Dust-laden Gas Research Articles

Theoretical analysis of dust removal from gas impinging on a confined liquid surface

Impingement of dust-laden gas on a confined liquid surface is analysed theoretically by assuming negligible viscous effect of the gas and no interaction of dust movement with the gas flow field. The analysis indicates that for a specific dust size there exists a certain nozzle size below which the dust is removed completely. The dust-removal efficiency η depends on the impaction number N I , the vertical gas velocity at the center of the nozzle exit λ, and the nozzle halfwidth r according to the equation λη + [ 2 (π − 2) ](1 − λ)(sin −1 η − η) = N I[( 1 r ) − η] .

EXPERIMENTAL INVESTIGATION OF A WET DUST SCRUBBER: DUST COLLECTION BASED ON TURBULENT DIFFUSION

ABSTRACT The “nozzle scrubber” is a wet scrubber in which the scrubbing water is dispersed in the dust laden gas stream by means of one or more pneumatic nozzles. This scrubber is distinguished by an excellent collection efficiency for submicron dust at an unusually low energy and water consumption. No well-defined theory exists for this process. The collection efficiency in the “nozzle scrubber” depends primarily on turbulent diffusion respectively on the interaction of particles and droplets induced by turbulence, and not on inertial separation as in the case of the venturi scrubber. A light scattering device was used to measure the particle distributions. The experimental set-up was built up in a technical scale. The influence of operation parameters, especially water consumption, residence time, and pressurized air, on the grade efficiency has been demonstrated by their systematic variation. The contribution of turbulent diffusion to the collection efficiency has been confirmed.

Capture of fine particles on charged moving spheres: a new electrostatic precipitator

A study of a novel electrostatic approach to fine-particle collection was carried out on a specially designed test apparatus. Copper spheres were first electrostatically and then dynamically suspended between two horizontal parallel plates using an applied DC electric field. The sizes studied were 74-88, 102-25, and 125-47 mu m. The charged spheres, which were larger by a factor of 10 to 100 than the dust captured (mean diameter 2.4 mu m), were then electrostatically propelled into an adjacent duct where they scrubbed a crossflowing dust-laden gas by the processes of impaction and electrostatic attraction. The total collection efficiency of the device and the target collection efficiency of the charged copper spheres were studied. Both were found to increase with the charging electric field strength, the target particle diameter, and the duct air velocity. This research suggests that electrostatic precipitation based on the scrubbing action of charged solid particles is possible, and that total collection efficiencies of up to 99% cleanup (or greater) should be attainable. Fly-ash collection performance exceeding that based on theory with regard to the particle target efficiency was observed. >

Pyrometric measurement of dust-laden gas temperature

Continuous measurement using radiation pyrometry under industrial conditions has often met with difficulties whose causes have been obscure. In this paper, the experience gained in utility boilers, with temperatures in the range of 800 °C to 1400 °C, is reported. It is shown that the inherent problems are best resolved by using a spectral pyrometer not sensitive to gas radiation. The radiation from the suspended particles thus provides the signal. A theoretical analysis is given. Heat fluxes originating from surrounding surfaces and reflected by the particles to the pyrometer play a major role. Several examples are given how problems may be resolved using process information and/or additional measurements, fed to and processed by an on-line microcomputer.

Investigation of a wet dust scrubber with a pneumatic nozzle: Dust collection based on turbulent diffusion

AbstractThe “nozzle scrubber” is a wet scrubber in which the scrubbing water is dispersed in dust laden gas stream by means of one or more pneumatic nozzle. This scrubber is distinguished by an excellent collection efficiency for submicron dust at an unusually low energy and water consumption. So far, the physical effects affecting the separation cannot be explained by a well‐defined theory. Therefore, it is sensible to investigate the collection efficiency with regard to the mechanisms of inertial impaction, turbulent diffusion and coalescence induced by turbulence. The experimental equipment is of a very simple design. A light scattering device was used to measure the particle distributions. In addition, electron micrographs were analyzed to obtain information about the submicron particles. The influence of operating parameters on grade efficiency has been demonstrated by their systematic variation. The contribution of turbulent diffusion to the collection efficiency has been confirmed; nevertheless, grade efficiencies were also measured when inertial impaction prevailed.

Combined inertial and thermophoretic effects on particle deposition rates in highly loaded dusty-gas systems

Little is yet known (theoretically or experimentally) about the simultaneous effects of particle inertia, particle thermophoresis and high mass loading on the important engineering problem of predicting deposition rates from flowing “dusty” gases. For this reason, we investigate the motion of particles present at nonnegligible mass loading in a flowing nonisothermal gaseous medium and their deposition on strongly cooled or heated sold objects by examining the instructive case of steady axisymmetric “dusty-gas” flow between two infinite disks: an inlet (porous) disk and the impermeable “target” disk—a flow not unlike that encountered in recent seeded-flame experiments. Since this stagnation flow/geometry admits interesting self-similar solutions at all Reynolds numbers, we are able to predict laminar flow mass-, momentum- and energy-transfer rate coefficients over a wide range of particle mass loadings, dimensionless particle relaxation times (Stokes numbers), dimensionless thermophoretic diffusivities, and gas Reynolds numbers. As a by-product, we illustrate the accuracy and possible improvement of our previous “diffusion model” for tightly coupled dusty-gas systems. Moreover, we report new results illustrating the dependence of the important “critical” Strokes number (for incipient particle impaction) on the particle mass loading and the wall/gas temperature ratio for dust-laden gas motion towards “overheated” solid surfaces. The present formulation and resulting transport coefficients should not only be useful in explaining/predicting recent deposition rate trends in “seeded”-flame experiments, but also highly mass loaded systems of technological interest, such as the deposition of opto-electronic materials by jet impingement, and fouling layers from ashladen fossil fuel combustion products.

DISTRIBUTIONS OF THE TANGENTIAL VELOCITIES ON THE DUST LADEN GAS FLOW IN THE CYLINDRICAL CYCLONE DUST COLLECTOR

ABSTRACT The authors measured the distributions of the tangential velocities of the feed dust concentration Co = 0. 6 ~15. 0 g/ m3 by the hot wire anemometers. The test dust was fly-ash which had the mean diameter Xpn = 2.03 μm. The used cylindrical cyclone had the diameter D1 = 140 mm, the diameter of the exit pipe D2 = 50 mn and the total length HT = 382 mm. In order to measure the fluctuating velocities and the time mean velocity on the dust Iaden gas flow and on the pure air flow, we had applied two kinds of the hot-wires. The flow Reynolds number Rec was Rec = Qo/HiV = 797~5582. The expert mental results sho-ed that there were no distinct differences of the time mean and fl uctuating velocities between the pure air flow and the dust laden gas flow. Then the authors have tried to explain those expert mental results by the response time of the fine solid particles and that of the scale of the nixing length in the turbulent rotational air flow.

Mechanical high-vacuum pumps with an oil-free swept volume

In both corrosive and clean duties the widely used oil lubricated rotary pump has certain disadvantages. In corrosive applications aggressive vapors and/or abrasive particles tend to contaminate the lubricant of the pump, which shortens its service life, while in clean applications the lubricant may cause contamination of the vacuum process. To overcome these problems a new multistage pump has been developed to work from atmospheric pressure to 0.01 mbar without any lubricant within its pumping chamber. The pump is a multistage positive displacement rotary machine in which a number of intermeshing rotor pairs of different profiles mounted on common shafts are held in correct phase relation without contact by a pair of timing gears. The construction is robust designed for industrial as well as for clean vacuum processes which can benefit by the exclusion of lubricant, sealing, or working fluid from the swept volume of the pump. Handling of condensable vapors or dust laden gas is assisted by gas ballast and versatile orientation of pump mounting. Pump sizes of 80 and 180 m3 h−1 displacement have been developed. The paper explains the reasoning that led to the choice of different mechanisms which have been combined in the pump, its performance and construction.

Existence of convergent Chapman-Jouguet detonation waves in a dust-laden gas

The author constructs an asymptotic solution to the problem of flow near a spatially curved Chapman-Jouget wave propagating in a polydisperse mixture of a fuel gas and chemically inert particles. It is shown that detonation wave can propagate in the dust-laden gas in this regime until their mean curvature exceeds the critical value H greater than O. A formulation of the problem is presented mathematically and a method of solution is given with results.

Effect of blowing on supersonic dust-laden gas flow round a blunt body

One of the main problems which arise in the design of high-speed aircraft is the protection of the streamline surfaces against the erosion effect of solid particles and drops occurring in the free stream. For this purpose it is possible to use the device of blowing cold gas. This leads to the formation of a boundary layer of high density in which the particles are decelerated [1]. The present study investigates the effectiveness of this method of erosion protection in the example of supersonic flow round a sphere.

4485684 Apparatus for extracting and analysing dust-laden gas samples : Rudolf Weber, Horst Riedel, Oelde, Federal Republic Of Germany assigned to Krupp Polysius AG

4485684 Apparatus for extracting and analysing dust-laden gas samples : Rudolf Weber, Horst Riedel, Oelde, Federal Republic Of Germany assigned to Krupp Polysius AG

Effect of particles on the rate of turbulent transport of a dust-laden gas

Simple approximations in equations of single-point second moments were used to analyze the effect of particles on the intensity of pulsative motion in a nonuniform turbulent flow.

Asymptotic laws of damping of weak continuous and shock waves in dust-laden gas

Analytic investigations into the damping of perturbations in dust-laden gas have been restricted to self-similar flows [1, 2] and flows with a symmetry plane, it being assumed in the latter case that thermal and velocity equilibrium of the phases is established instantaneously [3–6], i.e., the relaxation time of the medium is short. In the present paper, asymptotic laws of damping are obtained for plane, cylindrical, and spherical shock and continuous waves whose amplitude and width are such that the acceleration of the particles and the change in their temperature can be ignored. It is assumed that between the phases there is heat transfer proportional to the temperature difference and frictional momentum transfer proportional to the difference between the velocities of the phases. The obtained laws of damping of plane waves are found to be entirely analogous to the laws of damping of magnetohydrodynamic waves in a medium with finite conductivity, when the presence of Joule dissipation and the additional ponderomotive force in the traveling wave or in the gas flow behind the shock wave leads to exponential damping of the wave amplitude [7–9].

Applicability of approximate models in calculating the boundary region in dust-laden flows

The limits of application of “single-particle” and “quasihomogeneous liquid” models are established on the basis of analysis of Couette flow for a dustladen gas.

Technology Status of Particulate Removal from High-Temperature High-Pressure Combustion Gases

The adoption of coal-fired pressurized fluidized-bed combined-cycle combustion systems by utilities for electric power generation depends to a large extent on the development of an efficient and economic cleanup system between the combustor and gas turbine. To accomplish particulate removal from a dust-laden gas stream, a number of separation devices have been developed. These include conventional and augmented cyclones; porous metal, fabric, and ceramic membrane filters; fixed-, continuously moving-, and fluidized-bed granular filters; electrostatic precipitators; as well as several other novel separation devices. The results of tests of these devices are discussed in relation to pressurized fluidized-bed combustion applications.

Supersonic Flow of a Lightly Dust-Laden Gas past a Wedge

The flow field behind an attached shock wave produced by a wedge fixed in a uniform stream of a gas containing small dust particles is examined theoretically by a perturbation method. The shock wave causes a deviation from an equilibrium state and bends downstream as a result of interaction with Mach waves. Far downstream an entropy layer is formed near the surface of the wedge, across which the pressure is constant but other flow variables change appreciably. Numerical values of the temperature and the pressure of the gas are given for a few cases showing that these variables take then maximum in the relaxation region next to the shock wave.

Problems of the hydrodynamics of a bed fluidized by dust-laden gas

The results of an experimental investigation of the effect of the degree of dust contamination of the air on the hydraulic resistance and velocity of the start of fluidization of a bed of spherical particles with an average diameter of 2.00–9.35 mm are presented. Recommendations are given for calculating the frictional resistance coefficient and the velocity of the start of fluidization.

Further studies on the electro-aerodynamic precipitator

In an electro-aerodynamic precipitator a set of high voltage wire electrodes is installed at a finite distance from a set of collector plates of the opposite polarity. The direction of the electric field formed between the electrodes and the plates is aligned with the direction of the dust-laden gas flowing into the precipitator. These alternately charged plates mounted on a drum are then rotated below cleaning shoes where the dust is removed and drained into a sump. In such a configuration, electric wind and field have the same directions as the dusty gas flow. This, together with small passages which maintain laminar flow, eliminates reentrainment by electric wind and turbulence. The corona current also increases with dust loading, thus maintaining high operating efficiency at heavy loading. A small-scale preliminary experimental study has demonstrated that such a precipitator provides consistently high efficiency (above 99%), compactness, and light weight. When collecting fly ash it does not depend on the sulfur content of the ash for high operating efficiency. Highest efficiency obtained so far has been 99–98% with cigarette smoke.

Entropy losses in the flow of a dust-laden gas suspension in a nozzle

The unidimensional steady-state flow of a gas containing a system of solid particles in a nozzle is considered for the case in which the particles are characterized by a constant velocity lag; expressions are derived for the entropy losses in efficiency due to the friction between the gas and the particles and the irreversibility of interphase heat transfer.

Methods of intensifying the operation of electric filters for extracting high-resistance magnesite dust

Cyclones installed as the first stages of purification and operating with relative gas velocities of 6 m/sec, upon entry of the dust-laden gas containing 40–55 g/nm3, have an efficiency of 0.82, which is higher than the planned level (equal to 0.70).