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[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: • The outward helically corrugated tube is suitable for high pressure fluids. • The effects of corrugation height and pitch on turbulent flow are investigated. • The relationships among swirl, rotational flow and heat transfer are discussed. - Abstract: Concerning a novel outward helically corrugated tube manufactured through hydraulic forming under 290 MPa, a numerical study was conducted to investigate the mechanism of turbulent flow dynamics and heat transfer enhancement based on the Reynolds stress model (RSM) using the FLUENT software. A validation of the Reynolds stress model for turbulent flow over a wavy surface was performed, and the results were then compared with the results from a large eddy simulation (LES) model and with experimental measurements. The helically corrugated tubes with different corrugation height-to-diameter ratios and pitch-to-diameter ratios are then evaluated to explore their influence on turbulent flow and heat transfer. It was found that the intensity of swirl flow was enhanced with an increase in the corrugation height, and it increased with a decrease in the corrugation pitch, the intensification of the swirl flow strengthens the heat transfer and resistance characteristics. The intensity of rotational flow was enhanced with an increase in the corrugation height, and increased with an increase in the corrugation pitch; the enhanced rotational flow causes an inhibition effect on heat transfer and resistance. Moreover, the maximum values of the local Nusselt number and the friction factor along the walls were observed at the reattachment point, and their minimum values appeared at the core of the swirl flow. It is therefore reasonable to keep the corrugation height-to-diameter ratios be less than 0.1, and the pitch-to-diameter ratios be less than 2 to ensure that the growth rate of the heat transfer is greater than the growth rate of the flow resistance.
[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
[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] Highlights: • The thermoeconomic comparison of regenerative RORC and BORC is investigated. • The Pareto frontier solution with bi-objective compares with the corresponding single-objective solutions. • The three-objective optimization of the RORC and BORC is studied. • The RORC owns 8.1% higher exergy efficiency and 21.1% more LEC than the BORC under the Pareto-optimal solution. - Abstract: Based on the thermoeconomic multi-objective optimization by using non-dominated sorting genetic algorithm (NSGA-II), considering both thermodynamic performance and economic factors, the thermoeconomic comparison of regenerative organic Rankine cycles (RORC) and basic organic Rankine cycles (BORC) are investigated. The effects of five key parameters including evaporator outlet temperature, condenser temperature, degree of superheat, pinch point temperature difference and degree of supercooling on the exergy efficiency and levelized energy cost (LEC) are examined. Meanwhile, the Pareto frontier solution with bi-objective for maximizing exergy efficiency and minimizing LEC is obtained and compared with the corresponding single-objective solutions. Research demonstrates that there is a significant negative correlation between thermodynamic performance and economic factors. And the optimum exergy efficiency and LEC for the Pareto-optimal solution of the RORC are 55.97% and 0.142 $/kW h, respectively, which are 8.1% higher exergy efficiency and 21.1% more LEC than that of the BORC under considered condition. Highest exergy and thermal efficiencies are accompanied with lowest net power output and worst economic performance. Furthermore, taking the net power output into account, detailed investigation on the three-objective optimization for maximizing exergy efficiency, maximizing net power output and minimizing LEC is discussed
[en] Highlights: • A comprehensive model is developed for biomass entrained flow gasification. • Intrinsic reaction rate submodel for biomass char reactions is realized. • Effects of diffusion rate and kinetic rate are considered. • The relative errors between the simulated and experimental results are reasonable. - Abstract: Intrinsic reaction rate submodel is established in this study to consider the effects of diffusion rate and kinetic rate for simulating the char reactions due to their slow reaction rates and important controlling steps. The biomass gasification model for an entrained flow gasifier is developed with the Euler–Lagrange method using ANSYS FLUENT software. Gas phase is treated as continuous phase in standard k–ε model to close governing equations whereas biomass particles are treated as discrete phase in discrete phase model (DPM) to track the movement of particles. For homogeneous phase reactions, finite rate/eddy dissipation model is applied to calculate the reaction rates. For heterogeneous phase reactions, intrinsic reaction rate model is realized by coding the user-defined functions (UDFs) to calculate char reaction rates. The results obtained from this study show that the relative errors of volumetric concentrations are mainly within the range of 1–18% and the relative errors of lower heating value, gas production, cold gas efficiency and carbon conversion efficiency are within the ranges of 1–13%, 1–8%, 1–12% and 1–11%, respectively. The CFD model developed in this study can be used to simulate biomass gasification processes for entrained flow gasifiers.
[en] Based on the thermoeconomic multi-objective optimization and decision makings, considering both exergy efficiency and LEC (levelized energy cost), the performance comparison of low-grade ORCs (organic Rankine cycles) using R245fa, pentane and their mixtures has been investigated. The effects of mass fraction of R245fa and four key parameters on the exergy efficiency and LEC are examined. The Pareto-optimal solutions are selected from the Pareto optimal frontier obtained by NSGA-II algorithm using three decision makings, including Shannon Entropy, LINMAP and TOPSIS. The deviation index is introduced to evaluate different decision makings. Research demonstrates that as the mass fraction of R245fa increasing, the exergy efficiency decreases first and then increases, while LEC presents a reverse trend. The optimum values from TOPSIS decision making are selected as the preferred Pareto-optimal solution for its lowest deviation index. The Pareto-optimal solutions for pentane, R245fa, and 0.5pentane/0.5R245fa in pairs of (exergy efficiency, LEC) are (0.5425, 0.104), (0.5502, 0.111), and (0.5212, 0.108), respectively. The mixture working fluids present lower thermodynamic performance and moderate economic performance than the pure working fluids under the Pareto optimization. - Highlights: • The thermoeconomic comparison between pure and mixture working fluids is investigated. • The Pareto-optimal solutions with bi-objective function using three decision makings are obtained. • The optimum values from TOPSIS decision making are selected as the preferred Pareto-optimal solution. • The mixture yields lower thermodynamic performance and moderate economic performance.
[en] Exergy is an efficient tool for evaluating the quality of energy, and it plays an important role in reducing irreversible losses, improving energy efficiencies, saving energy sources and reducing pollution emissions for energy utilization systems. This study furthered an investigation of exergy distribution characteristics of NiFe_2O_4 solar-thermal dissociation in the solar reactor using User Define Function (UDF) technique in Fluent. The analysis was based on the previous results including temperature, velocity and species concentration during the solar thermal dissociation of NiFe_2O_4 in a solar reactor. In addition, the effects of reactant particle diameter, particle flow rate and aperture gas temperature on the physical and chemical exergy distributions were also studied. The results showed that with the increasing in particle diameter, a lower physical exergy region gradually forms around the axial centerline in the solar reactor and the value of chemical exergy decreases sharply. Both the increasing in particle mass flow rate and aperture gas temperature reduce the low value of physical exergy distribution and increase the value of chemical exergy due to the high conversion rate. There is an increasing limitation for mass flow rate of particles and gas temperature. But the increasing limitation needs further investigation by considering coupling the effects of the above operating parameters. The results obtained from this study gave basic knowledge of exergy distribution characteristics for solar-thermal dissociation processes and found a theoretical basis for structural optimization of solar reactors. - Highlights: • Exergy distribution characteristic in a solar reactor is presented. • Effect of different operating parameters on exergy distribution is analyzed. • Optimal operating parameter for a solar reactor is given.