Showing papers on "Diffuser (thermodynamics) published in 1995"

Experimental investigation of flow through an asymmetric plane diffuser

Abstract: There is a need for experimental measurements in complex turbulent flows that originate from very well-defined initial conditions. Testing of large-eddy simulations and other higher-order computation schemes requires inlet boundary condition data that are not normally measured. The use of fully developed upstream conditions offers a solution to this dilemma so that the upstream conditions can be adequately computed at any level of sophistication. The plane diffuser experiment by Obi et al. (1993) has received a lot of attention because it has fully-developed inlet conditions and it includes separation from a smooth wall, subsequent reattachment and redevelopment of the downstream boundary layer. The objective of this study is to provide careful qualification and detailed measurements in a recreation of the Obi experiment. The work will include extensive documentation of the flow two-dimensionality and detailed measurements required for testing of flow computations.

Turbomachinery with variable angle fluid guiding devices

Abstract: A turbomachinery is presented to provide stable operation at fluid flow rates much lower than the design flow rate without introducing surge in the device. This is achieved by providing a diffuser with variable angle vanes. The vane angle at low flow rates is adjusted so as to minimize the diffuser loss of the exiting fluid stream from the impeller. Since the flow angle of the exit flow of the impeller is a function only of the non-dimensional flow rates, and does not depend on the flow angle at the inlet the impeller, therefore, the vane angles can be regulated to achieve a stable operation of the impeller without producing surge of the turbomachinery at flow rates lower than the design flow rate. To optimize the performance of the turbomachinery, in addition to the variable angle vanes, an inlet guide vane having variable vane angle is provided so that the turbomachinery can be operated at the required flow rate and head pressure. The concept is demonstrated in a turbomachinery provided with variable diffuser vanes and an inlet guide vane.

Micro-fluid Flow In Microchannel

Abstract: General dynamic analysis for fluid flow in microchannel was presented. A microfluid flow measuring system is set up for experimental study, then the microfluid flow behaviour in microchannel with different cross sections and characteristic size is investigated under the pressure difference ranged from 0-200Kpa. The Silicon nozzle/diffuser elements are fabricated and the flow directing property of them is investigated under 0-10 Kpa . The experiment results agree well with the expected.

Apparatus for inflating an inflatable vehicle occupant restraint

Abstract: An apparatus (10) for use in inflating an inflatable vehicle occupant restraint (12) includes a pressure vessel (14), an actuator assembly (16), and a diffuser (20). The pressure vessel (14) defines a closed storage chamber (18) containing inflation fluid. The actuator assembly (16) opens the storage chamber (18) to release the inflation fluid from the storage chamber (18), and includes a charge of pyrotechnic material (100). The diffuser (20) directs the inflation fluid from the pressure vessel (14) into the vehicle occupant restraint (12) to inflate the vehicle occupant restraint (12). The apparatus (10) further includes a body (160) of combustible material which emits combustion products into the inflation fluid to increase the volume of the inflation fluid at a location outside of the storage chamber (18) by heating the inflation fluid outside of the storage chamber (18). The body (160) of combustible material is located outside of the storage chamber (18) between the storage chamber ( 18) and the vehicle occupant restraint (12).

Surge detection device and turbomachinery therewith

Abstract: A turbomachine having variable-angle diffuser vanes operates by regulating the angle of the diffuser vanes on the basis of the sensors disposed on the pump body or pipes of a surge detection device. The onset of surge can be forecast by measuring the fluctuations in the operating parameter(s) over a measuring interval of time computed on the basis of the operating characteristics of the impeller of the turbomachine. The onset of surge is prevented by adjusting the angle of the diffuser vanes in accordance with the sampling duration for parameter fluctuations over the measuring interval of time, and by adjusting the diffuser vanes to maintain the operating parameter fluctuations of the fluid machinery below a threshold value of the turbomachine derived from the design flow rate of the turbomachine. Application of the surge detection device in combination with fluid flow guide vanes and blades of the turbomachine enables full utilization of the potential capability of the turbomachine.

A Valve-less Planar Pump In Silicon

Abstract: A planar double chamber pump fabricated in silicon anodically bonded to glass and based on the diffuser/nozzle pump principle is presented. The diffuser/nozzle elements have a depth of 24 - 48 /spl mu/m and a neck width of 88 - 104/spl mu/m. The pump chamber diameter is 6 mm. Pump cavities and diffuser/nozzle elements are etched with an isotropic HNA-etch in the silicon. Pumps with three different diffuser lengths are compared reaching a maximum pump capacity of 230 /spl mu/min and a maximum pump pressure of 1.7 in H20 at a resonance frequency of 1318 Hz for methanol.

Automatic swimming pool cleaners and associated components

Abstract: Automatic swimming pool cleaning systems are disclosed. The systems include an automatic swimming pool cleaner (10) incorporating one or more curved wipers (30) protruding from its lower surface. The wipers (30) and flat bottom (34) of the cleaner (10) induce a vortex as fluid flows to its central inlet (50) entraining debris in the flow. A fluted throat (102A) surrounding the inlet (50) additionally assists the interior of the base of the cleaner (10) in retaining debris when opened for cleaning, additionally providing an improved flow path for enhanced debris pick-up. Also disclosed is a valve (200) useful for controlling and indicating the rate of fluid flow to a cleaner (10). Included within the valve (200) is a plunger (203) attached to a spring (202), with the spring force opposing fluid flow through the valve (200). Flow sufficient to overcome the spring force compresses the spring (202), however thereby moving the plunger (203) within the tube (201) and providing a dynamic indication of the rate of fluid flow. A diffuser (206) and fluid release ports (204A) permit the valve (200) to be adjusted to achieve a desired flow rate.

Fire protection nozzle

Abstract: A fire protection nozzle (10) has a base (14) with an inlet section (18) leading to an orifice (30). A diffuser element (40) is positioned coaxially with and downstream of the orifice (30) and one or more arms (24, 26) extend from the base (14) and support the diffuser element (40) in a position, where, when flow of the fire-retardant fluid from the inlet section (18) through the orifice (30) is established, the fire-retardant fluid emerges from the orifice (30) in a stream which impinges on a diffuser surface (42) defined by the diffuser element (40) to be distributed in a spray pattern. The diffuser surface (42) defined by the diffuser element (40) is generally spherical in shape in a region extending from an upstream end closest to the orifice (30) to at least downstream of an equatorial plane of the diffuser element (40) transverse to the conduit axis (Co).

Liquid Jet Pumps for Two-Phase Flows

Abstract: Isothermal compression of a bubbly secondary fluid in a mixing-throat and diffuser is described by a one-dimensional flow model of a liquid-jet pump. Friction-loss coefficients used in the four equations may be determined experimentally, or taken from the literature. The model reduces to the liquid-jet gas compressor case if the secondary liquid is zero. Conversely, a zero secondary-gas flow reduces the liquid-jet gas and liquid (LJGL) model to that of the familiar liquid-jet liquid pump. A ``jet loss`` occurs in liquid-jet pumps if the nozzle tip is withdrawn from the entrance plane of the throat, and jet loss is included in the efficiency equations. Comparisons are made with published test data for liquid-jet liquid pumps and for liquid-jet gas compressors. The LJGL model is used to explore jet pump responses to two-phase secondary flows, nozzle-to-throat area ratio, and primary-jet velocity. The results are shown in terms of performance curves versus flow ratios. Predicted peak efficiencies are approximately 50 percent. Under sever operating conditions, LJGL pump performance curves exhibit maximum-flow ratios or cut-offs. Cut-offs occurs when two-phase secondary-flow steams attain sonic values at the entry of the mixing throat. A dimensionless number correlates flow-ratio cut-offs with pump geometry and operating conditions.more » Throat-entry choking of the secondary flow can be predicted, hence avoided, in designing jet pumps to hand two-phase fluids.« less

Dual chamber inflator for side impact air bag

Abstract: An air bag inflator (20, 200) comprises first and second tubes (24, 26, 204, 206) defining respective first and second chambers (44, 46, 244, 246) for storing inflation fluid under pressure. A manifold (22, 202) has a manifold passage (42, 222). Each of the first and second tubes (24, 26, 204, 206) is secured to the manifold (22, 202) with the first and second chambers (24, 26, 204, 206) in fluid communication with the manifold passage (42, 222). A diffuser housing (28, 208) is fixed to the manifold (22, 202) and defines a third chamber (104, 242) for fluid communication with the manifold passage (42, 222). Passages (106, 244) in the diffuser housing (28, 208) provide fluid communication between the third chamber (104, 242) and an air bag. A rupturable wall (142, 248) is fixed to the diffuser housing (28, 208) to block fluid communication of the third chamber (104, 242) with the manifold passage (42, 222). An actuatable initiator (122, 246) is in the third chamber (104, 242) for, upon actuation, rupturing the wall (142, 248) to enable fluid communication of the third chamber with the manifold passage (42, 222).

Piston-type check valve with diffuser

Abstract: An axial recess is formed in the downstream end of the diffuser body. An annular tail member is mounted on the downstream end of the valve piston. The tail member retracts into the recess when the piston is in the closed position and provides an extension of the diffuser body when the piston is in the open position.

Calculation of laminar separated flow in symmetric two-dimensional diffusers

Abstract: This study is concerned with numerical analysis of laminar separated flow in symmetric, two-dimensional, straight-walled diffusers. With Reynolds numbers Re=56 and 114 and expansion ratios ER=3 and 4, totally, there are four cases considered. At the low Reynolds number and the low expansion ratio the flow in the diffuser is nearly symmetric to the center line, irrespective of the diffusion angle. As Reynolds number or expansion ratio increases, a large recirculation region forms at one side wall and a small one at the other side. For the case with Re=114 and ER=4 the small recirculating flow disappears at small main recirculation region for large diffusion angles. The pressure recovery reaches its peak value somewhere downstream of the reattachment point of the large recirculating flow. The effectiveness of the diffuser deteriorates as the diffusion angle increases, aprt from that at Re=56 the effectiveness increases from θ=15 to 30 deg. Symmetric flow solutions can be obtained by incorporating a symmetric relaxation method. The pressure recovery is higher for the symmetric flow than that for the asymmetric flow owing to the weaker recirculating strength in the former.

Varying permeability filter for airbag inflator

Abstract: A new and improved airbag inflator including a varying permeability filter. The inflator includes a housing for holding gas generating materials and has an outer diffuser wall with a plurality of discharge or diffuser ports arranged to direct the gas into an airbag for rapid inflation. The housing includes a ring of diffuser ports spaced apart around the periphery of the outer diffuser side wall and these ports are generally closer to one of the top or bottom end walls of the container than they are to the opposite end wall. A new and improved gas filter of varying permeability is mounted in the housing inside the periphery of the outer diffuser wall adjacent to the ring of ports. The annular filter has a gas permeability characteristic that varies between a maximum permeability rating in a first region farthest away from the ring of ports to a minimum permeability rating adjacent a second region closer to the ports. The varying permeability areas tend to equalize or even out the gas flow over the entire available flow cross-section of the filter so that blow-through or burn holes do not develop. Moreover, the more uniform and even flow through the entire available flow cross-section results in a maximum of filtering and entrapment of contaminants as well as excellent cooling of the hot gases with the least flow resistance overall to provide rapid inflation of the airbag during deployment.

Turbomachinery having a variable angle flow guiding device

Abstract: A turbomachinery having variable angle diffuser vanes is demonstrated with the use of a centrifugal pump. The performance of a diffuser is enhanced greatly by the use of adjustable angle diffuser vanes which can be set to a wide range of vane angles to provide a variable size of an opening between adjacent vanes. The demonstrated pumping system has a significantly wider operating range than that in conventional pumping systems over a wide flow rate, and is particularly effective in the low flow rate range in which known diffuser vane arrangements would lead to surge in the entire system and other serious operational problems. A number of concrete examples and formulae are given to demonstrate the computational methods used to select a vane angle for a given set of operating conditions of the turbomachinery.

Method and apparatus to achieve passive damping of flow disturbances in a centrifugal compressor to control compressor surge

Abstract: An apparatus achieves passive damping of flow disturbances to control centrifugal compressor surge. The apparatus includes a centrifugal compressor for compressing a low pressure fluid. The centrifugal compressor has an impeller, an inlet which communicates with an atmosphere and a discharge through which compressed air is supplied to a compressed air system. A fluid flow control is flow connected with the inlet for controlling the flow of a low pressure fluid to the compressor. A check valve is flow connected with the discharge for preventing high pressure fluid from back flowing to the compressor. A vane diffuser assembly fluidly communicates with the impeller. A spring-mass-damper system is coupled to any one or all of the fluid flow control, check valve or vane diffuser to dampen low amplitude flow disturbances of the compressible fluid.

Laser Anemometer Measurements of the Three-Dimensional Rotor Flow Field in the NASA Low-Speed Centrifugal Compressor

Abstract: A laser anemometer system was used to provide detailed surveys of the three-dimensional velocity field within the NASA low-speed centrifugal impeller operating with a vaneless diffuser. Both laser anemometer and aerodynamic performance data were acquired at the design flow rate and at a lower flow rate. Floor path coordinates, detailed blade geometry, and pneumatic probe survey results are presented in tabular form. The laser anemometer data are presented in the form of pitchwise distributions of axial, radial, and relative tangential velocity on blade-to-blade stream surfaces at 5-percent-of-span increments, starting at 95-percent-of-span from the hub. The laser anemometer data are also presented as contour and wire-frame plots of throughflow velocity and vector plots of secondary velocities at all measurement stations through the impeller.

Large volume beverage dispensing nozzle

Abstract: A beverage dispensing nozzle (10) which dispenses beverages at a high volume flow comprises a housing (11) which includes a body (52) and a cap (14) having an opening therein connected to the top of the body (52), a conduit (12) in the lower portion of the body (52), a diffuser (13) residing over the conduit (12), and a diffuser plate (44) positioned about the diffuser (13) The nozzle (10) connects to a standard electric valve which communicates with both a beverage syrup source and a mixing fluid source The diffuser (13) includes a passageway (27) which communicates with the beverage syrup source to deliver beverage syrup to a mixing chamber (51) The exterior body of the diffuser (13) and the interior surface of the body define a second channel (48) The diffuser plate (44) resides within the first channel (29) and includes a plurality of holes that produce a laminar flow in the mixing fluid stream as it enters the first channel (29) Upon exit from the first channel (29), the conduit (12) divides the mixing fluid stream into a first and second mixing fluid streams

Laser liquid level gauge with diffuser

Abstract: The liquid level sensor includes a modulated laser diode and collimating optics for collimating the diode output to produce an optical measurement signal beam transmitted to the fluid boundary through a diffuser to diverge the beam. A detector detects reflections of the beam from the fluid boundary to produce an output signal which is amplified and inverted to drive the modulator. The frequency of the modulation, which depends on the distance to the fluid boundary, is measured and the distance is derived from that measurement.

Airless spray gun diffuser

Abstract: An improved poppet valve seat having a predetermined through-bore restricting orifice operating in conjunction with a fluid diffuser and diffuser flow passages thereof reduce the velocity of fluid ejected from the spray gun below levels which are hazardous for potential injection into the skin of a user or the like when the spray tip and associated safety guards are removed from a spray gun. However, while the velocity of the paint passing through the spray gun is reduced to avoid the injection hazards, the atomization pressures required for cohesive and complete atomization of the fluid passing through the airless paint sprayer is maintained to provide an efficient and effective atomization of the fluid in an airless sprayer.

Cleaner for a rotating screen on a harvester

Abstract: A self-cleaning device for filtering air in a harvester includes a venturi ejector connected to a source of pressurized air which, in a cotton harvester, includes the cotton handling system blower. The ejector includes nozzle, a vacuum chamber shaped to minimize dead air zones and eddy currents, and a diffuser section. The vacuum chamber is supported near a rotating air inlet screen to vacuum debris from the inlet side of the screen. The debris is drawn into the negative pressure air stream upstream of the nozzle and is positively directed away from the engine compartment and other components on the harvester. The engine cooling fan receives only screened air, and the use of a source of air which is independent of the cooling fan, as well as any separate screen cleaning fan, helps to retain maximum engine cooling capacity.

Pressure Fluctuation Downstream of Diffuser Pump Impeller.

Abstract: Instantaneous pressures were measured on the casing wall of a guide vane passage and volute casing of a diffuser pump in order to investigate the interaction between impeller blades and diffuser vanes. The pressure fluctuations were discussed using statistical analysis as well as the ensemble-averaging technique. Frequency spectra of unsteady pressure data show that the dominant frequencies are the impeller blade passage frequency and its higher harmonics. Moreover, the ensemble-averaged pressure data show the spatial distribution of unsteady pressures, which illustrates the rotation of the circumferential pressure mode due to rotor-stator interaction.

Underbead Weld Shield Method and Apparatus

Abstract: A weld shield apparatus provides an atmosphere of inert gas within a conduit in an underbead region where an external weld joint is to be produced. The weld shield apparatus includes a diffuser structure having a convex exterior surface that is defined by a wall structure, the convex exterior surface is shaped to fit within the conduit and to be separated from an inner wall of the conduit by a clearance space. The diffuser wall structure is comprised of a porous material which enables gas communication between an inlet to the diffuser and the convex exterior surface. A seal is positioned immediately upstream from the diffuser structure and is positioned to engage the inner walls of the conduit so as to prevent the passage of gas in an upstream direction. A source of gas is coupled to the inlet to the diffuser structure and enables inert gas to pass through the porous material and to flow in a laminar manner, via the clearance space, past the underbead region and into the downstream region of the conduit. The resulting laminar flow and use of a neutral density gas enable small amounts of gas to be employed as a weld shield.

Regenerative combustor cooling in a gas turbine engine

Abstract: A combustor, for a gas turbine engine, employing regenerative combustor cooling. The combustion gas flow direction extends generally longitudinally aft of the combustor fuel nozzle. A coolant flowpath between the combustor casing and the combustor liner has: 1) a longitudinally aft inlet in fluid communication with a source of compressor-derived cooling air, of lower temperature and higher pressure than diffused air from the combustor diffuser; and 2) a longitudinally forward outlet in fluid communication with the combustor fuel nozzle for "spent" cooling air to be used for combustion.

Detailed Measurements on a Modern Combustor Dump Diffuser System

Abstract: An experimental investigation has been carried out to determine the flow characteristics and aerodynamic performance ofa modern gas turbine combustor dump diffuser The system comprised a straight walled prediffuser, of area ratio 135, which projected into a dump cavity where the flow divided to pass either into the flame tube or surrounding feed annuli In addition, a limited amount of air was removed to simulate flow used for turbine cooling The flame tube was relatively deep, having a radial depth 55 times that of the passage height at prediffuser inlet, and incorporated burner feed arms, cowl head porosity, cooling rings, and primary ports Representative inlet conditions to the diffuser system were generated by a single-stage axial flow compressor Results are presented for the datum configuration, and for a further three geometries in which the distance between prediffuser exit and the head of the flame tube (ie, dump gap ) was reduced Relatively high values of stagnation pressure loss were indicated, with further significant increases occurring at smaller dump gaps These high losses, which suggest a correlation with other published data, are due to the relatively deep flame tube and short diffuser length Furthermore, the results also focus attention on how the presence of a small degree of diffuser inlet swirl, typical of that which may be found within a gas turbine engine, can result in large swirl angles being generated farther downstream around the flame tube This is particularly true for flow passing to the inner annulus

Operating results and aerosol deposition of a venturi scrubber in self-priming operation

Abstract: This study deals with behaviour and washing efficiency of a venturi scrubber in self-priming operation. Usually the washing liquid is injected into the throat by means of a pump, in such a way that the amount of liquid added per cubic metre of gas is adjustable independent from the gas flow rate. In contrast to this kind of design, the venturi scrubber used works via a self-priming operation, i.e. the washing liquid is injected by means of a pressure difference between the inside and outside of the venturi throat as a result of the hydrostatic pressure of the liquid and the static pressure of the flowing gas. As is well known from the literature, the cleaning efficiency of a venturi scrubber improves with the amount of liquid added per volume of gas and with increasing gas velocity in the throat. However, high gas velocities and high charges of washing liquid cause a large pressure drop. Hence, the separation efficiency and energy consumption of the scrubber have to be optimized. It is shown that the separation efficiency could be improved by a multistage injection of the washing liquid. Due to the self-priming operation, the separation efficiency remains at a high level even if the gas velocity decreases, and thus requires no regulation from the outside. Liquid separation after the venturi scrubber is realized by an immersion tube in combination with swirl promotors in the diffuser section of the scrubber which increase the rotation of the gas—liquid flow. Thereby, droplets are pushed aside to the diffuser walls and are deposited.

Fluid flow measurement assembly

Abstract: A fluid flow measurement assembly is shown and described. In a preferred embodiment, a flow conditioning section comprised of straightening vanes and a diffuser, as well as temperature probes, pressure transducers, and a metering assembly, are all provided in an integral housing. The assembly is provided with different sized flanges on an inlet orifice and an exit orifice, such that the assembly may be installed in a fluid line in only one direction, thereby preventing incorrect installation. The flow conditioning section is comprised of a diffuser and straightening vanes, and serves to ensure that fluid flowing through the measurement assembly is uniformly scrambled and then normalized, such that the metering assembly will be substantially insensitive to upstream flow disturbances and will provide accurate and repeatable fluid flow measurements. The metering assembly is comprised of two counter-rotating turbines, a pickoff device or pulse detector being provided for each turbine. The assembly is provided with a single connector that is coupled to leads of the temperature probes, pressure transducers and pulse detectors, such that these components may be coupled to a processor external to the housing, simply and accurately. In calibrating the measurement assembly and in calculating the volumetric or mass flow rate of a fluid, a K factor for each turbine at a given frequency is determined and added together, the sum of the K factors being plotted against a sum of the frequencies for the two turbines, normalized by the viscosity of the fluid. The turbines are counter-rotating and are hydraulically coupled, such that a change in angular velocity in the upstream turbine will result in an opposite, or compensating change in the downstream turbine. By combining the K factors and frequencies for the turbines, therefore, flow or wear effects are averaged out, and higher resolution or accuracy is achieved given that a higher frequency is used as a reference point. The effects of the hydraulic coupling combined with tracking a calibrated difference in meter turbine frequency at any given flow rate allows for reliable validation of the gathered data, error detection and post measurement data correction.

Gas turbine engine and a diffuser therefor

Abstract: A gas turbine engine with first and second axial flow compressors and an intercooler therebetween is provided with a bend diffuser and a radial flow diffuser to diffuse the air leaving the downstream end of the first axial flow compressor at a first radial distance from the axis to the upstream end of the intercooler at a second radial distance from the axis. The bend diffuser comprises a first radially outer wall and a second radially inner wall. The first wall is elliptical and the second wall has a profile derived from a relationship between the local area ratio and the path length around the arc such there is rapid diffusion in the bend diffuser without fluid flow separation from the first wall.

Flow distribution and pressure drop in diffuser-monolith flows

Abstract: Most current automotive catalytic converters use diffusers to distribute the flow field inside the monolithic bricks where catalysis takes place. While the characteristics and performance of a simple diffuser flow are well documented, the influence of downstream brick resistance is not clear. In this paper, the trade-off between flow-uniformity and pressure drop of an axisymmetric automotive catalytic converters is studied numerically and experimentally for selected cases. The monolithic brick resistance is formulated from the pressure gradient of fully developed laminar duct-flow and corrected for the entrance effect. The monolithic brick downstream stabilizes the diffuser flows both physically and computationally. A distribution index was formulated to quantify the degree of nonuniformity in selected test cases. The test matrix covers a range of different diffuser angles and flow resistance (brick types). For simplicity, an axisymmetric geometry is chosen. Flow distribution within the monolith was found to depend strongly on diffuser performance, which is modified strongly by brick resistance. Pressure drop due to the headers and brick resistance and their relative roles is also identified. The implications of these data for converter design are discussed in terms of the trade-off between flow-uniformity and pressure drop.

Liquid treatment process with in-plant pressure monitoring of submerged diffusers

Abstract: The disclosure relates to obtaining dynamic wet pressure in a wastewater treatment plant. A difusser submerged in wastewater is connected to an air supply. This diffuser has an air flow regulator, a diffusion element to discharge the air into the wastewater, and a plenum situated between the element and the flow regulator. Pressure taps sense pressure in the pipe and the plenum. Manometers are used to determine hydrostatic head, pressure drop across the element, and pressure drop across the air flow regulator. Air flow regulator pressure drop and pressure/flow characteristics may be used, with other required information, to calculate dynamic wet pressure across the diffusion element.

Gas burner with an improved diffuser

Abstract: A gas burner of the type in which a inflammable mixture is caused to flow out of a diffuser, is formed of a plurality of parallel slots separated by grid elements, and distributed into a pair of grids separated by an elongate screen extending between the two grids. The burner includes at least one elongate slot extending perpendicular to said screen along the width of said grid pair, perpendicular to the screen, near the grids, and adapted to take up relative dimensional variations of said screen and the grid elements brought about by differential heatings of the screen and the burner walls surrounding the diffuser.