Showing papers on "Pressure drop published in 1999"

Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellent wall

Abstract: Drag reduction phenomena, in which 14% drag reduction of tap water flowing in a 16 mm-diameter pipe occurs in the laminar flow range, have been clarified. Experiments were carried out to measure the pressure drop and the velocity profile of tap water and an aqueous solution of glycerin flowing in pipes with highly water-repellent walls, by using a pressure transducer and a hot-film anemometer, respectively. The same drag reduction phenomena also occurred in degassed tap water when using a vacuum tank. The velocity profile measured in this experiment gives the slip velocity at the pipe wall, and it was shown that the shear stress is directly proportional to the slip velocity.The friction factor formula for a pipe with fluid slip at the wall has been obtained analytically from the exact solution of the Navier–Stokes equation, and it agrees well qualitatively with the experimental data.The main reasons for the fluid slip are that the molecular attraction between the liquid and the solid surface is reduced because the free surface energy of the solid is very low and the contact area of the liquid is decreased compared with a conventional smooth surface because the solid surface has many fine grooves. Liquid cannot flow into the fine grooves owing to surface tension. These concepts are supported by the experimental result that drag reduction does not occur in the case of surfactant solutions.

Prediction of Mass Transfer Columns with Dumped and Arranged Packings

Abstract: A good knowledge of the relationship between the two-phase countercurrent flow of a packed mass transfer column is essential for the design of rectification, absorption and desorption columns. Based on a physical model the authors describe their updated equations for calculating gas and liquid side mass transfer coefficients, pressure drop of dry or irrigated random and structured packings, their loading and flooding points as well as their liquid holdup. Based on one of the largest experimental databases in the world, the calculated results give only small deviations from the database

Two-phase pressure drop of refrigerants during flow boiling in small channels : an experimental investigation and correlation development.

Abstract: Two-phase flow pressure drop measurements were made during a phase-change heat transfer process with three refrigerants (R-134a, R-12, and R-113) at six different pressures ranging from 138 kPa to 856 kPa, and in two sizes of round tubes (2.46 mm and 2.92 mm inside diameters) and one rectangular channel (4.06 x 1.7 mm). State-of-the-art large-tube correlations failed to satisfactorily predict the experimental data. The data were used to develop a new correlation for two-phase pressure drop during flow boiling in small channels. The correlation was then tested against the experimental data for the three refrigerants; the error was {+-}20%.

Laminar fluid behavior in microchannels using micropolar fluid theory

Abstract: In this paper, we describe microchannel fluid behavior using a numerical model based on micropolar fluid theory and experimentally verify the model using micromachined channels. The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 50 to 600 μm and heights ranging from 20 to 30 μm. A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record pressure drop. The experimental data obtained for water showed an increase in the Darcy friction factor when compared to traditional macroscale theory, especially at the lower Reynolds number flows. The numerical model of the micropolar fluid theory predicted experimental data better than the classical Navier–Stokes theory and the model compares favorably with the currently available experimental data.

Characteristics of microencapsulated PCM slurry as a heat‐transfer fluid

Abstract: The hydrodynamic and heat-transfer characteristics of slurry containing microencapsulated phase-change materials (MCPCMs) were investigated experimentally for use as a heat-transfer fluid. Pressure drop and local convective heat-transfer coefficients of the slurry flows in a circular tube with uniform heat flux were measured. Slurries consisting of octadecane (C{sub 18}H{sub 38}) contained in 2--10-{micro}m-dia. microcapsules and pure water were used. The particle volume fractions in the slurry were varied up to 0.3. Results showed that increases in particle volume fractions caused the slurry flow structure to change from turbulent to laminar, and the pressure-drop reduction of the slurry flow relative to a single-phase water flow was under the same flow-rate conditions. The heat-transfer performance of the slurry also depended on the change in flow structure. When the MCPCMs melted, the local heat-transfer coefficients for turbulent slurry flows increased relative to those for nonmelting slurry. This phenomenon was influenced by the MCPCM fraction, the degree of turbulence, and the heating rate at the tube wall. The experimental data will be useful in the design of thermal-energy transportation systems using MCPCM slurry.

A fluid--structure interaction finite element analysis of pulsatile blood flow through a compliant stenotic artery.

Abstract: A new model is used to analyze the fully coupled problem of pulsatile blood flow through a compliant, axisymmetric stenotic artery using the finite element method. The model uses large displacement and large strain theory for the solid, and the full Navier-Stokes equations for the fluid. The effect of increasing area reduction on fluid dynamic and structural stresses is presented. Results show that pressure drop, peak wall shear stress, and maximum principal stress in the lesion all increase dramatically as the area reduction in the stenosis is increased from 51 to 89 percent. Further reductions in stenosis cross-sectional area, however, produce relatively little additional change in these parameters due to a concomitant reduction in flow rate caused by the losses in the constriction. Inner wall hoop stretch amplitude just distal to the stenosis also increases with increasing stenosis severity, as downstream pressures are reduced to a physiological minimum. The contraction of the artery distal to the stenosis generates a significant compressive stress on the downstream shoulder of the lesion. Dynamic narrowing of the stenosis is also seen, further augmenting area constriction at times of peak flow. Pressure drop results are found to compare well to an experimentally based theoretical curve, despite the assumption of laminar flow.

Measurements and modeling of PS/supercritical CO2 solution viscosities

Abstract: This paper presents a technology to determine the melt viscosity of a PS/super-critical CO 2 solution using a linear capillary tube die mounted on a foaming extruder. CO 2 was injected into the extrusion barrel and the content of CO 2 was varied in the range of 0 to 4 wt% using a positive displacement pump. Single-phase PS/CO 2 solutions were formed using a microcellular extrusion system and phase separation was prevented by maintaining a high pressure in the capillary tube die. By measuring the pressure drop through the die, the viscosity of PS/CO 2 solutions was determined. The experimental results indicate that the PS/CO 2 solution viscosity is a sensitive function of shear rate, temperature, pressure, and CO 2 content. A theoretical model based on the generalized Cross-Carreau model was proposed to describe the shear-thinning behavior of PS/CO 2 solutions at various shear rates. The zero-shear viscosity was modeled using a generalized Arrhenius equation to accommodate the effects of temperature, pressure, and CO 2 content. Finally, the solubility of CO 2 has been estimated by monitoring the pressure drop and the absolute pressure in the capillary die.

Extensional flow of a polystyrene Boger fluid through a 4 : 1 : 4 axisymmetric contraction/expansion

Abstract: The creeping flow of a dilute (0.025 wt%) monodisperse polystyrene/polystyrene Boger fluid through a 4:1:4 axisymmetric contraction/expansion is experimentally observed for a wide range of Deborah numbers. Pressure drop measurements across the orifice plate show a large extra pressure drop that increases monotonically with Deborah number above the value observed for a similar Newtonian fluid at the same flow rate. This enhancement in the dimensionless pressure drop is not associated with the onset of a flow instability, yet it is not predicted by existing steady-state or transient numerical computations with simple dumbbell models. It is conjectured that this extra pressure drop is the result of an additional dissipative contribution to the polymeric stress arising from a stress-conformation hysteresis in the strong non-homogeneous extensional flow near the contraction plane. Such a hysteresis has been independently measured and computed in recent studies of homogeneous transient uniaxial stretching of PS/PS Boger fluids. Flow visualization and velocity field measurements using digital particle image velocimetry (DPIV) show large upstream growth of the corner vortex with increasing Deborah number. At large Deborah numbers, the onset of an elastic instability is observed, first locally as small amplitude fluctuations in the pressure measurements, and then globally as an azimuthal precessing of the upstream corner vortex accompanied by periodic oscillations in the pressure drop across the orifice.

Centrifugal compressor surge and speed control

Abstract: Previous work on stabilization of compressor surge is extended to include control of the angular velocity of the compressor. A low-order centrifugal compressor model is presented, where the states are mass flow, pressure rise, and rotational speed of the spool. Energy transfer considerations are used to develop a compressor characteristic. In order to stabilize equilibria to the left of the surge line, a close coupled valve is used in series with the compressor. Controllers for the valve pressure drop and spool speed are derived. Semiglobal exponential stability is proved using a Lyapunov argument.

Lattice Boltzmann simulation of flows in a three‐dimensional porous structure

Abstract: The lattice Boltzmann method (LBM) with the fifteen-velocity model is applied to simulations of isothermal flows in a three-dimensional porous structure. A periodic boundary condition with a pressure difference at the inlet and outlet is presented. Flow characteristics at a pore scale and pressure drops through the porous structure are calculated for various Reynolds numbers. It is found that at high Reynolds numbers, unsteady vortices appear behind bodies and the flow field becomes time-dependent. Calculated pressure drops through the structure are compared with well-known empirical equations based on experimental data

Experimental optimization of confined air jet impingement on a pin fin heat sink

Abstract: A variety of nozzle configurations were tested to characterize and optimize the performance of confined impinging air jets used in conjunction with a pin-fin heat sink. Four single nozzles of different diameters and two multiple-nozzle arrays were studied at a fixed nozzle-to-target spacing, for different turbulent Reynolds numbers (5000/spl les/Re/spl les/20000). Variations in the output power level of the heat source and nozzle-to-target spacing were found to have only modest effects on heat transfer at a fixed Reynolds number. Enhancement factors were computed for the heat sink relative to a bare surface, and were in the range of 2.8-9.7, with the largest value being obtained for the largest single nozzle (12.7 mm diameter). Average heat transfer coefficients and thermal resistance values are reported for the heat sink as a function of Reynolds number, air flow rate, pumping power, and pressure drop, to aid in optimizing the jet impingement configuration for given design constraints.

Effects of rectangular microchannel aspect ratio on laminar friction constant

Abstract: In this paper, the effects of rectangular microchannel aspect ratio on laminar friction constant are described. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 150 micrometers to 600 micrometers and heights ranging from 22.71 micrometers to 26.35 micrometers . A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record the pressure drop. The experimental data obtained for water for flows at Reynolds numbers below 10 showed an approximate 20% increase in the friction constant for a specified driving potential when compared to macroscale predictions from the classical Navier-Stokes theory. When the experimental data are studied as a function of aspect ratio, a 20% increase in the friction constant is evident at low aspect ratios. A similar increase is shown by the currently available experimental data for low Reynolds number (< 100) flows of water.

Fluid flow controlling

Abstract: A method and system for controlling the rate of fluid flow. A flow restrictor having known pressure drop and flow rate characteristics provided in a passage through which the fluid, preferably a gas, flows. An upstream pressure sensor determines the pressure of fluid in the flow passage upstream of the flow restrictor. A downstream pressure sensor determines the pressure of fluid in the flow passage downstream of said flow restrictor. A pressure regulator adjusts the pressure of fluid upstream or downstream of the flow restrictor based on the pressure drop across the flow restrictor so that the actual pressure drop across the flow restrictor closely corresponds to the pressure drop associated with a desired rate of fluid flow.

Aphron-containing well drilling and servicing fluids of enhanced stability

Abstract: The invention provides a method and fluid for drilling or servicing a well in a subterranean formation containing lost circulation zones or depleted, low pressure reservoirs. A method of decreasing the density of the fluid circulated in a borehole, decreasing the invasion of fluid into formations contacted by the fluid, or decreasing the lost circulation potential of the fluid is provided by using as the fluid an aqueous liquid having dispersed therein a polymer which increases the low shear rate viscosity of the fluid to the extend that the shear thinning index of the fluid is at least about 10, a surfactant, and aphrons, wherein the aphrons are preferably generated by the turbulence and pressure drop as the fluid exits the drill bit in the vicinity of the formation, the fluid having a low shear rate viscosity of at least 10,000 centipoise. The fluid additionally contains an oligosaccharide mixture composed of pentosans and/or hexosans containing from one to about 10 combined sugar units selected from the group consisting of arabinose, mannose, galactose, glucose, xylose, cellobiose, and mixtures thereof. Preferably the oligosaccharide mixture is the water soluble portion of thermally hydrolyzed lignocellulose.

Effect of Column Diameter on Pressure Drop of a Corrugated Sheet Structured Packing

Abstract: The results of an experimental study indicating a significant effect of column diameter on the pressure drop and capacity of corrugated sheet structured packing are presented. The experiments were carried out with Montz-pak B1-250, using an air/water system at ambient conditions in perspex columns with internal diameters of 0.2 m, 0.45 m, 0.8 m and 1.4 m. The results clearly show that both the pressure drop increase and capacity decrease become significant when the column diameter approaches the value equal to that of the height of a packing element. This observation suggests that column diameter(s) based on small scale data can be too large.