3,066 research outputs found
The condensing cycle for heat engines with operating temperatures from 50 to 100C
No full textLow grade thermal energy with low temperatures of up to 150° C occurs e.g. as waste heat or waste steam in industry, or in in renewable energy systems such as solar thermal collectors or geothermal systems. The cost-effective utilisation of such low-grade thermal energy still constitutes an engineering challenge. Existing technology such as Organic Rankine Cycle systems is complex, and only cost-effective for power ratings of 500 kW and more. At Southampton University, the condensing steam cycle, which has an operating temperature of 100C and was originally proposed by James Watt in 1782, was re-examined. The theoretical efficiency at 100°C ranges from 0.064 for a simple condensing engine to 0.174 for a steam expansion ratio of 1:8. A 30 Watt model engine confirmed the efficiency increase. Further theoretical work indicates that operating temperatures as low as 50°C with an efficiency of 0.06 could be possible, still using water as working fluid. The condensing engine is a simple machine which could provide a cost-effective solution for low-grade heat conversion
Low pressure solar thermal converter
No full textThe current development of solar power converters with air as working fluid focuses mostly on concentrating collectors combined with hot-air engines, and on very low temperature solar tower concepts. Whilst concentrating collectors and Stirling engines need complex technology, solar tower converters have very low efficiencies and require large installations. Pressurized containers as energy converters offer the advantage of simplicity, but appear not to have been investigated in detail.In order to assess their performance potential, an idealised thermal pressure converter was analysed theoretically. Two improvements to increase the initially low efficiency derived from theory were found. Neglecting losses, maximum theoretical efficiencies ranged from 6.7% for a temperature difference of 60 K to 17.7% for a difference of 195 K. The low pressure solar thermal converter appears to offer development potential for low-tech solar energy conversion.<br/
Theoretical model for the onset condition of a steady hydraulic jump
No full textA hydraulic jump constitutes a transition from supercritical to subcritical flow. At the jump a roller forms, the water level suddenly increases, the flow velocity reduces, and power is dissipated. The type of hydraulic jump varies with the upstream Froude number. Experimental observations showed that for Froude numbers below F1 = 4.5 the jump is unstable and oscillatory, and from this value onwards it becomes steady. In this article, a novel theoretical model of the hydraulic jump is described which allows the transition point to be determined theoretically. The simple model assumes the power dissipation to occur through a counteracting moment at the roller. With an extended momentum balance approach, where the moments instead of the forces acting on the control volume are considered, the limit point is determined at F1 = 4.4. This correlates well with the experimentally derived value given in the literature of F1 = 4.5
Two simple theoretical models for teaching wave mechanics in coastal engineering
No full textWaves are an integral component of teaching in coastal engineering. Some aspects of wave theory are however complex and outside the scope of e.g. introductory courses, so that only the results of the theory are used. For other wave effects such as overtopping, no theory exists, and purely empirical formulas are employed. This limits the students’ understanding of the problems. At Southampton University, we developed simple models for wave effects to improve the teaching. The models rely on basic hydraulic engineering principles such as continuity, conservation of energy and momentum, with the condition that the results are reasonably close to those from more complex theories or from experiments. In this article, two such models for the propagation speed of a solitary wave, and for the shallow water breaking criterion, will be presented. The results from both models are surprisingly close to the textbook formulas or values
Energy dissipation through entrained air compression in plunging jets
No full textThe energy dissipation in plunge pools is often related to air entrainment in the plunging jet. Recent experimental work showed that the void fractionsbelow the impinging jet are significantly smaller than outside the jet impact. This was attributed to the compression of the entrained air bubbles. Inthis paper, the thermodynamics of air bubble compression is introduced as a novel energy dissipation mechanism in civil engineering hydraulics.For the analysis of reported experimental data, isothermal conditions were applied as a lower bound approximation. It was found that the energydissipation rates range from 2.7–9.7% for a jet velocity of 10ms−2 and void fractions of 0.11 and 0.31, to 15.9–36.0% for a jet velocity of 24.5ms−1and void fractions of 0.35–0.55. The results may explain the difference between energy dissipation rates in plunge pools at model and full scale
Vertical Jet Generation during a sudden Flow Deceleration
No full textDuring the emergency closure of a sluice gate at hydropower station inlets, or at the collision of a tsunami bore with a vertical wall, a sudden and complete deceleration of a moving water mass can occur. Experiments were conducted to model this sudden deceleration of a supercritical flow. It was observed that during such an event, a fast vertical jet can develop, with a vertical velocity of three times the horizontal velocity. A linearised theory was developed to describe the jet generation and characteristics. The theory employs the connected horizontal de- and vertical acceleration of a fluid body in contact with a vertical wall, governed by the requirement for conservation of energy and mass. The theory showed good agreement with the measurements, indicating its validity. With the theory, the additional pressures generated by the collision can be estimated
Effect of cliff shape on internal stresses and rock slope stability
No full textThe failure of coastal rock cliffs is attributed to a variety of factors such as weathering, wave action, ground water level, and rock and joint parameters. The wave-induced undercutting of the vertical cliff face is hereby assumed to play a critical role. Very little attention has, however, been paid to the gravity-induced stresses within the cliff face.
A numerical analysis of the stress distribution in idealised rock cliffs of varying slopes (50–90?) and in various stages of undercutting has been conducted. Investigated parameters included slope angle, cliff height, influence of a cliff base cavity, front face loading, and cliff surcharge loading. The analysis showed that stresses in the cliff face geometry immediately after a failure were insignificant. With the steepening of the cliff, stresses increased to reach very high values for a vertical cliff face. Additional undercutting of the cliff resulted only in a slight increase of those stress magnitudes. The steepened rock cliff can result in an unstable equilibrium condition, depending on cliff geometry and
the ratio of gravity-induced stresses (a function of cliff height) and material strength, which can lead to sudden cliff failure. The results of this study indicate that the most important parameter in cliff destabilization is wave-induced steepening of the cliff slope
The effect of using upper shroud on the performance of a breashoot water wheel
No full textThe cost-effective and ecologically acceptable exploitation of hydropower machine with small scale (low head and low flow rate) is still a challenging area. Breastshoot water wheel is one mode of the small scale hydropower machine that is common to use to generate power. The effect of using upper shroud on a breastshoot water wheel was evaluated. The research was done by examining two breastshoot water wheels with the outer diameter and hub diameter of 240 mm and 160mm respectively. Furthermore, the blade width is 40 mm. The wheel consist of 2 wheel which separated by 4mm thickness rim which the diameter the same as outer diameter of the outer wheel. Each side of the wheel consists of 12 blades. The experimental was done with two variations of wheel, i.e with upper shroud and without upper shroud. The torque was measured with prony brake method. The breastshoot wheel that was tested in this research is zuppinger water wheel type. The experimental result shows that the efficiency of the breastshoot water wheel with upper shroud is higher than water wheel without upper shroud in all speed range.</p
Simplified theory of the Archimedian screws
No full textThe Archimedean screw pump is one of the oldest hydraulic machines. Today, it is employed in pumping as well as - operating in reverse - in an energy conversion role. Despite its age, no consistent theory links the screw’s geometry with its mechanical efficiency. Based on geometrical parameters and an idealised energy conversion process for one turn of the helix, a theoretical model is developed and compared with experimental results. Theoretical maximum efficiencies were found to be a function of the screw’s geometry and matched reported experimental results well. The statement that the efficiency of a screw is a function of losses only could not be confirmed
Wave field mapping with Particle Image Velocimetry
No full textA number of different methods for the 3D measurement of the water surface in previous termwave fieldsnext term have been proposed in the literature, based mostly on refraction, photogrammetry and/or reflected light intensity. Although these methods can map the distorted water surface, they appear to be difficult in practical applications in hydraulic engineering laboratories. A novel previous termwave mappingnext term technique using previous termparticle image velocimetrynext term to measure surface flow velocities, and linear previous termwavenext term theory to determine previous termwavenext term heights from the measured velocities, has been developed and validated for regular previous termwaves.next term The method allows for the previous termmappingnext term even of complex previous termwave fieldsnext term with simple means and appears robust enough for application in the laboratory
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