J. H. Whitelaw

Flow of Newtonian and non-Newtonian fluids in concentric and eccentric annuli

Three components of mean velocity and the corresponding Reynolds shear stresses have been measured in fully developed concentric and eccentric annulus flows of a Newtonian fluid at bulk-flow Reynolds numbers of 8900 and 26600 and a weakly elastic shear-thinning polymer at effective bulk-flow Reynolds numbers of 1150, 6200 and 9600. The diameter ratio was 0.5 with eccentricities of 0, 0.5 and 1.0, and the use of a Newtonian fluid of refractive index identical to that of the Perspex working section facilitated the measurements by laser velocimetry. With the Newtonian fluid, the distribution of static pressure measurements on the outer wall is shown to be linear, with friction factors for concentric-annulus flows some 8% higher than in a smooth round pipe and for the eccentric flows of eccentricities of 0.5 and 1.0 it was lower by, respectively 8 and 22.5% than that of the concentric-annulus flow. In the former case, the law of the wall was confirmed on both inner and outer walls of the annulus at both Reynolds numbers. This was also the case for the outer wall in the eccentric-annulus flows, except in the smallest gap, but the near-inner-wall flow was not represented by a logarithmic region particularly in the smallest gap. The locations of zero shear stress and zero velocity gradient were displaced by amounts which were, like the secondary flows measured in the eccentric annulus of 0.5, almost within the measurement precision. In the eccentric-annulus flow with eccentricity of 1.0, there was a secondary flow with two circulation cells on each side of the plane of symmetry and with a maximum velocity of 2.2% of the bulk velocity. The measurements with the non-Newtonian fluid were less detailed since refraction limited the flow accessible to the light beams. The average wall shear stress coefficient was similar to that for the Newtonian fluid in the laminar region of the concentric-annulus flow and higher for the two eccentric-annulus flows. Transition was extended to an effective Reynolds number well above that for the Newtonian fluid with a drag reduction of up to 63%. The near-outer-wall flows had logarithmic forms between the Newtonian curve and that of the maximum drag-reduction asymptote, and all fluctuation levels were less than those for the Newtonian fluid, particularly the radial and tangential components.

Squish and Swirl-Squish Interaction in Motored Model Engines

Measurements of the three components of velocity and their corresponding fluctuations have been obtained by laser-Doppler anemometry mainly near TDC of compression in a model IC engine motored at 200 rpm with compression ratio of 6.7. The flow configurations comprised an axisymmetric cylinder head with and without upstream induced swirl and each of a flat piston and two centrally located, cylindrical and re-entrant, bowl-in-piston arrangements. In the absence of swirl and squish, the intake-generated mean motion and turbulence decayed considerably by the end of compression. The two piston-bowl configurations, however, resulted in a compression-induced squish motion with consequent formation of a toroidal vortex occupying the whole bowl space. Interacton of swirl, carried from intake and persisting through compression, with squish generated near TDC profoundly altered the axial flow structure. In the case of the cylindrical bowl, the sense of the vortex was reversed by swirl and, in the reentrant bowl, increased the number of vortices to two. The swirling motion inside the cylindrical bowl was close to solid body rotation while the re-entrant bowl gave rise to more complex flow patterns. Squish, in the presence or absence of swirl, did not augment the turbulent energy inside the cylindrical bowl contrary to the reentrant configuration where turbulence generation was observed.

Flow of Newtonian and Non-Newtonian Fluids in a Concentric Annulus With Rotation of the Inner Cylinder

Mean velocity and the corresponding Reynolds shear stresses of Newtonian and non-Newtonian fluids have been measured in a fully developed concentric flow with a diameter ratio of 0.5 and at a inner cylinder rotational speed of 300 rpm. With the Newtonian fluid in laminar flow the effects of the inner shaft rotation were a uniform increase in the drag coefficient by about 28 percent, a flatter and less skewed axial mean velocity and a swirl profile with a narrow boundary close to the inner wall with a thickness of about 22 percent of the gap between the pipes. These effects reduced gradually with bulk flow Reynolds number so that, in the turbulent flow region with a Rossby number of 10, the drag coefficient and profiles of axial mean velocity with and without rotation were similar. The intensity of the turbulence quantities was enhanced by rotation particularly close to the inner wall at a Reynolds number of 9,000 and was similar to that of the nonrotating flow at the higher Reynolds number. The effects of the rotation with the 0.2 percent CMC solution were similar to those of the Newtonian fluids but smaller in magnitude since the Rossby number with the CMC solution is considerably higher for a similar Reynolds number. Comparison between the results of the Newtonian and non-Newtonian fluids with rotation at a Reynolds number of 9000 showed similar features to those of nonrotating flows with an extension of non-turbulent flow, a drag reduction of up to 67 percent, and suppression of all fluctuation velocities compared with Newtonian values particularly the cross-flow components. The results also showed that the swirl velocity profiles of both fluids were the same at a similar Rossby number.

Particle velocity characteristics of dilute to moderately dense suspension flows in stirred reactors

Abstract Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler velocimetry in solid-liquid turbulent flows in fully baffled stirred reactors driven by Rushton-type impellers of different sizes at rotational speeds of 150, 300 and 313 rpm. The effects of particle size, density and volumetric concentration were investigated. The maximum particle concentration at which the solid-phase velocity measurements could be made was improved from 0.02 to 2.5% when the refractive index of the continuous-phase was matched to that of the dispersed particles. The results showed a steep particle concentration gradient in the vertical direction below the impeller and a mild one above the impeller, that the particles lagged or led the bulk fluid when the flow direction was upwards and downwards, respectively, and that the particle turbulence levels were in general lower than those of the single-phase flow levels, especially in the impeller stream and wall jet regions. Particle velocities decreased with an increase in particle concentration, while the particle turbulence levels remained the same. The apparent relative velocity of glass particles was higher than that of Diakon by up to 2.5 times and the effect of the particle size, at least for the sizes used in the experiment, was negligible.

Flow of Newtonian and non-Newtonian fluids in an eccentric annulus with rotation of the inner cylinder

Abstract Three velocity components of a Newtonian and a weakly elastic shear-thinning non-Newtonian fluid have been measured in an annulus with an eccentricity of 0.5, a diameter ratio of 0.5, and an inner cylinder rotation of 300 rpm. The results show that the rotation had similar effects on the Newtonian and non-Newtonian fluids, with a more uniform axial flow across the annulus and the maximum tangential velocities in the narrowest gap in both cases. The secondary flow circulation with the Newtonian fluid at a Reynolds number of 26,600 was in the direction of the rotation, with maximum values of 14% of the bulk velocity close to the inner pipe. With the 0.2% CMC polymer solution in laminar flow, rotation caused a narrow counter-rotating flow along the outer pipe wall, which was absent at a Reynolds number 9200. The turbulence intensities in the region of widest gap were uninfluenced by rotation, increased in the Newtonian fluid, and decreased in the non-Newtonian fluid in the region of the smallest gap. The flow resistance of both fluids increased with rotation at low Reynolds numbers and reduced with increasing values to become similar to those of nonrotating flows. Comparison between rotating results of the Newtonian and non-Newtonian fluids at a Reynolds number 9200 and the same inner cylinder rotation, showed effects similar to those of nonrotating flow with extension of nonturbulent flow, large reduction in turbulence intensities and drag reduction of the order of 61% for the CMC solution. The swirl velocities in both fluids were similar when the Rossby numbers were similar.

Flows through plane sudden-expansions

A calculation method has been developed and used to represent flows downstream of plane symmetric expansions with dimensions and velocities encompassing laminar and turbulent flows. Except for very low Reynolds numbers, the flows are time-dependent and asymmetric and the calculated results are appraised first in relation to published measurements of laminar flows and then to new measurements obtained at a Reynolds number of 26500. The time-dependent laminar simulations indicate that the critical Reynolds numbers are predicted with excellent accuracy for different expansion ratios and the details of the asymmetric velocity profiles are in good agreement with experimental measurements. The laminar flow calculations also show that increasing the thickness of the separating boundary layer leads to longer regions of separation and no dominant frequency for Reynolds numbers up to those at which the third separation region was observed. The turbulent flow simulations made use of the κ-e turbulence model and provided a satisfactory representation of measurements, except in regions close to the wall and within the recirculation regions. Also, the longer reattachment length was underestimated

Combustion Oscillations Close to the Lean Flammability Limit

The nature of premixed methane-air flames stabilised on a symmetric, plane sudden-expansion has been examined in terms of wall pressures and the chemiluminescence of the CH radical. Large-amplitude acoustic oscillations at the half-wave frequency of the entire duct were observed at high velocities and with near-stoichiometric mixtures, and instabilities with much lower-frequencies close to the lean and rich extinction limits. The emphasis is on these near-limit instabilities and, due to the asymmetry of the plane flow, the experiments were extended to round ducts in which the flammability and stability limits were similar to those in the plane duct. The branches of flame behind the steps of the plane expansion extinguished non-simultaneously, and gave rise to low-frequency flapping oscillations immediately prior to extinction of the first of these, and the remaining branch gave rise to lateral oscillations prior to its extinction. The oscillations were associated with axial movement of extinction along the shear layer due to the high strain rates close to the step, until the strain rate was sufficiently low and allowed upstream propagation of the flame through the recirculation region. The flames in the round duct gave rise to oscillations of the same nature. The frequency of the oscillations increased with flow velocity and flame speed, and the amplitude with heat release and much more with constriction of the duct exit due to coupling with a bulk-mode of the combustor cavity. These increased amplitudes caused the flammability limits to narrow due to the consequently higher strain rates and have implications for gas turbine operation.

Flow Characteristics of a Centrifugal Pump

Measurements of velocity have been obtained in a centrifugal pump in terms of angle-resolved values in the impeller passages, the volute, the inlet and exit ducts and are presented in absolute and relative frames. The pump comprised a radial flow impeller with four backswept blades and a single volute, and the working liquid had the same refractive index as the transparent casing to facilitate the use of a laser-Doppler velocimeter. The flows in the impeller passages wee found to depart from the curvature of the blade surfaces at off-design conditions with separation from the suction surface and from the shroud. Secondary flows from the suction to pressure surfaces were dominated by the influences of the relative motion between the shroud and impeller surfaces and the tip leakage. Geometric differences of 0.5 mm and one degree in spacing of the four blades caused differences in passage velocity of up to 6 percent at the lowest discharge. The flowrate from each impeller passage varied with volute circumferential position by up to 25 percent at an off-design flowrate. Poor matching of the impeller and volute at off-design conditions caused swirl and separation in the inlet and exit pipes.

Interpretation of cyclic flow variations in motored internal combustion engines

Cyclic variations and turbulence characterisation are considered in the context of in-cylinder flows. Three methods of data analysis, namely ensemble averaging, high pass filtering and cycle-by-cycle smoothing, are applied to velocity data obtained by laser Doppler velocimetry in a model axisymmetric and a realistic engine configuration. The interpretation of the results is supported by similar analysis of a steady-state simulation of in-cylinder flow with emphasis on the discrimination between random and deterministic flow variations. The results suggest that it is not always possible to identify cyclic variations as deterministic processes and that turbulence estimates based on filtering techniques may be misleading. The differences in the estimates of mean values are, however, small.

Control of Combustion Oscillations by Forced Oscillation of Part of the Fuel Supply

Abstract Naturally occurring oscillations in ducted premixed flames have been controlled by imposed oscillation of the fuel. The flames were stabilised behind a bluff-body and a step with methane as the main fuel and kerosene as the oscillated secondary fuel and with heat release rates of up to 150 kW. Some experiments with oscillation of gaseous fuel were performed to supplement and to complement earlier results Two types of actuator were used to oscillate gaseous fuels, one involving the oscillation of the mean fuel composition and the other the pulsed ignition of a fuel-air mixture. Liquid fuel flow was oscillated with a pintle-type injector with a jet of air to further atomise the fuel spray. The amplitude of the oscillated input is quantified for the different actuators in terms of the flow variables and frequency of oscillation Combustion oscillations were ameliorated by oscillating fuel at selected frequencies and more effectively by the controlled oscillation of fuel out of phase with the combusti...