Suchergebnisse

The dynamics of a laminar starting jet are explored in a series of laboratory experiments and numerical simulations. We identify new, objective methods for characterizing the leading vortex ring, enabling robust comparisons with results from a numerical model. Observations of circulation in the initial vortex ring and for the total jet are reported along with strain rate at the leading stagnation point. Growth and pairing of shear instabilities trailing the leading vortex ring is observed. Development of these secondary vortices and their subsequent interactions with the leading vortex has significant effects on the characteristics of the primary vortex ring. Strong fluctuations in strain rate at the leading edge are associated with the pairing of the initial vortex ring with a trailing secondary ring ; Support for this research was provided by the Spanish MEC and European Union under Projects # ENE2005-08580-C02-01 and DPI2005-08654-C04-01 ; Publicado

In this paper, energy and exergy analyses of a rotary dryer employed in a Hot Mix Asphalt (HMA) plant for heating and drying of the aggregates in the mixture is presented. In the analysis, the exergy method in addition to the more conventional energy analysis, is employed to identify and evaluate the ther modynamic losses. The results show that, at design conditions, the plant performs with energy and exergy efficiencies of 0.89 and 0.18, respectively. The energy losses are mainly due to the flue gases. The exergy distribution indicates that the combustion and the heat transfer at different temperatures in the burner yield the highest exergy destruction in the process. A parametric study is conducted for the plant under various operational production parameters, including different humidities of the aggregates and filler content in aggregates, working temperatures and ambient conditions, in order to determine the parameters that affect the plant performance. It is shown that the solids humidity has a great impact on energy requirements. A better and sustainable use of the heat source employed in the dryer is proposed to avoid the high irreversibilities found. Furthermore, operating corrections in the mix or in the exhaust gas temperature are proposed to optimize the performance of the plant ; This work has been has been partially funded by the Spanish Government, Project DPI2009 10518 and the Fenix project Research on new concepts for more safe and sustainable roads (CENIT 2007 1014). Also, the authors aknowledge the support provided by Intrame S.A ; Publicado

The importance of considering the water price in the analysis of the impact of dry versus hybrid condensing systems in the thermo economical performance of solar tower plants was demonstrated in this work. The dry condensing system consists of several induced-draft air-cooled condenser cells (ACCs) and the hybrid system consists of a parallel system where the condensing steam is split between the ACCs and a surface steam condenser where circulating water is cooled in a wet mechanical-draft cooling tower. The influence of the operating parameters of either the dry or wet cooling systems on the cooling load and fan power consumption were studied. Then, for a given condensing system (a system with a defined number of installed ACCs units and cooling tower units) and given the dry-air and wet-bulb air temperatures, the operating parameters were optimized to maximize the revenues of the power plant. This optimization depends on the water-to-electricity price ratio 𝑅, showing that at low ambient temperature when this ratio increases it is not profitable to turn on the cooling towers since the water cost is not counterbalanced by the higher cycle efficiency obtained with the lower condensation temperature. Finally, the annual operation and the LCOE and NPV of the CSP plant located in Dunhuang were analyzed for both dry and hybrid condensing systems with different number of ACCs and wet towers, showing that the most cost-effective configuration is the 16 ACCs with 3 wet cooling towers for water-to-electricity price ratio 𝑅 = 4 ($/m3)/($/kWhe) and 𝑅 = 5 ($/m3)/($/kWhe), but for 𝑅 = 10($/m3)/($/kWhe), the best option is with only 2 wet towers. ; This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE).

The solar tower receivers tend to experience rupture problems due to the high thermal gradients and the corrosion produced by the working fluid, typically solar salt. In this work we have developed a series of CFD simulations to study a new receiver design composed of bayonet tubes aimed to reduce the overheating the receiver in the most thermally demanded area. These simulations evaluate the thermal behavior of the tubes for different working fluids, i.e. molten salt, liquid sodium and supercritical CO2. The simulations show that, for all the working fluids analyzed, it is possible to reduce the high temperatures of the tube thanks to the asymmetries created when the bayonet tube has an eccentric configuration. Besides, the greatest reduction of temperature in bayonet tubes is achieved when the working fluid is liquid sodium due to its higher thermal conductivity. ; The authors would like to thank the financial support provided by the Spanish government through the project ENE2015-69486-R (MINECO / FEDER, UE).

Inverse heat transfer problems typically rely on temperature measurements for estimating unknown boundary heat flux, such as that in the water tubes of steam boilers or central receivers in solar tower power plants. In this work, an experimental facility consisting of a molten salt loop that simulates a tube of a solar tower receiver is presented to obtain the outer tube surface temperatures under solar tower power plant operating conditions. The external surface f the pipe in the test section is heated in a controlled manner with an induction heater, which provides a very high nonuniform heat flux. An inverse thermal method has been applied to obtain the incident heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient two-dimensional numerical model of a circular pipe flowing molten nitrate salt and subjected to a nonhomogeneous circumferential heat flux has been developed. The heat flux calculation with the inverse method is in accordance with the heat flux estimation based on the calibration of the induction heater. A good agreement between the experimental and calculated temperatures is observed. Furthermore, the deflection of the tube caused by the nonhomogeneous heat flux is measured and is compared to the deflection calculated from the radial temperature profile from the inverse problem solution, and a good agreement between both results is observed. ; This work was partially funded by the Ministerio de Eduación, Cultura y Deporte para la Formación de Profesorado Universitario (FPU-04941), the Ministerio de Ciencia, Innovacion y Universidades (Project RTI2018-096664-B-C21 (MICINN/FEDER, UE)) of the Spanish Government and the Project SBPLY/17/180501/000412 of the Regional Government of Castilla-La Mancha.