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Numerical Investigation of Nonlinear Effects in a Standing Wave Thermoacoustic Engine Using the Discontinuous Galerkin Method

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SSRN 2023年

作者： Yao, Chen Liu, Jiping Yan, Junjie State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong University Xi’an710049 China

Understanding the nonlinear effects in thermoacoustic heat engines operating under high-pressure amplitude conditions plays a crucial role in improving their operational performance and efficiency. The discontinuous Galerkin (DG) method has several advantages, including high accuracy, adaptability, efficiency, and the ability to handle the coupling of multiple physical fields, making it suitable for accurately simulating the flow and heat transfer characteristics in thermoacoustic engines. The simulation code was developed based on MFEM, an open-source C++ library for modular finite element methods. The fully compressible Navier-Stokes equations and Fourier heat conduction equation were solved to investigate the nonlinear effects, which cannot be captured by the linear theory. To verify the accuracy of the simulation code, the numerical investigation of the thermoacoustic wave propagations induced by thermal effects in a square enclosure was first conducted. The simulation results presented the details of the generation and propagation of thermoacoustic waves, which were largely consistent with the results obtained using the flux-corrected transport (FCT) algorithm. Second, a quarter-wavelength standing wave thermoacoustic engine was simulated. The simulation results captured the process of pressure wave generation in the thermoacoustic engine, which was consistent with the pressure wave generation process in the enclosure. The results revealed the flow and heat transfer characteristics and nonlinear effects in the thermoacoustic engine under high-pressure amplitudes, which were consistent with the results obtained using finite volume method. The simulation results showed that the DG method is an effective numerical method for simulating thermoacoustic engines. © 2023, The Authors. All rights reserved.

关键词： Navier Stokes equations

Enhancement of Scr Denitrification Control Strategy Considering Fluegas Temperature Fluctuation: Fundamental Principle and Performance Evaluation

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SSRN 2023年

作者： Gao, Wei Yin, Junjie Liu, Ming Zhao, Yongliang Wang, Chaoyang Yan, Junjie State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

Nitrogen oxides (NOx) is one of the main pollutants in coal-fired power plants, and selective catalytic reduction (SCR) is widely applied to reduce NOx emissions. Coal-fired power plants should frequently change their output power to help the power grid to accommodate high penetration of intermittent renewable power. However, NOx emission transient overshoot often occurs in coal-fired power plants when coal-fired power plants dynamically change their output power. In this study, the dynamic models for a coal-fired power plant and SCR denitrification are developed. The fluegas temperature fluctuation is revealed as a key cause for the NOx emission transient overshoot. The change law of SCR reaction temperature during load cycling processes is obtained, and its prediction model based on neural network algorithm is developed. Then, an enhanced control strategy considering temperature fluctuation of SCR inlet fluegas is proposed, and denitrification performance with the original and enhanced control strategies during load cycling processes are compared. The results show that the transient overshoot of NOx outlet concentration is avoided during load cycling processes, and the maximum value of NOx emissions during loading down and loading up processes with the enhanced control strategy are 48.62 and 49.44 mg m−3, which decreases by 7.36 and 4.63 mg m−3 compared with the original control strategy, respectively. Moreover, when the enhanced control strategy is adopted, the cumulative NH3 consumption decreases by 0.48 and 11.84 g m−3 during loading down and loading up processes, respectively. © 2023, The Authors. All rights reserved.

关键词： Coal fired power plant

Experimental Investigation on Heat Transfer of Supercritical Water flowing in a Horizontal Tube with Axially Non-Uniform Heating

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SSRN 2023年

作者： Hou, Zhenghui Huang, Xin Gao, Yu Zhang, Shengjie Wang, Haijun State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

For a heat exchanger, the heat flux is usually non-uniform in the axial direction, and the knowledge about heat transfer of supercritical water with axially non-uniform heating is scarce. To fill the gap, experiments were conducted in a horizontal tube to study the heat transfer characteristics of supercritical water with axially sinusoidal heating. The pressures varied from 23.5 to 26.5 MPa, mass fluxes 200 to 400 kg·m-2·s-1 and average heat fluxes 50 to 150 kW·m-2. Compared with uniform heat flux, the temperature difference between the top and bottom wall is larger than that of the axially sinusoidal heat flux. The inner-wall heat flux is non-uniform along the circumference due to the temperature difference, being higher at the side wall and lower at the top wall. A relative heat flux parameter is defined to describe the relative magnitude of the local heat flux. The relative heat flux parameter of the top generatrix, which is inversely proportional to the average heat flux, decreases linearly with the increase of the temperature difference between the top and side generatrix. The non-uniform degree of heat flux along the circumference is positively correlated with the average heat flux. Additionally, the local heat transfer characteristics should be considered in horizontal tubes. On the one hand, the heat transfer deterioration may occur when the fluid temperature near the top wall is higher than the pseudo-critical temperature and it can be suppressed with increasing Reynolds number. On the other hand, the local heat transfer can be enhanced if the side and bottom wall temperatures exceed the pseudo-critical temperature. © 2023, The Authors. All rights reserved.

关键词： Heat flux

Dynamic Characteristic Analysis of Sco2 Brayton Cycle Under Different Turbine Back Pressure Modes

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SSRN 2023年

作者： Zhao, Quanbin Xu, Jia yuan Hou, Min Zhang, Dalin Chong, Daotong Wang, Jinshi Chen, Weixiong State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

Closed supercritical carbon dioxide Brayton cycle has the advantages of high efficiency and high compactness. The compact structure and completely closed loop with supercritical state makes the dynamic characteristic different from that of steam Rankin cycle. This paper mainly focuses on the dynamic character of SCO2 cycle with different disturbance. Firstly, the dynamic characteristic of isolated heat exchangers is investigated under different disturbances. The response time of HTR and LTR is about 70 seconds, which is much longer than others. Then dynamic characteristics of SCO2 cycle are investigated with different turbine back pressure modes. Under cooled water flow rate disturbance, the operation parameter fluctuation amplitude and the response time with normal back pressure mode are slightly larger than those under fixed back pressure modes, and the effect of pressure iteration is not remarkable when cold water disturbance amplitude is small. But when water flow rate decreases 30%, the discrepancy becomes large and the tendency becomes opposite. Under heat fluoride flow rate disturbance conditions, the parameters iteration in closed loop restrain the non-coordinative of SCO2 pressure and temperatures. © 2023, The Authors. All rights reserved.

关键词： Brayton cycle

CO2 Emission Characteristics Modeling and Low-Carbon Scheduling of Coal-Fired Power Units Under Peak Shaving Conditions

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SSRN 2023年

作者： Yuan, Rongsheng Liu, Ming Chen, Weixiong Yan, Junjie State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

To accommodate the high penetration of renewable power, the operational flexibility of coal-fired power units is highly required by the power system, so coal-fired power units frequently operate under peak shaving conditions. The CO2 emission characteristics of a coal-fired power unit are significantly influenced by the load factor, and also by unit capacity and many other design parameters. Therefore, detailed CO2 emission characteristics of a coal-fired power unit should be modeled and considered in the low-carbon scheduling of coal-fired power units under peak shaving conditions. In this study, coal-fired power units are classified by live steam parameters, unit capacity, reheat stages, cooling mode, and operation mode, and the models of detailed CO2 emission characteristics are developed and validated. An indicator is defined and modeled to evaluate the CO2 emission reduction rate of a coal-fired power unit considering the renewable power accommodation, and an optimization method for low-carbon scheduling is proposed. The proposed optimization method reduces the computational time by ~80%. Finally, a reference case study, including zero, current, and 2030 renewable power accommodation scenarios, is conducted. Results show that the marginal carbon emission of the power system decreases by 123.33 g/kWh in the 2030 renewable power accommodation scenario. © 2023, The Authors. All rights reserved.

关键词： Carbon

Exergy-Based Control Strategy Design and Dynamic Performance Enhancement for Parabolic Trough Solar Receiver-Reactor of Methanol Decomposition Reaction

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SSRN 2023年

作者： Zhang, Peiye Liu, Ming Mu, Ruiqi Yan, Junjie State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

The parabolic trough solar receiver-reactors of methanol decomposition reaction (PTSRR-MDR) enable efficient hydrogen production with solar energy at users' end. Solar fluctuations affect the solar-to-chemical efficiency, hydrogen yield and system stability, indicating the necessity for an appropriate control strategy. In this study, dynamic and thermodynamic models of PTSRR-MDR with Cu/ZnO/Al2O3 as catalyst were developed. Results of thermodynamic analysis show that an optimal flow rate exists for methanol to achieve the maximum solar-to-chemical exergy efficiency of the PTSRR-MDR. The temperature increases sharply and exceeds the catalyst sintering temperature when the methanol flow rate decreases under its optimal value. Then, control objectives to achieve the maximum efficiency were obtained with a temperature of 3 °C to guarantee the PTSRR-MDR safety. Control strategies based on Dynamic Matrix Control (DMC), Forward-Feedback Control (FFC) and Ramp Control (RC) were evaluated under cloud shading/recovering and real radiation processes. Results show that strategies based on DMC, FFC, and RC increase the solar-to-chemical exergy efficiency from 14.76% to 16.31%, 16.28% and 15.65%, respectively. The Integral Squared Error (ISE) for methanol conversion rate under three methods are 1.46, 0.32 and 5.28, respectively, indicating that strategy based on FFC is considered most suitable for PTSRR-MDR operation control. © 2023, The Authors. All rights reserved.

关键词： Methanol

Experimental and Kinetic Study of Methanol Reforming and Methanol-Syngas Co-Oxidation at High Pressure

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SSRN 2023年

作者： Zhan, Haochen Li, Shuming Yin, Geyuan Gao, Yibo Hu, Erjiang Huang, Zuohua State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

Using engine exhaust heat to improve efficiency by performing co-combustion after partial reforming of methanol to syngas is a favored strategy to address the challenges associated with methanol engine applications. In the present work, experiments of methanol pyrolysis and methanol-syngas oxidation were conducted using high-pressure flow reactors at 1.0-5.0 MPa. The ratio of syngas to methanol varied from 0–50%. Mech15.34 was selected as the most reliable kinetic model to further kinetic analysis, which revealed that pressure decreases the decomposition of CH3OH to CH3, enhances the H-abstraction of CH3OH to CH2OH and CH3O, and increases the yield of H2 and CO. Furthermore, high pressure inhibits CH2OH = CH2O + H, thus reducing CH2O. The effects of blending syngas can be divided into dilution and chemical effects. The dilution effect refers to the reduction of CH3OH, which directly decreases the CH2O formation. The chemical effect refers to the competition of H2 and CO for OH radicals, which decelerates the CH3OH oxidation, decreasing the CH2O generation. Meanwhile, the reaction H2O2 + H = H2 + HO2 proceeds in reverse direction with H2 blending, generating more H radicals, which enhances the CH2O consumption through the reaction CH2O + H = HCO + H2. © 2023, The Authors. All rights reserved.

关键词： Oxidation

Simulation Study on Hydrogen-Heating-Power Poly-Generation System based on Solar Driven Supercritical Water Biomass Gasification with Compressed Gas Products as an Energy Storage System

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Journal of Thermal Science 2020年第2期29卷 365-377页

作者： JIN Hui WANG Cui FAN Chao State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong UniversityXi'an 710049China

Supercritical water gasification driven by solar energy is a promising way for clean utilization of biomass with high moisture content, but direct discharge of liquid residual causes energy waste and decreases energy efficiency. To reduce energy waste, a poly-generation system for hydrogen-rich gas production coupling heat supply and power generation based on supercritical water gasification of biomass driven by concentrated solar energy was established in this paper, which also provided a novel energy storage method to overcome the shortcomings of solar discontinuity. Thermodynamic model of the system was proposed and life cycle assessment(LCA) of the system was conducted. Influence of different parameters(temperature of 600℃ to 800℃, outlet temperature of heat exchanger of 42℃ to 56℃, biomass slurry concentration of 5% to 6.5%) on the gasification performance, energy and exergy efficiency, energy distribution and global warming potential(GWP) was discussed. The results indicated that hydrogen yield increased as gasification temperature increased since free radical reaction was enhanced which gas production reaction was classified into. Molar fraction of hydrogen increased as gasification temperature increased and reached 65.6% at 750℃. Energy and exergy efficiency of the system reached 74.84% and 34.87% at 700℃ and 600℃ respectively and that of gas production was 18.15% at 650℃, which was the highest. Increasing reaction temperature and decreasing biomass slurry concentration were effective ways to decrease GWP. Optimal operating parameter was reaction temperature of 650℃, outlet temperature of heat exchanger of 50℃ and biomass concentration of 5%.

关键词： biomass supercritical water gasification poly-generation system thermodynamics analysis energy storage life cycle assessment

Molecular dynamics revealing the non-monotonic variation of oil–water interfacial tension under the effect of cations and asphaltenes

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Journal of Molecular Liquids 2025年 431卷

作者： Zhang, Faxue Wen, Boyao Lu, Xi Wang, Haibo Luo, Zhengyuan Bai, Bofeng State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong University Shaanxi Xi'an710049 China Petroleum Exploration and Production Research Institute of Sinopec Beijing China

The cations can significantly affect the interfacial dynamics of asphaltene molecules, which greatly change the mechanics of oil–water interfaces and thus have a remarkable impact on the chemical flooding process of heavy oil. However, Due to the weak amphiphilic property of asphaltene, the effects of cations on the interfacial behaviors of asphaltene as well as the interfacial mechanics are still unclear. Here, we study the effects of different types and concentrations of cations (Na+, Ca2+ and Mg2+) on the interfacial tension of oil–water systems with different asphaltenes (C5Pe) concentrations by using molecular dynamics simulations. As asphaltene concentration rises, the stronger interactions between cation and asphaltene induce an interfacial enrichment of asphaltenes, resulting in a decrease of interfacial tension. The cations have a more prominent impact on the interfacial tension when the oil–water interfaces reach supersaturated adsorption state of asphaltene molecules. The interfacial tension shows a non-monotonic variation with cation concentration increasing, which has a minimum at intermediate cation concentrations in our simulations. We explain this non-monotonic dependence of interfacial tension on cation concentration by a coupling effect of cations and asphaltene molecules. Under low cation concentrations, the interaction between cations and asphaltenes gradually enhances with cation concentration increasing, leading to a weaker π–π interaction between asphaltenes. Consequently, the asphaltene aggregates disperse and individual asphaltene molecules can easily migrate to the interface. The surface density of asphaltene thus increases, causing a reduction in interfacial tension. At high cation concentrations, cations strongly shield the electrostatic attraction between asphaltene and water molecules, so that the adsorption ability of asphaltenes at interface will weaken or even desorb, thus the interfacial tension elevate. Our results reveal the under

关键词： Adsorption Aggregation Asphaltenes Cations Interfacial tension Molecular dynamics