Effect of modulations of opposed gas flow velocity on flame spread rate over a liquid surface

The effect of modulations of the velocity of the gas flow incident on the flame on the average flame velocity over a shallow liquid is studied. It is shown that the average flame velocity depends on the modulation frequency. If the modulation frequency is higher than the flame oscillation eigenfrequency, then, upon the imposition of the modulation, the flame velocity first increases and then gradually returns to the initial value. At frequencies close to the flame oscillation eigenfrequency, the average flame velocity is constant but is higher than the initial value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 11–17, January–February, 2009.     

Decrease in the etch rate of polymers in the oxygen afterglow with increasing gas flow rate

In this paper we report the variation of the etch rate of polymers in the afterglow of a radio frequency discharge in oxygen as a function of total flow rate in the range 2–10 cm³ (STP)/min. The measurements were made at ambient temperature with the O(³P) concentration held essentially constant. We report results on three polymers: cis-polybutadiene, a polybutadiene with 33% 1,2 double bonds, and a polybutadiene with 40% 1,2 double bonds. We have observed that the etch rate of these polymers decreases significantly with increasing flow rate, strongly suggesting that the vapor-phase products of polymer degradation contribute to the degradation process.     

Heat transfer of gas flow through a packed bed

This paper reports an experimental study of both the transient and steady-state heat transfer behaviour of a gas flowing through a packed bed under the constant wall temperature conditions. Effective thermal conductivities and convective heat transfer coefficient are derived based on the steady-state measurements and the two-dimensional axial dispersion plug flow (2DADPF) model. The results reveal a large temperature drop at the wall region and the temperature drop depends on the axial distance from the inlet. The 2DADPF model predicts the axial temperature distribution fairly well, but the prediction is poor for the radial temperature distribution. Length-dependent behaviour of the effective heat transfer parameters and non-uniform flow behaviour are proposed to be responsible. A comparison with previously published correlations and data in the literature shows that the relationships proposed by Bunnell et al. and Demirel et al. agree well with the measured effective radial thermal conductivity, whereas the wall-fluid heat transfer coefficient is better represented by the Li-Finlayson correlation.     

Numerical and experimental analyses of a stirred vessel for a large volumetric flow rate of sparged air

Computational fluid dynamics (CFD) and experimental analyses of some of the basic characteristics of air sparging in a tall stirred vessel equipped with a three-stage impeller are presented. The impeller was assembled from a radial ABT impeller as the lower, a turbine 6PBT45 as the middle and an axial Scaba-type 3SHP1 impeller as the upper. All the impellers were of the same diameter, i.e., 225 mm, while the vessel diameter was 450 mm. The impeller's rotational speed was 178 r·min^-1. The aeration regime was established with an air volumetric flow rate of 28.3 m³·h^-1. To the best of our knowledge, this study is the first to consider the very high gassing rate by means of CFD in a tank stirred by three-stage axial/radial impellers. The numerical simulation was performed using the ANSYS Fluent (R17.2, 2016) code for solving the governing equations of fluid dynamics in single- and multi-phase systems. While discussing the bubble size distribution, a discrete population balance model (PBM) was used. Adopting CFD, the stirring power and the total void fraction (the total gas holdup) were calculated. The results were in good agreement with the measured values using a laboratory experimental device.     

Hydrodynamic study of gas and solid flow through a screen-packing

Many drawbacks of conventional gas fluidized beds can be avoided by filling the column with a fixed wire-screen Raschig ring packing of high voidage. Such systems have been studied in batch and countercurrent continuous mode. Excellent fluidization occurs with relatively coarse particles but not with smaller particles. In batch systems, the expansion is given by a Capes and McIlhinney type power law and the contribution of the packing to support the bed can be up 20 percent of the weight of solid. In countercurrent systems, for each gas velocity two operating points can occur: the dilute or the dense phase bed in the same way as for gas liquid flow. All increases in the flow of the solid or in the dilution of the bed lead the packing to support a greater part of the grains.     

Measurement of diffusion and flow of gas through a porous solid

Experiments for measuring the flow or diffusion of a gas through a porous solid are described. The experiments allow calculation of an effective diffusion coefficient and the tortuosity factor for a porous sample of undefined pore characteristics.     

Numerical modeling study for the analysis of transient flow characteristics of gas, oil, water, and hydrate flow through a pipeline

This study presents the development of a four-phase, four-fluid flow pipeline simulator to describe simultaneous flow of gas, oil, water, and hydrate through a pipeline. The model has been equipped with a phase behavior model and hydrate equilibrium model to efficiently estimate thermodynamic and hydrodynamic properties of multicomponent mixtures. The governing equations are formulated for describing the physical phenomena of mass, momentum, and heat transfers between the fluids, and the wall. The equations are solved by utilizing the implicit finite-difference method on the staggered-grid system which can properly describe the boundary conditions as well as phase appearance or disappearance. The developed pipeline simulator has been validated against the field data presented by a previous investigator, and their matches are found to be relatively excellent. The model also has been applied to a multi-component, four-phase flow system in order to examine the transient flow characteristics in pipeline. Also, the potential and the location of hydrate formed in the pipeline have been studied by analyzing the flow characteristics. As a result, it was found that a pipeline system flowing gas, oil, water, and hydrate could be optimized by systematically investigating the hydrodynamic variables for the prevention of hydrate formation.     

基于峰值守恒法的油罐呼吸阀气通量计算

呼吸阀是减少油气排放量、保证储油罐使用安全性的必要措施之一.由于储油罐所处环境条件的不同,罐内物性参数随时间而发生变化,增加了呼吸阀设计条件的不确定性.影响呼吸阀气通量的各种物理量是多种多样的,其中温度变化是影响呼吸阀气量的主要因素之一.本文以温度变化作为研究呼吸阀气通量的出发点,提出了新的呼吸阀气通量计算方法——峰值守恒法.即将气相与液相的质量变化过程视为一个独立的质量守恒体,把该守恒体的气相介质分量变化过程作为研究对象,并设定其饱和蒸气压状态下的峰值变化过程为呼吸阀气通量,利用质量守恒原则,推导出了呼吸阀气通量计算方法.该方法与目前常用的公式方法相比,其计算气通量更接近实际情况,提高了呼吸阀的设计与选用的准确性.     

Longitudinal gas dispersion in transitional and turbulent flow through a straight tube

Longitudinal dispersion data are reported for a straight tube (internal dia. 1¼′ (2.77 cm)). Three binary gas systems were examined over the regime 3 × 10² < Re < 10⁴ with special emphasis on the transition from laminar to turbulent flow. Effect of Schmidt number is studied in detail over the transition regime, and practical variation with Sc for the lower turbulent regime is compared with the predicted effect based on existing theories of turbulence. In the laminar regime results are reported which diverge from Taylor's theory at Re lower than the normally accepted value for the onset of turbulence.     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

Colourless distributed combustion effects on a pre-mixed coke oven gas flame

Fuels consisting of high hydrogen concentration can be consumed under colourless distributed combustion (CDC) to suppress flashback tendency in premixed conditions. Higher NO_X challenges can also be overcome through CDC. For those purposes, coke oven gas was consumed under CDC within the scope of this study. To achieve CDC, ${\mathrm{N}}_2$ or ${\mathrm{CO}}_2$ diluents were introduced into the oxidizer so that oxygen concentration in the oxidizer would be decreased from 21% O₂ to its lean blow-off limits. Excess air ratios were determined as λ = 1.2 and λ = 1.5 along with a thermal power of 10 kW under premixed conditions. A commercial computational fluid dynamics code was used to predict temperature distribution and ${\mathrm{NO}}_X$, CO, and ${\mathrm{CO}}_2$ emissions. 162-step reactions created with GRI-Mech 3.0 chemical kinetics was integrated to the eddy dissipation concept combustion model. The temperature and ${\mathrm{NO}}_X$ profiles predicted were compared with the experimental results. Consistency was achieved at ~95% for temperature profiles, and almost 100% for ${\mathrm{NO}}_X$ profiles between the measured and the predicted ones. According to the results, it is concluded that a more homogeneous temperature distribution with the ~21% decrease in the maximum temperature was observed, and there was about 96% decrease in ${\mathrm{NO}}_X$ level. It was also demonstrated that using ${\mathrm{CO}}_2$ as a diluent is more effective in terms of temperature distribution over the combustor and ${\mathrm{NO}}_X$ reduction, while dilution with ${\mathrm{N}}_2$ is more effective in terms about 70% in decrease on CO.     

Dispersed oil–water–gas flow through a horizontal pipe

An experimental study of three‐phase dispersed flow in a horizontal pipe has been carried out. The pressure drop over the pipe strongly increases with increasing bubble and drop volume fraction. Because of the presence of drops the transition from dispersed bubble flow to elongated bubble flow occurs at a lower gas volume fraction. The gas bubbles have no significant influence on the phase inversion process. However, phase inversion has a strong effect on the gas bubbles. Just before inversion large bubbles are present and the flow pattern is elongated bubble flow. During the inversion process the bubbles break‐up quickly and as the dispersed drop volume fraction after inversion is much lower than before inversion, a dispersed bubble flow is present after inversion. (When inversion is postponed to high dispersed phase fractions, the volume fraction of the dispersed phase can be as high as 0.9 before inversion and as low as 0.1 after inversion.) © 2009 American Institute of Chemical Engineers AIChE J, 2009