Journal Of Applied Fluid Mechanics
Volume:13 Issue: 4, Jan-Feb 2020

The article presents a method of purification of synthesis gas (syngas) produced as a result of biomass gasification with the use of a spray scrubber. The authors focus on the presentation of how individual elements of the geometry of a spray scrubber can influence its particle removal efficiency. The paper examines cases of dry particle removal, the use of demisters in purification process and wet scrubber collection efficiency with the use of numerical fluid mechanics (CFD). General equations are also used by authors to determine the initial predictions. The key part of the article is to present the results of the velocity and pressure distribution depending on the scrubber construction, determine the effect of the number of demisters used on the particle removal efficiency and to determine the probability of coalescence depending on the size of liquid droplets. Moreover, the authors formulate a simplified formula for the collection efficiency of a scrubber which is consistent with the results obtained from CFD calculations. The numerical results of collection efficiency were compared with experimental data from literature.

The occurrence of turbulent flows is quite common in nature and several industrial applications. The accurate simulation of these complex flows is still a great challenge in science. Large Eddy Simulation (LES) is an efficient technique based on the elimination of all scales of a flow smaller than a characteristic length ∆, considering that the flow pattern in small scales is homogeneous and isotropic. Therefore, modeling of turbulence in such scales is universal and independent of the flow type. This work present PMLES, a new OpenMP CUDA Fortran solver for complex turbulent flows at high Reynolds numbers and large computational domains (about 1 × 108 cells), using a single GPU card. This was possible by using an economical numerical scheme associated with a robust and efficient solution method that requires little variable storage. Theoretical and numerical aspects are firstly discussed, and then details of the computational implementation are given. Finally, the developed code is tested and validated by simulating a turbulent jet, and comparing the results with experimental and computational data from the literature. An analysis of performance gain is also carried out, demonstrating the code’s ability to solve this class of problems with a considerable reduction in computational time.

In any supersonic intake, the flow decelerates from supersonic to subsonic speed through a constant or divergent channel “isolator” by a series of bifurcated compression shock waves referred to as a shock train. It is important to understand the characteristics of the shock train which occur inside the isolator to improve the performance of scramjet engines. In the present work, numerical simulations were carried out to investigate the characteristics of the shock train occurring in the divergent channels using coupled implicit Reynolds Averaged Navier-Stokes (RANS) equations along with the two-equation k-w SST turbulence model. Results show that the downstream pressure variation causes the shock train length to decrease and the shock structure phenomenon varies from Mach reflection to Regular reflection. The variation of the inlet Mach number has less influence on the shock train length and the location of the shock train is determined by the area ratio. In comparison with the constant area duct, the shock train structure phenomena varies from Mach reflection to regular reflection in the divergent channel. Also, the increase in divergent angle raises the total pressure loss.

This paper presents experimental and numerical investigations on the modification of local spanwise skinfriction over triangular riblets under the total drag reduction condition. Specifically, the mean and fluctuating vortical flow fields were measured using 2-components X-wires and computed using LES, respectively. Besides, the relationship between local skin-friction along the riblet spanwise and associated vortex evolution was also built using the vortex dynamic method. Based on these results, it was found that, compared with the smooth case, the impaired and enhanced vortex strength, and resultant viscous diffusion/energy dissipation, determined the reduction and augment of the viscous drag force over the local spanwise riblet groove, i.e., decreasing and increasing cases of local drag, respectively. Furthermore, the mean normal diffusion fluxes of normal and spanwise vorticities contributed more to the viscous drag under these two cases. Correspondingly, the relevant flow physics related to these phenomena was discussed in detail.

Swirling flow has been widely used in gas turbine and aero-engine combustor to stabilize the flame. However, accurate numerical prediction of the swirling turbulent flow is difficult due to complex vortex movement in the flow, and turbulence modeling is a key factor. To assess the turbulence modeling in predicting the swirling flow, numerical studies are conducted for a well-documented swirling flow case. Three turbulence models are applied in the framework of scale resolved models, i.e. a newly developed VLES (Very-large eddy simulation) model, two LES (Large eddy simulation) models including the WALE (Walladapting local eddy viscosity model) and CSM (Coherent Structure Method). Numerical results are compared with the experimental results including the mean and RMS velocities. It is found that VLES model performs best among the three models and the other two LES models give comparable predictions. The complex vortex structures are explored based on the unsteady simulation results. The study demonstrates the high potential of VLES modeling for accurate prediction of complex swirling flow.

The novel diesel engines with advanced fuel injection systems are equipped with solenoid injectors comprising multiple small nozzle orifices which makes considerable improvement in fuel spray characteristics and engine performance along with providing high pressure fuel injection system. On the other hand, poor fuel quality, impurities and heavy metal elements in the diesel fuel, and high temperature medium in the diesel engines combustion chamber lead to remarkable deposits formation in the small holes of the nozzle. In addition, it results in partial or complete nozzle hole obstruction which is called injector nozzle coking having detrimental effects on discharged spray ideal behavior and proper engine performance. In this work, the analysis of coking phenomenon influences on diesel spray macroscopic characteristics have been done. Initially, the coked injectors with different time operation and deposit amounts are prepared under experimental and specific operating conditions. Then, the images recorded from the spatial and temporal evolution of a diesel spray in various injection and chamber pressures, are processed through the extended code in MATLAB software in order to analyze discharged fuel spray characteristics. The SCHLIEREN Imaging Method with high speed camera has been utilized in a CVC (constant volume chamber) without combustion. Non-Destructive Electron Microscopy Method of SEM (Scanning Electron Microscope) imaging was utilized in order to analyze sediments quantity and construction changes during injector working in the real engine conditions. The results show that, sediments occupy 20, 40, 75 and 90% of the total hole opening surface, respectively in the injectors with 300, 700, 800 and 900 hours operating time. By increasing the injector operation time and accumulated sediment amount on the nozzle, the discharged injector spray exhibits a more inappropriate behavior. Moreover, The Results revealed that coking has considerable effects on the spray tip penetration at low injection pressures. As injection pressure increases, the decreasing rate of the penetration length alleviates gently. In other words, at high injection pressures (1500 bar and higher) the penetration length has minor drop compared with non-utilized injectors even at 900 hours operating time, but the spray projected area can be reduced up to 28% in high chamber pressures.

Erosion wear of the centrifugal pump impeller are the key issues since its operating in sediment-laden flow. Experiment and numerical method are adopted in this study to investigate the erosion wear of the impeller in a double-suction pump in the Jingtai Yellow River Irrigation Project (JYRIP). Observation during operation combined with morphology study is carried out to verify erosion mechanisms at the different sections of the impeller blade. For numerical study, the Eulerian-Lagrangian approach combined with the Mclaury model is employed to predict the trajectories of the particles under different particle parameters. Also the effects of particle size and concentration on impeller erosion wear is studied. Numerical results are consistent with experimental data. The surface wear of the blade changes obviously from circular shape to strip shape in the direction from the Leading Edge (LE) to the Trailing Edge (TE), and the wear damage area increases correspondingly. Trajectories of larger particles are more uniform and tend to cause more severe erosion wear damage. The wear damage tends to be slow after the particle size increases to a certain extent under the same sediment concentration. Besides, erosion rate increases with an increase in sediment concentration, and the erosion rate of the suction side is greater than the pressure side at the same position of the blade surface. These results can offer a good reference for the design of wear resistance impeller of the double suction pump.

Onsite utilization of wind energy in the urban environment is an effective solution to environmental protection and energy security. The typically micro wind turbines, including Savonius vertical axis wind turbine, H-type vertical axis wind turbine and micro horizontal axis wind turbine are more suitable for distributed generation, relative to centralized generation of large scale wind turbines. However, the wind in the urban environment characterized by low wind speed, high levels of turbulence and strongly unsteady direction and speed, directly affecting the aerodynamic characteristics of wind turbine. In the present work, wind tunnel tests have been conducted to investigate the low Reynolds number effect on aerodynamic characteristics for these three typical micro wind turbines, and the aerodynamic differences among them have been compared qualitatively and quantitatively. The experimental results show that micro horizontal axis wind turbine and H-type vertical axis wind turbine, belonging to lift-type wind turbine, have relatively higher startup wind speed and lower power coefficient due to deteriorative aerodynamic performance of airfoil at low wind speed. However, Savonius vertical axis wind turbine, as a drag-type wind turbine, exhibits excellent aerodynamic performance at low Reynolds number. The Savonius wind turbine has apparent output power at 5m/s, and the peak power coefficient exceeding 0.2 at 9m/s being superior to that of two other lift-type wind turbines at the same wind speed. In addition, in consideration of natural advantage of vertical axis wind turbine, Savonius wind turbine is the best option for applying at low Reynolds number urban environment.

Electroosmotic flow of salt-free power-law fluids through planar slit and cylindrical micro and nanochannels with fluid slip is theoretically analyzed. Analytical solutions are obtained to investigate the effects of flow behavior index, channel size, applied electric field strength, Gouy-Chapman length (or surface charge density), and fluid slip length on the velocity distribution and volumetric flow rate. The results show that the electroosmotic flow velocity and thereby the flow rate for shear-thinning fluids are many times larger than those for Newtonian and shear-thickening fluids for the ranges of applied electric field strength and surface charge density usually encountered in practice. Such augmentation can be further amplified by increasing the surface charge density, applied electric field strength and fluid slip. Furthermore, the electroosmotic flow velocity profile of shear-thinning fluids becomes more plug-like as the ratio of channel half-width (or radius) to GouyChapman length increases. However, such a profile for shear-thickening fluids always exhibits a parabolic-like flow pattern regardless of the ratios of channel half-width (or radius) to Gouy-Chapman length.

A single cylinder diesel engine was tested under different loading conditions with its piston crown coated with the Thermal Barrier Coating (TBC). The main objective of this work is to investigate the effect of the TBC on performance and emission characteristics in the diesel engine. The top surface of the piston was coated with 100 µm thick NiCrAl as lining layer by plasma spray method. A mixture of 88% Yttria stabilized Zirconia, 4% MgO and 8% TiO2 of 150 µm thick were coated over the lining layer. Exhaust emission (HC, NOx, CO and CO2) parameters were investigated using AVL exhaust gas analyzer. The results showed that the brake thermal efficiency was increased by 10% and brake specific fuel consumption was decreased by 9.8% for coated piston in comparison with the uncoated piston engine. It was also observed that, smoke, CO and HC emissions were decreased in the TBC engine as compared with the baseline engine. In addition carbon di oxide (CO2) and nitrogen oxide (NOx) emissions were partially increased.

Developing an accurate and reliable model for the pressure strain correlation is a critical need for the success of the Reynolds Stress Modeling approach. This is challenging because replicating the non-local effects of pressure using a modeling basis composed of local tensors is limiting. In this paper we use physics based arguments and analysis of simulation data to select additional tensors to extend this modeling basis for pressure strain correlation modeling to formulate models with improved precision and robustness. We integrate these tensors in the modeling basis and develop separate models for the slow and rapid pressure strain correlation. This complete pressure strain correlation model is tested for different turbulent flows and its predictions are compared to prior pressure strain correlation models. We show that the new model with an extended tensor basis is able to show improvements in accuracy and reliability.

The limitation of energy resources in the world has cut the attention of many researchers to find new resources of clean energy and develop methods for recovering exergy losses. Twin screw expander (TSE) is one of the positive displacement machines that has been widely applied in recent years to recover mechanical power from fluids due to its lower installation, operation, and maintenance costs in comparison with common expansion turbines (CET). In this paper, technical and economic conditions of a city gate station (CGS) were studied with the aim of recovering exergy loss using a TSE. A computational fluid dynamic code was used to simulate the three-dimensional fluid flow in the TSE. The simulation results for Shahroud station —a CGS located in the east of Iran— revealed that, unlike CET, flow and pressure fluctuations in different seasons of the year did not put any restrictions on the use of TSE whereby 85.3% of annual loss exergy was recovered. Economic studies showed that the internal rate of return (IRR) and payback period of TSE utilization were obtained of 27.6% and 4.4 years, respectively, making the investment in CGS stations more practical compared to the CET.

Sediment transport in the aquatic environment is one of the complex two-phase problems in flow mechanics and sediment hydraulics. In this study, the interaction between water and sediment is explored using the SPH method and developed SPHysics2D in which the pressure values are calculated using the state equation. In this study, the non-Newtonian rheological model µ(I) is used for modeling the sediment phase where it is developed based on the properties of granular particles. Also, the effective pressure is used for the study of sediment behavior. The method used in this research is compared with the methods used by other researchers. The Owen equation is utilized to determine the effect of viscosity within the two-phase area. The developed method is evaluated by the dam break on a dynamic bed and then, the experimental model of submerged sediment column collapse is investigated in the aquatic environment. The results of the modeling demonstrate the capabilities of the developed code for the use in the flow and sediment hydraulics.

This paper explores the use of Two-Dimensional sinusoidal surface features to delay transition and/or reduce drag. The authors, in this paper demonstrated that the presence of low amplitude sinusoidal surface features might damp the disturbances in the laminar boundary layer, reduce wall shear stress and maintain laminar flow for longer than a conventional flat plate. The hypothesis of the paper is inspired by the simplification of the dermal denticle on the surface of the shark-skin. Simulations are carried out using the Transition SST model in FLUENT based on the evidences of the transition model being suitable for a wider variety of high curvature scenarios. The surface waves are simulated for different amplitudes and wavelengths and their impact on transition onset and drag reduction are quantified at different velocities. Results presented in this paper indicate that a transition delay of 10.8% and a drag reduction of 5.2% are achievable. Furthermore, this paper adds credence to the notion that biomimicry is a very promising avenue for future drag reducing methods.

The attitude control of a rocket engine using the control surfaces becomes cumbersome particularly in larger rockets with high payload. In such cases, a more effective means of producing forces for controlling the flight is the deflection of exhaust gases, referred to as the gas-dynamic steering or the thrust vector control. In this study, the effect of a strut on the exhaust gas deflection, deployed at the locations; 0.62 L, 0.72 L and 0.8 L in the divergent-portion of a Mach 1.84 nozzle at over-expanded, correctly-expanded and under-expanded states of the jet, has been experimentally investigated. The level of expansion at the nozzle exit is varied by changing the settling chamber pressures from 4 bar to 8 bar, in steps of 2 bar. Further, to study the effect of aspect ratio, the height of strut is varied as 1.5 mm, 2.5 mm and 3.5 mm. The strut of height 3.5 mm, deployed at x/L = 0.72, is found to be the most effective thrust vector control at overexpanded conditions; with a maximum jet deflection of about 3.6o , obtained at a settling chamber pressure of 4 bar. The Schlieren flow visualization images confirm the findings of wall static pressure data.

In this work, a numerical study has been carried out in order to investigate the effects of a micro combustor size on the exergy and energy efficiencies of a premixed hydrogen/air for a micro thermophotovoltaic system. For this purpose, six combustors in different sizes are designed, in which geometry dimensional size gradually reduced. The effects of the combustor size on the entropy, exergy, radiation power, and energy conversion efficiency are investigated. Also, mean and uniform wall temperature are discussed. In order to compare the entropy generation of each micro combustor, a dimensionless entropy generation rate is defined. The hydrogen/air combustion with 9 species and 19 reversible elementary reactions were simulated by using the Eddy Dissipation Concept (EDC) model. Results indicate the micro-combustor geometry size has important effects. A reduction of the combustor geometry size dimensionality causes an increase in average wall temperature and makes it uniform. Moreover, by decreasing micro combustor size, the radiation power efficiency increases from 41.96 to 45.62% and total energy conversion efficiency from 6.46 to 7.02%. The highest exergy efficiency, 38.63%, is achieved in the smallest micro combustor while the minimum exergy efficiency 33.22%, is obtained in the largest micro combustor.

Extreme industrial conditions require a bearing which can withstand high-speed operations, heavy load, high stiffness and so on. Therefore in this study, the combined effects of fluid inertia forces and non-Newtonian characteristic with Herschel-Bulkley fluid as lubricant in an externally pressurized converging thrust bearing have been contemplated. Avoiding complex calculation, the term inertia in the momentum equation is estimated by the mean value average method across the film thickness. A mathematical model for converged thrust bearing has been introduced. Using appropriate boundary conditions, thickness of the core, velocity profile, film pressure and the load carrying capacity of the bearing for various values of Herschel-Bulkley number (N), Reynolds number (Re), Power-law index (n) and angle of convergence (φ) have been numerically computed. Having worked with an externally pressurized flow through a narrow clearance between two convergent disks symmetrical with respect to r and z axis, it is found that the converged bearing performance such as pressure distribution and load carrying capacity increases notably. The results obtained in this study is found to be in agreement with the results of Jayakaran et al. (2012), for a particular case.

Internal waves are abundant in stratified environments such as the atmosphere and the ocean. In this paper, the internal waves and turbulent wake effect of a cylindrical body in a linear stratified environment is investigated. In a glass tank with dimensions 3×1×0.5 (m), a linear stratification with buoyancy frequency (N) equal to 0.51 per second, was set up. Then a cylindrical body with 6 cm in diameter and 45 cm at length was towed in the fluid, using a computer controlled system. While changing the Froude (from 0.16 to 1.5) and Reynolds numbers of the flow, the effects of these changes were examined on the formation of the internal waves and wake of the cylinder. The internal waves generated were studied using shadowgraph technique and signal fluctuations were recorded. The results show that the presence of internal waves depend on changes of buoyancy frequency (N), Froude numbers, and Reynolds numbers of the flow. It was also found that with an increase up to the critical Froude number, the activity of internal waves and their wavelengths enhanced. Also, irregular long and short waves as well as turbulence of the environment were observed in the range of supercritical Froude numbers. In this study, the signals are recorded in domains of frequency and statistics. Using an ultra-fast salinity meter (densitometer) for recording signal frequencies it was found that increasing Froude number results in more combination of frequencies occurred in the environment. Based on the results of current experiments and previous studies, an equation was extracted to calculate the wavelength by parameters like velocity of body, maximum frequency value and propagation angle. The energy of wave spectrum increased up to a critical Froude number, and then decreased due to turbulence. The statistical distribution of signals recorded in most of the scenarios was normal.

In order to reduce the adverse effect of the tip leakage flow of cantilever stator on compressor performance, the impact of the axial position of endwall streamwise suction slot on tip leakage flow was numerically studied. The study on the overall performance of the compressor and the details of the flow field near the stator end region with and without suction showed that all suction schemes could weaken the tip leakage flow intensity to a certain extent, and the flow control effect was gradually enhanced with the increase of the suction flow rate. In the case of small suction flow rate, for example, 0.5%, the short slot schemes can improve the overall efficiency of the compressor by about 0.5%, which is more advantageous than the long slot scheme, and the overall efficiency improvement of the latter is about 0.3%. The advantage of the long slot scheme in flow control is reflected in the case of large suction flow rate, that is, 1.0%, which may improve the overall efficiency of the compressor by about 0.96%. The axial position of suction slot has a significant influence on flow control effect of the tip leakage flow. Compared with the downstream suction, which only modified the flow field by reducing the blocking effect generated by tip flow vortex, the upstream suction could better control the tip leakage flow by restraining the development of the initial stage of the leakage vortex. Besides, the endwall suction scheme with a full chord length slot has the greatest impact on the passage vortex, its effect on modifying the flow field near the end zone was determined by the combinatorial action of the enhancement of the passage vortex and the attenuation of the leakage vortex.

A supercritical airfoil is geometrically optimized using the new developed adjoint compressible lattice Boltzmann method. Minimizing the drag coefficient and eliminating the shock wave on the supercritical airfoil surface are considered as the cost function with constraint of fixed lift coefficient. The continuous adjoint method is applied to able designers to implement large number of design variables in actual optimization problems. The adjoint equation based on the specified cost function and constrains is successfully derived. Discretization of the governing equations is carried out using the finite volume approach and 3rd order of the MUSCL scheme. The supercritical SC(2)0410 airfoil, which has a strong shock on the top surface at transonic cruise conditions, is numerically optimized using the inviscid developed algorithm to eliminate the shock and reduce the wave drag. To validate the obtained results and show viscosity effect on the results, the base airfoil and optimized one are experimentally tested in a transonic wind tunnel at the same conditions. Pressure distribution on the surface of both the base and optimal airfoil are extracted from the experimental tests and compared with those of numerical simulations. The results indicate that the developed approach can be properly used for supercritical airfoil shape optimization for elimination the shock and reduction the wave drag.

The increased probability of fire occurrence in urban tunnels has led researchers to investigate this issue extensively. Although fire can occur at any point in a tunnel, the effect of fire source position on temperature distribution has not received considerable attention in most of previous investigations. In this research, the influences of varying horizontal fire source locations on temperature diffusion in particular maximum smoke temperature stratification beneath the ceiling has been investigated. A set of scale-down experiments was performed in a model tunnel [3 m (length) × 0.6 m (width) × 0.96 m (height)]. n-Heptane and gasoline were used as fuels in rectangular pools to generate a heat source. The analysis reveals that typical temperature curves have a similar trend when the fire source location changes. Furthermore, the temperature profile tip (maximum smoke temperature) is located between the burner and the origin of the tunnel. The modified model of maximum temperature, which considers the horizontal fire source location, is defined. The results here complement existing literature where the effects of variable fire position in a tunnel have not been considered.

A novel design of cooling air supply system with dual row pre-swirl nozzles (DRPM) is promoted and investigated. Simplified theoretical analysis and numerical simulation are used to estimate the total temperature reduction and mass flow rate characteristic of DRPM and compared with single row pre-swirl nozzle model (SRPM). The results show that, both models have similar flow structure and the variation of total temperature reduction and dimensionless mass flow rate with rotational Reynolds number and pressure ratio is also similar. Which have an inflection point with the increase of rotational Reynolds number but increases monotonically with the variation of pressure ratio. The pre-swirl system has the maximum flow rate and temperature reduction when the inflow Angle equal to 0 or the swirl ratio equal to 1 at the inlet of the receiver hole. The increase in pressure ratio improves the total temperature reduction and dimensionless mass flow rate as well. In the range of rotational Reynolds number calculated, DRPM can increase the dimensionless mass flow rate by 3.0% but the total temperature reduction decreased by 37.8% in average compared with SRPM. On the other hand, the dimensionless mass flow rate increased by 2.8% and total temperature reduction decreased by 14.9% in average in the range of pressure ratio calculated. Numerical results are in good agreement with the results calculated by simplified theoretical formulas.

Particle size normally varies over wide ranges in any commercial transportation of solids through the pipeline. In the present study, the three-dimensional numerical modeling of the conventional 90o bend transporting multi-sized particulate slurry using granular Eulerian-Eulerian model is performed. The mixture of water and six different sizes of zinc tailing particles ranging from 37.5 µm to 575 µm are considered. The effect of variation in velocity and concentration on pressure drop and flow field of the multi-sized particulate slurry is investigated. The simulations are performed in the velocity range of 2.25 m/s to 3.5 m/s for the weighted solid concentration range of 9.82 to 44.26%. The comparison of pressure drop data from the available experimental results and the present numerical modeling with multisized particulate slurry shows maximum deviation within ±6%. Further, the suspension behavior of different size particles in the multi-sized slurry flow inside the bend is analyzed with the variation in the flow velocity and solid concentration. The particles of different size in the multi-sized slurry showed different suspension characteristics.

Artificial supercavitation is one of the most prospective technique for underwater drag reduction, but it still faces some unsolved roadblocks, like cavity stability, noise, power etc. This article aims at investigating the instability of cavity with the strong jet impingement, and analyzing jet behavior in restricted space. A multiphase model using coupled VOF and level set method is adopted to capture the gas-liquid interface. By changing the position of jet nozzle exit and the jet intensity, a series of numerical simulation is performed. Firstly the transient evolution of the cavity interface under the effect of the high-speed jet is obtained. Numerical model is validated by comparing with the experimental data. Then the criterion to determine the transition between different jet/cavity interaction mechanisms is established based on a non-dimensional distance parameter. Furthermore, the entrainment mechanism inside the cavity is analyzed and provides useful insights on enhancing the cavity stability with strong jets.

The objective of present theoretical analysis is to study the combined influence of surface roughness and lubricant inertia on the steady performance of stepped circular hydrostatic thrust bearings lubricated with nonNewtonian Rabinowitsch type fluids. To take the effects of surface roughness into account, Christensen theory of rough surface has been adopted. Solution for momentum equation has been derived by means of average inertia approach. Analytic expressions for film pressure have been established for radial and circumferential roughness patterns. Results for film pressure, load carrying capacity of bearing and lubricant flow rate has been plotted and analyzed on the basis of numerical results. Due to surface roughness, significant variations in these properties have been observed.