Proceedings of the 1st National Aerospace Propulsion Conference NAPC-2017 March 15-17, 2017, IIT Kanpur, Kanpur
NAPC-2017-071 CRITERIA FOR SELECTION OF SOLIDITY IN DESIGN OF CONTRA ROTATING FAN STAGE. Nikhil Nayak1 Department of Aerospace Engineering, Indian Institute of Technology, Kanpur Kanpur, Uttar Pradesh, India
Chetan Mistry2 Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur Kharagpur, West Bengal, India - 45 mm blades, as compared to the baseline combination of 45 mm - 45 mm design, led to a considerable improvement by 7% increase in total pressure rise compared to base line design. Various combinations of number of blades on each rotor were tested with the same designed boundary conditions, and upon comparing the recorded rise in pressures, it could be seen that a combination of 18 blades on rotor-1 and 16 blades on rotor-2, with the new chord length combination of 50 mm and 45 mm respectively, gave a pressure rise by 3.5% improvement compared to baseline design. Key words: Contra rotating fan, chord, number of blades, solidity
ABSTRACT The present paper examines the methodology and computational analysis involved in the optimization of the geometrical parameters of contra-rotating fan stage design. The design optimization mainly focused on reduction of number of blade to achieve same performance duty as earlier design. The contra-rotating fan stage consisted of two rotors, named as rotor 1 and rotor 2, with opposing rotatory motions. The baseline design of the stage had rotor 1 outfitted with 19 and 17 number of blades for rotor-1 and rotor-2. Detailed designed and developed experimental setup at IIT Bombay has proven the performance benefits targeted while design.
NOMENCLATURE
Selection of number of blades always plays important role in terms of design, cost effectiveness, weight and drag relate to turbomachines. The design of axial flow compressor, solidity of the blades and as a result diffusion factor of the rotors plays key role in terms of overall performance of the stage. Main focus for the study is to reduce the number of blades for already designed contra rotating fan stage. The earlier designed blades have aspect ratio of 3 and as a result the chord length was selected as 45 mm. The small chord length puts limit on the diffusion process over the limited length of blades. This length restriction results into more number of blades for both the rotors of contra rotating fan stage. In the present study, the parametric study was carried out with change in solidity by changing chord length combinations for both the rotors and by changing the number of blades for both the rotors. The combination of rotor-1 and rotor-2 chords 50 mm
β δ DF θ DOR ζ i P0 Δ P0 ψ R V Cw σ
1Corresponding
author: Email:
[email protected] 2Corresponding author. Email:
[email protected]
1
Blade angle Deviation Angle Diffusion Factor Camber angle Degree of Reaction Stagger Angle Incidence Angle Total Pressure Total pressure rise across a rotor (P0x- P01) Total pressure rise coefficient (P02- P01)/0.5Ca2 Rotor Relative velocity absolute swirl velocity Solidity
1 2
Subscripts Rotor-1 Rotor-2
1
|
Where
INTRODUCTION
| 2
(1) (2)
The use of diffusion factor in design is a crude but effective way to account for the flow deceleration over the compressor blades, and the effective rise in pressure caused by the same deceleration [11].
A traditional axial flow compressor consists of stationary vanes (stators) and rotating blades, consecutively placed in rows in the path of the flow. A pair of rotating and stationary blades is called a stage. The rotating blades, also known as rotors, accelerate the fluid and the vanes convert the increased rotational kinetic energy into static pressure through diffusion and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. The cross-sectional area of the compressor is reduced in the direction of the flow, to maintain an optimal mass flow rate and air speed as the air is compressed.
Thus, the blade solidity, which can be effected by a change in the chord length, the diffusion factor can be correspondingly changed, leading to an improvement in blade performance [6]. Taking advantage of this improvement in blade performance, the number of blades in each stage can be reduced, which corresponds to a reduction in the weight of the setup, this being the desired output.
The idea of a contra-rotating compressor fan stage involves the usage of two shafts which rotate in opposite directions. Moving blades are located on these shafts successively without stators between them, i.e. the previous stage’s blades play the role of guiding vanes for the next one. This setup proves to be advantageous, as compared to a traditional design in terms of improvement in efficiency, wider operating range and improved thrust-to-weight ratios. The operation of contra rotating fan stage provides stall free operation for rotor-1 over wide mass flow spectrum, due to the strong suction effect produced by the second fan [1, 2, 3]. This arrangement of fans can also lead to improved pressure ratio with lower fan diameter, and it imply lower fan tips speeds, besides lower nacelle drag [4].
BASIC CASE STRUCTURE In considering the iterative study performed on the contrarotating fan, it is essential to first explain two primary elements that provide an improved understanding of the study. These are the structure and procedural approach carried out in each case of the study, and the default or basic case that is being improved upon. Therefore, in order to do the same, a breakdown of the design methodology and structure of the basic case can be considered as representative of the study as a whole. The default or basic case consists of a setup of low speed contra rotating fan blades, of 45 mm chord length each, in an arrangement of 19 blades on the first rotor and 17 blades on the second one, or a 19-17 setup. Detailed design mythology and selection of various geometrical parameters for initial case in discussed in detailed in Ref. 4.This setup had been chosen since a higher loading on rotor-1 as compared to rotor-2 provides a higher overall pressure rise and still maintains stall-free conditions for rotor-1, due to the inherent design benefit, i.e., the strong suction effect from rotor-2, which suppresses flow separation [1]. The performance of the fan depends on various factors, such as axial spacing, tip gap combinations, rotational speed combinations, velocity incidences, etc. In this study, the rotational speeds, the axial spacing and tip gaps have been maintained the same as original design. A uniform mass flow rate was also maintained, as well as a uniform inlet pressure, which have been detailed in a further subsection. Rotor-2 is assumed to discharge air axially, while the whirl component from rotor-1 is accentuated by the reverse spin direction of rotor-2, and provides a further advantage in terms of higher relative velocity at the inlet of rotor-2. The variables chosen for the iterative study are 1) the chord length and 2) the number of blades on each rotor, as detailed in further sections.
In this study, the consideration of one particular design of a contra-rotating fan, designed in IIT Bombay [1, 2, 3], fitted with C4 blade profiles has been undertaken, and it is aimed to examine the effects of varying the chord length of the blades on the pressure rise characteristics of the fan, while keeping all other parameters constant, and to further evaluate the effects of changing the number of blades of both rotors. The motivation in undertaking this study is threefold, it endeavors to achieve an improvement in the pressure rise over one single stage, which can be used to effect to a reduction in the total number of stages on the compressor, an effective reduction in the weight and size of the engine, and thereby, a reduction in the resultant drag in the engine [7, 8]. This improvement in design might therefore, be considered entirely beneficial, and may yield specifications for the selection of parameters for further design and development of the contra rotating fan, anticipating its feasible application in the design and development of future aircraft. The blade solidity ( ) is an important design parameter for the axial flow impeller and is defined as the ratio of blade chord length to pitch. As defined by Lieblein et al., the diffusion factor of the blades, which describes the tendencies for the boundary layer to separate under the influence of the pressure rise in the blade passage, is defined as follows
2
leading and the trailing edge region of the blades were mapped with the J-grid in the circumferential plane. Both rotating flow domains were captured initially with approximately 900,000 nodes each, with a sum of 1,800,000 nodes for both domains to get an optimally convergent solution.
GEOMETRY AND COMPUTATIONAL DOMAIN The computational domain for the test cases consists of a combination of flow passages. This domain can further be divided into two components, the rotating domains for rotor-1 and rotor-2, or R1 and R2, respectively. These two domains have been interfaced with a “Frozen Rotor” interface.
A mesh-sensitivity analysis has been performed on this case, by varying the number of nodes from 1.6 million to 3 million, in order to gauge any fluctuation in the results caused thereof. The optimal 180,00,000 elements setup has been found to gives almost constant total pressure rise and further increments in the number of elements yields no considerable change in the results. BOUNDARY CONDITIONS The boundary conditions chosen as per design are as total pressure inlet and design mass flow rate outlet condition. The boundary conditions have been specified in an absolute frame of reference. The SST model for turbulence has been used for its advantage in simulating the variations in Reynold’s stresses due to the Coriolis forces that appear in rotating frames of reference. These parametric constraints have been maintained as constant as well, in all the iterations performed hereafter, so as to provide a common basis for comparison. Figure 2 shows the total pressure rise plot along the stream wise direction. The total pressure rise by rotor-1 is in the range of 1000 pa and rotor-2 of the order of 750 pa. Figure- 4 shows velocity and static pressure contours at 50% span of the blade.
Figure 1 - Components of the domain for the basic case The hub of both domains has been given a radius of 67.5 mm from the axis line, and the shroud has similarly been placed at a radius of 203 mm [1]. The lengths of the component domains have been set as follows: The inlet of R1 has been placed at a distance of one chord upstream of the leading edge of the blade at the hub, with an additional clearance of 10 mm for disk thickness. The outlet of R1 has been placed at a distance of 20.25 mm from the trailing edge of the blade at the hub, which is 45% of the chord length of the blades. The inlet of R2 has been similarly placed at 20.25 mm from the leading edge of the blade of rotor 2 at the hub, so as to maintain a spacing of 0.9 chord [1] between the blades. This axial spacing of 40.5 mm has been maintained in all the various iterations throughout the study. The outlet of R2 is then placed 2 chords plus 10 mm clearance downstream. Also, a tip clearance of 3 and 3.5 mm has been maintained between the blade and the shroud, as per design. GRID GENERATION AND SENSITIVITY The commercial software package ANSYS CFX-14® has been used for analysis and grid generation in mapping the flow. The computational grid was generated by using ANSYS Turbogrid®. This software provides an opportunity to generate automated grids for specified geometry, like that of the domain detailed above, while also maintaining the tip clearance specified. The topology of Hex elements that was used to generate the mesh was a J-grid topology, with an embedded highdensity O-grid that surrounds the blade and provides a higher degree of accuracy for the gradients along the surface of the blade. The density of the mesh has been maintained about the span and pitch, with respect to the stream wise direction and the density of the mesh is higher at the hub and the shroud so as to provide greater accuracy in the convergence at those areas. The
Figure 2 - Total Pressure rise for initial design case Figure 3 (a) and (b) shows validation of total pressure rise coefficient variation along the span at design mass flow rate for different speed combinations. It is interesting to observe that the total pressure coefficient of rotor-1 matches fairly well up to 50 % span of rotor. Thereafter, it shows lower magnitude compared to the experimental results by about 2 to 4 %. This is likely to be because of the higher negative incidence near that region and subsequently is unable to capture the flow field in that location. Similarly for rotor-2 there seems to be a variation of total pressure coefficient along the span. Specifically near the tip
3
region where it shows lower magnitude compared to the experimental results. The validation of CFD results with Experiments build the confidence to carry out further study with targeted optimization parameters as change of chord length and number of blade counts.
(a) Velocity contours at 50% span
(a) Rotor-1
(b) Rotor-2
Figure 3 – CFD Validation of Total Pressure coefficient for rotor-1 and rotor-2 Results and discussion After obtaining a convergence of 10-5 based on the parametric constraints that were chosen, an analysis of the results was performed. As seen in figure 2, the mass calculated average of the rise in total pressure in a stationary frame of reference has been mapped against the stream wise location, providing a depiction of the workings of the contra rotating fan. The rise gained from the first rotor, as measured, is 958 Pa, and the rise from the second rotor is 577 Pa. The contra-rotating fan acts like two fans that are connected in series for pressure rise, as can be seen, and the overall pressure rise from the stage, therefore, is 1535 Pa at the exit of rotor-2. The pressure rise in rotor-1 is higher, due to a higher number of blades, which provides a higher diffusion of flow for a uniform mass flow rate. A rendering of the pressure and velocity contours for the setup, as seen in figure 3, provides a further glimpse into the reasons for the same, as the wake from rotor-1 can be seen to affect the flow in domain R2, and the higher relative velocity provided to rotor-2 due to the reversed direction of rotation is also clearly visible. This case is therefore assumed to be the base for future iterations, and an increase in the overall pressure rise and other performance factors from this basis would be seen as beneficial to design by adopting two approaches as discussed earlier.
(b) Static pressure contours at 50% span Figure 4: Pressure and velocity contours at 50% span for the basic case The variation in chord length has been observed to not affecting much to the camber and stagger angles, and only affects the solidity and diffusion factor, in turn affecting the performance of the fan. The number of blades on both rotors in each setup has been kept the same, as 19-17. As mentioned earlier, the spacing between the two rotor blades has been maintained at 40.5 mm at the hub, and the other design parameters have also been invariant for the purposes of comparison. As can be seen in figure-4, the variation in chord length produces significant change in the pressure rise and performance of both the rotors, both as individual and as an overall. The variations have been tabulated, for ease of comparison, and can be seen in table-1. The performance of the cases in which rotor 1 has a chord length of 50 mm and rotor 2 has a chord length of either 50mm or 45mm, or the 50-45 and 50-50 cases, is observed to be far better than the remaining cases, and a further increase in chord length does not yield any improvement, as can be seen from the 55-45 case. And as with the number of blades, an increase in the chord length of the first rotor is observed to affect the overall performance to be much as compared to variations in the chord length of the second rotor blade, as is evident from the 45-50 and 45-55 cases.
EFFECT OF CHANGE IN CHORD LENGTHS The first iterative study was performed by varying the chord length of the blades of both rotors, to observe the effects on the performance of the fan caused thereby. As discussed by Reneu et al. [5], the diffusion through 2-D passage is mostly affected by Area ratio, aspect ratio and diffuser angle of the passage. If we consider flow passage through blades as diffuser, the aspect ratio (chord length for rotor) change is one of the affecting parameters.
4
performance. Which clearly indicates the change in chord length of rotor-2 has no effect as an overall especially for rotor-2. At the same time as compared to 45-45 combination, the higher chord length of rotor-1 shows overall improvement all along the span. The combination of 50-45 chord length shows consistent performance in terms of pressure rise along the span for both the rotors. The blade loading for rotor-1 at 50% span and 95% span are as shown in Figure-7. As discussed earlier, the performance of rotor-1 is improving with change in chord length of both rotor-1 and rotor-2. The blade loading of rotr-1 clearly indicates the flow behaviour near the leading edge plays important role both on pressure and suction surfaces. With increase of chord length from 45-50 putting rotor-2 chord length constant indicates not much improvement at 50% span and 95% span. At the same time with increasing the chord length of rotor-2 shows loading to be lower at both 50% and 95% span of rotor-1. However, it is clearly visible that the 50-45 case has better blade loading than both the 55-45 and the 45-45 cases, regardless of the same chord-length that all three rotor-2 blades possess.
Figure 4 - Effect of change in chord length on Pressure rise Chord Length
45-45
50-45
50-50
45-50
45-55
55-45
Δp01 (Pa)
958
1010
1009
971
971
1038
Δp02 (Pa)
577
632
639
624
633
565
Δp0 (Pa)
1535
1642
1648
1595
1604
1603
Table 1 - Pressure rise from rotor-1 and rotor-2 and overall pressure rise This improvement in performance in terms of pressure rise can be explain as the change in the overall length for diffusion to be take place. For chord combination of 45-45 in earlier design, it shows the limited length and size for diffusion passage and as a result there was tight margin to achieve expected loading. In design, the rotor-1 is designed for higher loading compared to rotor-2 and increase in chord of rotor-1 provides scope for better diffusion to take place. The change in chord length directly affects the performance of individual rotor also, as is evident from figure-5(a) and (b).
(a) Total Pressure rise along the span for rotor-1
Figure 5 shows, the span wise variation of total pressure for rotor-1 with different chord length combinations. It shows with change of chord length for rotor-1 has more effect in terms of pressure rise by putting chord as 45 mm for rotor-2 as compare to vice versa. This clearly indicates the rotor loading changes throughout the span compared to original 45-45 mm chord length combination. The higher loading of rotor-1 demands for large diffusing passage compare to lower loading of rotor-2. This clearly indicates the requirement of higher chord length of rotor1 compare to rotor-2. However, in figure 5(b), which represents the span wise total pressure variation at the exit of rotor-2. The span wise variation of total pressure is showing different trend compared to exit of rotor-1. With increase of chord length of rotor-2 shows the improvement in performance above the 50% span of rotor but at the same time the lower half shows the deterioration in
(b) Total Pressure rise along the span for rotor-2 Figure 5: Variation in Total Pressure rise with change in chord length along the span
5
a result the design incidence is matching well for rotor-2 and it results into improvement in performance for rotor-2 and stage. Considering all the aforementioned cases, it is possible to narrow down the choice of the chord-length combination to use down to the 50-50 case and the 50-45 case. These two cases are not showing great variance in performance, as seen from the stream wise pressure rise charts too. However, considering the favorable blade loading of the 50-45 case, as well as the lower length of the second blade, which implies a lower overall length and weight for the rotor, it appears to be the judicious, efficient choice to make, and therefore, this design is chosen to go ahead with for further study.
(a) Blade loading of rotor-1 at 50% span
(a) Blade loading of rotor-2 at 50% span
(b) Blade loading of rotor-1 at 95 % span Figure 6: Variation in blade loading of rotor-1 with change in chord length Figure 6 shows the blade loading of rotor-2 at 50% and 95% span. As discussed earlier the performance of rotor-1 is not showing much variation along the chord after 40% of chord length. Increase of chord length for rotor-1 shows higher acceleration of flow near the leading edge and then effective diffusion along the chord. This higher acceleration on both sides of blade surface results into reduction in effective diffusion over a section. But as shown in figure 7 (b), for rotor-2 it can clearly be seen the chord length of rotor-1 play important role in terms of loading both at 50% and 95% span. The chord length in the range of 50-45 and 55-45, shows the improvement in flow behaviour near the leading edge of pressure surface. This is an indication of wake coming out of rotor-1 and striking on rotor-2 improves let the rotor-2 work under design flow conditions. The rotor-2 is working in aerodynamic matching with rotor-1 and as
(b) Blade loading of rotor-2 at 95% span Figure 7: Variation in blade loading of rotor-2 with change in chord length
6
for comparison. The goal for present study is to achieve nearly same pressure rise along the stage with less number of blades.
EFFECT OF CHANGE IN NUMBER OF BLADES As mentioned previously, the combination of a 50mm rotor-1 blade and a 45mm rotor-2 blade, which shows better performance than the default case, has been chosen for further study to understand the effect of selection of number of blades for both the rotors. The target here is to achieve low blade count for both the rotors so study is focus mare with low blade counts in order to reduce the effective weight of the fan, without compromising the performance of contra rotating fan stage.
(a) Total Pressure rise along the span for rotor-1
Figure 8 - Effect of change in chord length on Pressure rise Number of blades
19-17
18-17
18-16
17-17
17-16
17-15
Δp01 (Pa)
1010
994
994
977
975
974
Δp02 (Pa)
632
619
594
604
596
569
Total pressure rise (Pa)
1642
1613
1588
1581
1571
1543
(b) Total Pressure rise along the span for rotor-2
Table 2 - Pressure rise from rotor-1 and rotor-2 and net pressure rise
Figure 9: Variation in Total Pressure rise with change in blade count along the span
The effect of number of blade in comparison with base line case is as shown in figure 8. The figure clearly indicates the stream wise variation of performance of rotor-1 is in comparison with base line case while for rotor-2 there is an improvement in performance. The variations have been tabulated, for ease of comparison, and can be seen in table-2. The best performing case from the previous section, the 50-45 case, is here represented as the 19-17 case, in terms of the combination of number of blades on the rotors. The original case line represents the default or basic case as ‘Original’, and has been used to provide a baseline
Figure 9 (a) shows the performance of rotor-1 in terms of pressure rise along the span. It shows with 50-45 mm chord combination and same blades as 19-17 shows the improvement in performance above 50% span. The other blade combination shows improvement as compare to base line case. For low blade count 18-16 is also showing comparable improvement for rotor1. In line to rotor-1, for rotor-2 19-17 blade count shows improvement in performance along the span. For blade count combination of 18-16 shows comparative improvement above 50% span and has similar line pressure rise from hub to 50 %
7
span. The reduction in the number of blades leads to a direct effect on the pressure rising capacity of the fan, as well as each individual rotor. This can be attributed to the change in the shape of the flow passage. The reduction in number of blades on the first rotor is seen to affect the overall performance of the fan much more than changes to the second rotor, much like the variation in chord lengths. The 18-16 case seems to provide an adequate improvement in performance, as well as a significant reduction of weight, in terms of blade count.
Figure 12 (a) and (b) show the blade loading at 50% and 95% span of the rotor-2. The blade count has major contribution on blade loading capacity of both the rotors. Near Leading edge, the flow acceleration been affected by blade count change. The flow is getting diffused through rotor-1 and with slightly deceleration near leading edge of rotor-2 results into smooth deceleration of flow on blade of rotor-2. This results into reduction in wake size at the exit of rotor-1. The flow incident to rotor-2 in working in design flow angle mode due to good aerodynamic matching. This can clearly be seen with flow acceleration on the pressure and suction surfaces of the blade throughout the span.
Figure 11(a) and (b) show the blade loading at 50% and 95% span of the rotor-1. The blade count has marginal effect on performance at 50% span of the blade of rotor-1 for low blade counts of 18-16 but has improvement in compare with original base line case. The change of passage shape because of blade count slightly improves the performance of rotor-1 compare to earlier blade count of original and 19-17 with 50-45 chord combination.
(a) Blade loading of rotor-2 at 50 % span
(a) Blade loading of rotor-1 at 50% span
(b) Blade loading of rotor-2 at 95 % span Figure 12: Variation in blade loading of rotor-2 with change in number of blade count
(b) Blade loading of rotor-1 at 95 % span Figure 11: Variation in blade loading of rotor-1 with change in number of blade count
After this stage of iterations, considering the blade loading and span wise distributions, as well as the overall pressure rise from
8
the various cases, it appears to be a prudent decision to modify the basic design (45-45, 19-17) into the (50-45, 18-16) setup. This results into nearly same performance in terms of pressure rise observed for base line case. This can clearly be seen with improved suction effect in the rotor-2 blade and the blade loading charts. Though the design process discuss here seems to be simple with minor modifications but will give important design guidelines especially for the contra rotating stage as per the important criteria of reduction in number of blades. The performance can further be improved upon by optimizing the blade profile and flow angles tuning, to yield higher values of pressure rise with the improvement in weight and drag performances which is not been explore here in this paper.
REFERENCES 1. Mistry, C.S. and Pradeep AM. “Effect of variation in axial spacing and rotor speed combinations on the performance of a high aspect ratio contra-rotating axial fan stage”, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 23 January 2013. DOI: 10.1177/0957650912467453. 2. Mistry,C.S., “ Experimental investigations on performance of contra rotating fan stage under clean and distorted inflow conditions”,2014, PhD thesis, IIT Bombay 3. Roy, B., Raju, A., and Murthy, P., 1992. “Performance Enhancement of a Ducted Twin Rotor Contra-Rotating Axial Flow Fan with Suction Surface Serrations.” In ASME COGEN-TURBO 1992 Conf., pp. 529–534. 4. Talbotec, J., and Vernet, M., 2010. “SNECMA Counter Rotating Fan Aerodynamic Design Logic & Tests Results”. In 27th Int. Congr. Aeronaut. Sci., pp. 1–10. 5. Mistry, C.S. and Pradeep, A. M. “Design and performance analysis of a low-speed, high aspect ratio contra-rotating fan stage”. In: The 11th Asian international conference on fluid machinery, IIT Madras, Chennai, India, 21–23 November 2011, Paper no. 156, pp.1–10. 6. Reneau, L.R.; Johnston, J.P.; Kline, S.J. “Performance and design of straight, two-dimensional diffusers”. J. Basic Eng. March, 1967, 141–150. 7. A.J. Wennerstrom, “Highly Loaded Axial Flow Compressors: History and Current Developments”, ASME Journal of Turbo., October 1990, Vol. 112. 8. J.K. Schweitzer, J.E. Garberoglio, “Maximum Loading Capability of Axial Flow Compressors”, J. Aircraft, Vol. 21 No. 8, August 1984. 9. Lieblien S., “Incidence and Deviation-Angle Correlations for Compressor Cascades”, 1960, Journal of Basic Engineering, 575-584. 10. Lieblein, S., Schwenk, F. C., & Broderick, R. L. ,“Diffusion factor for estimating losses and limiting blade loadings in axial flow compressor blade elements”,1953, NACA R.M. E53 D01. 11. D.K. Hall, E.M. Greitzer, C.S. Tan, “Performance limits of Axial Compressor Stages”, Proceedings of ASME Turbo Expo, June 11-15, 2012, Copenhagen, Denmark 12. G.J Walker. “A Family of Surface Velocity Distributions for Axial Compressor Blading and Their Theoretical Performance”, 1976, ASME J. Eng. Power, 98, pp. 238–239. 13. C. L. Ladson, C. W. Brooks, A. S. Hill, and D. W. Sproles, “Computer program to obtain ordinates for NACA airfoils,” NASA, Houston, TX. USA, Tech. Rep. Memorandum 4741, Dec. 1996. 14. Dixon, S.L.; Hall, C.A. “Fluid Mechanics and Thermodynamics of Turbomachinery”, seventh ed.; Elsevier: Amsterdam, The Netherlands, 2014. 15. Saravanamuttoo, HIH, Cohen, H. and Rogers, G., 1996. “Gas Turbine Theory”, 5th ed. Longman Pub. Group.
CONCLUSIONS This study has discussed the methodology and computational analysis involved in the optimization of a contrarotating fan stage. The procedure included an analysis of the effects of varying the chord length of the blades, as well as the number of blade count for each rotor on performance. Computational analysis of the rotor was carried out using ANSYS CFX® Some of the important results derived from this study have been summarized as below. 1.
Study of the variation of chord lengths suggests an improved performance of the stage for a combination of 50mm-45mm blades, or 50mm-50mm blades. The stage shows an increase in net pressure rise of 7%, from 1535 Pa to 1642 Pa, and this can be attributed to the change in the diffusion factor of the blade, as well as the increased chord length of a high-aspect ratio blade, which provides larger flow passage area for effective diffusion to take place on.
2.
Upon implementing the 50mm-45mm combination into the design, and subsequently studying the effect of change of number of blades, it was found that a combination of 18-16 blades on the rotors still performs better than the initial case, providing an improvement of 3.5% in net pressure rise, at 1588 Pa, while accounting for a decrease in the number of blades on each rotor, and thereby, the net weight of the stage, as desired. This decrease in performance from the first case can be explained by the change in the shape of the flow domain and boundary layer thickness.
It is believed that the improvement in design provided by this study may yield specifications for the selection of parameters for subsequent design and development of the low speed, high aspect ratio contra rotating fan, anticipating its feasible application in the design and development of future commercial and/or military aircraft engines.
9
