Estimation of thermal and mass diffusivity in a porous medium of complex structure using a lattice Boltzmann method

The thermal and mass diffusivity in a porous medium of complex structure is studied by using the lattice Boltzmann method. The media under consideration include two-dimensional medium with an array of periodically distributed circular and square cylinders, three-dimensional granular medium of overlapping or non-overlapping spherical and cubical inclusions of different size, and randomly generated fibrous medium. The calculated effective diffusivities are in good agreement with existing analytical and numerical results when the inclusions, regardless of their shapes, are not overlapped. For the medium of overlapping inclusions, the effective diffusivity deviates from existing correlations as the inclusion fraction increases. In particular, the deviation increases dramatically if the thermal diffusivity of the inclusion is greater than that of the fluid in the medium for enhanced thermal conduction. A new empirical correlation between the effective diffusivity and the volume fraction for the medium of overlapping inclusions is proposed.     

自呼吸式直接甲醇燃料电池性能及其传质特性

针对有效面积为1 cm2的自呼吸式直接甲醇燃料电池（direct methanol fuel cell,DMFC）单电池,阳极采用燃料罐供液,将阴极侧集流体和夹具设计为一体式结构,并用自制的七合一膜电极组件对其进行测试,讨论了催化剂类型、扩散层材料、集流体结构等因素对其性能的影响,分析了电池内部的传质特性,优化了电池特别是其在中高电流密度条件下的性能。实验结果表明：采用Pt黑、Pt-Ru黑催化剂制作的自呼吸式DMFC能强化反应物的传质;采用碳布制作的膜电极更倾向于获得更高的极限电流密度;低电流密度时,因甲醇渗透电池电压随着甲醇浓度的增加而降低,但在中高电流密度下,电池性能随甲醇浓度的增大先升高后降低;平行集流体有利于阴阳极生成物的排出和反应物的传质,因此易获得较高的电池性能。     

Observation of premixed flame fronts by laser tomography

The principle of combustion field detection by using laser tomography, as well as exploitation of the laser tomography apparatus and the tool for image processing is described. An experiment detecting flame fronts by laser tomography was made by employing a V-shaped premixed flame. The results show that the instantaneous geometric shape of flame wrinkles within the light sheet can be clearly resolved. The contours of the flame fronts are precisely tracked through active contour models (ACM) from the digital images of laser tomography, laying the basis for the quantitative analysis of flame wrinkling and propagation. __________ Translated from Journal of Combustion Science and Technology, 2007, 13(3): 275–279 [译自: 燃烧科学与技术]     

微燃机与小燃机在南方地区分布式联供系统中的应用比较

建立了考虑设备最大最小出力限制、温度和出力对设备性能影响的分布式联供系统优化运行混合整数非线性规划（MINLP）问题求解方法。在负荷确定的条件下,可对不同配置的分布式联供系统进行优化运行计算及经济、节能减排、调峰特性分析。针对我国南方地区气候及能源价格特点,对办公楼、商场、医院和宾馆四类典型城市建筑比较了分别由微燃机和小燃机为主机组成的分布式联供系统的适用性。计算结果表明,与冷、热、电分供形式相比,除办公楼外,由微燃机和小燃机为主机组成的分布式联供系统均具有较好的经济性;分布式供能系统节能减排、调峰性能优越;微燃机联供系统和小燃机联供系统经济性相当,但小燃机系统一次能源利用率较高。     

Comparative energetic and exergetic performance analyses for coal-fired thermal power plants in Turkey

The purpose of this study is to analyze comparatively the performance of nine thermal power plants under control governmental bodies in Turkey, from energetic and exergetic viewpoint. The considered power plants are mostly conventional reheat steam power plant fed by low quality coal. Firstly, thermodynamic models of the plants are developed based on first and second law of thermodynamics. Secondly, some energetic simulation results of the developed models are compared with the design values of the power plants in order to demonstrate the reliability. Thirdly, design point performance analyses based on energetic and exergetic performance criteria such as thermal efficiency, exergy efficiency, exergy loss, exergetic performance coefficient are performed for all considered plants in order to make comprehensive evaluations. Finally, by means of these analyses, the main sources of thermodynamic inefficiencies as well as reasonable comparison of each plant to others are identified and discussed. As a result, the outcomes of this study can provide a basis used for plant performance improvement for the considered coal-fired thermal power plants.     

Investigation of coal-fired power plants in Turkey and a case study: Can plant

About 61% of the total installed capacity for electrical power generation in Turkey is provided by thermal resources, while 80% of the total electricity is generated from thermal power plants. Of the total thermal generation, natural gas accounts for 49.2%, followed by coal for 40.65%, and 9.9% for liquid fuel. This study deals with investigation of the Turkish coal-fired power plants, examination of an example plant and rehabilitation of the current plants. Studied plant has a total installed capacity of 2 × 160 MW and has been recently put into operation. It is the first and only circulating fluidized bed power plant in the country. Exergy efficiencies, irreversibilities, and improvement factors of turbine, steam generator and pumps are calculated for plant selected. Comparison between conventional and fluidized bed power plant is made and proposed improving techniques are also given for conventional plants.     

Model of a porous regenerator used for magnetic refrigeration at room temperature

A transient three-dimensional model of a porous regenerator operating at room temperature is presented. The solid magnetic material and the regeneration fluid are modeled separately. The fluid flows through interstitial channels formed by a regular matrix of solid particles. The velocity, pressure and temperature fields are obtained from the simultaneous solution of the Navier–Stokes and energy equations with variable properties in the solid and the Boussinesq approximation for the fluid. The magnetocaloric effect (MCE) is taken into account by the inclusion of a source term in the energy equation for the magnetic solid. Special numerical schemes are used to avoid unrealistic computation time and memory requirements. Typical velocity fields and transient temperature profiles are presented.     

Analytical method for evaluation of gas turbine inlet air cooling in combined cycle power plant

Gas turbine inlet air cooling technologies (GTIAC), mainly including chilling with LiBr/water absorption chiller and fogging as well, are being used during hot seasons to augment the power output. To evaluate the general applicability of inlet air cooling for gas–steam combined cycle power plant (GTCCIAC), parameters such as efficiency ratio, profit ratio and relative payback period were defined and analyzed through off-design performances of both gas turbine and inlet air cooling systems. An analytical method for applicability evaluation of GTCCIAC with absorption chiller (inlet chilling) and saturated evaporative cooler (inlet fogging) was presented. The applicability study based on typical off-design performances of the components in GTCCIAC shows that, the applicability of GTCCIAC with chilling and fogging depends on the design economic efficiency of GTCC power plant. In addition, it relies heavily on the climatic data and the design capacity of inlet air cooling systems. Generally, GTCCIAC is preferable in the zones with high ambient air temperature and low humidity. Furthermore, it is more appropriate for those GTCC units with lower design economic efficiency. Comparison of the applicability between chilling and fogging shows that, inlet fogging is superior in power efficiency at t_a = 15–20 °C though it gains smaller profit margin than inlet chilling. GTCC inlet chilling with absorption chiller is preferable in the zones with t_a > 25 °C and RH > 0.4.     

Photo-hydrogen production rate of a PVA-boric acid gel granule containing immobilized photosynthetic bacteria cells

The polyvinyl alcohol (PVA)-boric acid gel granule facilitates the light penetration and mass transport as it has the features of the transparency and adequate porous structure. In this work, a hydrogen production bioreactor with the indigenous photosynthetic bacteria (PSB) Rhodopseudomonas palustris CQK 01 immobilized in a PVA-boric acid gel granule is developed to enhance the rate of photo-hydrogen production. Particular attention is paid to exploring the effects of illumination wavelength and intensity, as well as the effects of concentration, flow rate, pH, and temperature of influent substrate solution on the hydrogen production rate. The immobilized PSB gel granule exhibited the maximum hydrogen production rate of 3.6 mmol/g cell dry weight/h in all tests. The experimental results show that the hydrogen production rate of an immobilized PSB granule illuminated at 590 nm is distinctly higher than that at 470 and 630 nm. Photo-inhibition of the gel granule occurs as the long-wavelength illumination intensity exceeds 7000 lux. In addition, there exists an optimal pH of 7.0 and temperature of 30 °C for PSB immobilized in the granule to produce hydrogen. More importantly, the feasibility of PSB immobilized in the PVA-boric acid gel granule for the enhancement of the photo-hydrogen production is demonstrated.