Stability of conducting viscous film flowing down an inclined plane with linear temperature variation in the presence of a uniform normal electric field

Weakly nonlinear stability analysis of a thin liquid film falling down a heated inclined plane with linear temperature variation in the presence of a uniform normal electric field has been investigated within the finite amplitude regime. A generalized kinematic equation for the development of free surface is derived by using long wave expansion method. A normal mode approach and the method of multiple scales are used to investigate the linear and weakly nonlinear stability analysis of film flow, respectively. It is found that both Marangoni and electric Weber numbers have destabilizing effect on the film flow. The study reveals that both supercritical stability and subcritical instability are possible for this type of film flow. It is interesting to note that both the Marangoni and electric Weber numbers have qualitatively same influence on the stability characteristics but the effect of Marangoni number is much stronger compare to the electric Weber number. Scrutinizing the effect of Marangoni and electric Weber numbers on the amplitude and speed of waves it is found that, in the supercritical region amplitude and speed of the nonlinear waves increases with the increase in Marangoni and electric Weber numbers, while in the subcritical region the threshold amplitude decreases with the increase in Marangoni and electric Weber numbers. Finally, we obtain that spatially uniform solution is side-band stable in the supercritical region for our considered parameter range.     

非线性弹性转子系统碰摩的动态特性

在同时考虑轴承油膜力、转轴非线性弹性力和碰摩发生时转静件问的相对速度对非线性摩擦力的影响基础上，构造了具有碰摩故障转子一轴承系统的动力学模型，对系统在运行过程中的非线性行为进行了数值仿真分析，发现在亚临界转速区，系统响应主要以周期运动为主，在半倍临界转速附近有短暂的混沌运动。在临界转速区，系统响应为周期运动，振幅相应增大。在超临界转速区，系统响应以混沌、周期分频和拟周期为主要运动形式。不同的速度影响因数对系统的响应也具有一定的影响。该结果为转子-轴承系统的故障诊断提供了一定的参考。     

350MW超临界火电机组技术经济性分析

近年,350MW超临界火力发电机组技术发展迅速,在中国大陆地区已有约20多台机组投运或正在建设中。与同容量等级的亚临界火力发电机组相比,超临界火力发电机组热经济性较高,但电厂初投资高。因此,有必要对350MW超临界参数的火力发电机组的技术经济性加以分析。通过对比亚临界参数火力发电机组,比较其技术特点和投资回报。     

Nonlinear analysis of tilted toroidal thermosyphon models

We analyze one-dimensional models for single-phase tilted toroidal thermosyphons for three different heating conditions: known heat flux, known wall temperature and mixed heating. For the first two the governing equations lend themselves to exact reduction to a set of three ordinary differential equations, while for the third the equations remain coupled as an infinite set. For all three cases, the tilt angle is stabilizing while the heat rate is a destabilizer. A nonlinear analysis is carried out using center manifold theory and normal form analysis. The known heat flux solutions lose stability through a supercritical Hopf bifurcation, while for the other two heating conditions the Hopf bifurcation is supercritical under some conditions and subcritical under others. Stable limit-cycle oscillations exist only for the supercritical cases, otherwise instability leads directly to chaos. Analysis also provides an estimate for the amplitude of oscillation for the supercritical conditions. Numerical experiments have confirmed the theoretical predictions qualitatively and quantitatively.     

Effects of supercritical environment on hydrocarbon-fuel injection

In this study,the effects of environment conditions on decane were investigated.Decane was injected in subcritical and supercritical ambient conditions.The visualization chamber was pressurized to 1.68 MPa by using nitrogen gas at a temperature of 653 K for subcritical ambient conditions.For supercritical ambient conditions,the visualization chamber was pressurized to 2.52 MPa by using helium at a temperature of 653 K.The decane injection in the pressurized chamber was visualized via a shadowgraph technique and gradient images were obtained by a post processing method.A large variation in density gradient was observed at jet interface in the case of subcritical injection in subcritical ambient conditions.Conversely,for supercritical injection in supercritical ambient conditions,a small density gradient was observed at the jet interface.In a manner similar to that observed in other cases,supercritical injection in subcritical ambient conditions differed from supercritical ambient conditions such as sphere shape liquid.Additionally,there were changes in the interface,and the supercritical injection core width was thicker than that in the subcritical injection.Furthermore,in cases with the same injection conditions,the change in the supercritical ambient normalized core width was smaller than the change in the subcritical ambient normalized core width owing to high specific heat at the supercritical injection and small phase change at the interface.Therefore,the interface was affected by the changing ambient condition.Given that the effect of changing the thermodynamic properties of propellants could be essential for a variable thrust rocket engine,the effects of the ambient conditions were investigated experimentally.     

Nonequilibrium Molecular Dynamics Modeling Of A Fuel Nanojet In Sub/supercritical Environments: Chamber Pressure Effects On Characteristics Of The Gas–liquid Interface

Molecular dynamics simulation is conducted to microscopically study the effects of environmental pressure on the injection process of a fuel nanojet. The united-atom force field and Lennard-Jones 12-6 potential are applied, which is validated by a single liquid droplet evaporation model against experiments. A gas–liquid–gas model represented by a simulation box for single droplet evaporation and an n-heptane nanojet injected into vacuum, lowly supercritical, and highly supercritical environments are investigated. Results indicate that at lowly supercritical conditions, the gas–liquid interface has been widened, and the interface tension still exists and the phase interface is detected. Only at highly supercritical conditions does the interface becomes sufficiently wide and a continuous phase transition without a distinct phase interface prevails. The most interesting conclusion from the molecular dynamics (MD) results is that the transition of a fluid from subcritical to supercritical states does not occur instantaneously when it goes across the critical point but is a gradual process that can be completed only when the temperature and pressure are sufficiently higher than the critical values. This microscopic analysis is well in accordance with the macroscopic measurements and observations in the literature.     

Evaluation on hydrothermal gasification of waste tires based on chemical equilibrium analysis

Massive amounts of waste tires are produced globally, which brings great challenges to the disposal and recycling of used tires. Hydrothermal gasification is a promising option for recycling waste tires. The hydrothermal gasification of waste tires was evaluated based on the chemical equilibrium analysis along with the response surface methodology (RSM) in terms of subcritical temperature range (250–300 °C), transition temperature range (350–400 °C), supercritical temperature range (550–600 °C), supercritical pressure (22.5–30.5 MPa) and feedstock concentration (5–20 wt%). CH₄ yield at 350 °C reached a maximum, 41.575 mmol/g. H₂ yield increased from 0.0283 to 53.602 mmol/g with increasing the temperature from 250 °C to 600 °C. CH₄ yield at the supercritical temperature increased with lifting the feedstock concentration, while H₂ yield decreased. The optimal parameters regarding maximum H₂ and CH₄ yields in the subcritical temperature range were 300 °C, 22.5 MPa and 12.5 wt%, respectively, while they in the supercritical temperature range were 550 °C, 30.5 MPa and 5.4 wt%, respectively. RSM was more suitable for predicting H₂ yield in the hydrothermal gasification of waste tires at subcritical and supercritical temperature ranges, but it was available for predicting CH₄ yield in three temperature ranges. This study can provide basic data for the hydrothermal treatment of waste tires.     

论发展超临界参数锅炉

结合国内超临及亚临界６００ＭＷ机组锅炉的运行的实绩,分析采用超临参数机组的可靠性及经济性,阐述超临压力参数锅炉比亚临界压力参数锅炉耗钢少的特点,指出发展临界压力参数锅炉的意义。     

Experimental investigation on heat transfer characteristics of low mass flux rifled tube with upward flow

An experiment for heat transfer of water flowing in a vertical rifled tube was conducted at subcritical and supercritical pressure. The main purpose is to explore the heat transfer characteristics of the new-type rifled tube at low mass flux. Operating conditions included pressures of 12–30 MPa, mass flux of 232–1200 kg/(m² s), and wall heat fluxes of 133–719 kW/m². The heat transfer performance and wall temperature distribution at various operating conditions were captured in the experiment. In the present paper, the heat transfer mechanism of the rifled tube was analyzed, the effects of pressure, wall heat flux and mass flux on heat transfer were discussed, and corresponding empirical correlations were also presented. The experimental results exhibit that the rifled tube has an obvious enhancement in heat transfer, even at low mass flux. In comparison with a smooth tube, the rifled tube efficiently prevents Departure from Nucleate Boiling (DNB) and delays dryout at subcritical pressure, and also improves the heat transfer of supercritical water remarkably, especially near pseudo-critical point. An increase in pressure or wall heat flux impairs the heat transfer at both subcritical and supercritical pressure, whereas the increasing mass flux has a contrary effect.     

汽油在超临界环境下的喷雾特性

利用超临界平台和定容燃烧弹系统,将低亚临界态(low sub-critical state,LS-CS)、亚临界态(sub-critical state,S-CS)和超临界态(super-critical state,Sup-CS)等不同状态的汽油,喷入到超临界环境(super-critical environment,Sup-CE)。依托纹影系统,得到汽油在超临界环境中的喷雾特性,并与常温常压环境(ambient temperature and atmospheric pressure environment,ATAPE)汽油喷雾特性进行详细对比。研究结果表明:在超临界环境下,同一喷射时刻,随着喷油温度的升高,单孔喷油器的喷雾贯穿距先增加,后平稳下降,但在超临界态一直攀升;喷雾面积先上升,在亚临界态初段达到峰值后波动下降。5孔喷油器的喷雾贯穿距先平稳上升,后急速下降到低于初始值;喷雾面积先增加,在亚临界态缓慢下降,在超临界态初期达到峰值后下降。此外,在喷射条件相同时,超临界环境下和常温常压环境下的喷雾形态发展有明显区别,由于超临界环境下喷雾与环境的相互作用,塌缩现象及气泡微爆现象主导喷雾的发展,喷雾贯穿距和喷雾面积随时间的变化量远小于常温常压环境下的喷雾贯穿距和喷雾面积。     

Phase changes of benzo(a)pyrene in supercritical water combustion

This paper presents new data on the behavior of benzo(a)pyrene (BaP) during supercritical water (SCW) combustion. It focuses on phase changes of the BaP throughout the transition from subcritical to supercritical regions. A sequence of images illustrates BaP’s phase change for the first time. They were obtained in situ using a hydrothermal diamond anvil cell coupled with Fourier transform infrared spectrometer as well as optical and infrared microscopes. Combustion/reaction at different oxygen concentration (0–49% H₂O₂), and pyrolysis experiments were conducted. The results show conclusively that (1) BaP is stable at pyrolytic conditions up to 452°C; (2) It can dissolve in supercritical water at 442–452°C forming partly decomposed globule. At extended reaction times above 500°C the globule undergoes carbonization while the dissolved compounds inhibit char formation. No complete dissolution was observed. (3) BaP combustion occurs simultaneously with dissolution in a single homogenous phase. At higher oxygen content, the dissolution and complete combustion takes place even in subcritical region (353° and 145 MPa). In such a case no melting phase is present. These observations may affect the design and organization of the SCW combustion process.     

Performance analysis in near-critical conditions of organic Rankine cycle

According to fluid critical temperature and heat source temperature, organic Rankine cycle (ORC) is recognized in two categories: subcritical ORC and supercritical ORC. For a given heat source, some organic fluids not only can be used in subcritical ORC, but also can be used in supercritical ORC. For heat source with temperature of 90 °C, HFC125, HFC143a and HF218 can be used in both ORCs. Performance of the three substances in both cycles, especially in near-critical conditions is studied with expander inlet temperature of 85 °C and hot water mass flow rate of 1 kg/s. The results show that when fluids go in supercritical ORC from subcritical ORC, cycle thermal efficiency varies continuously, while mass flow rate and net power generation vary discontinuously. Maximum net power generation in near-critical conditions of subcritical ORC is higher than that of supercritical ORC. For HFC125 and HFC143a, outlet temperature of hot water decreases with the increase of heating pressure ratio. For HF218, outlet temperature of hot water increases firstly and decreases secondly with the increase of heating pressure ratio, which leads to an increase of net power generation with the increase of heating pressure ratio in high heating pressure ratio conditions.     

MD STUDY ON INTERFACELIKE PHENOMENA IN SUPERCRITICAL FLUID

An interfacelike structure obser ved during heat transfer in fluid near the critical point has been analyzed using a molecular dynamics (MD) method for a simple fluid modeled by the Lennard-Jones (12-6) potential. Under the condition that a temperature gradient exists, MD simulations at supercritical pressure reproduced a structure which is similar to the normal liquid - vapor interface obser ved at subcritical pressure. Some characteristics of the interfacelike structure, such as density profile, interface thickness, and interfacial tension, are compared with those of a subcritical liquid - vapor interface. The predicted value of the supercritical interfacial tension is compared with the value that was estimated using a theory of hydrodynamic instability due to the interfacial tension.     

Effect of sulfide on corrosion behavior of stainless steel 316SS and Hastelloy C276 in sub/supercritical water

Stainless steel 316SS and Hastelloy C276, as the representatives of iron-based and nickel-based alloy, respectively, were employed to explore the corrosion properties under reducing subcritical and supercritical water containing sulfide. Experiments were executed at a pressure of 25 MPa, temperatures of 350 °C–520 °C, and sulfur concentrations of 1000 and 5000 ppm for 80 h. An isothermal equilibrium phase diagram involving the oxidation/sulfidation products of Fe, Cr, and Ni, was established by theoretical calculation in supercritical water system at 520 °C, in order to predict the corresponding products under various conditions and assist the discussion on corrosion mechanism. The results show that whether in subcritical water or in supercritical water, 316SS always exhibited better corrosion resistance relative to C276. In subcritical water at 350 °C, a portion of corrosion film peeled off from 316SS specimen, while numerous pores or cracks appeared on the surface of scale for C276. Under supercritical water at 520 °C, a compact scale grown on 316SS sample surface was composed of Fe₃O₄, FeCr₂O₄, and FeS. For C276, a duplex-layer scale formed on alloy surface. However, due to the higher content of Ni in C276, Ni-sulfide channels through the inner layer were developed, accelerating the sulfidation corrosion of alloys. Overall, the high-temperature alloys with high Cr content and low Ni/Cr ratio can be considered as the candidate material of equipment in supercritical water gasification of sulfur-containing coal.     

Minimization of classification samples for supercritical and subcritical patterns of supersonic inlet

In order to investigate sample minimization for classification of supercritical and subcritical patterns in supersonic inlet, three optimization methods, namely, opposite one towards nearest method, closest one towards the hyper-plane method and random selection method, are proposed for investigation on minimization of classification samples for supercritical and subcritical patterns of supersonic inlet. The study has been carried out to analyze wind tunnel test data and to compare the classification accuracy based on those three methods with or without priori knowledge. Those three methods are different from each other by different selecting methods for samples. The results show that one of the optimization methods needs the minimization samples to get the highest classification accuracy without priori knowledge. Meanwhile, the number of minimization samples needed to get highest classification accuracy can be further reduced by introducing priori knowledge. Furthermore, it demonstrates that the best optimization method has been found by comparing all cases studied with or without introducing priori knowledge. This method can be applied to reduce the number of wind tunnel tests to obtain the inlet performance and to identify the supercritical/subcritical modes for supersonic inlet.     

Calculation method of heat and fluid flow in a microreactor for supercritical water and its solution

A microreactor for supercritical water has been developed for chemical engineering process. Heat and fluid flow in a T-junction in this system are calculated using STREAM. Since the flow in the tube are in the wide range from laminar to turbulent flow, a nonlinear k–ε model taking into account the low-Reynolds-number effects (proposed by Abe, Kondoh and Nagano) are used. We assume that a supercritical water velocity is 2.0 m/s. The optimum condition for time fluctuation of temperature is when the substrate velocity is 0.4. In this case, the substrate temperature rises above the critical temperature immediately and the main reaction progresses efficiently. When v is 0.1, the temperature remains subcritical region for a while. On the other hand, when the v is larger than 0.4, the temperature does not reach the critical point.     

Thermochemical liquefaction characteristics of Cyanobacteria in subcritical and supercritical ethanol–water mixture

The thermochemical liquefaction of Cyanobacteria in subcritical and supercritical ethanol–water mixture was studied with different reaction temperature, reaction time, solvent composition, and solid–liquid ratio. Highest bio‐oil yield of 42.5% containing mainly fatty acid ethyl esters, phenols, pyrrolidinones, and pyridinols was obtained in ethanol–water mixture (4/6, v/v) at temperature of 320°C for 30 min, with solid–liquid ratio of 1 g/15 mL. Both solvent composition and supercritical state had great influence on the liquefaction of Cyanobacteria, while the synergetic effects of water and ethanol in co‐solvents were again verified. Copyright © 2017 John Wiley & Sons, Ltd.     

高效低污染超临界发电技术

本文介绍了超临界压力火电机组的一些概念以及形成超临界压力火电技术的理论基础,对超临界与亚临界火电机组在许多方面做了相应的比较;同时对超临界压力火电机组的技术特点进行了分析与总结。     

Numerical investigation of thermo-bioconvection in a suspension of gravitactic microorganisms

This paper investigates the effect of heating or cooling from below on the development of gravitactic bioconvection in a square enclosure with stress free sidewalls. The governing equations are the Navier–Stokes equations with the Boussinesq approximation, the diffusion equation for the motile microorganism and the energy equation for the temperature. The control volume method is used to solve numerically the complete set of governing equations. It was found that the suspension is destabilized by heating from below and stabilized by cooling from below. A transition from a subcritical bifurcation to a supercritical bifurcation was observed in the case of heating from below when the thermal Rayleigh number was increased.