Results 1 - 10 of 21
Results 1 - 10 of 21. Search took: 0.017 seconds
|Sort by: date | relevance|
[en] In the solid Breeder Blanket (BB) concepts both tritium release and heat recovery depend on the thermal performances of the breeding zone. Within the R&D activities of the Helium Cooled Pebble Bed (HCPB) breeding blanket, the knowledge of the thermal diffusivity of the breeder beds is of fundamental importance to model the transient heat transfer during the power pulses of the fusion machine. The aim of the present study is to investigate the thermal diffusivity of the breeder beds at BB relevant conditions; to this end the line heat source probe method was employed together with differential scanning calorimetry. An experimental facility, based on the line heat source probe method, was devised for the investigation of the thermal diffusivity of granular beds at breeder blanket relevant temperatures, mechanical state, purge gas type and pressure. Besides the experimental approach, literature values were used to estimate the specific heat capacity of the breeder materials starting from the specific heat capacity of the constituent compounds based on the mole fraction. In addition to the thermal diffusivity and heat capacity, a preliminary insight on the phase transitions of the reference and advanced ceramic breeder beds is given.
[en] The present work was carried out to evaluate the effect of rejuvenation heat treatment on recovery of the microstructure and hot corrosion resistance of a service-exposed nickel-based gas turbine blade. Different rejuvenation heat treatment cycles were carried out on the service-exposed blade. The microstructure of the blades was examined by scanning electron microscope (SEM) in as-received condition and after rejuvenation heat treatment. Both of the service-exposed and rejuvenated blades were subjected to cyclic hot corrosion tests to evaluate the resistance to hot corrosion. Microstructural investigations showed that the rejuvenation heat treatment cycle, including solution treatment at 1175 °C for 3h, followed by air cooling and two-stage aging treatments at 925 °C/1h and 845 °C/24 h was most successful to recover the microstructure of the service-exposed blade to its virgin condition. Moreover, it was found that the rejuvenation heat treatment has a significant effect on the hot corrosion resistance of the service-exposed blade via dissolution of continuous grain boundary carbide films and the redistribution of the elements in the alloy matrix. The results of the hot corrosion experiments showed that the rejuvenation heat treatment can improve the hot corrosion resistance of the service exposed blade up to about 60 percent. (paper)
[en] The effects of nanofluids (Al2O3–water) on the overall thermal performance of an annular enclosure (or jacket) are experimentally investigated which is used for recovering waste heat from a typical stack of a gas heater. In the initial stages of the heating process, the inner cylindrical wall becomes hotter, while the bulk fluid is nearly at the preceding uniform temperature; hence, the wall heat flux is strongly enhanced at the beginning. Afterward a decline in the wall heat flux is observed due to increasing Rayleigh number and correspondingly generating cellular flows in the annulus that leads to temperature enhancement of the liquid. Using nanofluids has the advantage of improving key parameters such as Nusselt number. Nanofluids with higher nanoparticle concentrations need less response time to react to any changes in thermal environment, and consequently they have smaller time constant. Higher convective heat transfer coefficient as well as greater temperature uniformity in the enclosure is achieved by selecting nanofluids with larger values of nanoparticle concentration. The results also reveal that convective heat transfer coefficient and Nusselt number of nanofluids are comparatively enhanced with time, since hotter base fluid results in higher effective thermal conductivity.
[en] High temperature gas-cooled reactors and very high temperature reactor (HTGR and VHTR) have a wide range of applications because of their inherent safety and high heat source temperature. Combined cycle can realize the cascade utilization of high-temperature heat source of HTGR and VHTR and improve their energy utilization efficiency. In this article, a theoretical optimization method for the combined cycle coupled with HTGR and VHTR is proposed. By optimizing the topping cycle and the bottoming cycle separately, the multivariate combined cycle efficiency expression is organized into the optimized relationship with three variables (reactor outlet temperature, main steam temperature and main steam pressure). Therefore, the problems of multiple variables, mutual coupling and unclear physical significance of the combined cycle coupled with HTGR and VHTR are solved. The theoretical optimization values can provide guidance for the design of combined cycle of HTGR and VHTR. (author)
[en] Considering water level and pressure variation in drum of a natural circulation heat recovery steam generator (HRSG) is very crucial to design HRSGs; in other words, if a sudden change occurs in input heat rate, steam and feed water flow rate, drum water level would increase or decrease rapidly. Therefore, due to their important role, drum inlet parameters, such as steam quality and pressure should be accurately controlled. In this study, a dynamic simulation of HRSG was carried out to investigate dynamic behavior of important key parameters involved in the steam generating process. The rate of produced pure water by brine concentration plant under varying gas turbine load and consequently input heat rate to the HRSG’s risers was investigated as well. Results showed that a rapid increase in input heat rate to the tube bundle increases evaporation rate and intensely increases steam quality as a result; therefore, too much excess heat could increase the chance of tube failure. Moreover, drum pressure drop and reduction in system water volume were occurred due to the increase in steam demand. By increasing input heat rate about 20%, the risers’ outlet steam quality increased significantly almost 45% of initial steam quality. In addition, a 5% increase in input heat rate led to approximately 20% growth in generated steam pressure, causing a more motive steam of higher quality being injected into the brine concentrator and finally generating 2% more pure water flow rate as a result.
[en] In this paper, combined heat and power frameworks employing solid oxide fuel cell power module and a small-scale gas turbine are presented. The offered system is utilized as heat and power supply for residential consumers with a carbon dioxide sorption circulating fluidized bed. As well a favorable solution for the high penalties associated with CO2 capture and reuse of the CO contents is offered. The combined heat and power system considered by a different arrangement in order to high proficiency, controllability, heat recovery and high capacity of energy. In the proposed system, the unburned product from the solid oxide fuel cell is re-extracted and utilized as a fuel source. The suggested system is analyzed by the first and second law of thermodynamics. During this study, comprehensive calculations of chemistry and thermal within the fuel cell are performed to get accurate results. The impact of various parameters, for example fuel and oxidant rate, carbon dioxide removal, operating pressure, compressor parameter on work and heat output of the cycle as well as the discharge of carbon dioxide contamination, is investigated. The optimal pressure ratio of the compressor to minimize the carbon dioxide production is found.
[en] Zinc oxides have attracted attention in high-temperature waste heat recovery due to their high melting points. This review aims to provide a comprehensive summary of the effects of dopant on thermoelectric properties of ZnO based bulk, thin films, nanowire as well as the effects of nanostructuring on ZnO based materials. In general, the thermoelectric performance can be enhanced by a single doping. Among single dopant elements, Al dopant seems to be effective dopant resulting in better thermoelectric properties. However, the solubility limit of the dopant in single doping limits the improvement of thermoelectric performance. Efforts in dual doping to improve the thermoelectric properties of ZnO materials have been conducted. As a result, the drastic improvement of the thermoelectric performance from Al-Ga dually doped ZnO bulk and Ga-In dually doped ZnO thin films shows strong evidence of this approach. In addition to doping effects, the thermoelectric performance can also be enhanced through structural effects such as mismatches between the film and substrate, morphology, thermal treatments or introducing nano-precipitated materials. In addition, the low-dimensional structure such as nanowire structure is promising for improving the thermoelectric properties of materials because of their strong quantum confinement effect. This leads to the increase of the Seebeck coefficient according to the Mott’s relationship without lowering the electrical conductivity. (review)
[en] In order to increase Renewable Energy (RE) in the power generation, the present technology of Nuclear Power Plants (NPPs) should be utilized. This paper introduces a ‘Thermal Hub’ concept which consists of the two hybrid systems with Concentrated Solar Power (CSP) for the heating steam in the reheater and Ocean Thermal Energy Conversion (OTEC) for waste heat recovery of discharged water from the NPP. These renewables were chosen based on the NPP’s thermodynamic characteristics. Both thermal hybrid systems will contribute to reduce the cost for the electricity of renewables. A useful chart is introduced for visualization of the relationship between nuclear and renewable energies in the various hybrid systems. (author)
[en] The present study investigates the thermal characteristics of a proposed porous heat exchanger (PHE). This heat exchanger consists of three sections, one high-temperature (HT) section and two heat recovery (HR) sections. Product of combustion as a high-temperature gas mixture enters to HT section in which enthalpy of gas flow is converted to thermal radiation, while in HR sections, the reverse phenomenon occurs. Simulation of fluid flow in porous medium generated by random and regular monodisperse and polydisperse particles is done using combination of the lattice Boltzmann method and smoothed profile method. Because of high-temperature variation in this system, effect of temperature on thermo-physical properties is also considered which has not been studied in previous research studies. Since the gas and solid phases are in non-local thermal equilibrium, separate energy equations are used for these phases. To obtain the radiative term in the solid energy equation, the radiative transfer equation is solved numerically by the discrete ordinates method. The influence of particles array and their sizes on the efficiency of the PHE system is studied. Finally, the effects of various parameters like optical thickness and scattering coefficient on the performance of PHE system are investigated.