Swirl Number Research Articles

Modal Decomposition of the Precessing Vortex Core in a Hydro Turbine Model

We report on the experimental study of a precessing vortex core (PVC) in an air model of a Francis turbine. The focus is placed on the modal decomposition of the PVC that occurs in the draft tube of the model turbine for a range of operation conditions. The turbulent flow fluctuations in the draft tube are assessed using stereo particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) is applied to the antisymmetric and symmetric components of the velocity fields to distinguish the dynamics of the azimuthal instabilities. The pressure pulsations induced by the PVC are measured by four pressure sensors mounted on the wall of the hydro turbine draft tube. Spatial Fourier decomposition is applied to the signals of the pressure sensors to identify the contributions of azimuthal modes, m=1 and m=2, to the total pressure fluctuations. The analysis based on velocity and pressure data shows similar results regarding the identification of the PVC. The contribution of the m=2 mode to the overall turbulent kinetic energy is significant for the part load regimes, where the flow rates are twice as low as at the best efficiency point (BEP). It is also shown that this mode is not the higher harmonic of the PVC, suggesting that it is driven by a different instability. Finally, we show a linear fit of the saturation amplitudes of the m=1 and m=2 oscillations to determine the critical bifurcation points of these modes. This yields critical swirl numbers of Scr=0.47 and 0.61, respectively. The fact that the PVC dynamics in hydro turbines are driven by two individual instabilities is relevant for the development of tailored active flow control of the PVC.

Effects of key structure parameters on flow field and sensitivity analysis for a bubble separator in MSR

In order to investigate the separation mechanism and provide some guidance for the separator design, the single-phase flow field of the bubble separator for the Molten Salt Reactor is simulated numerically by Reynolds Stress Model. The results show that a low-pressure and high-velocity region is induced under the obstruction of the guiding vanes in the flowing channel and the tangential velocity distribution resembles the Rankine vortex. Besides, the swirl number along the axial position increases first and then decreases due to the improvement of the first light phase outlet. Moreover, the effects of the key structure parameter of the generating swirler including deflection angle, the guiding vane number and the hub ratio on the flow field are investigated and their sensitivity analyses are conducted at the same time. It demonstrates that the deflection has the largest influence on the flow field and the hub ratio is the second.

Influence of key geometrical features on the non-reacting flow of a Lean Direct Injection (LDI) combustor through Large-Eddy Simulation and a Design of Experiments

Lean Direct Injection (LDI) emerged as an interesting concept to limit NOx emissions in aero engines at the cost of operating close to the flame lean blow-off limit. In this technology, fuel is injected into a swirled airstream that generates recirculating flow structures that stabilize the flame. It is then of paramount importance at the design stage to understand the effect of various features on these structures. The present investigation makes use of Eulerian-Lagrangian Large-Eddy Simulations (LES) previously validated against existing experimental data for a reference condition to study the liquid non-reacting flow inside the CORIA Spray LDI burner with the help of Adaptive Mesh Refinement (AMR). A Design of Experiments (DoE) is proposed to analyze the significance of several geometrical features on the flow field, namely the combustor width, the air swirler vane angle, the number of swirler vanes and the axial location of the fuel injector tip. The study covers the qualitative appearance of the flow and the quantitative characterization of the spray dispersion and fuel-air mixing process. In this way, the chosen response variables include the size of the relevant coherent flow structures (Central Toroidal Recirculation Zone induced by the Vortex Breakdown Bubble, Corner Recirculation Zone and Swirled Jet) and their associated velocities, spray features (global drop sizes and spray penetration), pressure drop across the swirler and induced swirl number. Besides, the Precessing Vortex Core (PVC) relevance and frequency content is studied through Proper Orthogonal Decomposition (POD). Results from the statistical analysis show that the number of swirler vanes and their angle are the geometrical parameters that most importantly influence the flow features: stronger recirculation zones leading to an improved atomization and mixing have been found both when decreasing the number of swirler blades and increasing the blade angle. However, both solutions also increase the pressure losses across the swirler. As far as the spectral analysis is concerned, the number of swirler vanes is the most influencing factor on both the frequency and intensity of the PVC modes, being crucial for the possible activation and the energetic content of a double-helix PVC mode.

Numerical study of kerosene spray and combustion characteristics using an air-blast atomizer

In this study, a computational model for a cylindrical combustor with an air-blast atomizer was created to investigate the effect of changing air to liquid mass ratio (ALR) on spray characteristics without combustion and the effect of changing ALR on combustion characteristics under different swirl numbers with constant kerosene fuel mass flow rate using computational fluid dynamics (CFD) code. The study aim is to define the ALR at which the pollutants concentrations at combustion exhaust will be the least. Eight values of ALR were considered and these include 0.3, 0.5, 0.7, 0.9, 1.0, 1.2, 1.4 and 1.5. The cold spray characteristics including the number of droplets, Sauter Mean Diameter, droplets penetration length and droplets contours were obtained in addition to combustion characteristics that were studied by describing the reverse flow zone (RFZ) boundary, flow pathlines, temperatures maps, outlet temperatures at combustor exit and species concentrations. The results indicate that increasing ALR and consequently spray droplet diameter decrease does not always translate into superior emissions performance and it depends on several parameters including swirling intensity where the modernistic capabilities of CFD code including flow visualization and calculated recirculated mass flow ratio are used to justify the reasons behind this exceptional behavior. In addition, the results show that the spray Sauter Mean Diameter decreases by increasing ALR but the droplet diameter reduction effect was steeper at lower ALR than higher ones. The length of the RFZ is decreasing with the increase of ALR and shifts upstream closer to the combustor swirler. The recirculated flow mass ratio and the outlet average temperature are decreasing with the increase of ALR.

Numerical investigation of the hydrogen swirl combustion flow in a micro gas turbine burner

The influence of swirl angle and hydrogen mixing ratio on the performance of micro gas turbine combustor is discussed by numerical simulation based on computational fluid dynamics (CFD) method, the parameters such as combustion characteristics and pollutant emissions are analyzed. The realizable k-epsilon model is adopted for turbulence simulation, the combustion model is general finite rate model. The results indicated that increasing swirl number reduces the temperature and NO emission. As hydrogen content in the fuel increases, flame length is slightly shortened, temperature in the combustion chamber rises, and temperature distribution becomes more uniform, NO emission at central axis becomes highly, while CO2 and CO emissions decrease. When hydrogen mixing ratio is 50%, the maximum NO emission is about 3 times that of pure methane combustion, and CO2 mass fraction is reduced by 43.9%, moreover, outlet CO emission is reduced by nearly 70%. The hydrogen mixing ratio has a greater impact on pollutant emissions, the sensitivity level of NO and CO emissions to hydrogen mixing ratio reaches 0.25 and 0.39 respectively. As hydrogen mixing ratio and swirl number increases, pressure loss grows, and average turbulent kinetic energy on central plane gradually rises, mainly in outlet region and recirculation zone. Increasing swirl number makes the synergy better, but hydrogen mixing has little effect on the synergy distribution.

Wake rotation impacts on wake decay

This study considers the behavior of a well-conditioned swirling turbulent wake. A custom swirling wake generator was used to produce a swirling wake absent of a tower that complicates the flow regime. Four swirling wakes were studied with swirl numbers ranging from 0.19 to 0.37 and compared to a non-swirling counterpart. Two-component Laser-Doppler Anemometry was used to measure mean and turbulent quantities in the wake. Measured profiles were analyzed based on similarity theory applied to a swirling wake. The results show that wake behavior was impacted by the initial swirl strength. Generally, wake growth rate and axial deficit decay rate increased with higher levels of swirl, and tangential velocity decay rate increased with lower levels of swirl. However, the wake that diffused fastest was the case with a swirl number of 0.27.

Effect of the conical diffuser angle on the confined swirling flow induced Precessing Vortex Core

This paper presents the effect of the conical diffuser angle (θ=0°,2°,4°) on the Precessing Vortex Core (PVC) induced in confined swirling airflows generated by axial swirlers with two swirl number (Sw=1.1,0.7). Planar Particle Image Velocimetry (2D-PIV) synchronized with unsteady wall pressure fluctuation measurements are performed to identify the Strouhal number of the precession frequency and pressure fluctuations amplitude induced by the PVC, as well as the PVC parameters related to its phase-averaged vortex structure. Furthermore, the distribution of pressure recovery rate based on the mass flow averaged pressure, and the relationship between the local axial pressure gradient and the local PVC structure is also investigated to understand the relationship between the PVC parameters and pipe geometries. These relationships are derived from transient simulations with the Shear Stress Transport Curvature Correction model, since the pressure field data cannot be obtained from the experiments conducted in this paper. Experimental and numerical results in pipe configurations 1 (diffuser followed by a straight pipe) and 2 (diffuser followed by an open exit) clarify the diffuser angle effect. As the diffuser angle increases, the Strouhal number decreases, whereas the pressure fluctuations amplitude increases. Moreover, the PVC parameters are also affected by increased diffuser angle: the vortex trajectory radius is enlarged, and the helical vortex pitch is shortened. In addition, the geometry change (from diffuser to straight pipe) leads to a sudden change in the vortex pitch and a modification of the pressure recovery rate evolution. The vortex structure in the conical diffuser is affected by a difference in the diffuser angle, but the latter does not affect the vortex structure in the downstream straight pipe. This highlights that the local vortex pitch depends only on the local pipe geometry. Finally, a theoretical model of the relationship between the pressure fluctuation amplitude and PVC parameters is proposed, and the relationship between the local pressure gradient and the local vortex trajectory radius gradient is clarified. The amplitude factor Cp∗ is a function of the dimensionless vortex trajectory radius r∗. Positive or negative local pressure gradient leads to the mitigation or promotion of the PVC development.

Sophisticated interplay of operating conditions governs flow field transition and optimal conversion inside tangentially fired gasifiers

A computational model is developed to simulate reacting environment inside the pilot-scale, tangentially-fired, Mitsubishi Heavy Industry entrained-flow coal gasifier. Using this comprehensive model, the impact of three operating parameters, namely, swirl number (SN), reactor pressure and mass throughput on the gasifier flow field and coal gasification process is investigated. We demonstrate that for given values of any two of these parameters, it requires a threshold value of the third parameter to form the central recirculation zone (CRZ), the absence of which lowers char conversion. Therefore, a modified SN incorporating the combined effect of conventional SN, reactor pressure and mass throughput on the onset of CRZ formation is proposed for tangentially-fired gasifiers. Furthermore, the impact of variation in reactor pressure and mass throughput on char conversion is shown to be mediated through the corresponding change in particle residence time and pressure effect on kinetics. The results demonstrate that the operating parameters interact in a complex state space that governs the gasifier flow physics and optimal operation.

Development of a small power generation system with a miniature-scale swirl burner, controlled heat transfer, and thermoelectric generators

Recent advances in small power-consuming electric devices have increased the demand for appropriate, compact, rapidly rechargeable, lightweight, and long-lived power sources with higher energy densities. Hydrocarbon fuels are capable of providing high specific energy density, so combustion-based power generators have received increasing attention as an interesting alternative to batteries. In this study, a small power generation system using a miniature-scale swirl burner, two thermoelectric generators (TEGs), a heat medium, and two water blocks as heat sink has been developed. Various swirl strengths ranging from to are studied to find the optimum swirl number (vane angle) for the blow-out limit. The swirl number related to the greatest stable region is found which is associated with the vane angle for the 3D printed axial swirlers. Because normal-butane is easily liquified and stored, and is in the gas phase at room temperature and atmospheric pressure, it is chosen as the fuel. This paper examines the effect of three thermal input powers of and along with various airflow rates on the performance of the power generation system at the vane angle swirler. The results show that about 3 min is needed for our system to reach a steady power output. Moreover, it is observed that the maximum output power is found at the load resistance of for studied operating conditions. Besides, it is shown that by considering the airflow rate fixed, the power output of the system increases with an increase in the fuel flow rate (thermal input power). Furthermore, a maximum power output of is obtained for the fuel flow rate and an airflow rate of () and respectively, which corresponds to a conversion efficiency of about at load resistance.

NOx emissions in a swirled-stabilized magnesium flame

Alternative sources of clean energy with reduced greenhouse gas emissions have to be considered. Among them, the combustion of metal fuels such as aluminum or magnesium is promising. Nitrogen oxides (NOx) are the only undesirable gases produced during such metal combustion. The present study analyzes the amounts of NOx (NO and NO2) produced by a swirled stabilized magnesium flame. Two geometric swirl numbers are considered: 0.7 (low swirl number) and 7.3 (high swirl number). Different air–fuel ratios – equivalence ratio ranging from 0.17 to 1 – are considered, being obtained by changing the mass flow rate of the injected fuel which is a pulverized fuel of pure magnesium particles with a size fraction 50–70 µm. Thermocouples and a bichromatic pyrometer are implemented to monitor the gas temperature and the flame temperature, respectively. Oxygen (O2) is measured using an in-line paramagnetic analyzers and NOx are measured using an IR-UV analyzer.Whatever the swirl conditions and the air–fuel ratio tested, a stabilized flame of Mg is obtained. The metal combustion exhibits very stable performances. NOx emissions versus equivalent ratio follow a bell-shape curve. Under the tested experimental conditions, the amount of NOx never exceeds 7 gNOx/kWh, which is lower than the maximal value of NOx emission from a gasoline engine. The NOx mole fraction increases when the air–fuel ratio increases, as the power dissipated by the flame increases. The temperature of MgO particles in the flame reaches 2250 °C and the gas temperature never exceeds 1400 °C. The thermal NOx formation is estimated applying Zeldovich’s model. The model indicates that NOx can be produced in the gas surrounding the hot MgO particles. For the highest equivalence ratio, even if the gas temperature is still increasing, the NOx production is disfavored in the rich mixture. Based on these results, a global scheme for NOx formation is proposed which includes four successive steps.

Dynamic characteristics and stability evaluation of cavitation-induced flow instability in a conical diffuser

Abstract It is known that cavitation surge, which is a self-excited vibration caused by cavitation volume fluctuation, may occurs in a draft tube of a water turbine when it is operated under overload. In this study, a cavitation surge was generated by a swirling flow into a Venturi tube, and its dynamic characteristics and stability were evaluated using a one-dimensional model and a transfer matrix. In addition, the variation of the cavitation volume was calculated using the binarization process with a high speed camera and compared with CFD. As a result, it is possible to evaluate the period of the cavitation volume fluctuation by the binarization process, but it is difficult to evaluate its volume quantitatively. The magnitudes of the cavitation compliance and the mass flow gain factor decrease with the increase of the cavitation coefficient. The magnitudes of the mass flow gain factor also tend to decrease with decreasing swirl number. The phase of the mass flow gain factor tends to delay as the cavitation coefficient increases. It is found that the phase of the mass flow gain factor and the cavitation compliance are also important parameters for the stability.

Analysis of Swirl Number Effects on Effusion Flow Behavior Using Time-Resolved Particle Image Velocimetry

Abstract The analysis of the interaction between the swirling and cooling flows, promoted by the liner film cooling system, is a fundamental task for the design of turbine combustion chambers since it influences different aspects such as emissions and cooling capability. In particular, high turbulence values, flow instabilities, and tangential velocity components induced by the swirling flow deeply affect the behavior of effusion cooling jets, demanding for dedicated time-resolved near-wall experimental analysis. The experimental setup of this work consists of a non-reactive single sector linear combustor test rig scaled up with respect to engine dimensions; the test section was equipped with an effusion plate with standard inclined cylindrical holes to simulate the liner cooling system. The rig was instrumented with a 2D time-resolved particle image velocimetry system, focused on different field of views. The degree of swirl for a swirling flow is usually characterized by the swirl number, Sn, defined as the ratio of the tangential momentum flux to axial momentum flux. To assess the impact of such parameter on the near-wall effusion behavior, a set of three different axial swirlers with swirl number equal to Sn = 0.6–0.8–1.0 were designed and tested in the experimental apparatus. An analysis of the main flow field by varying the Sn was first performed in terms of average velocity, root mean square, and Tu values, providing kinetic energy spectra and turbulence length scale information. In a second step, the analysis was focused on the near-wall regions: the strong effects of Sn on the coolant jets were quantified in terms of vorticity analysis and jet oscillation.

3D velocity and temperature distribution measurement and characteristic analysis of swirling combustion

The latest research directions of combustion flow field visualisation and combustion diagnosis have been extended to the development of non-contact, multi-parameter measurement methods for qualitative analysis and multi-dimensional visualisation. Velocity and temperature are important parameters that reflect the characteristics of the combustion flow field and affect the combustion dynamics and combustion mechanism, which have become the keys to characterising and diagnosing the combustion process. Three-dimensional (3D) velocity and temperature fields of combustion were reconstructed in combination with particle image velocimetry (PIV) and deflection tomography temperature measurement methods, and swirl combustion characteristics were studied. A non-premixed burner was designed to generate a swirling flame, and an experimental system was constructed for the simultaneous measurement of velocity and temperature to acquire tracer particles and Moiré fringe images. A 3D particle recognition technology and cross-correlation algorithm were used to reconstruct a 3D combustion velocity field. The fringe displacement analytical technique and deflection angle correction iterative tomography algorithm were used to reconstruct a 3D temperature field. The effects of different swirl numbers and swirl vane positions on the characteristics of the swirl flame shape, velocity distribution, and temperature distribution were investigated. The combustion reaction and field process evolution were explained. Certain characteristics, including heat and mass transfer and flow field transport of swirling combustion, were analysed. The measurement errors were discussed.

Combustion and emission characteristics of premixed biogas mixtures: An experimental study

In this study, effects of fuel composition, swirl number and hydrogen addition on combustion and emission characteristics of various biogas mixtures were experimentally investigated. To this end, a laboratory scale combustor and a swirl stabilized premixed burner were designed and manufactured. Later on, this combusting apparatus was equipped with flow, control, safety and measurement tools, hence entire test system was constituted. Combustion and emission characteristics of tested biogas mixtures were determined by measuring temperature and species (CO 2 , CO, O 2 and NO) distributions throughout the combustion chamber. Additionally, flame structures of tested biogas mixtures were evaluated by examining flame luminosity, visible flame length and flame thickness from instantaneous flame images. Results of this study showed that both radial and axial temperature distribution variations of tested biogas mixtures differently alter with hydrogen addition based on the gas composition. Although flame temperature increases with swirl number at burner outlet, it presents a non-monotonous dependence on swirl number outside the flame region because of the modified flow characteristics. This is also the case for emissions of CO 2 . • Premixed combustion characteristics of various biogas mixtures were investigated. • Gas composition, swirl number and hydrogen addition effects were examined. • Hydrogen addition differently altered temperature distribution based on the gas composition. • Flame temperature presented a non-monotonous dependence on swirl number.

Examination of combustion characteristics of oxygen enriched synthetic gases mixtures at various acoustic frequencies

Abstract Synthetic gases attract the attention of researchers due to the increase in energy demand. H2 and CO gases are the main components of synthetic gases. In this study, three different gas mixtures were tested according to their H2/CO (3, 1.5, 0.5) ratios. Thermal power (3 kW), swirl number (1) and equivalence ratio (0.7) kept constants for all experiments. Three different oxygen ratios selected for the oxygen enrichment experiments. First, the gas mixture was burned with standard air. After that, the oxygen concentration in the oxidizer was raised to 23 % and 25 % for examining the effect of oxygen enrichment. The stability results showed that an increase of hydrogen content in the synthetic gas mixtures caused decreasing in the oxygen enrichment limit. The mixture, which can be enriched up to 25.8 % O2 while the H2/CO ratios are 0.5, can be increased up to 25.2 % O2 at a medium H2/CO ratio (1.5). In the case where the H2/CO ratio was increased to the high level (3) by increasing the hydrogen level, the oxygen enrichment limit decreased to 24.1 %. A stable flame did not occur after these oxygen enrichment values. In addition, dynamic pressure data were recorded for all mixtures under external acoustic enforcement. Increases in the hydrogen and oxygen ratio at the same time caused the flame instabilities. Thus, the flame extinguished under acoustic enforcement. In addition to these results mentioned, the emission values showed that oxygen enrichment caused the decrease in CO emissions but the increase of NOx values for all synthetic gas mixtures.

Robust combustor design based on flame transfer function modification

Operation under stable conditions is an important prerequisite for gas turbine safety. While recent studies have focused primarily on acoustic design to avoid thermoacoustic instabilities, in the present study we shift the focus to improving stability margins by flame transfer function (FTF) modification. The flame transfer function of premixed flames is affected by various mechanisms such as variations in equivalence ratio, swirl fluctuations and shear layer instabilities. These mechanisms can be influenced by modifying parameters such as fuel distribution, injection location, swirl number or gas composition. Based on the Nyquist stability method we formulate criteria for how and at what frequencies the flame transfer function needs to be modified, in order to increase the stability margins of a thermoacoustic system. Gain and phase margin as well as the sensitivity function serve as measures of stability. The criteria are limited to single frequencies, which allows experimental FTF optimization with manageable effort. In the second part of this study it is shown that the Nyquist method can also be used as an efficient and compact way to determine whether the uncertainties of subsystems can affect the overall stability, without requiring eigenvalue calculations.

The flame displacement speed: A key quantity for turbulent combustion and combustion instability

Future combustion power and propulsion systems may operate in premixed regime enabling reduced fuel burn and reduced pollutant emissions. The turbulent premixed regime in those future combustion systems is likely to be in the corrugated regime where modeling the flame as a thin interface propagating into the fresh gas is made possible. The flame displacement speed is thus a key quantity for turbulent combustion in this regime. This quantity is also important for combustion instabilities. Indeed, the flame displacement speed [Formula: see text] combined to the flow speed [Formula: see text] determines the flame surface location by determining the flame surface speed [Formula: see text]. The flame surface location has shown to play a major role on combustion instabilities. Research work have also demonstrated the role of the flame displacement speed on the flame response which is used for subsequent combustion instability prediction. In this context, the derivation of flame speed models and flame transfer function models based on this quantity are required. This paper presents the theoretical derivation of flame transfer function coefficients for swirling premixed flames in this context. The derivation is based on the definition of the flame speed for turbulent flame, its perturbed form for oscillating flow, and the kinematic flame-flow speed budget. The obtained results are compared to previous literature data and discussed. The effect of the flame angle, id est the effect of the swirl number on the flame response is also investigated. This works motivates detailed local measurements and simulations to evaluate flow-flame speed budget terms.

Influences of heat release, blockage ratio and swirl on the recirculation zone behind a bluff body

ABSTRACT The recirculation zone created through vortex breakdown mechanisms in swirling flows plays a vital role for aerodynamic stabilization of turbulent flames in practical combustion systems. This zone interacts with the central recirculation zone (CRZ) of an upstream bluff body and this leads to a complex flow behavior that depends on the blockage ratio and swirl number. It has been previously observed that the vortex breakdown bubble (VBB) merges with the CRZ at large swirl number or blockage ratio. In this study, the influences of heat release on this flow structure and their physical mechanisms are explored through a series of large eddy simulations and experiments of bluff body stabilized premixed flames with swirling flows. Comparisons of simulation results with measurements are good. It is observed that in isothermal flows, as the swirl number or blockage ratio is increased, the vortex breakdown bubble moves upstream and its mean structure changes. The effect of heat release leads to considerable differences in the flow characteristics as the vortex breakdown bubble is pushed downstream due to dilatation. The critical swirl number, at which the VBB and CRZ merge, is observed to be higher in reacting flows for the same blockage ratio.

1D diesel engine cycle modeling integrated with MOPSO optimization for improved NOx control and pressure boost

In current investigation, one dimensional gas dynamic and chemical species prediction are simulated according to diesel engine with air cooler and catalyst. The results of engine cycle modeling are then verified with experimental data and the robust validation is obtained. The key parameters of air/fuel ratio, compression ratio, swirl number, coolant temperature, and air temperature of the heat exchanger are taken as input parameters for NOx reduction and pressure increase by multi-objective particle swarm optimization (MOPSO). According to merit function, NOx reduction is weighted two times the pressure increase, as a result out of 80 generated design ID78 is chosen as the best objective. The best design is characterized with high air temperature, swirl number, and compression ratio. It is found that swirl number is a dominant factor for NOx reduction and compression ratio for pressure increase. The best objective case is capable of 4.8% NOx reduction and 5.9% increase of pressure, however the best objective results in unfavorable torque decrease since the NOx reduction is biased. The best solutions also are benefited with 1.6% less wallheat loss due to lower diesel fuel mass injection.

Role of Three-Dimensional Swirl in Forced Convection Heat Transfer Enhancement in Wavy-Plate-Fin Channels

Abstract The influence of wall-corrugation-induced swirl flow on enhanced forced convection in wavy-plate-fin cores has been investigated. Three-dimensional computational simulations were carried out for steady-state periodically developed air flow (Pr ∼ 0.71; 50 ≤ Re ≤ 4000) with channel walls subject to constant-uniform temperature conditions. The recirculation that develops in the wall troughs and grows to have an axially helical character is scaled by the Swirl number Sw. As Sw increases with higher flowrate and/or corrugation severity, tornado-shaped vortices appear in the wave trough region midway of the interfin channel height, then extend longitudinally to encompass majority of the flow channel. The local wall-shear and heat transfer coefficient variations indicate that boundary-layer thinning upstream of the wave peak aids in intensifying momentum and heat transfer. However, the flow recirculation in wall trough impedes heat transfer at low Sw due to flow stagnation but promotes it at high Sw because of the vortices-induced augmented fluid mixing. The effects of this secondary flow are quantified by Φf(or j), which is seen to increase log-linearly as fin corrugation aspect ratio γ and/or fin spacing ratio ζ increases; the influence of cross section aspect ratio α is marginal. Moreover, the pressure drag penalty due to swirl critically affects overall pressure loss, and its proportion remains nearly constant when α varies, but grows as Sw, γ, and/or ζ increases and can be as much as 80% of the total pressure drop.