Optimal monolithic configuration for heat integrated ethanol steam reformer

A design concept for optimal design of monolith catalyst is presented through modeling of transport–kinetic interactions in a monolith catalyst. We argue that reactors employing monolithic catalysts should be based on its optimal choice of geometry. In line with that argument, we present a thorough analysis of the geometrical parameters influencing the performance of non-isothermal reactor operation. In this study, an optimal monolith configuration is estimated to be a combination (d_h, t_w) of (0.9 mm, 0.2 mm) for a compact ethanol reformer to produce hydrogen for portable applications where maximum volumetric reactor activity exists. A three-dimensional modeling framework is developed for the resulting optimal monolithic catalyst design that couples the reforming section with a suitable heat source in a recuperative way. As a result, greater ethanol conversion is obtained from the monolith channels near the periphery of the block. The coupling with combustion could predict the formation of cold and hot spots inside the reactor, their nature being dependent on the flow configuration. Further, the effect of altering the feed inlet operating conditions over the variation of ethanol conversion and temperature inside the reactor is also analyzed. The increase in reforming inlet velocity decreases the outlet conversion and shifts the cold spot, forward and deeper in co-flow configuration. The decreasing inlet feed temperature enhances the transfer of heat, eliminating the cold spot.     

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACT

This article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm^?2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm^?2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.     

Modeling and analysis of microchannel autothermal methane steam reformer focusing on thermal characteristic and thermo-mechanically induced stress behavior

The application of fuel cells boosts the hydrogen demand particularly for distributed hydrogen production facility. As a potential candidate of hydrogen supply, microchannel autothermal methane steam reactor operates at high temperature and results in high thermal impact, which would decrease its stability and lifespan. A three-dimension numerical model based on finite element method was developed to evaluate the thermal characteristic and thermo-mechanically induced stress behavior of the reactor. Three different potential manufacturing materials, Fe–Cr–Al alloy, ceramic and quartz, were chosen. The results indicate that the cold-spot temperature appears near reactor inlet while the hotspot temperature appears near reactor outlet for reactor manufactured by different materials. Corresponding to the hot spot temperature, the maximum Von Mises stress appears near reactor outlet. The difference is the maximum Von Mises stress appears in catalyst layer for quartz reactor while it appears in interconnect rib for both Fe–Cr–Al alloy reactor and ceramic reactor. Meanwhile the maximum Von Mises stress reaches 1830 MPa for ceramic reactor. While the maximum Von Mises stress is 1197 MPa for quartz reactor and 1760 MPa for Fe–Cr–Al alloy reactor respectively. It implies the outlet catalyst layer region is vulnerable for quartz reactor. While the outlet interconnect rib is most vulnerable for both Fe–Cr–Al alloy reactor and ceramic reactor.     

A study of methanol steam reforming on distributed catalyst bed

Heterogeneous catalytic fixed bed usually suffers from severe limitations of mass and heat transfer. These disadvantages limit reformers to a low efficiency of catalyst utilization. Three catalyst activity distributions have been applied to force the reactor temperature profile to be near isothermal operation for maximization of methanol conversion. A plate-type reactor has been developed to investigate the influence of catalyst activity distribution on methanol steam reforming. Cold spot temperature gradients are observed in the temperature profile along the reactor axis. It has been experimentally verified that reducing cold spot temperature gradients contributes to the improvement of the catalytic hydrogen production. The lowest cold spot temperature gradient of 3 K is obtained on gradient catalyst distribution type A. This is attributed to good characteristics of local thermal effect. Low activity at the reactor inlet with gradual rise along with the reactor flow channel forms the optimal activity distribution. Hydrogen production rate of 161.3 L/h is obtained at the methanol conversion of 93.1% for the gradient distribution type A when the inlet temperature is 543 K.     

Inverse Design of Cooling Arrays of Micro Pin-Fins Subject to Specified Coolant Inlet Temperature and Hot Spot Temperature

Given a micro pin-fin array cooling scheme with these constraints: (a) given maximum allowable temperature of the material (the hot spot temperature), (b) given inlet cooling fluid temperature, (c) given total pressure loss (pumping power affordable), and (d) given overall thickness of the entire micro pin-fin cooling array, find the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is removed by the cooling fluid. The goal was to create an optimum performance map for a cooling micro array having specified inlet coolant temperature and maximum temperature. Fully 3D conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto set of best trade-off solutions. These Pareto optimized configurations indicate the maximum physically possible heat fluxes for specified material and constraints. Detailed off-design performance maps of such Pareto-optimized cooling arrays of micro pin-fins were calculated that demonstrate superior on-design and off-design performance of pin-fins having symmetric convex cross sections as opposed to the commonly used circular cross sections.     

Hydrogen production by oxidative steam reforming of methanol over anodic aluminum oxide-supported Cu-Zn catalyst

Oxidative steam reforming of methanol (OSRM), which is a convenient reaction for producing hydrogen, suffers from the hot spot formation problem when conventional particle catalysts are used. Recently, an anodic aluminum oxide (AAO)-supported Cu-Zn catalyst was proposed as an OSRM catalyst for its high thermal conductivity through the aluminum metal body. In this study, OSRM was conducted in a prototype reactor packed with the AAO plate catalyst strips. It was verified that the high thermal conductivity of the catalyst effectively suppresses the hot spot formation and makes the temperature profile smooth along the reactor. The start-up time of the reactor depended on the preheating temperature and was very short (less than 2 min) for preheating over 503 K. The methanol conversion and reactor temperature increased with increasing O₂/CH₃OH mole ratio, indicating that the mole ratio can be used as a control variable to operate the reactor at desired conditions. Further, a reactor model was developed and verified, and the simulation showed that for a given total reactor volume, an optimal reactor configuration could be achieved by shortening the reactor length while widening the cross-sectional area.     

Minimizing hot spot temperature of porous stackings in natural convection

Due to their large surface of heat transfer per volume, porous structures such as metallic foams are considered as an interesting alternative to fins. In this paper, we investigate the optimal configuration of a porous medium structure with the objective to minimize the hot spot temperature in natural convection. The heat sink is adjacent to a heat-generating plate, and consists of a stacking of porous layers, in which a cooling fluid circulates strictly driven by natural convection. The objective of this work is to minimize the hot spot temperature of the system. The design variables are the porosity and the material of each layer. The thermal performance is evaluated with a CFD code based on a finite volume approach. The hot spot temperature minimization is pursued with a genetic algorithm (GA) under global mass and cost constraints. The GA determines the optimal porosity and selects the most appropriate material of each layer. Furthermore, the optimal total length of the stacking is indirectly determined by the GA as layers can be added or removed in order to improve the global performance and/or satisfy the constraints. A mapping of the designs generated by the GA as a function of the mass and cost constraint combination reveals that an appropriate distribution of porosity and material benefits the overall thermal performance of the layered porous medium.     

Comparative numerical evaluation of autothermal biogas reforming in conventional and split-and-recombine microreactors

A new microreactor design featuring embedded passive mixing elements was tested as a means to enhance autothermal reforming reaction of biogas over a novel ReNi/γ-Al₂O₃ catalyst. To determine an optimal condition that would result in completely converted biogas with H₂/CO product ratio of around one and minimal hot spot formation inside the reactor, use of various inlet O₂ and H₂O concentrations and inlet temperatures were numerically investigated. The influence of inlet reactant velocity on the reactor effectiveness was then studied at the optimal condition. Performance of a straight-channel microreactor was also studied and compared with that of the novel microreactor. The O₂:H₂O:CO₂:CH₄ ratio of 25:5:28:42% (v/v) and inlet temperature of 730 °C were noted as the optimal condition for the novel microreactor. Complete biogas conversion over a wider range of inlet Reynolds number, lower required catalyst loading to achieve the desired reactor performance, higher H₂ and CO selectivities and reduced hot spot formation were noted as the advantages of the novel microreactor.     

Thermal analysis of a micro tubular reactor for methanol steam reforming by optimizing the multilayer arrangement of catalyst bed for the catalytic combustion of methanolr

Packed bed tube reactors are commonly used for hydrogen production in proton exchange membrane fuel cells. However, the hydrogen production capacity of methanol steam reforming (MSR) is greatly limited by the poor heat transfer of packed catalyst bed. The hydrogen production capacity of catalyst bed can be effectively improved by optimizing the temperature distribution of reactor. In this study, four types of reactors including concentric circle methanol steam reforming reactor (MSRC), continuous catalytic combustion methanol steam reforming reactor (MSRR), hierarchical catalytic combustion methanol steam reforming reactor (MSRP) and segmented catalytic combustion reactor with fins (MSRF) are designed, modeled, compared and validated by experimental data. It was found that the maximum temperature difference of MSRC, MSRR, MSRP and MSRF reached 72.4 K, 58.6 K, 19.8 K and 11.3 K, respectively. In addition, the surface temperature inhomogeneity U_f and CO concentration of the MSRF decreased by 69.8% and 30.7%, compared with MSRC. At the same reactor volume, MSRF can achieve higher methanol conversion rate, and its effective energy absorption rate is 4.6%, 3.9% and 2.6% higher than that of MSRC, MSRR and MSRP, respectively. The MSRF could effectively avoid the influence of uneven temperature distribution on MSR compared with the other designs. In order to further improve the performance of MSRF, the influences of methanol vapor molar ratio, inlet temperature, flow rate, catalyst particle size and catalyst bed porosity on MSR were also discussed in the optimal reactor structure (MSRF).     

Distribution of heat sources in vertical open channels with natural convection

In this paper we use the constructal method to determine the optimal distribution and sizes of discrete heat sources in a vertical open channel cooled by natural convection. Two classes of geometries are considered: (i) heat sources with fixed size and fixed heat flux, and (ii) single heat source with variable size and fixed total heat current. In both classes, the objective is the maximization of the global thermal conductance between the discretely heated wall and the cold fluid. This objective is equivalent to minimizing temperature of the hot spot that occurs at a point on the wall. The numerical results show that for low Rayleigh numbers (∼10²), the heat sources select as optimal location the inlet plane of the channel. For configuration (i), the optimal location changes as the Rayleigh number increases, and the last (downstream) heat source tends to migrate toward the exit plane, which results in a non-uniform distribution of heat sources on the wall. For configuration (ii) we also show that at low and moderate Rayleigh numbers (Ra_M ∼ 10² and 10³) the thermal performance is maximized when the heat source does not cover the entire wall. As the flow intensity increases, the optimal heat source size approaches the height of the wall. The importance to free the flow geometry to morph toward the configuration of minimal global resistance (maximal flow access) is also discussed.     

The optimal spacing of parallel plates cooled by forced convection

This paper reports the optimal board-to-board spacing and maximum total heat transfer rate from a stack of parallel boards cooled by laminar forced convection. The optimal spacing is proportional to the board length raised to the power 1/2, the property group (μ)^1/4, and (ΔP)^−1/4, where ΔP is the pressure head maintained across the stack. The maximum total heat transfer rate is proportional to (ΔP)^1/2, the total thickness of the stack (H), and the maximum allowable temperature difference between the board and the coolant inlet. Board surfaces with uniform temperature and uniform heat flux are considered. It is shown that the surface thermal condition (uniform temperature vs uniform heat flux) has a minor effect on the optimal spacing and the maximum total heat transfer rate.     

Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion

Colorless distributed combustion (CDC) investigated here is focused on gas turbine combustion applications due to its significant benefits for, much reduced NOx emissions and noise reduction, and significantly improved pattern factor. CDC is characterized by distributed reaction zone of combustion which leads to uniform thermal field and avoidance of hot spot regions to provide significant improvement in pattern factor, lower sound levels and reduced NOx emission. Mixing between the combustion air and product gases to form hot and diluted oxidant prior to its mixing with the fuel is critical so that one must determine the most suitable mixing conditions to minimize the ignition delay. Spontaneous ignition of the fuel occurs to provide distributed reaction combustion conditions. The above requirements can be met with different configuration of fuel and air injections with carefully characterized flow field distribution within the combustion zone. This study examines four different sample configurations to achieve colorless distributed combustion conditions that reveal no visible color of the flame. They include a baseline diffusion flame configuration and three other configurations that provide conditions close to distributed combustion conditions. For all four modes same fuel and air injection diameters are used to examine the effect of flow field configuration on combustion characteristics. The results are compared from the four different configurations on flow field and fuel/air mixing using numerical simulations and with experiments using global flame signatures, exhaust emissions, acoustic signatures, and thermal field. Both numerical simulations and experiments are performed at a constant heat load of 25 kW, using methane as the fuel at atmospheric pressure using normal temperature air and fuel. Lower NOx and CO emissions, better thermal field uniformity, and lower acoustic levels have been observed when the flame approached CDC mode as compared to the baseline case of a diffusion flame. The reaction zone is observed to be uniformly distributed over the entire combustor volume when the visible flame signatures approached CDC mode.     

Unsteady thermal flow analysis in a heat regenerator with spherical particles

Heat regenerator occupied by regenerative materials improves thermal efficiency of regenerative combustion system through the recovery of sensible heat of exhaust gases. By using one-dimensional two-phase fluid dynamics model, the unsteady thermal flow of regenerator with spherical particles, were numerically analysed to evaluate the heat transfer and pressure drop and to suggest the parameter for designing heat regenerator. It takes about 7 h for the steady state in the thermal flow of regenerator, where heat absorption of regenerative particle is concurrent with heat desorption. The regenerative particle experiences small temperature fluctuation below 10 K during the reversing process. The thermal flow in heat regenerator varies with inlet velocity of exhaust gas and air, configuration of regenerator and diameter of regenerative particle. As the gas velocity increases with decreasing the cross-sectional area of the regenerator, the heat transfer between gas and particle enhances and pressure losses increase. As particle diameter decreases, the air is preheated higher and the exhaust gases are cooled lower with the increase of pressure losses. At the same exhaust gases temperature at the regenerator outlet, the regenerator length need to be linearly increased with inlet Reynolds number of exhaust gases. It is confirmed that inlet Reynolds number of exhaust gases should be introduced as a regenerator design parameter. Copyright © 2002 John Wiley & Sons, Ltd.     

Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments

Backfire, an abnormal combustion phenomenon, in a hydrogen fuelled spark ignition (SI) engine was analyzed using computational fluid dynamics (CFD) and experimental tests. One of the main causes of backfire origin is the presence of any high temperature heat source including hot spot in the combustion chamber of the engine during intake process. A CFD based parametric study was carried out by varying the temperature of hot spot and its location in the combustion chamber of the engine in order to analyze their effects on backfire origin and its propagation in the intake manifold of the engine. The temperature of hot spot was varied from 800 K to till the temperature of backfire occurrence. The minimum temperature of hot spot at which backfire occurred was observed as 950 K and beyond. The probability of backfire occurrence increases with increase in hot spot temperature. The CFD simulations were also carried out by varying the location of hot spot (spark plug tip and exhaust valve) and the results indicate that the location of hot spot does not influence the characteristics of backfire but it affects the timing of its origin. The average backfire velocity is 230 m/s based on the average turbulent flame velocity during backfire propagation in the intake manifold and the value agreed reasonably well with the experimental observations of backfire propagation on the engine with the transparent intake manifold. Backfire propagation is under the category of deflagration based on its velocity (subsonic), and the maximum pressure gradient (<0.3 bar). The backfire phenomenon is characterized into three stages namely ignition delay for backfire, backfire propagation and its termination. The study results provide a better in-depth understanding of backfire origin and its propagation and would be helpful for developing a robust control strategy. Based on this study, it is recommended that the spark plug and exhaust valves of hydrogen fuelled SI engine should be customized in such a way that the temperature of spark plug tip and exhaust valves should not exceed 900 K during suction process in order to eliminate backfire occurrence.     

Effects of catalyst segmentation with cavities on combustion enhancement of blended fuels in a micro channel

A novel design concept for combustion enhancement of H₂/CO, CH₄/CO, and H₂/CH₄ blended fuels in a micro channel using combined effects of catalyst segmentation and cavities is proposed. The enhancement and combustion characteristics are evaluated by numerical simulation with detailed heterogeneous and homogeneous chemistries. Effects of unsegmented and segmented catalysts with and without cavities are examined and discussed in terms of different multi-fuel mixtures. In general, it is found that the chemical process of conventional catalytic combustion is a competition for fuel, oxygen, and radicals between heterogeneous and homogeneous reactions. On the other hand, the purpose of using catalyst segmentation and cavities in a micro-reactor is to integrate advantages of heterogeneous and homogeneous reactions, to enhance fuel conversion, and to promote complete combustion in a short distance. In the proposed catalyst configuration, the heterogeneous reaction in a prior catalyst segment produces chemical radicals and catalytically induced exothermicity, and the homogeneous reaction can be subsequently ignited and anchored in the following cavity. H and OH radicals from both hydrogen and methane may obviously change the chemical pathway of CO oxidation. Full multi-fuel conversion and complete combustion can thus be achieved in a short distance. The existence of cavities appreciably extends the stable operational range of the micro-reactor for a wide range of inlet flow velocities. Moreover, cavities in a small-scale system can further stabilize the flame, and serve as a heat source to enhance the reaction. These features allow the proposed catalyst configuration to apply to various small-scale power, heat generation and propulsion systems.     

一次表面回热器进气结构对物流分配性能的影响

不合理的进气结构造成一次表面回热器(PSR)内部物流分配不均匀,影响同热器内部流体的流动和传热效果.本文提出了具有导流片的进气结构,并在尽可能地保证出口截面速度均匀分布的前提上,开展了不同进气结构对回热器芯体入口物流分配影响的研究.结果表明,在所研究的雷诺数范围内,改进后的进气结构中各通道出口截面速度的最大波动只有14...     

Numerical Investigation of Jet Impingement Cooling of a Flat Plate with Carbon Dioxide at Supercritical Pressures

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6 to 1.6 MW/m². The shear-stress transport turbulence model was used and the numerical model was validated by comparison with experimental results for jet impingement heating with hot water at supercritical pressures. Radial conduction in the jet impingement plate was also considered. The sharp variations of the thermal-physical properties of the fluid near the pseudocritical point significantly influence heat transfer on the target wall. For a given heat flux, the high specific heat near the wall for the proper inlet temperature and pressure maximizes the average heat transfer coefficient. For a given inlet temperature, the heat transfer coefficient remains almost unchanged with increasing surface heat flux at first and then decreases rapidly as the heat flux becomes higher due to the combined effects of the thinner high specific heat layer and the smaller thermal conductivity at higher temperature.     

Non‐isothermal multi‐phase modeling of PEM fuel cell cathode

In this study, numerical simulation has been carried out for the heat transfer and temperature distribution in the cathode of polymer electrolyte membrane fuel cells along with the multi‐phase and multi‐species transport under the steady‐state condition. The commercial software, COMSOL Multiphysics, is used to solve the conservation equations for momentum, mass, species, charge and energy numerically. The conservation equations are applied to the solid, liquid and vapor phases in the bipolar plate and gas diffusion (GDL) and catalyst layers of a two‐dimensional cross section of the cathode. The catalyst layer is assumed to be a finite domain and the water production in the catalyst layer is considered to be in the liquid form. The temperature distribution in the cathode is simulated and then the effects of the relative humidity of the air stream, the permeability of the cathode and the flow channel shoulder to channel width ratio are investigated. It is shown that the highest temperature change, both in the in‐plane and across‐the‐plane directions, occurs in the GDL, while the highest temperature is reached in the catalyst layer. The distribution of temperature in the bipolar plate is shown to be relatively uniform due to the high thermal conductivity of the plate. A decrease in the inlet relative humidity of the air stream results in the decrease of the maximum temperature due to the absorption of heat during the evaporation of liquid water in the GDL and catalyst layer. The non‐uniformity of the temperature distribution, especially in the catalyst layer, is observed with the increase of the permeability of the cathode. Similarly, the decrease of the channel shoulder to channel width ratio leads to a non‐uniform distribution of temperature especially under the channel areas. Copyright © 2009 John Wiley & Sons, Ltd.     

二甲醚均质压燃发动机燃烧特性的研究

二甲醚均质压燃发动机由一台单缸柴油机改造而成,其压缩比为10．7。二甲醚气体随进气进入气缸,形成均质混合气。通过试验采集分析缸内压力,结果表明二甲醚均质压燃燃烧是一个两阶段放热过程,分别发生在610K和900K左右。第一阶段放热量较少,约占10％,正常情况下第二阶段集中在上止点附近,释放出70％以上的燃料热量。发动机负荷对最大缸压力及其出现位置、压力升高率和放热率曲线形状等都有重要影响,而发动机转速对它们的影响比较小。     

Numerical investigation on the self-ignition and combustion characteristics of H2/air in catalytic micro channel

The self-ignition of hydrogen/air is an important process in the micro thermophotovoltaic system. The transient numerical models of gas-phase reaction and catalytic reaction in the various catalytic micro combustors were built and verified. The self-ignition process of gas-phase reaction caused by catalytic reaction in the catalytic micro channel with conventional heat dissipation was studied. The self-ignition process could be divided into four stages, fuel diffusion stage - pure catalytic reaction stage - flame front moving stage - stable combustion stage. The ignition time and temperature limit at different inlet temperatures, inlet velocities and channel heights were analyzed. The results showed that the wall quenching effect, thermal effect and flame propagation effect are dominant at low temperature, medium temperature and high temperature respectively. The catalyst length and the mixture internal energy were the main factor at low inlet velocity and high inlet velocity respectively. The steady-state time was also studied in the various operation conditions. Finally, the catalytic combustion characteristics in the stable combustion stage were analyzed. The influence of inert section length, inlet temperature and inlet velocity on the maximum temperature and fuel conversion ratio were investigated.