On vortex combustion

Known and previously unknown experimental observations of transfer, exchange, and combustion in vortex flames in high-velocity flows are analyzed. The evolution of our concepts of these processes with discoveries of new facts is traced. It is demonstrated that four regimes of homogeneous combustion of the vortex ball in a high-velocity flow are possible: deterministic (coherent), resonant (vibrational), stochastic, and pseudodetonation regimes. Pseudo-detonation combustion is understood as simultaneous homogeneous microturbulent frontal combustion of all layers of the vortex ball with the radius expansion rate several times greater than the entraining flow velocity. The intensity of injection of a homogeneous mixture or air into the burning vortex layer is found to be identical to the notion of intensity of homogeneous or diffusion combustion. It is demonstrated that the classical notions and the Shchelkin-Shchetinkov relations for burning surfaces and volumes are applicable, but only for certain phases and local zones of the burning vortex ball.     

Improvements in hydrocyclone design flow lines stabilization

Pressure distribution field within the separation chamber of hydrocyclones is qualitatively and quantitatively analyzed in the context of the design of compact plants for the on-board treatment of ballast water. Once the qualitative analysis has been carried out, the conclusions arising from this analysis have been evaluated by defining a parameter termed “asymmetry coefficient”, which provides quantitative information about the behaviour within the hydrocyclone. The influence of the vortex finder both on flow patterns and on air-core precession is successfully studied. As well as giving an insight into hydrocyclone flow lines behaviour, this article attempts to improve operational efficiency by modifying several design parameters in order to provide a stabilization of flow lines (reducing turbulence).     

Numerical modeling of heat and mass transfer processes upon the flowing of regular structures by gas-droplet stream

Approximate models are developed for calculating heat and mass transfer processes in gas-drop flows that pass over a regular packed bed with cylindrical elements. A number of different flow regimes are considered: flat laminar flow, the formation of the stationary vortex behind packing element and formation of nonstationary vortex wake. Criterial equations for calculating corresponding mass transfer parameters are obtained.     

THREE-DIMENSIONAL ELECTRICAL RESISTANCE TOMOGRAPHY IN A STIRRED MIXING VESSEL

Electrical resistance tomography (ERT) techniques under development appear to offer exciting possibilities for future non-intrusive, full-field measurement of gas-liquid and miscible liquid mixing characteristics. A 'full' 3-D electrical resistance tomographic analysis composed of a 3-D image reconstruction algorithm and a 3-D sensing strategy is being developed at UMIST to overcome the problem of using only a 2-D analysis for inherent 3-D problems. A diagonal sensor excitation and measurement protocol is being examined for the sensing strategy. The image reconstruction algorithm is based on the Newton-Gauss method together with a variant of Occam's regularisation. Experiments were conducted on a 2.7 m³ mixing vessel. The vessel is represented by a 3-D finite element model with 1536 voxels for an initial test with 4 sensing planes. A total 3969 voltage measurements can be obtained from the diagonal strategy from which only 2016 are independent measurements. Some initial results are presented with reconstructed images of multi-object phantom configurations and the resulting air-core vortex arising from stirring the mixing vessel with a Rushton impeller at 140 rpm.     

Orbitally shaken bioreactors—viscosity effects on flow characteristics

Phase‐resolved particle image velocimetry measurements were carried out to assess the flow dynamics occurring in orbitally shaken bioreactors of cylindrical geometry when working fluids of increasing viscosity are considered. Study of the phase‐resolved flow characteristics allowed to built a Re‐Fr map, where four quadrants associated to different flow regimes are identified: in‐phase toroidal vortex (low Fr and high Re), out‐of‐phase precessional vortex (high Fr and high Re), in‐phase single vortex (low Fr and low Re), out‐of‐phase counter‐rotating toroidal vortex (high Fr and low Re). Turbulence levels are found to be significant only in the top right quadrant (high Fr and low Re) and scaling of the turbulent kinetic energy obtained with fluid of varying viscosity is obtained using the ratio of the operating Froude number to the critical Froude number associated to the mean flow transition, . Estimates of the mean flow strain deformation as well as of the flow dissipative scale are provided, while a comparison is made between the flow circulation times obtained for different regimes. © 2014 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 60: 3951–3968, 2014     

近壁面静止圆球的尾迹特性

圆球在大空间均匀来流下的绕流特性已得到了广泛的研究,但是壁面对圆球绕流特性的影响还未清楚。通过实验方法研究了与壁面接触的静止圆球在明渠流中的绕流特性,重点关注了圆球后尾迹的特性。研究发现,圆球尾迹存在三种状态：稳定状态、非稳定对称状态和非稳定非对称状态。当Re<150时,尾迹呈稳定状态,尾迹关于垂直于壁面的平面对称;当150<Re<400时,尾迹呈对称状态,Reynolds数较高时,圆球后存在规则的、周期性的涡脱落,Strouhal 数为0.26~0.33,脱落涡关于垂直于壁面的平面对称,此时尾迹功率谱的频带分布较宽;而当Re>400时,圆球后的涡脱落不再对称,尾迹随着Reynolds数的增大呈现混沌特征。     

Transient three-dimensional heat transfer from rotating spheres with surface blowing

Transient heat transfer and thermal patterns around a rotating spherical particle with surface blowing are studied numerically for Reynolds numbers in the range 10?Re?300 and non-dimensional angular velocities up to Ω=1. This range of Reynolds number includes three distinct wake regimes: steady and axisymmetrical, steady but non-symmetrical, and unsteady with vortex shedding. The Navier-Stokes and energy equations for an incompressible viscous flow are solved numerically by a finite-volume method in a three-dimensional and time-accurate manner. The transient aspects of the thermal wakes associated with the aforementioned wake regimes have been explored. An interesting feature associated with particle rotation and surface blowing is that they can affect the near wake structure in such a way that an unsteady three-dimensional flow with vortex shedding develops at lower Reynolds numbers as compared to flow over a solid sphere in the absence of these effects, and thus, the temperature distributions around the particle are significantly affected. Despite the fact that particle rotation brings about major changes locally, the surface-averaged heat transfer rates are not influenced appreciably even at high rotational speeds; consequently, it is shown that the total heat transfer rates associated with rotating spheres with surface blowing can be calculated from heat transfer correlations developed for flow over evaporating droplets.     

Trapping region of impinging jets in a cross-shaped channel

This study follows our previous report (Zhang et al., Phys. Fluids, vol. 31, 2019, 034105) by describing the formation and evolution of the engulfment flow in the cross-shaped channel. First, the flow regimes were studied by planar laser induced fluorescence (PLIF). Results show the formation of a spiral vortex in the center of the chamber and the appearance of a well-mixed zone inside the spiral vortex. Second, we proposed a novel experimental method to analyze the residence time of the fluid in the chamber, and discover an unexpected trapping region inside the well-mixed zone. There is almost no fluid transport into or out of this region. Furthermore, three-dimensional numerical simulation is used to reveal the origination of this trapping region. Simulation results reveal that the fluid recirculates in the trapping region and the flow feature is caused by the bubble-type vortex breakdown.     

Mechanical Device Designed for Preventing Hydrocyclone Clogging and Air-Core Formation

The clogging of hydrocyclones as part of the solids control system in the oil industry is a recurrent operational problem. This occurs mainly due to the agglomeration of fine solids in the underflow duct and also to the incidence of higher-size-range particles than those specified for the equipment. Another common issue that results in poor performance is the formation of an air-core in the low-pressure zone of the separator. In this study, an innovative device coupled to the underflow output of the hydrocyclone was developed in order to minimize these problems. The aim is to mechanically adjust the apex area in a fast and accurate manner, and thus create oscillations in the inlet pressure and flows, constituting a safety system to prevent clogging and air-core formation.     

Dimensionless number for identification of flow patterns inside a T-micromixer

Results are presented from a numerical study examining the flow dynamics of the liquid phase inside T-type micromixers. The main aim of the study was to determine an identification number for the differentiation of the different flow regimes in the liquid phase in T-type micromixers. The critical value for the identification number at which the transition from vortex flow to engulfment flow occurs was obtained. The results were used to optimize the geometrical parameters and the operating conditions to achieve high mixing performance for the liquid phase in T-type micromixers. The model results were found to be consistent with experimental data for different T-mixers available in the literature.     

Detonation of a coal-air mixture with addition of hydrogen in plane-radial vortex chambers

Results of an experimental study of continuous and pulsed detonation of a coal-air mixture with addition of hydrogen in plane-radial vortex chambers 204 and 500 mm in diameter are presented. The tested substance is pulverized activated charcoal. A method of coal powder supply through narrow channels by means of adding the gas at the injector entrance is found. Stable regimes of continuous spin detonation with one or two transverse detonation waves moving with velocities of 1.8–1.6 km/sec are obtained for the first time in the combustor 204 mm in diameter. The frequency of pulsed detonation with radial waves is 4–4.8 kHz. The limits of continuous detonation in the combustor 500 mm in diameter are extended: regimes of continuous spin detonation with a large number (5–8) of transverse waves moving with velocities of 1.8–1.5 km/sec are obtained, the amount of hydrogen added to coal is reduced to 2.8%, and combustion of coarser fuel particles is ensured owing to an increased residence time of the mixture in the combustor. The wave structure and the flow in the vicinity of the waves are reconstructed in the combustor plane.     

Experimental study of flow regimes in three‐dimensional confined impinging jets reactor

Dynamic behaviors in a three‐dimensional confined impinging jets reactor (CIJR) were experimentally studied by a flow visualization technique at 100 ≤ Re ≤ 2000 and 2 ≤ D/d ≤ 12 (where D is the reactor diameter and d is the nozzle diameter). The effects of inlet Reynolds numbers (Re) and geometry configurations of the CIJR on the flow regimes have been investigated by a particle image velocimetry and a high‐speed camera. Results show that with the increasing Re, a segregated flow regime, a radial deflective oscillation, an axial oscillation and a vortex shedding regime emerge in turns in CIJR. A map of parameter space formed by the inlet Reynolds number (Re) and the normalized reactor diameter (D/d) has been presented. The effects of jet instability and confined boundary of the chamber on the flow regimes and their transition are also investigated and discussed. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3033–3045, 2014     

Investigation of the structure of a diffusion hydrogen plume in a supersonic high-enthalpy air jet

The results of investigation of the plume structure in the case of diffusion combustion of hydrogen in a cocurrent supersonic high-enthalpy off-design air jet are presented. Based on the registration of radiation within the wavelength range of 260–350 nm across and along the flame, a three-dimensional tomographic reconstruction of the plume was obtained, which confirmed the interrelation between the gas-dynamic structure and combustion intensity. A possibility of existence of combustion regimes with periodically repeated cycles of complete extinction and subsequent ignition of hydrogen in accordance with the barrel-shaped structure of the off-design jet is established experimentally. The existence of local peripheral regions of combustion is noted, which can indicate the presence of vortex structures. In the three-dimensional representation (obtained under the assumption of axial symmetry of radiation), these vortex structures have the form of annular zones. In a real flow, helical structures are possibly formed at the plume periphery. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 3–5, September–October 1999.     

T形微通道结构中的流体混合规律

对单级和双级T形微通道结构中流体的混合规律进行了实验和模拟研究。研究发现,随着流动Reynolds数的增加,微通道内依次出现层流、涡流、卷流3种不同流动状态;在单级T形微通道中,只有当流动处于卷流状态时,混合才得以强化;而对于双级T形微通道结构,流体接触面积增大,流向转变使得当最大尺度的混合通道中出现涡流时,混合即增强。在相同停留时间条件下,双级T形微通道结构中流体混合效率明显高于单级。研究结果充分说明,含有多级T形结构的树状分形仿生网络结构能够极大地强化流体混合,多级T形网络结构是一种高效的微观混合构型。     

Drying in active hydrodynamic regimes

It is shown that the activation of hydrodynamic regimes is one way of solving the problem of increasing the efficiency of drying processes, which includes solving the problems of the process intensity and cost effectiveness, as well as the quality of the final product. A method for estimating the degree of the activity of a hydrodynamic regime is recommended. A strategy for choosing the optimal apparatus-technological design of the drying process is developed that includes the complex analysis of the materials to be dried and the classification of wet disperse materials according to sorption-structural characteristics taking into account adhesion-autohesion properties. Standard apparatuses for each class of materials are recommended. The problem of drying materials with increased adhesion-autohesion properties is chosen. Drying in vortex apparatuses with simultaneous size reduction is considered as an example.     

Numerical and experimental investigations of mixing in T-shaped and cross-shaped micromixers

Mass transfer within the T-shaped and cross-shaped micromixers has been studied using CFD and confocal laser scanning microscopy methods. The concentration profiles, based on flow regimes, were used to compare the T- and cross-geometries. The cross-shaped micromixer tends to intensify the mixing and this is occurring for lower flow rates in comparison to the T shape. The improvement made by the cross geometry is attributed to the stronger vortex stretching and high shear rate, which reduces the liquid transfer length. The presence of a single outlet in the T-shaped micromixer induces a smaller degree of freedom for the fluid. A higher pressure drop is calculated in T-shaped micromixer than in cross-shaped micromixer.     

Numerical Investigation of Flow Patterns and Concentration Profiles in Y‐Mixers

The fluid flow patterns and associated concentration fields in Y‐mixers are investigated using lattice Boltzmann method‐based models. The focus lies on the impact of the mixing angle on the flow and concentration fields, with the mixing angle varying between acute (θ = 10°) and obtuse (θ = 130°) angles. Residence time distributions are determined to study the effect of the angles on the mixing and velocity patterns, in particular, different flow regimes, i.e., stratified laminar, vortex, and engulfment flow. The results from the simulations are validated with literature data and found to be in good agreement. Maximum mixing occurs in the 100° obtuse‐angle Y‐mixer, attributed to the extensive engulfment of flows in the mixing channel.     

Detonation combustion of coal

Results of an experimental study of continuous spin detonation of a coal-air mixture with addition of a certain amount of hydrogen in a plane-radial vortex chamber 500 mm in diameter are presented. The tested substance is fine-grained cannel coal from Kuzbass, which has a particle size of 1–7 μm and contains 24.7% of volatiles, 14.2% of ashes, and 5.1% of moisture. Stable regimes of continuous spin detonation with transverse detonation waves having velocities of 1.86–1.1 km/s with respect to the cylindrical wall of the combustor are obtained for the first time. The mass fraction of hydrogen is 1.5–0.88% of the air flow rate and 50–3.4% of the coal consumption rate. The maximum specific coal consumption rate of 106 kg/(s · m²) is obtained.     

粗糙表面不同粗糙元间局部流动与传热特性

用规则微小横肋模拟固体表面粗糙度,数值模拟和分析粗糙元间局部流动和换热特性.结果显示,在粗糙段各粗糙元间流动结构会发生变化,大体上可分为入口区、中间区和出口区.入口区流动变化比较快,不同粗糙元间壁面涡结构显示出很大不同;中间区则出现较稳定的单涡结构;与入口区类似,出口区流动结构又发生比较大的改变.阻力曲线从入口处粗糙元间的“W”形,逐渐演化到出口处正弦曲线形;换热曲线基本呈中间高、边沿低的趋势.研究表明,特定粗糙段各粗糙元间流态变化导致局部阻力和换热特性有所不同.     

The Effects of Vortex Finder Dimensions on the Natural Vortex Length in a New Cyclone Separator

The shape and structure of the vortex formed inside a cyclone separator are very important for the cyclone efficiency, because they mainly govern the separation process. There are many geometrical and operational parameters affecting the vortex. This paper presents experimental results on the effects of the vortex finder dimensions and the surface friction on the vortex length. The cyclone used in this investigation is cylindrical with no conical bottom. The cyclone pressure drop and the vortex length are recorded for each test performed using different flow rates. The results reveal that an increase of the cyclone height, i.e., of the frictional surface, leads to a decrease of the vortex length. It was also shown that the diameter and length of the vortex finder adversely affect the vortex length.