments. The droplet size between 0.40 to 0.95mm was examined in the experiments. Results showed that droplet burning is nonlinear in both a buoyant and a non-buoyant environment for the initial droplet diameters examined. Soot formation, which is influenced by Do may strongly affect the droplet burning process in both environments. The large droplet produces more soot and bums slowly whereas the small droplet bums fast because there is less soot.

Numerical Prediction of the Effects of Oceanic Flow Characters on the Evolution of CO2 Eniched Plumes

2004 ◽  
Author(s):  
Baixin Chen ◽  
Yongchen Song ◽  
Masahiro Nishio ◽  
Makato Akai
Keyword(s):  
Concentration Distribution ◽  
Droplet Diameter ◽  
Co2 Concentration ◽  
Injection Rate ◽  
Turbulent Intensity ◽  
Current Profile ◽  
Two Phase ◽  
Large Eddy Simulation Model ◽  
Initial Droplet ◽  
Co2 Ocean Sequestration

The near-field dynamics of CO2 rich plume draw attention of assessment of the local impacts of CO2 ocean sequestration on natural oceanic environment. In this study, we attempt to predict numerically the role of ocean flow characters, including the current profile and the turbulent intensity, and of the injection parameters, including the injection rate and initial droplet diameters, on the evolution of liquid CO2 (LCO2) droplet and CO2 enriched seawater plumes. The numerical model we used in this study is a two-phase large-eddy simulation model. From numerical experiments we found: 1). The plume height (both LCO2 plume and CO2 enriched seawater plume) is insensitive to ocean currents and turbulent intensity but do sensitive to initial droplet diameter. For releasing rates of 0.6kg/sec, the estimated plume heights at initial droplet diameters of 8.0 and 5.0 millimeter are approximately 170 and 80 meters for different oceanic flows. 2). The physics of CO2 enriched seawater plume, for instance CO2 concentration distribution and local largest concentration, however, are governed sensitively by seawater flow characters and alternatively by injection rate and initial droplet diameter. 3). Strong turbulence enhanced the dispersion and mixing of droplets and CO2 enriched seawater with fresh seawater to produce an improved CO2 concentration distribution.

scholarly journals Numerical Simulation and Experimental Verification of Droplet Generation in Microfluidic Digital PCR Chip

Micromachines ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 409
Author(s):  
Xiangkai Meng ◽  
Yuanhua Yu ◽  
Guangyong Jin
Keyword(s):  
Surface Tension ◽  
Flow Rate ◽  
Droplet Diameter ◽  
Digital Pcr ◽  
Droplet Formation ◽  
Droplet Generation ◽  
Two Phase ◽  
Factors Affecting ◽  
Microfluidic Technology ◽  
Fundamental Information

The generation of droplets is one of the most critical steps in the droplet digital polymerase chain reaction (ddPCR) procedure. In this study, the mechanism of droplet formation in microchannel structure and factors affecting droplet formation were studied. The physical field of laminar two-phase flow level was used to simulate the process of droplet generation through microfluidic technology. The effect of the parameters including flow rate, surface tension, and viscosity on the generated droplet size were evaluated by the simulation. After that, the microfluidic chip that has the same dimension as the simulation was then, fabricated and evaluated. The chip was made by conventional SU-8 photolithography and injection molding. The accuracy of the simulation was validated by comparing the generated droplets in the real scenario with the simulation result. The relative error (RE) between experimentally measured droplet diameter and simulation results under different flow rate, viscosity, surface tension and contact angle was found less than 3.5%, 1.8%, 1.4%, and 1.2%, respectively. Besides, the coefficient of variation (CV) of the droplet diameter was less than 1%, which indicates the experimental droplet generation was of high stability and reliability. This study provides not only fundamental information for the design and experiment of droplet generation by microfluidic technology but also a reliable and efficient investigation method in the ddPCR field.

Force Scaling Comparison of Transport Phenomena in Proton Exchange Membrane Fuel Cell Flow Channels

2021 ◽  
Vol 18 (3) ◽  
Author(s):  
Mehdi Mortazavi ◽  
Anthony D. Santamaria ◽  
Mahbod Heidari ◽  
Michael P. Doyle ◽  
Morgan A. Schrader ◽  
...  
Keyword(s):  
Surface Tension ◽  
Proton Exchange Membrane ◽  
Complex Geometry ◽  
Droplet Diameter ◽  
Pem Fuel Cells ◽  
Proton Exchange ◽  
Stoichiometric Ratio ◽  
Two Phase ◽  
Flow Channels ◽  
Exchange Membrane

Abstract Liquid–gas two-phase flow in flow channels of proton exchange membrane (PEM) fuel cells has been investigated extensively in the literature; however, a comparison between the order of the magnitude of the forces occurring within the flow channels has not been documented. A comparison is relevant due to increased interest in practical active and passive water management strategies. The present study compares the magnitude of the forces experienced by liquid water residing in the flow channels. An analytical model of a 20-cm-long flow channel was analyzed, and key forces were compared in the stream-wise coordinate. Results clearly reinforce the dominance of the surface tension forces over other forces applied in the channel while also demonstrating how they change with key variables. For a cathode stoichiometric ratio of 1, the surface tension effects were calculated to be three orders of magnitude greater than the gravitational effects, the second largest force scale, for a droplet diameter of 0.1 mm. For larger droplets, this difference becomes smaller but the surface tension effects remain dominant. The results are useful for flow-field designers where water removal using complex geometry and hydrophobic coatings are being explored.

Surface tension effects during low-Reynolds-number methanol droplet combustion

2006 ◽  
Vol 145 (4) ◽  
pp. 791-807 ◽  
Author(s):  
V RAGHAVAN ◽  
D POPE ◽  
D HOWARD ◽  
G GOGOS
Keyword(s):  
Surface Tension ◽  
Reynolds Number ◽  
Low Reynolds Number ◽  
Droplet Combustion ◽  
Low Reynolds

Theoretical Post-Dryout Heat Transfer Model

2018 ◽  
Vol 1 (1) ◽  
pp. 142-150
Author(s):  
Murat Tunc ◽  
Ayse Nur Esen ◽  
Doruk Sen ◽  
Ahmet Karakas
Keyword(s):  
Heat Transfer ◽  
Droplet Diameter ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Operating Pressure ◽  
Vertical Pipe ◽  
Two Phase ◽  
Experimental Values ◽  
Reactor Operating ◽  
Coolant System

A theoretical post-dryout heat transfer model is developed for two-phase dispersed flow, one-dimensional vertical pipe in a post-CHF regime. Because of the presence of average droplet diameter lower bound in a two-phase sparse flow. Droplet diameter is also calculated. Obtained results are compared with experimental values. Experimental data is used two-phase flow steam-water in VVER-1200, reactor coolant system, reactor operating pressure is 16.2 MPa. On heater rod surface, dryout was detected as a result of jumping increase of the heater rod surface temperature. Results obtained display lower droplet dimensions than the experimentally obtained values.

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