Showing papers on "Pressure drop published in 1994"

Convective boiling and condensation

Abstract: Introduction 1. The basic models 2. Empirical treatments of two-phase flow 3. Introduction to convective boiling 4. Subcooled boiling heat transfer 5. Void fraction and pressure drop in subcooled boiling 6. Saturated boiling heat transfer 7. Critical heat flux in forced convective flow - 1. Vertical uniformly heated tubes 8. Critical heat flux in forced convective flow - 2. More complex situations 9. Condensation 10. Conditions influencing the performance of boiling and condensing systems 11. Multi-component boiling and condensation Appendix Index

Simulation of Heat and Momentum Transfer in Complex Microgeometries

Abstract: In this article we present a time-accurate computational model based on the slip-flow theory to simulate momentum and heat transport phenomena in complex microgeometries, encountered in typical components of microdevices such as microcapillaries, microvalves, microrotors, and microbearings. In the first part, we present extensions to the classical Maxwell/Smoluchowski slip conditions to include high-order Knudsen number effects as well as to take into account the coupling of momentum and heat transfer through thermal creep and viscous heating effects. The numerical method is based on the spectral element technique; validation of the method is obtained by comparison of the numerical simulation results in simple prototype flows (e.g., channel slip-flows) with analytical results. Reduction of pressure drop in microchannels, reported in similar experimental studies, is investigated using slip-flow theory and simulations. In the second part, we consider model inlet flows and a slip-flow past a microcylinder. The effect of slip-flow on skin friction reduction and associated increase in mass flow rate as well as the variation of normal stresses is investigated as a function of Knudsen number. Finally, the effect of compressibility is examined and possible extensions of the current model to take into account such effect are discussed.

Dynamics of a drop in a constricted capillary tube

Abstract: Here we study the dynamics of a bubble or drop as it is driven by a pressure gradient through a capillary tube. For the case of a straight capillary, the drop can either approach a steady-state shape or the rear of the drop develops a re-entrant cavity. Also, depending on the initial conditions, the drop can break apart into smaller drops. For flow through a constricted capillary tube, depending on the physical parameters of the problem, the drop can either move through the constriction or break into two or more pieces as it moves past the constriction. We study this snap-off process numerically and determine the effect of the physical parameters on the dynamics of the drop.

Comparison of correction factors for shell-and-tube heat exchangers with segmental or helical baffles

Abstract: Heat transfer and pressure drop correction factors based on the Bell-Delaware method have been compared for an optimized segmental baffle heat exchanger and a helical baffle heat exchanger. In general, the results showed that properly designed helical baffles offer a significant improvement in heat transfer while providing a reduced exchanger pressure drop. The enhancement in heat transfer for helical baffles was reflected by the so-called turbulence enhancement correction factor, which accounted for the increase in heat transfer observed at a critical baffle inclination angle of 25°. As the baffle inclination angle was increased beyond this critical angle, the turbulence enhancement factor continued to increase and eventually produced a maximum heal transfer enhancement of 1.39 times that for ideal cross-flow conditions. The reduction in pressure drop due to the helical baffles was found to vary from 0.26 to 0.60 depending on the helical inclination angle.

On the vesicularity of pumice

Abstract: We develop a physical framework for the evolution of vesicular magma fragments after their formation by fragmentation in explosive eruptions. These fragments are carried by a gaseous jet through the volcanic conduit and into the atmosphere and are thus subjected to a decrease of pressure and temperature. We derive equations for the evolution of void fraction, internal gas pressure and temperature as a function of time and position in the fragment. The driving terms for the evolution are the pressure and temperature history external to the fragment. We show that a magma fragment can expand significantly before being quenched. The influence of melt viscosity is introduced as a dimensionless number that relates melt viscosity, pressure drop between fragmentation within the conduit and expansion in the atmosphere, and duration of the pressure drop. For melt viscosities larger than 109 Pa s, corresponding to high-silica-content rhyolitic melt with small amounts of dissolved water, expansion of vesicles is negligible so that quenched pumice samples record the state of vesicular magma at the moment of fragmentation. Over the viscosity range of 106 to 109 Pa s, the amount of vesicularity is a function of melt viscosity and the history of external pressure. For viscosities lower than 106 Pa s, melt viscosity is not important, and gas pressure within vesicles is always close to the external pressure. Available data for four Plinian deposits show that pumice vesicularity varies significantly both as a function of time during an eruption and between eruptions. For the magma compositions and volatile contents of the Bishop Tuff, United States; Taupo, New Zealand; and Minoan, Santorini, eruptions, the void fraction of pumice is predicted to be a decreasing function of melt viscosity, in agreement with observations. During an eruption, a change of fragmentation conditions will produce a small difference in pumice vesicularity, so that a systematic change in the average vesicularity within a pumice deposit may record a change of eruption dynamics.

Forced Convection in a Porous Channel With Localized Heat Sources

Abstract: A numerical study is performed to analyze steady laminar forced convection in a channel filled with a fluid-saturated porous medium and containing discrete heat sources on the bottom wall. Hydrodynamic and heat transfer results are reported for two configurations: (1) a fully porous channel, and (2) a partially porous channel, which contains porous layers above the heat sources and is nonporous elsewhere. The flow in the porous medium is modeled using the Brinkman-Forchheimer extended Darcy model. Heat transfer rates and pressure drop are evaluated for wide ranges of Darcy and Reynolds numbers. Detailed results of the evolution of the hydrodynamic and thermal boundary layers are also provided

Flow Through Porous Media of Packed Spheres Saturated With Water

Abstract: The existing literature on the flow of fluids through porous packed beds gives very limited quantitative information of the criteria employed in marking the applicability of the different flow regimes. It is the objective of this paper to provide experimental evidence for determining the demarcation criteria during the flow of water through a bed of randomly packed spherical beads. Two different sizes of glass beads, 3 mm and 6 mm, were employed as the porous matrix through which water flowed at rates varying from 5.07 [times] 10[sup [minus]6] m[sup 3]/s to 4,920 [times] 10[sup [minus]6] m[sup 3]/s. The dimensionless pressure drop data showed less variation when the characteristic length of the porous medium was taken to be proportional to the square root of the permeability over the porosity and not the bead diameter. Curves of properly nondimensionalized pressure drop plotted against the actual flow Reynolds number based on the porous medium permeability provided the following information. It was found that Darcy's law has very limited applicability and is valid for a small range of Reynolds numbers. This leads to a pre-Darcy flow that is valid for a much broader range of Reynolds numbers than expected. Alternatively, the range of validitymore » of the post-Darcy laminar Forchheimer flow is also found to be of much more limited applicability than previous studies have indicated. Transition to turbulence takes place earlier than expected and turbulent flow prevails from then on. The dimensionless pressure drop in both the Forchheimer and turbulent flow regimes can be modeled by an appropriately nondimensionalized Ergun's equation i.e., a first-order inertia term correction is sufficient in both flow regimes.« less

Heat Exchange Effectiveness and Pressure Drop for Air Flow Through Perforated Plates With and Without Crosswind

Abstract: Low-porosity perforated plates are being used as absorbers for heating ambient air in a new type of unglazed solar collector. This paper investigates the convective heat transfer effectiveness for low-speed air flow through thin, isothermal perforated plates with and without a crosswind on the upstream face. The objective of this work is to provide information that will allow designers to optimize hole size and spacing. In order to obtain performance data, a wind tunnel and small lamp array were designed and built. Experimental data were taken for a range of plate porosities from 0.1 to 5 percent, hole Reynolds numbers from 100 to 2000, and wind speeds from 0 to 4 m/s. Correlations were developed for heat exchange effectiveness and also for pressure drop. Infrared thermography was used to visualize the heat transfer taking place at the surface. 7 refs., 13 figs.

Two-Phase Electronic Cooling Using Mini-Channel and Micro-Channel Heat Sinks: Part 2—Flow Rate and Pressure Drop Constraints

Abstract: Increased rate of heat dissipation from electronic chips was explored by the application of flow boiling in mini-channel (D = 2.54 mm) and micro-channel (D — 510 \\x,m) heat sinks with special emphasis on reducing pressure drop and coolant flow rate. A pressure drop model was developed that accounts for the single-phase inlet region, the singleand two-phase heated region, and the two-phase unheated outlet region. Inlet and outlet losses associated with the abrupt contraction and expansion, respectively, were also accounted for, and so were the effects of compressibility and flashing within the two-phase region. Overall, the major contributor to pressure drop was the acceleration caused by evaporation in the channels; however, compressibility effects proved significant for the micro-channel geometiy. Based upon practical considerations such as pressure drop, erosion, choking, clogging, and manufacturing ease, the mini-channel geometiy was determined to offer inherent advantages over the microchannel geometiy. The latter is preferred only in situations calling for dissipation of high heat fluxes where minimizing weight and liquid inventory is a must.

On the pressure dependency of the viscosity of molten polymers

Abstract: A slit viscometer to measure the viscosity of polymer melts under processing conditions is described. Along the slit a pressure drop is generated by applying a pressure at both the entrance and the exit. In this way the pressure in the center can be controlled independently of the shear rate. The pressure gradient in the slit is measured by means of three pressure transducers which are mounted in the region of fully developed flow. Results of pressure-dependent viscosity measurements on polystyrene, polyacrylonitrile-butadiene-styrene, and polypropylene are presented in a shear rate range of five decades. The flow curves obtained at different pressures and temperatures can be shifted onto a master curve. The shear thinning behavior of the three materials is adequately described with the generalized Cross-Carreau equation, while the zero shear viscosity is modeled with a generalized Arrhenius-W.L.F. relationship, incorporating a pressure dependency. Alternatively, it is possible to describe the zero shear viscosity in terms of the free volume fraction and the temperature.

Concentric-tube airlift reactors: effects of geometrical design on performance.

Abstract: Pressure drops, gas holdup, and mass-transfer coefficients were measured in two concentric-tube airlift reactors of 30 and 300 L (nominal volume). The aspect ratio and the ratio of riser to downcomer cross section were the same for both reactors, but the geometry of the gas separation section was different. The influence of the bottom and top clearances was studied using water and carboxymethyl cellulose solutions and covering a range of effective viscosity from 10 -3 to 25×10 -3 Pa.s. The pressure drop results expressed as a Euler number were salisfactorily correlated with Fr, Ga, and a bubble disengagement group M. Correlations are presented also for the gas holdup in the riser, downcomer, and gas separator. The last-mentioned coincides with the correlation for the total holdup in the reactor. The gas-liquid mass-transfer coefficients for all the liquids and geometric variables in both reactors were successfully correlated as Sherwood numbers

Hydrodynamics of sawdust and mixtures of wood residues in conical spouted beds

Abstract: It has been proven that conical spouted beds allow for stable operation with sawdust and with wood residues, even with mixtures of these materials of wide particle size range and without being diluted with an inert solid. Peculiar hydrodynamic characteristics for sawdust have been observed; a great hysteresis in the pressure drop vs velocity curve, a pronounced peak of maximum pressure drop, and a difference between the velocity for which spout and fountain are formed and the velocity of the fully spouted bed. From the hydrodynamic study of sawdust, the ranges of the contactor geometric factors (cone angle, inlet diameter/base diameter ratio, inlet diameter/particle diameter ratio) for which operation is stable have been determined. Original correlations for calculation of minimum spouting velocity, of stable operation pressure drop, of maximum pressure drop, and of minimum voidage of complete spouting have been proposed.

Deagglomerators for dry powder inhalers

Abstract: The mouthpiece (30) comprises two parts (32 and 34), which are inserted into the mouth together. At low airflow rates, the air and powder stream through the channel (33, 36, 35) is forced to pass through the narrow gap (36) between protrusion (38) and the thin diaphragm (40), which seals region (42) off from the airflow. This narrow gap causes turbulence in the air stream, thereby deagglomerating the powder. At higher airflow rates, however, the pressure in region (42) is reduced with respect to the pressure in region (35) as the patient inhales harder. This causes the diaphragm (40) to bow upwards, widening the gap (36). The pressure drop across the channel (between the ends of the channel), and the turbulence through it, is thus lower than it would be if the variable geometry were not variable. The respirable fraction is thus more constant with airflow rated.

Characteristics of Swirling Flow in a Circular Pipe

Abstract: This paper examines experimentally the decay of swirl, the average dynamic, static and total pressures and the wall pressure in a pipeline 13 m in length and with an inside diameter of 80 mm for two Reynolds numbers and five different inlet swirls. The empirical correlations for the above quantities are derived, and by using these empirical correlations, the decay process and pressure distributions along the pipe for the swirling flow can be successfully computed by giving discharge velocity and a wall static pressure at any axial position.

Compressible viscous flow in slits with slip at the wall

Abstract: We study the time‐dependent compressible flow of a Newtonian fluid in slits using an arbitrary nonlinear slip law relating the shear stress to the velocity at the wall. This slip law exhibits a maximum and a minimum and so does the flow curve. According to one‐dimensional stability analyses, the steady‐state solutions are unstable if the slope of the flow curve is negative. The two‐dimensional flow problem is solved using finite elements for the space discretization and a standard fully implicit scheme for the time discretization. When compressibility is taken into account and the volumetric flow rate at the inlet is in the unstable regime, we obtain self‐sustained oscillations of the pressure drop and of the mass flow rate at the exit, similar to those observed with the stick‐slip instability. The effects of compressibility and of the length of the slit on the amplitude and the frequency of the oscillations are also examined.

Limits of temperature separation in a vortex tube

Abstract: The heating and cooling in a vortex tube is attributed to conversion of kinetic energy into heat and to the reverse process. A two-component model yields the upper limit for the temperature increase on the hot side: (Th-To)/T0 or=T0(1-X)( gamma -1/ gamma ), where gamma =1.4 for air, and X=(p0-pc)/p0 is the normalized pressure drop between the inlet (p0) and the cold exhaust port (Pc) Extensive experimental data for a vortex tube of 18 mm inner diameter with working fluid air all fall between these limits. The model predicts that the inlet velocity reaches the speed of sound for X=0.7. No values X>0.7 could be obtained in these experiments, indicating that the flow always remain subsonic.

Ductile creep and compaction: A mechanism for transiently increasing fluid pressure in mostly sealed fault zones

Abstract: A simple cyclic process is proposed to explain why major strike-slip fault zones, including the San Andreas, are weak. Field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock. Ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure. The increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic. The frictional slip associated with earthquakes creates porosity in the fault zone. The cycle adjusts so that no net porosity is created (if the fault zone remains constant width). The fluid pressure within the fault zone reaches long-term dynamic equilibrium with the (hydrostatic) pressure in the country rock. One-dimensional models of this process lead to repeatable and predictable earthquake cycles. However, even modest complexity, such as two parallel fault splays with different pressure histories, will lead to complicated earthquake cycles. Two-dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes. A possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip, stabilizing the fault, and then decreases with further slip, destabilizing it.

Motion of a deformable capsule through a hyperbolic constriction

Abstract: A model for the low-Reynolds-number flow of a capsule through a constriction is developed for either constant-flow-rate or constant-pressure-drop conditions. Such a model is necessary to infer quantitative information on the intrinsic properties of capsules from filtration experiments conducted on a dilute suspension of such particles. A spherical capsule, surrounded by an infinitely thin Mooney-Rivlin membrane, is suspended on the axis of a hyperbolic constriction. This configuration is fully axisymmetric and allows the entry and exit phenomena through the pore to be modelled. An integral formulation of the Stokes equations describing the flow in the internal and external domains is developed. It provides a representation of the velocity at any location in the flow as a function of the unknown forces exerted by the boundaries on the fluids. The problem is solved by a collocation technique in the case where the internal and external viscosities are equal. Microscopic quantities (instantaneous geometry, centre of mass velocity, elastic tensions in the membrane) as well as macroscopic quantities (entry time, additional pressure drop or flow rate reduction) are predicted as a function of the capsule intrinsic properties and flow characteristics. The results obtained for a capsule whose initial diameter is larger than that of the constriction throat show that the maximum energy expenditure occurs when the particle centre of mass is still upstream of the throat (typically 1 diameter away), and is thus due to the entry process. For large enough or rigid enough capsules, the model predicts entrance or exit plugging, in agreement with experimental observations. It is then possible to correlate the variation of the pore hydraulic resistance to the flow capillary number (ratio of viscous to elastic forces) and to the size ratio between the pore and the capsule.

A valve having means for generating a wireless transmittable indicating signal in case of a pressure drop within vehicle tires

Abstract: A valve which includes signal generating means for generating a signal representing a pressure drop occurring within a vehicle tire. The valve includes an absolute pressure sensor coupled to measure the pressure within the tire. A microprocessor is provided which upon activation following operation of the valve stores a value of the electric pressure signal from the pressure sensor in a memory. During operation of the vehicle, the stored reference electric pressure signal is compared with periodic measurements of pressure within the tire. The transmitter is activated at periodic intervals, whenever a given ratio between the compared values and a threshold value exceeds an upper or a lower limit. The system is activated by an acceleration sensor which can detect when the vehicle is moving, thus avoiding power consumption when the vehicle is stopped.