J. M. Nouri

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.

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.

Spray Characteristics of a Multi-hole Injector for Direct-Injection Gasoline Engines

Abstract The sprays from a high-pressure multi-hole nozzle injected into a constant-volume chamber have been visualized and quantified in terms of droplet velocity and diameter with a two-component phase Doppler anemometry (PDA) system at injection pressures up to 200 bar and chamber pressures varying from atmospheric to 12 bar. The flow characteristics within the injection system were quantified by means of a fuel injection equipment (FIE) one-dimensional model, providing the injection rate and the injection velocity in the presence of hole cavitation, by an in-house three-dimensional computational fluid dynamics (CFD) model providing the detailed flow distribution for various combinations of nozzle hole configurations, and by a fuel atomization model giving estimates of the droplet size very near to the nozzle exit. The overall spray angle relative to the axis of the injector was found to be almost independent of injection and chamber pressure, a significant advantage relative to swirl pressure atomizers. Temporal droplet velocities were found to increase sharply at the start of injection and then to remain unchanged during the main part of injection, before decreasing rapidly towards the end of injection. The spatial droplet velocity profiles were jet-like at all axial locations, with the local velocity maximum found at the centre of the jet. Within the measured range, the effect of injection pressure on droplet size was rather small while the increase in chamber pressure from atmospheric to 12 bar resulted in much smaller droplet velocities, by up to four-fold, and larger droplet sizes by up to 40 per cent.

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.

Particle motion and turbulence in dense two-phase flows

Abstract Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler anemometry in a descending solid-liquid turbulent flow in a vertical pipe with volumetric concentrations of suspended spherical particles of 270 μm mean diameter in the range 0.1–14%. Similar measurements were obtained in the flow downstream of an axisymmetric baffle of 50% area blockage placed in the pipe with volumetric concentrations of 310 μm particles up to 8% and of 665 μm particles up to 2%. In order to enable measurements in high particle concentrations without blockage of the laser beams the refractive index of the particles was matched to that of the carrier fluid. The results show that the particle mean velocity profiles become more uniform and the particle r.m.s. velocity decreases with increasing concentration in both flow cases. The particle mean velocity in the pipe flow also decreases with concentration and the relative velocity, the difference between the particle velocity and the fluid velocity in single-phase flow, decreases with increasing Reynolds number. The length of the recirculation region downstream of the baffle was shorter than in single-phase flow by 11 and 24% for particle concentrations of 4 and 8%, respectively. The particle mean velocities were hardly affected by size for concentrations up fo 2%, but the r.m.s. velocities were lower with the larger particles.

Spray stability of outwards opening pintle injectors for stratified direct injection spark ignition engine operation

Abstract The spray characteristics and spray stability from three prototype piezoelectric pintle-type injectors were investigated under different operating conditions in an optical direct injection engine designed for stratified combustion. The pintle-type outwards opening injector has the potential to address and overcome many of the typical problems related to close-spacing, spray-guided configurations owing to its hollow cone spray, exhibiting better air utilization than multihole sprays, with good penetration during early injection, and a spray angle almost independent of cylinder backpressure. The three prototype injectors have different nozzle exit geometries for optimization of spray stability under all engine operating conditions. The emerging fuel sprays for both single- and double-injection operation were visualized using Mie scattering and a high-speed CCD camera. The performance of the injectors was assessed by constructing mean and RMS images at different operating conditions of injection pressure, backpressure, injector needle lift, and engine speed. From these images, a spray angle analysis was performed by comparing the mean, standard deviation, maximum, and minimum cone angle under different operating conditions; the spray stability was quantified by analysing the mean and RMS images and the mean and RMS spray cone angles. Evaluation of the three prototypes has revealed that the positive-step inward seal band design produces the most robust spray angle ideally suited for stratified fuel/air mixture formation and combustion in spray-guided direct injection spark ignition (DISI) gasoline engines.

Internal flow and cavitation in a multi-hole injector for gasoline direct-injection engines

A transparent enlarged model of a six-hole injector used in the development of emerging gasoline direct-injection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras. Imaging of the flow inside the injector nozzle identified the formation of three different types of cavitation as a function of the cavitation number, CN. The first is needle cavitation, formed randomly at low CN (0.5-0.7) in the vicinity of the needle, which penetrates into the opposite hole when it is fully developed. The second is the well known geometric cavitation originating at the entrance of the nozzle hole due to the local pressure drop induced by the nozzle inlet hole geometry with its onset at around CN=0.75. Finally, and at the same time as the onset of geometric cavitation, string type cavitation can be formed inside the nozzle sac and hole volume having a strong swirl component as a result of the large vortical flow structures present there; these become stronger with increasing CN. Its link with geometric cavitation creates a very complex two-phase flow structure in the nozzle holes which seems to be responsible for hole-to-hole and cycle-to-cycle spray variations.

The application of a strain gauge technique to the measurement of the power characteristics of five impellers

The work described in this paper establishes the accuracy with which the power number can be measured within a mixing vessel and presents measurements for five impellers (Rushton, “bucket”, 45°-pitched blade, 60°-pitched blade and hyperboloid) as a function of rotational speed. The technique uses strain gauges and the telemetric transmission of the strain and is, therefore, more accurate than methods which involve mechanical determination of torque and that allows power measurements to be carried out in relatively small and more cost-effective models of mixing vessels. In the cases where previous data exists the results of this study compare well. This work provides new data for the “bucket”, 45°-pitched blade and hyperboloid (low Reynolds numbers) impellers and represented a systematic study for all five impellers with one measurement system.

Spray characterization of a piezo pintle-type injector for gasoline direct injection engines

The sprays from a pintle-type nozzle injected into a constant volume chamber have been visualised by a high resolution CCD camera and quantified in terms of droplet velocity and diameter with a 2-D phase Doppler anemometry (PDA) system at an injection pressure of 200 bar and back-pressures varying from atmospheric to 12 bar. Spray visualization illustrated that the spray was string-structured, that the location of the strings remained constant from one injection to the next and that the spray structure was unaffected by back pressure. The overall spray cone angle was also stable and independent of back pressure whose effect was to reduce the spray tip penetration so that the averaged vertical spray tip velocity was reduced by 37% when the back-pressure increased from 1 to 12 bar. Detailed PDA measurements were carried out under atmospheric conditions at 2.5 and 10 mm from the injector exit with the results providing both the temporal and the spatial velocity and size distributions of the spray droplets. The maximum axial mean droplet velocity was 155 m/s at 2.5 mm from the injector which was reduced to 140 m/s at z = 10 mm. The string spacing determined from PDA measurements was around 0.375 mm and 0.6 mm at z=2.5 and 10 mm, respectively. The maximum mean droplet diameter was found to be in the core of the strings with values up to 40 μm at z=2.5 mm reducing to 20 μm at z=10 mm.