Results 1 - 10 of 15
Results 1 - 10 of 15. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] Highlights: • Radiative properties of NiFe_2O_4 particles are obtained by experimental and numerical method. • A heat transfer model suitable for thermal dissociation process of NiFe_2O_4 is developed. • Conversion rate and temperature field inside the proposed solar reactor are investigated at different operating parameters. - Abstract: NiFe_2O_4 is a promising candidate that used in solar thermochemical hydrogen production due to its better redox performance. The solar-thermal dissociation processes of NiFe_2O_4 are analyzed by the Monte Carlo Ray Tracing Method (MCRTM) and Finite Volume Method (FVM), taking into account the conduction, convection, radiation and chemical reaction. More, the chemical co-precipitation method is employed to synthesize nanoscale NiFe_2O_4 and combine experimental and numerical method to determine its radiative properties. The metallic oxide dissociation solution obtained from the MCRTM and the FVM is compared with that from other methods in order to validate the MCRTM and the FVM for heat transfer analysis of solar-thermal dissociation. Finally, parametrical studies using the method are used to provide advices for structural designs of a new proposed solar reactor. The results can provide useful application for improving the efficiency of conversion from solar energy to chemical energy in the future.
[en] Highlights: • KH achieves better performance in solving the inverse geometry design problems. • The extinction coefficient has significant impact on the inverse design results. • The scattering property and emissivity mainly affect radiative heat flux. - Graphical Abstract: Display Omitted - Abstract: The krill herd (KH) algorithm is used to solve the inverse geometry design of a two-dimensional radiative enclosure filling with participating media. This inverse design problem aims to satisfy a uniform distribution of radiative heat flux over the design surface. The forward radiative heat transfer problem in irregular enclosures is solved by using the discrete ordinate method with a body-fitted coordinate system. Consequently, five kinds of KH algorithms are utilized to optimize the geometric positions of the control points, and Akima cubic interpolation is used to approximate the changing boundary through these points. Retrieval results show that the KH algorithm can be applied successfully to inverse geometry design problems, and KH is proved to be more efficient than the micro genetic algorithm and particle swarm optimization algorithms. The influences of radiative properties of the media and the number of control points on the retrieval geometry design results are also investigated.
[en] A quantitative evaluation of a radiative heat transfer is important in sodium combustion because a large amount of aerosol particles, which are produced as a result of the combustion, exists in a combustion region. In this study, a development of radiation model with aerosols and optical property measurement has been carried out for the purpose of evaluating radiative heat transfer based on a optical property, diameter, number density and statistical and spatial distribution of aerosol particles. In 2000 research, one dimensional analysis program of the Monte Carlo method has been developed. This program evaluates a radiative transmission intensity based on an optical property and a statistic and spatial diameter distribution of airborne particles. Using this program, an optical property can be estimated from experimental conditions (e.g. diameter distribution) and results (radiative transmission intensity). As a result of numerical analyses which evaluate an influence of a size parameter (relation between a particle diameter [D] and wavelength [λ] :=πD/λ) on the accuracy evaluation, an optical property can be estimated within 3% accuracy though an angle distribution measurement of radiative transmission intensity is necessary when the size parameter becomes large. (author)
[en] Radiative heat transfer in a room filled with aerosol is analyzed by the Monte Carlo method. The system treated is composed of non-gray gas and aerosol contained in a pair of infinite parallel black body walls. Mie scattering and size distribution of aerosol and non-gray gas are treated. As the result, present analysis gives the relation between complex refractive index of the aerosol and the characteristics of radiative transmission through aerosol layer. (author)
[en] Multi-objective optimal designs of high efficiency corrugated tube heat exchangers applied in nuclear plants are presented in this paper. Dimensionless corrugation pitch (p/D), dimensionless corrugation height (H/D), dimensionless corrugation radius (r/D) and Reynolds number (Re) were considered as four design parameters. The analyses of heat transfer characteristic, resistance characteristic and overall heat transfer performance for the corrugated tube were conducted based on CFD to acquire the objective functions (Nuc, fc, Nuc/Nus, fc/fs and η in this paper) for combination of process parameters. The Response surface methodology (RSM) with Central composite design (CCD) was used to identify the relationships between the objective functions and the design variables. Variance of the linear term, quadratic term and interactive term for design parameters in the response variables were analyzed. Response surface analyses were applied to visualize the effects of the interactive term. The results of optimal designs are a set of multiple optimum solutions, called ‘Pareto optimal solutions’. It reveals that augmenting the heat transfer performance caused by various design parameters in the optimum situation would lead to the increase of the resistance. The optimum values of fc/fs = 1.22 and η = 1.42 are obtained in the condition of specific (fixed) Nuc/Nus ≥ 1.2. - Highlights: • RSM is used to optimize the outward convex corrugated tube heat exchanger. • AVOVA is analyzed to verify the accuracy of quadratic model and effect sensitivity of factors. • The relationship between objective functions and designing parameters are constructed. • The response surface analysis is used to describe the regression equation. • The Pareto optimal curves are applied to acquire the optimal value of η
[en] Molten-salt-based nanofluids and ionic-liquid-based nanofluids are developed for thermal storage and heat transfer at relatively high temperatures, in the past few years. Preparation and stabilization techniques are briefly introduced firstly, and then, thermal properties, e.g., specific heat, thermal conductivity and viscosity, are summarized and discussed in detail. The properties are not only affected by the characteristics of nanomaterials and base fluids, but also affected by the synthesis method, such as the sonication intensity and duration. Some of the thermophysical property data are still incomplete, especially the thermal conductivity of molten-salt-based nanofluids, and properties of ionic-liquid-based nanofluids at high temperatures. While several literature works show that the Krieger–Dougherty model can well predict the viscosity, no general models for thermal conductivity and specific heat have been developed yet for both types of nanofluids.
[en] Highlights: • The model considering contact angle for a thermosyphon is developed. • The model with contact angle has lower relative error than without contact angle. • Mechanism of the effect of evaporator wettability on heat performance is discussed. • Mechanism of the effect of filling ratio on heat performance is discussed. • Heat performance is best at filling ratios of 20–30% for hydrophilic evaporator. - Abstract: A thermosyphon is considered an efficient heat dissipation device in engineering fields due to its low thermal resistance. The heat transfer mechanisms for thermosyphons at different evaporator wettability and filling ratios are not well detailed. A model considering evaporator wettability in terms of a contact angle is developed to detail the phase change process to explore the heat transfer mechanism for a thermosyphon in this study. The effects of evaporator wettability and filling ratio on the heat performances of a thermosyphon charged with water are investigated. It is observed that the simulated absolute temperatures with a contact angle are in better agreement with the experimental results with an average relative error of 0.15% than the simulation results without a contact angle (0.28%). The results show that a hydrophilic surface causes bubbles to easily depart the evaporator wall, thereby increasing the heat performance, whereas a hydrophobic surface causes bubbles to adhere to the evaporator wall, decreasing the heat performance. Further study shows that a low filling ratio of 12% will result in drying out, but a high filling ratio of 40% will prevent large bubbles from reaching the liquid surface, thereby decreasing the heat performance. The heat performance is best at filling ratios of 20–30% for an evaporator with a hydrophilic surface.
[en] Highlights: • Nu, f, and PEC of the shell side in the ACT is more obvious than the tube side. • A lower Re condition should be selected in the ACT for saving energy. • Various rl located at upstream does not influence on thermo-hydraulic performance. • Various rl located at downstream can increase thermo-hydraulic performance visibly. • rl located at downstream is more effective than upstream in shell side of the ACT. - Abstract: In the present study, heat transfer performance and flow characteristics of turbulent flow in asymmetrical corrugated tubes (ACT) are numerically and experimentally investigated. Experiments on a smooth tube and ACT were conducted for the validation of the numerical methods. Numerical simulations were then conducted to obtain an understanding of the physical behavior of thermodynamics and fluid flow in the ACT with the Reynolds number ranging from 12,000 to 66,000. Thermodynamic results between the tube side and the shell side of the ACT were then compared. Flow directions were defined as opposite, when large corrugation fillet radii (rl) located at the upstream or downstream. And the thermo-hydraulic performance and mechanism at the shell side, which were caused by two opposite flow directions, were presented and analyzed. The results show that, compared with the tube side of the ACT, the Nu, f, and the performance evaluation criterion (PEC) of the shell side in the ACT is more obvious and the maximum increment were 1.7, 1.13, and 1.26 respectively. It was also found that the value of various rl/D located at upstream does not influence on thermo-hydraulic performance. And a lower Re condition should be selected in the ACT for saving energy. While rl located at the downstream can significantly increase the overall heat transfer coefficient and decrease the Reynolds stress. The PEC was increased by 10–20% which is much more than the increase when rl was located at the upstream.
[en] Highlights: → Chemical energy values are 2.16-5.20 times as the physical energy values. → Chemical exergy values are 4.50-13.45 times as the physical exergy values. → Efficiencies mainly increase first and then decline when ER/temperature increases. → Higher carbon and hydrogen content generates higher energy and exergy values. → Higher ash content results in lower energy and exergy values/efficiencies. - Abstract: Biomass gasification with air in autothermal gasifiers is studied and compared with another fuel from thermodynamic aspect. The results indicate that the chemical energy values of product gases from biomass are 2.16-5.20 times as the corresponding physical energy values, while the chemical exergy values are 4.50-13.45 times as the corresponding physical exergy values. The energy and exergy efficiencies of biomass gasification are respectively in ranges of 52.38-77.41% and 36.5-50.19%, and mainly increase first and then decline when ER or gasification temperature increases. Higher carbon and hydrogen content in the ultimate analysis generates higher gaseous energy and exergy values, while results in lower energy and exergy efficiencies. Higher ash content makes biomass produce lower energy and exergy values/efficiencies.
[en] The sensitivity analysis for low temperature ORCs (organic Rankine cycles), as well as the thermoeconomic comparison between the basic ORC and regenerative ORC using Non-dominated sorting genetic algorithm-II (NSGA-II), are conducted in this paper. The derivatives of five system parameters on system performance are used to evaluate the parametric sensitiveness. The exergy efficiency and the APR (heat exchanger area per unit net power output) are selected as the objective functions for multi-objective optimization using R123 under the low temperature heat source of 423 K. The Pareto frontier solution with bi-objective for maximizing exergy efficiency and minimizing APR is obtained and compared with the corresponding single-objective solutions. The results indicate that the prior consideration of improving thermal efficiency and exergy efficiency is to increase the evaporator outlet temperature. A fitting curve can be yielded from the Pareto frontier between the thermodynamic performance and economic factor. The optimum exergy efficiency and APR of the regenerative ORC obtained from the Pareto-optimal solution are 59.93% and 3.07 m"2/kW, which are 8.10% higher and 15.89% lower than that of the basic ORC, respectively. The Pareto optimization compromises the thermodynamic performance and economic factor, therefore being more suitable for decision making. - Highlights: • The sensitivity analysis of the basic ORC is conducted. • The Pareto-optimal solution is compared with the single-objective solutions. • Evaporator outlet temperature should be preferentially considered. • 8.10% higher exergy efficiency and 15.89% lower APR for the regenerative ORC