Channel Gap Width Research Articles

ПРИМЕНЕНИЕ СИСТЕМЫ КОМПЬЮТЕРНОГО ЗРЕНИЯ ДЛЯ ЭКСПЕРИМЕНТАЛЬНОГО ИЗУЧЕНИЯ ДВИЖЕНИЯ ФРОНТА ПЛАМЕНИ

The article describes a method for processing of video recordings of flame front propagation in narrow channels using a computer vision library based on the open source code OpenCV. Using the results of machine processing, new data on flame sustainability and topology in a two-dimensional Hele-Shaw chan-nel were obtained. The dependence of apparent speed on the channel gap width emonstrates that the wider the gap, the faster flame propagation growth due to reduced influence of viscosity and heat loss.

Passive scalar transport in rotating turbulent channel flow

Passive scalar transport in turbulent channel flow subject to spanwise system rotation is studied by direct numerical simulations. The Reynolds number $Re=U_{b}h/\unicode[STIX]{x1D708}$ is fixed at 20 000 and the rotation number $Ro=2\unicode[STIX]{x1D6FA}h/U_{b}$ is varied from 0 to 1.2, where $U_{b}$ is the bulk mean velocity, $h$ the half channel gap width and $\unicode[STIX]{x1D6FA}$ the rotation rate. The scalar is constant but different at the two walls, leading to steady scalar transport across the channel. The rotation causes an unstable channel side with relatively strong turbulence and turbulent scalar transport, and a stable channel side with relatively weak turbulence or laminar-like flow, weak turbulent scalar transport but large scalar fluctuations and steep mean scalar gradients. The distinct turbulent–laminar patterns observed at certain $Ro$ on the stable channel side induce similar patterns in the scalar field. The main conclusions of the study are that rotation reduces the similarity between the scalar and velocity field and that the Reynolds analogy for scalar-momentum transport does not hold for rotating turbulent channel flow. This is shown by a reduced correlation between velocity and scalar fluctuations, and a strongly reduced turbulent Prandtl number of less than 0.2 on the unstable channel side away from the wall at higher $Ro$. On the unstable channel side, scalar scales become larger than turbulence scales according to spectra and the turbulent scalar flux vector becomes more aligned with the mean scalar gradient owing to rotation. Budgets in the governing equations of the scalar energy and scalar fluxes are presented and discussed as well as other statistics relevant for turbulence modelling.

Velocity and gas-void fraction in two-phase liquid-gas flow in narrow mini-channels

In the study, the values of velocity and gas void fraction in two-phase gas-liquid flow in narrow vertical mini-channels were experimentally determined. The influence of the physicochemical properties of liquid, the channel gap width and the superficial velocity of gas and liquid, on gas void fraction and gas bubble flow velocities in the flowing two-phase mixture in narrow, vertical mini-channels was investigated. The value of gas phase velocity was defined by means of a drift model, determining the values of distribution parameter C 0 . The relation describing the value of gas void fraction depended on the Bankoff coefficient and gas input volume fraction.

Fouling Control in a Submerged Flat Sheet Membrane System: Part I – Bubbling and Hydrodynamic Effects

Submerged flat sheet membranes are mostly used in membrane bioreactors for wastewater treatment. The major problems for these modules are concentration polarization and subsequent fouling. By using gas‐liquid two‐phase flow, these problems can be ameliorated. This paper describes a study of the use of gas‐liquid two‐phase flow as a fouling control mechanism for submerged flat sheet membrane bioreactors. The effect of various hydrodynamic factors such as airflow rate, nozzle size, intermittent filtration, channel gap width, feed concentration, imposed flux, and the use of membrane baffles were investigated. Experiments conducted on model feeds showed that fouling reduction increased with air flow rate up to a given value and beyond this flowrate no further enhancement was achieved. The effect of bubbling was also found to increase with nozzle size at constant airflow. Using intermittent filtration as an operating strategy was found to be more effective than continuous filtration and it also reduced energy requirements. The study showed the importance of the size of the gap between the submerged flat sheet membranes. As the gap was increased from 7 mm to 14 mm, the fouling became worse and the degree of fouling reduction by two‐phase flow decreased by at least 40% based on suction pressure rise (dTMP/dt). This is the first study which has reported the effects of baffles in improving air distribution across a flat sheet submerged membrane. It was found that baffles could decrease the rate of fouling by at least a factor of 2.0 based on the dTMP/dt data, and significantly increase critical flux.

Large eddy simulation of turbulent flows in a rotating concentric annular channel

Turbulent flow through a concentric annular channel rotating around its axis is investigated numerically by use of large eddy simulation (LES) coupled with a localized one-equation dynamic subgrid-scale (SGS) model (LDM). The objective of this study is to investigate the behavior of turbulent flow near the inner and outer walls of the rotating concentric annular channel and to examine the effectiveness of the LES technique for predicting the turbulent flow subjected to system rotation. The Reynolds number, based on the global friction velocity u τ and the annular channel gap width d, is 640, and the rotation number N = 0–20, which is defined as N = 2 Ωd/ u τ with Ω being the angular velocity of the rotating annular channel. To validate the present computation, LES on turbulent flows in a rotating pipe and in a concentric annular channel are carried out, which shows that the LES results are in good agreement with available experimental data and direct numerical simulation (DNS) results. Then, turbulent statistics in the rotating annular channel, including the resolved velocity, turbulence intensities, Reynolds stresses, turbulence skewness and flatness, and flow structures near the walls, are investigated. The budgets of turbulent kinetic energy and Reynolds stresses are calculated to examine the turbulence production rate, velocity and pressure-gradient correlation, turbulence diffusion, dissipation rate and Coriolis force term near the outer and inner walls.

Counter-current air-water flow in narrow rectangular channels with offset strip fins

Counter-current two-phase flows of air-water in narrow rectangular channels with offset-strip fins have been experimentally investigated in a 760 mm long and 100 mm wide test section with 3.0 and 5.0 mm gap widths. The two-phase flow regime, channel-average void fractions and two-phase pressure gradients were studied. Flow regime transition occurred at lower superficial velocities of air than in the channels without fins. In the bubbly and slug flow regimes, elongated bubbles rose along the subchannel formed by tins without lateral movement. The critical void fraction for the bubbly-to-slug transition was about 0.14 for the 3 mm gap channel and 0.2 for the 5 mm gap channel, respectively. Channel-average void fractions in the channels with fins were almost the same as those in the channels without fins. Void fractions increased as the gap width increased, especially at high superficial velocity of air. The presence of fins enhanced the two-phase distribution parameter significantly in the slug flow, where the effect of gap width was almost negligible. Superficial velocity of air dominated the two-phase pressure gradients. Liquid superficial velocity and channel gap width has only a minor effect on the pressure gradients.

An experimental study of the motion of concentrated suspensions in two-dimensional channel flow. Part 1. Monodisperse systems

A modified laser-Doppler velocimetry method is utilized to measure fully developed particle velocity and concentration profiles, as well as the mean-square amplitudes of velocity fluctuations (i.e. one component of the so-called particle temperature), for concentrated monodisperse suspensions across the narrow gap of a rectangular channel. A stable index-of-refraction match of the suspending and particulate phases in conjunction with short-focal-length focusing optics has enabled data acquisition up to particle volume fractions of 0.50. In general, the particle concentration distributions possess a maximum near the channel centreline and a minimum at the channel walls. Coupled to these concentration distributions were blunted velocity profiles, and particle velocity fluctuation distributions that had a sharp maximum at gap positions approximately 80% of the way from the channel axis towards the walls. The particle velocity distributions were consistent with the absence of slip between particles and the suspending fluid.The experimental data were compared with theoretical predictions from the diffusive flux model (Leighton & Acrivos 1987; Phillips et al. 1992), a model due to Mills & Snabre (1995), and the suspensions balance model (McTigue & Jenkins 1992; Nott & Brady 1994). The influence of bulk particle concentration, suspension volumetric flow rate, and ratio of channel gap width to particle diameter on the fully developed profiles was qualitatively consistent with the theoretical predictions from all three models. For the diffusive flux and suspension balance models, we used both literature values for model parameters, and values obtained from a best fit to our entire set of experimental data. Overall, the Mills & Snabre and suspension balance models were found to provide a better quantitative fit to the experimental data than the diffusive flux model.

Thermoacoustic streaming in a resonant channel: The time-averaged temperature distribution

The problem of thermoacoustic streaming in a plane parallel resonant channel, representative of the stack in a thermoacoustic engine, has been developed in a general dimensionless form. The utility of such a formulation and its wide ranging applicability to different solid–fluid combinations is demonstrated by which a consistent account of all the energy-exchange mechanisms can be made. Certain (wide-gap, thick-wall) simplifications are initially made to arrive at more manageable forms of the time-averaged temperature distributions of interest in both the fluid gap, and the channel walls. These simplifications clarify the origin of the thermoacoustic effect and provide a description of the responsible physical mechanisms based on which the validity of the “bucket-brigade” model is examined. The unexpected role of a little-known second-order thermal expansion coefficient is pointed out. It is shown that the conjugate wall–fluid coupling is crucial in yielding the large time-averaged axial temperature gradients that can be induced in the channel. In particular, the heat transfer rates at the ends of the channel are found to play an important role in determining the magnitude of these time-averaged gradients. The more general and practically useful case of arbitrary channel gap widths is also treated and it is found that for ideal gas working fluids there is an optimum channel gap width for which the axial thermal stratification in the channel is maximized. A comparison of the predictions from this study with available experimental data shows very good agreement.

Effects of Gap Width and Orientation on Two-Phase Flow in a Narrow Passage between Two-Flat Plates.

The effects of flow channel gap width and orientation on two-phase flow pattern, void fraction and frictional pressure drop have been investigated using an adiabatic air-water flow in a channel between two flat plates, 240mm long × 80mm wide, with gap widths of 1.465mm and 0.778mm. The six orientaions studied were vertically upward and downward, 45°inclined upward and downward, horizontal flow between horizontal plates and horizontal flow between vertical plates. The results showed that the effect of the flow channel orientation is small for both gap widths, except for the horizontal flow between vertical plates. In the last orientation, both void fraction and frictional pressure data showed a strong effect of mass velocity. Results of comparisons of several two-phase friction pressure drop correlations with the present data are also discussed.