Gas combustion in a narrow tube

A simplified model of flame propagation over a single capillary in the low-velocity regime is proposed. The model is based on the concept that the main features of flame propagation in the low-velocity regime are determined by the heat flux along the tube wall from the combustion products to the fresh mixture. Qualitative agreement with experimental results is obtained. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 22–26, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 99-03-32309).     

Gas combustion in a narrow channel at high pressure

This paper presents experimental results on the effect of pressure on the flame propagation velocity in a tube with diameter close to the critical diameter. An important feature of the investigated combustion regime is heat transfer along the tube wall from the combustion products to the fresh mixture. Methane-air and hydrogen-air mixtures were used. The experiments show that with increasing pressure, the burning velocity of methane-air flames decreases whereas the burning velocity of hydrogen-air flames is almost unchanged. This behavior is explained by the pressure dependence of the laminar burning velocity.     

Spin gas combustion in a narrow slot

Combustion in a gap between two thin circular plates with point injection of the gas in the center is experimentally examined. The existence of several combustion regimes is demonstrated. Unstable pulsed combustion is observed in the case of high flow rates. As the flow rate decreases, a transition to spin combustion occurs under certain conditions. If the distance between the plates is sufficiently small, the so-called excess-enthalpy flame can be realized in an expanding gas flow. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 23–26, May–June, 2006.     

不同宽度窄缝通道过冷沸腾

以去离子水为实验工质,在窄缝宽度δ=3、4 mm,质量流速G=143、300 kg·m^-2·s^-1,主流过冷度ΔT_sub=17、25℃,热通量q=1～20 W·cm^-2的参数范围内,对常压下竖直窄缝通道内向上流动过冷沸腾的换热规律进行了实验研究。对不同宽度窄缝通道内的同一区域过冷沸腾气泡演变过程进行了可视化实验分析,发现窄缝宽度因素对过冷流动沸腾的流动换热特性和壁面核化特性影响显著,其中包括沸腾起始点ONB,压降ΔP,传热系数h,汽化核心密度N_a,气泡脱离直径D_d,气泡脱离频率f等。     

Experimental investigation of gas combustion regularities in narrow tubes

Combustion wave propagation in flowing combustible gas has been studied experimentally in tubes with inner diameters larger and smaller than the critical diameters. Combustion wave velocity has been shown to depend on the material of the tube walls, the tube inner diameter, the mixture composition, and the gas flow rate. In a tube which has an inner diameter larger than the critical diameter, two flame-propagation regimes exist.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 1, pp. 42–47, January–February, 1996.     

Propagation and quenching of premixed flames in narrow channels

The shape and propagation of unsteady premixed flames in narrow channels with adiabatic and isothermal walls are numerically investigated in the present study. The flame chemistry is modelled by an one-step overall reaction, which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that both ignition methods and thermal boundary conditions affect flame formation and its shape. The flame keeps a single tulip shape in the whole process of propagating through the channel if plane ignition is used and a single mushroom shape if spark ignition is used. For isothermal cold walls, the flame cannot keep a single tulip shape or mushroom shape all the time. Under plane ignition, a transition from a tulip-shaped flame to a mushroom-shaped flame occurs as flames propagate from one end of channel to the other. Under spark ignition, the process is just the contrary. It is also shown that the heat loss at the cold walls has a dual effect on the formation of a tulip-shaped flame. Flame propagation and quenching in narrow channels of different heights are analyzed systematically, and a criterion is proposed to judge the flame states of partial extinction and total extinction. It is called the criterion of flame propagation and quenching in narrow channels. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 27–36, May–June, 2006.     

Rise of Taylor bubbles through narrow rectangular channels

Experiments have been performed to investigate the rise of Taylor bubbles in narrow rectangular channels (0.0051 m × 0.0027 m × 0.8 m and 0.01 m × 0.0027 m × 0.8 m). The studies conducted for both stationary and moving liquid have revealed definite influence of channel orientation, dimension and inclination on the propagation velocity of Taylor bubbles. The rise velocity first increases and then decreases as the channel is moved from the horizontal to the vertical position with its broad face always lying in a vertical plane. This is in agreement to the results reported in literature for circular as well as non-circular geometries. On the other hand, the rise velocity increases continuously with inclination when the channel is oriented with its broad face in a horizontal plane. The explanation for this difference in behavior has been obtained through visualization and photographic recording. It has also been noted that the bubble rise velocity in the vertical orientation could not be predicted by any of the existing correlations proposed for non-circular channels.     

Adiabatic two-phase flow in narrow channels between two flat plates

Adiabatic cocurrent flow of air and water through a narrow passage between two flat plates 240 mm long and 99mm wide with gap-widths of 0.778 mm and 1.465 mm was investigated for six different orientations: Vertically upward and downward, 45^° inclined upward and downward, and horizontal flows between horizontal plates and between vertical plates. Except for horizontal flow between vertical plates, the effects of gap width and flow channel orientation on flow pattern, void fraction and friction pressure drop were found to be small in narrow channels. The void fraction and two-phase friction multiplier data could also be reasonably correlated in terms of the Martinelli parameter. For horizontal flow between vertical plates, both the void fraction and friction multiplier data showed strong mass velocity effects. Several friction pressure drop correlations were tested for applicability to the narrow channels including a separated flow model proposed in this work.     

矩形双通道间脉动的非单值性试验研究

矩形平行通道流动不稳定性问题是影响紧凑装置安全运行的重要因素之一。文中在中国核动力研究设计院热工试验回路上进行了矩形双通道管间脉动试验。通过对各种热工水力参数和单通道加热的试验研究,得出结论:在低压低质量流速下,随着进口过冷度的增加,界限含汽质量分数先增加、后减小;随着系统压力的升高,质量流速脉动周期先变短、后变长;在低质量流速下,单通道加热时产生管间脉动的界限热流密度比双通道均匀加热情况下的高,在较高质量流速下,单通道加热时的界限热流密度比双通道均匀加热情况下的低。     

Mass transfer in narrow channels in the presence of turbulence promoters

A revised version of the “mesh step” model for the effect of a net type turbulence promoter on mass transfer in a narrow channel is presented. In the limit of a long channel the model is shown to yield a constant average boundary layer thickness the value of which is inversely proportional to the square root of the Peclet number. Mass transfer measurements using a ferro-ferri-cyanide electrochemical flow system show that the model correctly predicts the dependence of the mass transfer rate on flow velocity and Schmidt number, and that on the mesh step in the range of large mesh step values. In the range of small mesh step values, it is shown that the effect of mesh step on mass flux is negligible and, hence, that this need not be a major consideration in the optimization of spacer nets within the usual range of application.     

Computer simulation studies on overlapping polymer chains confined in narrow channels

Conformation of polymer chains strongly confined in a narrow channel space was studied over a broad range of polymer volume fractions φ using lattice Monte Carlo simulations. The longitudinal component of the chain dimension decreased in a power law of ∼φ⁻¹ as φ exceeded the overlap volume fraction. The conformation changed from the one extended along the channel to a random coil. The change occurred without much overlap between adjacent chains. As the conformational transition was completed, the chains started to penetrate each other. Contraction of the chains became more gradual, and eventually the longitudinal component of the chain dimension approached that of the unconfined chains with the overall chain dimension being smaller than that of the unconfined chains. Predictions of the scaling theory were thus confirmed with additional detailed information on the state of confined chains in each regime of characteristic φ dependence of chain dimensions.     

Special features of combustion of propane-air and hydrogen-air mixtures in a narrow tube

The special features of combustion-wave propagation in a narrow tube have been studied experimentally in low-velocity regimes for propane-air and hydrogen-air mixtures. For propane mixtures, the increase in the curvature of the flame surface correlates with the displacement of the maximum into the region of enriched mixtures in the dependence of the combustion velocity on the mixture composition. Combustion of lean hydrogen-air mixtures is accompanied by acoustic oscillations which lead to a narrowing of the range of flame, existence relative to the combustible-gas flow rate. For enriched mixtures, the flames are stable, and they exist at a hydrogen concentration close to the value for the upper concentration limit of flame propagation. Institute of Chemical Kinetics and Combustion, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 6, pp. 14–21, November–December, 1997     

Effect of wall potentials on the vapor and liquid flows in narrow channels

The flow of dense gases and liquids in narrow pores is theoretically described by the microhydrodynamic approach, which can be used to study the effect of wall potentials on the parameters of molecular flows. Calculations are made on the basis of Navier-Stokes equations in which the transport coefficients and the equation of state for a component are calculated using the simplest molecular model such as the lattice-gas model, which takes into account the own volume of molecules and their interaction. To use this approach, it is necessary to know only the interaction potentials between molecules themselves and between molecules and pore walls. The transport coefficients and the equation of state depend on the local values of vapor and liquid number densities and temperature. The approach can be used to study the processes in channels at a nanometer level. Dynamic flows of a monatomic gas (argon) in slit-like pores of different widths at a given pressure difference across the pore are studied. It is shown that the strong attraction between low-density argon atoms and the walls of the pore is accompanied by a highly anisotropic flow. The decrease in the attractive force between molecules and pore walls gives rise to a vapor or liquid slip flow at the walls.