1,720,956 research outputs found
Co-current air-water flow in downward sloping pipes: Transport of capacity reducing gas pockets in wastewater mains
No full textAir-water flow is an undesired condition in many systems for the transportation of water or wastewater. Air in storm water tunnels may get trapped and negatively affect the system. Air pockets in hydropower tunnels or sewers may cause blow-back events and inadmissible pressure spikes. Water pipes and wastewater pressure mains in particular are subject to air pocket formation in downward-sloping reaches, such as inverted siphons or terrain slopes. Air pocket accumulation causes energy losses and an associated capacity reduction. Whereas in horizontal and in upward inclined pipes all entrained air is transported with the water flow, the air in downward sloping pipes can move in both directions. Knowledge on air pocket motion in downward sloping pipes is essential for the proper venting of pressurized pipes and for the prevention of severe blow-back events. The motion of air pockets in downward sloping pipes, is closely related to liquid slugs in inclined pipes carrying gas with a small fraction of liquids (i.e. water, oil and gas condensate). The bubble-shaped interface and gas entrainment at the slug front are two features that are similar with air pockets in downward sloping pipes. Existing two-phase flow models have been validated mainly on data in horizontal and vertical pipes in which the gas phase drives the liquid phase. The performance of these models in inclined pipes, in which the liquid phase drives the gas phase, is not yet known. Despite its practical relevance in a variety of engineering fields, the literature on air-water flows in downward sloping pipes is scarce. The fundamental momentum balance that predicts when an elongated air pocket becomes stagnant in a downward inclined pipe, is yet to be developed. Lubbers was the first to systematically investigate the co-current flow of air and water in downward sloping pipes over the complete range of possible air accumulations. Like this thesis, Lubbers’ experimental research was part of the CAPWAT project on capacity losses in pressurised wastewater mains. The main research question, addressed in this thesis, is the development and validation of a total air transport model by flowing water, including the influence of pipe angle, length of sloping section, pipe diameter, surface tension, absolute pressure, pipe friction factor and viscosity. Furthermore, the air discharge by flowing water and the gas pocket head loss in wastewater will be compared with those in clean water. In order to quantify scale effects new measurements have been performed in laboratory facilities with internal pipe diameters of 0.08 m and 0.15 m and in a large-scale facility at a wastewater treatment plant with internal pipe diameter D = 0.192 m, a downward sloping length of L = 40 m (L/D = 209) and a downward pipe angle of 10°. Three series of experiments on co-current air-water flow have been conducted in the large-scale facility, each with its own specific objective in addition to the purpose of model validation: 1 Experiments with clean water, which provided quantitative information on the influence of the length of the downward sloping reach on the air pocket head loss and net air discharge. 2 Experiments with surfactant-added water for the assessment of the influence of surface tension on the air pocket head loss and net air discharge. 3 Experiments with untreated wastewater in order to determine the air pocket head loss and net air discharge in pipelines carrying wastewater. Obviously, these results have been compared with the first experimental series on clean water. The following main conclusions are drawn from this thesis: 1 A physically-based predictive model has been developed for the net air discharge by flowing water in downward sloping pipes. The model parameters include the length of the downward sloping reach and total length of the air pockets, pipe angle, pipe diameter, water (or liquid) discharge, viscosity, surface tension and pipe friction factor. 2 The model has been calibrated to a unique dataset of co-current air-water flows in downward sloping pipes. 3 The composition of wastewater, i.e. lower surface tension and solids content, does not enhance the air transport in comparison with the air transport in clean water. 4 A new velocity criterion for the occurrence of multiple air pockets in a downward sloping reach has been developed. This criterion defines whether the maximum gas pocket head loss may occur in practice. 5 A new momentum balance for elongated air pockets in downward sloping pipes has been developed. This momentum balance defines the clearing flow number. It is useful in practice to predict the direction and velocity of an elongated air pocket in a downward sloping pipe. The momentum balance and velocity criterion support the design of storm water storage tunnels and bottom outlets of hydropower stations for the proper venting of pipes and tunnels and for the prevention of severe blow-back events. Furthermore, two-phase flow models for the prediction of the transition to slug flow and its properties may benefit from these developments. 6 The required water velocity to start the transport of an elongated gas pocket to the bottom of a downward sloping pipe reach is 0.9?(gD)^1/2 (or Fw = 0.9) over a wide range of pipe angles (5° – 20°). This statement has been substantiated with experimental data at D > 0.19 m and the derived momentum balance. 7 A gas pocket detection method for the prediction of a gas pocket location has been extended with a total gas volume prediction. The detection method has been tested successfully in a field experiment.Sanitary EngineeringCivil Engineering and Geoscience
Meeting heat demands in Dutch existing homes using Low Temperature District Heating: Comparison study of several heating networks with Low Temperature Geothermal Heat as main source
No full textDutch homes use 70% of their total energy consumption for domestic hot water purposes and space heating. Currently, this energy is yielded through the use of natural gas. In order to meet Dutch Climate Agreement goals, which were agreed upon December 2018, Low Temperature District Heating (LTDH) is expected as being the most promising sustainable heat energy system solution for dense populated districts. To produce sustainable heat, Visser & Smit Hanab (V&SH) have developed a geothermal source which extracts heat from shallow surfaces, between 500 and 1250 meters. Currently, this system is only used to provide heat for greenhouses. Greenhouses are in need of heat supply for more than 5000 hours per year. This thesis presents which other technologies are required to make LTDH with Low Temperature Geothermal Heat (LTGH) as main source a success for heating in district areas. For the case, there are 5 LTDH concepts designed to supply heat to the livings with LTGH. The designed LTDH varies in storage methods, supply temperature in the network, and peak heat supply technologies. The used storage methods are Aquifer Thermal Energy Storage (ATES) and a water tank of 250 liters for each living. The used peak heat supply technologies are decentralized heat pumps, electrical heaters, and a biomass boiler. The following LTDH concepts are designed.1a. Collective peak supply 70 ⁰C2a. Decentral peak supply 70 ⁰C1b. Collective peak supply 50 ⁰C2b. Decentral peak supply 50 ⁰C3. Decentralized heat pumps using 30 ⁰C supply temperatureBased on Key Performance Indicators, these LTDH concepts are compared with each other. Throughout a thermodynamic and economic analysis, it appeard that LTDH concept 3, where decentralized heat pumps are used, scores the best on all the KPIs. concept. That is because the central heat pump, of the other concepts, consumes the most electricity of all the technologies in the system and is always on throughout the entire year. The average efficiency, over a year, of an air heat pump is higher than that of the central heat pump at the LTGH. This is because the air heat pump is switched off when there is no heat demand and with an LTDH, there are losses in the network and the ATES.Future research must be done on the electricity consumption of the central heat pump from the LTGH. If the heat pump can consume less electricity, the load on the electricity during cold hours can be reduced and throughout a year CO2 emissions can be reduced. Also, future research must be done on optimizing the LTDH concepts, so that the savings in CO2 emissions and LCOE will improve. In addition, an NPV analysis should be done to determine how the costs are distributed over the stakeholders.Electrical Engineering | Sustainable Energy Technolog
Air pocket removal from downward sloping pipes
No full textAir-water flow is an undesired condition in water pipelines and hydropower tunnels. Water pipelines and wastewater pressure mains in particular are subject to air pocket accumulation in downward sloping reaches, such as inverted siphons or terrain slopes. Air pockets cause energy losses and an associated capacity reduction. Despite its practical relevance, many phenomena associated with airwater flow in downward sloping pipe reaches are still poorly understood. Deltares and Delft University of Technology have investigated the co-current flow of air and water in twelve different large-scale facilities. Pothof and Clemens have recently developed a numerical model for the total air discharge by flowing water in downward sloping pipes. The model has been validated against the experimental data on co-current air-water flow and available literature. This paper presents new experimental data on the breakdown and removal of large air pockets. The experimental results are compared with the numerical model. The observed disagreement is analysed and discussed. The main conclusion is that the numerical model predicts the air pocket breakdown rate with reasonable accuracy.Water ManagementCivil Engineering and Geoscience
Innovative air vessel design for long distance transmission pipelines
No full textThe Shuweihat Water Transmission Scheme (SWTS, UAE) consists of a twin DN1600 DI PN25 pipeline transmitting 150 MIGD over 250 km from Shuweihat Desalination plant to Mussafah (Abu Dhabi city). The Scheme is divided in two subsequent systems, each with a tank farm and a pump station delivering water to downstream terminal reservoirs and direct consumers. The first system (Lot A) transmits water from Shuweihat to Mirfa (100 km). The second (Lot C) is from Mirfa to Mussafah (150 km). The pipeline follows roughly the UAE coastline and the profile is generally flat with a few local high points. The surge study of the Lot C system investigated a large number of scenarios and resulted in the design of surge protection equipments and control systems. The surge protection equipment consists of 16 x 121 m3 (1936 m3) innovative vertical non-vented air vessels, invented by Deltares. The innovation includes a passive air release valve at a strategic elevation on the air vessel. This air release valve opens if the water level drops below the float level. At this water level the air pressure is super-atmospheric so that air is released from the air vessel to prevent draining. This innovation was driven by the initial findings of the surge analysis where it became apparent that 22 x 220 m3 (4840 m3) air vessels would have been required. These large vessels were the result of a double constraint. On the one hand, the initial air mass had to be sufficient to allow a proper expansion of the vessels and thus the protection of the pipeline against excessive negative pressures. On the other hand, a very small initial air mass was required due to the lack of backpressure. This meant that the air pocket was expanding by a factor exceeding 20, resulting in the emptying of the vessels. In this paper, we will describe the hydraulic model of the hybrid air vessel, detail the benefits of the new design and discuss the cost savings compared to conventional air vessels. It is concluded that the hybrid air vessel has saved over 50% of the required air vessels total volume. Considering the size of air vessels initially required, the costs savings in terms of surge protection of the SWTS Lot C system were cut in excess of 60%.Water ManagementCivil Engineering and Geoscience
The effect of rational heat transfer on the behavior of surge vessels
No full textThe polytropic model to describe air expansion, has been used for years to describe the behavior of surge vessels. In this thesis the alternative to describe this behavior, the Rational Heat Transfer (RHT) equation developed by Graze (1968), is extended with an estimation of all heat terms in surge vessels. The RHT model is compared to the poly- tropic model for isolated and non-isolated vessels and is shown to give a good estimation for the air expansion, which has been checked with measurement data from Shuweihat, a location in Abu Dhabi in the United Arab Emirates where a large pumping station is located. Calculations for the size of the surge vessel point out that the minimal volume of a surge vessel calculated with the RHT model is 10 % smaller than if the volume is calculated with the polytropic model. Furthermore, the heating of the air pocket in a surge vessel has been modeled by a numerical solution of the one-dimensional heat equa- tion with a convective and a non linear source term. From this models can be concluded that the air in a surge vessel warms up slower than the expansion time of that vessel.Numerical MethodsApplied mathematicsElectrical Engineering, Mathematics and Computer Scienc
Wateropleidingen doet niet aan droge stof
No full textEnergieverlies leidingen beperkt door terugkoppeling praktijk naar ontwerp en beheer.Water ManagementCivil Engineering and Geoscience
Validation of the surge model and lessons learnt from commissioning of the Shuweihat water transmission scheme, UAE
No full textThis paper presents a validation of the surge modeling results as well as lessons learnt from the commissioning test of the Shuweihat Water Transmission Scheme in the UAE. The Scheme is divided in two systems, The first system (Lot A) transmits water from Shuweihat to Mirfa (100 km). The second (Lot C) is from Mirfa to Mussafah (150 km). The focus of this paper is on the model predictions and field data recorded by the SCADA system during full pump trip and valve closure events. The validation was performed during the commissioning test and showed a perfect match between the prediction and measurement of pressure and flow of the first wave. This paper also intends to highlight lessons learnt during the commissioning test and in particular the major risk that is caused by inaccurate manufacturer data. On the first day of the commissioning test, a near incident took place where the pressure during valve closure unexpectedly exceeded the pressure rating by 16%. Damage to the pipeline was avoided by pressing the emergency trip push-button of the pumps thereby preventing the pipeline from being exposed to the full pressure rise. The post-incident investigation revealed that the control valve characteristics provided and certified by the manufacturer were inaccurate. A difference in Kv of up to 20% was measured for valve positions below 40% open and the valves were fully closed at 5% opening. The control valve characteristics were recalibrated on site and the valve closure pattern was adapted. The commissioning test was resumed and completed flawlessly.Water ManagementCivil Engineering and Geoscience
Operational optimization of district heating systems with temperature limited sources
No full textFuture district heating systems (DHS) will be supplied by renewable sources, most of which are limited in temperature and flow rate. Therefore, operational optimization of DHS is required to maximize the use of renewable sources and minimize (fossil) peak loads. In this paper, we present a robust and fast model-predictive control approach to use the thermal mass of buildings as a daily storage without violating temperature constraints. The novelty of this paper includes two elements. First, the focus on an operational control strategy that explicitly accounts for temperature-limited renewable sources, like a geothermal source. Secondly, the optimization problem is formulated as a (nearly) convex optimization problem, which is required for adoption of model-predictive control in practice. The examples show that the peak heating demand can be reduced by 50%, if the thermal inertia of the buildings is used and the heating setpoints are adapted. Furthermore, the operational optimization finds the proper balance between benefits of pre-heating using renewable sources with limited capacity and costs of additional heat losses due to pre-heating.+ Corrigendum to “Operational optimization of district heating systems with temperature limited sources” [Energy Build. 226 (2020) 110347] - https://doi.org/10.1016/j.enbuild.2021.110861Support Process and Energ
Guidelines for transient analysis in water transmission and distribution systems
No full textWater ManagementCivil Engineering and Geoscience
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