Showing papers on "Pressure drop published in 1992"

Bubbles, Drops, and Particles in Non-Newtonian Fluids

Abstract: Preface Preface to the First Edition Acknowledgements About the Author INTRODUCTION, SCOPE, AND ORGANIZATION NON-NEWTONIAN FLUID BEHAVIOR Definition of a Newtonian Fluid Non-Newtonian Fluids: Definition, Examples of Different Types, Mathematical Models Dimensional Considerations in the Mechanics of Viscoelastic Fluids Experimental Techniques: Rheometry RIGID PARTICLES IN TIME-INDEPENDENT FLUIDS WITHOUT A YIELD STRESS Governing Equations for a Sphere Spherical Particles in Newtonian Fluids Spheres in Shear-thinning Fluids Spheres in Shear-thickening Fluids Light Spheres Rising in Pseudoplastic Media Pressure Drop due to a Settling Sphere Non-Spherical Particles RIGID PARTICLES IN VISCOPLASTIC FLUIDS Static Equilibrium of Particles Flow Field: Shape and size of flow zones Drag Force Role of Values of Yield stress used in correlations Time Effects RIGID PARTICLES IN VISCOELASTIC FLUIDS Flow over a sphere Flow over a cylinder Other Studies Involving Interactions Between Non-Newtonian Characteristics, Particle Shape, Flow Field, etc. FLUID PARTICLES IN NON-NEWTONIAN MEDIA Formation of Fluid Particles Shapes of Bubbles and Drops in Free Rise or Fall Terminal Velocity-Volume Behavior in Free Motion Drag Behavior of Single Particles Bubble and Drops Ensembles in Free Motion Coalescence of Bubbles and Drops Breakage of Drops Motion and Deformation of Bubbles and Drops in Confined Flows NON-NEWTONIAN FLUID FLOW IN POROUS MEDIA AND PACKED BEDS Porous Medium Definition, Examples and Characterization Flow of Newtonian Fluids Flow of Non-Newtonian Fluids Miscellaneous Effects Two Phase Gas/Liquid Flow FLUIDIZATION AND HINDERED SETTLING Two-Phase Fluidization Sedimentation or Hindered Settling Three Phase Fluidized Beds MOMENTUM, HEAT AND MASS TRANSFER IN BOUNDARY LAYER FLOWS Boundary Layer Flows Viscoelastic Effects in Boundary Layers Mass Transfer from Bubbles Mass Transfer from Drops Mass Transfer from Ensembles of Bubbles and Drops Heat and Mass Transfer in Fixed Beds Heat and Mass Transfer in Fluidized Beds Heat and Mass Transfer in Three Phase Fluidized Beds Heat Transfer from Tube Bundles WALL EFFECTS Definition For Rigid Spheres For Non-Spherical Particles For Drops and Bubbles FALLING OBJECT RHEOMETRY Falling Ball Method Rolling Ball Method Rotating Sphere Viscometer Falling Cylinder Viscometer *All Chapters contain Introduction, Summary and Nomenclature sections References Subject index Author index

Heat sink optimization with application to microchannels

Abstract: The equations governing the fluid dynamics and combined conduction/convection heat transfer in a heat sink are presented in dimensionless form for both laminar and turbulent flow. A scheme presented for solving these equations permits the determination of heat sink dimensions that display the lowest thermal resistance between the hottest portion of the heat sink and the incoming fluid. Results from the present method are applied to heat sinks reported by previous investigators to study effects of their restrictions regarding the nature of the flow (laminar or turbulent), the ratio of fin thickness to channel width, or the aspect ratio of the fluid channel. Results indicate that when the pressure drop through the channels is small, laminar solutions yield lower thermal resistance than turbulent solutions. Conversely, when the pressure drop is large, the optimal thermal resistance is found in the turbulent region. With the relaxation of these constraints, configurations and dimensions found using the present procedure produce significant improvement in thermal resistance over those presented by all three previous studies. >

Modulated bias units for steerable rotary drilling systems

Abstract: A modulated bias unit is provided for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations. The unit comprises a plurality of hydraulic actuators 132 spaced apart around the periphery of the unit and having movable thrust members 134 hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled. Each actuator 134 has an inlet passage 150 for connection to a source 124 of drilling fluid under pressure and an outlet passage 174 for communication with the annulus. A selector control valve 160,166 connects the inlet passages 150 in succession to the source of fluid under pressure, as the unit rotates, and a choke 172 is provided to create a pressure drop between the source of fluid under pressure and the selector valve. A further choke 176,180 is provided in the outlet passage from each actuator unit. The actuators and control valve arrangements may take a number of different forms.

Influence of Texture on Steady Foam Flow in Berea Sandstone

Abstract: We have studied the quantitative role of bubble size in the steady flow of strong foam through a 0.8-{mu}m{sup 2} Berea sandstone. Inlet and outlet textures are determined from photomicrographs taken of bubbles flowing through specially designed visual cells. Concurrent measurements of pressure profiles and liquid saturation profiles by microwave attenuation are acquired for gas velocities ranging from 1 to 3 m/day and covering a foam quality range from 70 to 90%. At steady state, liquid saturations remain constant near 35% independent of gas velocity. Moreover, measured foam-flow resistances practically do not vary with gas velocity over the studies range. Foam textures, however, do vary dramatically. For example, injected fine bubbles near 80 {mu}m in diameter exit the sandstone with sizes around 300 {mu}m. Powerful coalescence forces are the origin of this coarsening. Effluent bubble sizes increase slightly with increasing gas velocity, but are independent of the injected size. A simple one-dimensional foam population-balance model is outlined to quantify the observed flow and texture behavior. At steady state, generation and coalescence mechanisms alter the foam texture over distances of less than about a centimeter. 31 refs., 12 figs., 1 tab.

Induced stresses due to fluid extraction from axisymmetric reservoirs

Abstract: Earthquakes can be induced by fluid extraction, as well as by fluid injection.Segall (1989) proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction. Here, I present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries. The results ofGeertsma (1973) for a thin disk reservoir with uniform pressure drop are recovered as a special case. Predicted surface subsidence agrees very well with measured leveling changes over the deep Lacq gas field in southwestern France. The induced stresses are finite if the reservoir pressure changes are continuous. Computed stress changes are on the order of several bars, suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production.

The deformation and breakup of liquid drops in low Reynolds number flow through a capillary

Abstract: The deformation and breakup of a freely suspended, immiscible liquid drop are examined experimentally for low Reynolds number flow in a straight circular capillary tube. Results are reported for capillary numbers from 0.05 to greater than unity and for values of the viscosity ratio that span more than three orders of magnitude. In every case, the size of the drop is comparable to the tube diameter. The steady‐state shapes of the drops are compared with previous numerical studies of the problem. However, for a critical value of the capillary number that depends on drop size and viscosity ratio, drop breakup is observed in the experiment. When the viscosity ratio is small, i.e., the drop fluid is less viscous than the outer phase liquid, an indentation in the trailing edge of the drop grows and entrains outer phase liquid inside the boundaries of the original drop. When the viscosity ratio is O(1), the drop stretches along the axis of the tube until it breaks.

Flow control system for medical ventilator

Abstract: A system including the method employed, for controlling the flow of a pressurized gas through a variable orifice flow control valve in a pressurized gas conduit system, such as a ventilator, in response to either a desired flow rate/volume input signal, a desired pressure signal or both. With respect to the flow rate input signal, the system generates a flow signal indicative of the actual flow rate of gas through the flow control valve by measuring the pressure drop across the valve and using the actual orifice area based on a control valve position signal given to the valve. This flow signal is compared to the flow rate input signal to give a correction signal which is used to form a volume flow reference signal. With respect to the desired pressure signal, a signal indicative of the effective pressure at a set location in the system is compared to the desired pressure signal and processed to give a pressure flow reference signal. Depending on the mode desired, either the volume flow reference signal or the pressure flow reference signal or the greater of the two becomes a flow reference signal that is used to generate a control valve position signal based on a predetermined relationship. This signal is then used to vary the valve orifice area and correct the flow rate.

Experiments on convective heat transfer in corrugated channels

Abstract: Experiments have been performed to study convective heat transfer in the entrance region of corrugated channels. With water as the working fluid, two channel spacings were examined for a single corrugation angle of 20°. The flow rate was varied over the range I50 ≤ Re ≤ 4000. Flow visualization under low-Reynolds-number flow conditions suggested the presence of longitudinal vortices, while at somewhat higher Reynolds numbers, the existence of spanwise vortices was clearly revealed. For Re > 1500, Nusselt numbers in the corrugated channels exceeded those in the parallel-plate channel by approximately 140% and 240% for the two channel spacings, the corresponding increases in friction factor being 130% and 280%. Performance evaluations under the criteria of equal mass flow rate, equal pumping power, and equal pressure drop per unit length established both the corrugated channels as superior to the parallel-plate channel in intensifying heat transfer.

Dimple formation and behavior during axisymmetrical foam film drainage

Abstract: Draining foam or emulsion f h are generally of nonuniform thickness. A thick region or 'dimple" forme in the central part of a circular film. It is separated from the Plateau border by a thinner 'barrier ring". We have developed a new numerical model to simulate the entire drainage process, including the film formation. The model assumes that drainage is arieymmetric and that the fluid interfaces are immobile. The initial conditions are a pair of static hemispherical menisci. Fluid is withdrawn at a constant rate for a specified time to form a film. The condition for the transition from a nearly "plane-parallel" f i to a dimpled film in the absence of disjoining pressure was determined. The ratio of the minimum to maximum thickness in the film and a dimensionless rate of drainage are correlated with the ratio of the maximum possible curvature in the dimple to the curvature in the meniscus. The rate of drainage is always less than that given by the Reynolds theory for drainage between a pair of disks that is pressed by a pressure equal to the capillary pressure. When the film is approximately plane-parallel, the pressure drop from the center of the film to the Plateau border is less than half of that predicted by the Reynolds theory and there is a significant pressure gradient beyond the nominal film radius. When a dimple forms, most of the resistance to flow is in the thin barrier ring. The presence of disjoining pressure makes a qualitative difference in film drainage. Low electrolyte concentrations in a film containing ionic surfactant produce a repulsive disjoining pressure that inhibits formation of the thin barrier ring and thus of the dimple itaelf. The film drains rapidly to ita equilibrium thickness. For high electrolyte concentration, the disjoining pressure is dominated by van der Waals attraction. As a result a thin annular fiIm forms that forces the dimple into a lens with a finite contact angle. These types of behavior are observed experimentally.

Optimal thermal design of air cooled forced convection finned heat sinks-experimental verification

Abstract: D.B. Tuckerman and R.F.W. Pease (1981) showed that microchannels with water flow could be used to cool VLSI systems. Their work required the flow to be laminar, and the channel system, or fin array, was optimized analytically. Recently, it has been shown that, for some geometries and fluid pressure drops, a lower thermal resistance can be found if the channels are designed to allow turbulent flow. The current work uses the optimization scheme developed by R.W. Knight et al. (1991 and in this issue) to design three air-cooled aluminum finned arrays, which were built and tested experimentally. The thermal performances of the fin array designs, one containing 5 fins, one with 11 fins, and one with the predicted optimum of 8 fins, are compared. All arrays had turbulent flow and pressure drop across them, and all fins were the same length and width. The best thermal performance was obtained with the design predicted to be optimal. The scheme can be applied to a variety of heat sink design applications, including water-cooled microchannels. >

Polymeric flow through fibrous media

Abstract: Proper description of the resin flow through fibrous media is a very important input in the modeling of composite manufacturing processes. In this study, the complex phenomena associated with the flow of viscoelastic fluids through fibrous media are examined. In particular, the effects of fluid and fiber bed properties on the pressure drop observed during such flows were investigated, in conjunction with the specific conditions of typical composite manufacturing processes. To gain some insight into the problem, a theoretical and experimental investigation of Newtonian and non‐Newtonian flows through different geometric cylinder arrangements designed to simulate the actual fiber configurations was carried out. Particular care was taken to select the appropriate dimensionless flow parameters to demonstrate that the onset of the ‘‘excess’’ pressure drop due to fluid elasticity is independent of the flow geometry. Employment of this dimensionless group, which is related to the total strain on the macromolecules, yields superior predictions compared to previous results based on the Deborah number. In order to assess the applicability of the results in ideal fiber beds to actual manufacturing processes, flow of viscoelastic fluids through carbon fiber beds was studied under conditions that simulate the autoclave process.

Two-phase pressure drop of ammonia in small diameter horizontal tubes

Abstract: Data for pressure drop in adiabatic two-phase ammonia flows in small diameter horizontal tubes are presented. The data has direct application to the sizing of the flow-through radiator tubes in the Space Station Freedom heat rejection system. The data are compared to existing correlations for pressure drop and are found to be significantly lower than the most commonly used correlations. However, several of the less commonly used correlations predict the data accurately. Alternate pressure drop prediction methods are explored and a recommendation is made for a method to accurately predict the pressure drop in two-phase ammonia flows in small horizontal tubes.

Airfoil having multi-passage baffle

Abstract: A hollow impingement baffle includes a septum extending between its bottom and top and spaced between its forward and aft edges to define a forward manifold and an aft manifold. The baffle includes an inlet having a forward portion for channeling a first portion of compressed air to the forward manifold, and an aft portion for channeling a second portion of the compressed air into the aft manifold with a predetermined pressure drop for obtaining a lower pressure in the aft manifold relative to a higher pressure in the forward manifold. The baffle includes impingement holes for discharging the compressed air against the inner surface of a surrounding airfoil for the impingement cooling thereof.

Optimized offset strip fin for use in compact heat exchangers

Abstract: An offset strip fin for use in compact automotive heat exchangers is disclosed. The offset strip fin has multiple transverse rows of corrugations extending in the axial direction wherein the corrugations in adjacent rows overlap in order that the oil boundary layer is continually re-started. The fin dimensions have been optimized in order to achieve superior ratio of heat transfer to pressure drop along the axial direction. In one aspect, . .an.!. .Iadd.a .Iaddend.compact concentric tube heat exchanger has an offset set strip tin located in an annular fluid flow passageway located between a pair of concentric tubes. The preferred range of lanced lengths is determined to be between 0.035" to 0.075" for periodically developed flow. Maintaining the lanced length in the regime of periodically developed flow is advantageous in that it gives a higher heat transfer coefficient than is achievable with fully developed flow. This also provides the added advantage that variations in the shape of the flow passages from the rectangular do not impact negatively on the heat transfer. .Iadd.The corrugations have top portions and bottom portions of the same width. The height of the corrugations is greater than this width. .Iaddend.

An ultrasensitive silicon pressure-based microflow sensor

Abstract: An ultrasensitive silicon pressure-based flowmeter has been developed for use in measuring sub-SCCM gas flow in semiconductor process equipment. The device utilizes a capacitive pressure sensor to measure the pressure drop induced by flow across a micromachined silicon flow channel. The flowmeter is fabricated using a single-sided dissolved-wafer process and requires only six masks. The capacitive pressure sensor uses a thin (2.9 mu m) stress-compensated membrane, which enables the sensor to monitor differential pressures as low as 1 mtorr while withstanding overpressures greater than 700 torr. Creep and fatigue change the offset by >

Hydrodynamics of Two-Phase Flow Across Horizontal In-line and Staggered Rod Bundles

Abstract: This paper reports on void fraction and friction pressure drop measurements made for vertical two-phase flow of air-water across staggered and in-line rod bundles with different pitch-to-diameter ratios. All void fraction data showed a strong mass velocity effect and were significantly less than the values predicted by a homogeneous flow model, but were well correlated using the dimensionless gas velocity, j[sup [asterisk]][sub g]. The two-phase friction multiplier data could be well correlated with the Martinelli parameter for G [gt] 200 kg/m[sup 2]s. The correlations developed for void fraction and two-phase friction multiplier were successfully tested in predicting the total pressure drop in boiling R-113 experiments.

Effect of surfactants on the motion of drops through circular tubes

Abstract: The effect of surfactants on the motion and deformation of liquid drops in Poiseuille flow through circular tubes at low Reynolds numbers is examined Assuming no bulk transport of surfactant, the boundary integral method is used in conjunction with a convective–diffusion equation to determine the distribution of surfactant on the deformed surface of the drop The velocity and shape of the drop as well as the extra pressure loss due to the presence of the drop are calculated Increasing the surface Peclet number is found to produce large variations in surfactant concentration across the surface of the drop The resulting interfacial tension gradients lead to tangential (Marangoni) stresses that oppose surface convection and retard the motion of the drop as a whole For large Peclet numbers, Marangoni stresses immobilize the surface of the drop, leading to a significant increase in the extra pressure loss required to move the drop through the tube The accumulation of surfactant near the trailing end of the drop partially lowers the interfacial tension on that side, thereby requiring larger deformations to satisfy the normal stress balance At the same time, the increase in interfacial area associated with drop deformation causes an overall dilution of the surfactant, which, in turn, counteracts the effect of convective transport of surfactant at large Peclet numbers The effects of these coupled responses are studied over a wide range of the dimensionless parameters

Impingement Cooling of Electronics

Abstract: Experiments were conducted to determine the performance of a system of low-velocity air jets used to cool a simulated electronics package. The test model consisted of a uniform array of rectangular elements mounted to a circuit board. Each element was cooled by a cluster of four jets, and the spent fluid was vented at one end of the channel formed between the circuit board and the plate from which the jets were discharged. Reported are measurements of system pressure drop and convective heat transfer coefficients for elements at various sites within the array. Results indicate that (for the geometry tested) the largest portion of the total pressure drop occurs across the jet orifices. Further, the crossflow of spent air appears to enhance heat transfer for those elements near the exit end of the channel.

Developing Heat Transfer and Friction in a Ribbed Rectangular Duct With Flow Separation at Inlet

Abstract: The local heat transfer and pressure drop characteristics of developing turbulent flows in a rectangular duct with an abrupt-contraction entrance and repeated square-rib pairs on the two opposite walls have been investigated experimentally. Both entrance-region and periodic-fully-developed-region results were obtained. Laser holographic interferometry was employed in the local and average heat transfer measurements. The Reynolds number was varied from 5.0x10[sup 3] to 5.0x10[sup 4]; the rib pitch-to-height ratios were 10, 15, and 20; and the rib height-to-duct height ratio was kept at a value of 0.13. The results allowed the entry length to be determined and the regions susceptible to hot spots to be located. Semi-empirical heat transfer and friction correlations for the periodic fully developed region were developed. Moreover, performance comparisons between the ribbed and smooth ducts were made under two types of constraint, namely equal mass flow rate and equal pumping power. Finally, the effect of thermal entry length on the length mean Nusselt number was also investigated. The results showed that the length mean Nusselt number ratio was a function of only the duct length and independent of PR and Re, and could be further correlated by an equation of the form [bar N]u[sub m]/[bar N]u[sub p]=1more » + 1.844/(X/De).« less

Generalized method of pressure drop calculation across pipe components containing two-phase flow of refrigerants

Abstract: A generalized method is proposed for calculating the pressure drop across pipe components containing two-phase flow of refrigerants mainly through return lines of pump and gravity feed evaporators including inlet and outlet manifolds, as well as return bends of evaporator and condenser coils. The method involves recalculating single-phase resistance to obtain two-phase resistance by means of a two-phase multiplier that has been generalized from experimental data published over recent years. Specific properties of generalized two-phase flow in straight pipes published in Int J Refrig (1989) 12 314–322 have been used. The method is simple and its accuracy corresponds to the level achieved for two-phase flows in straight pipes.

Influence of soluble surfactants on the motion of a finite bubble in a capillary tube

Abstract: The steady motion of a long inviscid bubble displacing a viscous fluid in a capillary tube was first analyzed by Bretherton [J. Fluid Mech. 10, 166 (1961)] in the limit of small capillary number (Ca). While his theoretical prediction for the wetting film thickness left behind the moving front of the bubble showed good agreement with experimental observations for a moderately small Ca, it underpredicted the film thickness when Ca was smaller than about 10−4. In an attempt to resolve this discrepancy, several investigators studied the Marangoni effects of surface contaminants and predicted that the film thickness could be increased by a maximum factor of 42/3 if the surfactant transport is bulk‐diffusion controlled. This prediction, however, is limited to the case of a semi‐infinitely long bubble. The present study examines the motion of a finite length bubble in the presence of a small amount of surfactants. Both front and rear ends of the bubble are analyzed in the limit of small capillary number. The results indicate that due to the accumulation of the surfactant at the rear end of the bubble, the film thickening effect of the surfactant occurs only when the bubble length is larger than a certain critical value. It is also shown that the total pressure drop to drive the finite bubble through the capillary tube increases linearly with the length of the bubble as long as the bubble length is smaller than the critical value. When the bubble is longer than the critical value, the pressure drop does not increase any longer but remains to be constant. It appears that these results are capable of explaining the experimental observations of Schwartz and co‐workers [J. Fluid Mech. 127, 259 (1986)] in which the film‐thickening behavior at low Ca was shown to be dependent on the bubble length.