Numerical simulation of configuration and catalyst-layer effects on micro-channel steam reforming of methanol

A micro-reactor with eight non-parallel channels is proposed to improve the performance of micro-channel steam reforming of methanol. The widths of some channels in the micro-reactor vary gradually along the reactor length direction. The Zn-Cr/CeO₂-ZrO₂ catalyst is coated in the reformer with a certain porosity and permeability. The effects of micro-reactor structures and catalyst-coated manners on several factors are studied, including temperature distributions, velocity distributions, reactant concentrations and the methanol conversion rate. The results indicate that such a structure with a certain entrance inclination angle and channel inclination angle guarantees flow distribution uniformity in each reforming channel. Flow distribution uniformity is conducive to the increase of the methanol conversion rate. Besides, in order to measure strengths and weaknesses of different catalyst-coated manners, a wall-coated reformer and a packed-bed reformer are studied respectively. It is found that compared to the packed-bed reformer, the temperature and the methanol conversion rate in wall-coated reformer are far higher. It is necessary to find an optimal catalyst thickness that is able to reduce the CO concentration because the catalyst thickness can affect CO concentration in the product gases indirectly. The optimal inclination angles and the catalyst thickness are proposed based on the simulating results.     

Oxidative steam reforming of ethanol over Rh based catalysts in a micro-channel reactor

Oxidative steam reforming of ethanol (OSRE) was studied over Rh/CeO₂/Al₂O₃ catalysts in a micro-channel reactor. First, the catalyst support, Al₂O₃, was deposited on to the metallic substrate by washcoating and then the CeO₂ and active metal were sequentially impregnated. The effect of support composition as well as active metal composition on oxidative steam reforming of ethanol in a micro-channel reactor was studied at atmospheric pressure, with water to ethanol molar ratio of 6 and oxygen to ethanol molar ratio ranging from 0.5 to 1.5, over a temperature range of 350-550 °C. Ceria added to 1%Rh/Al₂O₃ showed higher activity and selectivity than 1%Rh/Al₂O₃ alone. Out of the various catalysts tested, 2%Rh/20%CeO₂/Al₂O₃ performed well in terms of activity, selectivity and stability. The OSRE performance was compared with that of SRE over 2%Rh/20%CeO₂/Al₂O₃ catalyst at identical operating conditions. Compared to SRE, the activity in OSRE was higher; however the selectivity to desired products was slightly lower. The H₂ yield obtained in OSRE was ∼112 m³ kg⁻¹ h⁻¹, as compared to ∼128 m³ kg⁻¹ h⁻¹ in SRE. The stability test performed on 2%Rh/20%CeO₂/Al₂O₃ at 500 °C for OSRE showed that the catalyst was stable for ∼40 h and then started to deactivate slowly. The comparison between packed bed reactor and micro-channel reactor showed that the micro-channel reactor can be used for OSRE to produce hydrogen without any diffusional effects in the catalyst layer.     

Simulation of subcooled flow boiling in a micro-channel

Bubble behavior was simulated to analyze the mechanism of subcooled boiling in a micro-channel. Bubble growth, condensation, and collapse in subcooled boiling, as well as the function of the degree of subcooling, lift-off diameter, heat flux, and mass flux are discussed. The influence of surface tension on interfacial heat transfer is likewise presented. In the calculated results, both onset of nucleate boiling and boiling heat transfer are influenced by micro-channel size. The simulated flow pattern corresponds with some experimental results. Findings on the boiling heat transfer from the simulation were compared with experiments.     

Microchannel heat exchangers for charge minimization in air-cooled ammonia condensers and chillers

This paper presents experimental results from a prototype ammonia chiller with an air-cooled condenser and a plate evaporator. The main objectives were charge reduction and compactness of the system. The charge is reduced to 20 g/kW (2.5 oz/Ton). This is lower than any currently available air-cooled ammonia chiller on the market. The major contribution comes from use of microchannel aluminum tubes. Two aluminum condensers were evaluated in the chiller: one with a parallel tube arrangement between headers and “microchannel” tubes (hydraulic diameter D_h = 0.7 mm), and the other with a single serpentine “macrochannel” tube (D_h = 4.06 mm). The performances of the chiller and condensers are compared based on various criteria to other available ammonia chillers. This prototype was made and examined in the Air Conditioning and Refrigeration Center in 1998, at the University of Illinois at Urbana-Champaign.     

大屏幕液晶电视微通道散热数值模拟

针对大屏幕液晶电视存在的散热问题,在综合考虑实际工况和MEMS技术基础上,设计了集成微通道的模拟液晶电视LED背光组件芯片.采用ANYSIS软件,对不采用微通道散热和采用40 μm微通道流速为0.000 1 m/s散热时的效果进行了数值模拟.在此基础上改变入口流速,模拟了不同入口流速对散热性能的影响.数值模拟结果证明,该方案设计具有高度的可靠性和可行性,可以在未来的电视设计技术中取得应用.     

Methane steam reforming: Kinetics and modeling over coating catalyst in micro-channel reactor

Kinetics of methane steam reforming for hydrogen production has been studied through experiment in a micro-channel reactor over coating catalyst. The catalyst coating prepared by cold spray on stainless steel substrate is based on a mixture of Ni–Al oxides which is normally employed in industry for methane primary steam reforming. Two kinetic laws namely parallel as well as inverse models have been derived at atmospheric pressure, and power law type kinetics have been established using non-linear least squares optimization. With the above kinetics, simulation study has been carried out to find out temperature distribution in the micro-channel over coating catalyst at two different types of boundary conditions. The results show a quite different “cold spot” character and reactants, products distribution character in the reaction channel due to its own distinct heat and mass transfer features. The kinetics and simulation study results can be applied in aid of micro-channel reactor design, and suggestion has been proposed for catalytic coating preparation and optimization.     

A pressure drop correlation for low Reynolds number Newtonian flows through a rectangular orifice in a similarly shaped micro-channel

Current microfabrication methods mean that rectangular orifices in similarly shaped micro-channels are often found in microfluidic devices. The power required to overcome the pressure drop across such orifices is often of importance. In the contribution reported here, numerical results for low Reynolds number incompressible Newtonian fluid flow through rectangular orifice in similarly shaped micro-channel have been used to develop a correlation for pressure drop arising from the orifice. The correlation, which was motivated by theoretical developments, indicates that the pressure drop is proportional to the average velocity through the orifice, and a function of the orifice contraction ratio, length-to-width ratio and, most particularly, aspect ratio.     

Study on the convective heat transfer and pressure drop in the micro-channel heat sink

Experiments have been performed to investigate the heat transfer characteristics and pressure drop in the micro-channel heat sinks under constant heat flux conditions. The experiments are performed for the Reynolds number and heat flux in the ranges of 200–1000 and 1.80–5.40 kW/m², respectively. The micro-channel heat sink with two different channel heights and two different channel widths are accomplished by wire electrical discharge machine. Effects of different geometrical configurations parameters of the micro-channel and heat flux on the heat transfer characteristics and pressure drop are considered. The micro-channel geometry configuration has significant effect on the enhancement heat transfer and pressure drop. The results of this study are expected to lead to guidelines that will allow the design of the micro-channel heat exchangers with improved heat transfer performance of the electronic devices.     

圆形和跑道形微通道内空气换热特性数值研究

采用数值模拟的方法对不同管径、不同长度微圆管的换热特性和换热能力进行了研究和对比。结果表明:对于相同管长的圆管,在相同压比条件下,除了内径为0.1mm的圆管外,单位流通面积的换热量随着管径的增大而逐渐减小;在相同压比条件下,除内径为0.1mm的圆管外,其他圆管的单位面积换热量均随管长的增加而增大,且较大管径的增大幅度明显大于较小管径的增大幅度。对当量直径相同的圆管和跑道形管进行对比发现,圆管的换热能力明显优于跑道形管道。     

基于微通道板的二维位置灵敏探测器的研制

介绍了基于微通道板的二维位置灵敏探测器的基本原理及其关键部分电阻阳极面板和数据读出系统的设计。通过测试,获得令人满意的线性和位置分辨。