Swirl Flow Research Articles

Experimental investigation on the effects of swirling flow on flow boiling heat transfer and instability in a minichannel heat sink

Flow boiling in micro/mini-channel heat sinks provides an effective thermal management solution for high-heat-flux cooling applications. However, the plain micro/mini-channel heat sink has some shortcomings, such as severe boiling instability and insufficiency for more stringent heat dissipation requirements. To overcome these shortcomings, this work designs a minichannel heat sink (MCHS) with twisted tape inducing swirling flow. The cross-section of minichannel is square, as well as the width, height and length of minichannel are 2 mm, 2 mm and 150 mm, respectively. The twisted tape length (Ltt = 50, 100 and 150 mm) and twist ratio (R = 3, 4 and 5) are selected as designed parameters to generate different swirling flow intensities. The flow boiling heat transfer and instability in such MCHS are experimentally examined under two mass fluxes (120.7 and 210.5 kg/(m2·s)), two inlet temperatures (70 and 80°C) and a range of heat flux (42.3–263.0 kW/m2). Results show that the swirling flow can mitigate the rapid growth for bubbles and suppress the coalescence of small bubbles effectively, as well as cause the bubbles to generate helical motion behavior. Especially, the micro-corners between the twisted tape and minichannel wall provide more effective nucleation sites to produce more small bubbles. Thus, the swirling flow induced by twisted tape can not only trigger onset of nucleate boiling (ONB) at low wall superheat and improve heat transfer performance, but also suppress the flow boiling instability. Smaller twist ratio or longer length of twisted tape results in stronger swirling flow, thus bringing greater heat transfer enhancement effect. Also, longer twisted tape presents better stable flow boiling, but the twist ratio has an insignificant effect on boiling instability. Compared to plain MCHS, the MCHS having twisted tape with Ltt = 150 mm and R = 3 achieves a 34.4% decrease in wall superheat of ONB and a maximum of 80.9% increase in heat transfer coefficient. Furthermore, this MCHS reduces the relative standard deviations of inlet pressure by 44.4% at heat flux of 100.5 kW/m2 and mass flux of 120.7 kg/(m2·s). These findings suggest that using twisted tape insert to induce swirling flow can improve overall performance for minichannel heat sinks effectively.

Experimental studies on combustion performance of beeswax-paraffin blended solid fuels in a hybrid rocket

The study intends to investigate the physical, chemical and thermal characteristics of paraffin blended fuels to determine their suitability as a solid fuel in a hybrid rocket. Wax fuels are a viable and efficient alternative to conventional rocket fuels, having excellent structural strength and thermal and mechanical properties. By utilizing both axial and swirl injection technique, the combustion performance of paraffin – beeswax blended fuels have been tested with a fabricated cylindrical grain in a laboratory-scale rocket setting along with oxygen. The test outcomes revealed solid fuel compositions of more beeswax content in paraffin wax on an oxygenated gaseous environment with a swirl-flow injection method has the highest average regression rate of 1.649 mm/sec at 181 kg/m2s mass flux. Axially injected oxygen with pure paraffin wax has the lowest value of 0.85 mm/sec at 96 kg/m2s. The regression rate comparisons revealed that oxygen injection by a swirl injector increased the regression rates by 40% for mass fluxes greater than 80 kg/m2s. Compared to other studies, the combustion efficiencies have been obtained in this study are good. Blended fuels can manage and increase combustion efficiencies for axial and swirl flow conditions. Swirl injectors outperform axial injectors for oxygen injection and allow for a higher proportion of Beeswax combined with paraffin. This study exclusively designed and manufactured an axial injector and swirl injector, according to the required dimensions of a lab-scale hybrid rocket's combustion chamber, injector, and exhaust nozzle, and their performances have been evaluated.

SPECIFICS OF STOICHIOMETRIC HYDROGEN-OXYGEN MIXTURE COMBUSTION IN A BIDIRECTIONAL SWIRLING FLOW

This paper considers thermodynamic features specific to flows with stoichiometric hydrogen-oxygen combustion in a steam environment. Four combustion chamber variants were evaluated to determine one with optimal combustion conditions: direct-flow combustion chamber, swirled flow combustion chamber, bidirectional combustion chamber, and bidirectional combustion chamber with a specially profiled bulkhead. Comparative analysis has shown that bidirectional combustion chambers achieve the lowest emissions levels, and a bidirectional combustor with a profiled bulkhead displays the highest combustion uniformity. Based on the results of the above study, a miniature bidirectional hydrogen-oxygen combustor was designed and tested, showing combustion completeness levels of 99.99% at the burner exit. Moreover, the combustor was cooled by natural convection with a maximum dimensionless efficiency of 1.5 on the flame tube surface.

SWIRLING FLOW AND BREAKUP CHARACTERISTICS AT HIGH REYNOLDS NUMBER IN A PRESSURE-SWIRL ATOMIZER FOR ARTIFICIAL SNOWMAKING

The pressure-swirl atomizer is used to provide atomized droplets, which is the main atomization component of the snowmaker. The formation of snowflakes is significantly affected by the effective collision between droplets and ice nucleus produced by ice nucleus generator. Whether the collision position moves forward or lags behind, it will both cause deterioration of the snow quality. Therefore, the flow and breakup process, breakup length, and variation law of atomization are very crucial for regulating the snow quality. The fully developed swirling flow at high Reynolds number in the pressure-swirl atomizer for outdoor snowmakers were investigated based on VOF-LES approach, and its accuracy was verified by experimental results. It was found that the Kelvin-Helmholtz instability caused by the velocity difference at gas-liquid interface decisively leads to the liquid film fragmentation. The primary breakup occurs after 5.4 ~ 5.8 times orifice diameter. As the injection pressure increases from 0.6 MPa to 1.4 MPa, the breakup length reduces by 7.7%, the jet velocity increases by 66.7%. It is critical to adjust the effective collision position and enhance the convection, so as to improve the quality of snow.

Substrate Characteristics of Multiple Arrays of Turbulent Swirling Impinging Jets

This research investigates the flow aspects and thermal behavior of multiple air jet impingements on a flat plate through two different geometrical arrangements. A variety of flow conditions and geometrical parameters are considered including Reynolds number, swirl number, impingement distance, and jet-to-jet separation. A numerical approach is adopted to conduct the analysis using the SST k-ω turbulence model in the commercial CFD software package ANSYS Fluent v17. Results reveal that the static pressure is uniformly distributed on the impingement surface for the higher nozzle-to-surface distance at a strong swirl flow, while for no-swirl and medium-swirl configurations, the static pressure is accumulated at the stagnation regions. A lower impingement distance ameliorates the cooling performance while a higher impingement distance accelerates the consistency in temperature distribution. Moreover, the staggered nozzle arrangement is found superior in terms of generating integrity in thermal cooling. Overall, the pressure coefficient enhances up to 126% for medium swirl (S = 0.3) and 67% for high swirl (S = 0.75) cases, and in terms of Reynolds number (Re), the pressure coefficient decreases by 12% and 15% for Re = 24600 and 35000, respectively, compared to Re = 11600. Besides, the average Nusselt number amplifies by 8% and 17% for medium (S = 0.3) and high swirl (S = 0.75) flow, respectively, while compared to Re = 11600, the average Nusselt number raises by 58% and 98% for Re = 24600, and 35000, respectively. The eddy viscosity increases strongly with the growth of swirl intensity, while a monotonous vorticity distribution is achieved at high swirl conditions.

An overview on thermal exchange and crisis of heat transfer release in the annular channels with flow swirl

An overview on thermal exchange and crisis of heat transfer release in the annular channels with flow swirl

An experimental study of friction stir welding parameters effect on joint properties of aluminium alloy and copper plate

Joining dissimilar metals like copper and aluminium is a complex area of research for the manufacturing industry. To effectively join the AA7075 with copper plates, an experimental investigation on friction stir welding was carried out. A straight cylindrical pin profile tool of H13 tool steel was chosen for this study. On the advancing direction, the AA7075 plates were fixed under the condition of zero tool offset. The tool was positioned at a 2° tilt angle for the entire experiment. Tensile test, microhardness test, SEM and EDS analysis were completed to investigate the effectiveness of the process parameters (tool traverse and rotational speed) in combination with a modified set-up on the microstructure, tensile strength and microhardness of the joint. Fractography was conducted on fractured specimens to identify the mode of failure. Complex microstructures of swirl and vortex-type material flow patterns were created in the stir zone. As a result of SEM analysis, one of the samples (S5) had finer grains at intermediate heat input, which produced excellent microhardness and tensile strength. Fractography identified ductile failure as the predominant mode of failure in the joints. The results of this study include an ultimate tensile strength value of 221.091 MPa with a joint efficiency of 95.05%, yield strength of 159.035 MPa, maximum elongation of 9.335% and a microhardness value of 271 HV for specimen welded at a traverse speed of 60 mm min−1 and a rotational speed of 1400 rpm.

Numerical analysis on hydrogen swirl combustion and flow characteristics of a micro gas turbine combustor with axial air/fuel staged technology

• 1 Axial air staging, axial fuel staging and jet stabilized technology are combined with swirl combustion. • 2 Outlet NO emission decreases from 20.2 ppm to 2.28 ppm as secondary air ratio grows from 0% to 40%. • 3 Fuel staging has a more significant effect on reducing NO emission than air staging. To analyze the effects of the two staging techniques on combustion characteristics and emissions, the influence of different secondary oxygen ratios (0-40%) and secondary fuel ratios (0-10%) on the combustion flow, emission characteristics, field synergy and other parameters of the micro gas turbine combustor are investigated by numerical simulation based on computational fluid dynamics (CFD) method. The realizable k-epsilon model is adopted for turbulence simulation, the combustion model is general finite rate model. The results show that with the increase of secondary oxygen ratio from 0% to 40%, scope of central recirculation zone becomes smaller, outlet velocity grows, OTDF becomes larger and pressure loss rises, the average outlet NO emission decreases from 20.2 ppm to 2.28 ppm, synergy angle β of the central plane becomes larger, and the change mainly occurs in the recirculation zone and the rear of the combustion chamber. As the secondary fuel ratio grows, outlet velocity decreases, pressure loss drops, and the outlet temperature distribution becomes more and more uniform. The synergy between velocity and temperature gradient of the central plane becomes better. When the secondary fuel ratio is 10%, the maximum NO emission on the central axis is 9.27 ppm, which is only 8.51% of the fuel unstaged value. The outlet NO emission does not drop monotonously with more secondary fuel. Fuel staging has a more significant effect on reducing NO emission than air staging.

FULL-SCALE CFD STUDY OF SWIRL NUMBER EFFECT ON PARTICLE AND FLOW BEHAVIOR IN INTERACTION WITH COMBUSTION DYNAMICS INSIDE COAL PULVERIZED BURNER

The present paper reports a numerical investigation of an atmospheric pulverized coal combustor. The main goal is to study the effects of flow behavior and gas-particle interaction on combustion dynamics for various swirl numbers (SN). This will help in better understanding of the combustion properties inside a large-scale facility. The RANS and quick discretization scheme are combined during this numerical simulation. For turbulence, the realizable k-ε model is adopted. Turbulence and chemistry interaction are modeled using the finite rate/eddy dissipation model along with reduced global reaction mechanism. Four swirl numbers were investigated. The numerical results are validated with previous experimental data. Good agreement between both results is found for temperature distribution and species concentration along the center axis of the nozzle. Flow topology, temperature, particle trajectory, and species concentrations in several locations downstream the injection nozzle are presented and analyzed for the considered swirl numbers. Analysis of the flow velocity and the particle trajectory showed that the combustion behavior and flame shape were directly related to the particle trajectory inside the nozzle. The numerical results also outlined that the variation of swirl number has changed the particle trajectory and consequently the flame topology. Thus, the swirl number significantly influences the burner efficiency.

REDUCTION OF TOTAL DRAG FOR FINITE CYLINDERS IN TURBULENT FLOW WITH A HALF-C SHAPE UPSTREAM BODY

The effect of an upstream half-C shape body on the reduction of the total drag of a finite circular cylinder is investigated experimentally. Five main cylinders, each with an upstream half-C shape body placed at five upstream positions, a total of 25 different configurations, are considered during the experimentation at a Reynolds number in the range of 1.2 × 10<sup>4</sup> to 1.6 × 10<sup>5</sup>. The aspect ratios of the main cylinders are 1.25, 2.09, 3.03, 3.78, and 4.90 and the upstream bodies are taken as 90° peripheral cut-outs of the respective main cylinder. The total configuration drag coefficient is calculated from the time-averaged drag force on the configuration and compared with the drag coefficient of the respective main cylinder. Results suggest that at least 20% of the total drag of the configuration might be reduced with the upstream half-C shape body, and the reduction varies with the main cylinders' aspect ratio and the upstream body placement distance. The maximum drag reduction is about 55% if the half-C shape body is placed at an upstream distance of 66% of the main cylinder diameter with an aspect ratio of 3.03 and at a Reynolds number of 1.8 × 10<sup>4</sup>. A critical distance ratio is found beyond which drag cannot be reduced by the upstream half-C shape body, and the attribution might be given to a formation of intermittent swirling flow in the cavity mode.

Study on the hydrodynamic characteristics of swirl flow device

The work examined the hydrodynamic situation, which is created by the interaction of reciprocal twisted flows and the distribution of axial velocity along the radius of the primary and secondary flow inputs and the radius of the exhaust pipe in order to create a complete physical picture. Determination of the radius distribution of the static pressure values determined by the height of the apparatus and the static pressure distribution in the exhaust pipe.

NUMERICAL INVESTIGATIONS OF A SWIRLING TWO-PHASE AIR-WATER UPWARD FLOW IN STRAIGHT AND CONVERGENT VERTICAL PIPE

In this study, a numerical study is presented to analyze the flow parameters such as longitudinal and transverse velocities, hydrodynamic pressure, and volume fraction inside a vertical pipe. A vertical ascending swirl flow is established with the specified boundary conditions and compared between straight and convergent geometry pipes. Normalized film thickness is found to vary between 0.4 and 0.6, where the numerical output data from the present study resemble wire-mesh sensor data from literature. Convergent pipe flow includes the variation of hydrodynamic pressure thereby affecting the slug and bubble flow region. Longitudinal and transverse velocities are plotted against time and compared at the three inspection planes near the inlet, mid-portion, and outlet, respectively. In order to understand the effectiveness of rotational effect of gas and liquid phases, the vorticity components are studied. Parameters such as Q-criterion and vortex stretching term indicate the straining and shearing flow near the peripheral and core regions. The temporal volume fraction variation at the output section indicates the increase in the output liquid yield of convergent pipe outlet by 17%.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Investigation on Circular Array of Turbulent Impinging Round Jets at Confined Case: A CFD Study

Jet impingement has immense applications in industrial cooling, such as glass tempering, turbine blades, electrical equipment, etc. The interplay in-between several jet arrangements and the effect of swirl intensity require enormous study to achieve steady heat transfer. This paper numerically investigates an inline array of 25 circular confined swirling air jets impinging vertically on a flat surface. In this regard, three-dimensional simulations are executed using the finite volume method for a number of control parameters, such as Reynolds number (Re = 11600, 24600, and 35000), impinging distance (H/D = 0.25, 0.5, 1), swirl number (S = 0.3 and 0.75) and jet-to-jet separation distance (Z/D = 2.5), where, D is the nozzle diameter. Impinging pressure distribution, flow velocity, surface Nusselt number, and Reynolds stresses are investigated for different operating conditions. The results reveal that both the wall pressure and surface Nusselt number are comparatively uniform in the case of high swirl flow. Moreover, distinct heat transfer behavior is observed from the unconfined condition for high swirl flow in which the heat transfer is constant after a certain radial distance. The Reynolds normal stress adjacent to the nozzle exit is more rigorous than the downstream regions while Reynolds shear stress varies unpredictably along the radial direction. In addition, an estimated 102 % enhancement in average Nusselt number is observed for high swirl flow, at a Reynolds number increment from 11600 to 35000. This enhancement is evident by 23 % in terms of thermal performance factor. Besides, the average Nusselt number and thermal performance factor augmented by 19 % and 8 %, respectively, for an increased swirl intensity at low a Reynolds number (Re =11600).

Investigating the Origins of Cyclic Variability in Internal Combustion Engines Using Wall-Resolved Large Eddy Simulations

Abstract Modern internal combustion engines (ICE) operate at the ragged edge of stable operation characterized by high cycle-to-cycle variations (CCV). A key scientific challenge for ICE is the understanding, modeling, and control of CCV in engine performance, which can contribute to partial burns, misfire, and knock. The objective of this study is to use high-fidelity numerical simulations to improve the understanding of the causes of CCV. Nek5000, a leading high-order spectral element, open source code, is used to simulate the turbulent flow in the engine combustion chamber. Multicycle, wall-resolved large-eddy simulations (LESs) are performed for the General Motors (GM), Transparent Combustion Chamber (TCC-III) optical engine under motored operating conditions. The mean and root-mean-square (rms) of the in-cylinder flow fields at various piston positions are validated using particle image velocimetry (PIV) measurements during the intake and compression strokes. The large-scale flow structures, including the swirl and tumble flow patterns, are analyzed in detail and the causes for cyclic variabilities in these flow features are explained. The energy distribution across the different scales of the flow are quantified using one-dimensional (1D) energy spectra, and the effect of the tumble breakdown process on the energy distribution is examined. The insights from this study can help us develop improved engine designs with reduced cyclic variabilities in the in-cylinder flow leading to enhanced engine performance.

The mechanism of switching direction of swirling sloshing waves

Swirling waves frequently occur in a three-dimensional tank under a nearly resonant excitation with oblique excitation angle. The oblique excitation produces two excitation components and the secondary component triggers rotational waves. The detailed mechanism of the switch in circular direction is clearly described in this study. The evolution of the hydrodynamic forces (Fz) on the tank walls presents a beating phenomenon and the switch direction always occurs at the peak and trough of the envelope of the Fz history. The external excitation moment changes the circulation intensity, and finally the swirling wave becomes a single-direction wave, but only for a short time. The profile of the single-direction wave was determined by the dominant sloshing mode and the instantaneous tank motion was found to be a key factor to determine the consequent swirling flow circulating direction.

Experimental and numerical study on an enhanced swirl cooling with convergent tube wall and local dimple arrangements

This paper experimentally and numerically investigates an enhanced swirl cooling structure, featuring multiple connected convergent swirl tubes and dimpled wall. The dimpled swirl tube adopts a novel local dimpled wall scheme, only arranging dimple arrays under the tangential jets, which takes advantage of the heat transfer potential of the individual dimple. A counterpart multistage convergent swirl tube without dimples is also studied as a comparison baseline to clarify the action of local dimple arrays on swirling flow. The spatially resolved heat transfer data are measured by applying transient thermochromic liquid crystal (TLC) technique. Four channel Reynolds numbers (Re = 20,000 to 50,000) are tested based on the maximum tube parameters. The results show that the enhanced dimpled swirl tube performs a synergistic reinforcement of the high-speed swirling flow over the dimple arrays, which produces many local high Nusselt number patterns. Under the individual injection slot, the upstream dimples have a better enhanced heat transfer performance. For the studied working conditions, the globally averaged Nusselt number of the dimpled swirl tube can be increased by 6.0%–11.5% in comparison with the baseline convergent swirl tube. Besides, the dimpled wall also suppresses the swirl intensity, which leads to the weakening of wall-fluid shear stress action and the disappearance of downstream vortex breakdown, and consequently reduces the pressure loss by 2.0%–10.2%. Based on the well-validated CFD results, the dimpled wall under incoming jets redistributes the contribution of the near-wall turbulence and the forced convection due to the near-wall flow motion to the heat transfer enhancement in the swirl tube, which achieve an ideal cooling performance of high heat transfer in combination with low pressure loss. This provides a new inspiration for the design and optimization of swirl cooling device.

Effect of various configurations of swirl generator system on the hydrothermal performance of the flat-plate solar collector

This is a numerical study that analysis the heat extraction potential of solar collector tubes by assembling a couple of nozzles at the sealed end of the pipe to make swirl flow. Swirl flow intensifies the turbulence rate which augments heat transfer by ruffling the boundary layer. To this end, several decisive factors including nozzle angle (A: 30°, 45°, 60°, 90°), tube diameter (D: 20 mm, 50 mm), nozzle edge size (N: 6.25, 12.5, 25 mm (for D50) and N: 2.5, 5, 10 mm (for D20)), and mass flow rate (M: 0.1, 0.5, 1 kg/s (for D50) and M: 0.04, 0.2, 0.4 kg/s (for D20)) were considered. Results demonstrated that all of the models of class ''A…/D20/N…/M…“ had higher heat extraction potential but lower friction factor compared with ”A…/D50/N…/M…“. Maximum and minimum values of heat flux extractions are 2113390 W/m2 and 59239 W/m2 that were obtained by ”A60/D20/N2.5/M0.4″ and “A30/D50/N25/M0.1″. The created friction factor by class ”A…/D50/N…/M…“ is higher than class ''A…/D20/N…/M…”. The highest friction factor is 3.51 (''A90/D20/N2.5/M0.04″) and the lowest friction factor is 0.019 (“A30/D20/N2.5/M0.2″). Overall, for all cases, class ”A…/D50/N…/M…“ bear the higher TPF compared with class ”A…/D50/N…/M…“ so that the greatest and lowest values of TPF are 5.09 and 0.49 achieved by ”A30/D50/N6.25/M1″ and “A90/D20/N5/M0.4″, respectively.

Two-phase flow pattern identification in horizontal gas–liquid swirling pipe flow by machine learning method

Gas–liquid two-phase swirling flow has been widely used in nuclear industry. Its flow pattern is fundamental to investigate the two-phase flow. Although flow patterns of non-swirling flow in a horizontal pipe have been investigated for a long history, flow patterns of swirling flow in the pipe are rarely reported. In this paper, gas–liquid two-phase flow patterns of swirling flow generated by a vane-type swirler inside a horizontal pipe were investigated by a visualization experiment. Five swirling flow patterns were observed and recorded by the backlight imaging method. Then image processing method was used to obtain void fraction, and the statistical analysis (CDF and PDF) of void fraction for each swirling flow patterns was performed. Owing to the distinguished and stable feature of PDF signals, four parameters describing the characteristics of PDF signals have been proposed as the indicator of machine learning method. Finally, five algorithms of machine learning method have been used to identify the swirling flow patterns, and RUSBoost tree algorithm performs best with an accuracy of 97.4%.

Oxygen transfer characteristics of swirl and non-swirl plunging jets

ABSTRACT In this study, two-blade and four-blade swirl inserts with 180°, 270° and 360° pitch angles, φ each were used for the first time to generate swirling flow in plunging water jets to enhance oxygen transfer. The swirl number, S estimated from the geometry varied from 0.4 to 1.0, denoting moderate to strong swirling flow. Oxygen transfer performance of swirling jets was compared to a non-swirl jet and higher values of oxygen transfer coefficient, K L a 20,, were observed in the former. In addition, K L a 20 increased with increasing φ and jet fall height h j . A strong power relationship was observed between K L a 20 and jet velocity v j . However, K L a 20 generally decreased with increasing water depth, h w . It was observed that any flow conditions which increased bubble plume size, penetration depth or swirl cone size and therefore contact time enhanced oxygen transfer and vice versa. Empirical relationships proposed for estimating K L a 20 based on influential jet and pool properties established in previous research had regression coefficient of determination R 2 values ranging from 0.85 to 0.99. Finally, comparison with multiple circular jets from previous studies indicated that higher oxygen transfer values can be achieved using swirl inserts for the same jet power per unit volume.