The discretized method of characteristics was used to develop a new numerical model for the analysis of the wave processes in the hydraulic ram. The model encompasses all parts of the device including, for the first time, the air chamber and the delivery pipe. This in turn allows a comprehensive study of the unsteady flows in the unit as the air chamber extensively interacts with the supply side of the ram. The program accounts for all possible flow states of the ram, i.e. with impulse and discharge valves closed, with both valves open and with either of the valves closed or open. The state of the partially or fully open discharge valve requires extensive and careful treatment of all physical phenomena involved, namely the transient boundary conditions, relevant to the air chamber and its connection to the drive pipe. The treatment is for the special case of constant sonic velocity in each of the ram pipes. The numerical model was validated against experimental data obtained from various test set-ups. Comparing the velocity and pressure head variations from numerical and experimental tests at selected points along the ram supply pipe performed the most stringent test. Excellent agreement was shown to exist, validating the analytical approach.
The historical background and operational principle of the Dynamic Pressure Exchanger (DPE) are outlined. The basic aerodynamic processes of cell-emptying and cell-filling are analysed by the ‘method of characteristics’ for air and for no temperature discontinuities in the unsteady flow pattern. The results of the analysis are then used to generalize performance qualitatively for overall pressure ratios up to the sonic threshold. It is shown that, for pressure wave effects to be fully utilized, a DPE rotor should run such that 8 is of the order of or less than 0.5, where δ is the ratio of the time taken to open or close a cell to the time taken for a sound wave to travel a cell length at the thermodynamic stagnation state of the primary or secondary fluid. In the case where the thermodynamic properties of the fluids vary considerably, it is suggested that 8 be referred to the gas which yields the highest sonic speed. In general, the extent to which the performance is affected by a change in δ, within the range 0 < δ < 0.5, is inappreciable. It is also shown that the use of a transfer passage may be expected to yield a significant improvement in performance and an increased range in overall pressure ratio. A number of applications are described and some recent developments are reviewed. It is also indicated that the main sources of loss can be incorporated within the method of characteristics used in the prediction of performance.
The computerized method of characteristics was used to analyse, for a single pipeline discharging into the atmosphere, the effects of valve-closure schedule and pipe length on optimum water-hammer strength. It was found that the criteria of optimum water-hammer utilization are a non-linear inherent valve schedule in which the bulk of the pressure surge occurs near the beginning or towards the end of valve closure, together with as small a value of dimensionless valve-closure time and as high a value of wide-open valve area as is consistent with cavitation-free operation. Also, a comparison of the results with hydraulic ram test data suggests that optimum drive pipe length may be based solely on optimum water-hammer strength, in the light of the relative effects of pipe friction and dimensionless valve-closure time. In general, optimum pipe length is not highly sensitive to inherent valve-closure schedule, water-hammer strength, pipe size or reservoir head.
Most of the states constituting the Arabian Gulf region are so deficient in renewable water resources that they now rely almost exclusively on seawater desalination. The present overview considers the various options that are used to link together electric power production and desalination. The benefits and shortcomings of these options are outlined and the influence of the relative demand for electricity and water on the optimum type of plant is examined. The operational and commercial characteristics of the various desalination processes are summarized. The future prospects of both power and desalination production are discussed and the subsequent paths that desalination technology will follow in the region are suggested. The environmental impacts of power generation and desalination are explored and possible mitigating measures covering each type of impact are proposed. Finally, it is submitted that the social benefits of power and desalination in the area cannot be considered as having been truly realized unless regulatory standards relevant to these impacts are set up and adhered to.
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