Turbulent Reynolds Number Research Articles

Scaling of thermal stratification or mixing in outlet plena of SFRs

Sodium-cooled fast reactors (SFRs) have a tendency for thermal stratification in the hot or outlet plena especially during shut-down and loss of forced convection transients. The formation of thermally stratified zones in the outlet plena can: lead to thermal fatigue in adjoining solid structures; and impact passive heat removal capabilities. There are limited number of experimentally validated models in literature for thermal stratification or mixing in liquid metal pools. A high fidelity scaled-down experimental facility is required to address these needs and capture the physical phenomenon close to what is expected in a real system. The design of experimental facility and instrumentation becomes complicated with use of liquid sodium, so a surrogate fluid can simplify the design and operation considerably, providing flexibility to obtain high quality measurements. Similarity analysis based scaling methodology is used to design a scaled-down model of outlet plenum with liquid gallium (Pr∼ 0.025) as a surrogate for liquid sodium (Pr∼0.005). However, reduction in size and different choice of coolant must be handled carefully to maintain relevance and reliability of the experimental facility. The possible differences in thermal stratification or mixing physics in the scaled-down model and the prototype SFR are evaluated by quantifying eddy thermal diffusivity and mixing efficiency. Empirical models are used to correlate these quantities as a function of Richardson number and turbulent Reynolds number, and verified using computational fluid dynamics simulations. The verified empirical model is used to construct mixing efficiency for the prototypical conditions and the scaled-down model conditions. For low flow rates in both geometries, the results show thermal profiles dominated by molecular thermal diffusivity resulting into a fully stratified interface. The full flow cases for both geometries show energetic mixing, characteristic of eddy thermal diffusivity dominated, mixed temperature profiles. These results provide evidence of accurate scaling analysis by capturing the thermal stratification or mixing physics representing the dominant physical modes of those of the prototype SFR.

Application of Hybrid RANS/LES turbulence models in rotor-stator fluid machinery: a comparative study

Purpose This paper aims to improve performance prediction and to acquire more detailed flow structures so as to analyze the turbulence in complex rotor-stator flow. Design/methodology/approach Hydraulic retarder as typical fluid machinery was numerically investigated by using hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) models CIDDES Algebraic Wall-Modeled Large Eddy Simulation (LES) (WMLES) S-Ω and dynamic hybrid RANS/LES (DHRL). The prediction results were compared and analyzed with a RANS model shear stress transport (SST) k-omega which was a recommended choice in engineering. Findings The numerical results were verified by experiment and indicated that the predicted values for three hybrid turbulence models were more accurate. Then, the transient flow field was further analyzed visually in terms of turbulence statistics, Reynolds number, pressure-streamline, vortex structure and eddy viscosity ratio. The results indicated that HRL approaches could capture unsteady flow phenomena. Practical implications This study achieves both in performance prediction improvement and better flow mechanism understanding. The computational fluid dynamics (CFD) could be used instead of flow visualization to a certain extent. The improved CFD method, the fine computational grid and the reasonable simulation settings jointly enhance the application of CFD in the rotor-stator flow. Originality/value The improvement was quite encouraging compared with the reported literatures, contributing to the CFD playing a more important role in the flow machinery. DHRL provided the detailed explanation of flow transport between rotor and stator, which was not reported before. Through it, the flow mechanism can be better understood.

Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram

This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity (DT=u′Lx) to the molecular diffusivity within the preheat layer (D*=SLδF,L). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

Tailoring incoming shear and turbulence profiles for lab‐scale wind turbines

AbstractAn active grid is used to generate a variety of turbulent shear profiles in a wind tunnel. The vertical bars are set to flap through varying angles across the test section producing a variation in the perceived solidity, resulting in a mean shear. The horizontal bars are used in a fully random operational mode to set the background turbulence level. It is demonstrated that mean velocity profiles with approximately the same shear can be produced with different turbulence intensities and local turbulent Reynolds numbers based on the Taylor microscale, λ. Conversely, it is also demonstrated that flows can be produced with similar turbulence intensity profiles but different mean shear. It is confirmed that the length scales and dynamics, the latter being assessed through the velocity spectra and probability density functions, do not vary significantly across the investigation domain. Such flows are of particular relevance for studies investigating the effect of in‐flow conditions on obstacles where these studies wish to decouple the effects of turbulence intensity and mean shear, a feat previously unattainable in experimental facilities. Given that the power output of wind turbines is known to be a function of both mean shear and turbulence intensity, the experimental methodology presented herein is invaluable to the wind turbine model testing community who, at present, cannot exert such control authority over the in‐flow conditions.

Effect of multiple arc protrusion ribs on heat transfer and fluid flow of a circular-jet impingement solar air passage

Results from experimental study of the heat and fluid flow characteristics of circular-jet impingement solar air passage are presented. The testing includes the turbulent flow Reynolds number (Re) from 5000 to 19,000, relative width ratio (WP/WAP) from 1.0 to 6.0, relative pitch ratio (PP/eP) from 8.0 to 12, relative height ratio (eP/dP) from 0.5 to 2.0, angle of arc (αa) from 35° to 75°, for the fixed values of stream wise pitch ratio(XS/dH) of 0.433, span wise pitch ratio(YS/dH) of 0.866 and jet diameter ratio (Dj/dH)of 0.065. The maximum enhancement in heat transfer and friction factor has been observed to be 6.29 and 9.25 times to that of smooth absorber plate. The optimum value of thermal performance has been found corresponds to WP/WAP=5.0, PP/eP=9.5, eP/dP=1.0, αa=55°, XS/dH=0.433, YS/dH=0.866 and Dj/dH=0.065. The optimum value of thermal hydraulic performance has been found to be 3.64 for Re of 13,000 within the range of the parameter investigated.

Turbulence intensity effects on heat transfer and fluid-flow for a circular cylinder in crossflow

The intrinsically unsteady heat transfer on the surface of a cylinder in crossflow has been investigated in detail by numerical simulation as a function of the freestream turbulence intensity and the Reynolds number. After a brief startup transient, a periodic steady state is established at all circumferential locations. The resulting timewise fluctuations were seen to be of different phase depending on where on the circumference they occur. On one side of the cylinder, maxima occurred at the same moment in time as minima occurred on the other side. This finding and comparisons of the magnitudes of the local heat transfer coefficients showed that side-to-side symmetry does not prevail in the presence of the unsteadiness. The fluctuation frequencies were found to be virtually uniform over the entire circumference of the cylinder and varied only slightly with the Reynolds number and the turbulence intensity. The full slate of results included: (a) timewise and circumferential variations of the local heat transfer coefficient, (b) timewise variations of the all-angle spatial-averaged heat transfer coefficient, (c) spatial variations of the timewise-averaged heat transfer coefficient, (d) spatial- and timewise-averaged heat transfer coefficients as a function of turbulence intensity and Reynolds number, (e) timewise fluctuation frequencies, (f) comparisons with the experimental literature, and (g) effect of the selected turbulence model. As expected, the magnitude of the heat transfer coefficient increases as the turbulence intensity increases.

The role of Reynolds number in the fluid-elastic instability of tube arrays

The onset of fluid-elastic instability in tube arrays is thought to depend primarily on the mean flow velocity, the Scruton number and the natural frequencies of the tubes. However, evidence from experiments suggests that the Reynolds number is also an important parameter, although the available data are insufficient to understand or quantify this effect. We use high resolution direct numerical simulations to solve the penalized two-dimensional Navier–Stokes equations in order to accurately model turbulent flow through tube arrays with a pitch to diameter ratio P∕D=1.5. To investigate the Reynolds number effect we perform simulations that independently vary the mean flow velocity and the Reynolds number at fixed Scruton number. Parameters are chosen so that the simulations are well outside the lock-in regime of resonant vortex excitation. Increasing Reynolds number and mean flow velocity both have strong de-stabilizing effects for rotated arrays. For in-line arrays the effect is weaker and not monotonic with increasing Reynolds number and mean flow velocity. This study clarifies how the onset of fluid-elastic instability depends on Reynolds number (and hence turbulence intensity) and reduces uncertainties arising from the experimental data, which usually do not account for the effect of Reynolds number. It also demonstrates the usefulness of two-dimensional direct numerical simulations to investigate fluid-elastic instability at turbulent Reynolds numbers.

Changes in divergence-free grid turbulence interacting with a weak spherical shock wave

The characteristics of divergence-free grid turbulence interacting with a weak spherical shock wave with a Mach number of 1.05 are experimentally investigated. Turbulence-generating grids are used to generate nearly isotropic, divergence-free turbulence. The turbulent Reynolds number based on the Taylor microscale Reλ and the turbulent Mach number Mt are 49≤Reλ≤159 and 0.709 × 10−3≤Mt≤2.803×10−3, respectively. A spherical shock wave is generated by a diaphragmless shock tube. The instantaneous streamwise velocity before and after the interaction is measured by a hot wire probe. The results show that the root-mean-square value of streamwise velocity fluctuations (r.m.s velocity) increases and the streamwise integral length scale decreases after the interaction. The changes in the r.m.s velocity become small with the increase in Reλ and Mt for the same strength of the shock wave. This tendency is similar to that of the streamwise integral length scale. The continuous wavelet analysis shows that high intensity appears mainly in the low-frequency region and positive and negative wavelet coefficients appear periodically in time before the interaction, whereas such high intensity appears in both the low- and high-frequency regions after the interaction. The spectral analysis reveals that the energy at high wavenumbers increases after the interaction. The change in turbulence after the interaction is explained from the viewpoint of the initial turbulent Mach number. It is suggested that the change is more significant for initial divergence-free turbulence than for curl-free turbulence.

Experimental investigation of the effect of droplet size on the vaporization process in ambient turbulence

This paper presents the results of an experimental investigation of the effect of droplet size on the fuel vaporization process in a turbulent atmosphere at standard ambient conditions. Single droplets of n-heptane and n-decane are formed and evaporated at the intersection point of two micro-fibers placed in the center of a fan-stirred spherical vessel which is maintained at room temperature and pressure. The droplet initial diameter varies in the range between 145 and 730µm. A controlled isotropic and homogeneous turbulent flow field with nearly zero mean velocity and intensity ranging between 0.25 and 1.5m/s is generated inside the vessel by means of eight axial fans. The results indicate that the droplet normalized squared diameter varies quasi-linearly with time, suggesting the applicability of the d2 law at all examined conditions. More importantly, the results reveal that the droplet initial diameter becomes a significant contributing factor in determining the evaporation rate in a turbulent environment, as the rate of vaporization is found to increase with both turbulent intensity and droplet size. However, droplet size produces no effect on the vaporization rate in quiescent (no flow) surroundings. The results also emphasize that as the ratio of the Kolmogorov length scale over droplet initial diameter approaches unity, the effect of turbulence becomes negligible, illustrating the importance of the small-scale eddies, and their relation to droplet size, in the transport/transfer of mass. The coupled effect of droplet size and turbulence on the vaporization rate is correlated in terms of either a turbulent Reynolds number or a vaporization Damköhler number.

Turbulent Flame Speed Dependencies in Lean Methane-Air Mixtures under Engine Relevant Conditions

Direct numerical simulations are carried out to study the behavior of turbulent flames propagating in lean methane-air mixtures of equivalence ratio 0.5 and for non-dimensional turbulence intensities (urms/SL) of 2-25. The turbulent Reynolds number (ReT) varies from 44-549, the Damköhler (Da) number from 0.26-3.2, and the Karlovitz number (Ka) from 1.1-49.4. The mixture pressure is 20 bar and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. A 13-species reduced mechanism and a global mechanism are employed in the study. It is shown that the normalized turbulent flame speeds (ST/SL) can be related to the flame area enhancement (AT/AL) resulting from turbulence interactions with the laminar flame and the efficiency factor (Io) which is close to unity (∼ 1.07 ± 0.04) when evaluated at the temperature of peak heat release rate. This area enhancement increases with increasing ReT and decreasing Ka. The effect of integral length scale on flame area enhancement is related to the effect induced by the spectrum of eddies of the turbulent cascade. A correlation is proposed for the turbulent flame speed. While the global mechanism is adequate for predicting flame speed and area enhancement, some differences in the structure of the flame are observed between predictions with the global and reduced mechanisms.

Simulation and feedback control of the Ahmed body flow exhibiting symmetry breaking behaviour

The present work considers the low-Reynolds-number wake flow behind a squareback Ahmed body, in close proximity to a ground. At low Reynolds numbers such wakes are known to undergo a series of bifurcations to a state that breaks reflectional symmetry. The symmetry breaking of the wake also persists at turbulent high Reynolds numbers, where it manifests as bi-modal behaviour with random switching between the asymmetric states. Thus far, it has only been possible to study the low-Reynolds-number sequence of bifurcations experimentally and mathematically. The present work presents the first numerical simulations capturing the sequence of symmetry breaking bifurcations that occur. A study of how the wake topology changes throughout suggests that interaction between the closer top/bottom pair of parallel shear layers can only dominate once there is sufficient underbody flow. When this occurs, the two main vortex structures in the wake switch from being horizontally to vertically aligned. A linear feedback control strategy, designed to attenuate base pressure force fluctuations, is then implemented. This causes an accompanying reduction in drag and re-symmetrisation of the wake. Analysis using the dynamic mode decomposition confirms that the wake shedding mode is re-symmetrised. This work motivates future attempts to capture wake symmetry breaking and bi-modality in numerical simulations, and application of a promising feedback control strategy at higher, turbulent Reynolds numbers.

Dissipation scaling in constant-pressure turbulent boundary layers

Using results from previous direct numerical simulations and experiments in the outer region of spatially evolving turbulent boundary layers, we compute the streamwise evolution and the wall-normal variation of the dissipation parameter CE, namely, the turbulent kinetic energy dissipation rate, normalized by appropriate powers of the local turbulent kinetic energy and integral length scale. For Reθ3 10000 (Reθ is a Reynolds number on the freestream velocity and the local momentum thickness), CE is essentially constant in the streamwise direction, but varies by up to 50% in the wall-normal direction. For Reθ<10000, CE is additionally found to vary in the streamwise direction and is inversely proportional to the local turbulence Reynolds number Reλ.

Scale-adaptive subgrid-scale modelling for large-eddy simulation of turbulent flows

The proportionality between the subgrid-scale (SGS) drain rate of kinetic energy and the viscous dissipation rate of the resolved motions is studied a priori by filtering a given fully resolved field and evaluating a generic form of the hypothesized energy spectrum. The ratio of the SGS drain to the resolved dissipation, on which a balance condition for the SGS dissipation across an arbitrary grid scale is founded, is shown to be independent of the turbulence Reynolds number, and can be described by a function in terms of the averaged mesh Reynolds number. Such a balance condition can serve as a physical constraint in the SGS modeling to account for the scale effects of the model coefficient(s). Scale-adaptive dynamic Smagorinsky-Lilly model and mixed nonlinear model are formulated for large-eddy simulation of transitional and/or turbulent flows in such a way that the constraint is satisfied. The newly proposed scale-adaptive dynamic SGS models are validated in simulations of homogeneous isotropic turbulence and turbulent channel flow, and prove to be superior over traditional dynamic SGS models.

The initial vortex aloft and helical structure near the surface

Formation of the helical vortices in different media is an important hydrodynamic problem. To study the evolution of the initial vortex aloft, we use a simple axisymmetric hydrodynamic model. The control parameters of the model are the initial height of the vortex and the turbulent Reynolds number. Simulations show that a vortex having initially only an azimuthal component of velocity is converted into a helical vortex. The initial vorticity drifts downwards to the surface. An external energy source is not included into the model for supporting the initial vortex, so that the energy of the system decreases with time. At sufficiently high turbulent Reynolds numbers, there is enough time to form an intense helical structure near the surface.

Towards colorless distributed combustion regime

Colorless distributed combustion (CDC) is a novel method for efficient and environmentally benign cleaner energy conversion of fossil and biofuels. CDC has been investigated in different configurations and geometries, with support to seek near zero emissions, uniform thermal field, energy savings, low pressure drop, and reduced combustion noise. In this paper, distributed combustion is investigated with focus on the flame-flowfield interaction and the different quantities that affect distributed combustion. The velocity field was obtained using particle image velocimetry (PIV) with focus on mean and fluctuating quantities. The flowfield information helped differentiate between the impact of increasing Reynolds number (through air dilution) and the impact of lowering oxygen concentration (through modeled entrainment). The flowfield information was further processed to give the integral length scale at the flame boundaries. The integral length scale along with the fluctuating velocity is critical to determine turbulence Reynolds number and Damköhler number. Together these numbers identify the combustion regime at which the combustor is operating. This information clearly distinguishes between traditional swirl flames and distributed combustion and helps explain the significant benefits of distributed combustion as it operates in a well-stirred reactor regime. The results revealed that major controllers of the reaction regime are flame thickness and laminar flame speed; both are significantly impacted by lowering oxygen concentration through entrainment of hot reactive species from within the combustor, which is important in distributed combustion. Understanding the controlling factors of CDC is critical for the development and deployment of this novel method for near zero emissions from high intensity combustors and energy savings using fossil and of biofuels for sustainable energy conversion.

A priori and a posteriori analyses of algebraic flame surface density modeling in the context of Large Eddy Simulation of turbulent premixed combustion

ABSTRACTThe flame surface density (FSD) based reaction rate closure is one of the most important methodologies of turbulent premixed flame modeling in the context of Large Eddy Simulations (LES). The transport equation for the Favre-filtered reaction progress variable needs closure of the filtered reaction rate and the subgrid scalar flux (SGSF). The SGSF in premixed turbulent flames has both gradient and countergradient components, where the former is typically modeled using eddy diffusivity and the latter can be modeled either on its own or in combination with the filtered reaction rate term using an appropriate wrinkling factor. The scope of the present work is to identify an explicit SGSF closure for the optimum performance in combination with an already established LES FSD model. The performance of different SGSF models for premixed turbulent combustion has been assessed recently by the authors using a Direct Numerical Simulation (DNS) database of freely propagating turbulent premixed flames with a range of different values of turbulent Reynolds number. The two most promising models have been implemented in the LES code. The modeling methodology identified based on a priori DNS analysis is assessed further a posteriori by comparing the LES simulation results of turbulent methane Bunsen flames with the well-documented experimental data. A significant change of the overall flame speed is not observed for different SGSF models. However, the flame shape and thickness respond to the modeling of SGSF. Considering the fact that the SGSF models have very different characteristics, the overall effect on the LES results in this work is smaller than expected. An extension of a previous a priori DNS analysis provides detailed explanations for the observed behavior.

Nonlinear development of the ekman layer structures

The nonlinear modes of coherent structure development in the Atmospheric boundary layer are investigated. Two-scale model of Atmospheric boundary layer is used in the calculation. The velocity field splits into large-scale profile of horizontal wind velocity and three-scale velocity field. The former depends only on the vertical coordinate. The latter is connected with roll circulation and is subject to the vertical coordinate and the coordinate perpendicular to the roll direction. The influence of turbulence is parameterized by turbulent viscosity. The modification of wind profile by rolls is taken into account. Depending on the Reynolds number, different types of the hydrodynamic instabilities specific to the Atmospheric boundary layer occurred. This appears at the relative orientation of the arising geostrophic wind and roll circulation, and also at the scales and space periods of the structures. As the Reynolds number grows, the mean energy and helicity increase. Within the range of the Reynolds number between 200÷300 the dependence is close to linear, which points to the possibility of utilizing weakly nonlinear theory methods, where perturbation amplitudes increase as Re1/2. The rise of the roll asymmetry followed by remarkable growing of the extreme amplitude of a longitudinal velocity component in the direction opposite to geostrophic wind compared to the amplitudes along the lines of geostrophic wind is detected. Increase of the positive component of helicity by contrast to the negative ones is observed simultaneously. A qualitative comparison between the modeling findings and the measured characteristics of the coherent structures observed in the Atmospheric boundary layer is carried out. In July 2007, these structures were measured by acoustic sounding methods in Kalmykia, where asymmetry in the distribution of longitudinal velocity component was observed as well. An apparent pattern of roll circulation begins to reproduce in the mesoscale atmospheric model RAMS under grid size about 500 meters. Reasonably good correspondence between simulation findings and observable vortex with centers lying about 1200÷1300 meters high is received. The values of turbulent viscosity and effective Reynolds number are typical for unstable stratification conditions.

Impingement jet array heat transfer with small-scale cylinder target surface roughness arrays

The present investigation considers special small-scale roughness patterns on impingement target surfaces to increase surface heat transfer augmentation levels of impingement jet array cooling. Utilized are three different heights of cylinder small roughness elements, which are mounted on three different test surfaces. The cylinder-shaped roughness is small-scale because diameter is 7.5 percent and maximum height is 25 percent of impingement hole diameter. Associated results are compared with spatially-averaged Nusselt number ratio distributions measured on target surfaces with triangle small roughness, or rectangle small roughness. Data are obtained for impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Nusselt number variations for the small cylinder roughness show different trends with streamwise development and changing roughness height, compared to target plates with small rectangle roughness and small triangle roughness. In general, this is because roughness elements which contain surface shapes with sharp edges generate increased magnitudes of vorticity with length scales of the order of the roughness element diameter. Such generation is not always present in an abundant fashion with the small cylinder roughness because of the smooth contours around each roughness element periphery. Such effects are illustrated by several data sets, including Nusselt numbers associated with the small cylinder roughness with a height of 0.250D at a turbulent Reynolds number of 11,000.

Measurements to determine the regimes of premixed flames in extreme turbulence

A new regime of extreme turbulence – defined as the ratio of turbulence intensity to laminar flame speed uʹ/SL from 25 to 243 – was characterized for six premixed flames using a new piloted Bunsen burner (called Hi-Pilot). The flames studied had uʹ/SL values several times larger than those of previous related studies and integral scales and turbulent Reynolds numbers as large as 41mm and 99,000, respectively. Layer thicknesses were determined from planar laser-induced fluorescence (PLIF) images of OH and formaldehyde. Preheat layer thickness was found to increase to sixteen times the laminar value. Residence time of eddies in the flame appears to be important, since the flame tip had preheat regions that were thicker than at the flame base. Reaction layers were not broadened, remaining below twice the laminar value. Four of the cases were predicted to lie in the Broadened Preheat - Thin Reaction layer (BP-TR) regime and the measurements confirmed that they had a BP-TR structure. However, two cases went far beyond the predicted boundary for the Broken Reactions (BR) regime but measurements showed that they were not broken but retained their BP-TR structure. Thus the regime of BP-TR is measured to persist over a wider range than previously predicted. One explanation is that the turbulent eddies may become weakened by the thick, viscous preheat layer before they arrive at the reaction front. Distributed reactions were not observed in the six cases that were selected.

Laser ignition and flame characteristics of pulsed methane jets in homogeneous isotropic turbulence without mean flow

The influence of turbulence on the minimum ignition energy (MIE) and ignited flame characteristics is investigated for pulsed methane diffusion jets ignited by laser-induced plasma. The methane jet is injected in a volume of homogeneous and isotropic air turbulence without mean flow, with the level of turbulence being controlled independently. The study is carried out for a range of fuel jet (Rejet) Reynolds number, namely 1000, 2000, and 3000, and a range of turbulent (Reλ) Reynolds number, namely 0–207. The results show that the position of the maximum intensity of flame emission was randomly scattered due to the fact that the ignited flame is deflected from the nozzle axis by the turbulent velocity fluctuations. The effect is more profound at higher Rejet. The value of the MIE, determined according to 50% ignitibility of mixture, increases by a factor of 2 for an increase of Reλ from 0 to 207 and by a factor of 5 for an increase of Rejet from 1000 to 3000. Two trends are observed on MIE with Reλ. For low Reλ, MIE is independent of Reλ. Past a critical value of Reλ, MIE increases as a linear function of Reλ. This transition occurs at critical values of Reλ,c = 158, 197 and 202 for Rejet= 1000, 2000 and 3000, respectively. The mean value of MIE for ignition before and after transition is a linear function of Rejet. The difference between the mean value of MIE before transition and after transition is around 5mJ for all considered Rejet.