Crespi F.Rodriguez de Arriba P.Sanchez D.Munoz A....
12页
查看更多>>摘要：This paper investigates the interest and potential of using working fluids based on Carbon and Sulphur Dioxide mixtures (CO2-SO2) in a transcritical Recompression cycle. In order to assess the actual thermodynamic potential of the concept proposed, the influence of dopant (SO2) content is assessed for two different turbine inlet temperatures (550 °C and 700 °C). The results obtained are compared with other CO2 mixtures already proposed in literature (CO2-C6F6 and CO2-TiCl4)) and for two alternative cycle layouts (Recuperated Rankine and Precompression). The results pf the analysis reveal that, at high ambient temperature, the Recompression cycle operating on CO2-SO2, with Sulphur Dioxide content between 20% and 30%(v), is a very interesting option for Concentrated Solar Power plants, able to achieve thermal efficiencies ≈45% and ?51% at 550 °C and 700 °C respectively. At a minimum cycle temperature of 50 °C, the proposed configuration leads to thermal efficiency gains of 6% and 2% with respect to the Brayton and Recompression cycles working on pure CO2. This performance enhancement of the Recompression cycle with CO2-SO2 is comparable to or higher than that enabled by other CO2 mixtures proposed in literature, but with significantly higher specific work (smaller footprint) and temperature rise across the solar receiver (lower installation costs).
查看更多>>摘要：This paper proposes a solution to the problems of lower heat transfer efficiency and higher consumption of raw materials in traditional single-tube row of tube evaporators, as well as the problems of higher food water loss and quality reduction under the forced convection of air cooler. By using the heat balance test method, the performance of three types of row of tube evaporators, namely single-tube double-fins row of tube evaporator (STDF-RTE), single-tube eight-fins row of tube evaporator (STEF-RTE) and single-tube double-fins row of tube evaporator with phase change material (STDF-RTE-PCM) is compared under different evaporation temperature. In addition, the insulation time of the STDF-RTE-PCM was also tested after the refrigeration unit was turned off. The results show that the STEF-RTE has 10.3% ～ 29.1% less consumables than the STDF-RTE for the same refrigeration capacity. The cold storage using the STDF-RTE-PCM has good temperature uniformity as the extreme difference of internal air temperature is within 2 °C under stable working conditions. After the refrigeration unit stops running, the PCM inside the STDF-RTE-PCM can still maintain the temperature of the cold storage in the ± 2 °C temperature range for 12 h. The experimental results provide an effective solution for improving the heat transfer performance of single-tube multi-fin row of tube evaporators for cold storage.
查看更多>>摘要：Trapezoidal and twisted trapezoidal tape elements which are used for heat transfer enhancement in tubes are experimentally investigated. The effects of the tapes on heat transfer, pressure drop, and overall performance factor are analyzed. Three different thread heights are used in the experiments, which are obtained by dividing the tape width by 4, 6, and 10. Pitch length is kept constant for the trapezoidal tapes, while the constant twisting ratio is used for the twisted trapezoidal ones. Tapes were held in the tube with no wall contact by stretching mechanically. Experiments were conducted under constant heat flux boundary condition and for Reynolds number interval of 5000–25,000, targeting turbulent flow at which overall performance factor values reported in literature are relatively small comparing to laminar flow. The experimental results show that trapezoidal tape elements enhance heat transfer and increase pressure drop with changing rates depending on thread heights. Overall performance factor in the tested interval is detected over unity for two twisted trapezoidal inserts, making them significant alternatives comparing to the literature. The best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height, while Nu is 90.52 and friction factor is 0.12. A maximum Nu of 118.46 is achieved with the proposed twisted trapezoidal tape, which is 1.9 times higher than the plain tube case and 1.57 times higher than the conventional twisted tape case. Since the twisted trapezoidal tapes perform better than the plain trapezoidal ones, it is concluded that the combination of the twisted and trapezoidal forms may be favorable in terms of heat transfer enhancement and overall performance factor regarding combination of swirl flow effect and periodical flow disturbance effect.
查看更多>>摘要：For a space heating ASHP unit, frosting on its outdoor coil would adversely impact its normal operation, requiring frequent defrosting and resulting in low operating efficiency and poor indoor thermal comfort. It has been shown that varying the speeds of both compressor and outdoor air fan in a variable speed (VS) ASHP unit can lead to different outdoor coil surface temperatures and thus different frosting suppression performances. However, this can also result in different output heating capacities from the ASHP unit. To establish the comprehensive relationship between frosting suppression performances and the total output heating capacity for a VS ASHP unit, an experimental study on developing the condensing-frosting performance maps for a VS experimental ASHP unit under different speed combinations of its compressor and outdoor air fan at three Test Conditions, was carried out and the study results are presented in this paper. The obtained performance maps clearly demonstrated that for the experimental ASHP unit, varying the speeds of its compressor and outdoor air fan would result in not only different outdoor coil surface temperatures, but also different total output heating capacities. A higher outdoor coil surface temperature and thus a better frosting suppression performance may be achieved, but usually at the expenses of losing some of its total output heating capacity. On the other hand, the experimental results also suggested that when the experimental ASHP unit was operated at different speed combinations that can lead to the same or similar total output heating capacities, its operating characteristics in terms of outdoor coil surface states and the total output heating capacity can be remarkably different. The obtained condensing-frosting performance maps may be used as the guidance for the design, operation and control of ASHPs for frost-free operation.
查看更多>>摘要：Air source heat pumps (ASHPs) have been widely used for heating in buildings. While in practice, the setting value of its supply water temperature is always far higher than the theoretical value owing to the delayed regulation, and this results in the low coefficient of performance. To solve this problem, an adaptive control method for the supply water temperature was proposed in present work. This control method could predict the best setting value of supply water temperature based on the heat-balance of the heat exchange amount in indoor fan-coils and building heating load, and the least squares method was used to achieve the adaptive identification of parameters. By adopting this control method in a field ASHP system, four sets of experiments were conducted. Results showed that this control method could quickly and accurately adapt to the actual project, and adjust the water temperature according to heating demand. When employing the new control method, the average supply water temperature was reduced by 8.4 °C, and lower supply water temperature made the coefficient of performance of ASHP unit increased by 21.16%; the building energy consumption was reduced by 34.24%, and the power consumption of the ASHP unit was reduced by 38.20%.
查看更多>>摘要：A simplified parallel power and refrigeration combined cycle based on the Kalina cycle (SPPRA-KC) using ammonia-water as working fluid is proposed and studied, in which refrigeration is taken as the main task and the refrigerant plays a leading role in improving the concentration of work solution. The turbine exhaust vapor is utilized to heat the work solution fed to the refrigeration sub-cycle, resulting in more refrigeration energy produced. Parameter analysis was conducted under temperatures of heat source/cooling water of 400 °C/30 °C from the perspective of energy quantity and quality. To obtain larger refrigeration capacity and power recovery efficiency, the dew point temperature of work solution and bubble point temperature difference in the boiler shall be as higher as possible and properly increased, respectively, which will cause more irreversible losses in the cycle, resulting in lower effective exergy efficiency. When optimizing the cycles with work concentration of 0.5 based on the effective exergy efficiency, the power recovery efficiency of the SPPRA-KC is 34.8% much higher than that of the related original cycle (PPRA-KC) due to much larger refrigeration capacity, and the effective exergy efficiency of the SPPRA-KC is 1.67% and 12.4% relatively higher than those of the PPRA-KC and the Kalina cycle, respectively. Besides, the main exergy destruction of the SPPRA-KC occurs in the boiler, absorbers, exhaust heat source and turbine.
查看更多>>摘要：Free-piston Stirling generators (FPSGs) show great potential in portable power sources for the advantages of high reliability, high efficiency, low noise, compact structure, long lifetime, etc. An FPSG aimed at 15 W electric power output is numerically and experimentally studied. By adopting the second method of Lyapunov, the onset characteristics of the FPSG are analyzed based on a thermodynamic model. The effects on the onset conditions of the interaction between the displacer and power piston are explored. It is shown that the optimal operating pressure and spring stiffness corresponding to the lowest onset heating temperature are 6.2 MPa and 5500 N/m, and the FPSG is easier to start when the damping of the displacer and power piston is smaller and the mass of the displacer and power piston is well matched. The experimental results of the effects of the pressure and spring stiffness on the onset temperature agree well with the calculated results. The FPSG can generate 15.5 W electric power with a thermal-to-electric efficiency of 16.0% in the experiment when the heating and chilling water temperatures are 820 K and 300 K, respectively. And the relative errors between the calculated and experimental PV power and their efficiencies are about 2.1%–6.8% and 24.4%–26.9%, respectively. This work can lay a good foundation for studying the onset characteristics and stable operating characteristics of FPSGs.
查看更多>>摘要：The use of clay as the combustor wall material for direct heat-to-electricity conversion using a Thermoelectric generator (TEG) was evaluated in a side-wall configuration burner. The influence of different fuel blends (Methane, Biogas and Biogas + H2) and flame work conditions in TEG power and efficiency are discussed. Flame–wall interaction (FWI) of the laminar side-wall quenching (SWQ) premixed V-flame was investigated. Heat release rate (HRR) and quenching distances were analysed with the chemiluminescence of excited OH* and CH* radicals. The gases velocity field was studied with Particle Image Velocimetry (PIV). For comparison, a galvanized steel (GS) plate was used in the same working conditions. We found that the efficiency (ηTEG) and electrical power (PTEG) losses between materials increases with flame temperature. Quenching distances (dq) were larger in the flame clay wall interaction. Flame quenching was governed by heat losses independently of the wall but the impact of convection was more significant when using the clay wall.
查看更多>>摘要：The numerical simulation results provided by a new multiphysics model of a magnetocaloric regenerator undergoing active magnetic regeneration (AMR) cycles inside an electromagnet air gap are compared with the experimental behavior of a similar built-in regenerator composed of gadolinium parallel plates. This multiphysics model is based on the coupling of an original semi-analytical magnetostatic model (reluctance network model) with a magnetocaloric model and thermo-fluidic model. The main objective of this work is to provide the first experimental validation of the multiphysics model by focusing on the production of the fluid temperature span by the magnetocaloric regenerator during long sequences of successive AMR cycles with an initial temperature below the Curie temperature of gadolinium. The case study offers a more extensive understanding of the thermal behavior of the regenerator under fully adiabatic conditions by theoretically and experimentally investigating the thermal inertia that governs this behavior. During this test, a trapezoidal alternating fluid flow was produced inside the regenerator by a controlled hydraulic cylinder that drove the calibrated magnetic field pulses generated by a highly effective electromagnet. An active magnetic refrigeration time constant of the magnetocaloric system is proposed based on a scaling analysis of the overall heat transfer during the transition to the stationary state. It effectively reflects the numerical and experimental behaviors. The numerical results are in good agreement with the experimental measurements and can facilitate further research into the estimation of magnetocaloric heating and cooling power as well as optimization of the design of plate regenerators of different magnetocaloric materials.
查看更多>>摘要：Frosting is widely seen in nature and many industry fields, and always plays negative effect. To accurately predict and control frosting process on cold plate surface, an experimental study on the frosting characteristics on the surface of horizontal cold plate is carried out, with the surface temperature varied at a range of ?30 ～ -10 °C under natural convection. Based on the experimental results, series of frosting characteristics are analyzed. As reported, the first three stages of frosting are short and account small proportion, within 160 s in the 1,200 s frosting process. The growth trend of frost thickness shows a leaf-shaped area, which could be used for predicting the frost thickness under different conditions. The surface temperature effect on the frosting rate is obvious, and directly influences the surface roughness. For a surface with temperature at ?27.5 °C, the peak of frosting rate is 9.59 × 10-6 m/s, the peak and fluctuation amplitudes of roughness at 90 × 10-6 m and 57.57 × 10-6 m, respectively. The periodic reverse melting directly influences the frost layer thickness and dynamic frosting density. On the ?27.5 °C surface, the lowest and highest densities are 22.08 kg/m3 and 65 kg/m3 at 368 s and 1229 s into frosting, respectively. Results of this study could be used for the control strategy optimization of anti-frosting and defrosting for energy equipment, such as air source heat pump.
NSTL
Elsevier