Cause analysis and suppression method of the vortex at the impeller outlet of the multiblade centrifugal fan

Indoor air conditioning systems play a crucial role in regulating the environmental conditions of enclosed spaces. The noise generated by such systems can significantly impact human comfort. The multiblade centrifugal fan, as a crucial component of indoor air conditioners, has a significant impact on the performance of the air conditioner. The vortex, which is the primary noise source inside the fan, hinders the aerodynamic noise reduction of the indoor unit. During experimental research and numerical simulation of the indoor unit, the researchers identified the presence of a vortex flow at the outlet of the multiblade centrifugal fan. By analyzing the velocity vector of different cross‐sections in the fan, the researchers determined the formation mechanism and influencing factors of the vortex at the fan's outlet. The addition of fairing sheet plates on the volute wall was explored as a vortex suppression method, which proved effective in weakening and suppressing the vortex. The test results showed that under the same air volume, the noise reduction amplitude of the fan equipped with fairing sheet blades was 1.2 dB at an outlet static pressure of 30 Pa and 0.7 dB at an outlet static pressure of 100 Pa. The research findings suggest that the addition of the fairing sheet can effectively reduce the vortex's intensity, improve the performance of the multiblade centrifugal fan, and reduce the noise.


| INTRODUCTION
With the growing popularity of the green economy and environmentally conscious lifestyles, the energy efficiency standards for electrical appliances are becoming increasingly stringent.Modern society expects essential equipment such as air conditioners and range hoods to meet higher energy efficiency and noise requirements.The multiblade centrifugal fan, which is a crucial power component in such equipment, plays a significant role in enhancing energy efficiency and reducing aerodynamic noise.
Numerous studies have been conducted to improve the energy efficiency and reduce the noise of multiblade centrifugal fans.Researchers have employed experiments and numerical simulations to investigate this area.Heo et al. 1 optimized the performance and noise of multiblade centrifugal fans using a radial basis neural network.Ver and Beranek, 2 Neise, 3 and Datong et al. 4 summarized the noise testing and noise reduction methods of multiblade centrifugal fans.Eisinger 5 conducted experimental testing on a forward multiblade centrifugal fan to thoroughly examine the noise characteristics within the fan.Velarde-Suárez et al. 6,7 Younsi et al., 8 and Lin and Huang 9 employed numerical simulations and experimental methods to analyze the internal flow of fans in the unsteady state.Sasaki et al. 10 discussed the impact of eddy currents on the broadband noise generated by a forward multiblade fan.Several studies have focused on enhancing the energy efficiency of multiblade centrifugal fans.Benchikh Le Hocine et al. 11 utilized an agent model and open-source database to automatically optimize certain structural parameters of a multiblade centrifugal fan, thereby improving its performance.Liu 12 and Weigang et al. 13 investigated the effect of the eccentric installation of collectors on the internal flow field and the performance of multiblade centrifugal fans.Hao and Xiaomin 14 conducted a comprehensive study on elliptical current collectors and recommended optimal installation methods for these components.Tao et al. 15 and Linghui et al. 16 utilized the multiobjective optimization method to optimize the structure of multiblade centrifugal fan components.Other researchers have used numerical simulation methods to investigate various aspects of multiblade centrifugal fans.Ballesteros-Tajadura et al. 17 and Xu and Amano 18 examined the pressure fluctuation inside the fan volute.Ting et al. 19 explored the profile design of the volute under constrained size.Ke et al. 20 improved the aerodynamic performance of a multiblade centrifugal fan by utilizing a bionic volute tongue structure.
Previous studies have also investigated the internal flow of multiblade centrifugal fans.Maeda et al. 21and Jiang et al. 22,23 examined the internal flow structure of multiblade centrifugal fans, exploring flow separation, secondary flow, inlet vortex, and other phenomena using experiments and numerical simulations.The study of the volute tongue is also crucial since it is the location where complex flow occurs in the multiblade centrifugal fan.Cau et al., 24 Cao and Chu, 25 and Gong 26 conducted thorough tests on the velocity field of the volute tongue using particle imaging velocimetry (PIV) and other methods.The research revealed that the structure matching of the volute tongue significantly affects the fan's performance.Neise 27 summarized the modification method based on the volute tongue of a multiblade centrifugal fan, aimed at reducing the pressure fluctuation in this area.Liu et al. 28 conducted experimental measurements and numerical analysis of the influence of a concave volute tongue on the aerodynamic performance and noise of a multiblade centrifugal fan.The results indicated that, compared with the traditional volute tongue, the concave volute tongue reduced the impact strength of the airflow on the volute tongue, thereby reducing the impact loss, the back pressure gradient at the volute tongue, and the noise.Nevertheless, studies about suppressing the vortex inside the fan are limited.Zhaoyang et al. 29 fitted a vortex-breaking device inside the volute to lessen the large-scale vortex and enhance the fan's internal airflow.Wang et al. 30,31 enhanced the blade structure to improve flow separation in the blade channel.They also modified the inlet collector's shape and installation position to minimize the vortex at the entrance by lowering the generation of separation vortices.During the smoke experiment on the indoor unit, the authors discovered a vortex at the outlet, as shown in Figure 1.Currently, there are only a few studies analyzing and explaining the reason behind the vortex at the volute outlet and its impact on the fan's performance and noise.
This paper aims to examine the cause of the vortex at the outlet of multiblade centrifugal fans and explore a suppression method.The internal flow structure of the multiblade centrifugal fan was analyzed through numerical simulation, and the cause of the outlet vortex was explored.The effectiveness of the fairing sheet (a long, thin sheet or square grid used in ventilation equipment and air supply pipelines to improve the internal flow by altering the airflow direction) in reducing the noise of the multiblade centrifugal fan is verified through numerical simulation and experimental comparison.The paper is organized as follows.Section 1 provides an introduction to the methods of improving energy efficiency and reducing noise in multiblade centrifugal fans, as well as related research on the internal flow of multiblade centrifugal fans.Section 2 discusses the research object and method, which encompasses the accuracy and error of the experimental equipment and testing instruments.Section 3 discusses the cause of the vortex at the outlet and the numerical simulation and experimental verification of the fairing sheet to suppress the vortex intensity and reduce the noise of the multiblade centrifugal fan.Finally, some conclusions are drawn in Section 4.

| Research objects
To investigate the cause of the vortex at the fan outlet, two different multiblade centrifugal fans were selected for numerical simulation: one with a single-inlet and the other with a dual-inlet.Both fans have different design flow rates and structural dimensions, which will allow us to draw a conclusive result.The multiblade centrifugal fan with dual-inlets has a design flow rate of 650 m³/h and a rated speed of 1200 rpm.On the contrary, the single-inlet fan has a design flow rate of 500 m³/h and a rated speed of 3000 rpm. Figure 2 displays the structure of the dual-inlet multiblade centrifugal fan, whereas Table 1 provides its key geometrical parameters.
According to Figure 3, the dual-inlet multiblade centrifugal fan has a singular inlet structure with narrower volute width compared with the single-inlet multiblade centrifugal fan.Table 1 lists the specific structural parameters.Ignoring the mutual interference of the left and right inlet airflow, the dual-inlet centrifugal fan outlet vortex has a symmetrical distribution similar to the single-inlet centrifugal fan due to their shared structural similarities.Numerical simulation subsequently confirmed this observation.

| Numerical method and verification
In the present study, the commercial software FLUENT was utilized to simulate the three-dimensional flow of the research object.The turbulence model selected to solve the internal flow control Navier-Stokes equation was the standard k-ε model.Mass flow inlet was chosen as the inlet boundary, and the flow and airflow directions of the current working condition were set for steady-state simulation.The inlet air was assumed to be fully developed turbulence, and its turbulence intensity and turbulence degree were set to 5% and 10%, respectively.The outlet was set as a static pressure outlet.The standard near-wall control equation was adopted for the near-wall equation, while the  "SIMPLE" algorithm was utilized for the pressure-velocity coupling equation.The "PRESTO!" algorithm was employed for the pressure interpolation correction.The secondorder upwind scheme was used for the discretization of the diffusion term, and the second-order upwind scheme was utilized for the discretization of the convection term.The calculation domain was meshed using polyhedral and hexahedral hybrid grids, with 10 layers of grids created in the near-wall area for encryption.The height of the first layer of grids on the wall was set to 0.05 mm to ensure that the calculation area Y plus was less than 100.To verify the grid independence of the model, five sets of models with different grid numbers were calculated, and the verification results are presented in Table 2.The validation of the numerical simulation results is shown in Figure 4, with a maximum error of 3.78% between the numerical simulation value and the experimental value, verifying the accuracy of the numerical simulation.The total number of grids selected was 9.02 million, with the grid number of the imported pipeline being 2.48 million, the grid number of the volute being 1.71 million, and the grid number of the impeller being 4.83 million.

| Experimental system
The tests were conducted in a semianechoic room and an air volume test room.The inside surfaces of the semianechoic chamber were lined with mineral wool wedges with a depth of 570 mm, which have excellent absorption characteristics at frequencies above 150 Hz.The chamber had a background noise level of 13.6 dB, which was significantly lower than that of the fan being tested.During the noise test, the speed of the multiblade centrifugal fan was adjusted using a remote controller.The sound signal of the fan was sampled by the microphone at the monitoring point, converted into a voltage signal by adjusting the amplifier, and then transmitted to the signal acquisition system.The signal was finally sent to the computer and processed by software to display the noise time domain and frequency spectrum data.
The fan performance test platform comprises mainly a variable frequency power supply, a static pressure box, a pressure sensor, a multinozzle flow meter, an auxiliary fan, and other components as shown in Figure 6 motor was measured using an RF9800 digital power meter with an error of 0.5%.The speed of the fan shaft is adjusted via software.
The test system needs to run for a minimum of 10 min to allow it to stabilize before data recording.The speed of the fan was adjusted through the motor program to meet the requirements of different static pressure conditions.It took approximately 5 min to run the test at different points to ensure that the test requirements were met.The test equipment operated in a constant temperature environment, and the experimental process was carried out in accordance with the test standard to ensure the credibility of the test results.

| Cause analysis
In the multiblade centrifugal fan smoke experiment (as shown in Figure 1), a symmetric vortex was observed at the fan outlet, prompting a need to study its generation process.However, conducting smoke or PIV tests in enclosed indoor units is challenging.Therefore, the author employed numerical simulation to display the flow situation in the multiblade centrifugal fan through the velocity vector and particle tracking of different cross-sections, study the cause of the vortex, and explore effective suppression methods.
First, numerical simulation of the multiblade centrifugal fan with a single-inlet was performed, and vortex diagrams of 7 different volute sections were intercepted to observe the generation process and characteristics of the vortex.
As depicted in Figure 7, the multiblade centrifugal fan is sectioned every 45°.Section S90 is located at the volute tongue, which is the smallest flow passage section in the volute.The flow at the volute tongue is relatively complex due to the volute tongue reflux.Figure 7 shows two obvious vortices, with the left side being the low-speed vortex (SV) at the step between the volute and the impeller and the right side being the vortex formed by the flow inertia when the air flows into the volute after the impeller rotates 90°.Moving from section S90 to section SV reveals the persistent existence of two vortices.The vortex on the left side gradually weakens, shifting its center inward, while the vortex on the right side expands in scope and eventually develops into the largest vortex.
For better visualization of the internal flow of the multiblade Centrifugal Fan, the particle tracking diagrams provided by the FLUENT software are utilized.Figure 8 illustrates the spiral flow of particles inside the multiblade centrifugal fan that can be observed using this technique.
Spiral flow is a distinct and regular vortex flow in turbulence that is commonly present in flow processes with large-angle transitions.The generation mechanism of spiral flow lacks a reasonable explanation.However, we postulate that the "Coanda" effect, the phenomenon in which high-speed airflow along the surface of an object can adhere to the surface at the corner, and flow instability are the reasons for the occurrence of spiral flow.On the basis of the observations made in Figure 7, it is evident that the flow direction in the multiblade centrifugal fan undergoes a significant angle change process.Consequently, the flow inside the volute exhibits a spiral flow pattern, and the vortex at the outlet is a specific manifestation of this flow.
Similarly, the multiblade centrifugal fan with a double inlet, depicted in Figure 2, is divided into four sections, and the velocity vector diagram of each section is shown in Figure 9.The section located near the volute tongue is denoted by S 90°, and four vortices can be observed from the velocity vector diagram of this section.If only half of the impeller area is observed, the sources of the two eddies are the lowspeed eddies at the step between the volute and the impeller and the eddies formed by the flow inertia after the impeller rotates 90°and flows into the volute.The vortex source is the same as that of the multiblade centrifugal fan with a single-inlet, which exhibits a symmetrical distribution.
In summary, Figure 7 shows the airflow entering through the inlet of the volute and changing direction by 90°as it passes through the impeller.As the airflow flows out of the impeller, it deflects to the walls on both sides of the volute due to the impact of the wall, forming two vortices inside the volute.As the impeller rotates, the vortices develop gradually and discharge at the volute outlet.The initial development of the vortex at the outlet of the multiblade centrifugal fan results from the large-angle change of the fan structure, causing the gas flow inside the fan to form a spiral flow.Additionally, the gap between the impeller and the volute generates a step flow, resulting in the formation of a low-SV.The spiral flow is primarily caused by the multiblade centrifugal fan's structure, and unless the internal structure of the fan is altered, the spiral flow phenomenon cannot be eliminated.However, various methods can be explored to suppress the spiral flow and reduce the vortex's intensity, leading to reduced noise.is the volute tongue.However, the clearance at the volute tongue is insufficient, and this could lead to impeller collision and reduced performance.Consequently, the fairing sheet is placed between the 180°a nd 270°sections, away from the volute tongue, as illustrated in Figure 10.
The structural parameters of the fairing sheet include b1, the height of the fairing sheet at the 180°s ection, W1, the radial distance from the impeller to the bottom of the volute, L1, the distance from the upper fairing sheet to the side of the volute, L2, the distance between the two fairing sheets at the 180°s ection, L3, the distance from the lower fairing sheet to the side of the volute, b2, the height of the fairing sheet at the 270°section, W2, the radial distance from the impeller to the bottom of the volute, L4, the side distance from the upper fairing sheet to the volute, L5, the distance between the two fairing sheets at the 270°s ection, and L6, the side distance from the lower fairing sheet to the volute.These parameters are shown in Table 3, which details the selected structural parameters of the fairing sheet.The fairing sheet thickness used here is 2 mm.
As depicted in Figure 11, the installation of the fairing sheet divides the airflow into two vortices passing through the fairing sheet.The number of vortices increases from 4 to 6 in comparison with the multiblade centrifugal fan without the fairing sheet.In the section near the outlet, the vortex becomes closer to the wall of the volute after the fairing sheet is installed.Figures 12 and 13 demonstrate the velocity contour and Q criterion vorticity contour of the volute outlet section.As shown in the figure, the number of vortexes at the volute's exit increases due to the vortex being broken by the fairing sheet's split flow.Breaking the larger eddies produces smaller eddies that are twice as numerous but less intense.The vortex cloud image confirms that the large vortex core breaks into smaller vortex cores and weakens the intensity after passing through the fairing sheet.Both Powell's vorticity equation and vortex sound theory illustrate that the generation of sound waves is closely associated with vortex size, strength, and interaction.Subsequent testing verified that reducing the strength of vortex cores decreases a fan's aerodynamic noise.
The installation of the fairing sheet in the multiblade centrifugal fan causes an increase in the number of vortices on the upper side of the volute outlet, but the intensity of the vortices decreases.On the other hand, the vortex on the lower side of the outlet increases in size but decreases in intensity.The broadband noise nephogram in Figure 14 provides a more intuitive representation of the intensity of the vortex on the upper side of the volute outlet being weakened and the noise being reduced after the installation of the fairing sheet.Overall, the installation of the fairing sheet leads to an increase in the number of vortices and a decrease in their intensity, resulting in a reduction in the broadband noise at the outlet of the volute.The effectiveness of the fairing sheet in reducing the noise of the multiblade centrifugal fan will be verified through experimental tests.
Figure 15 presents the distribution cloud map of the local total pressure loss coefficient at the inlet section of the fairing sheet to analyze the change in internal flow loss of the volute after installing the fairing sheet.The local total pressure loss coefficient is defined as follows: It is apparent from Figure 15 that the total pressure loss at the inlet of the fairing sheet increases due to the forced split when the airflow enters the fairing sheet.On the other hand, an overall section-wide perspective shows that the total pressure loss on both sides of the fairing sheet is lower than that of the original fan.Further, Figure 16 demonstrates that installing the fairing sheet significantly decreases the total pressure loss at the outlet of the volute in the lower left corner.The implementation of the fairing sheet reduces the flow loss inside the volute and improves the aerodynamic performance of the multiblade centrifugal fan on the outlet section.The fairing sheet plays a crucial role in breaking and splitting the vortex in the volute.Consequently, flow loss is inevitable.However, compared with the vortex's impact on the aerodynamic performance and fan's noise, the shunt loss of the fairing sheet is controllable to some extent.Installing a fairing sheet at the position where the vortex nucleus initiates can further reduce the shunt loss.This theory is supported by the total pressure loss of the inlet section of the fairing sheet.With the exception of the increase of the total pressure near the fairing sheet, the overall section's total pressure loss is lower after installing the fairing sheet.By combining the total pressure loss of the outlet section of the volute, the vortex strength is reduced via the fairing sheet, decreasing the airflow's loss inside the volute and enhancing the fan's efficiency.However, as the static pressure at the fan outlet increases, the impeller speed rises, and the fairing sheet's flow loss gradually increases.At this point, the vortex inside the volute predominates, and the fairing sheet's effect diminishes.

| Experimental verification
On the basis of the experimental equipment described in Section 2.3, a noise test was conducted on the multiblade centrifugal fan, and the volute with the fairing sheet was fabricated according to the design parameters shown in Figure 17.To eliminate the influence of air volume on the aerodynamic noise of the fan, the test was carried out with a constant air volume of 650 m³/h while adjusting the motor speed to measure the static pressure at the outlet of the fan under three different working conditions of 30, 50, and 100 Pa.sheet.The installation of the fairing sheet resulted in a significant reduction in the broadband noise of the multiblade centrifugal fan, particularly in the middle-and low-frequency range near 1000 Hz, due to the fairing sheet's ability to weaken the vortex intensity.This trend remained consistent across different static pressure conditions.
Figure 19 shows the static pressure efficiency and overall A-weighted SPL of the multiblade centrifugal fan at three different outlet static pressure conditions.
The experimental results reveal that the installation of the fairing sheet slightly improves the static pressure efficiency of the multiblade centrifugal fan under different working conditions while significantly reducing the aerodynamic noise.The maximum noise reduction value is observed to be 1.2 dB when the outlet static pressure is 30 Pa, and the minimum noise reduction value is also reduced by 0.7 dB when the static pressure is 100 Pa.The experiments effectively test the suppressive effect of the fairing sheet on the vortex at the fan outlet and the noise reduction effect on the indoor unit.One limitation of this method is that an increase in the static pressure at the fan outlet also increases the fan's speed, resulting in a gradual increase in the flow loss and flow separation at the fairing sheet.Ultimately, this improves the aerodynamic performance of the fan but reduces the effect of noise reduction.Additionally, the study investigated the effect of the number of fairing sheets on the multiblade centrifugal fan's performance, revealing that increasing the number of fairing sheets appears to cause higher aerodynamic losses.

F I G U R E 1
Screenshot of fan smoke test.RESEARCH METHOD 2.

F I G U R E 2
Geometrical model of fan with dual-inlets.T A B L E 1 Structural parameters of the fan.Parameter Value (dual-inlet) Value (single-inlet)

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I G U R E 3 Geometrical model of a fan with a single-inlet.T A B L E 2 Results of the mesh independency test.I G U R E 4 Validation of numerical simulation results.CFD, computational fluid dynamics.The electrical and acoustic performance and test methods of the sound level meter used in the tests were in accordance with Chinese standards GB/T 2888-2008 and GB/T 1236-2000 and met the requirements of JJG 176-2005 and JJG 188-2002.In the semianechoic chamber, a standard test system was established for testing the noise of the multiblade centrifugal fan.The indoor unit equipped with the multiblade centrifugal fan was hoisted at a height of 2 m from the microphone using a wire rope, and the microphone was positioned in the center of the indoor unit, 1.4 m from the ground.A static pressure test point was set at the exit position, and the SYT-2000 microcomputer digital pressure gauge was used.The measurement range was 0 to ±2000 Pa, and the basic error was ±1% full scale.The test was carried out according to Figure 5.The noise test equipment used included a microphone (model PCB 377B02) with an uncertainty of ±2 dB, a sound pressure amplifier (model PCB 426E01) with an uncertainty of ±0.47 dB, and a sound pressure calibrator (model Larson Davis CAL 200) with an uncertainty of ±0.1 dB and ±1 Hz.
. The sensor used to measure the static pressure of the air chamber had a range of 100 Pa and an error of 1.5%, while the sensor for measuring the flow rate had a range of 2000 Pa and an error of 0.25%.The input power of the F I G U R E 5 Diagram of the noise test device.F I G U R E 6 Diagram of air volume test component.
Using a fairing sheet is a common technique to improve the distribution of airflow and redirect airflow.It can also be used to mitigate the spiral flow caused by the large-angle change of the airflow in the multiblade centrifugal fan.Placing a fairing sheet in the spiral case can reduce the vortex's intensity.On the basis of the analysis presented in Section 3.1, the fairing sheet is expected to be most effective at the location where the vortex is the smallest, which

F I G U R E 7
Development of the internal vortex of the fan with a single-inlet.SV, speed vortex.F I G U R E 8 Particle tracking diagram.

F I G U R E 9
Development of the internal vortex of the fan with a dual-inlet.SV, speed vortex.

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I G U R E 10 The structure of the fairing sheet.T A B L E 3 Structural parameters of the fairing sheet.

F I G U R E 11
Development of the internal vortex of the fan with the fairing sheet.SV, speed vortex.F I G U R E 12 Diagram of velocity contour at the outlet.

Figure 18
presents the 1/3 octave spectrum of the multiblade centrifugal fan with and without the fairing

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I G U R E 13 Diagram of Q criterion vorticity contour at the outlet.F I G U R E 14 Diagram of broadband noise nephogram at the outlet.SPL, sound pressure level.F I G U R E 15 Contour of the local total pressure loss coefficient.F I G U R E 16 Contour of the local total pressure loss coefficient at the outlet.

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I G U R E 17 Volute with the fairing sheet.F I G U R E 18 The 1/3 octave spectrum with and without the fairing sheet.CPB, constant percentage bandwidth.LEI ET AL. | 3375Moreover, there is no positive relationship between the number of fairing sheets and the reduction of fan noise.4| CONCLUSION(1) The present study aimed to investigate and suppress the vortex phenomenon at the fan outlet in the indoor unit of a multiblade centrifugal fan.The cause of the vortex flow was identified through numerical simulations and experimental research.The fairing sheet was applied to suppress the vortex, and its effectiveness was verified through numerical simulations and experimental tests.The main findings of this study are summarized as follows: (2) The vortex flow in the volute of a multiblade centrifugal fan belongs to spiral flow, which is caused by the complex internal structure of the fan and the large-angle change of the airflow.Sudden expansion flow and secondary flow also contribute to the vortex phenomenon.Although the vortex phenomenon cannot be eliminated completely due to the fixed internal structure design of the indoor unit, certain methods can be used to suppress and weaken the spiral flow.(3) Numerical calculations revealed that the multiblade centrifugal fan with a single-inlet has two vortices, one caused by the large-angle change of the flow from the inlet to the outlet of the impeller and the other caused by the sudden expansion structure.The dual-inlet multiblade centrifugal fan has four vortices, which are mostly symmetrical.One pair is caused by the large-angle change of the flow from the inlet to the outlet of the impeller, and the other pair is due to the superposition of the sudden expansion structure and the secondary flow between the volute and the impeller.(4) The fairing sheet was used to weaken the vortex generated by the large-angle change of the airflow and reduce the aerodynamic noise of the indoor unit.The experimental results demonstrated that the installation of the fairing sheet led to an obvious noise reduction effect under three static pressure conditions.The noise reduction amplitude was 1.2 dB when the outlet static pressure was 30 Pa and 0.7 dB when the outlet static pressure was 100 Pa, providing an effective means for the noise reduction of the multiblade centrifugal fan.(5) The vortex flow caused by a sudden expansion structure or clearance secondary flow can be restrained by avoiding such a structure or reducing clearance in the design of indoor units.
Performance comparison of the fan under different working conditions.SPL, sound pressure level.
F I G U R E 19