High Turbulence Intensity Research Articles

Simulation of the interaction of unsteady high-intensity turbulence flow with heat and mass transfer in the boundary layer on the surface

Simulation of the interaction of unsteady high-intensity turbulence flow with heat and mass transfer in the boundary layer on the surface

Can turbulence within the field of view cause significant biases in radiative transfer modeling at the 183 GHz band?

Abstract. The hypothesis whether turbulence within the passive microwave sounders field of view can cause significant biases in radiative transfer modeling at the 183 GHz water vapor absorption band is tested. A novel method to calculate the effects of turbulence in radiative transfer modeling is presented. It is shown that the turbulent nature of water vapor in the atmosphere can be a critical component of radiative transfer modeling in this band. Radiative transfer simulations are performed comparing a uniform field with a turbulent one. These comparisons show frequency dependent biases which can be up to several kelvin in brightness temperature. These biases can match experimentally observed biases, such as the ones reported in Brogniez et al. (2016). Our simulations show that those biases could be explained as an effect of high-intensity turbulence in the upper troposphere. These high turbulence phenomena are common in clear air turbulence, storm or cumulus cloud situations.

High Reynold Number LES of a Rotating Two-Pass Ribbed Duct

Cooling of gas turbine blades is critical to long term durability. Accurate prediction of blade metal temperature is a key component in the design of the cooling system. In this design space, spatial distribution of heat transfer coefficients plays a significant role. Large-Eddy Simulation (LES) has been shown to be a robust method for predicting heat transfer. Because of the high computational cost of LES as Reynolds number (Re) increases, most investigations have been performed at low Re of O(104). In this paper, a two-pass duct with a 180° turn is simulated at Re = 100,000 for a stationary and a rotating duct at Ro = 0.2 and Bo = 0.5. The predicted mean and turbulent statistics compare well with experiments in the highly turbulent flow. Rotation-induced secondary flows have a large effect on heat transfer in the first pass. In the second pass, high turbulence intensities exiting the bend dominate heat transfer. Turbulent intensities are highest with the inclusion of centrifugal buoyancy and increase heat transfer. Centrifugal buoyancy increases the duct averaged heat transfer by 10% over a stationary duct while also reducing friction by 10% due to centrifugal pumping.

Adiabatic Effectiveness on High-Pressure Turbine Nozzle Guide Vanes Under Realistic Swirling Conditions

In modern lean-burn aero-engine combustors, highly swirling flow structures are adopted to control the fuel-air mixing and to provide the correct flame stabilization mechanisms. Aggressive swirl fields and high turbulence intensities are hence expected in the combustor-turbine interface. Moreover, to maximize the engine cycle efficiency, an accurate design of the high-pressure nozzle cooling system must be pursued: in a film-cooled nozzle, the air taken from last compressor stages is ejected through discrete holes drilled on vane surfaces to provide a cold layer between hot gases and turbine components. In this context, the interactions between the swirling combustor outflow and the vane film cooling flows play a major role in the definition of a well-performing cooling scheme, demanding for experimental campaigns at representative flow conditions. An annular three-sector combustor simulator with fully cooled high-pressure vanes has been designed and installed at THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion-cooled liners, and six film-cooled high-pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central airfoil aligned with the central swirler. In this experimental work, adiabatic film effectiveness measurements have been carried out in the central sector vanes, in order to characterize the film-cooling performance under swirling inflow conditions. The pressure-sensitive paint (PSP) technique, based on heat and mass transfer analogy, has been exploited to catch highly detailed 2D distributions. Carbon dioxide has been used as coolant in order to reach a coolant-to-mainstream density ratio of 1.5. Turbulence and five-hole probe measurements at inlet/outlet of the cascade have been carried out as well, in order to highlight the characteristics of the flow field passing through the cascade and to provide precise boundary conditions. Results have shown a relevant effect of the swirling mainflow on the film cooling behavior. Differences have been found between the central airfoil and the adjacent ones, both in terms of leading edge stagnation point position and of pressure and suction side film coverage characteristics.

Reconciling turbulent burning velocity with flame surface area in small-scale turbulence

A discrepancy between the enhancement in overall burning rate and the enhancement in flame surface area measured for high-intensity turbulence is addressed. In order to reconcile the two quantities, an additional contribution from the effective turbulent diffusivity is considered. This contribution is expected to arise in sufficiently intense turbulence from eddies smaller than the flamelet thickness. In the present work, the enhancement in diffusivity arising from these eddies is estimated based on a model energy spectrum; individual contributions from all turbulence length scales smaller the flamelet thickness are integrated over the corresponding portion of the spectrum. It is shown that diffusivity enhancement, estimated in this manner, is able to account for the measured discrepancy between the overall burning rate enhancement and flame surface area enhancement. The factor quantifying this discrepancy is formalized as a closed-form function of the Karlovitz number.

A new atmospheric boundary layer wind tunnel simulation methodology for wind effects on large cooling towers considering wind environment variations

The traditional atmospheric boundary layer wind tunnel model test practice employs wind fields, the flow characteristics of which are in accordance with the empirical formulae of the atmospheric turbulence presented in Codes of Practice and monographs. However, the empirical formulae presented in Codes of Practice and monographs cannot truthfully reflect the high variations of the realistic atmospheric turbulence which sometimes aggravates wind effects on structures. Based on model tests conducted in a multiple-fan actively controlled wind tunnel, it is found that most wind effects on large cooling towers change monotonically with the increase in free-stream turbulence, and the model test results are more unfavorable for a flow field of low turbulence intensity than for a flow field of high turbulence intensity with respect to the measured coherences. Thus, a new atmospheric boundary layer wind tunnel simulation methodology for wind effects on circular cylindrical structures is proposed to overcome the deficiency of the traditional atmospheric boundary layer wind tunnel model tests. The new simulation methodology includes the simulation of two realistic atmospheric boundary layer flow fields with the highest and the lowest turbulence intensities in the wind tunnel and the envelopment of model test results obtained in the two flow fields (e.g. the mean and fluctuating wind pressure distributions, the power spectral density, the coherence function, and the correlation coefficient). The superiority of the new atmospheric boundary layer wind tunnel simulation methodology over the traditional model test practice is demonstrated by comparing the model test results with the full-scale measurement data.

Modelling of the Interaction of Unsteady High-Intensity Turbulence Flow with Heat- and Mass-Transfer in the Boundary Layer on the Surface

The dynamic and thermal characteristics of unsteady near-wall flows are investigated numerically on the basis of two-parameter turbulence models under conditions of high-turbulence free stream and impact of perturbing heat- and mass-transfer factors in the boundary layer. The effect of mass-transfer parameters considered on the permeable section on the development of dynamic and thermal processes in the steady-state turbulent boundary layer is studied and the boundary layer structure along the surface is investigated. The mutual action of time harmonic oscillations of the velocity of outer inviscid free stream and the heat-transfer parameters on wall on the development of time-dependent heat-transfer characteristics in turbulent flow is analyzed. The numerical results are compared with experimental and theoretical data.

Mapping of the turbulent round jet developing region using a constant temperature anemometer (CTA)

The fully developed round turbulent jet has been extensively studied, whereas the developing region is much less understood. The high shear and turbulence intensities in the most interesting parts of the developing region make them inaccessible to common measurement techniques such as Constant Temperature Anemometry (CTA) due to the high demands on the measurement techniques for accuracy of the measurements. Turbulence measurements are therefore planned using our in-house laser Doppler anemometer (LDA) system based on its capability to provide accurate measurements and with its inherent ability to properly distinguish velocity components. A rigorous measurement with the intended LDA system however demands impractical processing time, so knowing the critical points at which measurement are to be taken will save valuable time. This information is herein acquired significantly faster and more practically, however less accurately, with single-wire CTA. A high-resolution measurement was done using a computer-controlled single-wire CTA with the wire probe mounted perpendicular to the incoming flow from the jet orifice. The measurements covered several points in the radial (r-direction) along x/D=10, x/D=15, x/D=20 and x/D=30 downstream (where D is the jet exit diameter), with spatial resolutions ranging from 1 to 3 mm between the points, depending on how far the measurement was from the jet centerline. A proper alignment was also conducted prior to measurement so that the same points can be reached again for LDA measurement on the same jet afterwards. The radial profiles of mean velocity and turbulence intensity at each downstream position are presented to show the statistics of the air flow inside and outside the jet. As expected from theory, the mean profiles display a nearly Gaussian shape, spread out and tapered with the downstream direction. The highest velocities are located at the centerline.

Numerical evaluation of heat/mass transfer analogy for leading edge showerhead film cooling on a first-stage vane

A numerical study was conducted to evaluate the applicability of the heat/mass transfer analogy to leading edge film cooling on a first-stage vane. A typical showerhead configuration was taken into account, thus including four staggered rows of evenly spaced cylindrical holes, angled at 45° towards the tip. The investigation was carried out at low speed (isentropic exit Mach number of Ma2is = 0.2), inlet turbulence intensity of Tu1 = 1.6% and 13%, and three different blowing ratios (BR = 2.0, 3.0 and 4.0). Steady-state simulations were conceived to mimic the pressure sensitive paint (PSP) technique aiming to measure the adiabatic film cooling effectiveness (η) in the leading edge region, under a density ratio of DR = 1 and 1.5. With the mainstream flow assumed to be air at ambient temperature, the desired DR was obtained by injecting coolant as air at different temperature from the mainstream (namely, heat transfer approach) or as isothermal foreign gas (namely, mass transfer approach). Despite the dominant effect of the DR on η, results indicated that the heat/mass transfer properties of the coolant are to be considered when comparing film cooling performance, at matched BR conditions. At low inlet turbulence intensity, the heat/mass transfer analogy was found to be applicable only to the lowest BR of 2.0. Its validity was extended to the medium BR of 3.0, in case of DR = 1.5, provided that the leading edge is approached by high turbulence intensity flow.

The ignition characteristics of the close dual-point laser ignition in a cavity based scramjet combustor

The ignition processes of single-point laser ignition (LI) and dual-point LI were experimentally investigated in a cavity-based Mach 2.52 scramjet combustor fueled with ethylene. The stagnation temperature and total pressure of the supersonic inflow were 1486 K and 1.6 MPa, respectively. The results indicate that the ignition performance of dual-point LI with a plasma distance of 3 mm is the same as the single-point LI. However, further increasing the plasma distance to 7 mm degrades the ignition performance of dual-point LI. Therefore, dual-point LI is not superior to single-point LI in the scramjet combustor, which is contradictory to the investigations performed in quiescent premixed combustible mixtures. The reason is that the high turbulence intensity and strain rate in the cavity recirculation flow (∼100 m/s) lead to serious plasma and flame quenching instead of enhancing the flame propagation. Therefore, expanding the spatial distribution of the plasma by dual-point LI method, which might result in significant radical loss and heat loss, is not recommended for igniting a scramjet combustor.

Optimal yaw strategy for optimized power and load in various wake situations

The interaction between nearby wind turbines in a wind farm modifies the power and loads compared to their stand-alone values. The increased turbulence intensity and the modified turbulence structure at the downstream turbines creates higher fatigue loading, which can be mitigated by wind farm and/or wind turbine control. To alleviate loads and maximize power possible strategies such as wake steering, where the turbine is yawed to redirect the wake such that it does not impinge the downstream turbine, have been studied. The work presented here focuses on situations where the wake is nevertheless affecting the downstream turbine, and more specifically how high loads can be avoided by yawing the wake-affected turbine. The analysis is conducted on a 2.3 MW machine, and the flow field is simulated using the Dynamic Wake Meandering model. The study investigates the impact on power and loads for different longitudinal interspacing and turbulence intensities. Optimal yaw strategies are defined for above rated regions where no power loss occurs. The potential load alleviation for different load sensors are studied, but the presentation is focussed on the blade root flapwise fatigue loading. For full wake at 3D interspacing and turbulence intensity of 5 %, around 35 % of load reduction on the 1 Hz Damage Equivalent Loads can be achieved at high wind speeds. Smaller reductions are achieved for higher atmospheric turbulence; the analogue case with 15 % turbulence intensity shows 17 % potential alleviation. The alleviation on the wind turbine lifetime is also calculated and compared for different turbulence intensities and mean wind speeds. Small reductions are achieved for sites with low mean wind speed and high turbulence intensity, but high reductions, of around 19 %, are accomplished in low turbulence intensity with high mean wind speed.

Compressibility in turbulent magnetohydrodynamics and passive scalar transport: mean-field theory

We develop a mean-field theory of compressibility effects in turbulent magnetohydrodynamics and passive scalar transport using the quasi-linear approximation and the spectral $\unicode[STIX]{x1D70F}$-approach. We find that compressibility decreases the $\unicode[STIX]{x1D6FC}$ effect and the turbulent magnetic diffusivity both at small and large magnetic Reynolds numbers, $Rm$. Similarly, compressibility decreases the turbulent diffusivity for passive scalars both at small and large Péclet numbers, $Pe$. On the other hand, compressibility does not affect the effective pumping velocity of the magnetic field for large $Rm$, but it decreases it for small $Rm$. Density stratification causes turbulent pumping of passive scalars, but it is found to become weaker with increasing compressibility. No such pumping effect exists for magnetic fields. However, compressibility results in a new passive scalar pumping effect from regions of low to high turbulent intensity both for small and large Péclet numbers. It can be interpreted as compressible turbophoresis of non-inertial particles and gaseous admixtures, while the classical turbophoresis effect exists only for inertial particles and causes them to be pumped to regions with lower turbulent intensity.

DESIGN AND STUDY OF EXCHANGERS WITH PRESSURE GRADIENT HEAT INTENSIFIERS

This paper is presented the numerical study of local heat transfer in the turbulent boundary layer with longitudinal pressure gradient. The study is based to free software with open source code (Salome and Code_Saturne) has been based by RANS approach and empirical models of turbulence. Validation of mathematical models and software is based by collation numerical results with the results of experimental study of fl ow characteristics in a turbulent boundary layer of longitudinal pressure gradient and high turbulence intensity (Epik E. Ya., NASc of Ukraine). The validation had a high qualitative coincidence of the fl ow characteristics determined as a result of the simulation with experimental data. We designed two constructive schemes of heat exchangers for air. This study presents a calculation plan for these heat exchangers. Results of the study are showed that the use of gradient heat exchange intensifi ers leads to an increase in the heat transfer coeffi cient from air to 17 %.

Effects of tangential inlet shape and orientation angle on the fluid dynamics characteristics in a biomass burner

The perennial crops are potentially used as renewable fuels in the boiler furnace. Due to its specific characteristics, the burner design for this biomass needs to be properly developed. The burner design proposed here is a cylindrical burner having an axial inlet and a pair of tangential injection. This study is aimed to investigate the effect of tangential inlet geometry on the burner performance, through numerical evaluation of fluid dynamics characteristics. The numerical evaluations are conducted using k-ε turbulence model under Ansys-Fluent software. The simulation results showed that, at certain orientation angles of tangential inlet, there are back flows from furnace to the internal burner. This phenomenon is responsible to the flame stability in the burner. The turbulence intensity and convective heat transfer coefficient are also influenced by the tangential inlet orientation angle. For same cross sectional area, the rectangular tangential inlet shape generated deeper backflow penetration and higher turbulence intensity than that was done by the circular ones. One of the rectangular shape shortcomings to the circular ones is to produce the higher static pressure, which correlate to the higher burner operating cost. This investigation study concluded that the burner with rectangular tangential inlet shape and orientation angle of 200 potentially produces the best burner performance.

Effects of pressure and Karlovitz number on the turbulence-flame interactions in lean premixed H2/air flames

This paper presents three-dimensional direct numerical simulations of lean premixed turbulent H2/air flames in the thin and distributed reaction zones, with the Karlovitz numbers at 60, 110, 150 and 1000, and pressures at 1 and 5 atm, respectively. Flame front structures and chemical pathways are examined in detail to investigate the effects of pressure and turbulence on flames. There is an increasing number of finer structures on the flame front with increased Karlovitz number. Eddy structures are observed downstream of the reaction zone under high turbulence intensity and thus Karlovitz number, indicating that the turbulent eddies are small and energetic enough to break through the distributed reaction zone. Statistical analysis indicates that the probability of high curvatures increases with increasing Karlovitz number at a constant pressure. When the Karlovitz number is kept constant, the probability of high curvatures is significantly higher at the atmospheric pressure than at elevated pressure. The approximation of Schmidt number (Sc = 1) in theoretical analysis introduces errors in the estimation of the smallest flow scale and the Karlovitz number. Accordingly, in the turbulent flame regime diagram, the boundary between the thin reaction zone and the distributed reaction zone should be modified at the elevated pressure. Moreover, the decorrelation of heat release and H2 consumption is directly related to turbulence intensity, and the decorrelation is reduced at the elevated pressure. Due to the enhanced radical transport at high Karlovitz number, chemical pathways can be locally changed, which is more significant at elevated pressure.

Experimental and Numerical Studies of Heat Transfer Characteristics for Impinging Jet on Dimple Surfaces

Experimental and numerical works have been carried out to investigate the effect of impingement dimples surface on heat transfer characteristics in a circular test plate. Impingement jet technique and dimple shape can enhances the heat transfer rate caused by the high turbulence intensity. The cylindrical shape of dimple was used in this work. The simulation results were validated with the experimental data obtained from 4 types test plates. The simulate data displayed the flow structure and contour temperature surface on the test plate which provided the complex flow and heat transfer characteristics. The experimental results showed that the case of the dimple diameter (d) equal to the jet diameter (Dj), and the distance between jet and the test plate (B) is 2 times of Dj, yielded the maximum heat transfer rate. The heat transfer rate enhanced up to 200 % compared to a flat plate at Rej = 14,500.

Indoor environment evaluation of a Dedicated Outdoor Air System with ceiling fans in the tropics – A thermal manikin study

Energy savings of the Air-Conditioning (AC) system is crucial in achieving energy conservation in buildings in the tropical region. A Dedicated Outdoor Air System with ceiling fans (DOAS-CF) aims to achieve energy savings arising from elevated room air temperature and minimizing the air-conditioning requirements. The objective of this study is to investigate the cooling effect produced by the DOAS-CF system at various locations in an AC room using a thermal manikin. The sedentary thermal manikin was set at three different locations such as under the ceiling fan, outside the fan blades and the center of the room at temperatures of 24 °C, 27 °C and 30 °C and at 60%RH with air velocities ranging from 0.02 m/s to 2.26 m/s at the height of 1.1 m. Two ceiling fans formed circulation airflows around each fan. High air speeds and low turbulence intensity were measured right under the ceiling fan and near the floor while low air speeds and high turbulence intensity were found at other locations. The cooling effect at the whole body of −3.3 °C was higher under the fan than that of −1.3 °C at other locations. The cooling effect was affected by temperature, location and fan speed mode significantly. The maximum cooling effect by manikin based equivalent temperature was higher than that by SET model due to an evaluation of the local cooling effect on each body part. The knowledge of the cooling effect is important and useful for a mechanical engineer to design the thermal environment produced by the DOAS-CF system.

Effects of unsteady wakes on heat transfer of blade tip and shroud

An experimental study has been conducted to investigate the heat-transfer characteristics of blade tips and shrouds with and without unsteady wakes. Depending on the presence of unsteady wakes, the local heat/mass-transfer coefficients of the tip and shroud were measured using the naphthalene sublimation method. Wakes from unsteady blades were modeled as wakes generated from moving cylindrical rod bundles. Test conditions were set to a Reynolds number of 100,000, based on an inlet velocity of 11.4 m/s and the axial chord length. The Strouhal number was varied from 0 to 0.22. For the case without unsteady wakes (St = 0), high heat/mass-transfer coefficients appeared in regions where various flow patterns, such as flow reattachment, swirling flow, and vortices, occurred. Unsteady wakes (St = 0.22) made high turbulence intensity of the tip leakage flow, and flow patterns ranging from flow reattachment to tip leakage vortex in the tip and shroud were changed and dispersed in the presence of unsteady wakes, thus changing the heat/mass-transfer distributions in these areas. Due to the high thermal load on the tip and shroud under unsteady wake conditions, more detailed cooling designs must be considered, ranging from film cooling holes to the installation of additional structures on the blade tip, especially in the leading edge region and mid-region I of the tip, and the mid-region I and trailing edge region of the shroud.

Strain rates, flow patterns and flame surface densities in hydrodynamically unstable, weakly turbulent premixed flames

Recent numerical and experimental studies have unveiled a potentially marked difference between the laminar as well as turbulent propagation of premixed flames exhibiting Darrieus–Landau (DL) (or hydrodynamic) instabilities from flames for which instabilities are inhibited. In this study we utilize two-dimensional numerical simulations of slot burner flames as well as experimental Propane–Air Bunsen flames to analyse differences in turbulent propagation, strain rate and induced flow patterns of hydrodynamically stable and unstable flames. We also investigate the effects of hydrodynamic instability on quantities which are directly related to reaction rate closure models, such as flame surface density and stretch factor. A clear enhancement of turbulent flame speed can be observed for unstable flames, generally mitigated at higher turbulence intensity, which is attributed to a flame area increase induced by the characteristic cusp-like DL-induced corrugation, absent in stable flames, which occurs concurrently and in synergy with turbulent wrinkling. Unstable flames also exhibit, both numerically and experimentally, a different correlation between strain rate and flame curvature and are observed to give rise to a channeling of the induced flow in the fresh mixture. Conditionally averaged flame surface density is also observed to attain smaller values in unstable flames, as a result of the thicker turbulent flame brush, indicating that closure models should incorporate instability-related parameters in addition to turbulence-related parameters.

Extinguishment of counterflow diffusion flame stabilized in turbulent airflow by polydisperse water mist

In the present study, the effect of turbulence on extinguishment of counterflow diffusion flame by polydisperse water mist was investigated experimentally. A counterflow methane-air diffusion flame was stabilized in the forward stagnation region of a porous cylinder. Water mist was supplied by a pressure-type atomizer. Uniform turbulence was produced in the airflow by a grid. Mist characteristics were measured by a phase Doppler particle analyzer and turbulence properties by a hot wire anemometer. For laminar flames, the critical stretch rate at extinguishment decreases with the mass fraction of water mist and the extinguishment is not affected by the Sauter mean diameter D32 even when D32 is as large as 70 µm. Turbulence reduces significantly the critical bulk (mean flow) stretch rate at extinguishment. Effect of turbulence on extinguishment is relevant to the turbulence stretch rate, which is the inverse of the Kolmogorov time scale. Sum of the bulk stretch rate and the turbulence stretch rate is the total stretch rate that actually exerts on turbulent flame. If the relative turbulence intensity v’/V is as low as 0.06 and D32 is smaller than 30 µm, extinguishment of turbulent diffusion flame is expressed uniquely by the total stretch rate, which coincides with that of laminar flame. On the other hand, for large water mist, D32 > 30 µm or high turbulence intensity, v’/V = 0.10, non-uniformity of mist dispersion appears, of which scale is of the order of integral scale of turbulence. The locally extinguished flame by a spot of dense water mist is reignited by surrounding flame and the suppression effectiveness of water mist is deteriorated when the turbulence intensity is high or the mist diameter is large.