High Inlet Velocity Research Articles

Combustion regimes of syngas flame in a micro flow reactor with controlled temperature profile: A numerical study

Flame characteristics of stoichiometric syngas/diluted-air mixture (CO:H2:O2:N2=1:1:1:7) in a micro-flow reactor with controlled wall temperature profile were numerically studied. In addition to single normal flame and single weak flame that respectively occur at high and low inlet velocities, two new combustion regimes, i.e., “coexistence of upstream normal flame and downstream weak flame (UNF-DWF)” and “uninterrupted flame with repetitive extinction and ignition (UFREI)”, were found at moderate inlet velocities. It is revealed that the coexisting downstream weak flame of UNF-DWF is attributed to incomplete consumption of CO by the upstream normal flame. Analysis of reaction path further demonstrates that the downstream weak flame of UNF-DWF is partially sustained by H2O through H2O+O=2OH due to H2O production in the upstream normal flame. To elucidate the dynamic process of UFREI, it is artificially divided into “upstream flame” and “downstream flame (actually an oscillating weak flame)”, and an entire period of UFREI is divided into four sub-regions. The temporal variations of CO mole fraction profile and heat release rate profile along the centerline of each sub-region are exhibited and analyzed. Briefly speaking, the formation of UFREIs is essentially a result of early re-ignition of residual CO from the upstream flame before the occurrence of its extinction. In summary, the present study sheds some light on the complicated combustion characteristics of syngas in the micro flow reactor with a prescribed wall temperature profile.

Performance analysis of an air rock thermocline TES tank for concentrated solar power plants using the coupled DEM–CFD approach

One of the most common solutions currently available to meet future energy needs in the world is concentrated solar power (CSP) plants combined with thermocline thermal energy storage (TES) tank subsystems. The air rock thermocline TES tank asserts to be an effective and inexpensive TES subsystem for CSP plants. In this study, a discrete element method (DEM) combined with a numerical model of computational fluid dynamics (CFD) was adopted to investigate the thermal performance of a scale model of an air rock thermocline TES tank. The study is carried out to optimize TES thermocline tank's thermal behavior by changing the heat transfer fluid (HTF) inlet velocity. The DEM–CFD coupled model is validated against analytical and experimental data. The results show that the use of the distributor reduced the influence of the wall effect by distributing the HTF in a regular way as the fluid passes through it, especially at high inlet velocity. Moreover, the results show that each TES tank subsystem has a critical velocity at which the tank operates most efficiently. Furthermore, the results demonstrated that the maximum value of the overall cycle efficiency was equal to 70.15% at 10 m/s, while the minimum value was equal to 42.5% at 2 m/s. The highest capacity and utilization ratio are 81.31 and 72.81% at 16 m/s, while the lowest capacity and utilization ratio are 52.01 and 44.75% at 2 m/s. The pumping energy needed to overwhelm the pressure drop rises when the HTF inlet velocity increases. This study can provide an effective reference for the efficient operation regulation and optimal design of the thermocline tank.

Improving efficiency of a square cyclone separator using a dipleg – a CFD-based analysis

The present study is mainly focused on proposing an effective way to improve the efficiency of a square cyclone separator. For this purpose, a dipleg is attached under the square cyclone to investigate its effect on the performance of square cyclone. A three-dimensional Computational Fluid Dynamics (CFD) simulation is done by solving the Reynolds-Averaged Navier-Stokes equations with the Reynolds stress model (RSM) turbulence model and applying the Eulerian-Lagrangian two-phase method. The turbulent dispersion of particles is predicted by the application of the Discrete Random Walk (DRW) model. The numerical results demonstrate that using dipleg although produced an increase in pressure drop but it positively enhances the separation efficiency of the square cyclone. Using dipleg significantly increases the separation efficiency of square cyclone especially at higher inlet velocity. This can be more obvious when using dipleg which is minimized the 50% cut size of square cyclone about 26.3%.

Analysis of thermo-hydraulic characteristics in the porous-wall microchannel with microencapsulated phase change slurry

Combining porous layer paved on the side wall and microencapsulated phase change material (MPCM) slurry as working fluid is presented in microchannel heat sink (MCHS), in which more surface area of channel matrix and larger temperature difference between coolant and heating surface during phase change will enhance heat transfer. The Brinkman-Forchheimer extended Darcy model together with energy equation in local thermal equilibrium account for heat transfer and flow in the porous layer as well as the equivalent heat capacity method for phase change of microcapsules under laminar flow. The effects of the materials of porous layer and substrate, heat sink design involving the porosity, thickness and height of porous layer as well as channel number under various inlet velocities and mass fractions of MPCM slurry on the thermo-hydraulic performance, which is evaluated with performance evaluation factor (PEF), in the porous-wall MCHS of MPCM slurry as coolant are analyzed in comparison to the non-porous mode with water passing through, and the 32.3% rise of performance evaluation factor can be obtained. The simulation results are validated with the experimental data. The lower thermal resistance occurs in the porous-wall mode of MPCM slurry as coolant with higher and thicker porous layer of higher porosity and thermal conductivity material as well as proper microchannel number in heat sink under higher inlet velocity than that in the non-porous MCHS with water flowing through, which is closely related to the phase change of MPCM in slurry, and influences PEF in consideration of flowing resistance. The MPCM mass fraction in coolant needs to be properly to avoid larger convection reduction and rise of flowing resistance, so as to obtain the better thermal and hydrodynamic performance of larger PEF.

Effects of micro-combustor geometry and size on the heat transfer and combustion characteristics of premixed hydrogen/air flames

major challenge in the development of micro-combustors is flame instabilities—resulting in a non-uniform wall temperature distribution and lower combustion efficiency. To overcome these issues, this research investigates the combustion characteristics of premixed hydrogen/air mixture in a micro-combustor with a cavity, bluff body, rib with bluff body and rib configurations. A detailed chemical reaction mechanism is also developed which consists of 13 species and 19 reactions. The obtained results are validated with published experimental findings. Having the model validated, a parametric study has been conducted to examine the effect of thermal conductivity coefficient, equivalent ratio, aspect ratio and geometrical configurations on heat transfer and combustion characteristics of hydrogen/air flames. It is demonstrated that increasing the thermal conductivity coefficient improves the preheating of the fresh mixture at upstream. However, this causes more heat loss from the outer walls to the surroundings. Moreover, increasing the equivalence ratio of a mixture reduces the negative effects arising from the heat losses on combustion stability. At higher inlet velocities, the location of a maximum temperature shifts towards downstream, which reduces the flame and average wall temperature. Among these configurations, a micro-combustor with bluff body is a more promising option to improve the flame stability and combustion efficiency.

Numerical analysis of thermal-hydraulic characteristics of steam-air condensation in vertical sinusoidal corrugated tubes

ABSTRACT The thermal-hydraulic characteristics of steam-air in the vertical corrugated tubes with sinusoidal waveform condensing wall of various amplitudes and wavelengths under the different inlet velocities, air mass fractions and wall subcoolings are numerically investigated, in the cases that the mass, species, energy and momentum sink terms are added in the cells adjacent to the condensing wall to simulate the condensation process. The mechanisms of turbulent flow and heat transfer enhancement of steam-air in condensation are also explored. The results show that the detached vortex characteristics and accumulated air in waveform domain influence the condensation heat transfer greatly, which are closely related to the amplitude and wavelength of basic waveform as well as inlet velocity. To evaluate the thermal-hydraulic performance of steam-air fluid in the corrugated tubes, the thermal-hydraulic performance factor, φ, describing the relation of heat transfer performance (i.e., η) to pressure drop characteristic (i.e., β), is defined. The better thermal-hydraulic performance can be obtained in the corrugated tubes with longer wavelength and higher amplitude waveform condensing wall under a low inlet velocity as well as with lower amplitude waveform condensing wall under a high inlet velocity. Besides, the corrugated tubes with A=0.5, 2 mm at L=21 mm respectively in the inlet velocity range of v=2-4 m/s and v=5-12 m/s are the better design of geometric parameters.

Combustion Turbulence Flow in Trapped Vortex Combustor with Guide Vane and Blunt Body

Numerical calculation was conducted to obtain the optimum structure parameters of the trapped vortex combustor (TVC) with the guide vane and blunt body. The results show that the optimum structure parameters of the guide vane are a/Hf=0.5, b/Li=0.2, and c/L=0.1, and the optimum structure parameters of blunt body are S/L=0.7, L2/L=0.1, and L1/Li=0.25. Then, the influence of different inlet conditions on the combustion turbulence flow was studied. The results show that high inlet temperature and low inlet velocity can effectively reduce total pressure loss; the equivalence ratio has little effect on total pressure loss. The study of unsteady flow shows that double vortices undergo the process of preliminarily forming-breaking down-forming again-being stable gradually.

MECHANICAL PROPERTIES OF 3D-PRINTED BLOOD VESSELS

The demand for vascular substitutes in clinical practice has increased, and 3D-printed blood vessels could be advantageous alternatives. Determining the mechanical properties of 3D-printed blood vessels is important to further improve the technology and clinical application. Here, dogbone test samples with and without cells were generated by 3D printing with combined UV curing and hydrogel ion curing. Tensile tests were performed to measure the mechanical parameters of the dogbones and build two kinds of fluid–solid interaction models. In accordance with hydrodynamics and momentum theorems, the velocities in accelerated ejection period, ejection peak, reduced ejection period, and early diastolic period were selected as inlet velocities. Velocity distribution on the symmetry surface of the fluid and the stress and strain of the solid tube under different velocities were analyzed. Results reveal that the velocity decreases gradually from the fluid center to the wall. At a high inlet velocity, the fluid velocity is high, while the stress and strain of the wall increase. During cardiac ejection peak, the stress on the wall reaches the maximum value of 2018 Pa, which is much lower than the ultimate stress. In addition, the strength and stiffness decrease for the vessels added with cells. This work provides a feasible method for measuring the mechanical properties of 3D-printed blood vessels. Keywords: 3D printing, mechanical properties, fluid–solid interaction

Three-dimensional numerical investigation of laminar flow in blind-tee pipes

Blind-tee pipes are often employed in process piping systems used in offshore and subsea oil and gas industry. The objective of the present study is to investigate the effects of blind-tee length, end-structure shape and flow velocity on the mixing condition inside blind-tee pipes. Three-dimensional numerical simulations are performed for the laminar flows in the pipes with blind-tee length varying from 1D to 5D at Re= 500, 1000, 1500 and 2000. Spherical and flat end-structure shapes are also investigated for the blind-tee section. Firstly, flow simulations in a straight pipe are performed, and the numerical results are compared with the analytical solutions. Then, the computed velocity profiles are used as the inlet velocity for the simulations of blind-tee pipes. Flow characteristics both inside and downstream the blind-tee section are numerically studied by analyzing the streamlines, pressure contours and velocity profiles. The results reveal that a higher inlet velocity and a blind-tee length no more than 3D can achieve a better mixing condition for the fluid flow in blind-tee pipes; while stagnant flows are more prone to occur towards the end of the blind-tee section at low velocities with the length longer than 3D, leading to potential fluid deposition in blind-tee pipes.

Additional criteria for MILD coal combustion

We proposed a theoretical basis for Moderate or Intense Low-oxygen Dilution (MILD) coal combustion based on the turbulent scalar energy spectra. This is motivated by the hypothesis that smallest scalar mixing length scales should be on the order of the particle size or smaller to ensure that mixing can occur to prevent formation of diffusion flames. Our proposed criterion is evaluated using several experimental datasets from the literature for coal combustion in both MILD and traditional combustion regimes. The experimental results confirm that the smallest mixing length scales should be of the order of or smaller than the particle diameter, ηmix⪅dp, to breakup the heat and mass transfer boundary layers around particles in MILD coal combustion. Results indicate that poor mixing of species with small Schmidt numbers around small particles leads to the high luminous intensity in the reactor. The effects of inlet velocity and jet diameter on the mixing length scales are analyzed. Higher inlet velocity and smaller jet diameter are expected to reach MILD regime. The proposed criterion can be used to guide experimental design to achieve MILD conditions for coal combustion.

Liquid Water Transport Behavior at GDL-Channel Interface of a Wave-Like Channel

This paper evaluates the liquid water at the gas diffusion layer-channel (GDL-channel) interface of reconstructed GDL microstructures with uniform and non-uniform fiber diameters in wave-like channels. A non-uniform GDL microstructure is reconstructed for the first time at the GDL-channel interface to evaluate droplet motion. The three-layer GDL microstructures are generated using the stochastic technique and implemented using the OpenFOAM computational fluid dynamics (CFD) software (OpenFOAM-6, OpenFOAM Foundation Ltd., London, UK). The present study considers the relationship between reconstructed GDL surfaces with varying fiber diameters, wettability, superficial inlet velocity and droplet size. Results show that the droplet detachment and the average droplet velocity decrease with an increase in the fiber diameter as well as the structural arrangement of the fibers. Under the non-uniform fiber arrangement, the removal rate of water droplets is not significantly improved. However, the choice of smaller fiber diameters facilitates the transport of droplets, as hydrophobicity increases even at slight surface roughness. The results also indicate that the average droplet velocity decreases under low inlet velocity conditions while increasing under high inlet velocity conditions. Therefore, the structural make-up of the GDL-channel interface influences the droplet dynamics, and the implementation of a non-uniform GDL structure should also be considered in the GDL designs.

Numerical investigations on the liftoff velocity of H2-air premixed combustion in a micro-cylindrical combustor with gradually changed section area

Sustaining and stabilizing flames are crucial issues in micro-combustion. In some micro-electro-mechanical systems such as the micro-thermophotovoltaic system, the flame should be formed in the combustion chamber, not outside it (combustion without liftoff). So, study of the liftoff phenomenon is important and vital in these systems. The aim of this study is to evaluate effect of changing combustor section area on the critical liftoff velocity in a micro-cylindrical combustor. For this purpose, the critical liftoff velocities are numerically identified for four combustor configurations (convergent, divergent, convergent-divergent and divergent-convergent combustion chamber). Premixed mixture of hydrogen-air has been used as reactants for the current investigation. Turbulence model implemented in this paper is RNG k-epsilon and combustion reaction was modeled with 10 species and 21 steps scheme using Eddy Dissipation Concept model. Two non-dimensional numbers d1/d2 (inlet to outlet diameter ratio) and d1/d3 (inlet to throat diameter ratio) are defined. For d1/d2 > 1.0, the combustion chamber is convergent, otherwise it is divergent. When d1/d3 > 1.0, the micro combustor is convergent-divergent and for d1/d3 < 1.0, the micro combustor is divergent-convergent. The results indicate that with increasing d1/d2, the liftoff occurs in a lower inlet flow velocity. With varying d1/d3, from 0.71 (2.0/2.8) to 1.0 (2.0/2.0), the liftoff velocity is reduced. Based on the numerical results, it can be said that the use of convergent and convergent-divergent combustion chamber decreases liftoff velocity. Meanwhile, the combustor with diverging and diverging-converging structure can enhance liftoff velocity. In the same condition, critical liftoff velocity of divergent-convergent micro combustor is the highest among all cases and this configuration is appropriate for Micro Electro-Mechanical Systems that work with high inlet velocity.

Investigation into convective heat transfer of nanofluids at the thermal boundary layer development region in micro-channels

The convective heat transfer at the thermal boundary layer development region in microchannels was theoretical and experimently investigated with de-ionized water and Al2O3-H2O nanofluids as the working fluids. It was found that nanofluids could effectively decreased the wall temperature and improve the heat transfer coefficient compared with the de-ionized water at the thermal boundary layer development zone. When the serious agglomeration of nanoparticles didn’t appear, increasing the concentration of Al2O3 nanoparticles could effectively increase the heat transfer enhancement effect. And the local heat transfer coefficient increase amplitude by using the nanofluids decreased along the direction of refrigerant flow. At the thermal boundary layer development zone, higher inlet velocity would get the larger increase amplitude of the heat transfer coefficient. At the thermal boundary layer development region, lower heat flux could achieve more pronounced heat transfer enhancement by nanofluids. When the thermal boundary layer developed, the heat transfer enhancement by nanofluids decreased and affected by heat flow become weak. When the mass flow rate increases, the better improvement can be achieved by nanofluids in the micro-channels. The nanoparticles have greater impact on local heat transfer coefficient in thinner thermal boundary layer, leading to the heat transfer enhancement is better at the thermal boundary layer development region in micro-channels.

Heat and mass transfer characteristics of three-dimensional bell-shaped polysilicon chemical vapor deposition reactor

Polysilicon, the crucial material of photovoltaic field, is commonly produced in a Siemens chemical vapor deposition (CVD) reactor with a complex physical and chemical phenomenon included inside as well as complicated geometry structure. In order to study the performance of silicon CVD reactor, a three-dimensional bell-shaped reactor model with 24 rods is constructed and verified to investigate the heat and mass transfer performance inside. The distributions of velocity, temperature, complex concentrations of six main components, silicon deposition rate, and energy of the reduction furnace at high and low inlet velocities are analyzed in detail. Results show that high inlet velocity is more conductive to obtain uniform distributions of velocity, temperature, complex concentration, and silicon deposition rate, leading to better silicon CVD performance. The inner rods are easier to collapse than the outer rods. In addition, radiation loss is the main energy consumption in the silicon reduction furnace and accounts for more than 60% of the total energy consumption. The convective heat transfer, chemical reaction heat, proportion of convective heat, and the proportion of chemical reaction heat increase with the rise in inlet velocity due to a fully developed flow field.

쉬라우드형 조류발전 시스템의 다중 채널구조에 따른 해수유동장 분석

Numerical analysis has been carried out to analyze seawater flow field and power generation characteristics of the tidal current power generation system for various multi channel shroud systems. Geometrical multi channel arrangement largely affects the flow field characteristics in the shroud system which power generation performance through turbine blade depends on. Sectional averaged velocity in front of the turbine blade which increases more than 2 times compared with channel inlet is much influenced as well as the flow from the rear with curl. And flow variation results in high inlet velocity in horizontal arrangements of multi channels with mechanical output of the turbine. These results are expected to be used as applicable data for the development of the tidal power generation system with shrouds.

Numerical analysis of flow field and particle motion in a dynamic cyclonic selector

Computational Fluid Dynamics is employed to investigate the flow field and the fate of particles in a dynamic cyclonic classifier which is used to separate fine particles of dried sludge, produced as waste by pulp and paper-making processes. The cyclonic classifier is equipped with a rotating impeller, which improves the tangential flow, and a circular baffle, which distributes the inlet stream of gas and particles. Unsteady Reynolds-averaged Navier-Stokes equations are solved for the continuous phase, addressing the impeller motion though the Sliding Mesh approach, whereas Lagrangian tracking is employed for the particles. Surprisingly, the removal efficiency is found to be non monotonic with particle size, instead presenting a fish-hook shape. This is partly imputed to the presence of the circular baffle that promotes, in the bottom region of the cyclone, the formation of a nearly toroidal recirculation zone which entrains small particles, subsequently separated at the bottom. Moreover, too high inlet velocities were found to hamper the action of impeller rotation with a resulting detrimental effect on removal efficiency.

Numerical investigation on the performance of bluff body augmented micro cavity-combustor

Bluff body augmented micro-cavity combustors are proposed in this study to enhance the combustion efficiency under high inlet velocities. The original and two types of bluff body supplemented micro cavity combustors, namely CAVITY, TRIANGLE, and FIN combustors are numerically investigated. The results show that the combustion efficiency (ηc) of bluff body augmented combustors are higher than that of the CAVITY combustor, and the largest increment of 10.8% is obtained in the FIN combustor. The underlying mechanisms for the improvement in ηc of the FIN and TRIANGLE combustors have been analyzed. It is demonstrated that the altered flow field structures and transvers reactant transport induced by the bluff body play dominant roles. Moreover, the local Damköhler number (DaL) is introduced, which is defined as the ratio of the local hydrogen reaction rate and the convective mass transport rate. The variation of ηc along the streamwise direction in the proposed combustors is determined by the value of DaL.

Flame dynamics and stability of premixed methane/air in micro-planar quartz combustors

The flame dynamics and stability of methane/air premixed combustion in micro-planar quartz combustors were numerically studied with detailed chemical reaction mechanism. The computed flame characteristics were quantitatively and qualitatively in good agreement with experimental phenomena. The oscillating flame was observed from the perspective of three-dimensional simulation for the first time. Parametric analysis associated with channel height and channel length was performed. Results indicated that the blowout limit was found to increase first and then decrease as increasing the channel height due to the tradeoff relationship between increased areas for heat flux and the extended timescales for energy diffusion. The change in channel length can lead to the variation in residence time of mixture in channel and heat loss from the external wall. It was found that there existed a critical point of channel length, above which the flame stability was improved marginally. Furthermore, it was shown that wall thermal conductivity had effect on both the flame structure and flame location, especially at relatively high inlet velocities. Further analyses indicated that the mechanism of enhanced flame stability with sufficient high wall thermal conductivity can be attributed to the lower strain rate and smaller deflection of streamlines in flame front.

Numerical investigation of two-phase flow in a horizontal porous evaporator with localised heating using non-Darcian flow and two equations model

In this article, a new mathematical model, based on the modified enthalpy formulation of Two-Phase Mixture Model (TPMM), is developed by considering non-Darcy flow and Local Thermal Non-Equilibrium (LTNE) conditions to describe numerically the two-dimensional problems of the complete evaporation process inside an asymmetrically heated porous evaporator. The governing equations have been discretised using Finite Volume Method (FVM) on a fixed non-staggered grid layout. The in-house code has been validated against experimental data and the results show good agreement between them. Three different models have been employed for the partitioning of the wall heat flux. Only the effect of the heat flux has been analysed, while all other parameters and properties have been kept fixed during the present investigation. The numerical simulations indicated that the locally high heat flux significantly influences the working fluid flow performances, and lead to extend the superheated vapour region towards the exit of the evaporator. The results also show that the two-phase zone significantly expanded in the axial direction and does not occupy a large portion inside the channel due to the energy transport through the two-phase region is significantly higher in the flow direction as compared to the transverse direction, caused by the decrease in the effective diffusion coefficient in this region and higher axial velocity. Numerical analysis reveal that the different models for the partitioning of the wall heat flux have almost no influence on the overall predictions, except in the vicinity of the wall. The results clearly demonstrate that the numerical simulations with the modified h-formulation by considering non-Darcy law and LTNE model are indispensable for the problem of complete evaporation process, particularly under the high both heat flux and inlet velocity conditions.

Thermo-economic performance evaluation of emerging liquid-separated condensation method in single-pressure and dual-pressure evaporation organic Rankine cycle systems

Liquid-separated condensation is an emerging and effective heat transfer enhancement method to reduce the condenser cost, which can help with the widespread application of organic Rankine cycle (ORC) systems. However, few related studies have focused on the shell-and-tube condenser even if it is widely used in ORC systems. The thermo-economic performance superiorities and applicable working conditions of shell-and-tube liquid-separated condenser should be investigated, especially in novel ORCs. This study introduced the liquid-separated condensation method into single-pressure and dual-pressure evaporation ORC systems with shell-and-tube condenser. R245fa, R600a, and R1234ze(E) were used as working fluids. Optimal liquid-separated thermodynamic states and decrements in condenser heat transfer area of one to four stages liquid-separated condensations were provided. Decrements in specific investment cost (SIC) of ORC system by adopting the liquid-separated condensation method were evaluated. Results show that the liquid-separated condensation method is more suitable for low tube diameter, low inlet velocity in tubes, high inlet velocity outside tubes, and low cooling water temperature rise. Optimal liquid-separated thermodynamic states remain constant with increasing the tube diameter and inlet velocities. The SIC of ORC system can be reduced by 4.6% at most by adopting the liquid-separated condensation over the conventional condensation method. Increasing the mass flow rate of heat source fluid and pinch point temperature difference during heat absorption process is beneficial to increase the superiority of liquid-separated condensation method. Compared with dual-pressure evaporation ORC, the effects of liquid-separated condensation method on reducing the SIC are more remarkable in single-pressure evaporation ORC.