Addition Of Swirl Research Articles

Influence of twisted tape insert on the coolant flow characteristics in swirled film cooling

The present paper discusses film cooling behavior through numerical simulation in the presence of a twisted tape insert inside the film hole. The twisted tape insert imparts a swirl to the coolant flow. Coolant swirl intensity is controlled by varying the pitch of the twisted tape resulting in swirl numbers of 0.0289, 0.116, and 0.168. The film cooling performance is evaluated using area-averaged effectiveness and heat transfer coefficient for blowing ratios of 0.5, 1.0, 1.5, and 2.0. Results revealed a significant amount of improvement in averaged effectiveness with the addition of swirl. Coolant swirl predominantly modifies the jet trajectory resulting in a reduced jet penetration and increased lateral expansion. Further investigation on the effect of twisted tape thickness on the coolant distribution has been found to be negligible. Pressure losses occurring due to the insertion of twisted tape inside the film hole is evaluated through the coefficient of discharge which indicated the necessity of higher pumping power than the film cooling case with no-swirl.

Non-Premixed Combustion Simulation with Variation of Swirl Burner Slope using Turbulence Modeling K-ɛ

Combustion is a very complex matter. Therefore, we need to know the phenomenon that occur in combustion apart from the temperature distribution. This research uses quasi-research. The background in using quasi-experimental is because it has advantages in data collection. Moreover, the phenomena that occur in more detailed research by means of a numerical approach could also be observed. This research was conducted with an approach (simulation) of diffusion combustion using methane (CH4) fuel and air containing 29% oxygen (O2), 71% nitrogen (N2) and the addition of swirl in the air ducts. k-ɛ turbulence modeling was aaplied in this somulation. This variation of the fuel velocities are 3 m/s, 5 m/s, 7 m/s, where the air velocity was varied for 3 m/s, 5 m/s, 7m/s. In this simulation, room pressure of 1 atm was applied for the surrounding. The results showed that, the greater the angle of inclination of the swirl burner, the shorter the flame. And on the variation of fuel velocity, we get the result that the higher fuel velocity, increase fire height; however, the temperature of the fire height will decrease.

Squirmers with swirl at low Weissenberg number

Abstract

Heat Transfer Augmentation in Solar Collectors Using Nanofluids: A Review

Background: Enhancing the heat transfer rate in solar collectors is an essential factor for reducing the size of the system. Yet, various methods have been presented in the literature to increase the heat transfer rate from an absorber to the heat transfer fluid. The most important methods are: the use of evacuated receivers, addition of swirl generators/turbulators and use of various nanofluids as the heat transfer fluid. Objective: The current study reviews the achievements in the enhancement of solar collectors’ heat transfer process using various types of nanofluids. The review revealed that the most widely employed nanoparticles are Al2O3 and Carbon nanotubes (CNTs) and the most popular base fluid is water. Most of the investigations are performed on indirect solar collectors, while recently, the researchers focused on direct absorption methods. In the indirect absorption collectors, the thermal conductivity of the working fluid is essential, while in a direct absorption collector, the optical properties are also crucial. Optimization of the optical parameters along with the thermophysical properties of the nanofluid is suggested for the applications of solar collector.

Two-step numerical simulation of the heat transfer from a flat plate to a swirling jet flow from a rotating pipe

PurposeImpinging jets have been widely studied, and the addition of swirl has been found to be beneficial to heat transfer. As there is no literature on Reynolds-averaged Navier Stokes equations (RANS) nor experimental data of swirling jet flows generated by a rotating pipe, the purpose of this study is to fill such gap by providing results on the performance of this type of design.Design/methodology/approachAs the flow has a different behaviour at different parts of the design, the same turbulent model cannot be used for the full domain. To overcome this complexity, the simulation is split into two coupled stages. This is an alternative to use the costly Reynold stress model (RSM) for the rotating pipe simulation and the SST k-ω model for the impingement.FindingsThe addition of swirl by means of a rotating pipe with a swirl intensity ranging from 0 up to 0.5 affects the velocity profiles, but has no remarkable effect on the spreading angle. The heat transfer is increased with respect to a non-swirling flow only at short nozzle-to-plate distances H/D < 6, where H is the distance and D is the diameter of the pipe. For the impinging zone, the highest average heat transfer is achieved at H/D = 5 with swirl intensity S = 0.5. This is the highest swirl studied in this work.Research limitations/implicationsHigh-fidelity simulations or experimental analysis may provide reliable data for higher swirl intensities, which are not covered in this work.Practical implicationsThis two-step approach and the data provided is of interest to other related investigations (e.g. using arrays of jets or other surfaces than flat plates).Originality/valueThis paper is the first of its kind RANS simulation of the heat transfer from a flat plate to a swirling impinging jet flow issuing from a rotating pipe. An extensive study of these computational fluid dynamics (CFD) simulations has been carried out with the emphasis of splitting the large domain into two parts to facilitate the use of different turbulent models and periodic boundary conditions for the flow confined in the pipe.

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.

Reduction of Jet Impingement Noise by Addition of Swirl

Abstract Experimental investigations are carried out with an objective of reducing impingement jet noise using coaxial swirl jets. Two nozzle-to-plate distances have been considered and the Mach number values ranged from 0.95 to 1.83. The vane angles of the coaxial swirlers ranged from 0 deg to 60 deg, corresponding to swirl numbers that ranged from weak to high. Flow visualization has also been conducted to understand the shock-cell structure for each case. The results indicate that swirl jets are highly efficient in the control of impingement noise. Transonic, screech, and impinging tones are completely eliminated by the swirl jets. A weak swirl seems efficient for lower Mach numbers while higher amounts of swirl are efficient at higher Mach numbers.

Scalar structure of turbulent stratified swirl flames conditioned on local equivalence ratio

In a recent paper (Kamal, et al., 2015), we reanalyzed single shot species and temperature measurements from non-swirling flames stabilized on the Cambridge/Sandia stratified burner by conditioning measurements on the local equivalence ratio, and found that the state space structure of the flames was closely approximated by that of a laminar flame at the given equivalence ratio. In the present communication, we show that the same state space relationships remain robust for species CH4, O2, H2O, and CO2 in premixed and stratified flames under high swirl. Conditioned mass fractions of CO and H2 in the stratified swirl flame show a greater effect of stratification than was observed in the non-swirling cases, and this is attributed to larger gradients in equivalence ratio that occur with the addition of swirl. Aside from this modest effect of stratification, major species mass fractions in the swirling flame are also closely approximated by laminar flame results at the local equivalence ratio.

Dynamic mode decomposition of forced spatially developed transitional jets

In this paper, the dynamic mode decomposition (DMD) is employed to study the database of a swirling jet simulated by large eddy simulation (LES), and the results are compared with its non-swirling counterpart in detail, in purpose of understanding swirl-induced changes of flow dynamics at transitional stage. The two jets have identical Mach number (Ma=0.9) and Reynolds number (Re=9×104), and eigenmode forcing is applied at the inflow. For both jets, the neutrally/least stable dynamic modes are found illustrated by DMD-spectrum. The prevalent temporal frequencies from a global perspective are obtained by DMD, reasonably agree with that of LES. The dynamic modes capture the physical processes like vortex roll-up and merging accurately, including spatial structures and corresponding locations. It is found that the dominant mode is varied, more pertinent frequency contents are found, nonlinear growth rates are enhanced and nonlinear interactions are strengthened with the addition of swirl. Moreover, proper orthogonal decomposition (POD) presents some similar spatial structures to that of DMD, but differences between DMD and POD are also found in analyzing such transitional flows.

Improving the performance of shell-and-tube heat exchangers by the addition of swirl

Heat exchanger is a component which is used to transfer the heat from one medium to another efficiently. Generally, they occupy a large space compared to other components and such bulky designs are not attractive in the modern industrial applications due to several constraints. Therefore, it is invaluable to develop compact heat exchangers but with the improved performance. In this work, an investigation was made on the possibility of reducing the size of a shell-and-tube heat exchanger by addition of swirl. Swirl was generated by using a twisted-tape which inserted inside tube and the effects of these tapes on the heat transfer rate and pressure drop were theoretically studied. The results showed that a half-length regular spaced twisted-tape insert gave the lowest Nusselt number while a full-length twisted-tape insert gave the maximum Nusselt number and hence the highest rate of heat transfer. The length of the heat exchanger could be reduced by 13.3% with a full-length twisted tape and this would be result in 6.8% of reduction of the fabrication cost. Therefore, addition of swirl into the fluid flow should help to design compact and low cost heat exchanges with improved performance but the pressure drop increased leading to an increase of the required pumping power. A prototype shell-and-tube heat exchanger was designed and fabricated based on the theoretical results. Studies are underway to experimentally investigate the overall effectiveness of the use of twisted-tape inserts for enhancing the heat transfer rate by considering all the related benefits and drawbacks.

Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport

The flow field of a turbulent jet emerging from a straight round pipe into a laminar crossflow is investigated by means of large eddy simulations. The concentration of a passive scalar, introduced with the jet, is calculated in order to quantify the mixing of the jet and the crossflow. In the jet, swirl is introduced by means of body forces and a range of jet swirl numbers from S=0 up to S=0.6 is studied. The impact of the jet swirl on the flow field, on the coherent structures, and on the mixing efficiency is investigated and quantified by means of various analyses. It is found that for all swirl numbers larger than zero a clear asymmetry appears in all quantities studied. Additional to the two hanging vortices at both sides of the jet a third vortex is introduced by the swirling pipe flow which interacts with the former. This feature is described in detail as it is not mentioned in the literature. For the strongest swirl investigated a recirculation zone near the jet exit is observed. Despite the asymmetry and even with a recirculation zone at the outlet, the counter-rotating vortex pair still exists in all cases in the downstream flow, where it entrains a large amount of crossflow fluid into the jet. The near field, however, is altered by the jet swirl in several respects. The jet more and more approaches the bottom wall with increasing swirl. As a result, the entrainment is gradually attenuated due to the larger blocking of the secondary flow by the wall. Increased swirl increases both the turbulent kinetic energy in the pipe and the vorticity of the average flow field near the jet exit, and thus stimulates the mixing in these regions. However, this stimulating effect is overwhelmed by the closer position of the jet trajectory to the wall of the channel with increasing swirl, which in turn reduces entrainment of fresh crossflow fluid into the jet. As a final result of these two competing effects, the overall mixing efficiency of a jet into a crossflow is merely unchanged with the addition of swirl. Various mixing indices, both spatial and temporal, are used for this analysis. Their respective advantages and disadvantages are discussed and detailed illustrations provide a sound understanding of their behavior.

Scalar Transport in a Swirling Transverse Jet

The scalar transport in a swirling jet in a crossflow has been investigated in water tunnel experiments. The jet to freestream velocity ratio was varied from 4.9 to 11.1, and the jet swirl numbers ranged from 0 to 0.17. The jet exit Reynolds number was kept at 1.3 x 10 4 during the experiments. Planar laser-induced fluorescence was utilized to measure planar cross sections of the mean concentration field of the jet up to 68 jet diameters downstream of the exit. The jet penetration depth, half-value radius, and maximum concentration were determined from these concentration fields. For jets without swirl, measured cross-sectional mean concentration distributions have symmetric double-lobed kidney shapes that are consistent with the counter-rotating vortex pair that is known to exist in the far field of the jet. The addition of swirl causes the far-field distributions to become nonsymmetric, with one of the lobes increasing in size and the other decreasing, resulting in a comma shape. Swirl is also observed to decrease jet penetration but not to significantly affect the decay of maximum mean concentration for the range of swirl numbers investigated.

Mixing of swirling jets in a supersonic duct flow

Hydrogen fuel injected into a scramjet combustor must mix rapidly if complete combustion is to occur within a reasonable stream wise distance. An experiment has been conducted to determine whether the addition of swirl will improve the mixing of a supersonic jet of fuel simulant (helium or air) injected at 30 deg to the wall into a confined Mach 2 airflow. The swirling jets were created by injecting the fuel simulant tangentially into a cylindrical chamber and accelerating it through a convergent-div er gent nozzle. The flow was visualized by imaging Rayleigh scattering from a laser light sheet, and the plume penetration and cross-sectional area were obtained. The plumes from the swirling and nonswirling jets had comparable penetration and area, but the swirling jets contained substantially less mass flow, suggesting better mixing efficiency. Interaction of streamwise vorticity within the plumes of the swirling jets with their images in the duct wall caused the plumes to be inclined laterally to the freestream.

The Prediction of Swirl and Inlet Turbulence Kinetic Energy Effects on Confined Jet Mixing

Mixing of a turbulent jet with a coaxial slower-moving secondary stream in a constant diameter tube was investigated. Of special interests were the effects of swirling the jet and initial turbulence kinetic energy. The analysis involved a numerical solution of the governing flow equations which were simplified by the Prandtl boundary layer assumptions. The two unknown turbulent stresses in the flow equations were modeled by defining an isotropic effective viscosity. The effective viscosity was calculated from a two-equation model of turbulence. The turbulence model was modified for swirling flows. Predicted results were compared with experimental results of several investigators. Good agreement was obtained when calculated results were compared with mean velocity and wall pressure data. The addition of swirl to the jet increased the rate of spread of the jet and resulted in decreasing the axial length required for mixing. The initial turbulence levels of the streams were found to have a significant effect on the distribution of mean velocity and pressure. This dependence has not been considered by most investigators and makes it imperative that experimentalists include turbulence information in their presentation of results.