High Turbulence Intensity Research Articles

On the turbulence amplification in shock-wave/turbulent boundary layer interaction

The mechanism of turbulence amplification in shock-wave/boundary layer interactions is reviewed, and a new turbulence amplification mechanism is proposed based on the analysis of data from direct numerical simulation of an oblique shock-wave/flat-plate boundary layer interaction at Mach 2.25. In the upstream part of the interaction zone, the amplification of turbulence is not essentially shear driven, but induced by the interaction of the deceleration of mean flow with streamwise velocity fluctuations, which causes a rapid increase of turbulence intensity in the near-wall region. In the downstream part of the interaction zone, the high turbulence intensity is mainly due to the free shear layer generated in the interaction zone. During the initial stage of turbulence amplification, the characteristics of wall turbulence, including compact velocity streaks, streamwise vortices and an anisotropic Reynolds stress, are well preserved. The mechanism proposed explains the high level of turbulence in the near-wall region observed in some experiments and numerical simulations.

Investigation on the flow characteristics of a novel multi‐blade combined agitator by time‐resolved particle image velocimetry and large eddy simulation

AbstractWe investigated the flow characteristics in a tank of H/T = 1.5 stirred by a novel multi‐blade combined agitator (MBC) by using time‐resolved particle image velocimetry and large eddy simulation approach. The predictions were assessed by Y+ values, Taylor microscale and power spectrum analysis, as well as experimental validation of velocity distributions. Results demonstrate that the MBC agitator can load the energy into the system effectively with a power number of 12.5 in a turbulent regime, resulting in improved axial and radial mass exchange. The upper and lower short blades produce an axial down‐flow in the top half and an axial up‐flow in the bottom half, respectively. Part of axial flows change to radial flows by the radial pumping of the long blades, meanwhile, the impingement of two axial flows improves the axial mass exchange. These flow characteristics lead to an obvious improvement in the turbulent kinetic energy distribution uniformity with higher turbulent intensity.

Effects of Turbulence and Temperature Fluctuations on Knock Development in an Ethanol/Air Mixture

The effects of turbulence on knock development and intensity for a thermally inhomogeneous stoichiometric ethanol/air mixture at a representative end-gas autoignition condition in internal combustion engines are investigated using direct numerical simulations with a skeletal reaction mechanism. Two- and three-dimensional simulations are performed by varying the most energetic length scale of temperature, $$l_T$$ , and its relative ratio with the most energetic length scale of turbulence, $$l_T/l_e$$ , together with two different levels of the turbulent velocity fluctuation, $$u'$$ . It is found that $$l_T$$ / $$l_e$$ and the ratio of ignition delay time to eddy-turnover time, $$\tau _{ig}/\tau _t$$ , are the key parameters that control the detonation development. An increase in either $$l_T$$ or $$l_e$$ enhances the detonation propensity by allowing a longer run-up distance for the detonation development. The characteristic length scale of the temperature field, $$l_T$$ , is significantly modified by high turbulence intensity achieved by a large $$l_e$$ and $$u'$$ . The intense turbulence mixing effectively distributes the initial temperature field to broader scales to support the developing detonation waves, thereby increasing the likelihood of the detonation formation. On the contrary, high turbulence intensity with a short mixing time scale, achieved by a small $$l_e$$ and a large $$u'$$ , reduces the super-knock intensity attributed to the finer broken-up structures of detonation waves. Either $$\tau _{ig}/\tau _t$$ less than unity or $$l_e = l_T$$ even with a large $$u'$$ is found to have no significant effect on super-knock mitigation. Finally, high turbulent intensity may induce high-pressure spikes comparable to the von Neumann spike. Increased temperature and pressure by combustion heating, noticeably after the peak of heat release rate, significantly enhance the collision and interaction of multiple emerging autoignition fronts near the ending combustion process, resulting in localized high-pressure spikes.

Experimental investigation on flow-induced vibration of flexible multi cylinders in atmospheric boundary layer

Experimental investigations of the flow-induced vibration (FIV) of flexible multi cylinders in tandem, side-by-side and staggered arrangements were conducted in an atmospheric boundary layer wind tunnel over a wide spacing ratio and incidence angles. All cylinders were cantilever supported and allowed to vibrate in cross-flow and inline directions. The vibration characteristics and transition features are identified among different configurations, and the responses dependent on reduced velocity under each configuration are discussed. Four regimes of the FIV of tandem cylinders are classified, where cylinder 1 vibrates divergently similar to galloping in Regime Ⅰ (l < 1.6, α = 0°) but is suppressed in Regime Ⅱ (1.6 ≤ l < 3, α = 0°), cylinder 2 always vibrates obviously in all four regimes, and cylinder 3 has a distinct vibration in only Regimes Ⅱ, Ⅲ (3 ≤ l ≤ 5, α = 0°) and Ⅳ (l >5, α = 0°), in which l is the nondimensional center-to-center distance and α is the incidence angle. There are two regimes for side-by-side cylinders, in which a divergent vibration of cylinder 1 is observed in Regime V (l < 1.6, α = 90°) and only cylinder 3 vibrates significantly in Regime VI (1.6 ≤ l ≤ 3.2, α = 90°). The aerodynamic properties analyzed through a prediction method are used to explain the vibration mechanism, in which the negative aerodynamic damping ratio branches sustain the divergent vibration in Regime I. Based on the comprehensive study above, the FIV phenomenon of flexible multi cylinders can be understood in atmospheric environment with nonuniform inflow, high turbulence intensity and subcritical Reynolds numbers. Finally, the potential of harvesting wind energy via this technique, showing the highest efficiency 52% but limited at Ur≥30, is discussed.

Wake Characteristics of a Tidal Stream Turbine under Combined Wave and Current

Lin, X.; Zhang, J.; Guan, D.; Zhang, J.; Zhang C., and Gan, M., 2020. Wake characteristics of a tidal stream turbine under combined wave and current. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1558-1562. Coconut Creek (Florida), ISSN 0749-0208.Tidal stream energy has been considered as one of the most promising renewable and clean resources to ease energy shortage. This study experimentally investigates wake characteristics of a tidal stream turbine subject to combined wave and current, using an advanced acoustic Doppler velocimeter to measure its wake properties. The phase-space filter is used to remove spikes in raw data, and the phase-averaged method is adopted for further data analyses. The experimental results indicate that the mean wake characteristics under combination of wave and current share a similar trend with that under pure current. However, it's found that the wave-current interaction fastens the wake recovery and increases the turbulence intensity level. Besides, the supporting structure is found to play an important role in the near wake. A region with low velocity and high turbulence intensity is observed in the lee side of the supporting structure. Furthermore, the wake zone is found to shift upward to the free surface due to wake merging of the turbine rotor and the supporting structure.

Nutrient Patchiness, Phytoplankton Surge-Uptake, and Turbulent History: A Theoretical Approach and Its Experimental Validation

Despite ample evidence of micro- and small-scale (i.e., millimeter- to meter-scale) phytoplankton and zooplankton patchiness in the ocean, direct observations of nutrient distributions and the ecological importance of this phenomenon are still relatively scarce. In this context, we first describe a simple procedure to continuously sample nutrients in surface waters, and subsequently provide evidence of the existence of microscale distribution of ammonium in the ocean. We further show that ammonium is never homogeneously distributed, even under very high conditions of turbulence. Instead, turbulence intensity appears to control nutrient patchiness, with a more homogeneous or a more heterogeneous distribution observed under high and low turbulence intensities, respectively, under the same concentration in nutrient. Based on a modelling procedure taking into account the stochastic properties of intermittent nutrient distributions and observations carried out on natural phytoplankton communities, we introduce and verify the hypothesis that under nutrient limitation, the “turbulent history” of phytoplankton cells, i.e., the turbulent conditions they experienced in their natural environments, conditions their efficiency to uptake ephemeral inorganic nitrogen patches of different concentrations. Specifically, phytoplankton cells exposed to high turbulence intensities (i.e., more homogeneous nutrient distribution) were more efficient to uptake high concentration nitrogen pulses (2 µM). In contrast, under low turbulence conditions (i.e., more heterogeneous nutrient distribution), uptake rates were higher for low concentration nitrogen pulses (0.5 µM). These results suggest that under nutrient limitation, natural phytoplankton populations respond to high turbulence intensities through a decrease in affinity for nutrients and an increase in their transport rate, and vice versa.

Influences of complex multi-channel turbulator on hybrid nanoparticle transportation and thermal behavior

Turbulator with new shape is utilized in current research to improve thermal efficiency. Hybrid nanomaterial has been involved as carrier fluid and k–ɛ turbulence model was selected for modeling. Simplified equations were solved via FVM and outputs in terms of contours were presented. Stronger secondary eddy is a result of increasing in inlet velocity and revolution which can provide thinner boundary layer and increase temperature gradient but at cost of greater pressure drop. Due to better mixing of nanomaterial in greater Re, higher Nu can be achieved. Higher turbulent intensity can be obtained with insertion of turbulator and convective heat transfer improves.

Evaluation of Turbulent Spot Production Rate in Boundary Layers Under Variable Pressure Gradients for Gas Turbine Applications

Abstract The paper presents several results from an experimental data base on transitional boundary layers developing on a flat plate installed within a variable area opening endwall channel. Measurements have been carried out by means of time-resolved particle image velocimetry (PIV). The overall test matrix spans three Reynolds numbers, four freestream turbulence intensity levels, and four different flow pressure gradients. For each condition, 16,000 instantaneous flow fields have been acquired in order to obtain high statistical accuracy. The flow parameters have been varied in order to provide a gradual shift of the mode of transition from a by-pass process to separated flow transition. In order to quantify the influence of the flow parameter variation on the boundary layer transition process, the transition onset and end positions, and the turbulent spot production rate have been evaluated with a wavelet-based intermittency detection technique for every condition exhibiting a complete transition process. The by-pass transition mode has the longest transition length that decreases with increasing the Reynolds number. The transition length of the separated flow case is smaller than the by-pass one, and the variation of the flow parameters has a similar impact. The variation of the inlet turbulence intensity has a small influence on this parameter except for the condition at the highest turbulence intensity that always shows the lowest turbulent spot production rate because a by-pass type transition occurs. This large amount of data has been used to develop new correlations used to predict the spot production rate and the transition length in attached and separated flows.

Numerical studies on turbulent flame propagation in premixed gas deflagration inside a tube

Numerical simulation on turbulent flame propagation in premixed gas deflagration process in a tube will be reported in this paper, aiming at identifying the key factors affecting flame shape and flame velocity. Large eddy simulation with premixed gas combustion model is used to obtain results validated by full-scale experimental data. The effect of flow velocity and turbulence on flame propagation is discussed. The flow velocity of premixed gas is observed to be one of the main factors determining flame shape and affecting flame propagation process. The velocity difference of different parts of the flame front, both in magnitude and direction, will lead to tulip-shaped flame. Turbulence would accelerate the propagation of flame periodically. The cause of flame acceleration of low-intensity turbulence originates from two factors, namely, combustion and flow field, which transfer the heat and mass of chemical reaction from diffusion to vortex transport. As high-intensity turbulence will not affect the chemical reaction and the turbulent burning velocity, flame acceleration is controlled only by the characteristics of the flow field.

Study on benefit of guide vane for vertical axis wind

Guide vane was introduced to overcome the problem of high turbulence intensity due to the high rise building in the urban area. The guide vane is a fixed aerofoil that directs air into the moving wind turbine. Guide vane borders the Vertical Axis Wind Turbine (VAWT) and improves the VAWT performance by increasing the wind velocity, decreasing turbulence intensity and increasing torque because it acts as wind concentrator and wind shielding to an optimum angle before it interacts with turbines. This project is based on the VAWT on the College of Engineering (COE) building in Universiti Tenaga Nasional (UNITEN). This VAWT had been fabricated in CREO software to study the benefit of guide vane onto it and the design of guide vane also was constructed by using similar software. SOLIDWORKS Flow simulation was used to analyze the performance of the VAWT. The simulation had been done by simulation of VAWT without and with guide vane to study the benefit and analyze the flow characteristic of the guide vane towards wind turbine. The simulation result shows that the presence of guide vane increase wind speed by maximum 0.0051 m/s and torque increment is about seven times from VAWT without guide vane. Other than that, decreasing of turbulence intensity value also detected in flow simulation result. To sum up, guide vane had the potential to be installed in the urban area because it improves the power output of the wind turbine.

The merging of Kelvin–Helmholtz vortices into large coherent flow structures in a high Reynolds number flow past a wall-mounted square cylinder

Flows at tidal-stream energy sites are characterised by high turbulence intensities and by the occurrence of highly energetic large and coherent flow structures. The interaction of the flow with seabed roughness is suspected to play a major role in the generation of such coherent flow structures. The problem is introduced with canonical wall-mounted square obstacles representing abrupt changes of bathymetry, with high Reynolds number flow (Re = 250000). Two methods are used: a numerical model, based on the LBM (Lattice Boltzmann Method) combined with LES (Large Eddy Simulation) and an experimental set-up in a circulating tank. The numerical model is validated by comparison with experimental data. In the case of a wall-mounted square cylinder, large-scale turbulent structures are identified in experiments where boils at the free surface can be observed. LBM simulation allows their three-dimensional characterisation. The dynamic of such large-scale events is investigated by temporal, spatial and spectral numerical analysis. Results show that periodical Kelvin–Helmholtz vortices are emitted in the cylinder wake. Then, they merge to form larger and more coherent structures that rise up to the surface. A wavelet study shows that the emission frequency of the Kelvin–Helmholtz vortices is not constant over time.

Model of the flow on Columnar vortex generator

Flow control devices are one of the most commonly studied fields of aerodynamic research in the world. This is because improvements in the aerodynamic performance of any vehicle lead directly to save fuel and money and reduce their operational costs. Another reason is that flow control can reduce the risk of aerial vehicles operating near complex structures which can generate high turbulence intensity levels, supposing a risk for its maneuvers. A passive flow control device used for improving the flow on the ramp of an aircraft carrier is the columnar vortex generator (CVG). Using this device, encouraging results to improve the flow for aerial vehicles which operate on those ships, have been obtained. However, CVG detailed working and vortex generation is not fully known. For that reason, the aim of this paper is to present the results obtained in wind tunnel tests of this passive flow control device. The flow behavior at the front, rear and especially inside it, was analysed. A smoke visualization has been made to the CVG model as a first approach to the flow structure surround and inside the CVG. Flow inside the CVG and specifically its vortex generated has been also analysed using velocity maps obtained by particle image velocimetry (PIV). Tangential and axial PIV planes have been obtained to characterise the vortex and its movement inside the device. These resultant velocity maps have been used to compare the vortex with theoretical models (Rankine and Oseen-Lamb vortex) and establish a new model that predicts the velocity profiles of the vortex generated inside the CVG. Finally, new possible applications using the CVG to improve the flow over a building roof and over the flight deck of a frigate under stern wind is tested by means of PIV in the wind tunnel.

Particle history from massively parallel large eddy simulations of pulverised coal combustion in a large-scale laboratory furnace

A study on the coal particle history during combustion in a large-scale furnace using large eddy simulation is presented. The massively parallel execution produces a high-resolution representation of the fluid mixing and particle dispersion throughout the whole computational domain. The coal combustion is modelled using well-established, cost-effective combustion models. A specific feature of the devolatilization model is the optimisation of the kinetic constants for the furnace operating condition, which were obtained through an iterative procedure between particle heating rates from full large eddy simulation runs and the advanced model Chemical Percolation Devolatilization. In a previous work, we showed that the classical coal combustion models, when used in a high-resolution massively parallel large eddy simulation, lead to satisfactory predictions of the in-flame gas properties, namely gas temperature and gas species concentrations. In this work, we went beyond the comparisons between gas phase measurements and predictions. Single particles were tracked over time and instantaneous ensambles were collected to obtain a better understanding of the conditions that coal particles are subjected to in the investigated test case. The particles trajectory, combustion history and instantaneous state distribution were analysed. The volatile flame features were related with the characteristic trajectory of different sized particles. The combustion history revealed that particles are subjected to large variations of heating rates, including very short sequential periods alternating between heating and cooling during the early stages of combustion, due to the high turbulence intensity in the near burner region. Finally, the state distribution of the ensamble provided a global picture of the instantaneous coal combustion process.

End-wall ignition of methane-air mixtures under the effects of CO2/Ar/N2 fluidic jets

Methane is a fuel with wide applications in daily life and industrial production, mainly due to its suitable performance in producing more heat per mass and less carbon dioxide per unit of heat released. Methane is flammable, and therefore, its safety issues in the process of storage and transportation due to its potential explosion risk must be prioritized first and foremost. Due to wind or obstacles in the explosion process, most explosions take place in a turbulent environment; hence, investigations related to the effect of turbulence on the explosion process require in-depth examination. In this study, a methane-air mixture under standard conditions (T0 = 298 K, p0 = 100 kPa) is selected as the test mixture, and the effect on explosion parameters, i.e., pmax, τe and (dp/dt)max, is investigated by the turbulence produced by three different inert additives (CO2, Ar, and N2). Explosion tests are first performed under quiescent conditions and then compared with cases under turbulent conditions. Under quiescent conditions, the explosion overpressure in the end-wall ignition case is approximately 40% lower than that in the central ignition case. In end-wall ignition, the incident precursor shock is weaker and the heat loss is higher than those in central ignition. By introducing turbulence, the value of pmax is greatly enhanced and τe is reduced, indicating that turbulence greatly improves the explosion hazard. By adjusting the pressure difference in the fluidic jet and spherical chamber, i.e., pJ0/p0, the turbulence intensity is accordingly changed. Under the condition of a higher pJ0/p0, the higher turbulence intensity better promotes the chemical reaction and explosion process, and the values of explosion parameters increase linearly with the turbulence intensity. The turbulence-enhancing effect on explosion is more prominent near the flammability limit than under stoichiometric conditions. Compared with that under fuel-rich condition, explosion depends less on the turbulence properties at the fuel-lean side. Turbulence generated by CO2 has the best effect on enhancing explosion when approaching the fuel-rich limit.

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH4/H2/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.

Aerodynamic Characteristics of Different Airfoils under Varied Turbulence Intensities at Low Reynolds Numbers

A numerical study was conducted on the influence of turbulence intensity and Reynolds number on the mean topology and transition characteristics of flow separation to provide better understanding of the unsteady jet flow of turboelectric distributed propulsion (TeDP) aircraft. By solving unsteady Reynolds averaged Navier-Stokes (URANS) equation based on C-type structural mesh and γ - Re ˜ θ t transition model, the aerodynamic characteristics of the NACA0012 airfoil at different turbulence intensities was calculated and compared with the experimental results, which verifies the reliability of the numerical method. Then, the effects of varied low Reynolds numbers and turbulence intensities on the aerodynamic performance of NACA0012 and SD7037 were investigated. The results show that higher turbulence intensity or Reynolds number leads to more stable airfoil aerodynamic performance, larger stalling angle, and earlier transition with a different mechanism. The generation and evolution of the laminar separation bubble (LSB) are closely related to Reynolds number, and it would change the effective shape of the airfoil, having a big influence on the airfoil’s aerodynamic characteristics. Compared with the symmetrical airfoil, the low-Reynolds-number airfoil can delay the occurrence of flow separation and produce more lift in the same conditions, which provides guidance for further airfoil design under TeDP jet flow.

Numerical aero-thermal study of high-pressure turbine nozzle guide vane: Effects of inflow conditions

Accurate predictability of high-pressure turbine nozzle guide vane aero-thermal performance is highly desired in the development campaign due to the exposure of the component to a frequent and high heat load. In this paper, the representative vane profile in modern aero-engines is numerically studied. Aerodynamics and aero-thermal validations of the blade profile have been performed in comparison with the available experimental data. It has been shown that a satisfactory agreement could be achieved with the use of the transitional turbulence model shear stress transport γ–θ due to its superiority in capturing the laminar–turbulent transition. Sensitivity studies on the increase in the inlet turbulence intensity, inlet endwall boundary layer thickness, and inlet total temperature profile have been performed to understand the impact of inflow conditions’ uncertainty on the aero-thermal predictability. Increasing the inlet turbulence intensity increases the pressure surface heat transfer coefficient and induces an earlier transition onset on the suction surface. Due to the rapid decay of turbulence intensity in the numerical model, the use of an artificially high inlet turbulence intensity has been shown to be effective in the prediction improvement. On the other hand, the change in the inlet boundary layer thickness influences the formation and strength of the secondary flow, namely, horseshoe vortex and passage vortex. These secondary flow phenomena affect the local blade surface heat transfer coefficient in the near-endwall region although the most significant rise in heat transfer is found on the endwall. The temperature distortion amplitude of a hot streak and its relative clocking position with the vane significantly affect the heat flux distribution. In contrast, the heat transfer coefficient is less sensitive to the change in hot streak conditions. However, it has been shown that increasing the temperature distortion amplitude could induce a larger difference among different clocking configurations. In addition, decreasing the difference between the fluid and wall temperature would delay the transition onset and stabilize the boundary layer. Further analysis of the unsteady effects has been carried out by comparing the steady and time-averaged flow solutions. It has been observed that the discrepancy between these solutions is attributed to the flow field nonlinearity. Thus, a significant discrepancy can be found in the laminar–turbulent transition as well as in the trailing edge region. However, since the contribution of these regions on the total area-averaged heat transfer is small, their influence on the total vane heat transfer is limited.

Dissipation of Turbulent Kinetic Energy in the Oscillating Bottom Boundary Layer of a Large Shallow Lake

AbstractMixing rates and biogeochemical fluxes are commonly estimated from the rate of dissipation of turbulent kinetic energy ε as measured with a single instrument and processing method. However, differences in measurements of ε between instruments/methods often vary by one order of magnitude. In an effort to identify error in computing ε, we have applied four common methods to data from the bottom boundary layer of Lake Erie. We applied the second-order structure function method (SFM) to velocity measurements from an acoustic Doppler current profiler, using both canonical and anisotropy-adjusted Kolmogorov constants, and compared the results with those computed from the law of the wall, Batchelor fitting to temperature gradient microstructure, and inertial subrange fitting to acoustic Doppler velocimeter data. The ε from anisotropy-adjusted constants in SFM increased by a factor of 6 or more at 0.2 m above the bed and showed a better agreement with microstructure and inertial method estimations. The maximum difference between SFM ε, computed using adjusted and canonical constants, and microstructure values was 25% and 50%, respectively. This difference was 30% and 55%, respectively, for those from inertial subrange fitting at times of high-intensity turbulence (Reynolds number at 1 m above the bed of more than 2 × 104). Comparison of the SFM ε to those from law of the wall was often poor, with errors as large as one order of magnitude. From the considerable improvement in ε estimates near the bed, anisotropy-adjusted Kolmogorov constants should be applied to compute dissipation in geophysical boundary layers.

Numerical modelling of a dual-rotor marine current turbine in a rectilinear tidal flow

In this study, numerical simulation is used to investigate a counter-rotating dual-rotor marine current turbine (MCT) that is aligned for a rectilinear tidal current. Results of power and thrust coefficients and the mean axial velocity in the wake are compared with that of the blade element momentum (BEM) method coupled with the Park wake model. For a single-rotor MCT, small discrepancies are observed for front rotor, and larger discrepancies for rear rotor when comparing the CFD and BEM results. The mean axial velocity in the wake agrees better with the higher turbulence intensity (TI). CFD results shows that the power coefficient (CP) of rear rotor depends on the ambient turbulence intensity. The maximum CP of dual-rotor turbine is 5% higher than that of just the front rotor. Streamlines show that a large vortex is formed behind the rear rotor. The numerical simulations give more credibility to the BEM Park model, but also points to its sensitivity to the incoming turbulence intensity.

Heat transfer characteristics of impinging jet on a hot surface with constant heat flux using Cu2O–water nanofluid: An experimental study

Abstract The experimental investigations are carried out on a circular nanofluid jet impingement for cooling an aluminum disk with constant heat flux. The study is performed to understand the effects of different factors such as Reynolds number and nanoparticle concentration on the fluid flow and characteristics of heat transfer. The target surface has a circular shape and is kept at constant heat flux with the value of 1414.71 W/m2. The copper oxide-nanoparticle concentrations are changed from 0.03 to 0.07 wt%. The experimental results show that the Cu2O nanofluid increases the heat transfer efficiency of the impinging jet cooling system. Compared to the case of using the base fluid, the nanofluid increases the convective heat transfer by 45% at 0.07 wt% concentration at Reynolds number of 7330. The center of the target surface, i.e. the stagnation zone, has the highest turbulence intensity because of impinging of the fluid flow on the surface center, while the endpoint of the target surface has the minimum turbulence intensity. Indeed, the turbulence intensity decreases along the radial direction, which augments the effect of employing the nanofluid because the heat transfer due to the turbulence diminishes and merit of using the nanofluid enhances.