Measurement of velocity-temperature correlations in a turbulent diffusion flame

The paper reports on the nonintrusive, simultaneous measurement of velocity and temperature fluctuations in a turbulent jet diffusion flame. Velocity fluctuations were measured using laser Doppler anemometry (LDA), whereas coherent anti-Stokes Raman spectroscopy (CARS) was used for temperature measurements. The simultaneous measurements were affected by both density bias and velocity bias because the LDA imposed a form of biased sampling on the CARS. The measured velocity-temperature correlation coefficients indicated that the gradient-diffusion hypothesis is reasonably accurate for the radial direction. However, for the axial direction the gradient diffusion hypothesis is accurate only in the central region of the flame, while countergradient diffusion is found in the outer region.     

Time and spatial correlation measurements in the turbulent wake of a circular cylinder using Laser Doppler Anemometry

The present paper reports Laser Doppler Anemometry (LDA) measurements in the turbulent (R _d =1300) wake of a circular cylinder (d=2 mm). On several typical locations in the wake, at cross-section x/d=125, temporal and lateral space velocity correlations have been measured. Temporal autocorrelation functions were determined from the randomly sampled LDA data by means of the Slotting Technique. The autocorrelation functions which were obtained were too low, the discrepancy giving an indication of the influence of noise in the measurements. The influence of noise generated by the photomultiplier tubes, which appeared to be the most important noise source, could be reduced by a cross-correlation technique. A partly new LDA system for two-point velocity correlation measurements was developed, consisting of an elongated measuring volume and a two-point detection system including a semireflecting mirror to divide the scattered light. The results of this investigation clearly demonstrate that LDA is a powerful tool for measuring time and spatial correlations and related properties of turbulence.     

Investigation of a compressible turbulent boundary layer and a region of separation at M=5 by a laser Doppler anemometer

Results are cited of an experimental investigation of the structure of a compressible turbulent boundary layer on a thermally insulated cylinder placed longitudinally in the flow. The experiments were conducted at M=5 and Re_x10⁷. In order to establish a longitudinal positive pressure gradient and a region of separation at the end of the cylinder, a tailpiece in the shape of an axisymmetric isentropic compression surface, or conical flaps with various half angles, were mounted. Profiles of the longitudinal velocity component were measured using p_o' and t_o probes, and also using a laser Doppler anemometer (LDA) with a Fabry-Perot interferometer. In the absence of a longitudinal pressure gradient the velocity profiles measured by the different methods were in satisfactory agreement among themselves and with the results of calculations. In the presence of a longitudinal positive pressure gradient, the velocity profiles become less filled and the static pressure, calculated according to the results of measurements of the velocity with the aid of the LDA and the pressure p_o', varied across the thickness of the boundary layer. In the separated region, the recirculating velocity of the flow was measured with the aid of the LDA.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 175–178, March–April, 1976.     

Natural convection from a thin heated wire situated on the surface of a liquid

The temperature and velocity fields associated with the free convection of a liquid near a thin heated wire situated close to the horizontal surface of the liquid were studied experimentally. The temperature field was analyzed by the shadow method using a Svil' 80 instrument, and the velocity field by observing the motion of light-scattering particles. Universal profiles of the horizontal velocity and vertical temperature gradient were derived by making scale transformations of the spatial profiles measured in various cross sections of the heated zone for several values of the power developed by the wire.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 139–142, November–December, 1970.     

Measurement of the Combustion Temperature of a Solid Propellant by the Cars Method

To measure the solidpropellant combustion temperature, a procedure was used based on examination of the temperature dependence of the Qbranch of the coherent antiStokes Raman scattering (CARS) spectrum due to nitrogen contained in combustion products. The measurements were carried out at a pressure of 4 MPa, under which the spectrum demonstrated a substantial overlap between spectral lines. CARS intensities at two frequencies were registered; their ratio was used to determine the combustion temperature of a stoichiometric ammonium dinitramide–polycaprolacton mixture.     

A new method to measure particle turbulent dispersion using laser doppler anemometer

A laser doppler anemometer (LDA) was used to measure local dispersion coefficients of particles in turbulent flow. The experimental set-up is described along with the data acquisition equipment and processing procedures. Results for 5 particles dispersing from a point source in pipe flow are shown. A second estimate for the diffusivity was obtained from mean square dispersion measurements.List of symbols A projected area of LDA measuring volume, normal to pipe axis - B ₀ – B ₂ coefficients used in concentration curve fits - C particle number density, concentration - d _f fringe spacing - f _B Bragg cell frequency shift - f _D frequency of Doppler signal - H LDA measuring volume dimension in pipe axial direction - h random variation in H - J particle flux - J ₀, J ₁ bessel function of zeroth and first order - r radial location - t time - U axial velocity - u fluctuating component of axial velocity - v_p average particle radial velocity - x axial coordinate - y position of particle in the direction normal to the mean flow relative to the centerline - mean square dispersion Greek Symbols _2,3 roots of Bessel functions - _p turbulent diffusivity of particles - laser wave length - laser beam intersection angle     

Flow Structure of Lifted Swirling Jet Flames

Swirling combustion is widely applied in various applications such as gas turbines, utility boilersor waste incinerators. This article contributes to the ongoing research by providing experimentaldata that are gathered in the mixing zone of a lifted swirling premixed natural gas flame. Theobjective of this paper is fivefold: (1) to introduce the lifted swirling flame featuring lowNO _x emissions (2) to provide experimental data such as major species distributions, temperature and streamlines of the flow pattern, (3) to report on velocity bias in probability density function (PDF) distributions and to present PDF sequences of velocities in medium scale swirling flows, (4) to make an assessment on the local small-scale turbulence that is present in the swirling mixinglayer and (5) to provide new experimental data for model verification and development.The PDFs are corrected in order to compensate for the velocity bias phenomenon, which is typicalfor randomly sampled LDA data. Sequences of axial PDF data are presented and measurement locationsof interest are selected to look at the PDF characteristics of the internal and externalrecirculation zones, the mixing layer and the onset of the reacting flow into detail. The mixinglayer PDFs covered a wide velocity range and revealed bimodality; even the concept ofmulti-modality is suggested and explored. Analysis showed that a sum of two Gaussian distributionscan accurately envelop the experimental PDFs. The reason for this broadband turbulence behavior isto be found in combination of precessing and flapping motion of the flow structures, and also incombustion generated instabilities of the lifted flame. As a result, the flame brush is wide (largescale motion) and the mixing (small-scale turbulence) flattens any high temperatures in thecombustion process.The multi-scale turbulence concept is subsequently used to make anassessment of the local turbulence characteristics in the mixing layer.The idea is that the PDFs capture both contributions of the flow-inherent fine grain turbulence (u _l ) which is superposed on slowlarge scale fluctuating structures. It is this u _l that will be of interest in continued research on the classification of the lifted flame into acombustion regime diagram (e.g. Borghi diagram). Finally, the bimodalitycharacter in reacting flows and the prediction of large-scale structuresmay be a challenge for LES researchers.     

Simultaneous particle image velocimetry and chemiluminescence visualization of millisecond-pulsed current�Cvoltage-induced perturbations of a premixed propane/air flame

The effects of millisecond-wide, pulsed current?Cvoltage-induced behavior in premixed laminar flames have been investigated through the simultaneous collection of particle image velocimetry (PIV) and chemiluminescence data with particular attention paid to the onset mechanisms. Disturbances caused by applied voltages of 2?kV over a 30-mm gap to a downward propagating, atmospheric pressure, premixed propane/air flame with a flow speed near 2?m/s and an equivalence ratio of 1.06 are investigated. The combined PIV and chemiluminescence-based experimental data show the observed disturbance originates only in or near the cathode fall region very close to the burner base. The data also suggest that the coupling mechanism responsible for the flame disturbance behavior is fluidic in nature, developing from the radial positive chemi-ion distribution and an ion-drift current-induced net body force that acts along the annular space discharge distribution in the reaction zone in or near the cathode fall. This net body force causes a reduction in flow speed above these near cathodic regions causing the base of the flame to laterally spread. Also, this effect seems to produce a velocity gradient leading to the transition of a laminar flame to turbulent combustion for higher applied current?Cvoltage conditions as shown in previous work (Marcum and Ganguly in Combust Flame 143:27?C36, 2005; Schmidt and Ganguly in 48th AIAA aerospace sciences meeting. Orlando, 2010).     

Intermittency and scaling in turbulent convection

Both the velocity and temperature measurements taken in turbulent Rayleigh-B‘‘enard convection experiments have been analyzed. It is found that both the velocity and temperature fluctu-ations are intermittent and can be well-described by the She-Leveque hierarchical structure. A positive correlation between the vertical velocity and the temperature differences is found both at the center, near the sidewall and near the bottom of the convection cell, supporting that buoyancy is significant in the Bolgiano regime. Moreover, the intermittent nature of the temperature fluctuations in the Bol-giano regime can be attributed to the variations in the temperature dissipation rate. However, the relations between the velocity and temperature structure functions and their correlations implied by the Bolgiano-Obukhov scaling are not supported by experimental measurements.     

Droplet motion in an electrohydrodynamic fine spray

The results of a combined experimental and numerical study on droplet behavior within an electrohydrodynamic fine spray are presented. The fine spray exists in the transition region between the multiple cone-jet and rim emission spray modes. Experiments were conducted specifically to characterize the motion of droplets within the spray. Light-sheet visualizations and measurements of droplet speed and velocity using laser-based, single-particle counters were obtained. Additionally, a numerical simulation of the droplet motion within the spray was made and compared to the experimental results. The electrohydrodynamic fine spray of ethanol droplets ( 1 to 40 m diameter) was generated using a typical capillary-plate configuration, with a capillary tip electric field intensity of 10⁶ V/m and a spray charge density of 70 C/m³. Acquired images of the spray revealed a zone of rapid expansion near the capillary followed by a more gradual expansion farther from the capillary. In situ laser-diagnostic measurements confirmed these observations. Measured droplet speeds decreased rapidly with increasing axial distance from the capillary, but then increased beyond the spray's axial mid-plane as a result of a change in the sign of the axial internal electric field. Droplet axial velocity components behaved similarly. The radial velocity components exhibited a maximum value off of the spray's centerline in the near-capillary region. Farther away from the capillary, they increased monotonically with increasing radial position. These trends identified the significant role that the radial internal electric field plays in spray expansion. The numerical simulation of the normal spray verified the inferred change in sign of the axial internal field and underscored the dominant contribution of the external electric field in the near-capillary region and of the internal electric field farther away.     

Spatial correlation measurement in turbulent flows using pulsed wire anemometry

Details of a new technique in pulsed wire anemometry, developed to allow measurement of two-point spatial velocity correlations in highly turbulent flows, are described. An outline of the interface devices necessary for linking two anemometers to a microcomputer is given and examples of the use of the technique are presented. Firstly, measurements of spatial correlations with and without time delay in the near wake of a nominally two-dimensional cylinder normal to a uniform stream confirmed the viability of the technique. Secondly, measurements in the highly turbulent, separated region behind a normal flat plate fitted with a downstream splitter plate are presented as a demonstration of the effectiveness of the technique in such difficult flows. We believe that these are the first direct measurement of spatial correlation functions within a separated flow.     

The Problems of the Turbulent Burning Velocity

The paper reviews the practical problems in measuring a turbulent burning velocity that gives the mass rate of burning. These largely centre on identifying an appropriate flame surface to associate with the turbulent burning velocity, u _t , and the density of the unburned mixture. Such a flame surface has been identified, in terms of the mean reaction progress variable, $\bar {c}$ , for explosive flame propagation in a fan-stirred bomb. Measurement of $\bar {c}$ makes possible an estimation of the flame surface density, ??, from the relationship ${\it \Sigma} =k\bar {c}\left( {1-\bar {c}} \right)$ . It is shown that in such explosions, mass rates of burning derived from the measured total flame surface area agreed well with those found from the measured turbulent burning velocity. Flamelet considerations identify appropriate dimensionless correlating parameters for u _t . As a result, correlations of turbulent burning velocity divided by the effective rms turbulent velocity, are plotted against the turbulent Karlovitz stretch factor, K, for different values of the Markstein number for flame strain rate, Ma_sr. These plots cover a wide range of variables, including pressure and fuels, and are indicative of different regimes of turbulent combustion. At the lower values of K, there is some evidence of increases in u _t and k due to high-frequency flame surface wrinkling arising from flame instabilities. These increase as Ma_sr becomes more negative. It is found from the developed value of the mean flame surface density throughout the flame brush that, to a first approximation, an increase in u _t for a given mixture is accompanied by a proportional increase in the volume of the brush. The analysis shows that the volume fraction of the turbulent flame brush that is reacting is quite small.     

Evaluations of SGS Combustion,Scalar Flux and Stress Models in a Turbulent Jet Premixed Flame

A newly developed fractal dynamic SGS (FDSGS) combustion model and a scale self-recognition mixed (SSRM) SGS stress model are evaluated along with other SGS combustion, scalar flux and stress models in a priori and a posteriori manners using DNS data of a hydrogen-air turbulent plane jet premixed flame. A posteriori tests reveal that the LES using the FDSGS combustion model can predict the combustion field well in terms of mean temperature distributions and peak positions in the transverse distributions of filtered reaction progress variable fluctuations. A priori and a posteriori tests of the scalar flux models show that a model proposed by Clark et al. accurately predicts the counter-gradient transport as well as the gradient diffusion, and introduction of the model of Clark et al. into the LES yields slightly better predictions of the filtered progress variable fluctuations than that of a gradient diffusion model. Evaluations of the stress models reveal that the LES with the SSRM model predicts the velocity fluctuations well compared to that with the Smagorinsky model.     

Thermally induced velocity gradients in electroosmotic microchannel flows: the cooling influence of optical infrastructure

An axially non-uniform temperature distribution is shown to induce a disturbance to the electroosmotic flow field in microchannels, causing a significant deviation from the ideal plug-like velocity profile. Such axial temperature gradients are shown to be induced passively by the increased dissipation of Joule heat through the optical infrastructure of a viewing window. A combination of caged-dye-based molecular tagging velocimetry (to determine the cross-stream velocity profiles), fluorescence-based thermometry (to determine the in-channel fluid temperatures), and electrical current measurements are employed. The temperature visualization experiments demonstrate that the fluid is locally cooled in the viewed region, resulting in a local increase in the electric field strength. When large fields are applied, measurements indicate that the fluids temperature in the viewed region can be as much as 30°C less than in the remainder of the capillary. Despite an increase in viscosity, this local cooling results in a locally increased electroosmotic wall velocity which induces a concave velocity profile in the viewed portion and a convex velocity profile elsewhere. Experimentally determined profiles exhibit a variation in velocity across the channel of up to 5%. The cause of this velocity profile curvature is confirmed by comparing the velocity profiles obtained at a range of fields to an analytical solution that includes the effects of temperature on the liquid conductivity and viscosity.     

Multi-mode fibre laser Doppler anemometer (LDA) with high spatial resolution for the investigation of boundary layers

A novel LDA system using laser diode arrays and multi-mode fibers in the transmitting optics is presented. The use of high numerical aperture multi-mode step-index fibres results in measurement volumes with, for example, 80 µm length and minimal speckle effects. Because of the high spatial resolution and low relative fringe spacing variation of d/d5×10^–4 the multi-mode fibre LDA is predestined for investigating turbulent flows. Boundary layer measurements carried out show excellent agreement with theoretical velocity profiles.     

Mathematical models for mass transport through arcs and flames

Summary The transport of particles, caused by axial and radial diffusion and axial flow of convection, will be considered in a D.C. arc and in a laminar flame. The following mathematical model will be discussed. Assuming a steady state in both cases the mass transport may be described in cylindrical coordinates (r, z) by the following partial differential equation where C means the particle concentration, D the coefficient of diffusion, and W the axial velocity. D and W are taken to be constant; various boundary conditions, corresponding to different approximations of the physical situation, are considered. Solutions of (0.1) are obtained in an explicit form.     

Stationary modes of combustion in a fine-scale turbulent stream

The majority of models of the turbulent combustion of gases are based mainly on intuitive concepts concerning the processes occurring in the flame. The characteristics of a turbulent flame are estimated from considerations of dimensionality and similarity. A detailed review of works on turbulent combustion is given in [1]. Problems on the calculation of the combustion rate in a turbulent stream as a proper value of the equations of heat and mass transfer and of the corresponding boundary conditions have recently been raised. Here too one must rest on assumptions of a semiempirical nature, which in large measure is connected with the inadequate level of development of turbulence theory. In the present work the equation of propagation of the zone of chemical reactions in the stream is averaged statistically by analogy with studies of turbulent flows. Correct averaging is possible at scales of hydrodynamic disturbances smaller than the flame thickness (fine-scale turbulence). The temperature pulsations are related with the size of the heat flux using the theory of mixing lengths. The main influence is specific to effects arising during averaging of the heat release function. Two stationary modes, distinguished by the normal propagation velocity ₁, are isolated within the framework of the Cauchy problem with a given initial mixture temperature and zero heat flux in the burned gas. A heat conduction mode occurs with a stream velocity > ₁ and an induction mode with < ₁. An expression is found for ₁ which reflects the principal effects in the flame and which in the limit coincides with the equation of Zel'dovich and Frank-Kamenetskii for a laminar flame. In those cases when the distorting effect of the heat release function is small, the turbulence affects the combustion rate through mechanisms of intensification of transport processes.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 118–124, September–October, 1973.     

The effect of initial conditions on the exit flow from a fluidic precessing jet nozzle

The fluidic precessing jet (FPJ) is a member of a family of self-excited oscillating jet flows that has found application in reducing oxides of nitrogen (NO_x) from combustion systems in the high-temperature process industries. Its flow field is highly three-dimensional and unsteady, and many aspects of it remain unresolved. Velocity data, measured close to the exit plane, are presented for a variety of FPJ nozzles with three different inlet conditions, namely, a long pipe, a smooth contraction and an orifice. The results indicate that jet inlets that are known to have nonsymmetrically shedding initial boundary layers, namely those from the orifice or long pipe, cause jet precession to be induced more easily than the smooth contraction inlet, which is known to have a symmetrically shedding initial boundary layer. The nature of the exit flow is dominated by the degree to which a given configuration generates precession. Nevertheless, the three different inlet conditions also produce subtle differences in the exit profiles of mean velocity and turbulence intensity when the flow does precess reliably. Roman symbols d diameter of inlet (m) - D₁ diameter of FPJ chamber (m) - D₂ diameter of FPJ chamber exit lip (m) - E expansion ratio D₁/d - f frequency (Hz) - f_p precession frequency (Hz) - h step height (D₁-d)/2 (m) - n power law index to describe pipe inlet jet (dimensionless) or nth sample passing through LDA probe volume - N total number of bursts sampled (dimensionless) - r radial distance from FPJ chamber axis (m) - rms root-mean-square or fluctuating velocity component, (m/s) - R₁ radius of FPJ chamber (m) - R₂ radius of exit lip (m) - Re Reynolds number u_id/ (dimensionless) - S(f) arbitrary power spectrum (m²/s) - St Strouhal number, f_ph/u_i (dimensionless) - t_n residence (or transit) time of a particle moving through the LDA probe volume (s) - u axial component of mean velocity (m/s) - u_cl axial component of mean centreline velocity (m/s) - u_i bulk inlet velocity near the inlet plane (m/s) - u_n velocity of the nth particle through the LDA probe volume (m/s) - u_vc axial component of mean velocity in the region of the vena contracta (m/s) - u axial component of rms velocity (m/s) - v radial component of mean velocity (m/s) - v radial component of rms velocity (m/s) - w tangential component of mean velocity (m/s) - w tangential component of rms velocity (m/s) - x axial distance from FPJ chamber inlet plane (m) - x axial distance from FPJ chamber exit plane (m)Greek symbols kinematic viscosity of air at 21°C, 14.7×10^-6 m²/s     

The effect of temperature gradient on the correlation of temperature fluctuations

In this paper we present the experiment results of the space correlations of temperature fluctuations when there is and is not temperature gradient. From those results we can see clearly that the temperature fluctuation field is isotropic without temperature gradient, and it is obviously anisotropic with temperature gradient. The temperature correlation along gradient direction is obviously larger than the vertical to direction. Our experiment results agree with the theory results of D.W. Dunn and W.H. Rend^[1].     

Three-dimensional flow in axial flow fans of non-free vortex design

Three-dimensional laser Doppler anemometer (LDA) measurements were carried out downstream of isolated axial fan rotors of non-free vortex design in order to investigate the role of radial velocity components in design. The structure of secondary flows due to non-free vortex operation was studied in detail. It is pointed out that the tangential gradient of radial velocity at midspan is nearly in direct proportion with the spanwise gradient of ideal total head rise prescribed in design. Design criteria have been established for the neglect of torsion of stream surface segments inside the blading. A linear relationship was proposed in order to estimate the pitch-averaged radial velocities at the rotor exit. Using this relationship, a proposal has been put forward for taking the radial velocity components into account in non-free vortex design with the assumption of conical stream surfaces through the blading.