Thermal stability of metal-supported catalysts for reduction of cold-start emissions in a wood-fired domestic boiler

The aim of the present work is to develop a catalyst based on a mixture of manganese oxides and platinum supported on a metallic monolith for abatement of emissions from wood combustion, particularly during the cold-start phase. The activity and the thermal stability of the catalysts have been studied in the laboratory, before performing tests in a wood-stove. The effect of the hydrothermal treatment at 900°C on the adherence of the washcoat onto a metallic substrate was studied using scanning electronic microscope. It revealed well-adhering washcoat onto the metallic support due to the growth of the alumina whiskers during the treatment. The influence of the amount of washcoat, as well as the influence of the concentration of manganese oxides in it (Mn: 5 to 20 mol%/Al₂O₃) on the activity of fresh and hydrothermally-treated catalysts were studied. The activity tests were carried out using a mixture of carbon monoxide, naphthalene and methane in the presence of air, steam and carbon dioxide to resemble the flue gases from wood combustion. On the fresh catalysts, containing the same total amount of manganese, a high concentration of manganese oxides in the washcoat favoured the oxidation of carbon monoxide and naphthalene, whereas a lower concentration of manganese oxides in the washcoat gave higher activity for the oxidation of methane. An increased total amount of manganese oxides in the catalysts, which had the same amount of washcoat, resulted in an increase in activity for the oxidation of the three combustibles. After thermal treatment at 900°C for 270 h in steam, most of the manganese oxide catalysts were activated for the oxidation of carbon monoxide and naphthalene while only being slightly deactivated for the oxidation of methane. The addition of manganese oxides in the washcoat, however, lowers the temperature of the γ- to -alumina phase transformation. Platinum (0.5 mol%) was added to the manganese oxide (10 mol%) catalyst to improve its activity. A platinum catalyst was also tested for comparison. The platinum and the mixed catalysts showed similar activity for the oxidation of carbon monoxide and naphthalene, while the mixed catalysts were more active for the oxidation of methane. A similarly mixed MnO_x–Pt (10–0.5 mol%) catalyst supported on Al₂O₃ stabilised with 3% lanthanum, but at larger scale, was tested in a wood-stove. The possibility of pre-heating the catalyst during the start-up phase was studied. The tests revealed a strong decrease of the carbon monoxide and unburned hydrocarbons emissions during the start-up phase when the catalyst was pre-heated with hot air compared with no pre-heating or no catalyst.     

基于用户浏览轨迹的商品推荐

随着电子商务的迅速发展,推荐系统在这些网站中得到了广泛的应用。目前应用最广泛的个性化推荐算法是协同过滤推荐算法,但是该方法存在稀疏矩阵与冷启动问题。根据用户浏览记录推荐商品是缓解这些问题的一个重要研究方向,这些方法根据用户在电子商务网站的访问日志,提取出用户的浏览路径序列,即用户浏览轨迹,为用户推荐偏爱商品。目前,通过分析用户浏览路径为用户推荐商品的方法主要依据用户浏览轨迹模式匹配或者从用户浏览轨迹中商品与下一个商品关系的角度进行考虑。而本研究从浏览轨迹中被浏览商品与最终被购买商品关系的角度出发,并以此为基础建立用户浏览轨迹偏爱模型,挖掘用户偏爱,为用户推荐商品。实验表明,所提方法能够在一定程度上解决因为新用户缺少历史购买及评分记录而引起的新用户冷启动问题,提高了推荐方法的准确度与召回率。     

冷启动时PTC-base喷油器内高压瞬变流场传热研究

汽车在冷启动时有害物质排放很高,这和燃油温度有很大关系。本研究提出一种基于PTC的喷油器,该喷油器是在普通喷油器内部油路周围嵌入PTC材料,并以车载电源供电。当温度低于居里温度时,PTC材料的电阻很小,因此可产生大量热量,这些热量可以使流经管路的燃油得到加热。当温度达到居里温度时,PTC材料电阻变得很大,材料停止发热。利用PTC材料的这种特殊的阻温特性,我们可以在车辆启动时把喷油器出口的燃油温度控制在一个理想的范围。本研究的主要工作是对PTC-base喷油器建立模型并对不同条件下该喷油器的燃油传热进行分析。分析结果表明,冷启动时,在PTC-base喷油器中,燃油可以得到很好的加热。这对降低汽车冷启动排放将产生重要的作用。     

Numerical analyses of non-isothermal self-start behaviors of PEM fuel cells from subfreezing startup temperatures

In this paper, a multiphase multidimensional PEM fuel cell model for cold-start simulations has been employed for numerical analyses of the non-isothermal self-start behaviors of a PEM fuel cell from subfreezing startup temperatures, focusing on the coupled phenomena of the ice formation and temperature increase inside the cell. The roles played by many key influential parameters, including the water vapor concentration in the cathode gas channel, the initial water content inside the membrane, the operating current density, and the startup cell temperature, are carefully examined. Numerical results indicate that decreasing the interfacial water vapor concentration at the gas diffusion layer and gas channel surface on the cathode side of the cell would delay ice precipitation and prolong the cell operation time. Decreasing the operation current density and the initial water content inside the membrane, and increasing the startup cell temperature are beneficial for the non-isothermal cold starts of the PEM fuel cell and could lead to successful self-starts.     

Numerical investigation of cold-start behavior of polymer electrolyte fuel cells in the presence of super-cooled water

In this study, a mathematical model has been developed to simulate the transient cold-start processes of polymer electrolyte fuel cells. The super-cooled water is assumed to exist within the cell. The non-equilibrium water transfer between the membrane and the catalyst layer is considered. The models of water freezing and ice melting in the catalyst layer and gas diffusion layer have been established. For the first time, the randomicity of the freezing process is captured by introducing a freezing probability function. Based on this model, the cold-start processes of a single polymer electrolyte fuel cell starting at various operating and initial conditions have been simulated numerically. The results indicate that the cold-start performance of the cell is determined by the water storage potential of the electrolyte in cathode catalyst layer. For each startup temperature and operating current load, there is a most appropriate initial membrane water content, which corresponds to the longest cell shutdown time. When the cold-start process is failed, the ice is mainly accumulated in the cathode catalyst layer. The ice distribution becomes more non-uniform as the cold-start temperature is lower.     

基于耦合相似度的矩阵分解推荐方法

随着因特网和信息技术的高速发展,信息过载现象越来越严重。推荐系统能够给个人和商家(例如电子商务和零售商)提供个性化的推荐。数据稀疏性和分数预测质量问题被公认为是现存推荐系统中的主要挑战。当前绝大多数推荐系统技术都依赖于协同过滤方法,它主要利用用户-项目评分矩阵来表示用户和项目之间的关系。一些研究利用附加信息来提高推荐准确性,但是,绝大多数现存的引入项目之间关系的方法并不能很好地用于预测和推荐,因为其假设项目属性之间是独立同分布的,而实际上项目(或用户)的属性之间是存在耦合关系的。由此提出了基于属性耦合关系的矩阵分解模型,它能有效地刻画项目之间的耦合相关性,从而更加合理 地预测用户对项目的评分。实验结果表明,所提出的模型在热启动和冷启动的推荐准确性方面均优于传统的推荐算法。     

Shared cache architectures for decision support systems

In this paper we evaluate two shared-cache architectures for small-scale multiprocessors. We vary shared cache sizes from 8MB to 1GB, under various block sizes, cache organizations and sizes, and strategies for IO transactions. We use 12 bus trace samples obtained during the execution of a 100GB TPC-H on an eight-way multiprocessor.

To deal with the cold-start misses at the beginning of each sample, we identify the sure misses which are known to be misses in the full trace. The difference between the total number of misses and the number of sure misses is the zone of uncertainty, which may be hits or misses in the full trace. It turns out that the zone of uncertainty is small enough in most cases that useful conclusions can be drawn.

Our conclusions are that a single-cluster configuration with a shared cache—even a very small one—can be very effective for TPC-H. We also show that the coherence traffic between shared caches in a multiple cluster system is very high in the context of TPC-H.     

Imaging of desaturation of the frozen gas diffusion layers by synchrotron X-ray radiography

The visualization of the thawing and desaturation process on an initially saturated, frozen gas diffusion layer (GDL) with a serpentine gas flow channel was performed based on synchrotron X-ray computed tomography images. High speed CT scanning during the experiments allowed the dynamic desaturation process to be quantified under the cold-start with air purging condition. The saturation profiles and the desaturation rates were studied over the entire GDL domain, through-plane, and in selected regions of interest for localized behavior. Sigracet 35AA and 35BA GDLs were selected for the experiments to study the effects of GDL hydrophobicity. Along with the real-time saturation profiles, the average desaturation rates for the entire GDL domain over the whole purging process were 0.000186 μL cm^?2 s^?1, 0.000470 μL cm^?2 s^?1, 0.000516 μL cm^?2 s^?1 and 0.000901 μL cm^?2 s^?1 with the superficial gas velocity of the purging air at 2.88 m/s, 4.26 m/s, 5.98 m/s and 9.02 m/s, respectively. In addition, the dynamic saturation contours and 3-D GDL geometry models were constructed to show the liquid water movement through a GDL. Although the GDL desaturation curves for each experiment share similar trends, the results show that different conditions including air flow rate, GDL geometric location, initial water saturation, and GDL boundary condition could cause heterogeneous desaturation behavior on both overall and localized GDL regions. These data provide valuable information for future modeling studies that involve the thawing process in the GDL, and could be used to optimize the cell design and develop cold-start protocols.     

One-dimensional thermal model of cold-start in a polymer electrolyte fuel cell stack

A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component's relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0 °C was found to be the most effective way to achieve the rapid start-up.     

Cold-start hydrocarbon emissions in port-injected gasoline engines

An analysis is made of the sources of the high engine-out hydrocarbon (HC) emissions during cold starting of port-injected gasoline engines. A cycle-by-cycle analysis of the different parameters, which affect engine-out HC emissions, is made during the startup process. The contribution of each cylinder of a four-stroke V6, 3.3 l production engine in the total HC emissions is investigated. The HC emissions were measured in the exhaust port using a fast response flame ionization detector (FID). The effect of the initial startup position of the piston and valves in the cycle on combustion and HC emissions is examined. The mass of fuel injected, burned and emitted was calculated for each of the first 120 cycles. Different approaches to reduce engine-out and tailpipe HC emissions during cold-start are discussed.