This paper proposes the use of plasma actuator to suppress boundary layer separation on a compressor blade suction side to increase axial compressor performance. Plasma actuators are a new type of electrical flow control device that imparts momentum to the air when submitted to a high AC voltage at high frequency. The concept presented in this paper consists in the positioning of a plasma actuator near the separation point on a compressor rotor suction side to increase flow turning. In this computational study, three parameters have been studied to evaluate the effectiveness of plasma actuator: actuator strength, position and actuation method (steady versus unsteady). Results show that plasma actuator operated in steady mode can increase the pressure ratio, efficiency, and power imparted by the rotor to the air and that the pressure ratio, efficiency and rotor power increase almost linearly with actuator strength. On the other hand, the actuator's position has limited effect on the performance increase. Finally, the results from unsteady simulations show a limited performance increase but are not fully conclusive, due possibly to the chosen pulsing frequencies of the actuator and/or to limitations of the CFD code.
The aerodynamic performance of compressors and fans is essentially measured in terms of pressure ratio and efficiency. An increase in either one of these two parameters will inevitably result in an overall benefit to the engine. For example, a pressure ratio increase could allow a reduction in the number of stages for axial compressors and thus a reduction in weight, length and mechanical complexity of the engine. The weight and length reductions are even more important given that a great proportion of the engine weight and length is attributed to the axial compressors [
The pressure ratio is related to the air deflection in each blade row of the stage. The higher the deflection, the higher the pressure ratio and imparted power to the air will be. However, the achievable pressure ratio at a given rotational speed is limited by the growth of the airfoil surface and end-wall boundary layers. When the deflection is too high, the suction surface boundary layer separates and the pressure ratio and efficiency decrease rapidly.
This paper presents the results of a preliminary computational investigation of the ability of a new flow control technology, plasma actuation, to control the separation of the airfoil surface boundary layer in highly-loaded compressors. Successful implementation of this technology would ultimately lead to compressor stages with pressure ratio significantly higher than those achievable with conventional designs while maintaining high efficiency.
In the past decades, significant research on boundary layer control technologies in compressors has been carried out. Among them, Loughery et al. [
Even though the above mentioned flow control methods showed interesting results from an aerodynamic point of view, the manufacturing and maintenance cost associated with air recirculation and aspiration/injection systems may limit their implementation in real engines.
The advent of the Single Dielectric Barrier Discharge (SDBD) actuator (referred to hereafter as plasma actuators) could provide an interesting alternative. Plasma actuators are a relatively new flow control technology with very promising preliminary results in a variety of low-speed turbomachinery applications such as laminar separation control on turbine blades [
A plasma actuator consists of two offset electrodes that are separated by a layer of dielectric material as shown in Figure
SDBD plasma actuator.
Being electric and without any moving part, plasma actuators offer a response capability over a wide range of frequencies. This capacity allows them to be easily used in steady (continuous) mode as well as in unsteady (pulsed or duty cycle) mode. Moreover, their simplicity and low power consumption also suggest potentially low integration, maintenance and operating costs.
The present work proposes the use of plasma actuators to prevent flow separation on the blade suction side to allow a more aggressive design of compressor blades. A plasma actuator is positioned upstream of the separation zone to add momentum in the boundary layer. Two mechanisms can be used to prevent the separation. The first one is direct momentum addition in the boundary layer through steady (continuous) actuation, while the second aims to enhance the mixing between the high-momentum fluid in the upper part of the boundary layer and the low-momentum fluid adjacent to the surface through resonant excitation of turbulent flow structures inside the boundary layer. This second approach is performed through the operation of the actuator in pulsed mode at appropriate resonant frequencies and could perhaps achieve the same separation suppression as continuous actuation while requiring only a fraction of the power [
The objective of this paper is to make a preliminary assessment through CFD simulations of the proposed concept to obtain a reference relatively to the required actuator strength (force generated by the actuator), location and actuation power.
The next section will present the methodology used for this study and the design procedure for the rotor blades. Section
A computational approach is the most appropriate way to carry out this preliminary study for two reasons. First, for evaluation of new concepts where the actuator’s parameters (actuator strength, position, actuator geometry, etc.) are unknown, it is a lot less expensive and time consuming to carry out CFD simulations than to empirically set up and run experiments. Second, with plasma actuators being still at the research stage, the flow acceleration they can induce is still relatively small to be tested in realistic turbomachinery flow conditions. Consequently, a numerical approach will allow the evaluation of the required actuator strength instead of being limited to the range available with current actuators. However, a numerical approach requires the modeling of the plasma actuator behavior. This section briefly reviews existing SDBD actuator models and describes the model used in this study and its implementation into a CFD code. Subsequently, a brief description of the CFD code and the blade design process will be presented.
The modeling of the plasma actuator’s effect on the flow is a very active area of research. As such, many models have been and are being developed to simulate their effect on the flow in aerodynamic applications.
These models can be divided in two main categories: (
The models in the first category mainly include those of Roy and Gaitonde [
The models in the second category form the majority of the plasma actuator models. They integrate different levels of complexity and as such their computational time varies greatly (from a few seconds to a few hours). The most recent ones model the actuator as a spatial body force distribution. Among the simplest of these models, that of Shyy et al. [
As the level of complexity in the models increases, more and more plasma features are taken into account. However it is important to mention that no model (even the more sophisticated one) can pretend to be able to generate the exact body force distribution corresponding to specific actuator’s geometry and input voltage. Moreover, from an analysis of the time scales associated with plasma actuator operation made by Orlov et al. [
Thus, the approach taken in this preliminary study is to simulate the plasma actuator using a time-averaged (over an AC cycle) body force distribution that is as realistic as possible when compared to the body force distribution produced by scientific models.
It was found that a time-averaged body force distribution presenting the same global features as the ones obtained from a scientific model [
Equations and boundary conditions for the hybrid model: equation (
However, the electric potential distribution (
Subdivision of the air domain over the covered electrode into
The addition of the potential distribution on the dielectric surface over the covered electrode serves two purposes. The first is to reproduce the phenomenon that is seen in plasma actuators which is that charges arrange themselves in such a way as to cancel as much as possible the electric field [
Electric circuits used to compute the properties on the dielectric surface (only circuits 1 and
Once the resistances and capacitances corresponding to those of the air and dielectric for each circuit (
Once the boundary conditions are specified, the spatial distributions of electric potential and charge density are solved, at each time step, in the computational domain from (
The actuator geometry used with the hybrid model to obtain the spatial force distribution used throughout the project has 0.0254 mm thick electrodes and their respective lengths are 5 mm (exposed) and 12.7 mm (covered). Figure
Time-averaged spatial force distribution generated by a plasma actuator from the “hybrid” model.
Spatial distribution of the force.
Zoomed region 1.
Zoomed region 2.
The plasma actuator model is implemented in the CFD code as the spatial body force distribution obtained in Section
The simulations carried out in this study have been performed using UNSTREST [
Mapping of the actuator’s force distribution onto a blade suction surface mesh.
To generate the different blades used throughout this project, a through-flow program has been developed to offer the flexibility to design the blade according to a prescribed spanwise loading coefficient with control on the cross section profiles used along the span. This program was used to create 3-D blades and the associated gas paths and to predict the approximate performance from a series of common parameters such as: inlet temperature and pressure, stage dimension (r
As such, a spanwise distribution of load coefficient was used to create the different blade cross sections. Losses and deviation correlation used in the program were obtained from results presented in [
To assess the effectiveness of plasma actuator to increase the performance of axial compressors, two types of simulations have been carried out. The first type aims to establish the performance of rotor blades that do not integrate flow control method on their surface. These rotors will be defined as “conventional rotors” and will be used as a reference to evaluate the effectiveness of plasma actuators. The second type of simulations concerns rotor blades with plasma actuators on their suction surface to prevent flow separation. All the simulations carried out in the project have been performed on a single rotor blade passage configuration using a
Computational domain used in the simulations (only one blade passage is used in the simulation) and approximate position of the actuator (the actuator is on the suction side only).
All rotor blades have been designed using the methodology presented in Section
Due to the relative limitation in the performance of plasma actuator (in the near future) and to avoid unnecessary complexity brought about by shock, subsonic rotors have been chosen in this preliminary study. The hub-to-tip ratio, flow coefficient, aspect ratio and solidity of the blades have been chosen to correspond approximately to those found in the first stages of low pressure compressors. Finally, to isolate the effect of flow turning on rotor performance, it has been decided to use the same cross section profile (modified NACA 65006) all along the blade span and the same stacking point (45% of the camber line). This methodology also limits the number of design parameters to only the spanwise loading coefficient distribution which is of interest as the objective of this project is to evaluate the benefits of plasma actuators on flow turning capability and resulting performance enhancement. The parameters that have been used to design the blades are presented in Table
Design parameters of the blades.
Tip Mach number | 0.5 |
Tip Radius | 0.3 m |
Inlet hub-to-tip ratio | 0.70 |
Inlet absolute flow angle | 0° |
Mid Span Solidity | 1.16 |
Aspect Ratio | 1.15 |
Mean Flow coefficient | 0.6 |
Inlet Total Pressure | 101300 Pa |
Intel Total Temperature | 300 K |
The following two Sections (
Ten rotor blade geometries have been simulated to establish the performance range that can be achieved with conventional blades. The lowest pressure ratio blade (Figure
Pressure ratio and isentropic efficiency of conventional blades.
Three output parameters have been studied to evaluate the performance of the blades: the pressure ratio (total-to-total), the power transmitted to the air by the rotor and the isentropic efficiency (as defined by (
The results show that the power increase significantly with pressure ratio, while the opposite is observed for the efficiency. This situation is explained by the fact that for the blades having a pressure ratio at or above 1.204, the flow on the suction side of the blade begins to separate, thus increasing the losses and consequently decreasing the efficiency. The separation then becomes more important as the pressure ratio increase due to the higher loading coefficients.
The results presented in Figure
The analysis of the flow field for the blade with the lowest efficiency reveals that it presents separation zones all along its span. This blade then constitutes a good candidate for the evaluation of plasma actuators to increase the performance of compressor blades. This study will be presented in the next section.
Figure
Loading coefficient distribution and cross sections for the blade used in Section
Section
As mentioned in Section
Figure
Axial velocity contour lines (m/s) and the positions at which plasma actuators are positioned.
Nine simulations have been carried out with continuous actuation to evaluate the effect of actuator strength and position on the performance increase. For each actuator location, three actuator strengths have been simulated: 1 N/m, 2 N/m and 4 N/m. Although, the actuator strength used in this project are significantly higher than those of the first generation of plasma actuators [
Six simulations have been performed with pulsed actuation. Their objective is to evaluate the effect of the pulsing frequency on the mixing enhancement, wake reduction and separated zone suppression. All the simulations used an actuator strength of 2 N/m. Only two actuator positions have been simulated: position 2 and 3. The pulsing frequency has been established from a method that has been validated experimentally by Huang [
From [
To define the forcing frequency, the flow properties at 60% of the span have been used due to its large separation zone along the blade span. From these flow properties the frequency corresponding to a Strouhal number of 1 has been established to be on the order of 2000 Hz. To take into account the potential uncertainty on this value, simulations have also been carried out with frequencies of 1000 Hz and 400 Hz. The duty cycle (fraction of the time during which the actuation is on over a period) has been held constant at 25% throughout the simulations, based on results presented by Huang [
To evaluate the influence of plasma actuation on the rotor performance, three parameters have been considered for comparison with the reference case: the total-to-total pressure ratio, the isentropic efficiency and the power that must be provided to the actuator relative to the increase in power imparted to the air by the rotor. The results are presented in Table
Simulated cases and results.
Simulations | Results | ||||||
---|---|---|---|---|---|---|---|
Case | Actuator | Actuator | Pulsing | Total-to-Total | Isentropic | Rotor | Actuator |
Number | Position | Strength | Frequency | Pressure | Efficiency | Power | Power |
(Figure | (N/m) | (Hz) | Ratio | (%) | (kW) | (kW) | |
1 | Reference case (no actuation) | 1.249 | 95.6 | 308.4 | — | ||
Steady Actuation (continuous actuation) | |||||||
2 | 1 | 1 | — | 1.250 | 95.8 | 309.1 | 0.12 |
3 | 1 | 2 | — | 1.252 | 95.9 | 309.9 | 0.29 |
4 | 1 | 4 | — | 1.254 | 96.2 | 311.7 | 0.70 |
5 | 2 | 1 | — | 1.250 | 95.8 | 309.0 | 0.11 |
6 | 2 | 2 | — | 1.252 | 96.0 | 310.1 | 0.26 |
7 | 2 | 4 | — | 1.254 | 96.2 | 311.6 | 0.64 |
8 | 3 | 1 | — | 1.250 | 95.8 | 309.1 | 0.21 |
9 | 3 | 2 | — | 1.252 | 96.0 | 309.9 | 0.45 |
10 | 3 | 4 | — | 1.254 | 96.3 | 311.5 | 0.99 |
11 | 2 | 2 | 400 | 1.250 | 95.7 | 308.3 | 0.18 |
12 | 2 | 2 | 1000 | 1.250 | 95.7 | 308.6 | 0.18 |
13 | 2 | 2 | 2000 | 1.250 | 95.7 | 309.0 | 0.20 |
14 | 3 | 2 | 400 | 1.250 | 95.7 | 309.2 | 0.40 |
15 | 3 | 2 | 1000 | 1.250 | 95.8 | 309.3 | 0.40 |
16 | 3 | 2 | 2000 | 1.250 | 95.7 | 308.6 | 0.40 |
Figure
Effect of actuator strength and position on compressor performance for steady actuation.
Table
Actuation strength has a significant impact on all three parameters used to measure the blade performance. Figure
Effect of actuator strength on the separated region for cases 1 to 4.
Evolution of the separated flow region (on the suction side) all along the span for an actuator located at position 1.
The analysis of the power that must be given to the actuator relatively to the power increase of the rotor (see Table
The effect of the actuator position on the pressure ratio, efficiency and power increase is almost imperceptible. However, its effect is very important on the power that must be submitted to the actuator. The main reason for this power gap between positions 1, 2 and 3 is the fact that at position 1 and 2 the fluid velocity is relatively small because of the recirculation zone, while at position 3 the flow begins to decelerate but its velocity is still important. Therefore, at position 3, the actuator must impart momentum to a larger amount of fluid particles over a given time than at position 1 and 2.
Even though in the present simulations, actuators positioned at locations 1 and 2 seem to be more effective than at location 3 (from an energy point of view), practical consideration may limit this advantage. Compressor blades being relatively thin near their trailing edge, structural constraints may inhibit the integration of plasma actuators in this region. For that reason, position 3 could offer a better location. Therefore, the optimal position could be a compromise between energy and structural/geometrical considerations. In addition, the insensitivity of the increase in pressure ratio and efficiency with respect to actuator position allows for a more radial actuator along the span rather than one that needs to follow the shape of the suction side boundary layer separation line.
Even though the results from unsteady actuation show a certain increase in the performance of the blade on a time-averaged basis, an analysis of the flow field at different times during the duty cycle period reveals that the objective pursued by pulsed actuation is not reached. While unsteady actuation aims to bring in momentum from the outer fluid to the fluid adjacent to the surface through enhanced mixing, the simulations indicate that the only momentum imparted to the fluid comes from the actuator itself. This conclusion is confirmed by the analysis of the variation of the flow properties over several cycles of actuation as shown in Figure
Variation of the flow properties over time for case 12 (solid line) and comparison with the reference case (case 1) (dashed line).
Consequently, the results only show a significant impact when the actuator is on and almost nothing when it is off.
Two factors could explain the observed situation. The first one is that the frequencies simulated are not close enough to the resonant frequencies. The second factor is related to the mesh and CFD code used. Through their turbulence model, RANS CFD codes average the effect of the small turbulent structures whose size are smaller than that of the mesh. Therefore, if those are the structures that are excited by pulsed actuation, then the effect cannot be captured. Similar conclusions have been obtained previously by Lemire and Vo [
The analysis of the results carried out in Section
According to Figure
Thus, significant improvement can be achieved by the integration of plasma actuator into the design of high pressure ratio compressor blades.
This paper presents a preliminary study, through CFD simulations, of the potential of plasma actuators to suppress the flow separation over a compressor blade in order to increase its pressure ratio, efficiency and power. Simulations have been carried out to evaluate the effect of three parameters: actuator strength, actuator position on the blade and actuation method (steady versus unsteady).
Of the three parameters taken into account, actuator strength is the one that has the most significant impact on the performance increase of the blade. Moreover, an almost linear correlation has been found between the actuator strength and the pressure ratio, efficiency and rotor power. The effect of the position is relatively negligible on the performance increase but has a significant impact on the power supplied to the actuator. The further upstream the actuator is from the beginning of the separation zone the more power will have to be supplied to it. Finally, the assessment on the actuation method is not conclusive possibly due to the frequencies used to excite the turbulent structure of the boundary layer or to the size of the excited flow structures being too small to be resolved with the mesh used in the present RANS CFD code.
From an energy stand point, plasma actuators have shown to be very efficient. They allow significant power increase of the rotor while requiring a relatively small amount of power.
Even though simulations were carried out on compressor blades, the concept studied in this project is also applicable to fan blades.
Due to the limitation encountered with pulsed actuation, future work should focus on the evaluation of other forcing frequencies and on the establishment of the capability of CFD codes to capture the resonant effect of the forcing frequency. To define the forcing frequencies, numerical probes monitoring the fluctuation of the relative velocity on the blade surface could be used. From these signals the dominant (resonant) frequencies can be identified at which pulsed actuation can be applied. Low-speed simulations using simple 2D geometry might allow to validate existing experimental results of pulsed actuation as well as to evaluate other turbulence models that could capture the resonant effect. In concert with the development of more powerful actuators, the experimental testing of the concept in cascade should also be started.
In conclusion, this paper shows that plasma actuators are a promising technology in the development of more aggressive compressor blade design.
Area of a mesh
Length of the separated region or distance from the actuator to the trailing edge
Electric field
Forcing frequency
Force vector
Enthalpy
Electrical current
Leading edge
Actuator power
Strouhal number
Time
Trailing edge
Velocity vector
electrical potential, local freestream velocity
Time step
Relative permittivity (compared to air)
Electric potential
Charge density
Debye length (plasma property)
Loading coefficient.
Inlet condition
Outlet condition
Stagnation condition
The authors would like to thank the National Research Council of Canada (NRC) and the Natural Sciences and Engineering Research Council of Canada (NSERC), whose funding made this research possible, Mr. O. Toukal for his assistance with the computational resources and Mr. P. Versailles whose experimental work on plasma actuator allowed us to have confidence in our plasma actuator model.