Search

This paper presents new results for numerical predictions of air flow and pressure distribution in two commonly used elbows: (1) 90[degrees] mitered duct elbows with turning vanes having 0.05 m radius, 0.038 m vane spacing and (2) 90[degrees] mitered duct elbows without turning vanes, in 0.2 x 0.2 m (8 in. X 8 in.) duct cross section using the STAR-CD computational fluid dynamics (CFD) code. A k-[epsilon] turbulence model for high Reynolds number and k-[epsilon] Chen model were used for that purpose for comparative purposes. The simulation used 13 different Reynolds numbers chosen between the range of 1 X [10.sup.5] and 2 x [10.sup.6]. To validate the CFD results, the results of two experimental papers using guided vanes were compared with simulated vane runs under the same condition. The first experimental study used a 0.6 x 0.6 m (24 in. X 24 in.) square elbow with 0.05 m radius, 0.038 m vane spacing and air velocities at 2.54 m/s (500 fpm) and 25.4 m/s (5,000 fpm), the second experiment used a 0.81 X 0.2 m (32 in. X 8 in.) rectangular elbow geometry with 0.05 m radius, 0.038 m vane spacing with air velocities from 10.16 m/s (2,000 fpm) to 13.97 m/s (2,750 fpm). For Reynolds numbers (1.00-2.00) x [10.sup.5] the pressure drop difference between vaned and unvaned elbows was found to be 35 Pa as compared to 145 Pa. The simulations also agreed reasonably well with published experimental results. For the 0.6X0.6 m (24 in. x 24 in.) square elbow and 0.81 X 0.2 m (32 in. X 8 in.) rectangular elbow with vanes, the difference in pressure drop was 3.9 and 4.1% respectively and indicates that CFD models can be used for predictive purposes in this important HVAC applications area. DOI: 10.1061/(ASCE)0733-9402(2009)135:4(119) CE Database subject headings: Flow simulation; Velocity profile; Predictions; Air flow.

Cyclic loading has been known to induce fluid flow and thus mechanotransduction in bones. In the past, four-point bending tests have been used exclusively in studying fluid flow in bones. In order to better understand the mechanism of deformation and fluid flow under loading, compression tests were done on trabecular bone specimens under drained and undrained conditions. In the drained tests, the volume change was observed, whereas in the undrained tests, excess pore fluid pressure was measured. Cyclic loading tests were conducted in addition to monotonic loading tests to observe the permanent volume change or excess pore fluid pressure with loading cycles. A fast loading rate gave a sharp rise in the excess fluid pressure compared to a slow loading rate. The strength and stiffness of the specimens appeared to deteriorate with an increased speed of loadings, but there was no appreciable difference between the results obtained from drained and undrained tests. The drained and undrained tests as described allowed a better understanding of bone behavior under loadings for a coupled stress-flow analysis. DOI: 10.1061/(ASCE)0733-9399(2009) 135:5(375) CE Database subject headings: Human factors; Biological properties; Mechanical properties; Deformation; Cyclic loads.

Pressure drop through anisotropic porous mediumlike cylinder bundles is experimentally examined in turbulent flow regime. Three porosities, [epsilon] = 0.66, 0.82, and 0.90, are considered. The flow blockage by the cylinder bundles is varied, with the yaw angle ([alpha]) used as an anisotropic measure. When the yaw angle is fixed while the porosity is varied, the pressure drop behaves as predicted by the force balance model, consistent with the classic observation: The pressure drop is proportional to the square of the flow velocity with the empirical proportionality as a function of (1 - [[epsilon].sup.1/2])[[epsilon].sup.2] obtained from the force balance model compared to that of (1 - [epsilon])/[[epsilon].sup.3] from the hydraulic radius theory. On the other hand, for a given porosity, topological anisotropy of the cylinder bundles causes the sinusoidal response of the pressure drop to the variation of yaw angle. At [alpha] = 0 deg with a 60 deg period, the lowest pressure drop occurs from the most open configuration of the cylinder bundle whereas the largest flow blockage at [alpha] = 30 deg causes the highest pressure drop. This variation appears to result from an increase in the drag coefficient of each cylinder element in a harmonic manner.

Numerical analysis of flow distribution inside a compact ceramic high temperature heat exchanger and chemical decomposer (thereafter, heat exchanger), which will be used for hydrogen production, wherein the sulfur iodine thermochemical cycle is performed. To validate the numerical model, experimental investigation of the heat exchanger is accomplished. The study of the flow distribution in the base line design heat exchanger shows that the design has large-flow maldistribution and the reverse flow may occur at poor inlet and outlet manifold configurations. To enhance uniformity of the flow rate distribution among the heat exchanger internal channels, several improved designs of the heat exchanger manifolds and supply channels are proposed. The proposed designs have a sufficiently uniform flow rate distribution among the internal channels, with an appropriate pressure drop. [DOI: 10.1115/1.2911680] Keywords: CFD, pressure drop, fluid flow distribution, channel flow, gross flow maldistribution

A simple model is developed here to predict the pressure drop and discharge coefficient for incompressible flow through orifices with length-to-diameter ratio greater than zero (orifice tubes) over wide ranges of Reynolds number The pressure drop for flow through orifice tubes is represented as two pressure drops in series; namely, a pressure drop for flow through a sharp-edged orifice in series with a pressure drop for developing flow in a straight length of tube. Both of these pressure drop terms are represented in the model using generally accepted correlations and experimental data for developing flows and sharp-edged orifice flow. We show agreement between this simple model and our numerical analysis of laminar orifice flow with length-to-diameter ratio up to 15 and for Reynolds number up to 150. Agreement is also shown between the series pressure drop representation and experimental data over wider ranges of Reynolds number. Not only is the present work useful as a design correlation for equipment relying on flow through orifice tubes but it helps to explain some of the difficulties that previous authors have encountered when comparing experimental observation and available theories.

An experimental investigation has been made on a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake centerline. The length of the splitter plate, l, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of the base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate without the splitter plate. A significant increase in the base pressure coefficient was achieved with the splitter plate (l/h [greater than or equal to] 1), fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, the mean velocity increased more than that in the upstream position in the case without the splitter plate. With the splitter plate, however, the base pressure rise made the mean velocity distribution more closely approach that of a fully developed turbulent boundary layer. Keywords: flat plate, boundary layer, separation, splitter plate, base pressure

Friction factor data for adiabatic cross flow of water in a staggered tube array were obtained over a Reynolds number range (based on hydraulic diameter and gap velocity) of about 10,000-250,000. The tubes were 12.7 mm (0.5 in.) outer diameter in a uniformly spaced triangular arrangement with a pitch-to-diameter ratio of 1.5. The friction factor was compared to several literature correlations and was found to be best matched by the Idelchik correlation. Other correlations were found to significantly vary from the test data. Based on the test data, a new correlation is proposed for this tube bundle geometry, which covers the entire Reynolds number range tested. [DOI: 10.1115/1.2903817] Keywords: friction factor, tube bundle, pressure drop, staggered array

Three-dimensional oscillatory airflows induced by a rotating grass-cutting blade in a cylindrical chamber are studied experimentally and numerically in this paper. Experimental pressure results are obtained using a sound pressure transducer and a data acquisition system. The measured pressure data contain background noise and high frequency sound signals due to the blade vibrations. The background noise is separately measured; its effect on the signal is determined from a spectral subtraction algorithm. A time-accurate finite volume numerical solution to the three-dimensional incompressible unsteady Navier--Stokes equations is also sought using the sliding frame technique and the unstructured tetrahedral mesh. Convergence studies are conducted using various combinations of mesh sizes and time increments to ensure the stability of the numerical scheme. The experimental and numerical pressure results are in good agreement.[DOI: 10.1115/1.2842225] Keywords: computational fluid dynamics (CFD), cutting blade, oscillatory airflow, slidingframe, air pressure, rotation

Air in water flow is a frequent phenomenon in hydraulic structures. The main reason for air entrainment is vortices at water intakes, pumping stations, tunnel inlets, and so on. The accumulated air, in a conduit, can evolve to a different flow pattern, from stratified to pressurized. Among different patterns, slug is most complex with extreme pressure variations. Due to lack of firm relations between pressure and influential parameters, study of slug flow is very important. Based on an experimental model, pressure fluctuations inside a circular, horizontal, and inclined pipe (90 mm inside diameter and 10 m long) carrying tow-phase air-water slug flow has been studied. Pressure fluctuations were sampled simultaneously at different sections, and longitudinal positions. The pressure fluctuations were measured using differential pressure transducers (DPT), while behavior of the air slug was studied using a digital camera. The objective of the paper is to predict the pressure variation in a pipeline or tunnel, involving resonance and shock waves experimentally. The results show that the more intensive phase interaction commences stronger fluctuations. It is shown, that the air-water mixture entering the pipe during rapid filling of surcharging can cause a tremendous pressure surge in the system and may eventually cause failure of the system (e.g., the maximum pressure inside the pipe would reach up to 10 times of upstream hydrostatic pressure as suggested by others too). Relations for forecasting pressure in these situations are presented as a function of flow characteristics, pipe geometry, longitudinal, and cross-sectional positions and head water [DOI: 10.1115/1.2375134] Keywords: two-phase flow, water tunnel, pressure fluctuation, experimental model slug flow

A method, based on that developed for single-phase flow, is proposed for the correlation of two-phase frictional pressure drop data. It is validated using air-water data obtained on small horizontal passages of rectangular and circular cross-section for values of total mass flux G in the 50-2000 kg/m squared s range. The pressure drop for air-water mixtures can be predicted from the proposed correlations provided the critical quality (or superficial gas Reynolds number) and the critical pressure gradient for transition from bubble/plug-to-slug flow are known. A comparison of the proposed method with that of Lockhart and Martinelli is presented and discussed.

A new pressure loss correlation predicts flow through screens for the wire Reynolds number range of 10 to the -4 to 10 to the fourth power using the conventional orthogonal porosity and a function of wire Reynolds number. The correlation is extended by the conventional cosine law to include flow that is not perpendicular to the screen. The importance of careful specification of wire diameter for accurate predictions of porosity is examined. The effective porosity is influenced by the shape of the woven wires, by any local damage, and by screen tension.

Measurements of the mean pressure distributions and Strouhal numbers on partially grooved cylinders with different groove subtend angles were made over a Reynolds number range of 2.0 X [10.sup.4] to 1.3 X [10.sup.5] which was within the subcritical regime of smooth cylinder. The Strouhal number, pressure distributions, and their respective coefficients were found to be a function of the groove subtend angles. In general, a progressive shift of the flow regime to lower Reynolds number was observed with higher subtend angle and a subtend angle of 75 deg was found for optimum drag reduction. With the configuration of asymmetrical groove surface, lower drag, and higher lift coefficients were obtained within the same Reynolds number range. Wake traverse and boundary layer results of the asymmetric grooved cylinder indicated that the flows at the smooth and groove surfaces lied within different flow regimes and a downward shift of the wake.

The classical criterion for cavitation of bubbles is based on the assumption that the bubbles are subjected to a quasi-steady pressure change and that the non condensible gas within the cavity behaves isothermally. No simple criterion accounting for the dynamic terms in the Rayleigh-Plesset equation is presently available. In this paper, a criterion for cavitation of bubbles, which takes into account the contribution of the dynamic terms in the Rayleigh-plesset equation, is obtained and, presented in a new useful form which allows rapid computation of the critical pressure whatever the fluid and the nuclei initial conditions are. The impact of these results on different flow situations are analyzed and discussed.

A study is conducted to determine the additional total pressure losses that result from combining flows through various sharp-edge three-leg junctions. Results show that the largest possible Mach number for the combined flow is 0.66. Flow visualization identified normal shock waves downstream of the junction for duct widths between 1 to 1.5. Good agreement was also found between experimental and theoretical results.

The operating principles, as well as the technical aspects of the implementation of a new computer-controlled pressure standard are presented. The instrument can have dual use: either as a pressure source or as a pressure sensor. The device is intended mostly for use in problems where small differential pressures are of interest, i.e., 0-2.5 KPa and high accuracy is desired. Such a pressure range encompasses, for example, most of the pressure measurement applications in subsonic wind-tunnel testing. The device interfaces to a PC and is ideal for fully-automated pressure transducer calibration applications. The accuracy of the pressures produced or measured by the device is 0.08 percent F.S. (Full Scale).

This paper reports on the measurement of fluid (water) pressure distribution at a soft (polyurethane) pad/steel interface. The distribution of the interfacial fluid pressure has been measured with a specially-designed fixture over the typical range of normal loads and velocities used in the chemical mechanical polishing/planarization of silicon wafers. The results show that, for most cases, the leading re, o-thirds of the fixture exhibits a subambient pressure, and the trailing third a positive pressure. The average pressure is sub-ambient and may be of the order of 50~100% of the normal load applied. An analytical model has been developed to predict the magnitude and distribution of the interfacial fluid pressure. The predictions of this model fit the experimental results reasonably well, especially for low sliding velocities. [S0742-4787(00)00902-4]