查看更多>>摘要：The present work focuses mainly on the effects of heat input, filling ratio, inclination angle, tube diameter and coolant temperature on the thermal performance of a wraparound heat pipe charged with R134a. Results show that thermal resistance decreases with the increase of heat input when the filling ratio is larger than 40%. An optimal filling ratio for the heat pipe with the best performance exists between 50% and 60%. The pressure of working fluid in the heat pipe exceeds 1.6 MPa in the 70% and 80% filling ratios experiments. For larger inclination angles (theta > 10 degrees), the thermal resistance decreases with increasing the heat input and finally tends to a stable value. For heat loads of 420 W and greater, the values of thermal resistance are 0.056, 0.07, 0.034 and 0.027 K/W for outer diameters of 8, 10, 12 and 16 mm, respectively. No significant difference in thermal resistance at different coolant temperatures is observed for heat inputs greater than 300 W. In all experiments, for a 22 degrees inclination angle, an outer diameter of 16 mm, and a filling ratio of 50%, the best performance of heat pipe is observed and the lowest value of thermal resistance is 0.027 K/W.
查看更多>>摘要：In this research, thermal resistance of the modified inflated aluminium plate that features a closed thermosiphon under natural convection was tested. HFE-7000 dielectric fluid and graphene nanofluid were used as the working fluids for different filling ratios (30%, 50%, and 70%), graphene concentrations (0.3 wt%, 0.5 wt%, 1 wt%), and supplied powers (30 W, 60 W, 90 W). The effect of uniform and non-uniform heat source on the thermal resistance are examined and a separate flow visualization is also conducted to understand the boiling phenomenon. It is found that a 70% filling ratio offers the best heat transfer performance. At a supplied power of 90 W and filing ratio of 70%, the thermal resistance is reduced by around 4% relative to the filling ratio of 30% for the same supplied power of 90 W. In terms of input power, the decrease in thermal resistance was found as the power increases. The uniform heat source has a better heat transfer performance than the non-uniform one. From the visualization experiment, appreciable graphene was entrained from evaporator to condenser due to boiling especially at a higher concentration of 1%, causing the blockage and raising the thermal resistance. Therefore, the best concentration of graphene should be kept to below 0.5 wt% in which the size of the bubbles was sufficiently large enough to facilitate latent heat transfer effectively for supplied powers of 60 and 90 W.
查看更多>>摘要：ABSTR A C T Miller cycle has been gradually applied on marine low-speed engines to reduce NOx emission, due to its convenient implement with lower cost. However, current studies about relevant effects, influence mechanism and turbocharger re-match all have neglected the coupling relationship between engine and turbocharger, which will have non-negligible impact on final results. Therefore, based on this relationship, the influence of Miller cycle is studied through a numerical simulation. Then, a thermodynamic analysis is conducted to summarize the relevant impact mechanism and provide guidance for turbocharger re-match. Numerical results show that the in-cylinder charger temperature is actually reduced, but the combustion duration is prolonged. Hence, the exhaust gas temperature is increased, causing compressor operating points moving to the direction with higher pressure ratio and mass flow rate. Furthermore, from the thermodynamic analysis, the fundamental reason of this phe-nomenon is the uniflow scavenging adopted by marine low-speed engine: for this scheme, the more the Miller cycle is applied, the more the mass will flow across the cylinder. For compensating the losses caused by Miller cycle, achieving the original cylinder compression pressure is an effective method, hence a higher boost pressure is needed and the turbocharger should be re-matched. Finally, based on the thermodynamic analysis, the demand of Miller cycle for turbocharger is summarized and the relevant guidance for turbocharger re-match is proposed: the relationship between boost pressure and Miller cycle should follow the original poly-tropic process in cyl-inder; the high efficiency area of compressor should include the operating point with maximum Miller cycle, where the pressure ratio and the mass flow rate are decided by the maximum Miller cycle and the index of original poly-tropic process.
查看更多>>摘要：The organic Rankine cycle (ORC) is an efficient power generation technology that has been widely used in renewable energy utilization and industrial waste heat recovery. The thermal stability of working fluids has a significant effect on the fluid selection and system design of ORC systems. In this study, an off-design model of an ORC system was established with hexamethyldisiloxane (MM) as the working fluid, and the effects of the MM thermal stability on the system were analyzed. The results showed that the effect of MM thermal decomposition on the cold source and working fluid pump was limited. The outlet temperature of the evaporator decreased with MM decomposition, which might lead to incomplete evaporation of working fluids and possible damage to the expander. The outlet temperature of the heat source also decreased with MM decomposition, which led to lower outlet temperatures than the acid dew point limit temperature. Both of these results can affect the safe operation of ORC systems. The net power and thermal efficiency of the system decreased with increasing thermal decomposition ratios of MM. The net power and thermal efficiency decreased by 7.48% and 10.72% respectively in the model of this study with a 10% decomposition ratio.
查看更多>>摘要：Transcritical CO2 air-source heat pump has been widely applied for its high efficiency in producing domestic hot water. However, it preforms poorly for space heating specially in cold region, because of the high return water temperature and single-phase heat transfer in gas cooler. To resolve this issue, a modified integrated CO2 heat pump system is proposed, which can operate in heating-water combined mode and space heating mode with radiator followed by fan-coil unit in space heating circuit. To improve its thermal comfort and reduce economic cost simultaneously, multi-objective optimization for maximizing space-heating energy and minimizing total annual cost is conducted, based on a new optimizing model, considering two modes simultaneously as a whole and employing new concept of artificial neural network group. The best individual in Pareto front is selected, with 6109.2$/year cost and 5.87 x 10(8)J heating energy. Corresponding optimal geometric parameters of components are obtained as follows, 0.378 m(2) high-temperature gas cooler transfer area, 0.468 m(2) low-temperature gas cooler heat transfer area, and 0.0015 m(3)/s compressor displacement. Moreover, effects of important factors (electric fee, water demand, and ambient temperature) on system performance are also investigated. This work presents a new optimizing model and reveals the excellent application potential of the new system.
查看更多>>摘要：In this paper, a new approach to calculate the transfer functions (g-functions) for simulating the thermal performance of large diameter, shallow bore helical Ground Heat Exchangers (He-GHE) is proposed using mean fluid temperature rather than borewall temperature. The g-functions are generated using a validated numerical Capacitance Resistance Model-Helical GHE (CaRM-He) for different bore diameters, bore depth and helical pipe pitch. Mathematical formulation is presented which allows calculation of the combined g-function for multiple bores using the g-functions for individual cases. A simplified resistance-based model which enables the calculation of traditional borewall temperature-based g-functions using the mean fluid g-functions for different mass flowrates is also presented. Finally, mean fluid temperature is calculated for an array of eight He-GHEs for Sacramento climate zone using (a) CaRM-He model, (b) mean fluid temperature-based g-function and (c) borewall temperature-based g-functions for maximum (0.128 kg s(-1)), reference (0.063 kg s(-1)) and minimum (0.057 kg s(-1)) mass flowrates. For the reference mass flowrate case, the predicted mean fluid temperature Root Mean Square Deviation (RMSD) between the CaRM-He simulation and the g-function approaches is less than 5% and 6% of the yearly average temperature difference between the inlet and outlet for the mean fluid and borewall based g-functions respectively. For the other mass flowrates, the RMSD in mean fluid temperature varies between 9% and 17% of the yearly average temperature difference between the inlet and outlet temperature. Overall, the proposed g-function approach can be used effectively to estimate the performance of large diameter helical GHEs.
查看更多>>摘要：Albeit the exhaust gas recirculation (EGR) is widely used to reduce the nitrogen oxides (NOx) emissions from large marine two-stroke engines, several challenges emerge for the engine-turbocharging system matching considering the contradictory requirements of the engine and its subsystems operation. Such challenges become more pronounced in complex engine configurations that include parallel turbochargers and the EGR system along with cut out and bypass branches. This study aims at parametrically investigating a large marine twostroke engine equipped with an EGR system, two parallel turbochargers of different size, and cut out branches. The turbocharging system characteristics are selected targeting the minimisation of the engine specific fuel consumption whilst ensuring compliance with the respective NOx emissions limits and satisfying imposed constraints for the compressors operation. A detailed model of the zero/one dimensional type is developed in the GT-SUITE software and used to simulate the investigated engine along with its subsystems. Simulation runs are performed to investigate the engine with four different turbocharger configurations of varying capacity ratio and under various operating conditions in terms of the EGR rate and engine load. The simulation results are analysed to reveal the impact of the turbocharger selection of the engine performance and emissions parameters. Furthermore, modulation schemes with EGR blower speed control, exhaust gas bypass and cylinder bypass are investigated to overcome the mismatch on the engine components flow rates and avoid turbocharger operational issues. The derived results demonstrate that the lowest weighted BSFC is achieved for the case of 70:30 capacity ratio between the large and small turbochargers, whilst the engine operation with the EGR is associated with a 2.6% penalty in the weighted BSFC. The EGR blower speed control is found sufficient to avoid the compressor overspeed at high engine loads exhibiting the lower BSFC penalty, whereas the cylinder bypass control is appropriate for controlling the compressor speed at low engine loads. This study contributes on delineating the underlying parameters and interactions between the engine components for the investigated marine two-stroke engine and provides recommendations for the engine-turbocharging system matching procedure.
查看更多>>摘要：Adsorption-based atmospheric water harvesting has received significant interest owing to its promise of decentralized water supply and a wide applicability. In this study, the adsorbent bed is specially designed and optimized for efficient moisture capture. A transient three-dimensional non-equilibrium model has been developed that takes both internal and external mass transfer resistances into account. It is found that the simulation results agree better with the experimental results, which indicates the reliability of the model. Then this model was applied to investigate the influence of the adsorbent bed structure, and the operating parameters on the transient distributions of the adsorbent temperature, adsorption kinetics, dynamic up-taken moisture capture, mass transfer resistance and relative capture efficiency of the adsorbent bed. ASLi30 (Activated carbon fiber + Silica gel + 30 wt% LiCl)-vapor is selected as the working pair. It has been found that generally, the effects of the air channel aspect ratio on the investigated indexes are negligibly small. A thinner layer thickness and a smaller air channel length characterize an excellent moisture capture, small vapor transport resistance and faster adsorption kinetics. By considering different parametric variations for performance optimization and the effect of operating conditions, these results provide important insights and design guidelines of adsorbent bed for efficient moisture capture.
查看更多>>摘要：Supercritical CO2 Brayton cycle (SCBC) is widely used in high-temperature waste heat recovery combined systems with sequential or cascade configurations due to its compact structure and high efficiency. However, since the heat source conditions of these systems are not clearly defined, there is no practicable design approach to comprehensively adapt the waste heat with different temperatures, especially for the ships. This paper aims to propose an effective thermodynamic configuration method to facilitate the design and application of recuperative SCBC combined systems for the high-temperature waste heat recovery of marine engines. To this end, the factors that may affect the system configuration are investigated, and it reveals that the recuperator effectiveness can not only influence its own performance but also determine the heat source conditions, which will finally affect the performance of the entire system. Therefore, a new system configuration method is proposed based on the recuperative effects, and a novel combined system is designed as a case study for further illustration and multi-objective optimization. The results indicate that an optimal value of the recuperator effectiveness exists in the preliminarily determined range, 0.56-0.8. Under the system optimal operating conditions, the total energy output and electricity production cost are 538.97 kW and 5.34 cent/kWh, respectively, and the corresponding thermal and exergy efficiencies research up to 33.17 % and 61.93 %, respectively. It proves that the configuration method proposed in this paper can realize an efficient design of recuperative SCBC combined system, and provide a reference for other relevant systems.
查看更多>>摘要：Low-temperature preheating to achieve effective thermal management for lithium-ion batteries is a crucial enabler for the efficient and safe operation of electric vehicles in cold conditions. Effective heating is yet challenging due to its implementation complexity and a tricky balance of the heating performance. Here, we develop a lightweight compound self-heating system involving two external light aluminum heaters, which recycle the discharge energy contributing to external heating. Basic electrical and thermal modeling for the compound selfheating system is performed and experimentally validated. We adopt four key but conflicting heating metrics: heating time, heating efficiency, battery degradation, and temperature uniformity, to optimize the resistance of external heaters with the adaptive particle swarm optimization method. We thus propose a rapid compound selfheating strategy that can conveniently warm the battery up with 32.49 degrees Cmin(-1). Experimental results under different states-of-charge and temperatures confirm the good adaptability of the proposed heating strategy. Comparison experiments with the unheated battery demonstrate the proposed heating strategy improves discharge power, charge power, and discharge energy by over 7.4 times, 19.0 times, and 109.9%, respectively. With the optimal external aluminum heaters, battery available discharge energy is enhanced by above 70.4%, implying a huge step forward to boost battery performance.
NSTL
Elsevier