Inlet Velocity Ratio Research Articles

Numerical Simulation and Response Surface Analysis of Esterification of Monobutyl Chlorophosphate with n-Butanol in a Microchannel Reactor

Microreactors are essential for microchemical reactions owing to their high mass transfer efficiency, precise control of reaction time, easy amplification, and good safety performance. These characteristics provide several advantages, including shortened reaction times and enhanced chemical reaction conversion rates, rendering microreactors particularly significant in chemical production. In this study, a finite-rate model was developed for the esterification of monobutyl chlorophosphate (MCP) and n-butanol in a microchannel reactor. This study investigates the impact of the microchannel’s length-to-diameter ratio, the mass ratio of n-butanol to MCP at the inlet, and the inlet flow ratio on the entire reaction system through numerical simulations. The findings indicate that increasing the length-to-diameter ratio and reducing the inlet flow rate effectively prolongs the residence time of materials in the microreactor, thereby enhancing the conversion rate of the reactants. Optimal results are achieved with a moderate n-butanol/MCP mass ratio, which facilitates MCP transformation. Moreover, this study employs response surface analysis to investigate the influence of independent factors, such as the microchannel’s length-to-diameter ratio, component ratio, and inlet velocity ratio, on MCP conversion rates. A prediction formula with conversion rate as the dependent variable and microchannel length-to-diameter ratio, component ratio, and inlet velocity ratio as independent variables was established.

Stability and combustion characteristics of dual annular counter-rotating swirl oxy-methane flames: Effects of equivalence and velocity ratios

An experimental study was carried out to investigate flow/flame interactions, stability, combustion, and emissions properties of CH4/O2/CO2 stratified flames stabilized on a dual annular counter-rotating swirl (DACRS) burner for gas turbine combustion applications. The effects of two targeted parameters, namely equivalence ratios (for primary - ϕp, secondary - ϕs, and global - ϕg streams) and velocity ratio (Vr = Vp/Vs: primary to secondary stream inlet velocity ratio), are studied at fixed volumetric oxygen fraction (OF) in the oxidizer mixture (O2 + CO2) of both primary and secondary streams of 34 % at fixed Vp of 5 m/s. The experimental results indicated that no flame flashback was recorded in the domain of any operational ϕp up to stoichiometric operation of the secondary stream (ϕs = 1.0). At near stoichiometric operation of the primary stream, the main secondary flame can persist in extremely lean conditions (ϕs = 0.45 @ Vr = 3), thereby extending the thresholds of flame blowout. Moreover, raising ϕp from 0.4 to 1.0 results in significant reduction of ϕs at blowout from 0.595 to 0.456, corresponding to reducing the combustor ϕg at blowout from 0.577 to 0.499. At lower ϕp, the oxy-methane flame tends to lift and extinguish earlier.

Experimental study on the propagation characteristics of non-premixed H2/air flames in a curved micro-combustor

In this study, propagation characteristics of non-premixed H2/air flames in a curved micro-combustor were experimentally investigated. It was found that after cold-state ignition at the combustor exit, flames can propagate upstream and form a stable flame over a wide range of average inlet velocity (Vave,in) and nominal equivalence ratio (φ). The top-wall temperature distribution demonstrated that a flame separation phenomenon occurs when φ ≤ 1. High temperature zone of the combustor wall expanded and shifted downstream with an increasing Vave,in; however, with an increase in φ, it expanded initially and then shrank, and moved toward the air-side. The maximum wall temperature varied non-monotonically with both the Vave,in and φ, and they peaked at Vave,in = 1.5 m/s and φ = 1.6, respectively. Both lower and upper propagation limits (i.e., propagable velocities) exhibited non-monotonic tendencies versus φ. Specifically, the lowest and highest propagation limits are 0.3 m/s and 7.0 m/s, respectively, and they both occur at φ = 1.4. Empirical correlations of the propagation limits and propagable velocity range with φ were obtained. In summary, the present study demonstrated the feasibility of cold-state ignition of non-premixed H2/air for curved micro-combustors, and revealed the main flame propagation characteristics.

Combustion performances of a novel micro-combustor with a backward facing step and an exhaust gas recirculation channel

A novel micro-combustor with a backward facing step and an exhaust gas recirculation (EGR) channel was developed to operate at low mass flow rates of H2/air mixtures (1.33–5.34 × 10−6 kg/s). The function of EGR was realized by a high-speed jet issued from the combustor inlet with a diameter of 0.2 mm. Effects of inlet velocity (Vin) and equivalence ratio (ϕ) on combustion performances and EGR ability were investigated numerically. With the increase of inlet velocity, flame moved downstream and splitted at a critical velocity, and the flame temperature and outer wall temperature rise simultaneously. An increase in the equivalence ratio led to a larger reaction zone and higher flame temperature and wall temperature. The mass fractions of NOx emissions are very low with an order of magnitude of 10−10. Both the mass recirculation ratio (εm) and enthalpy recirculation ratio (εh) increased with the increase of incoming mass flow rate. However, for an identical mass flow rate, both εm and εh decreased with equivalence ratio. A dimensionless number, Fa, which was defined as the ratio of pressure difference between the inlet and outlet of EGR channel to the dynamic head of the jet flow, was introduced to evaluate the driving force of EGR. The Fa number decreased with the increase in the mass flow rate of jet flow, but exhibited a non-monotonic variation with equivalence ratio. The variation trends of εm, εh and Fa were interpreted by analyzing the variation regularities of parameters involved in their definitions.

Effect of Surface Reaction on the Distribution Characteristics of Temperature and OH Radicals in Microchannel Combustion

Microchannel burners suffer from low combustion efficiency and poor stability in applications. In order to explore the effect of wall reaction on methane/air premixed combustion performances in the microchannel, the effects of wall activity, inlet velocity, pressure, and equivalence ratio on the temperature and radical distribution characteristics were studied by CFD computational simulations. It is found that as the reaction pressure increases, there are more free-radical collisions, causing the reaction temperature to rise. The OH radicals participate in the reaction at the active near wall so that the mass fraction of the OH radical on the active wall is lower than that on the inert wall. As the equivalence ratio increases from 0.6 to 1.2, the high-temperature regions increase but the maximum temperature decreases. The mass fraction of OH radical increases with the increase of the equivalence ratio, and the increase of OH radical near the inert wall is larger than that of the active wall. As the flow rate increases, the disturbance increases, and the combustion reaction becomes more intense, resulting in an increase in the temperature and the mass fraction of OH radicals. The mass fraction of H, O, OH, and CH3 radicals in the inert wall was slightly higher than that in the active wall, in which the peak mass fraction of CH3 radical appeared at the axial position closest to the entrance, while the other three radicals reached the peak at about the same axial position. This study provides a reference for combustion stability in microcombustors.

Impact of H2 Blending of Methane on Micro-Diffusion Combustion in a Planar Micro-Combustor with Splitter

An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions.

Evolving a Novel Blade Shape of a Savonius Wind Rotor Using an Optimization Technique Coupled With Numerical Simulations and Wind Tunnel Tests

Abstract The global adoption of Savonius wind rotors as an eco-friendly means of small-scale power production is on the rise. However, their suboptimal performance remains a significant challenge due to the generation of higher unproductive torque. This paper aims to address this issue by obtaining an optimal blade profile considering the power coefficient (CP) as an output function using optimization techniques. The objective function includes the overlap ratio, intermediate points on the curve, inlet velocity, and tip speed ratio (TSR) as the optimization geometric parameters. To achieve this, the simplex search method and the non-dominated sorting genetic algorithm II are opted to develop the blade profile. The blade profile is developed using a natural cubic spline curve with fixed end points and variable intermediate points along with other parameters. The computational analysis is done using ansys fluent software with shear stress transport k−ω turbulence model. The solver setup employs the finite volume method to simulate the transient 2D flow around the blade profile. A direct comparison is made between the optimized blade profile and the conventional semicircular one over a range of TSRs. The results clearly indicate the superior performance of the former, exhibiting a higher CPmax by 23% compared to the conventional one at TSR = 0.8. Finally, experiments have been conducted in a wind tunnel to find the practical feasibility of the optimized blade profile generated through the simplex search method.

Multi-objective optimization of micro planar combustor with tube outlet by RSM and NSGA-II for thermophotovoltaic applications

The pressure loss (Δp), output power (Qtot) and system efficiency (ηMTPV) are the most important performance parameters of micro combustor for thermophotovoltaic applications. However, the above performance paremetres are conflicted in the optimization of micro planar combustor with tube outlet for thermophotovoltaic applications. In this work, RSM (Response Surface Methodology) and Non-dominated Sorting Genetic Algorithm II (NSGA-II) is combined to make a trade-off among the above performance parameters. With the inlet velocity (vin) and equivalence ratio (φ), tube length (L9), tube diameter (D) and tube distance (L10) selected as design variables, central composite design is used to arrange simulation cases. The regression model about Δp is quadratic, while the regression models about Qtot and ηMTPV are linear. Then, the significance of each term in the regression models is determined and arranged by variance (ANOVA). Fianlly, according to the Pareto optimal solution obtained from NSGA-II, the optimal values of the objective function are Δp = 977.87 Pa, Qtot = 6.59 W and ηMTPV = 2.72 %, and the corresponding design variables are vin = 7.09 m/s, φ = 0.99, L9 = 6.71 mm, D = 1.75 mm and L10 = 3.97 mm, respectively.

3D Numerical Modelling of Turbulent Flow in a Channel Partially Filled with Different Blockage Ratios of Metal Foam

The aim of the present research work is to understand the intricacies of fluid flow through a rectangular channel that has been partially filled with a metal foam block of different blockage ratio (0.16-1), with a pore density (5–30 Pores Per Inch i.e. PPI), along with varying inlet velocity (6.5–12.5 m/s). For the porous region, numerical solutions are acquired using the Darcy Extended Forchheimer model. The Navier-Stokes equation is used in the non-porous zone. Different flow behaviours were seen as a function of PPI, height, and inlet velocity. The pressure drop increases with inlet velocity, PPI, and block height, with a maximum value of approximately 4.5 kPa for the case of 30 PPI, 12.5 m/s, and a blockage ratio of 1. Results show that the existence and location of the formation of eddies depends on the inlet velocity, PPI, and blockage ratio. Such studies have been reported less and will aid research on forced convection through a channel partially filled with metal foam and optimisation studies between increased heat transmission and the additional pressure drop for the same by providing a detailed fluid flow analysis.

An investigation of the cavitation and vibration phenomena in a cylindrical cyclone

Cylindrical cyclones are a popular choice for oil-water separation and sewage treatment in the petroleum industry. Here, we investigate the cavitation and vibration phenomena in a cylindrical cyclone with a vortex finder by using various cyclone operating parameters and conducting multiphase flow numerical simulations. The lowest point of pressure on the cross section of the main cylinder is defined as the pressure center point and is used to understand the generation of the flow field oscillation, which is shown to exhibit an approximately circular motion. This circular oscillation of the flow field is an important characteristic that influences the overall performance and behavior of the cyclone system. Cavitation analysis results show that an increase in the inlet velocity leads to more intense cavitation and an expansion of the cavitation area. At lower split ratios, cavitation primarily occurs at the junction of the inlet pipe and the main cylinder, while at higher split ratios, the cavitation area gradually extends into the overflow pipe. Additionally, structural response analysis demonstrates that increasing the inlet velocity and overflow split ratio significantly enhances the vibrational degree of the structure, leading to greater stress levels. Adjusting the oil content at the inlet has a relatively minimal impact on the structural response compared with the influence of the inlet velocity and split ratio. Furthermore, dimensional analysis is used to analyze the change of wall pressure, and the wall pressure that induces structural vibration can be calculated using the inlet velocity and the split ratio.

Analysis of entropy generation and exergy efficiency of the methane/dimethyl ether/air premixed combustion in a micro-channel

This work presents a numerical investigation on the entropy generation and exergy efficiency in a methane/dimethyl ether/air-fueled micro combustor. To this end, a three-dimensional numerical model established is first validated by comparing its predictions with experimental data in the literature. It is, then, applied to assess the effects of the inlet velocity and blending ratio on the entropy and exergy generation. As far as the entropy generation caused by chemical reactions is concerned, there exists a reaction step that contributes most to the overall entropy. The contributing rate of such a reaction is inhibited by increasing the blending ratio. Meanwhile, for a given fuel composition, the entropy generation due to chemical reactions and heat conduction is found to exhibit a monotonic trend with the inlet velocity, whereas the entropy induced by mass diffusion shows a non-monotonic dependence. Furthermore, the exergy destruction ratio induced by the chemical reaction plays a dominant role in the total exergy generation, irrespective of inlet velocity and blending ratio. Both the exergy loss and efficiency are shown to be increasing as the inlet velocity is elevated, along with the increase in the combustor wall temperature. In general, this work identifies how blended combustion affects the entropy and exergy generation in methane/dimethyl ether combustion systems and sheds light upon the underlying mechanism.

Parametric optimization of vortex generator configuration for flow control in an intake duct for waterjet propulsion

Vortex generators (VG) are simple miniature devices that suppress flow separation in aeronautical elements. In this study, VG device operation in a submerged flush waterjet inlet was numerically investigated to improve the flow quality entering the jet pump at low Inlet Velocity Ratio (IVR). Steady Reynolds-averaged Navier-Stokes equations are solved using the SST k-ω turbulence model and used to simulate the flow field, with the effects of the orientation, angle of attack, height and installation position of VGs thoroughly analyzed. The hydrodynamic performance of the intake duct was assessed over the operational range. The results show that VGs arranged in a divergent configuration better suppress flow separation. The zonal magnitude of the high turbulent kinetic energy on the ramp side was reduced, with the non-uniformity in the pump face plane reduced from 0.41 to 0.26. Excessive angles of attack and VG heights have negative effects. Through parametric studies, VGs were found to have best performance in the target duct with an angle of attack of 40⁰ and a height measuring 20% of the bottom boundary layer thickness. In addition, VGs installed along the keel of a ship can effectively suppress the flow distortion and improve the overall efficiency of the intake duct.

Multi-objective optimization of an axial flow hydraulic turbine with a collection device to be installed in an open channel

In this study, a multi-objective optimization design method was developed by combining, among other methods, single-phase flow analysis and the optimization method to simultaneously optimize the runner and collection device of an axial-flow hydraulic turbine with a collection device, using the power coefficient and axial thrust coefficient as objective functions. To verify the effectiveness of this design method, the characteristics of the optimized turbine were investigated through verification experiments and multiphase flow analysis, and were compared with the results of single-phase flow analysis. The results of single-phase flow analysis showed that the axial thrust coefficient of the optimized turbine was equivalent to that of the original turbine, while the power coefficient improved dramatically. However, the results of the multiphase flow analysis showed that although the axial thrust coefficient of the optimized turbine was increased compared to that of the original turbine, the power coefficient was significantly higher than that of the results in the single-phase flow analysis, due to a significant increase in the efficiency of the turbine, as well as an increase in the loading coefficient and the inlet velocity ratio, unlike the single-phase flow analysis. This indicates that this design method is effective in terms of output improvement.

Experimental investigation on flame stability and emissions of lean premixed methane–air combustion in a developed divergent porous burner

Flame stability and pollutant emissions are issues of the greatest concern for low-calorific-value gas combustion. Although the conventional straight porous burner is conducive to improving the flame stability, it still cannot meet the industrial demand. To improve the flame stability and reduce the pollutant emissions during fuel-lean combustion, a divergent double-layer porous burner was creatively proposed in this study. The influences of the inlet velocity and equivalence ratio on the flame stability, temperature distributions, the emissions were experimentally investigated in detail. The results showed that the flame front gradually moved downstream, the preheating zone length increased as the inlet velocity increased. Compared with the traditional straight porous burner, the divergent porous burner extended the blowout limit by 300%. Since the burner was operated under fuel-lean conditions (the equivalence ratio range was 0.45–0.6), the CO emissions were relatively high when the inlet velocity was less than 0.4 m/s. When the inlet speed was equal to or greater than 0.4 m/s, the maximum CO emissions concentration was 132 ppm. The burner showed superior performances in terms of NO emissions, with emissions concentrations of less than 20 ppm under all test conditions. In summary, the great advantage of the divergent porous burner lies in extending the flame stability limit. For clean combustion it is recommended that the burner be operated at a higher velocity (greater than or equal to 0.4 m/s) to obtain lower CO and NO emission levels.

Oil–water two-phase flow-induced vibration of a cylindrical cyclone with vortex finder

Cylindrical cyclones play an important role in oil–water separation and sewage treatment in the petroleum industry. Here, we describe the characteristics of vibration induced by a two-phase rotational flow in a cylindrical cyclone. The cyclone operating parameters together with a dimensional analysis and multiphase flow numerical simulation were used to understand the flow field characteristics. The frequency and amplitude of pressure fluctuation were obtained by measuring pressure changes at points on the axis of the device. It shows that the pressure in a cylindrical cyclone varies periodically during separation and that fluctuation frequency and amplitude are related to the inlet velocity and flow split ratio. The effect of the overflow split ratio on the pressure fluctuation frequency is negligible, but increasing the overflow split ratio will cause greater fluctuation of the flow. For a cylindrical cyclone, the pressure fluctuation frequency can be calculated from the inlet velocity. Adjusting the inlet velocity and the overflow split ratio changes the mechanical response of the structure. The results of a modal analysis show that the structural vibration response is consistent with the response state of the lowest point of the internal central-vortex pressure and that both are in approximate circular motion. Furthermore, the frequency of pressure fluctuation induced by the flow is close to the intrinsic frequency of the structure with a single bottom constraint, which can cause unwanted resonance easily. Therefore, an appropriately added constraint on a cylindrical cyclone should be taken into consideration to avoid the resonance frequency.

Investigation of Inlet Conditions in The Mixing Process of Nanoparticles and Blood in a T-Shaped Microfluidic Reactor with Small Rectangular Cavities.

During the metastasis of cancer cells, circulating tumor cells (CTCs) are released from the primary tumor, reach the bloodstream, and colonize new organs. A potential reduction of metastasis may be accomplished through the use of nanoparticles in micromixers in order to capture the CTCs that circulates in blood. In the present study, the effective mixing of nanoparticles and the blood that carries the CTCs are investigated. The mixing procedure was studied under various inlet velocity ratios and several T-shaped micromixer geometries with rectangular cavities by using computational fluid dynamics techniques. The Navier-Stokes equations were solved for the blood flow; the discrete motion of particles is evaluated by a Lagrangian method while the diffusion of blood substances is studied by using a scalar transport equation. Results showed that as the velocity ratio between the inlet streams increases, the mixing rate of nanoparticles with the blood flow is increased. Moreover, nanoparticles are uniformly distributed across the mixing channel while their concentration is decreased along the channel. Furthermore, the evolution in time of the blood substances in the mixing channel increases with the increase of the velocity ratio between the two streams. On the other hand, the concentration of both the blood substances and the nanoparticles is decreased in the mixing channel as the velocity ratio increases. Finally, the differences in the dimensions of the rectangular cavities seems to have an insignificant effect both in the evolution in time of the blood substances and the concentration of nanoparticles in the mixing channel.

Optimum design of S-shaped diffuser by studying the effect of inlet velocity, turning angle and area ratio

Optimum design of S-shaped diffuser by studying the effect of inlet velocity, turning angle and area ratio

Analysis of Thermal and Flow Characteristics in a Combined Cross-Flow and Jet-Flow Configuration with Flow-Guiding Fins

This study investigated the enhanced cooling of electronic components at high temperatures with cross-flow and jet-flow combinations. The cooling performance of four different model geometries (Models 1, 2, 3, and 4) of an electronic component was analysed by considering different jet-to-channel inlet velocity ratios (Vj/Vc) and ratios of the distance between the jet and impinging surface to jet diameter (H/D). The Vj/Vc and H/D ratios were varied in the 0–3 and 2–4 ranges, respectively, in the computational fluid dynamics analysis. The thermal and flow characteristics were revealed through a comparative result analysis, also considering results from the literature. The heat transfer improved, the Nusselt number increased, and the electronic surface temperature decreased with an increase in the Vj/Vc ratio. However, the Nusselt number decreased with an increase in the H/D ratio. Models 2 and 4 had higher heat transfer from the electronic component than the other models. A low H/D ratio and low Vj/Vc ratio yielded higher heat transfer in Model 3 than in Model 1.

Ignition studies of hydrogen-air mixtures over hot wire

Hot surface ignition of combustible gas mixtures poses a safety threat in many engineering applications. Gaseous mixtures ignite when hot surface temperature reaches the ignition threshold. In the present work, two-dimensional numerical simulations with detailed reaction mechanism are performed to simulate the flow of stoichiometric hydrogen-air mixture over a stationary hot wire. The effect of heating rates, wire diameters, mixture inlet velocities, and mixture equivalence ratios on the ignition threshold is investigated. In all the cases investigated, ignition is found to occur at the rear stagnation point, and wire heating rate did not influence the ignition phenomenon significantly. With an increase in mixture inlet velocity and mixture equivalence ratio, the ignition threshold increases, whereas the threshold has been found to decrease with increasing wire diameters. The role of the local equivalence ratios at the ignition point and reaction rates prior to the ignition process has been studied to help better understand the ignition phenomenon under different conditions.

Stratified Flames in Dual Annular Counter-Rotating Swirl Burner for Wider Operability Gas Turbines

Abstract This study investigates numerically the effects of equivalence ratio (Φ) on flow/flame interactions and emissions of stratified oxy-methane (CH4/O2/CO2) flames in a dual annular counter-rotating swirl (DACRS) burner for wider operability and environmental-friendly gas turbines. The flow mixture entering the combustor is split into two coaxial streams of different equivalence ratios. The central stream is characterized by higher Φ to continuously ignite the flame for enhanced flame stability, whereas the annular stream is a highly lean mixture to sustain the environmental performance of the combustor. The partially premixed combustion model is adopted in the ansys-fluent 2021-r1 software to model the reaction kinetics of the generated stratified flames in the two-dimensional axisymmetric domain. Nine cases of the same inlet velocity ratio (primary stream to secondary stream) of 3.0 are examined at a fixed oxygen fraction (OF: volumetric percentage of oxygen in the O2/CO2 mixture) of both streams of 30%. Flame stratification is achieved by varying the equivalence ratios of the primary (Φp = 0.9, 0.8, and 0.7) and secondary (Φs = 0.7, 0.55, and 0.4) streams. The results indicate effective flame/flow interactions, complete combustion, and reduced emissions for the DACRS stratified flames.