查看更多>>摘要：Heliostat field aiming strategy significantly affects the safety and operational efficiency of the Solar Power Tower (SPT) system. Heliostats installed in fields are likely to show deviations of optical properties from design specification to some extent. Such deviations generally reflect in the surface curvatures and normal directions of heliostat facets. These differences can give rise to the inevitable degradation of optical performance, impacting the execution effect of the pre-designed aiming strategy. Focusing spot distortion with an inappropriate aiming strategy possibly leads to overheated local areas or component damages. To cope with this error and to improve the light concentration performance, an aiming strategy optimization approach with post installation calibration was proposed in this paper. An equivalent parameter model was developed based on heliostat parameters, producing a more accurate description of optical performance. The adopted heliostat parameters were obtained by the parameter identification method. The parameter identification was formulated by photographed solar flux images and simulated flux distributions. The aiming strategy optimization model of heliostat field was built with obtained identification parameters, and the optimization goal considers utilized solar power absorbed by the heat transfer fluid and flux distribution uniformity. The simulation results showed substantial improvement from the proposed aiming strategy.
查看更多>>摘要：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 ($/m(3))/($/kWh(e)) and R = 5 ($/m(3))/($/kWh(e)), but for R = 10 ($/m(3))/($/kWh(e)), the best option is with only 2 wet towers.
查看更多>>摘要：The trans-critical transition of multi-component sprays under the ECN Spray-A conditions has attracted extensive interest. However, the drastic change of critical points of multi-components makes this controversial. In this study, the trans-critical transition of sprays before and after the end of injection is evaluated by numerical methods, and new findings have been obtained. First, the robust and accurate thermodynamic equilibrium solver and multi-component droplet evaporation model were developed and implemented into the OpenFOAM. These models have been validated against the measured phase change diagram and isolated droplet evaporation rate. Then, the mixture critical temperatures of n-dodecane/nitrogen and multi-component diesel/nitrogen were calculated under a wide range of pressures based on the thermodynamic equilibrium solver. The mixture critical temperature decreases almost linearly with the increase of the pressure. Following that, extensive sprays of ndodecane and multi-component diesel under high temperature and pressure conditions were simulated, and the predicted spray liquid penetrations were validated against the experimental data. The results found that the dilute sprays after the end of injection are more prone to transition to the supercritical mixing regime, while the trans-critical possibility of dense sprays before the end of spray decreases due to the cooling effect of extensive evaporation. Finally, the trans-critical transitions of multi-component diesel sprays before and after the end of injection were evaluated and compared with those of n-dodecane sprays. Due to the low mixture critical temperature and evaporation rate of diesel, its sprays are more likely to transition to the supercritical regime. Therefore, it can conclude that the dilute sprays after the end of injection cannot represent the thermodynamic state of dense sprays. Moreover, the multi-component diesel and n-dodecane sprays follow a different transcritical transition pathway.
查看更多>>摘要：To solve the shortcomings of small cooling capacity of the current Joule-Thomson (J-T) cryocooler, a multi-layer microchannel J-T cryocooler is designed and fabricated by printed circuit board technology. The characteristic of the cryocooler is that there exists heat transfer while throttling. The coupling effect of heat transfer and throttling affects the overall performance of cryocooler. Therefore, the steady-state model was established for each part of the cryocooler. The P-h diagram of N-2 was calculated by the model. Considering the variation of J-T coefficient (mu(jt)) in the process, the coupling performance of heat transfer and throttling in the J-T cryocooler were analyzed. A new parameter, J-T efficiency (eta(jt)), was proposed to evaluate the J-T effect. Additionally, according to the dependence of mu(jt) with temperature and pressure, the refrigeration performance of the cryocooler under different working conditions was simulated. The results show that the heat transfer in the throttle can enhance the throttling effect. When the inlet temperature is 285.0 K and the inlet pressure increases to 18.00 MPa, the N-2 temperature at the cold end reaches 116.3 K, and the gross cooling capacity is 4.55 W. When the inlet pressure is 6.00 MPa and the inlet temperature reduces to 195.0 K, the temperature at the cold end can reach to the saturated temperature of 103.2 K, and the maximum gross cooling capacity is 7.68 W. For the two groups of working conditions, the heat transfer in the throttle can enhance the J-T effect. eta(jt) reaches 95.87% and 94.76%, the proportion of temperature drop caused by J-T effect is 56.10% and 68.03% of the total temperature drops, respectively. Finally, the variation laws of inlet temperature and eta(jt) are analyzed when N-2 reaches saturated temperature under different pressures to guide the selection of inlet parameters.
查看更多>>摘要：Flash boiling atomization has been considered a promising approach to enhance spray atomization. Increasing the fuel temperature or reducing the ambient pressure can promote the phase-change of the spray, and boiling can be achieved during this process, so that spray atomization can be enhanced. However, the strong evaporative feature of flash boiling atomization induced a high vapor concentration in the plume thus changes spray characteristics. To understand the impacts of the vapor phase on spray properties in the aspects of radial expansion and drop sizing, this investigation is carried out based on a customized single-hole fuel injector with practical fuel injection settings. High-speed backlit imaging and Phase Doppler Interferometry are used to quantify the characteristics of flash boiling sprays under various boundary conditions. Macroscopic and microscopic spray morphologies were captured and it was shown that the geometry of the flash boiling spray plume was significantly impacted by the expansion of the compressed vapor phase. The evaporation of the droplets was studied by the single droplet evaporation model and the results were compared against drop sizing measurements. It was found that the evaporation and elimination of liquid droplets might affect the statistical interpretation of the drop sizing results, which is a notable feature for flash boiling sprays.
查看更多>>摘要：Elastocaloric cooling represents a potentially higher efficiency cooling technique that is also potentially more environmentally friendly than conventional technologies. Elastocaloric devices have now been investigated for some time for this purpose, with materials that are Cu-based, Fe-based or Ni-based. These devices are highly promising, but their main drawback is the lifetime of the devices. A solution could be to activate the devices by compression instead of tension. An active elastocaloric regenerator operating under compression composed of eight NiTi tubes was built and tested in a flow system to evaluate its performance and lifetime. Two regenerator configurations were tested, one with only the NiTi tubes in the regenerator and one with the addition of a flow distributor to improve the thermal exchange between the tubes and the heat transfer fluid. The flow system used was also optimized and modified to reach the best operating conditions. A maximum temperature span of 5 K could be reached by this regenerator and a temperature span of 0.4 K for a cooling power of 1071 W kg- 1. Moreover, under compressive cycling, the regenerator could withstand more than 100 000 cycles without failure.
NSTL
Elsevier