Porous Screen Research Articles

Methods of monitoring energy processes

The effect of porous materials to conduct heat transfer processes is maximal when used in the focuses of elliptical bodies of revolution. The executed experiment, testing rapidly occurring processes in ellipsoid models of revolution and elliptical cylinders, made it possible to find out opportunities for monitoring energy processes. A holographic pulse installation with a synchronizing device was used. A focused stress wave in a transparent cube was reported. The source of the wave was the second focus of the ellipsoid of revolution. The efficiency of the behavior of a porous screen is shown. It has been established that the effect of energy of incandescent gas leads energy effects in the stress wave and that the lead value may be regulated by concentrator parameters. The article shows the opportunity of dividing energies of the stress wave and gas, thereby obtaining their concentration and transportation in the wave guide, which permits monitoring energy processes. The oscillogram of the stress waves obtained on field samples has shown that the pressure of a focused wave in the second focus is two to three times higher than that of a stress wave and that the time of effect is up to three times longer. In the future the authors plan to extend the field of application of the results obtained.

Comment on 'Wakes of three axisymmetric bodies at zero angle of attack'

the 70% porous glass fiber screen performed best. The roughness of the screen affected the velocity field but had little effect on the temperature field because of the turbulent boundary layer. More details are provided in the references.' The screen was removed when velocity measurements were made with a hot-wire anemometer. We have been investigating the heated-wire technique. The difficulty is that the flowfield of interest involves mixed convection with buoyant flow near the jet edges. However, the technique is of great interest. We appreciate the experience and recommendations of Gartenberg and Roberts.

Cooling of stacks of plates shielded by porous screens

This paper addresses the question of how to cool a stack of parallel, heat-generating boards when the flow is impeded by electromagnetic screens placed upstream and downstream of the stack. Four separate designs are considered: (1) forced convection cooling of a stack with board-to-board spacing selected to minimize the stack-coolant thermal resistance; (2) forced convection cooling of stack with fixed board-to-board spacing; (3) natural convection cooling of a vertical stack with spacing selected to minimize the overall thermal resistance; and (4) natural convection cooling of a vertical stack with fixed spacing. The optimal spacings in designs (1) and (3) are determined by intersecting the known asymptotic solutions for stacks with small spacings and stacks with large spacings. The results of parts (1) and (2) are extended to applications where the stack is cooled by immersion in a free stream. We show that the effect of the screen is controlled by a single dimensionless group, which is identified for each class of designs; namely, forced versus natural convection, and high- versus low-screen Reynolds number. Engineering results are reported for the design of screens made of wire meshes, or perforated plate with square (sharp) edges.

An analytical method to evaluate the efficiency of porous screens as wave dampers

The linear theory for water waves impinging obliquely on dissipative multilayered media is used to evaluate the reflection and transmission coefficients. The case of periodic medium consisting of alternating layers of upright porous walls and water of equal or different thicknesses is examined. Wave propagation in these media exhibits Bragg reflection. Using a plane wave assumption, which involves neglecting the evanescent eigenmodes that exist near the structure boundaries (to satisfy matching conditions), the problem can be reduced to a matrix equation which relates the complex amplitudes of the incident and reflected plane waves to the complex amplitude of the transmitted wave. The problem of wave propagation is analysed for an infinite channel, and for a wave flume in which a paddle generates the waves and a backwall limits waves propagation. The variation of the magnitude of the reflection coefficient, | R|, with k 1Λ is discussed, where k 1 is the wavenumber in the water and Λ is the width of a unit cell consisting of two layers, one porous and another water. Increasing the porous layer width or decreasing the wave period broadens the range of k 1Λ values for which resonance occurs. Furthermore, with increase in the angle of wave incidence, θ, the value of | R| decreases and the dependence of | R| on k 1Λ is damped; for large angle of incidence the reflection is almost constant and negligible. On increasing the number of absorber units, an overall decrease in the reflection coefficient is achieved, but an increase in the number of oscillations between resonant peaks and in the peak amplitude also occurs.

Kinetics of mass transfer across a porous screen under vacuum deposition conditions

Abstract The effect of a porous partition on the characteristics of a molecular flux in a vacuum is studied. Free molecular and transitional cases are considered by the Monte Carlo method. It is shown that the partition temperature and location of the condensation zone substantially affect the molecular flux .

Effects of midheight heating or cooling through a thermal screen on natural convection in a porous layer.

This paper deals with the effects of midheight cooling on natural convection and entropy generation in a porous layer heated from below. A porous screen, which is called as thermal screen in this paper, is adopted for the intermediate cooling with fluid flowing through internal passages formed within its solid matrix. It is shown from a numerical and analytical examination that the midheight cooling, reducing nonuniformity in horizontal temperature distributions, can control convective patterns and the associated heat transfer rate. Also, it is found that the midheight cooling can reduce the net entropy generation through the whole layer for certain convection regimes, and that the additional increase in the entropy generation due to convection is evaluated in terms of the porous Rayleigh number and the temperature level of the thermal screen.

The use of porous screens as wave dampers in narrow wave tanks

Expressions are derived for the reflection of plane incident waves from the closed end of a narrow wave tank when a number of thin vertical porous screens are introduced to damp the waves. The results may have application to the design of wave tanks where a small amount of beach reflection is desirable.

Trapping and absorption of sound waves I a screened sphere

Linear acoustic waves associated with a sphere covered by a concentric porous screen are studied. The effects of a porous screen on the radiation of sound generated by the surface vibration of a sphere, as well as the scattering of an incident plane wave by a stationary sphere, are analyzed based on the method of separation of variables. The acoustic power far away from a screened sphere is compared to that associated with a sphere without a screen. The present results indicate that, as the ratio of the sphere radius to the wavelength, r b/λ, increases, the relative acoustic power becomes a periodic function with respect to the gap-to-wavelength ratio d/λ, white the amplitude is dependent on the screen Reynolds number Re. The optimum gap distance, which minimizes the relative acoustic power, is found to be a quarter wavelength plus a multiple of the half-wavelength. As the radius-to-wavelength ratio r b/λ tends to infinity, the optimum screen Reynolds number, which minimizes the relative acoustic power, is 1.0 for the scattering of a plane incident wave.

Activity of carboxypeptidase a bound to a modified cellulosic matrix

Carboxypeptidase A immobilized on acid chloride of oxidized cellulose showed the following features: (a) as indicated by the linearity of reaction kinetics, the immobilized enzyme action is not diffusion controlled; (b) greater flow rates are achievable with less clogging during continual usage since the enzyme is attached to a porous screen; (c) ease of handling; and (d) no apparent electrostatic interaction with the support material that is uncharged. The immobilized enzyme retained 60% of the original activity. The half-life of free enzyme was only 20 min, whereas for immobilized enzyme it was enhanced up to 2 h 48 min. It could be recovered and repeatedly used.

Continuous conductometric sensor for carbon dioxide

A continuous conductometric sensor is described for the determination of carbon dioxide in the atmosphere and in nonacidic and nonbasic gases. Gaseous carbon dioxide diffuses through a hydrophobic, gas-porous membrane into a thin layer of pure water. The back wall of the thin water layer is a porous screen that separates the water layer from a mixed-bed ion exchanger column. The ion exchanger continuously removes ionic species from the water layer including the products from the dissociation of CO/sub 2/(aq). Conductivity electrodes positioned in the thin water layer measure its conductance. The diffusion of carbon dioxide through the membrane into the thin water layer and the removal of ionic species through the screen by the mixed-bed ion exchanger establish a steady-state concentration gradient of CO/sub 2/. This gradient is proportional to the partial pressure of CO/sub 2/ in the gas phase, and cell conductance is proportional to P/sub CO/sub 2///sup 1/2/. This device was evaluated for use in the determination of carbon dioxide levels up to approx. 1% and conformed to the theoretical predictions.

A Portable Well Point Sampler for Plume Tracking

AbstractFor regions of shallow ground water, a shallow well point sampler can provide cost‐effective sampling of contaminant plumes. A miniature well point sampler allows rapid sampling of ground water down to a maximum depth of 12 feet below grade. A high ratio of striking force to cross‐sectional area of the shaft enables the well point to penetrate soils grading from silt through sand to fine gravel. Three slot sizes have proved satisfactory for the slotted well point. For fine gravel through silty sand, a receiver flask serving as a silt trap with a hand pump can withdraw water samples. For silt or clay, a special shielded point is used to expose a porous screen upon withdrawal of the shaft. The points can be left imbedded in the strata and a vacuum drawn to recover a sample under saturated or unsaturated conditions.Such a sampler has been shown to be effective in tracing plumes from individual on‐site septic systems and hazardous waste sites. The text describes a simple procedure for plume analysis and description of the extent of dewatered strata near the point based upon different sample volumes. The probe has been used with conductivity meters and fluorometers for profiling subsurface plumes. A cost comparison is shown between monitoring well installation and the portable well point for varying sample frequency and depth.

Interaction of shock waves in air with a porous screen

Interaction of shock waves in air with a porous screen

Asymmetric Distortion Generation in a Variable Height Annulus

POROUS screens were placed in the compressor intake to create quasisteady pressure maldistributed inlet flows. This study was aimed at creating a single-lobe circumferential sine-wave velocity profile with no radial components at the compressor inlet. A method relating screen blockage to predicted velocity levels was devised. The method yields velocity levels close to those for which the screen was designed. Contents Quasisteady pressure maldistributed compressor inlet flows are normally created during testing by the inclusion of porous screens in the compressor intake, but interactions between a row of compressor rotor blades and upstream placed screens in such experiments can alter the shape of the generated distortion.l Suppression of this effect may be realized by remote positioning of the screen upstream of the compressor leading-edge plane. Often, an intake may not be a constant annulus, so flow differences between the compressor leadingedge plane and the plane of the screen call for differences in the screen design whose generated distortion is itself modified in passing through the annulus to the compressor. The aim of this study was to present to a compressor inlet a single-lobe circumferential sine-wave velocity profile with no radial components, the distortion being generated remotely upstream in a duct dissimilar to the annulus at the compressor intake. Based upon small perturbation assumptions, a method of screen design is proposed which superimposes a generated distortion upon the undistorted potential flow solution of the developed annulus. The example used was a bellmouth and nose-bullet sited coaxially close to the compressor leadingedge plane and separated only by a short length of the constant height annulus. The screen was set at the bellmouth intake, perpendicular to the axial direction. While this example used symmetrical ducting, the principles involved could be applied to a nonsymmetrical installation.

Displacement reactions in the solid state

Simple displacement reactions in the solid state are considered with the purpose of predict-ing the morphologies and the reaction rates from a knowledge of pertinent thermodynamic and diffusion data. The theoretical predictions are substantiated by experimental observa-tions for four reaction couples [Ni/Cu2O, Co/Cu2O, Fe/Cu2O, and Fe/NiO] reacted at 1000°C. Displacement reactions are classified according to the product morphology; layered and ag-gregate arrangements of the product phases were observed, with two modifications (lamellar and interwoven) occurring within the aggregate morphological class. Parabolic kinetics for the growth of the product phases are observed for each couple. The magnitudes of the para-bolic rate constants for the couples which exhibit the layered arrangement are comparable with calculated values. A technique for controlling the product morphology is discussed, and a process for producing porous metal or oxide screens is introduced.

The Stability of Flow Through Porous Screens

In many cases of the flow through porous screens, one may consider it to be made up of a number of jets passing through the openings of the screen. These jets are separated by a series of wakes behind the solid parts of the screen. The majority of investigations on the flow through such screens have been concerned with the measurement of pressure drop and its variation with different flow conditions; it has been assumed that the pressure is discontinuous at the screen itself and that the pressure drop coefficient Δp/½ρυ2 provides sufficient information, where Δp is the pressure drop across the screen, ρ the density of the fluid, and υ the velocity of approach to the screen.

High Speed Flow through Perforated Plates

The flow through porous screens has been widely studied from both the theoretical and experimental points of view. The most widely used types of screen are the wire mesh and the perforated plate, and the majority of the literature has been concerned with the former. Several attempts have been made to correlate the parameters governing the flow through such screens, i.e. the pressure drop, the flow conditions and the geometry of the mesh.

WINDSCREENS FOR HARBOURS

Big ships may sometimes be prevented from mooring when piers are not shielded against gusts from cross winds. Forces large enough to break the mooring cables can also develop. 
 
 To prevent these difficulties, a windscreen had to be built in 1947 in Le Havre harbour. It was a plain concrete wall, 240 m long and 21.5 m high, with a curved side facing the most frequent wind. Quite a few wide-apertures were provided across the wall, so as to feed, in a way, the wake downstream from the wall.
 
 The question of building a protecting screen has been raised again recently in Marseille, to provide a shield along one of the piers which is open to cross winds (Mistral). I have suggested designing a porous screen, made of a plane thin wall with numerous small holes regularly spaced, as in a perforated plate. I had reasons to believe, and it has been confirmed by experiment on small scale models, that a porous screen could provide a shield as efficiently as a plane wall. On the other hand a porous screen would offer many advantages when compared to a plain wall, as a) reduced stresses from gust effects, b) lighter and simpler structure, and c) eddies in the wake would not be as large.

Acoustic Impedance Measurement of Very Porous Screen

A new procedure is presented for the measurement of the normal specific acoustic impedance of extremely porous screen as a function of frequency. The method, restricted to this case, is based on a special application of the Transmission-Characteristic Method of impedance measurement and can be applied to screens having impedances even less than 1/100 the characteristic impedance of air. Relatively simple experimental equipment and computation is required. Measured values of the impedance for two samples of extremely porous screen, determined by this procedure, are given.

Acoustic Impedance Measurement of Very Porous Screen

A new procedure is presented for the measurement of the normal specific acoustic impedance of extremely porous screen as a function of frequency. The method (based on a special application of the transmission-characteristic method of impedance measurement) is restricted to this case. Equations are derived which express the impedance of the screen in terms of the acoustical characteristics of a small enclosure, i.e., a rectangular chamber or a closed cylindrical pipe. Good accuracy can be obtained with relatively simple experimental equipment and with relatively little computation. Measured values of the impedance for samples of extremely porous cloth, determined by this procedure, are given.

Measurement of Sound Absorption by a Modified Tube Method

Measurements of the sound absorbed by small samples of material one foot square were obtained for five pitches,—4096, 2048, 1024, 512, 256, with numerical calculations of coefficients for the three higher pitches. Diffraction effects at the pitches 512 and 256 made the calculations uncertain. Sound from an electrodynamic loudspeaker passed through a cast iron tube 8 inches in diameter and 2 feet long where it impinged on the material placed at an angle of 45° to the axis of the tube. It was thus reflected so as to pass at right angles to its original direction through a side opening in the tube to a condenser microphone where it was measured. The path from the material to the microphone was enclosed by a tube of ground cork 2 inches thick held in place by porous metal screening. Additional measurements were taken with the sample removed (theoretically, 100 percent transmission, 0 percent reflection) and also with a thick glass plate replacing the material (100 percent reflection). Further experiments are in progress to get coefficients at the lower pitches.