Showing papers on "Pressure drop published in 2000"

Contact line deposits in an evaporating drop

Abstract: Solids dispersed in a drying drop will migrate to the edge of the drop and form a solid ring. This phenomenon produces ringlike stains and occurs for a wide range of surfaces, solvents, and solutes. Here we show that the migration is caused by an outward flow within the drop that is driven by the loss of solvent by evaporation and geometrical constraint that the drop maintain an equilibrium droplet shape with a fixed boundary. We describe a theory that predicts the flow velocity, the rate of growth of the ring, and the distribution of solute within the drop. These predictions are compared with our experimental results.

Properties of ceramic foam catalyst supports: pressure drop

Abstract: Ceramic foams are prepared as positive images of corresponding plastic structures and exhibit bed porosities as high as 80–90%. This makes them attractive as catalyst supports in processes where high pressure drop in the reactor tube is a problem. In this research, pressure drop relationships were examined for 10, 30, 45 and 65 pores per inch (PPI) ceramic foam samples made from 92.0 and 99.5% α-Al2O3 and from ZrO2 stabilized with Mg, Ca, and La2O3. Pore distributions were determined with imaging analysis, using digital techniques. Pressure drop measurements confirmed that ceramic foams follow the Forscheimer relationship and may be interpreted with the Ergun model, in which the pressure drop is the sum of viscous and inertial terms. The Ergun parameters, α and β, are not constant, α decreases from 8.05 to 2.88 and β increases from 0.0338 to 0.111 as the pore density increases from 10 to 65 PPI. Empirical equations were developed for these parameters in terms of the mean pore size and the bed porosity, and these indicated a dependence on the media properties. Calculated pressure drop from these equations were within 15% of measured values. Up to 15 wt.% γ-Al2O3 washcoat was added to 30 PPI samples of α-Al2O3 foams. Nitrogen BET surface areas increased from about 2 m2 g−1 in the unwashcoated samples to almost 15 m2 g−1 at the highest loading. Both α and β increase linearly with the BET surface area, α by only about 50% but β by a factor of 8. This suggests that roughness introduced by the washcoat plays a dominant role in the turbulent resistance.

Monolithic silica columns for HPLC, micro-HPLC, and CEC

Abstract: Two types of monolithic silica columns derivatized to form an ODS phase, one prepared in a fused silica capillary (SR-FS) and the other prepared in a mold and clad with an engineering plastic (poly-ether-ether-ketone) (SR-PEEK), were evaluated. The column efficiency and pressure drop were compared with those of a column packed with 5-μm ODS-silica particles and of an ODS-silica monolith prepared in a mold and wrapped with PTFE tubing (SR-PTFE). SR-FS gave a lower pressure drop than a column packed with 5-μm particles by a factor of 20, and a plate height of 20 μm at a linear velocity below 1 mm/s. SR-PEEK showed higher flow-resistance than the other monolithic silica columns, but they still showed a minimum plate height of 8-10 μm and a lower pressure drop than popular commercial columns packed with 5-μm particles. The evaluation of SR-FS columns in a CEC mode showed much higher efficiency than in a pressure-driven mode.

Pressure drop measurements in a microchannel

Abstract: Recent developments in micro-energy and micro-chemical systems have produced a need for greater understanding of flow in small channels. Several recent studies of friction factors and transition Reynolds numbers in rectangular microchannels have produced results that differ from classical theory. In this work, friction factors and laminar flow friction constants were determined for water flowing in high aspect ratio channels with depths ranging from 128 to 521 μm. Reynolds numbers were between 60 and 3,450. Pressure drops were measured within the channel itself to exclude entrance and exit losses. Transitions to turbulence were observed with flow visualization. Uncertainties in measured variables were quantified and propagated into the estimated friction constants. Friction factors were also determined in a 1,050- μm-deep channel that served as a control. After considering experimental uncertainties and systematic errors, significant differences remained between the results and classical theory

Damping force control type hydraulic shock absorber

Abstract: In a damping force control type hydraulic shock absorber, the flow path area of a port is changed by moving a spool according to an electric current supplied to an actuator, and thus the flow path area of a passage between cylinder upper and lower chambers is directly changed, thereby controlling orifice characteristics. Moreover, the pressure in a pilot chamber is changed according to the resulting pressure loss so as to change the valve opening pressure of a disk valve, thereby controlling valve characteristics. This enables the damping force characteristic control range to be widened. The pilot chamber is formed by the side wall of a valve member, the disk valve, a seal disk, and a seal member, also, the seal member has no sliding portion. It is therefore possible to minimize the leakage of hydraulic fluid and to obtain stable damping force characteristics. It is also possible to minimize variations in damping force with temperature changes.

A numerical study of the motion of drops in Poiseuille flow. Part 1. Lateral migration of one drop

Abstract: The cross-stream migration of a deformable drop in two-dimensional Hagen–Poiseuille flow at finite Reynolds numbers is studied numerically. In the limit of a small Reynolds number (< 1), the motion of the drop depends strongly on the ratio of the viscosity of the drop fluid to the viscosity of the suspending fluid. For viscosity ratio 0.125 a drop moves toward the centre of the channel, while for ratio 1.0 it moves away from the centre until halted by wall repulsion. The rate of migration increases with the deformability of the drop. At higher Reynolds numbers (5–50), the drop either moves to an equilibrium lateral position about halfway between the centreline and the wall – according to the so-called Segre–Silberberg effect or it undergoes oscillatory motion. The steady-state position depends only weakly on the various physical parameters of the flow, but the length of the transient oscillations increases as the Reynolds number is raised, or the density of the drop is increased, or the viscosity of the drop is decreased. Once the Reynolds number is high enough, the oscillations appear to persist forever and no steady state is observed. The numerical results are in good agreement with experimental observations, especially for drops that reach a steady-state lateral position. Most of the simulations assume that the flow is two-dimensional. A few simulations of three-dimensional flows for a modest Reynolds number (Re = 10), and a small computational domain, confirm the behaviour seen in two dimensions. The equilibrium position of the three-dimensional drop is close to that predicted in the simulations of two-dimensional flow.

Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes

Abstract: This paper presents a critical review of correlations to compute heat transfer coefficients and pressure drop, for refrigerants condensing inside commercially available tubes with enhanced surfaces of various types, and a theoretical analysis of the condensation phenomenon. Predictions from some of the above equations are compared with experimental data. In addition, information is presented about the influence of small amounts of compressor oil on the condensation of refrigerants in enhanced tubes.

The onset of flow instability in uniformly heated horizontal microchannels

Abstract: Onset of nucleate boiling and onset of flow instability in uniformly heated microchannels with subcooled water flow were experimentally investigated using 22-cm long tubular test sections, 1.17 mm and 1.45 mm in diameter, with a 16-cm long heated length. Important experimental parameter ranges were: 3.44 to 10.34 bar channel exit pressure; 800 to 4,500 kg/m{sup 2}s mass flux (1 to 5 m/s inlet velocity); 0 to 4.0 MW/m{sup 2} channel wall heat flux; and 7,440--33,000 Peclet number at the onset of flow instability. Demand curves (pressure drop versus mass flow rate curves for fixed wall heat flux and channel exit pressure) were generated for the test sections, and were utilized for the specification of the onset of nucleate boiling and the onset of flow instability points. The obtained onset of nucleate boiling and onset of flow instability data are presented and compared with relevant widely used correlations.

High-temperature ultrafast liquid chromatography.

Abstract: A novel liquid chromatographic system which enables high temperature ultrafast liquid chromatography (HTUFLC) has been designed through the careful consideration of heat transfer, band broadening, and pressure drop Studies of the effect of linear velocity on the HETP show that column efficiency at high velocity, especially of well-retained solutes, dramatically improves at higher temperatures At 150 °C, at a flow rate of 15 mL/min with a 5 cm by 46 mm (id) column packed with 3 μm polystyrene-coated zirconia porous particles, long chain alkylphenones were completely resolved, and the analysis time could be decreased by a factor of 50 compared to that at room temperature (25 °C) at a conventional flow rate (4 mL/min) In addition, using pure water as the mobile phase, five phenols were separated in less than 30 s

Transient fluidization and segregation of binary mixtures of particles

Abstract: Fluidization of binary mixtures of particles belonging to group B of the Geldart classification of powders was studied. Beds tested were prepared by mixing in different proportions particles with almost equal density (≈2,500 kg/m3) and dissimilar size (125 μm silica sand and 500 μm glass beads). Experiments were carried out using a segmented fluidization column equipped with multiple pressure transducers. Experimental procedures included continuous monitoring of pressure drop at different locations along the bed during quasi-steady or stepwise changes of gas superficial velocity, and characterization of particle-size distributions in each segment of the fluidization column after fluidization of the bed for given times. Three ranges of gas superficial velocity were recognized for each solids mixture. At low velocity the bed behaves as a fixed bed. At high velocity, it is fully and steadily fluidized. In an intermediate velocity range, transient fluidization takes place: an initially uniform fluidized bed eventually undergoes segregation, giving rise to a defluidized bottom layer rich in the coarser solids and to a “supernatant” fluidized layer where finer particles prevail. The thresholds between these velocity ranges are rather sharp and were characterized as functions of initial bed composition. Rates at which the defluidized solids layer builds up from initially uniform beds, and the ultimate compositions of the defluidized bottom and fluidized top layers are characterized for beds with different compositions at variable gas superficial velocity.

High-pressure rheology of polystyrene melts plasticized with CO2: Experimental measurement and predictive scaling relationships

Abstract: A high-pressure extrusion slit die rheometer was constructed to measure the viscosity of polymer melts plasticized by liquid and supercritical CO2. A novel gas injection system was devised to accurately meter the follow of CO2 into the extruder barrel. Measurements of pressure drop, within the die, confirm the presence of a one-phase mixture and a fully developed flow during viscosity measurements. Experimental measurements of viscosity as a function of shear rate, pressure, temperature, and CO2 concentration were conducted for three commercial polystyrene melts. The CO2 was shown to be an effective plasticizer for polystyrene, lowering the viscosity of the polymer melt by as much as 80%, depending of the process conditions and CO2 concentration. Existing theories for viscoelastic scaling of polymer melts and the prediction of Tg depression by a diluent were used to develop a free volume model for predicting the effects of CO2 concentration and pressure on polymer melt rheology. The free volume model, dependent only on material parameters of the polymer melt and pure CO2, was shown to accurately collapse the experimental data onto a single master curve independent of pressure and CO2 concentration for each of the three polystyrene samples. This model constitutes a simple predictive set of equations to quantify the effects of gas-induced plasticization on molten polymer systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3168–3180, 2000

An analysis of loop seal operations in a circulating fluidized bed

Abstract: The operation of a loop seal in a circulating fluidized bed is studied on the basis of pressure balance of the circulation loop. The sharp-crested theory of free surface flow is applied to analyze the solids flow rate through a loop seal. Factors, which influence the solids flow rate through the loop seal, include loop seal air velocity, initial bed inventory, standpipe size, loop seal slit size and particle size. The solids flow could occur only between two limiting values of each of those parameters. The analysis also presented pressure distributions along the loop for different circulation rates. Results from above theoretical analyses were compared with experimental results. A good agreement between the results confirmed the validity of the present analysis.

Impact of tube-to-particle-diameter ratio on pressure drop in packed beds

Abstract: Discussions in the literature about the influence of (1) the operating parameters Re 0 (Reynolds number) and D/d p (tube-to-particle-diameter ratio), and (2) the physical phenomena of wall friction and flow maldistribution on the pressure drop in packed tubes are in conflict. This influence is analyzed by applying the extended Brinkman equation to cylindrical beds of particles of nearly (but not perfectly) spherical shape and moderate-size dispersity.

Compressible Flow Through Solar Power Plant Chimneys

Abstract: Chimneys as tall as 1500 m may be important components of proposed solar chimney power plants. The exit air density will then be appreciably lower than the inlet density The paper presents a one-dimensional compressible flow approach for the calculation of all the thermodynamic variables as dependent on chimney height, wall friction, additional losses, internal drag and area change. The method gives reasonable answers even over a single 1500 m step length used for illustration, but better accuracy is possible with multiple steps. It is also applicable to the rest of the plant where heat transfer and shaft work may be present. It turns out that the pressure drop associated with the vertical acceleration of the air is about three times the pressure drop associated with wall friction. But flaring the chimney by 14 percent to keep the through-flow Mach number constant virtually eliminates the vertical acceleration pressure drop.

Flow enhancement of a rough fracture

Abstract: We study experimentally and numerically the permeability of a rough fracture at laboratory scale when viscous forces are dominating (low Reynolds number). The experimental setup includes a granite fracture surface (10 × 10 cm²) opened in mode I. It allows a continuous opening of the fracture parallel to its mean plane. The fracture roughness is measured and characterized in terms of self-affine heterogeneities. Its isotropy is checked. The departure from the cubic law is measured as a function of the aperture opening (for mean separations between 4.0 and 10.3 mm) and the pressure drop orientation by rotating the fracture by 90 degrees. A strongly anisotropic hydraulic behavior is observed and results from the geometrical heterogeneities that exist up to the fracture macro-scale.

Heat exchanger design targets for minimum area and cost

Abstract: A methodology is proposed to determine the feasible region for shell and tube heat exchanger designs on a pressure drop diagram. By accounting for operating as well as geometrical constraints, the feasible region is de®ned so as to eliminate trial-and-error during the design activity. Every point on this plot of shellside versus tubeside pressure drop corresponds to a unique design in terms of tube length, shell diameter and baffle spacing. Furthermore, curves may be plotted for designs corresponding to a specified heat exchange area or a given total annual cost. Such curves permit screening of various design options prior to detailed rating of the exchangers, and allow heat exchanger design targets to be established for minimum area or cost. The area target ensures an exchanger of the smallest size with minimum capital cost, whereas the cost target yields the optimum pressure drops accounting for the tradeoff between power consumption and heat exchanger area. The methodology is equation-based and can be conveniently implemented on a computer.