Swirl Generator Research Articles

Influence of perforated triple twisted tape on thermal performance characteristics of a tube heat exchanger

One of the challenging tasks in the development of heat exchangers is to select an appropriate geometry that will consume minimum energy. This could be achieved by designing a suitable enhancing device which will boost the thermal performance characteristics of heat exchangers. Twisted tape swirl generator, a heat transfer enhancement device equipped in a tube heat exchanger would induce turbulence and superimposed vortex motion causing a reduction in the hydrodynamic or thermal boundary layer thickness which results in the improvement of convective heat transfer. In this study, a new design of perforated triple twisted tape (PTTT) insert was applied as a swirl flow device with four different porosities (Rp) of 1.2, 4.6, 10.4 and 18.6% to enhance the convective heat transfer of a tube heat exchanger. The effects of porosity (Rp) of PTTT swirl generators on heat transfer, fluid friction and thermal performance characteristics of a tube heat exchanger were experimentally investigated. The experiments were conducted under constant heat flux condition using air as the working fluid in turbulent flow regime for variation in Reynolds number (Re) from 7250 to 49,800. The Nusselt number of the tube installed with PTTT inserts for porosity ranging from 1.2 to 18.6% was found to be 88–320% higher, whereas, the friction factor was achieved to be 112–355% higher in comparison to the plain tube. The highest heat transfer result of 320% was obtained at a porosity of 4.6% with PTTT insert compared to the smooth tube. Despite the significant enhancement in heat transfer, the friction factor was obtained to be 355% higher in comparison to the plain tube at a porosity of 4.6% with PTTT insert. The experimental results demonstrated that Nusselt number, friction factor, and thermal enhancement efficiency (TEE) were increased with decreasing the porosity of the tape inserts except for 1.2%. The heat transfer performance was evaluated based on constant blower power and the values were found to be 1.13–1.5 relative to the plain tube. The maximum TEE of 1.5 was achieved using PTTT insert with a porosity of 4.6%. Based on the experimental data, the empirical correlations of heat transfer, friction factor and TEE were developed in terms of Rp, Re and Prandtl number (Pr) to predict the heat transfer, friction factor and TEE. The optimal design of PTTTs based on the evaluation of thermal performance with suitable porosity could be an excellent heat transfer enhancement device for many industrial applications.

POD analysis of a vortex rope with transient boundary conditions

The paper presents the results of a POD analysis of the flow in a swirl generator. The generator has two separate inlets – one straight axial inlet and one tangential inlet ended by a spiral case. This allows to set different operating points, which are in turn connected with different shapes of the vortex rope present in the flow field. In this case, the investigation is aimed at linearly varying flow rates at the boundaries, namely, the ratio of the tangential flow rate is increased from 40 % up to more than 80 % of the total flow rate, which is maintained constant. Substituting the POD modes into the energy equation provides additional insight into the energy fluxes between the modes and consequently into a major part of the turbulent flow energy cascade as the input data were gained from a CFD simulation using scale-resolving Stress-Blended Eddy Simulation model. The POD modes can be used to construct a simplified model of the vortex rope dynamics covering a wider operating range rather than just one point.

Numerical study on pressure drop and filtration efficiency of gas–solid flow through axial cyclone separators

The separation of particulate matter from incoming gas flow is necessary for different industrial applications, and this filtration process is primarily done by cyclone separators. In axial inlet cyclone separators, a swirling motion is imparted to the incoming flow by means of a swirl generator and particle separation is achieved by centrifugal separation. In this paper, the gas-particle flow through an axial inlet cyclone separator is analysed by the Eulerian–Lagrangian CFD approach using the open-source package OpenFOAM. The turbulent flow field is modelled using Reynolds-averaged Navier–Stokes equations with RNG k–e model, and the particle trajectory is calculated by discrete phase modelling. The study mainly focuses on the modelling and simulation of the dispersed flow, the development of the turbulent flow field inside the cyclone filter and the influence of the solid phase on the flow field. The simulation results show that the solver effectively models the flow field and the centrifugal separation of particles. The analysis of particle trajectory and the effect of particle density on the flow with both uniform and varying inlet particle distribution are also discussed.

Experimental investigation of vortex ring formation as a consequence of spiral vortex re-connection

The unsteady shape variation of a cavitating spiral vortex is experimentally studied pointing out the repetitive creation of vortex ring (or vortex loop). To generate the spiral vortex structure, a newly designed vortex generator is employed. This generator is used to study the decelerated swirling flow leading to the spiral vortex formation as a consequence of spiral vortex breakdown, which is considered to be the main triggering mechanism for the occurrence of the coherent vortex rope structure in the Francis turbine draft tube operated at part load conditions. Thanks to its design, the vortex generator enables to change the ratio between fluxes of axial momentum and tangential moment of momentum of generated swirl. Using this set-up, the behavior of the vortex structure changes in a similar way as the flow rate variation in the draft tube of Francis turbine. At certain flow conditions, the spiral vortex movement is characterized by sudden spiral entanglement leading to disconnection of a vortex ring and followed by spiral reconnection. Thanks to the transparent diffuser of the swirl generator apparatus, both high-speed camera recording of the cavitating vortex and PIV measurements of velocity fields at cavitationfree conditions are employed. The main aim of this paper is to link the visual observation of the above described vortex dynamics with the velocity fields measured in one longitudinal and one cross-sectional planes.

A Novel Passive Method to Control the Swirling Flow with Vortex Rope from the Conical Diffuser of Hydraulic Turbines with Fixed Blades

In this paper, we introduce a novel passive control method to mitigate the unsteadiness effects associated to the swirling flows with self-induced instabilities. The control method involves a progressive throttling cross-section flow at the outlet of the conical diffuser. It adjusts the cross-section area with a diaphragm while maintaining all positions of the circular shape centered on the axis. It improves the pressure recovery on the cone wall while the pressure fluctuations associated with the self-induced instability are mitigated as it adjusts the cross-section area. It can adjust the diaphragm in correlation with the operating conditions of the turbine. We investigated the passive control method on a swirl generator, which provides a similar flow as a hydraulic turbine operated at a partial discharge. The plunging and rotating components are discriminated using the pressure fluctuation on the cone wall to provide a clear view of the effects induced by this passive control method. As a result, the novel proof of concept examined in this paper offers valuable benefits as it fulfils a good balance between the dynamical behavior and the hydraulic losses.

Silver–water nanofluid flow and convective heat transfer in a microfin tube equipped with loose-fit twisted tapes

Heat transfer enhancement and performance of compact heat exchangers have been extensively studied in the past century for the purpose of promoting energy efficiency. Microfin tubes in single/two/multiple-phase flow heat exchangers into which twisted tape swirl generators are installed can promote heat transfer with a moderate pressure loss penalty. This article reports on the enhanced heat transfer of silver–water nanofluids in a microfin tube into which loose-fit twisted tapes are installed in a counter-flow arrangement. The experiments were carried out using nanofluids with various silver concentrations (0.007–0.03 vol%), loose-fit twisted tapes with clearance ratios (c/D) of 0.0 (tight-fit), 0.05, 0.075 and 0.1, for a twist ratio, y/W, of 2.0. The results indicate that the heat transfer rate (Nu) and pressure drop (f) increase with a decrease in clearance ratio (c/D) and increase in silver (Ag) nanoparticle concentration. Additionally, the thermal performance factor tends to increase with the decrease in Reynolds numbers.

Similarity of the asymmetric swirl generator and a double bend in the near-field range

An asymmetric swirl generator (ASG) is considered to replace the current swirl generator in the upcoming 2020 revision of the standard for heat meters EN 1434. While recent studies have shown its superiority with respect to a reproducible representation of the double bend out-of-plane (DB) flow disturbance in the far field, there are still open questions regarding the similarity in the near-field range and the determination of an optimum testing distance. In this paper, we examine the performance of an ASG in the potential testing range and investigate an increase of the segmental orifice plate. Laser Doppler anemometry (LDA) measurements downstream of the ASGs and a DB carried out at Reynolds numbers (Re) of 5×104 and 5×105 are evaluated based on a comparison of the flow patterns and a quantitative analysis by means of performance indicators. The results indicate, that the original version of the ASG does not reach the level of asymmetry and swirl provided by the DB in the near-field range. An increase of the orifice height yields higher swirl and asymmetry, hence, a better similarity of the flow characteristics. A maximum degree of resemblance with the crescent-shaped velocity patterns of the DB is found seven diameters downstream of the modified ASG. In contrast, a testing distance of two diameters or less does not represent the DB, as the early flow development bears the traces of the disturber's geometry. The results may be considered for the implementation of the test procedure in EN 1434 and the next revision of the standards for water meters.

Improving performance of a trapezoidal-trough thermosyphon solar collector using peripherally wing-cut swirl generator

ABSTRACTIn this present study, the performance of a newly modified solar collector named trapezoidal-trough solar collector (Tra-trough) has been examined using peripherally wing-cut swirl generator (PSG). The experiment has been executed at Government College of Engineering, Salem, Tamilnadu, India (Latitude: 11°66´N and Longitude: 78°15´E) with three different wing shapes including PSG with triangular wings (PSG-Tri), rectangular wings (PSG-Rec) and trapezoidal wings (PSG-Tra). The obtained results are contrasted with plain tube Tra-trough (TT-plain) solar collector under the same working condition. The experimental trials are made by keeping swirl ratio (Y = 3), wing-span ratio (b/W = 0.3) and wing-chord ratio (d/W = 0.4) as constant. The results obtained from the TT-plain are verified with the fundamental equations and found the deviations within ±11.21% for Nusselt number and ±8.24% for friction factor. From the experimental result, the PSG provides higher heat transfer rate over the TT-plain with/without typical swirl generator (TSG). The results also exposed that the PSG with trapezoidal wings (PSG-Tra) offers higher heat transfer rate and thermal efficiency than those given by other type of wing shapes. The thermal performance of trapezoidal wings (PSG-Tra) is enhanced up to 137% over the TT-plain. In addition, correlations for predicting the Nusselt number and friction factor have been developed. The deficiency of the predicted value for Nusselt number and friction factor are within ±10.31% and ±11.21%, respectively.

Addressing the adverse cold air inflow effects for a short natural draft dry cooling tower through swirl generation

Short natural draft dry cooling towers (NDDCTs) are susceptible to cold air inflow. A transient simulation on Gatton tower is carried out to study the time-dependent cold air inflow characteristic. Our results show that, the cold air inflow penetrates inside the tower after a short period of pseudo-steady state, and a steady state with the cold air inflow is finally formed. We also investigate the possibility of inducing swirling motions to counter the cold inflow. The results demonstrate that, by reducing the local vortices caused by the specific tower structure, and thinning the boundary layer thickness, swirl is able to decrease the cold air inflow effect. Finally, feasibility of the suggested approach was verified by comparing the energy required to create the swirl with the extra heat transfer from the heat exchangers in the tower which would have not been materialized because of the cold inflow. It was observed that, an extra 40 kW heat transfer gain can be anticipated if only 1 W is spent to induce the swirl.

The effects of short length and full length swirl generators on heat transfer and flow fields in a solar air heater tube

The effects of short length (SL) and full length (FL) swirl generators on heat transfer and flow fields in a solar air heater tube have been investigated experimentally. Thermal energy transport coefficient performance for different values of the spring ratio, twist ratio, tape thickness ratio and diameter ratio (DR) was studied for Reynolds numbers (Re) ranging between 6000 and 20,000. The tube with swirl generator inserts augments the thermal energy transport coefficient mostly, which is accompanied by a larger pressure penalty. The twisted and spring tape (FL) leads to larger thermal energy transport coefficient than spring and twisted tape (SL). It was found that by using spring and twisted tape (FL) at constant pumping power, heat duty increases up to 22% and 27% in comparison with spring and twisted tape (SL), respectively. Similarly, by using spring and twisted tape (FL) at constant heat duty, pumping power rises up to 27% and 33% in comparison with spring and twisted tape (SL), respectively. A large database has been generated for the thermal energy transport coefficient and thermal–hydraulic performance which is beneficial for the design of solar thermal heaters and heat exchangers.

Orthogonal experimental design of an axial flow cyclone separator

In order to improve the performance of the separation device, a novel kind of cyclone separator is designed. The effects of the structure parameters and operation parameters on pressure drop and separation efficiency are investigated based on orthogonal experimental through the test platform. In addition, the internal flow characteristics in the optimized swirl generator are investigated by numerical simulation. Accordingly, the pressure drop of the optimized swirl generator is 170.9 Pa and its separation efficiency is 68.23% when extraction ratio is 5%. The blade shape and the blade outlet angle can influence the pressure drop and efficiency. The diameter of guiding cone has influence on pressure drop, but has no influence on the efficiency. When the extraction ratio is increased to 10%, the efficiency could be raised to 79.97%, and it has no significant increase at higher extraction ratio. The separation efficiency can reach to 88.10% when it is used to separate 180–250 u m sand. Finally, the simulation reveals that the flow features and distribution in the optimized swirl generator. Therefore, the new-type cyclone separator, which is expected to be applied in separation fields, has great actual using value.

Characteristics of heat transfer of impingement system with swap swirl generator

AbstractThis article presents the computational fluid dynamic simulation of the heat transfer characteristics induced by a swap swirl air‐jet generator on the impingement surface. The study was carried out for conventional and swap twist tape inserts of twisted ratio y = 2.93 with various swap angles (α = 30°, 60°, 90°) at a constant distance of nozzle diameter impingement plate (L = 2D). The results show that the Nusselt number of the swirl impingement air‐jet depends on the twisted tape swap angles and airflow rate. The results also showed that the swap angle of 90° gave notable uniform local heat transfer distribution compared with the typical twist tape and other swap twist tapes (α = 30°, 60°). In addition, the predicted results of the local heat transfer coefficient help explain the local turbulence intensity and generation to assist the industrial applications of swirl impingement air‐cooling jet.

Experimental Study of the Effects of Inclusion of Rectangular-Cut Twisted Tapes on Heat Transfer and Pressure Drop in a Round Tube

Twisted tape inserts were used as swirl generators in the flow through a tube for enhancing heat transfer. Twisted tapes have commonly been applied in many thermal applications used in industry. Several modified twisted tapes have been investigated for improving the thermal performance of heat exchangers. The effects in tubes with modified rectangular-cut twisted tape (RC-TT) inserts with various free-spacing ratios (s/w = 2.0, 2.5, 3.0, 3.5 and 4.0) and edge–width ratios (t/W = 0.1, 0.15, 0.2 and 0.25) on heat transfer and pressure losses are reported. The RC-TT used in this work had a fixed thickness of 1.2 mm, width of 60 mm, length of 2000 mm and twist ratio (y/W) of 3.0. Experiments were performed over a range of Reynolds numbers, 10,000 ≤ Re ≤ 20,000, using air as a test fluid. A tube with a classical/typical twisted tape was also tested for comparison. In all cases, the Nusselt number tended to increase with the Reynolds number, while the friction factor and thermal performance showed the opposite trend. For the rectangular-cut twisted tape inserts, the friction factor and Nusselt number decreased, while thermal performance increased with increasing free-spacing ratios (s/w) and edge–width ratios (t/W). Furthermore, the maximum thermal performance was achieved at the lowest free-spacing ratio (s/w), edge–width ratio (t/W) and Reynolds number (Re).

Nozzle exit conditions and the heat transfer in non-swirling and weakly swirling turbulent impinging jets

Investigations have been conducted into turbulent impinging jets but the exact flow dynamics and mechanisms leading to the observed heat transfer distributions at the impingement plane remain outstanding. In particular, use of different swirl generators (vanes, twisted inserts) means the role of varying inflow conditions (at the nozzle exit plane x/D = 0) should be studied to resolve its role on the observed convective heat transfer trends. The present paper studies axisymmetric turbulent weakly swirling (S = 0.31) jets (D = 40 mm) impinging onto a heated plate. Parameters varied include inflow conditions and the effects of impingement distance (H/D = 2, 4, and 6). The Reynolds Averaged Navier Stokes (RANS) equations are used to model the jets using the k-kl-ω turbulence model, which is benchmarked against other models. Three azimuthal ( ) velocity profiles at a Reynolds (Re) number of 24,600 are used at the nozzle exit plane: Uniform (UP), Solid Body Rotation (SBR), and Parabolic Profiles (PP). The start of the wall jet region, designated through elevated levels of turbulent kinetic energy correlates well with the widely observed first peak in Nu distribution. This is however extremely sensitive to the imposition of any swirl, with the application of even weak swirl (S = 0.31) minimally modifying flow dynamics (in the upstream jet region) and leading to recirculation zones stabilized at the impingement plane. This occurs in near-field impingement (H/D) for some inflow conditions (S031-UP), but not others thereby highlighting the significance varied nozzle and swirl generation methods on trends observed in the literature. The imposition of elevated levels of turbulence at the nozzle inflow (x/D = 0) appreciably modifies the heat transfer distribution, particularly in far-field impingement (H/D = 6).

Wake Analysis of an Aerodynamically Optimized Boxprop High-Speed Propeller

The Boxprop is a novel, double-bladed, tip-joined propeller for high-speed flight. The concept draws inspiration from the box wing concept and could potentially decrease tip vortex strength compared with conventional propeller blades. Early Boxprop designs experienced significant amounts of blade interference. By performing a wake analysis and quantifying the various losses of the flow, it could be seen that these Boxprop designs produced 45% more swirl than a conventional reference blade. The reason for this was the proximity of the Boxprop blade halves to each other, which prevented the Boxprop from achieving the required aerodynamic loading on the outer parts of the blade. This paper presents an aerodynamic optimization of a 6-bladed Boxprop aiming at maximizing efficiency and thrust at cruise. A geometric parametrization has been adopted which decreases interference by allowing the blade halves to be swept in opposite directions. Compared with an earlier equal-thrust Boxprop design, the optimized design features a 7% percentage point increase in propeller efficiency and a lower amount of swirl and entropy generation. A vortex-like structure has also appeared downstream of the optimized Boxprop, but with two key differences relative to conventional propellers. (1) Its formation differs from a traditional tip vortex and (2) it is 46% weaker than the tip vortex of an optimized 12-bladed conventional propeller.

Analysis of coherent structures in the near-field region of an isothermal free swirling jet after vortex breakdown

This work is focused on the near-exit region of a round isothermal free swirling jet to characterize the effect of swirl on the flow field and dominant coherent structures. Most of the work in this field investigated jets produced by vane swirl generators, whereas in our case an axial plus tangential swirl generator without a center-body was used. Stereoscopic PIV has been employed to capture the instantaneous flow field for Reynolds number equal to 21,800 and two swirl levels (0 and 1.4). A proper orthogonal decomposition (POD) applied to the instantaneous velocity vector fields allowed to identify the dominant flow structures associated with the precessing vortex core (PVC). The time coefficients of the most energetic first two POD modes were used to reconstruct the phase of the fluctuating motion in the swirling flow. The phase information was used to conditionally average the instantaneous velocity fields and swirl strength maps; this allowed to reconstruct the 3D structure of the PVC from the 2D data. Overlapping phase slots were used to improve the quality of the reconstructed 3D structure. The minima of negative swirling strength revealed the presence of two co-winding helical structures located in the inner and outer shear layers. The helical structures wind opposite to the base flow rotation and disappear at an axial location of approximately x/D=1.5. Detailed analysis of the mean, phase averaged and POD modes velocity fields are also reported to further highlight the flow field features. The third and fourth transverse POD modes evidenced the presence of a m=2 azimuthal wave buried in the dominant m=1 wave; such wave belong to a vortex pair superimposed to the precessing vortex core. Close to the nozzle exit, the analysis of the coherent and random parts of the velocity fluctuations evidenced that the PVC slightly increases the stochastic velocity fluctuations, but the latter resulted to be more isotropic respect to the non-swirling case. Our results put into evidence that the combined application of PIV, POD and phase averaging techniques can provide detailed observations of coherent fluctuations induced by vortex precession. The latter can help to improve the understanding of PVC and VB and to find possible strategies to prevent/control their formation. Finally, they can be of interest to develop numerical models for the simulation of these complex flows.

Off-Design Analysis of Transonic Bypass Fan Systems Using Streamline Curvature Through-Flow Method

Abstract The two-dimensional streamline curvature through-flow modeling of turbomachinery is still a key element for turbomachinery preliminary analysis. Basically, axisymmetric swirling flow field is solved numerically. The effects of blades are imposed as sources of swirl, work input/output and entropy generation. Although the topic is studied vastly in the literature for compressors and turbines, combined modeling of the transonic fan and the downstream splitter of turbofan engine configuration, to the authors’ best knowledge, is limited. In a prior study, the authors presented a new method for bypass fan modeling for inverse design calculations. Moreover, new set of practical empirical correlations are calibrated and validated. This paper is an extension of this study to rapid off-design analysis of transonic by-pass fan systems. The methodology is validated by two test cases: NASA 2-stage fan and GE-NASA bypass fan case. The proposed methodology is a simple extension for streamline curvature method and can be applied to existing compressor methodologies with minimum numerical effort.

Experimental Investigation of Nox Emission in a Combustor with One and Two Sequential Combustion Stages

This article is the first of a cycle dedicated to the experimental investigation and process optimization of the combustion of an air-fuel mixture with gas temperatures at exit from the combustion chamber of approximately 1700 °C, while satisfying the requirements relating to emissions. The article is divided into two stages. The first stage examines the possibilities of obtaining minimal NOx and CO concentrations when using a traditional (single-stage) system for combustion of a prepared air-fuel mixture (AFM). It presupposes the presence of one burner unit (BU) through which fuel and air enter the combustor (COMB), and the combustion process is executed in flame stabilization zones that are formed in it due to swirl generators and/or bluff bodies. For such a system, the effect on NOx and CO emissions over the range of exhaust temperatures from 1300 to 1700°C (CC excess air coefficient from 2.2 to 1.6) from the following was studied: the quality of preparation of the AFM, the pilot burner air, the flow speed in the COMB, and the air temperature at intake. The second stage examined the possibility of reducing harmful emissions due to the use of a two-stage sequential plan of fuel combustion. This presupposes the presence of two BUs, one of which, a traditional one (BU1), is placed at the inlet face of the COMB; and a second, sequential one (BU2), on the side walls of the COMB below the flow. Here, results are produced in testing BU2, in the form of an annular sudden flow constrictor with fuel-injection apertures arranged on its circumference. The total fuel flow is partitioned between the stages. In the first stage, combustion occurs for fuel in the form of the AFM, and in the second the remaining fuel is combusted in the hot flow of gases of the first stage, depleted by oxygen. The effect of the AFM temperature intake to the first stage and the speeds of its feed to the COMB on NOx and CO emissions over the range of exhaust temperatures from 1300 to 1700°C is studied for this system. The results were obtained in tests on a model CC at a test station of JSC VTI. Multivariate parametric studies on this subject were published earlier [1–3].

On the influence of circulation on fire whirl height

This work presents a numerical study of laboratory fire whirls. The Reynolds Stress Transport turbulence model RST is used together with Eddy Break-Up combustion and radiation models. The unsteady 3D fire whirl prediction assume the same swirl generator used in the laboratory consisting on enclosure walls to constrict airflow to enter tangentially in the facility. A low momentum fuel jet is located at the bottom centre of two halves of an offset hollow cylinder. A detailed comparison between available experimental data and numerical predictions, ranging from 2 to 300 kW is presented. The results are in good agreement with the available experimental data. The flame height dependence on circulation was deduced using scaling laws and validated with experimental data. For each heat release considered, a critical circulation was found that results in a maximum flame height. Turbulence fields predictions are discussed in light of the role of turbulent suppression and Richardson parameters.

Burner geometry effect on coherent structures and acoustics of a confined swirling partially premixed methane flame

The effect of the mixing tube length and fuel nozzle geometry on coherent structures and acoustics’ characteristics of partially premixed swirling methane flame is experimentally studied. Particle image velocimetry (PIV) is used to document the flowfield inside the transparent confinement tube. Proper orthogonal decomposition along with phase averaging are used to capture coherent structures. High-speed imaging is carried out to shed light on the flame front dynamics within the outer shear layer (OSL). The acoustic waves are measured using a Bruel & Kjaer type 4189 microphone. A radial-type swirl generator with a swirl number (S) of 1.15 is adopted. Two different lengths of the mixing tube and two different geometries of the fuel nozzle with a similar equivalent diameter are tested. The swirling airflow and fuel flowrates are kept constant. The results showed a significant difference in the energy of coherent structures between the two central nozzle geometries with the same mixing length. Both, the mixing length and central nozzle geometry, are found to alter the integral length scale and the central recirculation zone (CRZ). The mixing length is found to alter the amplitude of the acoustics modes when using the same central nozzle geometry. However, keeping the same mixing tube and varying the central nozzle geometry has the same effect. The flame front dynamics within the outer recirculation zone (ORZ) is found to depend on both the central nozzle geometry and mixing tube length (i.e., level of mixing). High amplitude coherent structures are found to contribute to flame roll-up within the outer shear layer. The results showed that the mixing length is not the only parameter to mitigate combustion instabilities. That is, combustion stability can be improved using a single-orifice nozzle with a long mixing length or a multi-orifice nozzle with a short mixing length.