查看更多>>摘要：The circulating fluidized bed (CFB) boiler is highly recommended in peak shaving for its wide load regulating range compared with the pulverized coal (PC) boiler. However, the research on dynamic flowsheet model of the CFB boiler is quite limited due to its complex flow regime. In this paper, both steady state and dynamic flowsheet model of a supercritical 350 MWe CFB boiler was developed based on Aspen Plus and Aspen Dynamics software coupled with comprehensively user defined combustion kinetics and heat transfer coefficient with FORTRAN subroutines. This model was then validated by the operating data of the 350 MWe supercritical CFB boiler. Based on this model, the dynamic characteristics of three typical step signal disturbances, coal flow rate, primary flow rate and secondary air flow rate, were further analyzed to provide theoretical references to dynamic operation. Also, the quantitative relation between the stabilization time and step signal amplitude was calculated. The calculated dynamic characteristics demonstrates that the heat distribution does not vary much after adding step signal disturbances of coal flow rate, primary air flow rate and secondary air flow rate. The step signal disturbance of secondary air flow rate has less influence to the CFB boiler system compared with that of primary air flow rate. The superheated steam temperature is more sensitive to step signal disturbances compared with reheated steam temperature. The higher step signal amplitude results in longer stabilization time. Above all, this dynamic flowsheet model can not only be used to conduct dynamic behavior prediction and analysis but also provide transfer functions to the further development of CFB automatic control system.
查看更多>>摘要：Gasoline evaporation from vehicle fuel tanks is a major source of VOCs, that represents a serious threat for both human health and environment. To limit these emissions, the most adopted strategy is to use a carbon canister filter to store fuel vapors and then burn them inside the engine along with the fresh charge. However, the canister saturation level is usually unknown, and it can easily reach the full saturation, especially in particular conditions. As hybrid vehicles are becoming more popular in the market, this issue becomes even more important, since canister purging phase has even less time to be performed. In this activity, a 1D transient, non-isothermal, non-adiabatic model has been developed to specifically simulate the carbon canister filter of a common EVAP system and analyze its behavior during fuel vapor adsorption, with particular attention to the mutual influence of carbons adsorption performance and temperature variation. The model presented is based on an adsorption isotherm derived from the potential theory of adsorption, and it has been developed to be usable with only few canister properties. A system of two coupled PDEs has been coded and solved in MATLAB (R) environment, and results have been compared to experimental data in terms of mass and internal temperature variation on a common European canister, found in a previous study, with a standard loading flow rate of n-butane mixture and standard environmental temperature. The model has then been analyzed by using the Design For Six Sigma (DFSS) method, which has allowed a good optimization on calibration parameters with a relatively low amount of tests. Results show a significant increment in the predictive capabilities of the optimized model (up to 90% of the experimental value), with respect to the first simulations. This model helps to understand and evaluate the influence of environmental temperature on the canister filter performance, and can be used for both canister design and control of the purging strategies performed by an on-road vehicle.
查看更多>>摘要：Significant efforts are currently underway to transform the transportation industry from a fossil fuel-based industry to a hydrogen-based industry to achieve the goal of zero carbon emissions. In this study, hydrogen direct injection (DI) is implemented using three mixture formation modes: homogeneous charge, lean-homogeneous charge, and lean-stratified charge (LSC). The main objective is understanding the effect of the hydrogen mixture mode on the efficiency and emission characteristics of the hydrogen DI engine. Accordingly, hydrogen was used as the fuel in a spray-guided single-cylinder research engine. The results revealed that owing to the high heat loss characteristics of hydrogen, the optimized combustion phasing angle was retarded. The LSC mode minimized heat transfer loss by reducing the high-temperature area near the cold cylinder wall. Furthermore, it had the highest indicated thermal efficiency (ITE) of 34.09 %, especially under low load conditions. However, the stratified rich hydrogen in the LSC mode resulted in high nitrogen oxide emissions (6.68 g/kWh). Heat management is vital to efficiently extract energy from hydrogen in an internal combustion engine. Heat loss reduction (13 %) contributes more to high ITE than pumping loss improvement (2 %) in the LSC mode.
查看更多>>摘要：Aimed at learning the variation law of injection pressure with coolant mass flux and promoting the active control of phase-change transpiration cooling, this study proposes a new transient mathematical model by simplifying the two-phase mixture model (TPMM) with the introduction of modified mixture enthalpy and enthalpy ratio. The smooth change with time of mixture enthalpy and enthalpy ratio improves the convergence and stability of TPMM. Experiment is first carried out to verify the accuracy of the new model. Then the two-phase flow and corresponding fluid-structure coupling heat transfer processes of transpiration cooling are studied with the new model under both steady and transient conditions. Besides, the relationship link from coolant adjustment, corresponding pressure and mass flux to the cooled wall temperature is discussed. The steady results show that injection pressure is closely related with phase change location, and the huge change of fluid kinematic viscosity and pumping effect of capillary force induces the non-linear variation of injection pressure with coolant mass flux. The transient results reveal that injection pressure changes with different laws when the mass flux increases and decreases. The changing speed of mass flux and scale of external heat flux also have a vital influence on the fluctuation of injection pressure, the faster the mass flow rate changes and the greater the heat flux is, more tempestuously the injection pressure varies.
Shelke, Ashish, VBuston, Jonathan E. H.Gill, JasonHoward, Daniel...
13页
查看更多>>摘要：Combined numerical and experimental studies are conducted to characterise 21,700 cylindrical lithium-ion battery (LIB) thermal runaway (TR) induced by nail penetration. Both radial and axial penetrations are considered for 4.8 Ah 21,700 NMC cell under 100% state of charge. Heat generation from the decomposition of the cell component materials are analysed. The maximum cell surface temperature rise and time to reach it in both types of penetration tests are compared. Snapshots from the video footages captured by three high definition and one high speed cameras shade light on the dynamic processes of spark ejection and flame evolution. A generic predictive tool is developed within the frame of the in-house version of open-source computational fluid dynamics code OpenFOAM for nail induced TR. The code treats the cell as a lumped block with anisotropic thermal conductivities and considers heat generation due to nail induced internal short circuit resistance, exothermic decomposition reactions and heat dissipation through convective and radiative heat transfer. Validation with the current measurements shows promising agreement. The predictions also provide insight on the magnitudes of heat generation due to internal short circuit resistance, decompositions of solid electrolyte interphase layer (SEI), anode, cathode and electrolyte. Parametric studies further quantify the effects of cell internal short circuit resistance, contact resistance between the nail and cell, convective heat transfer coefficient and cell surface emissivity on TR evolution.
查看更多>>摘要：Solar energy storage via a thermochemical approach is a promising method to realize the efficient utilization of discontinuous sunlight. Traditional solar thermochemical conversion with the assistant of hydrocarbon requires the purification process of products, and a relatively high reaction temperature limits its thermodynamic efficiency. In this work, a thermodynamic study on solar-driven methanol steam reforming reaction in a Pd-Ag membrane reactor has been conducted. The partial pressure, conversion rate, and thermodynamic efficiency are studied and analyzed under different reaction temperatures (150-250 C) and permeate pressures (10-3-1 bar). Via the membrane reactor, the equilibrium of reaction shifts forward and the conversion rate of methanol can reach as high as above 99.9% in 150-250 C, and purified hydrogen and carbon dioxide can be collected separately. Under the optimized reaction temperature and pressure, the solar-to-fuel efficiency and exergy efficiency can reach as high as 55.2% and 74.79%, respectively. Due to the utilization of solar energy and membrane reactor, the annual coal saving rate and carbon dioxide reduction rate are predicted to be 0.63 t/m2 and 1.53 t/m2, respectively. This thermodynamic research provides an efficient approach for solar energy conversion and storage without CO2 emission.
查看更多>>摘要：The recompression supercritical carbon dioxide Brayton cycle is one of the most promising candidate power blocks for the solar power tower plant. As an important component, the main compressor can be driven by an electric motor or a turbine. The layout with the turbine-driven main compressor has the advantages of low cost, high efficiency, and safety during loss-of-load conditions. For such a system, the main compressor and the turbines are connected with the synchronous generator and assumed to be operated with a constant shaft speed, which has a big challenge for the operation under off-design conditions with various ambient temperatures, and the corresponding optimal operation parameters and control schemes are lacking. Especially in the condition of low ambient temperature, the traditional control scheme, which usually maintains the main compressor inlet temperature at the design-point value, obviously underestimates the efficiency of the solar power tower system. In the present study, an integrated model of solar power tower coupling with the Brayton power cycle is developed, and the particle swarm optimization algorithm is utilized to search for the optimal operation parameters and control schemes under off-design conditions with various ambient temperatures. Finally, annual performance analyses are conducted to investigate the actual effects of the proposed control schemes. The results indicate that: in the case with low ambient temperature, the proposed control scheme improves the power cycle efficiency by 0.78%; the total power output of 390 MW.h (accounting for 1.02% of the yearly power output) can be saved. In the case with high ambient temperature, the power cycle efficiency experiences an inevitable decrease, which causes the total wasted power output of 253 MW.h (accounting for 0.66% of the yearly power output); the losses caused by high ambient temperature are restrained in a small range by the proposed control schemes.
查看更多>>摘要：Solid particles are often added to heat transfer fluids to improve their thermal conductance. Such dispersions are characterized by measurement of the zeta potentials in numerous scientific papers. This short review discusses common misconceptions in these publications. The examples presented here refer to the fluids based on ethylene glycol and its mixtures with water, but the same misconceptions may appear in studies involving other nonaqueous solvents and their mixtures with water.
查看更多>>摘要：In this paper, a cascaded lattice Boltzmann (CLB) method for investigating solid-liquid phase-change heat transfer is developed. In the CLB method, the enthalpy methodology is adopted to capture the solid-liquid phase interface implicitly. Mesoscopically, the enthalpy is defined as the basic evolution variable of the CLB equation of the temperature field so as to consider the phase-change effect in simulations without iteration procedure, and to achieve this purpose, the cascaded collision process is artfully designed by modifying the shifting procedure. Numerical results demonstrate that the enthalpy-based CLB method can serve as an accurate and reliable numerical tool for simulating solid-liquid phase-change heat transfer.
查看更多>>摘要：A novel concept of the turbo-piston combined cycle (TPCC) engine was proposed to perfectly satisfy the complex and multiple performance requirements of many heavy vehicles. It has a unique function of free mode (cycle) switching between three operating modes, which are the diesel engine (DE), the turboshaft engine (TE), and the DE-TE cyclic combined modes. The cyclic coupling between the baseline DE and TE cycles in the combined mode was supposed to help improve the performances of the two baseline engines simultaneously, and prototype experiments confirmed the feasibility and effects of this assumption. However, the cyclic coupling mechanisms in the combined cycle that are crucial for the R & D of the TPCC engine remain unclear. This study mainly focuses on investigating the fundamentals of the cyclic coupling mechanisms and impacts in the combined cycle of the TPCC engine via theoretical (ideal) thermodynamic cycle modeling analysis and one-dimensional (1-D) cycle modeling simulation. The results mainly show that from theoretical thermodynamic analysis, the TPCC engine's cyclic coupling can significantly improve the specific net work of the combined cycle and even exceed the sum of the isolated DE and TE cycles, and only the temperature increase ratio parameter of the TE cycle can generate a tradeoff influence between the specific net work and the theoretical thermal efficiency of the combined cycle. In addition, from the 1-D cycle simulation, the performance of the baseline DE cycle in the combined cycle is dramatically improved by the baseline TE in terms of its high intake pressure and exhaust pressure conditions, and the operating process of baseline TE is simultaneously improved by the baseline DE in terms of its intake suction and exhaust-mixing effects. Based on certain simulation cases, the intake mass flow rates of the baseline DE and TE (i.e., the total intake mass flow rate of the combined cycle) are respectively increased by 150% and 1.1%, and their power outputs are respectively increased by 8.5% and 3.1%. Further, the brake specific fuel consumption rate (BSFC) of the baseline DE and TE respectively decreases by 8.0% and 2.9%, and the operating point of the baseline TE on the turbomachinery performance maps is slightly moved to the more efficient and higher performance area. This investigation is consistent with experimental results on cyclic coupling effects, and it can provide a timely and preliminarily understanding of the TPCC engine's fundamentals.
NSTL
Elsevier