Unsteady Numerical Simulations of the Flow Related to the Unstable Energy-Discharge Characteristic of a Medium Specific Speed Double Suction Pump
Regions of positive slope in the pressure-discharge characteristics can result in a reduction or even lack of damping of system instabilities. They are therefore one of the major concerns in design and operation of centrifugal pumps as they are limiting the admissible operating range. The considered preliminary hydraulic design of an industrial double suction pump of medium specific speed n = 0.410 (0.205 per impeller side) shows a marked saddle in the energydischarge characteristic associated to a sudden drop of efficiency versus discharge at part load. Unsteady RANS type flow simulations are performed using hexahedral meshes with 2.5 million nodes to model the inlet casing, the shrouded double sided impeller with 2 x 7 blades, the diffuser with 12 blades and the volute. In unsteady solution monitoring, low frequency (below blade passing frequencies) phenomena are noticed. Therefore, simulation times of up to 10 impeller revolutions at each flow rate are requested to achieve statistically steady behavior of the flow judged on global performance numbers and circumferential flow rate distribution. The numerical simulations emphasize a drop in the characteristic, though at a lower flow rate than found on the test rig. It is shown to be associated to a one-sided separation in the diffuser, further leading to an unbalanced flow rate distribution of about 10% of flow rate between both sides of the impeller. There is a region of hysteresis, where both configurations with balanced and unbalanced flow rate distributions can be obtained for the same global flow rate. The asymmetric flow distribution leads to asymmetric velocity profiles at the impeller-diffuser interface which results in a strongly vortical flow in the diffuser channels, where an important amount of energy is dissipated in regions of increased viscous and turbulent shear.
Flow in a Centrifugal Pump Impeller at Design and Off-Design Conditions—Part I: Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) Measurements
Detailed optical measurements of the flow inside the rotating passages of a six-bladed shrouded centrifugal pump impeller of industrial design have been performed using particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). Results include instantaneous and ensemble averaged PIV velocity vector maps as well as bin-resolved LDV data acquired in the midplane between hub and shroud of the impeller. The flow is surveyed at both design load and at severe off-design conditions. At design load, Q =Q d , the mean field of relative velocity is predominantly vane congruent, showing well-behaved flow with no separation. At quarter-load, Q=0.25Q d , a previously unreported two-channel phenomenon consisting of alternate stalled and unstalled passages was observed, with distinct flow congruence between every second of the six passages. A large recirculation cell blocked the inlet to the stalled passage while a strong relative eddy dominated the remaining parts of the passage. The stall phenomenon was steady, nonrotating and not initiated via the interaction with stationary components
Chapter 6 Rift Basins of the Sudan
Abstract The Sudanese rift structures form intracontinental basins, bordered on all sides by anorogenic terrain. These basins are seen as the result of a multistructural system of rifts which appear to have been activated several times since the Palaeozoic. Improved methods of dating and extensive geophysical surveys have led to a better understanding of the mobile belts and the Basement rocks in the general environs of the basins, and of the elevated blocks and highs surrounding the sedimentary basins. Detailed mapping and extensive drilling in the sedimentary basins led to a better understanding of the Phanerozoic. The rapid rate of uplift and subsidence have assisted in the rapid accumulation and filling of the basins with unconsolidated sediments, ranging from a few hundred metres to some thousand metres. The rift structures contain sediments of several age groups, origin and mode of deposition. Some of them are as old as the Palaeozoic passing through to Mesozoic, Tertiary and Quaternary (Bahr El Arab rift and Blue Nile rift). The thickness of the Tertiary sediments exceeds 15 km in Bahr El Arab rift. Palaeozoic sediments were found to occupy NE-SW grabens. Jurassic? and Cretaceous sediments were deposited in NW-SE troughs. Tertiary sediments filled grabens that range in thicknesses up to 15 km. Highly saline ground-water bodies which coincide with hydrogeologically closed basins occupy the flowing end of each of the rift basins. These saline bodies have been interpreted as buried saline lakes, sabkhas or playas. Alternating wet and dry periods eventually filled up the basins with intercalating layers of fresh and saline layers. The filling up of the basins led to the interconnection of the hydrological regime and the development of the integrated River Nile. Rift structures in Sudan form the major ground-water basins, outside these basins ground water is found only in small quantities and is of poor quality. The rift basins also act as reservoirs for hydrocarbon accumulation; all the known oil fields of inland Sudan are located in these basins.