Hydrodynamics and heat and mass transfer at chemical conversions in slot reactors

In this paper, experimental and numerical investigations of the hydrodynamics and heat transfer in a disk slot heat exchanger-reactor for a radial flow of a gas mixture reacting on the channel walls are described. Data for the coefficients of heat transfer from the wall being heated to the gas flowing inside the reactor are presented. The temperature field of a catalytically active reactor plate at heat release on it has been investigated experimentally. Calculations of the flow and heat transfer in a slot reactor element for a catalytic reaction with heat release have been performed. Partial oxidation of methane in an oxygen medium with the formation of a hydrogen-containing synthesis gas in a two-dimensional microchannel has been investigated numerically. Data for the extent of the chemical conversion of methane versus the initial mixture consumption and reaction temperature are presented.     

Modelling of methanol-to-hydrogen steam reforming with a heat flux distributed along a microchannel

The flow of reacting mixture of methanol and steam in a 2D microslot was studied numerically at activation of the reactions on the channel wall. This modelling was carried out in the framework of Navier — Stokes equations for a laminar flow of multicomponent compressible gas. Correlations between thermal, diffusion, and physical-chemical processes were studied under the conditions of intense endothermic reaction and external heat supply distributed along the channel. It is shown that not only the amount of heat supplied to the reaction zone is essential, but also the mode of heat supply along the channel length is important, which allows optimization of the compact reactor for hydrogen production.     

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.     

Mass transfer in a two-phase flow in a curvilinear channel

In this paper, the results of an experimental investigation of the distribution of local coefficients of mass transfer on a pipe wall for a flow of a two-phase gas-liquid mixture with a turn of 90° are presented. The mass transfer coefficients were measured by using an electrodiffusion method. It has been shown that the distribution of local mass transfer coefficients on the inner surface of a curvilinear channel is nonuniform both in length and along the perimeter and depends on the content of gas in the two-phase flow.     

Experimental observations of methane–oxygen diffusion flame structure in a sub-millimetre microburner

We examine the structure of confined, laminar methane–oxygen diffusion flames in an alumina microburner with a sub-millimetre dimension. To minimize termination of gas-phase combustion via surface radical quenching, the reactor walls are chemically treated and annealed. We show, through chemiluminescent images, that gas-phase methane–oxygen diffusion flames exist in the microburner without the need for catalytic reaction. However, their structure differs from the continuous laminar diffusion flame profiles that we would expect in a similar burner configuration on a macroscopic scale. Instead, we observe a sequence of isolated reaction zones structures (flame cells) that form along the length of the microburner combustion channel aligned in the direction of the gas flow. This form of cellular diffusion flame instability appears to be unique to wall-confined combustion in microscale devices. The number of flame cells observed depends on the inlet gas velocities and initial mixture strengths.     

Laminar free convection heat transfer between vertical isothermal plates

The paper represents results on numerical investigation of flow and heat transfer between two isothermal vertical plates under laminar natural convection. A system of complete Navier–Stokes equations is solved for a two-dimensional gas flow between the plates along with additional rectangular regions (connected to inlet and outlet sections), whose characteristic sizes are much greater than the spacing between the plates. The calculations were performed over very wide ranges of Rayleigh number Ra = 10 ÷ 10⁵ and a relative channel length AR = L/w = 1 ÷ 500. The influence of the input parameters on the gas-dynamic and thermal structure of thermogravitational convection, the local and mean heat transfer, and also the gas flow rate between the plates (convective draft. We determined sizes of the regions and regime parameters when the local heat flux on the walls tends to zero due to the gas temperature approach to the surface temperature. It is shown that the mean heat transfer decreases as the relative channel length AR grows, whereas the integral gas flow rate (convective draft) and Reynolds number in the channel Re = 2wUm/ν increase. The use of a modified Rayleigh number Ra* = Ra · (w/L) (Elenbaas number) leads to generalization of calculation data on mean heat transfer. These data are in good agreement with the correlations for heat transfer [1, 2] and gas flow rate [3]. The reasons of variation of the data in the range of low Rayleigh numbers are discussed in detail.     

Calculation of the drag and heat transfer from a sphere in the gas flow in a cylindrical channel

A numerical experiment on the simulation of heat transfer from a sphere to a gas flow in a cylindrical channel in the Stokes and transient flow regimes has been described. Radial and axial profiles of the gas temperature and the dependences of drag coefficient C_d of the body and Nusselt number Nu on Reynolds number Re have been calculated and analyzed. The problem of the influence of the early drag crisis for a sphere on its heat transfer to the gas flow has been considered. The estimation of this phenomenon has shown that the early drag crisis of the sphere in a strongly turbulent flow causes a reduction in heat transfer from the sphere to the gas by three to six times (in approximately the same proportion as for its drag coefficient).     

Experimental investigation of subcooled flow boiling of a 50/50 ethylene glycol/water mixture in finned rectangular aluminum channels

Subcooled flow boiling heat transfer experiments were performed with a 50/50 ethylene glycol/water mixture in a finned aluminum channel. The channel represented a hybrid electric vehicle power electronic cold plate receiving a 50/50 mixture from the radiator at 105°C and 2 atmospheres. Experiments used a range of mixture flow rates and both top- and bottom-heating situations. Boiling curves were generated, and subcooled flow boiling heat transfer coefficients were determined including the test channel fin effects. Subcooled flow boiling heat transfer coefficients showed a 25–30% increase compared to single-phase convection.     

Numerical Investigation into the Development Performance of Gas Hydrate by Depressurization Based on Heat Transfer and Entropy Generation Analyses

The purpose of this study is to analyze the dynamic properties of gas hydrate development from a large hydrate simulator through numerical simulation. A mathematical model of heat transfer and entropy production of methane hydrate dissociation by depressurization has been established, and the change behaviors of various heat flows and entropy generations have been evaluated. Simulation results show that most of the heat supplied from outside is assimilated by methane hydrate. The energy loss caused by the fluid production is insignificant in comparison to the heat assimilation of the hydrate reservoir. The entropy generation of gas hydrate can be considered as the entropy flow from the ambient environment to the hydrate particles, and it is favorable from the perspective of efficient hydrate exploitation. On the contrary, the undesirable entropy generations of water, gas and quartz sand are induced by the irreversible heat conduction and thermal convection under notable temperature gradient in the deposit. Although lower production pressure will lead to larger entropy production of the whole system, the irreversible energy loss is always extremely limited when compared with the amount of thermal energy utilized by methane hydrate. The production pressure should be set as low as possible for the purpose of enhancing exploitation efficiency, as the entropy production rate is not sensitive to the energy recovery rate under depressurization.     

入口段协同式翅片强化单相对流换热数值模拟

对二维平行平板通道入口段内设置协同式折流翅片的层流换热和流动特性进行了数值模拟。研究了翅片倾角以及通道长高比L／H对换热和阻力特性的影响。研究的Re数范围为100-1000。在翅片倾角β=0°-21.8°范围内, 通道内平均Nu数随翅片倾角β的增大而单调增大,随通道长度增大而单调减小。如果从相同泵功下强化效果来评价, 则是小倾角翅片较优,并且随Re数增大强化效果减弱。另外,分析表明,场的协同确实与换热率密切相关。     

Experimental study of incomplete oxidation of methane in a ring channel

Chemical transformations at incomplete methane oxidation in the air medium were studied experimentally at reaction activation on the wall of an annular microchannel. Methane was oxidized incompletely on a rhodium catalyst deposited on an inner wall of the channel. Concentrations of the products of chemical transformations were measured in the outlet gas mixture for different reactor temperatures and stay times. We have determined the range of channel wall temperatures and stay times of the mixture corresponding to an increase in the portion of hydrogen and carbon dioxide indicating transition from predominant methane combustion to cascade chemical reactions with activation of steam and carbon dioxide methane conversions. It is shown that the kinetic model of chemical transformations of methane in the air medium depends significantly on the temperature of channel walls and stay time of the mixture. The effect of outer diffusion deceleration on the rate of chemical transformations at incomplete methane oxidation under the strained conditions is determined. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 05-08-65526).     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.     

Methane steam reforming in an annular microchannel with Rh/Al2O3 catalyst

Regularities of methane conversion in the presence of water steam were obtained experimentally while activating chemical conversions on the inner convex wall of an annular microchannel. The steam methane reforming was done on the Rh/Al₂O₃ nanocatalyst with the heat applied through the microchannel gap from the outer wall. Concentrations of the products of chemical reactions in the outlet gas mixture are measured at different temperatures of the outer microchannel wall. The range of channel wall temperatures at which the ratio of hydrogen and carbon oxide in the outlet mixture grows substantially is determined. Data on the composition of methane conversion products for the ratio H₂O/CH₄ = 1.77 and the activation energy of methane steam reforming at reactor outer wall temperatures of up to 880°C are obtained. The effect of the radiation heat exchange and the external diffuse limitation on the rate of chemical conversions in methane steam reforming in an annular microchannel with external heat supply is determined.     

Deposition of diamond-like films from a high-velocity flow of a rarefied gas mixture

In contrast to the widespread deposition method of carbon films with the activation of gas mixtures on a hot wire, activation (dissociation of hydrogen and methane) is performed in a high-temperature channel formed by a tungsten coil in this study. From this channel, the gas is expanded to the substrate where deposition occurs. A film containing diamond crystals and hexagonal crystals having characteristic features of lonsdaleite is deposited at the channel temperature of 2100°C and the surrounding pressure of 20 mmHg. The diamond-like film is deposited from a high-velocity high-temperature un-ionized flow for the first time.     

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...     

Vertical forced convective boiling of refrigerant binary mixtures

The present research focused on the forced convective boiling heat transfer of a binary mixture of refrigerant R 12/R 114. The test section is an industrial stainless steel pipe (AISI 316), circular duct, uniformly heated (Joule effect) over a length of 2 300 mm, with an inner diameter of 7.57 mm and a wall thickness of 0.975 mm. The test section instrumentation consists of 0.5 mm, K-type insulated thermocouples distributed along the tube on the wall and inside the channel, and connected to give directly, at each location, the temperature difference between the wall and the bulk fluid. The fluid flow is upwards, with subcooled inlet conditions. The experiments were performed using three different mixture compositions, four different pressures ranging between 1.0 and 2.5 MPa, three specific mass flow rates between 500 and 1500 kg.m⁻².s⁻¹ and three heat fluxes between 18 and 40 kW.m⁻². The experimental results show a relevance of the heat transfer regime identification in the evaluation of mixture heat transfer performance as referred to the pure components.     

2D mixture fraction measurements in a high pressure and high temperature combustion system using NO tracer-LIF

Mixture fraction measurements in a jet-in-cross flow configuration at high pressures (15 bar) and temperatures (above 1000 K) were performed using planar laser induced fluorescence of nitric oxide (NO-PLIF) as trace species. The goal was the evaluation of this tracer LIF technique for the characterization of the mixing of fuel and hot exhaust gas in the mixing channel. The fuel (natural gas (NG) or H₂/N₂/NG mixture) along with the tracer were injected into the crossflow of the exhaust gas and PLIF measurements were performed in different planes. In order to relate the measured NO-LIF signal to fuel concentration and mixture fraction, effects of pressure, temperature and species concentration were taken into account. Numerical calculations and spectroscopic simulations that mimic the experimental conditions were performed to identify excitation schemes that give optimum correlations between the NO-LIF signal and the mixture fraction. The measured NO-PLIF images were transformed into mixture fraction plots using the computed correlations. The paper reports on the experimental challenges encountered during the measurements and the steps taken to overcome those difficulties. Examples of mixture fraction distributions are presented and discussed. The paper concludes with a detailed analysis on the accuracy of the measured mixture fraction values.     

Evaluating the efficiency of thermo-electric conversion of heat from gas combustion in a small-scale system with counterflow heat exchange

The efficiency of thermoelectric conversion of heat from gas combustion was evaluated in a small-scale system consisting of two channels with opposing gas flows and thermocouples located in the separating wall. Combustion occurred in the chamber fed with fresh mixture heated by combustion products through heat-conducting walls of the channel. In the channel walls, there were thermoelectric converters. It has been shown that in this system, the maximum conversion efficiency of heat from gas combustion may be close to the maximum efficiency of thermoelectric conversion calculated by the maximum acceptable working temperature of the hot side of the converter. This conclusion is valid in the case when the adiabatic combustion temperature of the gas mixture is below the maximum allowable operating temperature of the hot side of the thermoelectric converter. The considered system is promising for the burning of low-calorific gas mixtures and does not require additional energy for cooling the cold side of the thermoelectric converter.     

Coaxial radiative and convective heat transfer in gray and nongray gases

Coupled radiative and convective heat transfer is investigated for an absorbing gas flowing in a finite length channel and heated by blackbody radiation directed along the flow axis. The problem is formulated in one dimension and numerical solutions are obtained for the temperature profile of the gas and for the radiation escaping the channel entrance, assuming both gray and nongray absorption spectra. Due to radiation trapping, the flowing gas is found to have substantially smaller radiation losses for a given peak gas temperature than a solid surface that is radiatively heated to this temperature. A greenhouse effect is also evident whereby radiation losses are minimized for a gas having stronger absorption at long wavelengths.     

Heat exchange at laminar flow in rectangular channels

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.