查看更多>>摘要：Automotive air-conditioning (AAC) system performance must be improved to reduce energy consumption and promote energy efficiency. Efficiency of the AAC system can be increased by applying the right lubricants. With a combination of different metal oxide components and composition ratios, composite nanolubricants are expected to outperform single-component nanolubricants in improving AAC system performance. The present work was undertaken to investigate the performance of the AAC system using combination of Al2O3-SiO2/PAG composite nanolubricants with composition ratio of 60:40 for 0.005 to 0.06% volume concentrations. The experiment was undertaken with an initial refrigerant charge of between 95 and 155 g and a compressor speed of between 900 and 2100 rpm. The efficiency of the AAC system was assessed by the determination of cooling capacity, compressor work, coefficient of performance (COP) and power consumption. The highest average COP enhancement was recorded 28.10% at 0.015% volume concentration. Al2O3-SiO2/PAG composite nanolubricants attained the highest COP value of 9.19 for 155g at compressor speed of 900 rpm. In addition, for 0.015% Al2O3-SiO2/PAG composite nanolubricants, the cooling capacity was enhanced up to 65.21% while the compressor work and power consumption were reduced up to 25.26% and 19.70% respectively. Therefore, 0.015% Al2O3-SiO2/PAG nanolubricants was highly recommended for the maximum efficiency in AAC system performance.
查看更多>>摘要：Microchannel evaporator cooling systems are widely studied for their ability to remove high heat fluxes. However, flow instabilities during microchannel boiling, like pressure drop oscillation and flow maldistribution, can cause an uneven temperature variation and deteriorate the cooling performance. This study analyzes the effect of using an oscillatory valve opening on the flow distribution and channel temperatures across a two-channel evaporator. Experiments and prediction models indicate that a specific range of valve oscillation amplitude and frequency may cause synchronization resulting in balancing the flow distribution and temperature across the channels, even when they are thermally isolated from each other. Analysis of system performance at different conditions indicates that larger oscillation amplitudes in the valve opening may allow a wider frequency range for temperature synchronization. For a fixed valve opening amplitude, using either significantly small or large frequencies may not be helpful. Moreover, for small amplitudes in valve opening, synchronization may never occur for any frequency. These observations lead us to believe that deliberate oscillations in the valve opening can serve as an active control strategy to mitigate flow maldistribution in parallel channels, suppress temperature oscillation, and improve the overall cooling performance.
查看更多>>摘要：The increasing use of light-emitting diodes (LEDs) requires advances in heat dissipation. Previous applications of heat pipes for LED cooling faced the challenges of contact thermal resistance and insufficient heat transfer areas. In this study, a novel heat sink based on a mini-heat pipe array (MHPA) is proposed, which can overcome these challenges. The thermal performance of the heat sink was experimentally tested. The substrate temperature can be reduced below 70 °C when the input power is 100 W. MHPA shows good temperature uniformity. When the input power is 100 W and 200 W, the maximum temperature drops of MHPA in the vertical direction are 0.6 °C and 1.1 °C, respectively. A numerical model was established and validated. The effect of the heat sink structure was investigated using numerical methods. A higher fin height and more cross-cuts can enhance heat transfer. When the fin pitch is less than 10 mm, increasing the pitch is beneficial to heat transfer. Appropriate use of variable fins can enhance heat transfer. Orthogonal tests are designed to find the optimal heat sink structure. The optimal structure has a fin pitch of 8 mm, a fin height of 43 mm, a fin thickness of 0.5 mm, 40% of the variable fins, and 6 cross-cuts.
查看更多>>摘要：Layering magnetocaloric materials with different transition temperatures is an effective way to improve the cooling performance of magnetic cooling systems. The optimum layering designs, however, remain unclear. This paper developed a one-dimensional transient model to investigate the cooling performance of the multi-layered active magnetic regenerator filled with the first-order magnetic phase transition materials. In addition to the type of the materials, the number of layers and the transition temperature interval between layers are crucial parameters that influence the cooling performance of the magnetic cooling system. Moreover, the length proportion of each layer is investigated where traditional wisdom fails to determine the optimum length proportion. A new graphical tool is proposed to properly size the length proportion of the multi-layered regenerator. With the recommended type of magnetocaloric materials, the number of layers, transition temperature intervals, and length proportions, this study could promote the development of future multi-layered active magnetocaloric regenerators.
查看更多>>摘要：Oxy-fuel combustion with H2O as diluent is regarded as one of the most promising technologies to realize the carbon capture process in power plants. The phase state of H2O has a significant influence on system performance. In this paper, a novel oxy-fuel combustion system with the utilization of liquefied natural gas (LNG) cold energy is proposed. The effect of H2O phase states for improving potential and economy of the system is quantified using the exergy and exergoeconomic analysis methods. Results show that an O2/water system operates better than an O2/steam system, achieving an overall exergy efficiency of 36.35%, which is 4.7% higher than that of the latter. In the O2/water system, 11.99% of the exergy destruction can be avoided; this is, considerably higher than the 8.79% avoidable exergy destruction in the O2/steam system. In terms of economic feasibility, the total investment in the O2/water system is 1.86×105$/h, which is distinctly lower than the 3.47×105$/h investment in the O2/steam system. Moreover, the avoidable total cost rate in the O2/water system exceeds 53.36%, which is significantly higher than the 33.61% proportion in the O2/steam system. These results reveal that water is more suitable than steam for moderating the combustion temperature in oxy-fuel systems in terms of economic feasibility and exergy efficiency. This study is anticipated to have a positive influence on the global transition to a low-carbon future.
查看更多>>摘要：Homogeneous charge compression ignition (HCCI), also known as Controlled Auto-Ignition (CAI), enables combustion with high fuel conversion efficiency and low NOX emissions. It can benefit both spark and compression ignition engines and lessens the demand for expensive exhaust after-treatment systems. However, the HCCI application is restricted due to a limited operating range. Water injection can tackle this problem, which enables higher loads without excessive heat release rates. Although already reported in spark-ignited engines, water injection is still a novelty for HCCI combustion. The water injection strategy is not fully developed for HCCI engines, especially fueled with renewable fuels and using different methods to achieve CAI conditions. Hence, this paper studied the performance, combustion, and emissions characteristics of an ethanol HCCI engine using water injection and total exhaust gas recycling from an adjacent diesel cylinder. HCCI combustion was attained via direct recirculation of exhaust gases from a diesel cylinder, serving as a heat and active radical source for the ethanol charge auto-ignition. Water injection was used for combustion phasing control and load expansion. Detailed combustion and heat release analyses were performed. During the experiments, combustion phasing (CA50) ranged from 2 to 8 crank angle degrees (CAD) ATDC. Water injection reduced the heat release rates and prevented knock, keeping the in-cylinder temperatures below 1850 K. The results indicated that water injection effectively controlled combustion phasing and heat release rate, increasing the maximum operational load threefold. The combustion duration increased with water injection, varying from 6.6 CAD at high loads to 17.0 CAD at low loads. The indicated conversion efficiency was 27% at low loads and achieved the maximum of 39% at 6 bar of indicated mean effective pressure (IMEP). The NOX emissions formed in ethanol HCCI combustion were lower than 0.8 g/kWh.
查看更多>>摘要：In this paper, an improved thermodynamic model for the design of a small two-phase ejector is proposed by using the constant-pressure mixing modeling method. Particularly, the mixing process modeling is associated with the area variation of the mixing section, where a corresponding area ratio of the mixing section entrance to exit is introduced. Meanwhile, the mixing pressure is determined by solving model equations under given operating conditions. Furthermore, the effects of the mixing area ratio, operating conditions, ejector components efficiencies on the ejector performance and geometry parameters are analyzed by the proposed model. The results show that the entrainment ratios increase from 0.47 to 1.14 as mixing area ratios range from 1.0 to 1.2 under the given conditions. The mixing area ratio has significant influences on the ejector performance at lower pressure lift ratio, higher primary fluid temperature and lower secondary fluid temperature. It can be concluded that the entrainment ratio, mixing pressure and mixing velocity of ejector could be improved with the mixing area ratio increasing. And it is further confirmed that the constant-pressure mixing ejector provides better performance than the constant-area mixing ejector under design operating conditions. The research results can provide theoretical guidance about the ejector design and performance predictions.
查看更多>>摘要：This study proposed a ground source heat pump system using capillary networks as the front-end heat exchanger, to alleviate thermal pollution and utilize waste heat generated in subway tunnels and the surrounding rock. We conducted onsite measurement of the capillary ground source heat pump system based on a real engineering project involving a subway tunnel. Subsequently, the heat transfer performance of the capillary heat exchanger and energy efficiency of the system were analyzed. The heat transfer capacity of the capillary heat exchanger was approximately 45–80 and 100–110 W/m2 in a test with a duration of 7 h in winter and a test with a duration of 14 h in summer, respectively. The average coefficient of performance of the heat pump unit and coefficient of performance of the system were 3.72 and 3.18, respectively, in winters, and 3.19 and 2.64, respectively, in summers. Under a 4-day test with an intermittent operation of 8 h per day in summer, the performance of the system was improved and remained stable for four days during the test. The average coefficient of performance of the heat pump unit and of the system for intermittent operation were 3.67 and 2.96, respectively, which were larger than those for 14 h continuous operation in summer. In practical applications, intermittent operation or additional auxiliary cooling sources are recommended in summer conditions.
查看更多>>摘要：Numerical modeling of solar thermal storage systems is often challenged with limitations on the computational effort due to their transient non-linear behavior that dictates accurate modeling of the physics over long-term operations (i.e., annual). Such challenges result into scarcity of literature on comprehensive design guidelines for solar thermal storage systems. This study presents a framework through which the potential of artificial neural network (ANN) modeling of a hybrid solar thermal storage system involving phase change materials is extensively investigated. An experimentally validated numerical model for the system is used to generate the training and testing datasets for the ANN model. The effect of changing the sampling method and the number of training samples is studied on the ANN model prediction. The results show that Sobol sequence sampling is superior to other sampling methods especially for low number of samples. The best sampling method is utilized to generate the training dataset with which the hyperparameters of the learning algorithm are optimized. The optimized ANN model is ultimately used to predict the system solar fraction under various design conditions to develop design maps that offer better visualization and sizing guidelines for the hybrid solar thermal storage systems. ANN is shown to offer a potential candidate for accurate and computationally efficient modeling of complex thermal systems. Upon proper configuration and training, the ANN model can accurately (i.e., a coefficient of determination of 0.9999) predict the performance of the hybrid thermal storage system with approximately five orders of magnitude reduction in computational time compared to conventional numerical models.
查看更多>>摘要：Methane/oxygen rocket engine is considered to be one of the most promising reusable rocket engines in future space activities. Adequate understanding and accurate prediction of heat transfer characteristics are considered key points for the development of reliable methane engines. In this paper, a methane combustion chamber with 7-elements is simulated using Reynolds averaged Navier-Stokes (RANS) method with Eddy Dissipation Concept (EDC) combustion model. The investigation reveals that the near-wall coupling effects of flow and chemistry have a significant influence on the wall heat load, and the coupled wall function developed by direct numerical simulation (DNS) is modified, validated, and incorporated in the RANS frame to consider the aforementioned coupling effects. The results show that the deviation of the wall heat load compared to experimental data is reduced from 25% to 5% for the wall of high temperature when chemistry effects are considered. The influence of the coupled wall function is limited near the wall and the properties of main flow are generally independent of the wall models adopted. The investigation also reveals that the turbulent flux of chemical enthalpy near the wall is comparable to the turbulent flux of sensible enthalpy in case of a methane combustion chamber. Finally, the effects of chemistry on the wall heat flux can be attributed to the coupling impacts of the chemical equilibrium shifting caused by the large temperature gradient near the wall and the non-uniform radial velocity brought by the powerful vortex system in the chamber.
NSTL
Elsevier