Jet Reynolds Number Research Articles

Maximum Wall Stress on a Smooth Flat Plate Under Planar Jet Impingement

Abstract This paper investigates the maximum wall shear stress value τmax and its location xmax as measured on a smooth flat plate impinged upon by a normal planar jet. τmax and xmax are found to be closely related to the stagnation pressure Ps and the half-width of the mean wall pressure profile bpw. The measurements were made by two different techniques: a Stanton probe and oil film interferometry. The maximum wall shear stress location xmax is found to be independent of the jet Reynolds number. At a small nozzle-to-plate distance H≲6 Djet, xmax is related to the jet slot width as xmax≈1.1Djet. At a large nozzle-to-plate distance H≳6 Djet, the maximum wall shear stress location is related to the mean wall pressure half-width as xmax≈1.4 bpw. A new Reynolds number, referred to as the stagnation Reynolds number, is defined as Res=def2bpwPs/ρ/ν, where ρ is the fluid density and ν is the kinematic viscosity. The maximum wall shear stress is found to be strongly influenced by the stagnation Reynolds number, and the dependence as measured by Stanton probes is approximated by a power law of τmax/Ps≈0.38/Res0.38. The solution of the laminar flow equations in the Appendix gives an alternate relation for τmax, which is in better agreement with the oil film interferometry measurements. Dimensional analysis is performed to gain insight into the empirical findings.

Effects of the nozzle contraction angle on particle flow behaviors in a gas-particle two-phase jet

The particle dynamics of a conical jet with different contraction angles (the cone angle of the contraction wall, α = 20°, 40°, 60°, 80°) were investigated using particle image velocimetry (PIV) and optical fiber probes. The axial and radial distributions of particle-phase velocity and concentration were measured within a confined chamber at the mass loading ratio mp = 0.3–3.3 and jet Reynolds number Re = 11,843–104,497. Due to the nozzle contraction effect, the particle dynamics of conical jet are significant differences from the pipe jet and the smooth contraction jet. In the axial direction, there is a strong slip velocity between two phases near the nozzle exit (x/D = 0–6), which leads to the occurrence of particle acceleration. In the radial direction, the particles preferentially converge towards the centerline, forming the particle contraction, which intensifies the momentum transfer from the axial to radial direction and thus increases the particle dispersion. The particle acceleration and contraction are more pronounced with the increase of contraction angle. Furthermore, the order of the velocity decay rate and the spreading rate for different contraction angles are 40° > 80° > 60° > 20°. The nozzle contraction angle of 40° shows the best entrainment and mixing characteristics. In addition, the particles contraction near the nozzle exit as well as the particles accumulation in the wall region (r/R = 0.83–1.00) increase the non-uniformity of particle concentration.

Experimental Study of a Novel Twin-Nozzle Configuration for Reacting Jet in Hot Crossflow

Abstract Advanced gas turbine adopts axially staged combustion to achieve the goal of increasing turbine inlet temperature while limiting NOx emissions. The premixing effect of secondary fuel injection has a significant influence on secondary combustion organization and flame-dynamic characteristics. In this paper, we proposed a novel twin-nozzle configuration for secondary fuel injection. Secondary fuel is injected from the front nozzle, and air is injected from the rear nozzle. Operation condition studied includes the diameter (d) of front and rear nozzle from 1 mm to 3 mm, jet Reynolds number from 1900 to 5700, the jet spacing L ranges from 2d to 4d, and the equivalence ratio of primary stage from 0.72 to 0.59. This flexible configuration controls the injection of fuel and air separately and allows fully lifted flame front organization, which is crucial for fuel/air mixing and NOx control. Using high-speed CH* imaging, the effects of primary stage equivalence ratio, nozzle diameter, and rear air injection ratio on the dynamical characteristics are investigated. We discussed the flame propagation mechanism, flame base pulsation frequency, ignition delay distance, and heat release distribution. We found that when the jet Reynolds number is reduced from 5700 to 1900, the flame pulsation frequency rises from 176 Hz to 586 Hz. When the rear air injection ratio increases from 0 to 3, the pulsation frequency decreases from 586 Hz to 88 Hz, the flame lift-off height increases, and the ignition delay distance decreases.

Effect of Pin Fins on Jet Impingement Heat Transfer Over a Rotating Disk

Abstract Many engineering applications consist of rotating components that experience high-heat load. Applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low-pressure region created due to the radially outward pumping of fluid close to the rotating surface. The present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas-turbine disk. The experiments were conducted at rotational Reynolds numbers (ReR) of 5487–12,803 based on the disk diameter (D) and jet Reynolds numbers (Re) of 5000–18,000 based on the jet diameter. The effects of nozzle to target spacing (z/d=2−6), eccentricity of impingement (ε = 0−0.67), angle of impingement—both toward and away from the center (θi and θo=0 deg−20 deg), and the pin fin height (Hf=3.05 mm−19.05 mm) were studied. Steady-state temperature measurements were taken using thermocouples embedded in the disk close to the target surface, and area average Nusselt number (Nu) was calculated. The results have been compared with those for a smooth aluminum disk of equal dimensions and without any pin-fins. The average Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower values of Re, and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re=5000. In the impingement dominant regime, the effect of disk rotation was minimal for a smooth disk, but the heat transfer increased with rotational speed in case of pin-fins. There was no impact of eccentricity on Nu for ε = 0 and 0.33. For ε = 0.67, the maximum reduction in enhancement over a smooth surface (21.95%) was observed when compared with coaxial impingement for stationary impingement for Re=18,000 and z/d=4. The effect of inclination angle was insignificant, and no clear trend could be established. Higher heat transfer rates were observed for z/d=6 with the increasing Re, and this effect diminished with the increase in the rotational speed. With the increase in pin-fin height, especially at higher values of Re, there was in increase in the value of Nu. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the heat transfer enhancement.

Vortex breakdown in variable-density gaseous swirling jets

Theoretical predictions and numerical simulations are used to determine the transition to bubble and conical vortex breakdown in low-Mach-number laminar axisymmetric variable-density swirling jets. A critical value of the swirl number $S$ for the onset of the bubble ( $S^*_B$ ) and the cone ( $S^*_C$ ) is determined as the jet-to-ambient density ratio $\varLambda$ is varied, with the temperature dependence of the gas density and viscosity appropriate to that of air. The criterion of failure of the slender quasi-cylindrical approximation predicts $S^*_B$ that decreases with increasing values of $\varLambda$ for a jet in solid-body rotation emerging sharply into a quiescent atmosphere. In addition, a new criterion for the onset of conical breakdown is derived from divergence of the initial value of the radial spreading rate of the jet occurring at $S^*_C$ , found to be independent of $\varLambda$ , in an asymptotic analysis for small distances from the inlet plane. To maintain stable flow in the unsteady numerical simulations, an effective Reynolds number $Re_{eff}$ , defined employing the geometric mean of the viscosity in the jet and ambient atmosphere, is fixed at $Re_{eff}=200$ for all $\varLambda$ . Similar to the theoretical predictions, numerical calculations of $S^*_B$ decrease monotonically as $\varLambda$ is increased. The critical swirl numbers for the cone, $S^*_C$ , are found to depend strongly on viscous effects; for $\varLambda =1/5$ , the low jet Reynolds number (51) at $Re_{eff}=200$ delays the transition to the cone, while for $\varLambda =5$ at $Re_{eff}=200$ , the large increase in kinematic viscosity in the external fluid produces a similar trend, significantly increasing $S^*_C$ .

Momentum boundary-layer characterisation from a pulsed impinging jet

The development of the unsteady boundary layer produced from an impinging jet with a temporal puslation forms part of the complex flow physics occurring within an internal combustion engine and its characterisation is crucial for improved modelling with lower-fidelity tools. The momentum boundary layer prediction of these flows currently relies on wall-modelling based on the logarithmic law of the wall for equilibrium zero pressure gradient boundary layers. However, it is not fully understood how these non-equilibrium boundary layers evolve and whether the reliance on the law of the wall is justified for modelling such flows. Thus, in this paper, we attempt to characterise the momentum boundary layer developing from a pulsed impinging jet through the use of highly-resolved large eddy simulations of a canonical setup. The simulations are conducted for three peak jet Reynolds numbers of 10,000, 20,000 and 50,000, achieved at the peak of the temporal pulse. The resulting boundary layer produced by the jet impingement is examined for the mean flow, the scaling of the outer-layer and the boundary layer parameters, such as the boundary layer thickness, the friction Reynolds number and the wall-scaled profiles. The effect of the jet Reynolds number on the different parameters is also evaluated statistically as well as temporally. The results show that the boundary layer’s development is highly unsteady, even statistically, with a large variation in the friction Reynolds number over the statistical sampling period. However, most of the values were found to be lower than the values seen for turbulent boundary layers. This leads to an at least 40% overprediction in the wall-scaled velocity when using the equilibrium law of the wall for modelling these boundary layers. The predictions of the wall-friction using the equilibrium law of the wall and select turbulence models was also tested and it was found that the latter could be useful as a recourse to the near-wall modelling of these flows.

Experimental investigation of heat transfer characteristics of an inverse diffusion flame in a coaxial tube burner for with and without swirl

Heat transfer by flame jet impingement is widely used in many of the industrial and domestic applications like heating metal bars, scrap melting, shaping the glass, metal slab cutting, domestic cooking and others. Aim of the present experimental work is to study an effect of swirl on a local heat flux distribution of an inverse diffusion flame (IDF) jet impinging on a flat target surface in a coaxial tube burner. The twisted tape of twist ratio 3 (corresponding to the swirl number, S = 0.52) is used to create a swirl in the central air jet of the burner. The flame shapes and heat flux distributions are compared for the with and without swirling IDF under the different air jet Reynolds number (Rea) of 1000 to 2500, equivalence ratio (ϕ) of 0.4 to 1.3 and a burner-to-impingement plate distance (H) of 10 to 100mm. The distributions of heat fluxes are studied within a radius of 75 mm from the point of stagnation on an impingement plate. Results show that the swirling IDF helps in clean combustion of the fuel with much shorter flame height. Swirling in the flame jet enhances the peak heat flux for the higher air jet Reynolds number for slightly fuel rich conditions of ϕ = 1.1 at the optimal burner-to-target plate distance of 40 mm.

A fully dynamic mixing time‐scale model for the sparse Lagrangian multiple mapping conditioning approach

Multiple mapping conditioning / large eddy simulation (MMC-LES) is a promising model for turbulence-chemistry interactions, successfully applied to simulate different combustion regimes involving gaseous, liquid, and solid fuels. MMC-LES attempts to produce accurate molecular mixing by localising mixing in an independent, composition-like reference space. In the MMC mixing model, mixing between two stochastic particles is defined based on the Lagrangian mixing time-scale τL, which is defined as the ratio of the Lagrangian subgrid-scale mixture fraction variance (fL′2˜) and the scalar dissipation rate (χL), typically modelled algebraically, based on fixed values of global constants at all spatial locations. Accurate modelling of τL is essential towards predicting the conditional scalar fluctuations, which is otherwise highly challenging if the same set of static constants are used at all spatial locations for complex flames with varying extinction/re-ignition. Herein, a fully dynamic mixing time-scale model (dyn-aISO) is proposed within the MMC-LES framework, which dynamically evaluates fL′2˜ and χL, essentially based on a local variance coefficient and variable turbulent Schmidt number, respectively, eliminating the need for specifying tuneable model constants. This novel dyn-aISO model is used to simulate the pilot-stabilised Sandia flame series with increasing local extinction levels. The conditional and unconditional profiles of compositional scalars from the new dynamic model are compared with those from a fully static model (st-aISO) and a model which uses a static constant for fL′2˜ (stCf-aISO). Numerical results from different timescale models deviate from each other for Sandia flame F having a high jet Reynolds number. With the proposed improvement of the micro-mixing time-scale model, the dyn-aISO model accurately captures the transient behaviour of the complete flame series, which could otherwise be obtained by the tuning of static constants. The role of molecular and turbulent mass diffusivities, coefficient of residual variance of mixture fraction, mixing distance, and the mixing time-scale on the predictions of the reactive scalars is also explored.

Heat Transfer Enhancement of Impingement Cooling by Different Crossflow Diverters

Abstract Impingement cooling can effectively disperse local heat load, but its downstream heat transfer is always reduced due to crossflow effect. In this study, the flow and heat transfer characteristics of impingement cooling with semi-circular (SC), semi-rectangular (SR), semi-diamond (SD), and semi-four-pointed star (SFS) crossflow diverters are compared over the ReD ranging from 3,500 to 14,000 by solving three dimensional Reynolds-averaged Navier–Stokes equations with SST k–ω turbulence model. It is found that the arrangement of crossflow diverters changes the distribution of local jet Reynolds number (ReD,j/ReD) and reduces the mass velocity ratio of downstream crossflow to jet (Gcf/Gj), so the impingement heat transfer is enhanced significantly. However, friction loss also increases. Overall evaluation reveals that all crossflow diverters can improve the comprehensive heat transfer performance parameter (Φ), and the maximum increases of Φ are 11.0%, 14.3%, 12.2%, and 14.7% for SC, SR, SD, and SFS cases, respectively. It is noted that the Nusselt number of heated SFS-shaped diverter surface is also the highest. Besides, the influences of streamwise location (L) and thickness (t) of SFS-shaped diverter are also investigated. Results show that when the L increases from 2D to 3D, the heat transfer and friction loss change slightly; when the L increases from 3D to 4D, the heat transfer decreases sharply, and friction loss increases seriously. As for the t, it has almost no effect on the flow field and heat transfer.

Effect of Jet-to-Jet Distance and Pipe Position on Flow and Heat Transfer Features of Active Clearance Control Systems

Abstract The goal of this work is to investigate the effect of supply pipe position on the heat transfer features of various active clearance control (ACC) geometries, characterized by different jet-to-jet distances. All geometries present 0.8 mm circular impingement holes arranged in a single row. The jets generated by such holes cool a flat target surface, which is replicated by a metal plate in the experimental setup. Measurements are performed using the steady-state technique, obtained by heating up the target plate thanks to an electrically heated Inconel foil applied on the side of the target opposite to the jets. Temperature is also measured on this side by means of an IR camera. Heat transfer is then evaluated thanks to a custom-designed finite difference procedure, capable of solving the inverse conduction problem on the target plate. The effect of pipe positioning is studied in terms of pipe-to-target distance (from 3 to 11 jet diameters) and pipe orientation (i.e., rotation around its axis, from 0 deg to 40 deg with respect to target normal direction), while the investigated jet Reynolds numbers range from 6000 to 10,000. The obtained results reveal that heat transfer is maximized for a given pipe-to-target distance, dependent on both jet-to-jet distance and target surface extension. Pipe rotation also affects the cooling features in a nonmonotonic way, suggesting the existence of different flow regimes related to jet inclination.

Pulsed impinging jets: Momentum and heat-transfer

With increasing efficiencies of internal combustion engines, the accurate prediction of the heat transfer across walls, such as from temporally varying impinging jets, is of significant importance. To avoid the excessive cost of fully resolved simulations, this prediction often relies upon the modelling of the resulting non-equilibrium boundary layer using wall functions. Given the unsteady nature of the problem, it is important to understand the development of these boundary layers under non-equilibrium conditions and assess whether existing wall functions are suitable for modelling such flows. Thus, in this paper, the momentum and thermal boundary layers developing from a single pulsed impinging jet are investigated through the use of highly-resolved large eddy simulations of a representative canonical set-up. The simulations are conducted for three jet Reynolds numbers of 20,000, 50,000 and 100,000, based on the peak velocity of the temporal pulse. The jets are simulated at temperature ratios of 0.25 and 0.8, where the ratio is defined as the temperature of the wall to the centreline temperature of the jet. The resulting boundary layers produced by the jets’ impingement are examined in terms of the mean flow, the wall shear stress, wall heat-flux, and the wall-scaled profiles of velocity and temperature. Where possible, the results are used to test select wall functions, and it is observed that the momentum boundary layer is over-predicted with the classic log-law of the wall while the thermal boundary layer can be predicted to a certain degree with certain wall functions, particularly in cases where the Reynolds analogy is valid for the data used to compute the temperature profile.

Jet Impingement Heat Transfer Characteristics with Variable Extended Jet Holes under Strong Crossflow Conditions

In this paper, detailed flow patterns and heat transfer characteristics of a jet impingement system with extended jet holes are experimentally and numerically studied. The jet holes in the jet plate present an inline array of 16 × 5 rows in the streamwise (i.e., the crossflow direction) and spanwise directions, where the streamwise and spanwise distances between adjacent holes, which are normalized by the jet hole diameter (xn/d and yn/d), are 8 and 5, respectively. The jets impinge onto a smooth target plate with a normalized distance (zn/d) of 3.5 apart from the jet plate. The jet holes are extended by inserting stainless tubes throughout the jet holes and the extended lengths are varied in a range of 1.0d–2.5d, depending on the jet position in the streamwise direction. The experimental data is obtained by using the transient thermochromic liquid crystal (TLC) technique for wide operating jet Reynolds numbers of (1.0 × 104)–(3.0 × 104). The numerical simulations are well-validated using the experimental data and provide further insight into the flow physics within the jet impingement system. Comparisons with a traditional baseline jet impingement scheme show that the extended jet holes generate much higher local heat transfer levels and provide more uniform heat transfer distributions over the target plate, resulting in the highest improvement of approximately 36% in the Nusselt number. Although the extended jet hole configuration requires a higher pumping power to drive the flow through the impingement system, the gain of heat transfer prevails over the penalty of flow losses. At the same pumping power consumption, the extended jet hole design also has more than 10% higher heat transfer than the baseline scheme.

Effect of channel orientation on heat transfer in a rotating impingement cooling channel

The jet impingement cooling technology is usually applied in the turbine blade leading edge which suffers high thermal loads. In a high-speed rotating turbine blade, however, the channel orientation (β, the angle between the direction of the jet and the rotating plane) could alter the jets and heat transfer. Therefore, the effects of channel orientation (from 90∘ to 180∘) on heat transfer of confined jet impingement cooling are experimentally studied under an iso-thermal boundary. Both the jet-to-target surface distance and the jet-to-jet spacing are 3 times of the jet diameter. The jet Reynolds number varies from 5000 to 15,000, and the maximum jet rotation number reaches 0.24. The results show that the rotational effect on heat transfer is different in each channel orientation. In the cases of 90∘≤β≤ 135∘, a large vortex generated by the Coriolis force in the supply channel significantly changes the mass-flowrate distribution in each jet orifice. Hence, the heat transfer in the middle part decreases with the rotation number whereas the heat transfer at both ends increases, resulting in a “U-shape” pattern of heat transfer in the impingement channel. In the cases of 135∘<β≤ 180∘, however, the heat transfer throughout the stagnation zone decreases with the rotation number. The rotation-induced jet diversion is the primary factor that weakens the heat transfer, and jets are driven away from the target surface at high rotation numbers. Moreover, the heat transfer correlation on the stagnant zone is developed, which considers the effects of channel orientation and rotation number.

Comparison of the mean skin friction with the heat transfer and the flow development in turbulent planar wall-attaching jet flows.

Measurements of the mean skin friction below turbulent planar wall-attaching offset jet flows were performed using oil-film interferometry for jets initially offset (Hs) 0.2 to 1 times the jet height (Hj) from the wall. The results were compared with turbulent flow field measurements and with previous heat transfer measurements. The profiles of the mean skin friction coefficient below the offset or preattachment region were similar when compared relative to the distance to the reattachment location (Xr). The initial recovery length of the mean skin friction downstream of the reattachment location was proportional to the reattachment length similar to the decrease in the wall pressure fluctuations in this region. The longer recovery and initial decay of the mean skin friction coefficient appeared to collapse in terms of (x-Xr)/Hj; as did the initial change in the mean static pressure, the measures of the mean flow development, and the Stanton number downstream of the reattachment point. The decay in the maximum of the turbulent stresses associated with the attached shear layer also collapsed in terms of (x-Xr)/Hj, unlike the fluctuating pressure and initial recovery of the mean skin friction. The change in the location of the minimum and zero in the turbulent shear stress as the flows transitioned to the wall jet region collapse in terms of x-Xr/Hj similar to the longer recovery and initial decay of the mean skin friction. The mean skin friction for flows with Hs of Hj and jet Reynolds numbers (Rej) of 21800 to 54500, varied as Rej-0.2 below the attaching jet, in the reattachment region, and in the downstream wall jet region.

A detailed analysis of flow and heat transfer characteristics under a turbulent intermittent jet impingement on a concave surface

A computational study is carried out of the 3-D flow field and heat transfer under a turbulent intermittent circular jet impingement on a concave surface. The control-volume procedure with the SIMPLE algorithm is employed to solve the unsteady RANS (use full form) equations. The RNG k-? model is implemented to simulate turbulence due to its success in predicting similar flows. The numerical results are validated by comparing them with the experimental data. The effects of jet Reynolds number and oscillation frequency on the flow and heat transfer are evaluated. The profiles of instantaneous and time-averaged Nusselt numbers exhibit different trends in axial, x-directions and circumferential, s-directions. It is found that increasing frequency from 50 to 200 Hz results in considerable time-averaged Nusselt number enhancement in both axial and curvature directions. The intermittent jet at a frequency of 200 Hz enhances the total average Nusselt number by 51.4%, 40%, and 33.7% compared to the steady jet values at jet Reynolds numbers of 10000, 23000, and 40000, respectively. In addition, a correlation for the average Nusselt number is proposed depending on the Reynolds number and the Strouhal number.

Thermocouple response time estimation and temperature signal correction for an accurate heat flux calculation in inverse heat conduction problems

Many heat transfer processes do not allow temperature measurements with advanced instruments like IR cameras, and thus require the use of thermocouples – often inserted sheathed thermocouples – in order to measure the temperature in some key-regions. Nevertheless, thermocouples have an intrinsic response time that can dampen substantially temperature measurements and, when applicable, affect heat flux estimations by inverse methods. In this study, a thermocouple measurement correction method is proposed, especially for fast thermal transients like quenching, to avoid underestimation of the boundary heat fluxes due to delayed temperature responses. A simplified energy balance at the hot junction allowed modeling the temperature measurement by a thermocouple, in which heat losses and the response time were taken into account so they can be estimated using the least squares method on experimental data. A series of tests was performed to validate the present method with a heated jet impinging on a copper body instrumented with two thermocouples: one ungrounded and sheathed, then with high response time (”slow”); and another with exposed welded wires, hence with low response time (”fast”). After having estimated the slow thermocouple response time and heat loss parameters, the model allowed a good reconstruction of the fast thermocouple signal using the slow thermocouple measurements. Also, the heat flux estimation by the inverse method using the reconstructed signal resulted in practically the same results obtained using the fast thermocouple data. A parametric sensitivity analysis showed the heat loss parameters can be neglected for heat flux estimations, while simulations showed that temperature noises degrade the response time estimation but data filtering can mitigate this noise effect. Finally, the present method was applied to a large scale jet-cooling of a hot plate – near industrial conditions – and showed a substantially higher dissipated heat flux than the initially estimated. In fact, the results using reconstructed signals demonstrated that the heat flux dissipated by the jet has a higher dependence on the jet Reynolds number than the observed with the original thermocouple measurements, showing how important the temperature measurement correction is to evaluate fast thermal processes.

Experimental study of the heat transfer of single-jet impingement cooling onto a large heated plate near industrial conditions

Jet impingement cooling has been intensively studied in the past by many authors because of its important application in the metallurgical industry; however, most of the experiments in the literature are at laboratory scale and, in some cases, not near industrial conditions. In this study, we performed cooling experiments near industrial conditions in a new experimental apparatus with a large nickel plate as test sample, which was heated until 850 ∘C before being cooled by a single circular water jet. Five experimental results are presented with different jet Reynolds numbers, from 9,800 to 120,000, obtained by varying the water flow rate and the nozzle diameter. The presented results are: temperature evolutions during cooling, dissipated heat flux (estimated by solving a 2D inverse heat conduction problem), transverse heat flux at the heat-exchanging surface, and the characteristics (position and velocity) of the rewetting and maximum cooling rate fronts. The increase in the jet Reynolds number increased slightly the heat flux at the stagnation zone, but increased it substantially for positions farther from the impact location. The transverse heat flux increased with the passage of the rewetting front and then decreased, and its magnitude was practically the same regardless of the jet Reynolds number. The change in the nozzle diameter did not affect significantly the heat transfer nor the rewetting front growth, although the heat dissipation was slightly higher with the smaller nozzle, possibly because of the higher jet impact velocity. Finally, we compared the present results with some found in the literature in similar conditions, showing that laboratory experiments are valuable to provide detailed information, especially near the stagnation zone, but large-scale experiments allow obtaining macroscale data of the cooling process.

Shape and stability characteristics of hydrogen-enriched natural-gas oxy-flames in a micromixer burner

ABSTRACT This study investigated experimentally the effects of H2-enrichment on premixed oxy-methane flames stabilized on a micromixer-like burner for clean energy production in gas turbines. The stable combustion zone was established over ranges of fuel hydrogen fraction (HF: 25–70% by vol.) and equivalence ratio (ϕ: 0.1–1) at fixed oxidizer oxygen fraction (OF = 30% by vol.) and fixed jet velocity (5.2 m/s). The blowout limit was plotted within the HF-ϕ space, superimposed on the contours of some operating parameters, namely adiabatic flame temperature (AFT), combustor power density (PD), jet Reynolds number (Re), and unburnt-mixture density (ρmix). Furthermore, images of visible flame shape were acquired at distinct points within the stable zone to study the effects of ϕ, HF, AFT, PD, Re, and ρmix on flame macrostructure. Results show that H2-enrichment extends the blowout limit from ϕ = 0.35 down to ϕ = 0.1, thereby widening the stable combustion zone. The blowout limit has little correlation with AFT and PD but correlates strongly with ρmix, i.e., occurs consistently at the same ρmix. The analyses of blowout limit and flame images were used to infer that the flames blow out possibly at constant ignition delay.

Turbulent jet ignition of ultra-lean methane/air mixture under engine-like condition

Turbulent jet ignition (TJI) is a promising technology that enables engines to operate stably at ultra-lean-burn conditions for improved thermal efficiency. In the present work, TJI of an ultra-lean methane/air mixture (excessive air ratio λ = 2.0) under an engine-like condition (temperature of 900 K and pressure of 5.8 MPa) was numerically studied, with particular interest on the ignition patterns and the critical/favorable conditions for successful ignition/rapid combustion. A high-fidelity large eddy simulation was performed, with the linear eddy model being incorporated to account for turbulence–chemistry interaction. To represent the different amount/state of energy released from a pre-chamber, a variety of turbulent jet temperatures, inlet velocities, and orifice diameters were covered. The results identified two successful ignition patterns, including a flame propagation pattern and a jet re-ignition pattern. An ignition regime was also established, with the boundary between ignition success/failure identified by a global Damköhler number (Da) of 0.1. In terms of the combustion rate, heat release in the propagative flame is governed by the growth of the flame surface, which is primarily controlled by stretch and enlarges with the jet Reynolds number (Re). The present results suggest that, for low-reactivity fuels such as methane, the pre-chamber combustion in a TJI system should pursue complete heat release and increase Re and Da simultaneously for the injected turbulent jet into the main combustion chamber.

Heat transfer and flow velocity study of a row of jets emerging from a perforated pipe at a low Reynolds number

Aerodynamic and thermal experimental investigations were carried out on a row of circular low Reynolds jets emerging from a perforated pipe. Particle Image Velocimetry and Infrared Thermography were performed to determine flow velocity fields and local Nusselt number variations along the impinged plate. Jet Reynolds number (1500<Rej<5000), upstream crossflow in the pipe (1.5<VR<9.7), injection-to-plate distance (3<H/d<7) as well as center-to-center jet spacing (2.25<p/d<5.8) were all taken into account. Measurements were compared to those of a fully developed circular jet issuing from a long tube under the same conditions (3000<Rej<5000 and 3<H/d<7). Experimental results showed that jets tend to emerge with an effective cross-sectional area reduced to a crescent shape unlike that of axisymmetric fully developed jets; their specific structure was attributed to flow deflection from the pipe to the holes and resulted in heat transfer specificities differing from those of a fully developed jet. Heat transfer rates were found to be primarily dependent on injection Reynolds number, injection-to-plate distance and center-to-center spacing, and they were found to be relatively low compared to a fully developed jet.