Showing papers in "Journal of Applied Fluid Mechanics in 2020"

Numerical Investigation of Sinusoidal and Trapezoidal Piston Profiles for an IC Engine

Abstract: This paper is aimed at a comparative investigation on two different velocity profiles for piston movement namely Sinusoidal and Trapezoidal Profiles for an IC Engine. In conventional IC Engine, velocity profile of piston motion is Sinusoidal. It has many disadvantages such as high mean velocity that leads to high inertial force, frictional losses, wear and high rate of heat leakages. Nearly 20% of the total power produced by the engine is dissipated into heat because of friction. Of this 20%, about 75% is due to friction of piston rings on the cylinder walls. This is an irreversible loss and can be seen as a consequence of high mean piston velocity associated with the existing Sinusoidal Piston Velocity Profile. In addition, varying velocity profile can cause rapid acceleration and finally jerks which lead to considerable mechanical vibration and noise. As a result the mechanical strength of engine material will be high to withstand the inertial force, friction and wear. To overcome these difficulties, an extensive attempt is made to improve the piston movement by restructuring the piston velocity profile with reduced mean velocity which is constant for most of the crank angle. A comprehensive experimental examination is conducted for the Sinusoidal velocity profile, which are utilized in arriving at an optimal CFD procedure through validation study. A proposed connecting rod configuration with internal gear and pinion arrangement is proposed to achieve different Trapezoidal Profiles. The optimum CFD procedure found from validation study is used to analyze and understand the engine with modified Trapezoidal Velocity Profiles. There is almost 20% reduction of mean piston velocity that considerably improves hydro-thermo dynamic and mechanical characteristics of the existing engine.

Optimization of the Splitter Blade Configuration and Geometry of a Centrifugal Pump Impeller using Design of Experiment

Abstract: Centrifugal pumps are among the most applicable machines in a wide variety of industrial systems for fluid pumping and transportation. Therefore their optimization has always been of great importance. Pump impellers play an important role in these machines as the energy transfer takes place in this part. In the present study, the impeller of a centrifugal pump is optimized by investigating the effect of adding splitter blades and modifying their geometry. A centrifugal pump is experimentally tested and numerically simulated and the characteristic curves are obtained. In the first stage, two different sets of splitter blades with different lengths are added to the impeller and the effect of splitter blade lengths on the results are explored. The case with the highest total head and overall efficiency is selected for the optimization process. The main blade and the splitter blade leading edge position and also the splitter blade distance between two successive blades are selected for the optimization process in the second stage. Efficiency and total head of the pump are considered as the optimization objectives. Using Design of Experiment (DoE) technique, the design space is created and response surface method is utilized to find the optimum geometry. The results show adding splitters can improve total head by about 10.6% and by modifying the geometry using DoE technique it could increase further by 4.4% with the negligible effect on the pump overall efficiency.

Experimental Investigation of Thermal Barrier (8YSZ-MGO-TIO2) Coated Piston used in Diesel Engine

Abstract: 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.

Performance Comparison of Novel Single and Bi-Diaphragm PZT Based Valveless Micropumps

Abstract: A commercial micropump should provide properties that justify the simple structure and miniaturization, high reliability, simple working principle, low cost and no need for complex controller. In this study, two novel piezoelectric actuated (lead zirconate titanate-PZT) valveless micropumps that can achieve high flow rates by pumping chambers and fixed reservoirs were designed and fabricated. Extensive experiments were conducted to investigate the effects of hydrodynamic and electromechanical on flow rates of the Single Diaphragm Micropump (SDM) and the Bi-diaphragm Micropump (BDM). BDM had two actuators facing to the same chamber at 180-degree phase shift. The primary features of the proposed designs were the high flow rates at low driving voltages and frequencies with the help of innovative design geometry. 3D-printing technique providing one-step fabrication for integrated micropumps with fixed reservoir was used. The micropump materials were biocompatible and can be used repeatedly to reduce costs. Mechanical parameters such as tensile test for silicon diaphragm, surface topography scanning by microscopy techniques and drop shape analysis for hydrophobic property were investigated to reveal surface wetting and flow stability. In addition, the effect of reservoir height was investigated and the calibration flow rates were measured during the inactive periods. The maximum diaphragm displacements were obtained at 45 V and 5 Hz. The maximum flow rate of SDM and BDM at 45 V and 20 Hz were 32.85 ml/min and 35.4 ml/min respectively. At all driving voltage and frequency levels, BDM had higher flow rates than of SDM.

Experimental Investigation on the Effect of Increasing the Collar Thickness on the Flow Pattern around the Oblong Pier in 180˚ Sharp Bend with Balanced Bed

Abstract: In this paper, the effect of increase collar thickness has been investigated by considering other parameters affecting the flow pattern around the oblong bridge pier located at 90 ̊ from the 180 ̊ bend constant. Data acquisition was performed using the Vectrino 3D velocimeter. The leveling of the collar in both tests is constant and 0.4 times the pier diameter above the initial bed surface. The flow conditions were close to the motion threshold and bed balanced. The results showed that with an increase in the thickness of the collar by a factor of 4, the barrier surface is located in the flow path and the flow is deviated to the bed by colliding with the edge of the collar. This flow deviation that was due to the increase the collar thickness, led to 30% increase in the maximum depth of the scour hole upstream of the pier. Also, with the increase in the thickness of the collar, the tendency of the streamlines upstream of the pier to the inner bank increased, resulting in a 20% increase in the maximum sedimentation near the inner bank. The maximum turbulent kinetic energy at 80 degrees of the bend, corresponds with a protected pier with 3 mm thickness, equal to 620 cm/s.gr, which is reduced by 60% with increasing collar thickness.

Numerical Study on Shock Train Characteristics in Divergent Channels

Abstract: 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.

Aerodynamic Optimization of Unmanned Aerial Vehicle through Propeller Improvements

Abstract: This paper aimed at presenting a number of suggested improvements that can enhance the performance of a multi-rotor Unmanned Aerial Vehicle. Evaluating each suggestion in terms of the added benefits and feasibility concluded a final choice, which is incorporating a sinusoidal leading-edge profile to the propeller. This choice was numerically investigated with ANSYS Fluent 16.1 through the SST K-Omega turbulence model. The performance of the modified propeller was assessed by comparing the lift and drag results to the same propeller with a straight leading-edge under the same conditions. Both models were studied at pre-stall and post-stall conditions to see the performance effect with respect to the angle of attack. The findings of this research showed 7% increase in the lift force and coefficient that were associated with the addition of the sinusoidal leadingedge including improved recovery from stall spanning from angle of attack that extends between 10° to 25°. This research also provides more insights into how the delayed stall and improved lift help the multirotor to extend flight time and carry heavier payloads. It allows for the exploration of the inner working of the sinusoidal leading-edge and its relationship with the flow field over the propeller.

A Moore-Greitzer Model for Ducted Fans in Ground Effect

Abstract: Ducted fans are widely used in unmanned aerial vehicles due to their high propulsive efficiency and safety. The aerodynamics are complex within the vicinity of the ground, including take-off, landing and hovering. In the present study, the aerodynamic stability of the ducted fan was studied with a modified Moore-Greitzer model to estimate and analyse the stability in ground effect. The model was validated and compared with threedimensional unsteady simulations. The results indicated that the rotating stall occurred for the ducted fan in ground effect. The model results can be used to guide the design and control of the vehicles, to ensure the stability and safety of both the ducted fan and vehicles.

CFD-DEM Simulation to Predict the Critical Velocity of Slurry Flows

Abstract: The hydraulic transport of solids has been adopted for many years. However, deposition of the solid particles in the pipe can cause equipment failure, pipeline erosion, and excessive pressure drop, resulting in financial and environmental problems. The critical velocity, also known as critical deposition velocity, is the limit of particle deposition when a moving bed of particles starts to form on the bottom of the pipe. The determination of the critical deposition velocity is an essential step in slurry pipeline`s design and operation. This paper assesses the influence of the velocity in the particle deposition of slurries containing coarse particles of apatite and hematite industrial concentrates and quartz. We used CFD to solve Navier-Stokes for the fluid phase and DEM to solve Newton`s equations governing the granular particles. CFD and the DEM modules adopted OpenFOAM and LIGGGHTS. Numerical results were compared with experimental data from the literature.

Influence of Impermeable Elevated Bottom on the Wave Scattering due to Multiple Porous Structures

Abstract: The significance of multiple porous structures with finite spacing upon elevated seabed in the presence and absence of the leeward wall is examined under oblique wave impinging. Fluid propagation is assumed over the impermeable elevated bottom, and the fluid realm is separated into open water and porous structure regions. Continuity of the dynamic pressure and mass fluxes at the interfaces of the porous structure and the open water regions are adopted. The resistance and reactance due to the presence of the porous structure are taken into account using the porous structure dispersion relation. The numerical model is developed based on the eigenfunction expansion method along with matched velocity potentials at the interfaces of open water and the porous block regions. The wave reflection and transmission characteristics, energy damping and wave force impact on the leeward wall is analysed. The significance of the porosity, structural width, angle of incidence, width between the two structures and water chamber length is studied considering multiple porous blocks with finite spacing under oblique wave impinging in the presence and absence of leeward wall. The numerical results obtained in the present study agrees well with the theoretical and experimental results available in the literature. The present study illustrates that, with the increase in the number of porous blocks and gap between the porous blocks, the resonating trend is observed in the wave transformation and the influence of the elevated step height is revealed for the wave trapping.

Prediction of base pressure in a suddenly expanded flowthe processes at supersonic Mach Number Regimes using ANNand CFD

Abstract: The sudden expansion of flow in a supersonic flow regime has gained relevance in the recent pasts for a wide run of applications. A number of kinematic as well as geometric parameters have been significantly found to impact the base pressure created within the suddenly expanded stream. The current research intends to create a predictive model for base pressure that is established in the abruptly extended stream. The artificial neural network (ANN) approach is being utilized for this purpose. The database utilized for training the network was assembled utilizing computational fluid dynamics (CFD). This was done by the design of experiments based L27 Orthogonal array. The three input parameters were Mach number (M), nozzle pressure ratio (NPR) and area ratio (AR) and base pressure was the output parameter. The CFD numerical demonstrate was approved by an experimental test rig that developed results for base pressure and used a nozzle and sudden extended axisymmetric duct to do so. The ANN architecture comprised of three layers with eight neurons in the hidden layer. The algorithm for optimization was Levenberg-Marquardt. The ANN was able to successfully predict the base pressure with a regression coefficient R2 of less than 0.99 and RMSE=0.0032. The importance of input parameters influencing base pressure was estimated by using the ANN weight coefficients. Mach number obtained relative importance of 47.16% claiming to be the most dominating factor.

Effect of Air Quantity Distribution Ratio on Flame Height of Flue Gas Self-Circulation Burner

Abstract: It is a difficult scientific problem of applied fluid mechanics that the flame is too long and does not match the furnace chamber in a small restricted heating space. This paper aims to investigate the effect of the air quantity distribution ratio on the flame height of a flue gas self-circulation burner. In order to obtain a better combustion emission effect and a shorter flame height, a burner head structure with a small flue gas selfcirculation was designed. Numerical simulation was employed to investigate the effect of the different distributions of central air, swirling air and secondary air on flame height. The periodic boundary condition model was adopted and the numerical model was compared and validated by experiment. Correlation analysis was used to determine the influence of the air inlet ratio of each part on the flame height and recirculating flue gas ratio (RFGR). The results show that the influence of different air quantity distributions on flame length is very significant. A reasonable central air ratio is a necessary condition for the good combustion of this flue gas self-circulation burner. Secondary air can effectively increase the RFGR, and flame height was significantly shorter with the increase of RFGR, but when it increased to more than 12%, the flame length was basically no longer shortened. On the premise of stable combustion, when the ratio of central air, swirling air and secondary air are respectively 25%, 35% and 40%, the shortest flame length is achieved. This work reveals an influence mechanism of the flame height of a small burner with a flue gas circulation structure. These results can provide theoretical support and an engineering design basis for the short flame problem in a small restricted space.

Rankine Vortex Formation during Draining: A New Twin Port Suppression Strategy

Abstract: This paper reveals the results of a study of vortex air core formation (Rankine vortex) when a rotated liquid (water) column in a cylindrical vessel is drained through two ports located at equal eccentricity (e) at the vessel base (diameter, d1and d2) simultaneously; d1is fixed whereas d2 is varied. Just before draining, a rotation (n rpm) is provided to the liquid column in controlled conditions. As draining progresses, when the liquid level reaches certain height called critical height (hc), initially a surface dip forms which further develops in to a vortex extending down till the drain port. Results show that critical height increases as the fluid rotation rate increases at the lowest eccentricity. But, at higher eccentricities, hc , exhibits more or less an increasingdecreasing trend in most of the cases studied. Critical height is observed to be minimum for the largest value of d2 (equal to d1 ) irrespective of the values of the speed of fluid rotation, liquid initial height and port eccentricity. To particularly note, at the highest eccentricity, vortex formation is found to be completely suppressed for all values of port diameter (d2) and initial fluid rotation (n) as indicated by the near-zero critical height values. The tangential velocity measurements using Particle Image Velocimetry are also reported. PIV results obtained for certain cases with induced fluid rotation (normal draining and faster draining) correlate well with the changes in the efflux (axial) velocity (deduced analytically) in these cases studied. The tangential velocity along radial direction obtained (PIV) also indicated the type of vortex formed in normal and faster draining cases. Video visualization of vortex formation carried out reveals that, vortex air core switching takes place between the drain ports maintaining an arched or curvilinear surface profile apart from demonstrating the nature of outlet flow discharge. All the vortex air core formation studies so far carried out were invariably with single drain port except the preliminary novel study by the same author group and the present study is a detailed extension of that novel study.

Active Fluids: Effects of Hydrodynamic Stress on Growth of Self-Propelled Fluid Particles

Abstract: Under the current global energy crisis the interests in developing a third generation of biofuels produced from non-food feedstock such as microalgae and cyanobacteria have clearly increased. Hydrodynamic stress, always present in cultivation process of these microorganisms, is an essential factor to ensure mixing inside bioreactors; however the importance of its intensity is usually ignored by applying a random agitation (energy consumption) which is unnecessarily overestimated. In this work, two types of agitation, stirring in agitated photobioreactors (APBR) and bubbling air in draft tube airlift photobioreactors (DPBR), are applied to study the effects of hydrodynamic stress on the growth and pigment content evolution of the cyanobacteria Synechocystis sp. PCC 6803, a self-propelled microorganism. The range of applied shear stress was between 0 and 400 mPa. Similar results are obtained for both agitation mechanisms, indicating that the effects of shear stress are limited to the breakdown of the cell colonies; once they are broken down any further increase in shear stress has no significant effect on their growth rate. Moreover, the variation in pigments concentration appeared to be linear with the cellular concentration and independent from shear intensity.

A Reliable Pressure Strain Correlation Model for Complex Turbulent Flows

Abstract: 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 Characteristics of Self-Resonating Jet Issuing from the Helmholtz Nozzle Combined with a Venturi Tube Structure

Abstract: Self-resonating waterjet is a new type of waterjet technology that has been widely used for many practical applications. In order to further improve the performance of self-resonating waterjet, the Helmholtz nozzle was improved by replacing the upper part of a traditional contract structure with a venture tube one. This composite nozzle of a venturi tube structure and a Helmholtz resonator was proposed based on the working mechanism of self-resonating waterjet nozzles and the instability of cavitation flow in venturi tubes. Furthermore, the results were also compared with those generated by a conventional Helmholtz nozzle under the same conditions. The frequency of the pressure pulsation in the oscillating cavity and at the outlet was obtained and analyzed by the classical Fast Fourier transform (FFT) method. The results showed that the main frequency of the pressure oscillation rises to 2362.78Hz, and the peak and average values of the pressure are increased by 45% and 12.5% respectively at the outlet of the composite nozzle. In the oscillating cavity of composite nozzle, the pressure oscillations in the central region have higher frequencies and amplitudes, while near the wall are reversed.

Parametric Investigation of Wavy Rectangular Winglets for Heat Transfer Enhancement in a Fin-and–Tube Heat Transfer Surface

Abstract: In this paper numerical simulations were performed utilizing Computational fluid dynamics code Fluent to investigate the thermo-fluid performance of a wavy rectangular winglet supported fin-and-tube heat exchanger with five inline rows of circular tubes. The influence of wave height, number of waves, wavy winglet length and winglet attack angle on the thermo-fluid performance of the fin-and-tube heat transfer surface has been examined under laminar flow conditions. Further the Plain and wavy rectangular winglets are placed together over different tube locations and their effect on heat transfer and flow resistance is also examined. An enhancement factor has also been discussed to summarize the overall thermo-fluid performance. The results show that increase in the wave height increase both heat transfer and pressure drop, and an optimum wave height could be decided based on the enhancement factor. It is also found that the increase in wavy winglet length guides the flow more effectively towards the tubes wake region. It is also observed that with increase in number of waves the heat transfer performance initially increases and then decreases as the wave pitch becomes very small. For wavy winglet supported heat exchanger the optimum attack angle is found out for maximum enhancement factor.

Analytical and Numerical Investigation of the Airflow in Face Masks used for Protection against COVID-19 Virus - Implications for Mask Design and Usage

Abstract: The use of face masks for the general public has been suggested in literature as a means to decrease virus transmission during the global COVID-19 pandemic. However, literature findings indicate that most mask designs do not provide reliable protection. This paper investigates the hypothesis that the impaired protection is mainly due to imperfect fitting of the masks, so that airflow, which contains virus-transporting droplets, can leak through gaps into or out of the mask. The fluid dynamics of face masks are investigated via analytical and numerical computations. The results demonstrate that the flow can be satisfactorily predicted by simplified analytical 1D-flow models, by efficient 2D-flow simulations and by 3D-flow simulations. The present results show that already gap heights larger than 0.1mm can result in the mask not fulfilling FFP2 or FFP3 standards, and for gap heights of ca. 1mm most of the airflow and droplets may pass through the gap. The implications of these findings are discussed and improvements to existing mask designs are suggested.

Experiments and Large-Eddy Simulations of Lobed and Swirling Turbulent Thermal Jets for HVAC's Applications

Abstract: The mixing improvement by passive control is of wide practical interest. The lobed diffuser, which mixes the primary and secondary streams with high efficiency, has been widely used for heat and mass transfer in the field of fluid engineering. In addition, the jets through lobed generate streamwise vortices, which mix the ambient air and the jet fluid more effectively. The main objective of the present work is to develop new air diffusers for heating, ventilation, air conditioning (HVAC) systems using different jet geometries, in order to improve the users’ thermal comfort. Three free jets of air diffusers emitted from a tubular lobed, with six and five lobes, and from a swirl nozzle have been both studied experimentally and numerically. All diffusers have the same throat diameter. It turns out that the results obtained with the LES/WALE and LES/K-ET turbulence models are respectively in good agreement with the experimental results of the lobed and swirling jets. These results indicate that the best mixture is obtained using the six-lobed nozzle with respect to the five-lobed nozzle and the swirling nozzle. In addition, the importance of the jet type on the mixing capacity is highlighted.

A numerical study of the flow interference between two circular cylinders in tandem by scale-adaptive simulation model

Abstract: Fil: Grioni, Mauro. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ingenieria. Instituto de Mecanica Estructural y Riesgo Sismico; Argentina

Heat conduction characteristic of rarefied gas in nanochannel

Abstract: Nonequilibrium molecular dynamics simulations is applied to investigate the simultaneous effect of rarefaction and wall force field on the heat conduction characteristics of nano-confined rarefied argon gas. The interactive thermal wall model is used to specify the desired temperature on the walls while the Irving– Kirkwood expression is implemented for calculating the heat flux. It is observed that as the temperature differences between the walls increases by lowering the temperature of the cold wall, the number of adsorbed gas atoms on the cold wall increases notably due to the increment in the residence time of the gas atoms. Consequently, the interfacial thermal resistance between the gas and the cold wall reduces which results in a reduction of the temperature jump. Meanwhile, the increase in the temperature of the hot wall leads to a reduction of the residence time of gas atoms in the near-wall region which decreases the number of absorbed gas atoms on the hot wall. This results in an increase in interfacial thermal resistance which leads to a higher temperature jump. It is observed that the bulk, wall force field and interface regions form approximately 10%, 45% and 45% of the total thermal resistance, respectively. Furthermore, unlike the interfacial thermal resistance, the bulk and the wall force field thermal resistance are approximately independent of the implemented temperature difference.

Design for a Squirrel Cage Fan with Double Arc Blade

Abstract: Squirrel cage fans are commonly used in HVAC (heating, ventilation, and air conditioning) systems. The single arc blade model is commonly used in this type of fans since it can be shaped simply only by two parameters of inlet and outlet angle of blade. However, the efficiency of the fans is much lower than that we expected. In this paper, the single arc blade is replaced by double-arc blade in order to optimize the blade model and to improve the static pressure efficiency and total pressure efficiency of the fan. Numerical results show that the design with double arc blade is able to improve the internal flow, and to enhance the aerodynamic performance of squirrel cage fans.

Experimental and Numerical Investigation of Air Flow Motion in Cylinder of Heavy Duty Diesel Engines

Abstract: In recent years, numerical simulations have become key tool for diesel engine combustion system development due to the requirement of the shorter development duration for the improved performance and better emission levels. In this study, an approach, which integrates numerical and experimental methods in order to characterize the flow field in diesel engine cylinder, is presented. The steady-flow port bench testing, PIV (Particle Image Velocimetry) measurements and numerical simulation methods are used to determine the flow behavior inside the cylinder. Numerical simulation method is validated by using experimental results in terms of mass flow rate and swirl ratio in cylinder. Mass flow rate values predicted within 5 percent error and swirl ratio values predicted within 10 percent error. This proves the viability of numerical method as an important alternative to port bench measurements. In addition to that, cylinder-to-cylinder variation and effects of surface roughness are investigated by swirl ratio measurements and optical diagnostic. Results showed that surface quality and manufacturing problems have significant effects on the swirl ratio in cylinder.

Experimental investigation on the influences of different horizontal fire locations on smoke temperature stratification under tunnel ceiling

Abstract: 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) x 0.6 m (width) x 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.

Transition delay and drag reduction using biomimetically inspired surface waves

Abstract: 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.

Numerical Study on a Novel Type of High Gravity Rotary Gas-Liquid Separator

Abstract: In textile printing and dyeing industry, a novel type of separator called high gravity rotary gas-liquid separator (HGRGS) is designed, which includes a rotary drum with multi-layer fins and an impeller. First, the structure and separation principle of HGRGS are introduced in this paper. Then, the flow field and separation efficiency are studied by CFD techniques. To ensure the accuracy of the numerical simulation, the results are verified by the available experimental data. Compared with the typical cyclone, the maximum pressure drop reduction rate in HGRGS is 64.7% when the gas enters at 10 m/s. Besides, for droplets less than 5 μm, the separation performance in HGRGS is more efficient and it will be greatly improved by 30% for 1 μm droplets. The numerical results also show that the tangential velocity inside the rotary drum is linear with the radius and the higher the rotating speed, the greater the tangential velocity. Moreover, the maximum tangential velocity between the forced and quasi-free vortex has moved to the vicinity of the outer wall, which is beneficial for droplets to move outward. Additionally, the droplets in HGRGS can be captured with enough residence time owing to the lower axial velocity than that in a typical cyclone.

Michell’s Thin Ship Theory in Optimisation of Warp-Chine on Pentamaran Configuration

Abstract: This study was conducted to optimise warp–chine pentamaran configurations in wave cancellations to a significant total resistance reduction for a wide range of speed. The optimisation of a pentamaran with a warp–chine hull form was performed by a computer program Godzilla based on Michell's theory and validated by the towing test. The distance parameters of the outrigger were evaluated to select the lowest resistance generated. Computational analysis depended on the Michell–based tool compared to a commercial Computational Fluid Dynamics (CFD). The comparison of the measurement test of the total resistance and Michell's calculation results of all configurations showed a suitable trend, especially at Fn ≥ 0.4. However, it was not satisfactory for CFD trend. The illustrated of far-field wave pattern by the Michell-based instrument also consistent with the wave spectrum that captured in the test. The results of the analysis and observations revealed that the test measurement for all configuration models in the same estimated error (uncertainty) range of the total resistance. This optimisation has confirmed the stagger at the range of 0.36L–0.42L where the front outriggers and the after outriggers not in line of clearance as in arrow formation significant in wave cancellation and resistance reduction.

Rheological Characterization and Performance Evaluation of Magnetorheological Finishing Fluid

Abstract: The novel controllable behaviour of magnetorheological (MR) fluid is the backbone of magnetorheological fluid-based finishing processes. MR fluid-based finishing processes facilitate better control over finishing forces as the stiffness of MR finishing fluid used in these processes can be controlled in accordance with the applied magnetic field and MR finishing fluid composition. Therefore, a detailed experimental investigation was carried out to find the effect of MR finishing fluid constituents on its yield stress through the Taguchi Design of Experiments. Rheological data obtained from a magneto-rheometer (MCR-102) was characterised by using Bingham plastic, Herschel–Bulkley and Casson’s fluid constitutive modelling. The coefficient of regression (R) values of Herschel–Bulkley model were found to be best suited for all compositions of MR finishing fluid. Analysis of variance (ANOVA) has been used to find the contribution of selected parameters for improving the response characteristics. The optimized fluid has been then used for the finishing of biocompatible stainless steel AISI 316L, and the finishing results show that the average surface roughness value decreases down to 58 nm.

Mitigation of Flow Maldistribution in Minichannel and Minigap Heat Exchangers by Introducing Threshold in Manifolds

Abstract: In the present paper, a detailed numerical investigation has been carried out to analyze the flow maldistribution in 50 parallel rectangular cross-section (1 mm depth and 1 mm width) minichannels and minigap section (1 mm depth and 99 mm width) with rectangular/trapezoidal manifolds in Z-type flow configuration. The author carried out numerical investigation with various mass flow rates, namely 0.05 kg/s, 0.1 kg/s and 0.2 kg/s which results in Reynolds number of 1532, 3064, 6128 respectively. A novel approach for the mitigation of non-uniform flow has been proposed introducing threshold at the entrance of the minigeometry section. The conventional case without threshold (as reference) and 1 mm, 3 mm and 7 mm threshold were introduced. The threshold has been employed by making a manifolds’ depth bigger than section’s depth. The maldistribution coefficient can be reduced twice in minigap section or three times in the minichannel section already with the 1 mm threshold as compared to the arrangement without threshold. It is found that rectangular manifold gives lower maldistribution coefficient than trapezoidal manifold which corresponds with actual state of the art. The distribution is more uniform in minichannel section than in minigap section for the same inlet parameters. To obtain uniform distribution of fluid flow should be stabilized already at the inlet manifold, at the entrance to the minichannel or minigap section. That was done by introducing the threshold in the manifolds, which is novelty of this study.

Analysis on the Performance Improvement of Reciprocating Pump with Variable Stiffness Valve using CFD

Abstract: As a key component of reciprocating pump, the valve has a significant influence on its performance. However, it is difficult for the existing valve to simultaneously solve the problems such as fatigue, erosion and cavitation in engineering application. In this paper, a solution to these problems of using variable stiffness spring is proposed. And three new structures of the valve are designed. Furthermore, based on Computational Fluid Dynamics (CFD) method, a three-dimensional dynamic simulation model considering fluid-structure interaction in the suction stroke of reciprocating pump is established by using dynamic grid technique and User-Defined Functions (UDF). The performance of these new valves are compared with that of conventional valve respectively. The result shows that the new valves have significant influence on the motion characteristics of the valve disc, flow field distribution and cavitation. Besides, the simulation and experimental results of the maximum lift are compared, and it is found that they are basically in agreement. The new structures provide a new research direction for improving the performance of reciprocating pump. Simultaneously, the above simulation method can also provide guidance for valve design, structural optimization and service life improvement.