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Modeling of Transition from Stratified flow to Condensation-Induced Water Hammer in Horizontal Steam-Water Pipes

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SSRN 2023年

作者： Liu, Wenbing Wang, Zichen Zhao, Quanbin Chong, Daotong Yan, Junjie State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an710049 China

Kelvin-Helmholtz (K-H) instability theory is extensively used to predict the transition from stratified flow to slug flow. This study developed a one-dimensional (1-D) model based on K-H theory and considering the variation of axial wave amplitude to predict the transition from stratified flow to condensation-induced water hammer (CIWH). The prediction results of the present model were compared with the experimental data and the results of Taitel’s model. The comparison shows that the present model could reasonably predict critical instability, which is the boundary between stratified flow and CIWH. Critical instability is divided into three types according to the wave amplitude of critical instability. Critical instability occurs at the upper limit, middle, and starting point of the exponential growth of wave amplitude under low, medium, and high liquid levels. The effects of major parameters, including liquid level, water temperature, pipe diameter, and pipe length, on the critical inlet steam velocity and position of critical instability were analyzed. When the liquid level increases, the critical inlet steam velocity decreases, then increases, and then decreases, and the position of critical instability moves downstream, then upstream, and finally turns to the steam inlet. The effect of water temperature, pipe diameter, and pipe length on the critical instability depends on the liquid level and is complex. © 2023, The Authors. All rights reserved.

关键词： Condensation

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

Numerical investigation on the drag characteristics of supercritical water flow past a sphere

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Science China(Technological Sciences) 2020年第8期63卷 1509-1519页

作者： WU ZhenQun OU GuoBiao REN YiFei JIN Hui State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong UniversityXi'an 710049China

Numerical simulation has become a useful approach to explore the complex physical and chemical process inside the supercritical water(SCW)fluidized bed *** to the drastic and non-linear variation of water properties near pseudocritical point,the flow field and drag coefficient of SCW flow past a cold particle are considerably different from that of conventional ***,in this work,particle-resolved numerical investigation is conducted to study the flow field and drag characteristics under the influence of the special water properties *** varying the enthalpy corresponding to the particle surface temperature,which will be called the particle surface enthalpy for short,in a wide range,it realizes the study under different properties variation *** simulated results demonstrate that the variations of drag coefficient versus Reynolds number(Re)and particle surface enthalpy are in good regularity when taking the freestream temperature as reference *** drag coefficient is enhanced greatly at high enthalpy difference between particle surface and inflow,and the enhancement is mainly contributed by that of the frictional drag ***,the drag coefficient variations under different pressures are in good agreement when cases are set with the same inflow and particle surface enthalpies,*** fitting the simulated results,a new drag coefficient correlation is developed for Re range of 10–200 with consideration of the influence of the special properties *** fitting quality is good that the maximum deviation between the new correlation and simulated data is 13.92%,and the mean deviation is 3.67%.

关键词： supercritical water numerical simulation sphere drag coefficient

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

An Investigation on Characteristics of Severe Slugging in an Offshore S-Shaped Riser with a Long Pipeline

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SSRN 2023年

作者： Yu, Haiyang Xu, Qiang Huang, Bo Cao, Yeqi Guo, Liejin State Key Laboratory of Multiphase Flow in Power Engineering Xi 'an Jiaotong University Xi’an710049 China

Long pipelines and risers are essential for transporting oil and gas from the seabed to offshore processing facilities. However, most research focuses on riser systems with short pipelines (≤ 400 m). A 29.1 m high S-shaped riser connecting a 1657 m horizontal pipe, which is comparable to the scale of industrial fields, is utilized for investigating the characteristics of severe slugging. Based on the visual data of gas-liquid interface evolution and the fluctuation of the riser's pressure, the flow pattern map is drawn. Long pipelines delay the liquid accumulation and prolong the severe slugging period. A new correlation for predicting the slugging period is proposed with a deviation of ±15%. The long pipeline provides sufficient compression space and aggravates the blowouts of liquid slugs. The maximum blowout velocity reaches 23 times the mixing velocity. The gas accumulated in the pipeline expands sharply, which causes the instability of the gas-liquid interface in the horizontal pipe and induces a large number of liquid slugs to flow into the riser, where the continuity blowdown of slugs appears. A slug-tracking model considering pressure and liquid holdup is established to precisely forecast the fluctuation of riser pressure during the slug continuity blowdown. © 2023, The Authors. All rights reserved.

关键词： flow patterns

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 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