Array Of Orifices Research Articles

Development of a multi-jet polishing process for inner surface finishing

High-precision inner surfaces are difficult to be machined to a sufficiently high surface quality. This paper presents the development of a novel multi-jet polishing process for precision polishing of inner surfaces through adopting a rod-shaped nozzle designed with a linear array of orifices at its side face. The material removal characteristic on inner cylindrical surface was modelled based on computational fluid dynamic method. Four groups of material removal experiments were conducted to validate the proposed material removal model and investigate its material removal characteristics. Moreover, the surface generation model was also developed and validated based on the material removal model. A series of polishing experiments were conducted on 304 stainless steel cylindrical inner surfaces. The results show that the proposed multi-jet polishing process with the newly designed nozzle is able to achieve high efficiency and precision inner surface finishing on the inner surface.

Flow through a cylindrical pipe with a periodic array of fractal orifices

We apply direct numerical simulation (DNS) of the incompressible Navier–Stokes equations to predict flow through a cylindrical pipe in which a periodic array of orifice plates with a fractal perimeter is mounted. The flow is simulated using a volume penalization immersed boundary method with which the geometric complexity of the orifice plate is represented. Adding a periodic array of orifice plates to a cylindrical pipe is shown to increase the mixing efficiency of the flow in the laminar regime. The average stretching rate is shown to increase by a factor of up to 5, comparing pipe flow without orifice plates to flow passing through an orifice plate derived from the Koch snowflake fractal. The dispersion rate is shown to increase by a factor of up to 4. In laminar flow, the viscous forces are most important close to the walls, causing orifice geometries with the largest perimeter to exhibit the largest axial velocities near the centerline of the pipe and the largest pressure drop to maintain the prescribed volumetric flow rate. The immersed boundary method is also applied to turbulent flow through a ‘fractal-orifices pipe’ at Re = 4300. It is shown that the pressure drop that is required to maintain the specified volumetric flow rate decreases by about 15% on comparing orifice plates with a circular opening to orifice plates with more complex shapes that contain several corners such as triangles, squares, stars and the Koch snowflake.

Heat transfer in a channel with inclined target surface cooled by single array of centered impinging jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with heated target plate inclined at an angle cooled by single array of equally spaced centered impinging jets for three different jet Reynolds numbers (Re=9300, 14400 and 18800). Air ejected from an array of orifices impinges on the heated target surface The target plate forms the leading edge of a gas turbine blade cooled by jet impingement technique. The work includes the effect of jet Reynolds numbers and feed channel aspect ratios (H/d = 5, 7, 9 where H=2.5, 3.5, 4.5 cm and d=0.5 cm) on the heat transfer characteristics for a given orifice jet plate configuration with equally spaced centered holes with outflow exiting in both directions (with inclined heated target surface). In general, It has been observed that, H/d=9 gives the maximum heat transfer over the entire length of the target surface as compared to all feed channel aspect ratios. H/d=9 gives 3% more heat transfer from the target surface as compared to H/d=5 (for Re=14400). Also, it has been observed that the magnitude of the averaged local Nusselt number increases with an increase in the jet Reynolds number for all the feed channel aspect ratios studied.

Equivalent Circuit Modeling of Film Pressure in Air Bearing with Orifice Array

For the turbulent flow in the air bearing,the Reynold's equation has no closed-form solution.Computational fluid dynamics(CFD) simulation is time-consuming for large scale orifice arrays.An approximate analytical model for the pressure distribution of the air film in orifice arrays is proposed,which combines equivalent circuit modeling and segmented nonlinear interpolation.Based on the pressure distribution of the air film derived from CFD simulation,a multi-port equivalent circuit model for the steady-state pressure distribution of an orifice unit is established by using linear resistor network,and the resistances in the network are determined.The air film of an orifice unit is subdivided into multiple regions,and a nonlinear segmented interpolation function is fitted by using the PSpice circuit simulation data.The load capacity of an orifice unit is calculated from the interpolation function with an error of 3.70% as compared with the CFD results.The equivalent circuit model for a 4?4 orifice array is obtained by interconnecting 16 orifice units,and the load capacity of the orifice array can be calculated efficiently.As compared with the CFD simulation,the error of the equivalent circuit model for the 4?4 orifice array is 8.74% in terms of load capacity while the computational speed is improved by over 3 orders of magnitude.

ELECTRICAL AND SPRAY CHARACTERISTICS OF A MULTIORIFICE CHARGE-INJECTION ATOMIZER FOR ELECTRICALLY INSULATING LIQUIDS

Electrostatic atomization of electrically insulating liquids has a number of potential uses with respect to combustion applications and the food processing industry. For charge-injection atomizers that generate well-dispersed spray plumes of this class of liquids, spray-specific charge is inextricably linked to the orifice diameter such that the product of these two variables is approximately constant. This, for a single-orifice atomizer, constrains the maximum flow rate against the mean spray drop diameter for a given pressure drop and spray-specific charge. A relaxation of this requirement is possible using a novel evolution of the high-voltage (HV) emitter electrode surface, permitting an orifice array to be used. This permits high spray charge densities at high flow rates without the precision alignment issues that face charge-injection atomizers that possess a pointed high-voltage electrode. Results show that current-voltage characteristics are very similar to a point-plane system, and both electrical and spray measurements revealed that the multiorifice atomizer is able to provide finely atomized jets even though there is a slight degradation in spray performance when introducing more holes.

Mass Transports in an Air-Breathing Cathode of a Proton Exchange Membrane Fuel Cell

Mass transport in an air-breathing cathode of a proton exchange membrane (PEM) fuel cell was investigated numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. The diffusions of reactant species in the porous cathodes are described using the Stefan–Maxwell equation. The electrochemical reaction on the surfaces of the porous cathode is modeled using the Butler–Volmer equation. Gas flow in the air-breathing porous cathodes is described using isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are handled using charge conservation based on Ohm’s law. A finite-element scheme is adopted to solve these coupled equations. The effects of electrode porosity (0.4<ε<0.6) on the fluid flow, mass transport, and electrochemistry are examined. Detailed electrochemical/mass characteristics, such as flow velocities, species mass fraction, species flux, and current density distributions are presented. These details provide a solid basis for optimizing the geometry of a PEM fuel cell stack that is run in passive mode.

Three-Dimensional Fluid Flow in the Processing of Fine Fibers

In this paper, a multidimensional numerical simulation has been used to investigate the creation of fine fiber by the melt-blown process. The problem involves highly complex fluid flow and convective heat transfer processes. The fine fiber is created by the use of high-velocity, obliquely impinging air jets whose high shear forces stretch a polymer extrudate in the partially fluid state. High-temperature air is used to maintain the fluidity of the polymeric material as it exits the die. The model which was developed and implemented for the simulation closely reflects the physical situation employed in the actual production of fine fibers. The actual configuration of fine fiber production is a linear array of orifices from which the partially fluid polymer emerges. The array contains inherent symmetries which enable the problem to be tractable. Notwithstanding this, the numerical simulation still required multimillions of control volumes to achieve results of practical relevance. The results of the simulation indicate that, contrary to physical intuition, the shear exerted by the airflow is relatively uniform around the circumference of a given fiber. Although the interfiber spacing is small, it does not promote unfavorable fiber-to-fiber interactions.

Effect of breathing-hole size on the electrochemical species in a free-breathing cathode of a DMFC

A three-dimensional numerical model is developed to study the electrochemical species characteristics in a free-breathing cathode of a direct methanol fuel cell (DMFC). A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of orifices. The radius of the orifice is varied to examine its effect on the electrochemical performance. Gas flow in the porous cathode is governed by the Darcy equation with constant porosity and permeability. The multi-species diffusive transports in the porous cathode are described using the Stefan–Maxwell equation. Electrochemical reaction on the surfaces of the porous matrices is depicted via the Butler–Volmer equation. The charge transports in the porous matrices are dealt with by Ohm's law. The coupled equations are solved by a finite-element-based CFD technique. Detailed distributions of electrochemical species characteristics such as flow velocities, species mass fractions, species fluxes, and current densities are presented. The optimal breathing-hole radius is derived from the current drawn out of the porous cathode under a fixed overpotential.

Pneumatically actuated micromachined synthetic jet modulators

Results presented in the synthetic jet literature have focused on the demonstration of and application of one or more single-orifice synthetic jet actuators in jet vectoring and other aerodynamic applications. For these applications, amplitude and phase modulation techniques are often used in conjunction with the oscillatory nature of the synthetic jet flow to achieve the desired results. In this work the authors present a multi-orifice synthetic jet actuator and investigate the feasibility of using integrated microvalves for dynamic orifice output modulation. A multi-orifice synthetic jet consists of a micromachined orifice array with integrated microvalves for flow modulation and a shared membrane actuator for synthetic jet generation. Individual orifice output modulation using microvalves could be used to compensate for manufacturing-induced or spatial variations in orifice output in a multi-orifice synthetic jet. Moreover, dynamic orifice output modulation could also be used to alter locally the zero net-mass-flux nature of the synthetic jet flow, producing localized flow regions over the multi-orifice synthetic jet in which the net-mass-flux is positive or negative instead of zero as in the case of traditional single-orifice synthetic jets. For instance, if a single oscillatory actuator is used to generate in parallel synthetic jet flow from an array of orifices, the microjet modulator associated with a particular orifice could be sequenced to open only during the intake (or exhaust) stroke of the actuator, creating a localized low (or high) pressure region at the given orifice. The use of dynamic modulation to create these localized regions with non-zero net mass flux could be harnessed to improve the efficiency synthetic jets in jet vectoring, flow control, or other applications. This paper discusses the fabrication and characterization of pneumatically actuated, micromachined synthetic jet modulator arrays and demonstrates the use of these synthetic jet modulators for dynamic modulation of multi-orifice synthetic jet flows. The multi-orifice synthetic jet presented here utilizes a traditionally machined synthetic jet actuator for generation of synthetic jet flows (5–20 m/s) and an array of individually addressable micromachined synthetic jet modulators for manipulation of the resulting synthetic jet flows. Included are qualitative and quantitative experimental results that demonstrate static on-off modulation and dynamic flow modulation at the jet generation frequency. Continuous variation of the output of individual jets from suction-only operation to exhaust-only operation was achieved by changing the phase of the modulation signal relative to the jet generation signal. Also presented is phase formulation of the modulated synthetic jet flow, which compares favorably with measurements of the exit pressure of the modulated synthetic jet flow. A sample application of the pneumatic microjet modulator array, a lateral air pump, is also presented to demonstrate the use of dynamic synthetic jet modulation to create localized regions with non-zero net mass flux.

Mass/electron co-transports in an air-breathing cathode of a PEM fuel cell

Mass/electron transports in a passive cathode of a proton exchange membrane (PEM) fuel cell have been studied numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. Diffusions of reactant species in the porous cathodes are described by the Stefan–Maxwell equation. Electrochemical reaction on the surfaces of the porous cathode is depicted via the Butler–Volmer equation. Gas flow in the air-breathing porous cathodes is governed by isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are dealt with the charge conservations based on the Ohm's law. A finite-element method is employed to solve the above-coupled equations. The effect of overpotential on the fluid flow, mass transport and electrochemistry is examined. Detailed electrochemical/mass characteristics such as flow velocities, species mass fraction, species flux and current density distributions are presented. They can provide a solid basis for optimizing the geometry of the PEM fuel cell stack running with a passive mode.

A microstructured silicon membrane with entrapped hydrogels for environmentally sensitive fluid gating

In this paper, we report on the fabrication and characterization of a new hydrogel-based microvalve. The basic structure is a silicon membrane having an array of orifices with an internal structure designed to anchor the hydrogel while allowing it to gate the flow across the membrane. Each orifice (140 μm diameter) has a central post suspended by four tethers on each side of the membrane. A stimuli-sensitive hydrogel is polymerized inside each orifice. In the swollen state, the hydrogel completely occupies the void space of the orifice, completely blocking pressure-driven fluid flow. In the shrunken state, the hydrogel contracts around the post, allowing fluid to flow through an opened annular gap. Fabrication of the microstructured silicon membrane requires only two masking steps and involves a combination of deep trench and KOH etch. Two different hydrogels, based on N-isopropylacrylamide (temperature-sensitive) and phenylboronic acid (pH and glucose-sensitive) were trapped and tested in this microvalve. The measured response times were 10 s (temperature), 4 min (pH), and 10 min (glucose). The maximum pressure drop the microvalve can sustain before breakage of the hydrogel is 21 kPa and 16 kPa for temperature-sensitive and (pH/glucose)-sensitive hydrogels, respectively.

Micro Resolution

This article focuses on researchers at Eastman Kodak Co. who are working toward a new generation of inkjet printheads that are based on microelectromechanical (MEMS) technology. The prize is that the MEMS scale has the potential to advance the quality of inkjet images by permitting larger and denser arrays of smaller ink orifices, to increase resolution and printing speed. In addition, the company expects that economies of scale of MEMS manufacturing will make printers more affordable by decreasing production costs. Currently, inkjet printheads use one of two approaches for ejecting ink. One uses piezoelectric channels filled with ink. The printer applies a voltage across each channel, which deforms to squeeze out the ink. But limitations in the manufacturing process make it difficult to increase the size of the orifice array. The biggest challenge in using CFD to simulate the inkjet printing process is keeping track of the drops free surface. Modeling problems such as Delametter’s, where the surface tension plays an important role, require accurate resolution and tracking of fluid surfaces. They also require an evaluation of surface curvatures and sensing where and how fluid adheres to solids.

Effect of Outflow Orientation on Heat Transfer and Pressure Drop in a Triangular Duct With an Array of Tangential Jets

Experiments are conducted to study the heat transfer and pressure drop characteristics in a triangular duct cooled by an array of tangential jets, simulating the leading-edge cooling circuit of a turbine blade. Coolant ejected from a high-pressure plenum through an array of orifices is aimed at the leading-edge apex and exits from the radial outlets. Three different outflow orientations, namely coincident with the entry flow, opposed to the entry flow, and both, are tested for various Reynolds numbers 12600⩽Re⩽42000. A transient liquid crystal technique is used to measure the detailed heat transfer coefficients on two walls forming the leading-edge apex. Flow rate across each jet hole and the crossflow development, which are closely related to the local heat transfer characteristics, are also measured. Results show that increasing Re increases the heat transfer on both walls. The outflow orientation affects significantly the local heat transfer characteristics through influencing the jet flow together with the crossflow in the triangular duct. The triangular duct with two openings is recommended since it has the highest wall-averaged heat transfer and the moderate loss coefficient among the three outflow orientations investigated. Correlations for wall-averaged Nusselt number and loss coefficient in the triangular duct have been developed by considering the Reynolds number for three different outflow orientations. [S0022-1481(00)01204-4]

Injection of bubbling liquid jets from multiple injectors into a supersonic stream

Multiple (12 injectors) bubbling liquid jets (helium microbubbles in water) were injected transverse to a M = 2.4 airflow. Penetration and spray plume spreading angle were measured directly using nanoshadowgraphs and front-lighted pictures, respectively. The experiments were performed at two conditions, i.e., the constant supply pressure condition and the constant liquid mass flow rate condition. For the case of a parallel arrangement of the injector orifices to the airflow, the penetration of the jet array increased steadily from front to back. The last jet (12th jet) has over 5 times the penetration of the first jet for the water only case. The usual similarity law for the penetration, h * q°-59 was approximately valid also for the multiple water-only jets. For the bubbling jet case, the penetration of the first jet doesn't change with increasing gas concentration y, but the rear jets have less penetration height than that of liquid-only jets at the constant injection pressure condition. For the constant injection pressure condition, the resulting penetration of the jet plume decreased with increasing y. On the other hand, for the constant liquid mass flow rate condition, the penetration of the multiple bubbling jets increased a little with increasing y. Straight coherent jets just coming out of orifice were observed for the y = 0 case. Conical jet plumes were obtained for the bubbling jet case. Therefore, the width of the jet plume increased by using the bubbling jet. The effects of the angle between the orifice array and the freestream direction and the surfactant concentration on the penetration and mixing of multiple bubbling jets were also clarified.