Modeling of methane steam reforming in a microchannel with a heat flow distributed in length

Numerical modeling of a flow of a high-temperature mixture of methane with water vapors in a two-dimensional plane microchannel with activation of chemical conversions on the channel wall has been performed. The modeling was performed within the framework of Navier-Stokes equations for a laminar flow of a multicomponent compressible gas. The influence of the external heat flux supplied to the gas mixture and its distribution along the channel length on the properties of the methane steam reforming have been investigated. It has been shown that not only the amount of heat supplied to the reaction zone but also the method of heat supply along the channel length are important. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for obtaining a synthesis gas.     

Thermal regime of slotted channel with moving incompressible liquid under microwave conditions

The article studies mathematical simulation of microwave heating of flow in a slotted channel. The internal heat sources, which are proportional to the absorbed microwave energy, obey the Bouguer law. The stationary temperature distributions in the liquid, wall inner surface andmiddle of the wall along the channel have been found from the balance of heat supply and heat release. The maximum temperature values over channel cross sections have also been determined. The heat transfer to the flow was realized in the nonboiling convective regime. The microwave power was selected such that themaximum temperatures and heat fluxes did not exceed themaximumallowable values for the materials.     

Modeling of methane steam reforming in a microchannel subject to multicomponent diffusion

Catalytic methane steam reforming in a slot microchannel under external heat supply to the mixture reacting on walls is considered based on numerical simulation of a complete system of Navier-Stokes equations. Three ways of heat supply to channel walls are represented, namely, a uniform heat flux, a heat flux linearly decreasing in channel length, and a heat flux following the reaction rate profile of the main reaction. The thermophysical parameters of the mixture depend on its temperature and composition. Two diffusion models are considered, namely, models with equal and different diffusion coefficients for each mixture component. It is shown that consideration of multicomponent diffusion does not practically affect the concentration of the components and the methane reforming at the outlet. For the above-mentioned ways of heat supply, the methane reforming with a heat flux linearly decreasing in channel length is most significant.     

Infrared thermography as a tool for thermal surface flow visualization

Infrared (IR) thermography is a two-dimensional, non-contact technique of temperature measurement which can be usefully exploited in a vast variety of heat transfer industrial applications as well as research fields. The present work focuses attention on thermal surface flow visualizations of several types of fluid flow studied by means of the IR imaging system and in particular: the flow over a delta wing at angle of attack; the flow generated by a disk rotating in still air; air jets impinging on a flat wall; the flow inside a 180deg turn in astatic channel with, or without, turbulence promoters; the flow inside a 180deg turn in arotating square channel. Each flow visualization is illustrated through thermographic images and/or Nusselt number maps. The emphasis is on the capability of the infrared system to study: laminar-to-turbulent transition and location of primary and secondary vortices over the delta wing at angle of attack; the spiral vortical structure developing at transition over the disk; azimuthal structures arising for certain jet conditions; the influence of the channel aspect ratio (width to height ratio) on the heat transfer coefficient distribution along the 180deg turn, as well as the influence of ribs, in the case ofstatic channel; the influence of rotation for the rotating channel.     

微针肋槽道内去离子水换热特性

设计了槽肋比为1:2和2:1的矩形大长宽比微针肋散热器,并实验研究了去离子水在其内的流动换热性能。结果表明:当进口温度为40 ℃、微针肋槽道在雷诺数小于650、最高壁面温度低于77 ℃时,单位面积散热量可达21.32 W/cm2。当雷诺数一定时,同一个槽道壁面温度沿着流动方向不断增加、同一个位置壁面温度随着加热功率的增加而增大,局部努谢尔数沿着流动方向先减小后逐渐增加并趋于定值。当针肋流动换热长度较长时,其入口效应可以忽略,槽道平均努谢尔数随着雷诺数的增大而增大,与加热功率无关;为了更好地表达微针肋槽道内的换热特性,考虑了槽肋比、流动雷诺数等影响,拟合了去离子水在微针肋槽道内的对流换热关系式。     

Edge flames stabilized in a non-premixed microcombustor

The dynamics of an edge flame confined in a non-premixed microcombustor model is studied numerically within the context of a diffusive-thermal model. Fuel and oxidizer, separated upstream by a thin plate, flow through a channel with a prescribed velocity. At the tip of the plate, the fuel and oxidizer mix and, when ignited, an edge flame is sustained at some distance from the plate. The objective in this work is to consider the effects of confinement, differential diffusion, and heat loss on the dynamics of an edge flame in a narrow channel. We consider a wide range of channel widths and allow for changing Lewis numbers, and both adiabatic conditions and heat losses along the channel walls. The results illustrate how the flame shape and standoff distance are affected by the channel width, by mixture composition through variations in Lewis numbers and by heat losses. Conditions for flame stabilization, flame oscillations and flame extinction or blowoff are predicted.     

Numerical heat transfer analysis in turbulent channel flow over a side-by-side triangular prism pair

Heat transfer and flow behavior in a channel fitted with a transverse triangular prism pair is numerically investigated in the turbulent flow regime for the Reynolds number ranging from 10000 to 50000. The aspect ratio of channel height to the prism base is fixed at 4.0 throughout the study. The Navier-Stokes equation, along with the energy equation, is solved using a finite volume method with the SIMPLE technique and the QUICK numerical scheme for coupling the discretized equations while the standard k-ɛ turbulence model is used for closure of the problem. The numerical result reveals that heat transfer augmentation in the channel with the built-in prism pair can be obtained. It is observed that as compared to a channel, the heat transfer is enhanced by about 17% for a single triangular prism and by some 85% for a triangular prism pair mounted on the channel wall. Effects of the clearance between the prisms on the heat transfer augmentation are presented. The heat transfer enhancement is due to the vortex formation or recirculation zone downstream of the prism elements. However, the presence of the prisms also leads to higher values of friction loss over the channel.     

Modeling of vapor condensation in a longitudinally finned minichannel

Heat transfer with vapor condensation inside a longitudinally finned tube is numerically studied. The proposed model considers vapor condensation on two initial flow areas, namely, annular and rivulet. The model allows prediction of pressure difference along the tube length, vapor velocity profiles in the central channel and an interfin groove, and also a velocity profile in the condensate rivulet at the bottom of the interfin channel, local heat transfer coefficients at different fin points, and average heat transfer coefficients over tube section and length. The calculations showed that in the case of vapor condensation in longitudinally finned tubes of a small diameter it is of fundamental importance to divide the flow tube section into a central channel and interfin channels. The governing vapor velocities in these channels may differ by more than an order of magnitude. The reduced vapor velocity, used in engineering calculations, does not reflect the character of dynamic vapor impact on a condensate film on the most part of the heat transfer surface. For tubes with relatively large fins the proposed model describes vapor condensation almost completely,meanwhile, the mass vapor quality by the time of filling of the grooves reaches 0.01–0.05. The highest heat transfer intensification was obtained for “sharp fins” with a high value of the fin head curvature. Comparison of results of calculation by the model with results of the known experiments on water vapor condensation yields a good qualitative and quantitative agreement for low vapor velocities at the channel inlet (under 30 m/s). The wall thermal conductivity coefficient value affects significantly the condensation efficiency.     

The numerical investigation of heat transfer and pressure drop of turbulent flow in a triangular microchannel

In this presentation, the flow and heat transfer inside a microchannel with a triangular section, have been numerically simulated. In this three-dimensional simulation, the flow has been considered turbulent. In order to increase the heat transfer of the channel walls, the semi-truncated and semi-attached ribs have been placed inside the channel and the effect of forms and numbers of ribs has been studied. In this research, the base fluid is Water and the effect of volume fraction of Al₂O₃ nanoparticles on the amount of heat transfer and physics of flow have been investigated. The presented results are including of the distribution of Nusselt number in the channel, friction coefficient and Performance Evaluation Criterion of each different arrangement. The results indicate that, the ribs affect the physics of flow and their influence is absolutely related to Reynolds number of flow. Also, the investigation of the used semi-truncated and semi-attached ribs in Reynolds number indicates that, although heat transfer increases, but more pressure drop arises. Therefore, in this method, in order to improve the heat transfer from the walls of microchannel on the constant heat flux, using the pump is demanded.     

圆管管翅散热器肋侧主流方向绝对涡通量研究

本文以圆管管翅式散热器为研究对象,采用数值模拟的方法对圆管管翅式散热器肋侧空气通道主流方向绝对涡通量的特性进行了探讨。给出了肋侧空气通道主流方向绝对涡通量无量纲化参数,二次流雷诺数;横向管间距、肋侧空气流动雷诺数对二次流雷诺数及努塞尔数的影响。     

Vortex pattern of the turbulent flow around a single cube on a flat surface and its heat transfer at different attack angles

Experimental results on convective heat transfer from a single cube on a flat surface are presented for different attack angles to the incident flow and Reynolds numbers. The character of vortex formation and the effect of flow structure on heat transfer at detached flow around a cube were studied by visualization. Local heat transfer and heat transfer averaged over the separate faces and the whole lateral surface of the cube were studied. Contribution of separate cube faces to total heat transfer depending on the attack angle was estimated. Data obtained were compared with those published in literature. The reasons for observed differences caused by the effect of many factors: boundary layer thickness, turbulence level of the incident flow, channel constraint, etc., are analysed.     

Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct

In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.     

Experimental study of temperature field evolution in the channel with corrugated wall at a jump-like change in the heat flux

Results of experimental investigation of nonstationary temperature field in the flow and on the surface of the channel with corrugated wall are presented at a jump-like change of heat release in time. The changes were performed at the air flow in a rectangular channel, whose one heated wall was made as a plate with triangular corrugation oriented at 45° relative to the main flow direction. The microthermocouples with the thickness of below 10 microns were used to perform these changes. Investigations revealed typical regularities of temperature evolution in the flow and along the perimeter of the heated rib. Experimental data on time-temperature dependence are approximated well by the exponential function.     

Experimental Investigation of the Effect of Flow Arrangements on the Performance of a Micro-Channel Heat Sink

Abstract

An experimental study is carried out to investigate the effect of entrance and exit conditions that prevail due to different flow arrangements on the thermal performance of a copper micro-channel heat sink. Three flow arrangements—U-type, S-type, and P-type—were considered for the analysis with a test piece having inlet and outlet plenum dimensions of 10 mm × 30 mm × 2.5 mm with an array of parallel micro-channels having an individual width of 330 μm and a uniform channel depth of 2.5 mm. Performance evaluations for different flow conditions at inlet and outlet plenums were made by maintaining constant heat supply at 125 W, 225 W, and 375 W with varying Reynolds number ranging from 224 to 1,121. Nusselt number and pressure drop were computed by measuring temperature difference and pressure drop across the inlet and outlet plenum for various test combinations. Maximum heat transfer was observed for the U-type flow arrangement, followed by the P-type and S-type; maximum pressure drop was noted for the S-type flow arrangement, followed by the U-type and P-type arrangements for a constant Reynolds number. A detailed analysis of the experimental results indicate that from a pressure drop point of view, the P-type flow arrangement is preferred, whereas from the heat transfer point of view, the U-type is found to be a better option.     

甲烷入口条件对紧凑型重整器燃烧管道化学反应与产热特性影响

紧凑型甲烷重整器燃烧管道由燃料气体通道、多孔层以及固体平板组成.采用三维数值模拟方法,对甲烷入口速度、温度等对催化燃烧反应以及产热特性影响进行了研究.结果显示,甲烷入口速度由2.5 m/8增大到10 m/s时,最大化学反应速率提高了20.4%,CH4利用率下降了41.2%,最大热流量提高了11.8%;温度由873 K升...     

Experimental investigation of wall temperature evolution in a compact heat exchanger at a drastic change in the heat flux

Results of experimental study of the nonstationary temperature field on the surface of a complex shape channel at a jump-like change in heat release with time in the wall of packing with one-way heat supply are presented. Measurements were carried out in the air flow between two plates with triangular corrugations directed at 90° relative each other. Measurements were carried out by thermocouples with the thickness of less than 10 microns. The effect of Reynolds number and supplied power on the temperature change over the heat exchanger surface with time caused by a jump-like change in supplied electric power at a turbulent air flow is analyzed. Investigations have revealed typical features of temperature evolution over the perimeter of the heated cell. Experimental data on time dependence of temperature can be approximated well by the exponential function.     

Laminar free convection heat transfer and separated flow structure in a vertical channel with isothermal walls and two adiabatic opposing fins

Results of numerical study of laminar free convection and heat transfer in a vertical plane-parallel channel with two thin adiabatic fins on its walls are presented. The channel has the open inlet and outlet, and its surfaces are maintained at the same temperature. The channel height is unchanged with elongation parameter A = L/w = 10, and the fins are located in the middle of the channel toward each other. Fin height l/w = 0 ÷ 0.4 and Rayleigh number Ra = 10² ÷ 10⁵ are varied in calculations. The effect of these parameters on the flow structure, temperature field, local and integral heat transfer, and gas flow caused by gravitational forces are analyzed in detail. Numerical analysis is based on solving the full Navier–Stokes and energy equations in twodimensional statement and Boussinesq approximation. To determine the dynamic and thermal parameters at the inlet and outlet, the calculation is carried out with two large volumes attached to the inlet and outlet. The features of the flow and heat transfer at separated flow around the channel fins are studied in detail in this work.     

Effect of longitudinal heat conduction on the catalytic ignition of carbon monoxide in a boundary layer

The catalytic ignition of dry carbon monoxide and air in a boundary layer flow over a palladium plate is studied in this paper. The heterogeneous reaction mechanism is modelled with the dissociative adsorption of the molecular oxygen and the non-dissociative adsorption of CO, together with a surface reaction of the Langmuir-Hinshelwood type and the desorption reaction of the adsorbed product, CO₂(s). The critical condition for catalytic ignition, represented by the ignition Damköhler number, has been deduced using high activation energy asymptotics of the desorption kinetics of the most efficiently adsorbed reactant, CO(s). Longitudinal heat conduction along the plate has been considered and its influence on the ignition temperature has been evaluated. This influence is rather weak, indicating that the flat plate boundary layer flow configuration is a robust device to determine the critical conditions for catalytic ignition.     

三分螺旋折流板换热器壳侧流场温度场模拟

建立了三分螺旋折流板换热器壳侧流动与传热的数学模型,对其流场、温度场和压力场的数值模拟结果在多个纵剖面和横切片上进行了展示。可见在螺旋通道的轴心和靠近壳体的折流板外缘区域局部速度较高,在折流板背流面呈现出有利于强化掺混作用的回流区,流道内几乎没有流动死区;壳侧流体温度为稳步均匀下降趋势,并呈现从圆周的外缘向轴心方向逐渐递减;而压力场则呈现明显的周期性和阶梯性。     

Thermal Analysis for Velocity,Kinetics, and Enthalpy Reaction Measurements in Microfluidic Devices

Abstract

The aim of this work is to present new devices for the measurement of velocity, kinetics, and enthalpy of chemical reactions occurring in a microfluidic chip, co-flow, or droplets flow. The thermal analysis goes from the macroscopic approach by microcalorimetry to microscopic analysis inside the microchannel by IR thermography. Concerning microcalorimetry, the enthalpy is deduced from the measurement of the global heat flux dissipated by the chemical reaction as a function of the molar flow rate. A validation is presented on a well-known acid-base reaction. This device can be combined with an IR camera for local characterization. The processing of the measured temperature fields allows the estimation of properties of great importance for chemical engineers, such as heating source distribution (i.e., the kinetics) of the chemical reaction along the channel. A validation experiment of a temperature field processing method is proposed with the Joule effect. From such a previous experiment, a Peclet field is estimated and used in a further step in order to study an acid-base co-flow configuration. Finally, a first tentative of thermal characterization inside droplets flow during an acid-base chemical reaction is also presented.