Preliminary thermodynamic study for an efficient turbo-blower external combustion Rankine cycle

This research paper presents a preliminary thermodynamic study of an innovative power plant operating under a Rankine cycle fed by an external combustion system with turbo-blower (TB). The power plant comprises an external combustion system for natural gas, where the combustion gases yield their thermal energy, through a heat exchanger, to a carbon dioxide Rankine cycle operating under supercritical conditions and with quasi-critical condensation. The TB exploits the energy from the pressurised exhaust gases for compressing the combustion air. The study is focused on the comparison of the combustion system’s conventional technology with that of the proposed. An energy analysis is carried out and the effect of the flue gas pressure on the efficiency and on the heat transfer in the heat exchanger is studied. The coupling of the TB results in an increase in efficiency and of the convection coefficient of the flue gas with pressure, favouring a reduced volume of the heat exchanger. The proposed innovative system achieves increases in efficiency of around 12 % as well as a decrease in the heat exchanger volume of 3/5 compared with the conventional technology without TB.     

一种新的海洋浅层水合物开采法——机械-热联合法

天然气水合物是国家的战略能源之一.天然气水合物分解相变使其开采难度高于常规化石能源.国际天然气水合物试验性开采表明通过降压、注热等方法难以满足商业化开采的需求,尤其在水合物位于浅层、软土情况下,持续稳定且高效率的热量供给是其瓶颈.天然气水合物机械-热联合开采是一种新概念模式,即通过粉碎水合物沉积物通过管道输运并在内部分解,这样既增加了传热的表面积,又利用海水热量和对流传热提高了能量供给效率.分析表明:利用该方法开采时水温过高会导致水合物分解过快而产生不稳定流,温度过低又导致水合物二次生成或结冰;水流流速既要能使被粉碎的水合物沉积物颗粒悬浮和流动,又不能导致流动失稳.为了实现高效安全的机械-热水合物开采,经过初步分析提出原位水合物地层粉碎的颗粒直径设定在0.1~1.0 cm之间,控制水流速度为0.22~0.67m/s,温差保证在5K以上,混合物中水的体积分数大于0.85.      

Transport of Three-Phase Hyper-Saline Brines in Porous Media: Examples

This paper presents a number of applications of a new code which can simulate the transport of high temperature three-phase (gas, liquid, solid) hyper-saline fluids in a porous medium. The examples presented demonstrate that multiple phase changes occur as the fluid state evolves across the H₂O–NaCl phase diagram. Multi-phase flows occur in a variety of situations, including a horizontal domain with fluid withdrawal, a vertical counter-flowing salt-pipe, and a horizontal domain with saturation shocks and expansion waves. The code is also used to simulate heat, water and salt flows in a large scale model (10s of km) of the Taupo Volcanic Zone, New Zealand.     

Piecewise linear model for water column oscillator simulating reactor safety system

A water column oscillation system, which is simulating the pressure balanced injection system in an advanced nuclear power plant, consists of free surfaces in a main-tank and a sub-tank. A cover gas simulating residual heat vapor generation is injected into the main-tank constantly and ejected intermittently. The water level can be described by a set of two piecewise linear ordinary differential equations with four independent parameters. The solution shows chaotic features under specific conditions. The behavior and bifurcation are examined.     

有气体溶解和没有气体溶解的水中石墨烯间的疏水引力

凝聚物理学界发现溶解在水中的气体会在疏水表面吸附,由此认为当疏水物体间距足够小的时候,吸附在疏水表面的气体会相互联通形成纳米气泡桥,纳米气泡桥连结疏水物质形成疏水引力,但是关于纳米气泡桥的形成过程和形态,力学界还没有一个清晰的描述.采用分子动力学方法,研究了在有气体溶解的水中和没有气体溶解的水中两片石墨烯间的引力作用,分析了两种情况下各相密度分布的变化过程、结构相图的变化过程和平均力势的变化过程,详细阐明了纳米气泡桥的形成和消失过程,并定量计算了纳米气泡桥的作用效果和作用距离.模拟结果表明:两片石墨烯在有气体溶解的水中和无气体溶解的水中的疏水引力都是由纳米气泡桥引起的.当石墨烯间距小于0.5nm时,无论水中是否有气体溶解,疏水引力由真空纳米气泡桥引起;当石墨烯间距大于0.5nm时,在没有气体溶解的水中,疏水引力由水蒸气纳米气泡桥引起;而在有气体溶解的水中,疏水引力由所溶气体形成的纳米气泡桥引起.      

Analysis of evaporative cooling and enhancement of condenser efficiency and of coefficient of performance

The graphical method to determine with the aid of a Mollier i - x diagram (psychrometric chart) combined heat and mass transfer is simulated by a computer program. Heat rejection rates from a plate-fin tube type condenser are determined for various flow rates and inlet state conditions of air and for different degrees of wetting of the heat transfer surfaces. The presence of water and the cooling by latent heat makes it possible to exchange more heat than the unwetted exchanger would even for idealized conditions of infinite heat transfer coefficient of the air. The evaporative cooled condenser also can exchange heat with ambient air which has much higher temperature than the condensing fluid. Evaporative cooling increases heat transfer by a factor of more than three for saturated inlet air and greater than five for lower inlet humidities. Wetted heat exchangers require less extended surfaces and can operate effectively with bare tubes only. Wetting the condenser of a refrigeration or heat pump system makes it possible to exchange the condenser load at lower temperatures. This yields an increase of COP of the order of 30 to 60% and therefore a substantial decrease in compressor power and its energy consumption.     

Jet impingement on cylindrical cavity: Conical nozzle considerations

In laser gas-assisted processing, the assisting gas emerges from a nozzle and nozzle geometric configurations alter the flow structure and heat transfer characteristics in and around the section processed. In the present study, the influence of nozzle geometric configurations, cavity diameter and depth, on flow structure and heat transfer rates from the cavity is investigated. A cylindrical cavity with two diameters and varying depths is accommodated in the simulations. Air is used as assisting gas while steel is employed as workpiece material. A numerical scheme using a control volume approach is accommodated to discretize the flow and energy equations. It is found that flow structure changes significantly for large diameter cavity. The influence of the nozzle cone angle on heat transfer and flow structure is more pronounced as the cavity depth increases.     

Prevention of fog in a condenser by simultaneous heating and cooling

By partial condensation the vaporous component of a vapour–gas mixture can be separated. This process plays an important part, especially in the recovery of solvents when the solvent is a vapour and mixed with a gas. The only drawback is, however, the frequent occurrence of undesired fog formation. This fog consists of a large number of small solvent droplets and only by a large effort can it be separated again. Through good modelling of the processes of heat and mass transfer the causes for the formation of fog and a method for its prevention can be found. The solution seems to be paradoxical: to avoid the formation of fog the condenser has to be cooled and heated simultaneously. If fog can be prevented, the degree of separation of the vapour–gas mixture even increases. The heating of the condenser may be accomplished by internal energy recovery, thereby simplifying the apparatus concept and energy supply.     

Cell model of nonisothermal gas absorption in gas-liquid bubbly media

A model for combined mass and heat transfer during nonisothermal gas absorption in a two-phase gasliquid bubbly medium with a high gas content and/or large times of gas-liquid contact is suggested. Diffusion and thermal interactions between bubbles is taken into account in the approximation of a cellular model of a bubbly medium whereby a bubbly medium is viewed as a periodic structure consisting of identical spherical cells with periodic boundary conditions at a cell boundary. Distribution of concentration of dissolved gas, temperature distribution in liquid and coefficients of mass and heat transfer during nonisothermal absorption of a soluble pure gas from a bubble by liquid are determined. In the limiting case of absorption without heat release the derived formulas recover the expressions for isothermal absorption.     

Plant growth – A thermodynamicist's view

The synthesis of glucose requires an increase of enthalpy and a decrease of entropy such that the Gibbs free energy increases. This is impossible by the laws of thermodynamics unless there is an accompanying, compensating process that decreases the Gibbs free energy. Schr?dinger [1] has suggested that the accompanying process should be the absorption and reemission of radiation. This process supplies the heat of reaction and the entropy increase of radiation is more than enough to offset the chemical decrease of entropy. And yet we are not satisfied with Schr?dingers proposition, because we see no connection between the entropy increase of radiation and the physiology of the plant. Therefore we propose an alternative: The accompanying process consists of the transpiration of water and the mixing of water vapour with air; in this view radiation only furnishes the heat of reaction. Received February 14, 1997     

Joule–Thomson Effects on the Flow of Liquid Water

We present a revised form of the energy balance for the coupled thermodynamics of liquid water flowing in porous media and give examples of situations where a commonly used formulation based on transport of enthalpy leads to erroneous results. Assuming negligible contribution from kinetic energy as well as sources and sinks such as energy from radioactive decay, total energy conservation is reduced to a balance between changes in internal energy, enthalpy, conductive heat flux, and gravitational potential energy. The Joule–Thomson coefficient is defined as the change in temperature with respect to an increase in pressure at constant enthalpy. Because liquid water has a negative Joule–Thomson coefficient at low temperatures, at a constant gravitational potential water cools as it compresses and heats as it expands. If one ignores the gravitational energy, transport of enthalpy alone leads to water heating by 2 \(^\circ \) C per kilometer as it is brought up from depth. The corrected energy balance transports methalpy, which is enthalpy plus gravitational potential energy. Although the simpler form leads to small changes in the temperature profile for typical simulations, there are several instances where this effect may prove to be important. The most important impact of the erroneous form is probably in the field of geothermal energy production, where the creation of a few degrees of heat in a simulation could lead to miscalculation of power plant efficiencies.     

An analysis of the effect of the solid layer for the modified chemical vapor deposition process

The heat transfer problem relative to the modified chemical vapor deposition process has been analyzed and the effects of solid layer thickness, torch speed and tube rotation are studied. The quasi-steady three-dimensional energy equations have been solved for the temperature fields in the gas and the solid layer with a Gaussian heat flux boundary condition on the outer surface. Of particular interest is the effect of the solid layer thickness and the torch speed on inner surface temperature, gas temperature and thermophoretic velocity. The large change of the axial temperature distribution of the surface occurs for different solid layer thicknesses or torch speeds. The presence of the solid layer and tube rotation reduce the effects of nonuniform torch heating in the circumferential direction and the resulting surface temperatures are very uniform in this direction.     

2D numerical simulation for simultaneous heat, water and gas migration in soil bed under different environmental conditions

A general mathematical model for investigating simultaneous heat, water and gas (air plus vapor) transfer in unsaturated porous soil under different environmental conditions is presented based on the volume-averaging method. Two-dimensional numerical simulation in steady state is conducted for obtaining accurate images of field characteristics in a confined soil bed, which might be useful to provide necessary information for agricultural applications. Various effects of environmental parameters, such as ambient temperature, relative humidity, radiative heat flux and wind speed, on transport processes in soil without plant roots are analyzed through the calculating results in the present paper. Received on 13 January 1998     

Numerical investigation of premixed combustion in a porous burner with integrated heat exchanger

In this paper, we perform a numerical analysis of a two-dimensional axisymmetric problem arising in premixed combustion in a porous burner with integrated heat exchanger. The physical domain consists of two zones, porous and heat exchanger zones. Two dimensional Navier–Stokes equations, gas and solid energy equations, and chemical species transport equations are solved and heat release is described by a multistep kinetics mechanism. The solid matrix is modeled as a gray medium, and the finite volume method is used to solve the radiative transfer equation to calculate the local radiation source/sink in the solid phase energy equation. Special attention is given to model heat transfer between the hot gas and the heat exchanger tube. Thus, the corresponding terms are added to the energy equations of the flow and the solid matrix. Gas and solid temperature profiles and species mole fractions on the burner centerline, predicted 2D temperature fields, species concentrations and streamlines are presented. Calculated results for temperature profiles are compared to experimental data. It is shown that there is good agreement between the numerical solutions and the experimental data and it is concluded that the developed numerical program is an excellent tool to investigate combustion in porous burner.     

Hydrodynamics and heat transfer characteristics of G–S magnetically stabilized beds consisting of admixtures of shale oil and magnetic particles

The hydrodynamic and heat transfer behavior of a bed consisting of magnetic and shale oil particle admixtures under the effect of a transverse magnetic field is investigated. The phase diagram, bed void fraction are studied under wide range of the operating conditions i.e., gas velocity, magnetic field intensity and fraction of the magnetic particles. It is found that the range of the stabilized regime is reduced as the magnetic fraction decreases. In addition, the bed voidage at the onset of fluidization decreases as the magnetic fraction decreases. On the other hand, Nusselt number and consequently the heat transfer coefficient is found to increase as the magnetic fraction decreases. An empirical equation is investigated to relate the effect of the gas velocity, magnetic field intensity and fraction of the magnetic particles on the heat transfer behavior in the bed.     

基于CFD-DEM的高炉风口回旋区形状和传热特性数值模拟研究

采用计算流体力学-离散单元法(CFD-DEM)耦合方法,对高炉风口回旋区进行了数值模拟研究。首先通过与实验结果对比,验证了CFD-DEM模型的正确性;然后考察了不同气速对风口回旋区形状和传热特性及颗粒接触的影响。数值模拟结果表明:风口回旋区的大小和形状均受气速影响较大,在较大进气速度下,颗粒受到的曳力大于颗粒间的摩擦阻力并破坏颗粒间的桥力,形成较大尺寸的回旋区;且颗粒之间接触力较小,形成较大的空隙结构,更有利于热气体向周围扩散以强化传热。目前考察的三种气速结果表明:当气速为11m/s时,热量向下方传递速度最快;当气速为13m/s时,热量向上方传递速度最快;而当气速为15m/s时,热量向右方传递速度最快;此外,气速越大流态化越明显,颗粒间接触越少,接触力也越小。     

Simulation of the primary breakup of a high-viscosity liquid jet by a coaxial annular gas flow

The conversion of low-grade fossil and biogenic energy resources (petcoke, biomass) to a synthesis gas in a high pressure entrained flow gasification process opens a wide spectrum for high efficient energy conversion processes. The synthesis gas can be used for production of methane (SNG), liquid fuels (BtL, CtL) or as fuel for operation of a gas turbine in a combined cycle power plant (IGCC). The production of a tar free high quality syngas is a challenging objective especially due to the fact that typical liquid or suspension fuels for entrained flow gasifiers feature viscosities up to 1000 mPas. Fuel droplet conversion at typical entrained flow gasification conditions is characterized by heat up, evaporation and subsequent degradation of the vapour phase. To guarantee a high fuel conversion rate in the gasifier an efficient atomization of the fuel is required. Mainly twin-fluid burner nozzles are used for atomization of those typically high viscous fuels. The present study is focused on the assessment of the accuracy of CFD computations for the primary breakup of high-viscosity liquids using an external mixing twin fluid nozzle. In a first step experiments were performed with a Newtonian glycerol-water-mixture featuring a liquid viscosity of 400 mPas. Jet breakup was investigated using a high speed camera as well as PIV and LDA-System for a detailed investigation of the flow field. In a second step the experimental results serve as reference data to assess the accuracy of CFD computations. Compressible large eddy simulations (LES) were performed to capture the morphology of the primary breakup as well as the important flow field characteristics. A Volume of Fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Comparison of experimental and numerical results showed good agreement with respect to breakup frequency, velocity fields and morphology.     

深海天然气水合物机械?热联合开采方法研究综述

天然气水合物由于储量大、污染低等优点, 已成为我国非常重要的战略能源, 世界各国也加快了天然气水合物的勘探和开发工作. 经济高效的开采方法以及相关的灾害控制和环境保护是对天然气水合物进行商业化开采必须要解决好的两个关键问题. 目前, 注热法和降压法的联合使用被认为是最为有效的天然气水合物开采方法. 在降压法和注热法中, 天然气水合物开采涉及传热、相变、渗流和变形等物理过程和效应, 而传热最慢且相变会消耗大量的热量, 无法直接采用常规的单纯依靠渗流原理的油气开采方案来开采天然气水合物. 我国南海的天然气水合物主要赋存于粉砂质黏土和粉细砂等类型的沉积物中, 胶结性差且埋深较浅. 常规的开采方法还不适合我国南海的水合物开采, 需要考虑新型的开采方式, 这其中提高沉积层中的热传导效率是天然气水合物开采的关键. 郑哲敏提出了机械?热联合开采的新概念方法, 利用无穷无尽表层海水的热量, 基于对流传热的原理和管道输送技术, 并兼顾类似采煤挖掘可能导致的深海浅软地层安全问题. 天然气水合物机械?热联合开采法是一种新的概念模式, 具有开采可控、高效且能有效降低地层安全性风险的优点. 本文针对该新方法的能量、装备、经济可行性进行综合评估, 阐述了针对核心问题管道含相变气液固多相流动、地层安全方面的研究进展, 展望了未来推广应用的空间.      

Boiling heat transfer enhancement of water on tubes in compact in-line bundles

In desalinization devices and some heat exchangers making use of low-quality heat energy, both wall temperatures and wall heat fluxes of the heated tubes are generally quite low; hence they cannot cause boiling in flooded tube-bundle evaporators with common large tube spacing. However, when the tube spacing is very small, the incipient boiling in restricted spaces can generate and results in higher heat transfer than that of pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes and test pressures on the boiling heat transfer of water in restricted spaces of the compact in-line bundles consisting of smooth horizontal tubes. The experimental results show that tube spacing and tube position have significant effects on the boiling heat transfer in a compact tube bundle. There is an optimum tube spacing that provides the largest heat transfer coefficient at the same heat flux.