Principles of flow field diagnostics by laser induced biacetyl phosphorescence

A new method for measuring the density, temperature and velocity of N₂ gas flow by laser induced biacetyl phosphorescence is proposed. The characteristics of the laser induced phosphorescence of biacetyl mixed with N₂ are investigated both in static gas and in one-dimensional flow along a pipe with constant cross section. The theoretical and experimental investigations show that the temperature and density of N₂ gas flow could be measured by observing the phosphorescence lifetime and initial intensity of biacetyl triplet (³A_u) respectively. The velocity could be measured by observing the time-of-flight of the phosphorescent gas after pulsed laser excitation. The prospect of this method is also discussed.     

用丁二酮激光诱导磷光测量流场可能性的研究

对混合在氮气中丁二酮三重态~3Au的激光诱导磷光性质的实验结果表明,磷光寿命是温度的函数,与密度及浓度无关;而初始磷光强度是密度的线性函数,对温度变化不敏感,用观察氮气中少量丁二酮磷光寿命与磷光初始强度的方法可以测量流场的温度与密度分布,磷光寿命长的特点可以用来测量速度分布,它在很大程度上克服了多普勒测速中因固体微粒质量大而滞后于气流的问题,因而可以在具有高加速度的流场中使用,本文还对磷光衰减机理进行了分析,结果与实验一致,上述同时测温度与密度的方法尚未见诸文献报道。     

Simultaneous measurement of temperature,density and velocity in gas flows by modulated photoluminescence

A novel possibility to determine the temperature, density and velocity simultaneously in gas flows by measuring the average value, amplitude of modulation and phase shift of the photoluminescence excited by a temporally or spatially modulated light source is investigated. Time-dependent equations taking the flow, diffusion, excitation and decay into account are solved analytically. Different experimental arrangements are proposed. Measurements of velocity with two components, and temporal and spatial resolutions in the measurements are investigated. Numerical examples are given for N ₂with biacetyl as the seed gas. Practical considerations for the measurements and the relation between this method and some existing methods of lifetime measurement are discussed.     

PIV measurements for gas flow under gradient magnetic fields

Particle Image Velocimetry (PIV) techniques were developed to measure the convective N2-air flow under gradient magnetic fields. The velocity fields were calculated by the Minimum Quadratic Difference (MQD) algorithm and spurious vectors were eliminated by Delaunay Tessellation.The N2-air flow was measured as the magnetic flux density varying from 0～1.5T. A strengthened vortex flow of air was observed under the condition that the magnetic field was applied, and the velocity of N2 jet rose with the increase of the magnetic density. The experimental results show that the magnetic force will induce a vortex flow and cause a convection flow of the air mixture when both gradients of the O2 concentration and the magnetic field intensity exist.     

Investigation of the inverted medium of a quasi-stationary CO2 laser with pulsed excitation

The results of an investigation of the inverted medium of a quasi-stationary CO₂ laser is presented. The medium is distinguished by the fact that the time of flight of individual molecules through the discharge gaps is less than the relaxation time of the 00 °1 CO₂ laser level. The emitted power, the gain, the saturation intensity, and the gas temperature are measured. Using the experimental data, the distribution of the molecules in the vibrational and rotational states of the inverted medium is calculated. The maximum power density attained in this experimental model is 25 W/cm³. For comparison, the characteristics of a model in which cold CO₂ is added to the flow of excited nitrogen are investigated. It is shown that in this case the output power level is determined by the efficiency with which the jets are mixed.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 23–29, January–February, 1973.The authors thank V. M. Fedorov for useful discussions and A. A. Borynyak for his help with the experiments.     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows

 Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine. Received: 29 August 1997/Accepted: 16 September 1998     

Combined phosphor and CARS thermometry at the wall–gas interface of impinging flame and jet systems

For the determination of surface normal temperature gradients, a generic system was built up consisting of two opposed, vertical nozzles impinging onto a flat, horizontal copper plate. From below, the plate was heated by non-reacting, turbulent air jets (Re = 5,000) and by a laminar flame (λ = 0.7, Re = 350), respectively. For well-defined boundary conditions, the plate was cooled by a turbulent cold jet from above in both cases. Wall temperature as well as gas temperature distributions within and outside of the thermal boundary layer of the hot side of the system were determined. The radial surface temperature profile of the plate was measured by coating it with thermographic phosphors (TP), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm). The temporal decay of the phosphorescence intensity was measured pointwise by a photomultiplier tube. In this case, the 659 nm emission line of Mg₄FGeO₆:Mn was monitored. Non-intrusive point measurements of the gas temperature close to the surface were performed by rovibrational coherent anti-Stokes Raman spectroscopy (CARS) of diatomic nitrogen. Beams from a seeded, frequency-doubled Nd:YAG laser (532 nm) and from a modeless broadband dye laser (607 nm) were phase-matched into a surface-parallel, planar-boxcars configuration. The temperature data could be collected as close as 300 μm to the surface. Thermographic phosphors as well as CARS proved to be consistent for wall temperature and boundary layer measurements in all test cases. The results and challenges of this approach are discussed.     

Modeling of the oscillating field effects on the Heβ line emission in short pulse laser-produced plasmas

Strong oscillating fields may induce strong modifications of the emission spectra of ions. We discuss here the possibility of observing such effects in actual laser experiments where space- and time-integration effects can easily mask their existence. Focusing on the Al He_β transition, we first discuss the calculation of its spectral broadening in the presence of a strong laser field. Then, starting from 1D hydro-simulations of short, intense, laser pulse-produced plasmas that provide the density, temperature and laser intensity profiles as a function of time, full integrated collisional-radiative calculations of the laser field-dependent emissivity of the Al He_β line, are presented.     

Aerosol formation in an isothermal stagnation flow

An experiment has been performed in a laminar stagnation point flow in which two non-premixed reactants produced an aerosol of sub-micron particles. The reactants were NH₃ and HCl. The rate of mixing of the reactants was determined by the velocity gradient or strain rate of the flow; the response of the aerosol dynamics to the flow field was measured with a laser light scattering technique. Laser Doppler Spectroscopy was used to measure the particle size. It was found that the particle size was independent of the strain rate of the flow. On the other hand, the particle number density decreased as the strain rate increased. It is argued that the intensity of light scattered from the aerosol is, therefore, a measure of the amount of product of the relatively slow NH₃-HCl reaction.     

Effect of buoyancy on the free surface flow past a permeable bed

Laminar steady free surface flow having one permeable bounding wall is investigated in the presence of buoyancy force. The experimental results of Rajasekhara [1] were found to be in good agreement with our theoretical results based on a model which admits slip-velocity at the porous material. The effect of buoyancy force is to increase the velocity distribution in the case of greater heat addition (N_o>0) and to decrease it by a greater cooling (N_o<0). As a result, the mass flow rate increases and the friction factor decreases for N_o>0 and the opposite is true for N_o<0. We further find that the effect of buoyancy force on the temperature distribution is to increase its magnitude. In particular, we find that the rate of heat transfer at its nominal surface is increased in the case of heating (N_o>0) of flow.     

Laser-induced fluorescence modulation techniques for velocity measurements in gas flows

Broadband fluorescence of iodine, excited at 514.5 nm by a single-mode argon-ion laser tuned to the quasi-linear part of an absorption line, was used to detect the Doppler shift and hence the velocity of iodine molecules seeded in a nitrogen jet flow. The slope of the absorption line profile was measured directly using a frequency shift introduced by acoustooptic modulators (AOMs). A velocity of 36 m/s was measured in a jet of N₂ at 60 Torr in 2 ms with an accuracy of 11%. To reduce experimental noise, the laser beams were switched at 125 KHz and signal-tuned amplification was used.     

Discontinuous solutions of the Navier-Stokes equations for compressible flow

We prove local existence and study properties of discontinuous solutions of the Navier-Stokes equations for one-dimensional, compressible, nonisentropic flow. We assume that, modulo a step function, the initial data is in L² and the initial velocity and density are in the space BV. We show that the velocity and the temperature become smoothed out in positive time, and that discontinuities in the density, pressure, and gradients of the velocity and temperature persist for all time. We also show that for stable gases these discontinuities decay exponentially in time, more rapidly for smaller viscosities.     

A numerical study of pulsatile laminar flows in a pipe with a ring-type constriction

Numerical simulations have been carried out to study pulsatile laminar flows in a pipe with an axisymmetric ringtype constriction. Three types of pulsatile flows were investigated, namely a physiological flow, a pure sinusoidal flow and a non-zero mean velocity sinusoidal flow. The laminar flow governing equations were solved by the SIMPLE algorithm on a non-staggered grid and a modified Crank-Nicolson approximation was used to discretrize the momentum equations with respect to time. The maximum flow Reynolds numer (Re) is 100. The Womersley number (N_w) ranges from 0 to 50, with the corresponding Strouhal number (St) ranging from 0 to 3·98. The constriction opening ratio (d/D) and thickness ratio (h/D) are fixed at 0·5 and 0·1 respectively. Within the time period investigated, all these pulsatile flows include both forward and backward flows. The unsteady recirculation region and the recirculation points change in size and location with time. For N_w ≤ 1 and St≤ 1·56 x 10^?3 the three pulsatile flows have the same simple relation between the instantaneous flow rate and pressure loss (Δp) across the constriction and the pressure gradient in the axial direction (dp/dz) in the fully developed flow region. The phase angles between the flow rate and pressure loss and the pressure gradient are equal to zero. With increasing N_w and St, the phase angle between the flow rate and the dp/dz becomes larger and has its maximum value of 90° at N_w = 50 and St = 3·98. The three pulsatile flows also show different relations between the flow rate and the pressure gradient. The pure sinusoidal flow has the largest maximum pressure gradient and the non-zero mean velocity sinusoidal flow has the smallest. For larger N_w and St the fully developed velocity profiles in the fully developed flow region have a smaller velocity gradient along the radial direction in the central region. The maximum recirculation length increases for N_w ranging from 0 to 4·2, while this length becomes very small at N_w = 50 and St = 3·98. The deceleration tends to enlarge the recirculation region and this effect appears for N_w ≥ 3 and St ≥ 1·43×10^?2. Linear relations exist between the flow rate and the instantaneous maximum values of velocity, vorticity and shear stress.     

Determination of density and concentration from fluorescent images of a gas flow

A fluorescence image analysis procedure to determine the distribution of species concentration and density in a gas flow is proposed. The fluorescent emission is due to the excitation of atoms/molecules of a gas that is intercepted by an electron sheet. The intensity of the fluorescent light is proportional to the local number density of the gas. When the gas flow is a mixture of different species, this proportionality can be used to extract the contribution associated with the species from the spectral superposition acquired by a digital camera. In particular, the fact is exploited such that the ratio between a pair of color intensities takes different values for different gases and that different linear superpositions of different color intensities yield a ratio that varies with the species concentration. This leads to a method that simultaneously reveals species concentrations and mass density of the mixture. For the proper working of a continuous electron gun in a gas, the procedure can be applied to gas flow where the pressure is below the thresholds of 200∼300 Pa and the number density is no greater than 10²³ m⁻³. To maintain the constancy of the emission coefficients, the temperature variation in the flow should be inside the range 75–900 K (above the temperature where the probability to meet disequilibrium phenomena due to rarefaction is low, below the temperature where visible thermal emission is present). The overall accuracy of the measurement method is approximately 10%. The uncertainty can vary locally in the range from 5 to 15% for the concentration and from 5 to 20% for the density depending on the local signal-to-noise ratio. The procedure is applied to two under-expanded sonic jets discharged into a different gas ambient—Helium into Argon and Argon into Helium—to measure the concentration and density distribution along the jet axis and across it. A comparison with experimental and numerical results obtained by other authors when observing under-expanded jets at different Mach numbers is made with the density distribution along the axis of the jet. This density distribution appears to be self-similar.     

Peristaltic flow of a nanofluid in a non-uniform tube

The present analysis discusses the peristaltic flow of a nanofluid in a diverging tube. This is the first article on the peristaltic flow in nanofluids. The governing equations for nanofluid are modelled in cylindrical coordinates system. The flow is investigated in a wave frame of reference moving with velocity of the wave c. Temperature and nanoparticle equations are coupled so Homotopy perturbation method is used to calculate the solutions of temperature and nanoparticle equations, while exact solutions have been calculated for velocity profile and pressure gradient. The solution depends on Brownian motion number N _b , thermophoresis number N _t , local temperature Grashof number B _r and local nanoparticle Grashof number G _r . The effects of various emerging parameters are investigated for five different peristaltic waves. It is observed that the pressure rise decreases with the increase in thermophoresis number N _t . Increase in the Brownian motion parameter N _b and the thermophoresis parameter N _t temperature profile increases. Streamlines have been plotted at the end of the article.     

An experimental study of a two-dimensional plane turbulent wall jet

 Laser-Doppler measurements were conducted in a plane turbulent wall jet at a Reynolds number based on inlet velocity, Re ₀, of 9600. The initial development as well as the fully developed flow was studied. Special attention was given to the near-wall region, including the use of small measuring volumes and the application of specific near-wall data corrections, so that wall shear stresses were determined directly from the mean velocity gradient at the wall using only data below y ⁺=4. It was possible to resolve the inner peak in the streamwise turbulence intensity as well as the inner (negative) peak in the shear stress. Limiting values of (u′)⁺ and uv ⁺ were determined. Turbulence data from the outer region of the flow were compared to earlier hot wire measurements and large differences in the normal turbulence intensity and the shear stress were found. These differences can be attributed to high turbulence intensity effects on the hot-wires. Received: 17 October 1996 / Accepted: 8 December 1997     

Velocity and turbulence measurements in combustion systems

A laser-Doppler velocimeter is used in the measurement of high-temperature gas flows. A two-stage fluidization particle generator provides magnesium oxide particles to serve as optical scattering centers. The one-dimensional dual-beam system is frequency shifted to permit measurements of velocities up to 300 meters per second and turbulence intensities greater than 100 percent.Exiting flows from can-type gas turbine combustors and burners with pre-mixed oxy-acetylene flames are described in terms of the velocity, turbulence intensity, and temperature profiles.The results indicate the influence of the combustion process on turbulence.List of Symbols A exit area of combustor or burner (m²) - A/F mass air-fuel ratio - D exit diameter of combustor or burner (m) - M mass flow rate of gases (kg/s) - N _D number of Doppler bursts used in each velocity measurement - Q volumetric flow rate at T _r (m³/s) - R exit radius of combustor or burner (m) - R _1/2 distance from centerline to radius where the velocity is one-half of the local centerline velocity (m) - Re exit Reynolds number based on cold flow, QD/A - r distance from centerline of flow (m) - T temperature (°C) - T _CL centerline temperature (°C) - T _r inlet (cold) air temperature of combustor or burner (°C) - T.I. turbulence intensity, - mean velocity (m/s) - U _i instantaneous velocity individually realized by LDV (m/s) - mean velocity at centerline of flow (m/s) - mean square velocity fluctuation (m²/s² - x distance along centerline downstream of exit (m) - absolute viscosity at T _r (kg/(ms)) - density at T _r (kg/m³)     

Experimental study of the initial growth of a localized turbulent patch in a stably stratified fluid

We present a laboratory experiment of the initial growth of a turbulent patch in a stably stratified fluid. The patch is created due to a localized source of turbulence, generated by a horizontally oriented and vertically oscillating grid much smaller than the tank size and far from solid boundaries. Synchronized and overlapping particle image velocimetry(PIV) and planar laser induced fluorescence (PLIF) measurements capture the evolution of the patch through its initial growth until it reached a maximum size. The simultaneous measurements of density and velocity fields allow for a direct quantification of the distribution of kinetic energy, buoyancy and degree of mixing within the patch. We can also relate the propagation speed of the turbulent/non-turbulent interface and its thickness to the properties of the turbulent fluid inside the evolving patch. The velocity measurements in this setup indicate significant transient effects inside the patch during its growth. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity w_e as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. The detailed information about the growth of localized sources of turbulence in stratified environment might be of use in stealth design of autonomous underwater vehicles.     

Supercritical nitrogen free jet investigated by spontaneous Raman scattering

The injection of fuel and oxidizer in high pressure rocket combustion chambers is simulated in a model experiment dedicated to investigate the phenomenology of coaxial injection at supercritical pressures. Cryogenic nitrogen is injected in a pressurized reservoir of N₂ at ambient temperature at 4 Mpa and 6 Mpa, above the critical pressure for nitrogen of 3.4 Mpa. The radial density distribution of the N₂ is determined by spontaneous Raman-scattering, from the measured densities temperature distributions are derived. The radial density and temperature pro les are analyzed as a function of the distance from the injector for x/D = 1 to 30. The influence of density gradients, internal field effects and interference effects on the performance of the Raman technique when applied to turbulent high density flows is discussed. Received: 5 August 1998/Accepted: 10 January 1999