Exploration of acceptable operating range for a compression system in a double bypass engine

Ruoyu WANG, Xinjun YU,b,*, Bojie LIU,b, Gungfeng AN,b

a Research Institute of Aero-Engine, Beihang University, Beijing 102206, China

b National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Beijing 102206, China

KEYWORDS

Abstract The variable cycle engine is distinguished by its highly adjustable compression system,whose aerodynamic characteristic is extremely complex.To explore the regulation range of a double bypass engine compression system, a multi-dimensional analysis method is developed, through which the coupling mechanism between the compressor component and the bypass is examined.The operation zones of the compressor components and the bypass system are proposed, and the operation range of the compression system is obtained by calculating the overlapping part of the operation zones.The results show that in the double bypass mode, there exists a minimum mode selector valve area and a minimum core driven fan stage stall margin that ensures a feasible bypass flow, the two parameters correspond to each other.Under the given fan and core driven fan stage conditions, the maximum value of the inner bypass ratio is restricted by the upper limit of the forward variable area bypass injector and the maximum Mach number in the total bypass, while the minimum value of the inner bypass ratio depends on the lower limit of the forward variable area bypass injector geometry and the system recirculation margin.The single bypass mode is a unique condition of the double bypass mode, as the operation zone of the compressor component degenerates from a two-dimensional surface to a straight line.There are multiple bypass states available in the single bypass mode,while the regulation range of the bypass ratio is jointly restricted by the operation range of the high pressure compressor and the aerodynamic boundary of the forward variable area bypass injector.

1.Introduction

The Variable Cycle Engine(VCE)combines the advantages of the turbojet engine and the turbofan engine by adjusting the bypass ratio tremendously, hence is an ideal choice for the next-generation aircraft.1,2To realize the alternation of the thermodynamic cycle, the compression system of the variable cycle engine employs a unique multi-connected aerodynamic layout, including more compressor components than the conventional engine,3,4not to mention the particular bypass regulation components such as the Mode Selector Valve(MSV) and the Variable Area Bypass Injector (VABI).5–7As a result, the matching law of the variable cycle compression system becomes extremely complicated.

As the fundamental unit of the variable cycle engine,extensive investigation has been conducted currently at the component level.Zeng and Wei8studied the influence of load and pre-swirl distribution on the aerodynamic performance of a Flade fan and proved the advantage of positive pre-swirl.Ma9and Zhang10et al.investigated the aerodynamic performance of the Core Driven Fan Stage (CDFS) and the High Pressure Compressor (HPC), and the variations of interstage flow characteristics are discussed in their studies.In recent years,the increasing interest in the bypass flow has highlighted the importance of the regulation components,thus giving rise to a series of models for the VABI,with the studies by Song,11Huang,12Wang,13,14and Xu15et al.as representatives.

On the basis of component research, the researchers have also explored the overall matching of the VCE, of which the mainstream works resort to the engine equations.By introducing simplified component models to the engine control equations, the aerodynamic performance of the engine is simulated, based on which the matching working characteristics of VCE are analyzed.Berton et al.16performed a comprehensive comparison of six propulsion systems of high-speed civil transports and urged the importance of controlling VCE weight and complexity.Denney17and Tai18et al.evaluated the performance of the Double Bypass Engine (DBE) using the NASA NPSS program and noted that well-organized compressor flow is a prerequisite for variable cycle engines to achieve their benefits.Moreover, the regulation laws of the variable cycle engine are explored using the optimization algorithm, such as the work by Jia,19,20Chen,21Simmons22and Patel23et al.

So far, although detailed investigations have been done at component level,24,25these efforts are limited to local performances and require further guidance from the context of the whole engine.Nevertheless, although the overall matching studies have the advantage of quickly obtaining the matching law of VCE compression systems,26–29the component models in these works are often oversimplified and fail to achieve sufficient accuracy.Since it is now impractical to perform threedimensional numerical simulations during preliminary engine design, new approaches are needed to balance efficiency and accuracy.Obviously, the development of multi-dimensional methods that operate dynamically between the highdimensional flow field analysis and the low-dimensional performance analysis will help to solve the problem.Furthermore,under complex aerodynamic conditions,the mutual constraint between the complex bypass and the compressor components is a unique question in the variable cycle engine, and it is necessary to reveal its mechanism while clarifying the regulation range of the VCE compression system.The above aspects are rarely mentioned in the open literature, which is what the present study aims to do.

This paper focuses on the compression system of the classic double bypass engine, which includes not only the traditional fan and compressor parts but also the bypasses and adjustment valves, and seeks to reveal the system matching mechanism while exploring its acceptable operating range at different working modes.The remainder of the paper goes as follows:first,a multi-dimensional model customized for the VCE compression system is developed; next, the regulation range of the compressor components and the bypass ducts are analyzed,respectively; then, examine the coupling relationship between compressor components and bypass configuration; finally,the comprehensive operation regions of the compression system under different operation modes are summarized to provide the theoretical basis for the design process.

2.Model configuration and multi-dimensional analysis method

2.1.DBE compression system

As shown in Fig.1, the DBE compression system in the present study contains three compressor components: the fan,CDFS, and the high pressure compressor.The fan is installed on the low-pressure shaft, while the CDFS and the high pressure compressor are installed on the high-pressure shaft.The outer bypass between the fan and CDFS decouples the mass flow rate of these two components, and the inner bypass between the CDFS and high pressure compressor enables their mass flow rates to be adjusted independently.The MSV at the entrance of the outer bypass can adjust the through-flow area of the outer bypass, whereas the Forward Variable Area Bypass Injector(FVABI)at the juncture of the inner and outer bypass can conveniently regulate the flow combining process.The bypass downstream of the FVABI is called the total bypass.

Under the multi-connected aerodynamic configuration, the mass flow distribution of the compression system is described by inner bypass ratio, the outer bypass ratio, and the total bypass ratio:

where minner, mouter, and mtotdenote the mass flow rate of the inner bypass, outer bypass, and total bypass, respectively.mHPCand mCDFSrepresent the mass flow rate of the high pressure compressor and the core driven fan stage.

Regulation of the bypass ratio is accomplished by geometric adjustment of the MSV and FVABI.In this paper,the area of the MSV(denoted by AMSV)is normalized by its maximum geometric area, and the corresponding variation range is 0–1.The opening condition of the FVABI is defined by the ratio of the through-flow area of the inner bypass and the outer bypass,the regulation range is AFVABI=0.05–0.45.Generally,the Mach number in the bypass should not exceed 1.0 to avoid excessive flow loss.Moreover, the flow paths of the inner bypass and total bypass are designed to be contracted along the axial direction, hence the maximum Mach number will appear at the outlet.

2.2.Multi-dimensional analysis method

Fig.1 Geometric configuration of DBE compression system.

As we know, the aerodynamic coupling is extremely strong in the VCE compression system,so it is necessary to consider the whole compression system when investigating its matching mechanism.In the previous work,the authors have established specialized models for both the bypass configuration and the compressor components, through which their matching rules are revealed.13,14

2.2.1.Using through-flow method to predict aerodynamic characteristics of compressor components

The core of the component analysis model is an in-house through-flow program called VICT.VICT is a highly versatile through-flow program with satisfying accuracy which can not only be calibrated with various model parameters(e.g.loss, blockage, and deviation) but also solve the computational domain in a modularization way.Verification of VICT has been performed on different kinds of compressors(from fan to compressor, from low speed to high speed), considering both experimental and CFD results.30Based on VICT, a novel analysis method is developed to capture the component performance deviation during the coupling working process, and the new method is proved to be highly accurate as well.As shown in Fig.2, the characteristic maps of each compressor component are first generated by VICT,thus providing a database for the coupling analysis.Then,operating points of each component are calculated based on the given restrictions and the aerodynamic relations of the upstream and downstream components.For example,under the Single Bypass Mode (SBM), the fan and the CDFS are coupled by the rotating speed, while under the Double Bypass Mode (DBM), the fan and the CDFS are coupled by both the mass flow rate and the rotating speed.On the other hand, given the fact that the inner bypass cannot be fully closed and the physical rotating speeds of the CDFS and HPC are identical, these two components restrict each other via mass flow rate.

The most interesting aspect of the component matching process is that the operating conditions will multiply along the streamwise direction.See Fig.2, at each given fan operating point,the operating condition of the CDFS corresponds to a constant speed line, whereas the feasible zone of the HPC extends to a surface.The above phenomenon stems from the accumulation of the system regulation freedom, and requires additional restrictions(e.g.,the bypass ratio or the system total pressure ratio) to further locate the component matching point.

Fig.2 Component coupling working process in DBE compression system.

2.2.2.Using control volume models to predict effects of bypass and adjustable valves

As shown in Fig.3, the bypass analysis model is made up of basic flow models of the dividing flow process and the combining flow process.Both models are established based on the control volume theory and could be calibrated according to the realistic bypass geometry, the effects of adjustable valves(MSV for dividing flow and FVABI for combining flow) are also modeled.To consider the coupling effect in the complex bypass, the basic flow models are joined together skillfully in light of the real-life geometric configuration.The calibration method and validation results of the bypass model are narrated specifically in Refs.13,14

One thing to be emphasized about the bypass system is that several special conditions should be avoided in the bypass operating process.As shown in Fig.4 (a), the outer bypass recirculation is a unique phenomenon during the engine mode transition process which manifests as a reverse flow at the MSV.The outer bypass recirculation will redistribute the mass flow in the compression system and induce component performance deviation,17,31hence is an aerodynamic boundary of the bypass system.The authors have investigated the influence of bypass recirculation in a former study,31and the recirculation margin is defined as follows:

In Eq.(4),poutdenotes the actual backpressure at the outlet of the total bypass,while pcris the theoretical bypass backpressure when the outer bypass ratio is critical to recirculation.In other words,pout=pcrcorresponds to the condition where the outer bypass ratio Bout=0(i.e.,RM=0),and pcris the maximum bypass backpressure that ensures a non-recirculation operating point.The value of pcrcan be calculated using the bypass program developed previously.13Fig.4 (b) presents a validation of the bypass recirculation criterion.Threedimensional simulation results are demonstrated from an integrated DBE compression system model.Details about the simulation model can be found in Ref.31, where the bypass recirculation phenomenon is captured successfully under the coupling working environment.Obviously, the outer bypass ratio should be larger than 0 when the compression system is operating normally, while Bout< 0 implies the occurrence of the outer bypass recirculation.Fig.4(b)shows that the operating points with and without bypass recirculation correspond to the recirculation margins of RM < 0 and RM > 0, whereas the near-critical points have the recirculation of approximately 0, thus proving the feasibility of the recirculation margin proposed in the present study.

Fig.3 Theoretical models for characteristic bypass flows.

Besides the outer bypass recirculation, choking conditions are also disfavored in the bypass system, as it will not only affect the through-flow capacity but also generate large amounts of loss and reduce the engine efficiency.13Fig.5 (a)demonstrates a flow field with inner bypass choking, whereas choking may also appear at the total bypass in real life.A further discussion of the bypass choking boundary is given in Fig.5 (b), using the in-house bypass program.The umaxon the ordinate of Fig.5(b) denotes the maximum injection ratio(u = mouter/minner) of the FVABI, and results are classified by the isolines of the injection pressure ratio π (π = p*inner/p*outer).Interestingly, the limiting factor for the FVABI extreme injection ability correlates strongly to the injection pressure ratio:at high injection pressure ratios(π>1.3),umaxis restricted by the inner bypass choking;while at π=1.1,the total bypass will reach the choking condition prior to the inner bypass.The opening condition of the FVABI will also influence the bypass choking boundary, but the effect is not as strong as the pressure ratio.

In a word, the bypass analysis model in this study could detect the aerodynamic boundaries of the bypass configuration and help to guide the compression system matching design,which will be demonstrated in the following sections.

2.2.3.A multi-dimensional method for coupling effect of compressor and bypass configurations

As shown in Fig.6, the multi-dimensional model of the DBE compression system in this paper is essentially an organic combination of the component analysis model and the bypass analysis model, through which the coupling operating mechanism of the VCE compression system could be disclosed.

To be specific, the multi-dimensional model first calculates the compressor aerodynamic performance and then transmits the total pressure and total temperature to the inlet boundaries of the inner and outer bypass.Based on the inlet conditions provided, the bypass simulation module calculates the bypass aerodynamic and geometric conditions according to the conservation of mass flow rate.The modeling process can be summarized as a three-step procedure:

(1) Determine the operating state of each compressor component using the component analysis model.

(2) Determine the inlet and outlet conditions of the bypass system according to component aerodynamic performance.

(3) Calculate the aerodynamic performance of the bypass system using the integrated bypass model.

The multi-dimensional analysis method can provide a quick performance estimation for the whole compression system,which will benefit the preliminary design and performance optimization of the variable cycle engine.

The VCE compression system is essentially a combination of compressor components and bypass configurations.This section discusses the matching working mechanism from the perspective of component and bypass separately, through which aerodynamic boundaries will be revealed.

3.1.Operating boundaries of fan and compressor parts

Fig.4 Special flow conditions of bypass system: Outer bypass recirculation.

Fig.5 Special flow conditions of bypass system: Bypass choking.

First, consider the matching between the fan and the CDFS.The Variable-camber Inlet Guide Vane (VIGV) is installed at the CDFS inlet to achieve a large regulation of the flow angle.When the operating point of the fan is determined, the inlet condition of the CDFS remains unchanged.Therefore, if the condition of the VIGV and the physical rotating speed of the CDFS is decided, the CDFS corrected rotating speed is definite, and its operating point can only move along a fixed constant speed line.Under the constraint of the stall margin(here we select 5%

Then consider the matching between the CDFS and high pressure compressor.It is now assumed that the matching state of the fan and the CDFS remain unchanged.Since the high pressure compressor and the CDFS are seated on the same shaft, their physical rotating speeds are identical.Consequently, under the current constraints, the operating point of the high pressure compressor can only move on a fixed constant-speed line, and its flow adjustment range is affected by its stall margin.Given that the inner bypass ratio of the compression system depends directly on the physical flow of the CDFS and the high pressure compressor, the inner bypass ratio can be calculated using Bin=(mCDFSphy-mHPCphy)/mHPCphy.Therefore,under the above constraints,the variation range of the inner bypass ratio of the compression system is uniquely determined, with the maximum and minimum value of Binbeing restricted by the stall boundary and the choke boundary of the high pressure compressor, respectively.

According to the above analysis, the adjustment range of the DBE compression system is affected by the operating range of the fan, CDFS, and the high pressure compressor.Taking the upper and lower limits of the component stall margin as the boundaries,the adjustable range of the system bypass ratio of can be drawn in Fig.7, where the normalized fan rotating speed is kept at nFancor= 1.0, the normalized CDFS rotating speed is nCDFScor= 0.9, and the angle of the VIGV is αVIGV= 20°.The vertical lines in Fig.7 represent the isolines of the CDFS stall margin, which are also the isolines of the outer bypass ratio.The maximum and minimum values of Boutdesignate the near-stall and near-choke conditions of the CDFS, respectively.On the other hand, the horizontal lines in Fig.7 represent the isolines of SMHPC, whose upper and lower boundaries represent its near-stall and near-choke conditions.The actual adjustment range of the bypass ratios is related to the mass flow characteristics of the components at the corresponding rotational speeds, thus determined by the design of the compressor.

3.2.Restrictions of bypass and control valves on operating boundaries of compression system

Turning now to the matching condition of the bypass, as shown in Fig.7, each point in the component operation zone corresponds to a set of the fan and CDFS operating state,hence the target mass flow rate of the outer bypass is definite.To achieve the target mass flow rate, in the first place, the through-flow area of the MSV has to be large enough to ensure the flow capacity of the outer bypass.On this basis, the inner bypass ratio can be adjusted within a certain range by controlling the FVABI opening condition and the backpressure at the outlet of the total bypass.Referring to Fig.1, for the bypass system, the following criteria need to be satisfied:

(1) MSV inflow Mach number below 1.0 (Fig.1, Plane 1).

(2) FVABI inflow Mach number at the outer bypass duct(Fig.1, Plane 2) below 1.0.

Fig.7 Acceptable bypass ratio adjustment range determined by operating ranges of CDFS and HPC(nFancor=1.0,SMFan=25%,nCDFScor = 0.9, αVIGV = 20°).

(3) FVABI inflow Mach number at the inner bypass duct(Fig.1, Plane 3) below 1.0.

(4) FVABI outflow Mach number at the total bypass duct(Fig.1, Plane 4) below 1.0.

(5) The bypass system has enough recirculation margin.

Based on the five criteria above, the specific requirements for the complex bypass system are analyzed, starting from the MSV.As shown in Fig.8, with the operating points of the fan and CDFS determined, there exists an MSV inlet area that makes the inlet Mach number(Ma1)equal to 1.0,which is the minimum MSV condition for the required mass flow rate(Criterion 1).On the premise of achievable MSV geometry,check the Mach number (Ma2) at the outer bypass duct.If Ma2> 1.0, it is necessary to increase the opening of MSV so that the Mach number falls back to a reasonable level(Criterion 2).On the basis of a reasonable flow field in the outer bypass, the Mach number at the inner bypass is calculated using the combining flow model, and improve the MSV area again if the Mach number exceeds 1.0 (Criterion 3).With the first three criteria satisfied,check the Mach number at the total bypass(Ma3).Since former investigations have proved that the Mach number in the total bypass will decrease with the closure of the FVABI,13the condition with the minimum FVABI area(AFVABI-min) is examined in the first place, and circumstances with Ma3> 1.0 are eliminated (Criterion 4).After ensuring the first 4 criteria, examine the recirculation margin of the bypass system.If the value of RM is smaller than its lower limit (e.g., RMmin= 5.0%), then increase the opening of the FVABI and check Criteria 4 and 5 respectively until the recirculation margin reaches the standard,thus the minimum MSV opening that makes the duct flow solvable under the current working state of the components can be obtained (denoted by AMSV-c).However, if the opening of FVABI reaches the maximum (Amax) and still cannot meet the requirement of the recirculation margin, or the total bypass Mach number exceeds 1.0 during the regulating process,the current matching state is unattainable for the bypass system.

A graphic explanation of the bypass restriction is displayed in Fig.9, where the operating boundaries of the inner and outer bypasses are depicted on the system performance map.According to Fig.9,the MSV limits the CDFS working range by its choking and recirculation boundaries under specified fan and CDFS settings,and the restriction is removed as the MSV opens up.Moreover,the restriction of the MSV will extend to the downstream HPC,cutting off an area from its operational zone.Furthermore, upon the given inner bypass ratio, the FVABI choking situation will affect the HPC operating zone,with the level of restriction determined by the FVABI geometry.

Since the operating range of the compressor component is already obtained in terms of the acceptable bypass ratio(Fig.7), superimposing the bypass limit onto it turns out to be the most convenient method.See Fig.10(left side),analysis in Fig.8 has proved that with operating conditions of the fan and the CDFS determined,there exists a minimum MSV opening area (denoted as AMSV-c) that makes the bypass flow achievable, so along each CDFS constant speed line, AMSV-cis in one-to-one correspondence with the outer bypass ratio Bout.What’s more, since each outer bypass ratio corresponds to a definite CDFS matching point (at constant fan operating point and CDFS rotational speed),the outer bypass ratio Boutis in one-to-one correspondence with the CDFS stall margin SMCDFS.The combination of the above relation reveals that AMSV-cis in one-to-one correspondence with SMCDFS.That is to say,under the given AMSV,there exists a maximum outer bypass ratio(Bout-max)that can be achieved by the compression system,while the CDFS will have to operate above a minimum stall margin (SMCDFS-min).

Fig.8 Feasible condition of bypass system.

Fig.9 Operating boundaries of bypass configurations on system performance map.

Fig.10 Restriction of bypass for acceptable bypass ratio adjustment range.

As shown in Fig.10 (right side), by projecting the MSV constraint to the component operating zone, the space is separated into the ‘‘feasible zone” and the ‘‘infeasible zone”.The infeasible zone is beyond the scope of the DBE compression system, yet the realizability of the feasible zone requires further validation.In fact, the ‘‘upper” and ‘‘lower” limits of the bypass (dashed line) depend on the regulation ability of the FVABI, which is what the next section endeavors to disclose.

4.Interaction between compression components and bypass configurations

After clarifying the matching working mechanisms of both components and bypass,this section will consider the coupling effect between them and explore the actual matching working boundary of the compression system.The following analysis is performed according to the system operation mode.

4.1.Variations of system operating range: Double bypass mode

4.1.1.Using FVABI to adjust system operating range

According to Fig.8,in the feasible zone,the bypass system will naturally satisfy the matching Criteria 1–3,whereas regulation of the FVABI will affect the Mach number of the total bypass(Criterion 4)and the system recirculation margin(Criterion 5).The variation range of the inner bypass ratio depends on these two criteria.To get an overview of relevant parameters,Fig.11 demonstrates the variations of the bypass aerodynamic performance with the regulation of the FVABI, while the other control parameters are kept constant.It can be seen from Fig.11(a)that the inner bypass ratio will increase monotonically with the increase of AFVABI, hence under ideal conditions, the upper and lower limits of Bincorrespond to the maximum and minimum opening state of the FVABI, respectively.As shown in Fig.11(b), the Mach number at the outlet of the total bypass will increase with the increase of AFVABI, meaning that supersonic phenomenon may occur in extreme cases.Thus, Criterion 4 should be considered when calculating the maximum Bin.Moreover, see Fig.11(c), the system recirculation margin tends to increase with the increase of AFVABI,so the recirculation boundary(i.e.,Criterion 5)will not restrict the upper limit of the inner bypass ratio.On the contrary, the lower limit of Binwill be restricted by Criterion 5 and Criterion 4,respectively.

In a word,in the feasible zone,the maximum value of Binis restricted by the upper limit of AFVABIand the Mach number in the total bypass, while the minimum value of Bindepends on the lower limit of AFVABIand the system recirculation margin.

To further reveal the coupling effect between the bypass ducts and the compressor components, the aerodynamic performance for the Case 2 in Table 1 is analyzed using the multi-dimensional method.The blue shadow in Fig.12 represents the operation zone of the compressor components,whose boundaries consist of the lower and upper bounds of the compressor stall margin.Meanwhile,the red shadow designates the adjustment range of the bypass restricted by the five criteria in Section 3.2.According to Fig.12, although the compressor component can operate with the outer bypass ratio approaching 0.48,the maximum outer bypass ratio that the bypass system can achieve is only 0.46 due to the geometry limit of the MSV.As for the conditions where Bout< 0.46, results show that the adjustment range of Binby the bypass system is wider than that that by the components, thus the bypass will not impair the operating range of the compression system.In fact,the overlapped area of the component operation range and the bypass operation range is what the compression system can substantially achieve, which is named the ‘‘comprehensive operation zone” in the present study.

A closer inspection of the bypass operation range (Fig.12)shows that the upper and lower bounds of the inner bypass ratio are not monotonous with the increase of the outer bypass ratio: the maximum value of the inner bypass ratio (Bin-max)first increases and then decreases with the increase of Bout,the turning point is at Bout=0.35;on the other hand,the minimum value of the inner bypass ratio (Bin-min) first decreases and then increases slowly with the increase of Bout, with the minimum value achieved at Bout= 0.28.The reason for the above phenomenon is that the factors that restrict the aerodynamic performance of the bypass have changed with the variation of the system matching state.For example, when Bout< 0.35, the restriction factor for Bin-maxis the maximum opening of the FVABI, yet at Bout> 0.35, the Mach number at the total bypass will become the obstruction.As for Bin-min,for the cases with Bout< 0.28 and Bout> 0.28, the limiting factors are the recirculation margin and the minimum opening of the FVABI, respectively.

4.1.2.Using MSV to adjust system operating range

Fig.11 Effect of AFVABI on bypass aerodynamic performance(nFancor=1.0,SMFan=25%,nCDFScor=0.9,αVIGV=20°,Bout=0.27,AMSV = 1.0).

Table 1 Calculation scheme to investigate influence of bypass ducts.

Fig.12 Operation range of compression system under influence of bypass.

To unveil the influence of the MSV geometry on the operation range of the compression system, the comparison of the cases in Table 1 is demonstrated in Fig.13.Since the four schemes to be considered share the same fan and CDFS operating condition,the component operation zones of the four cases are identical.With the closure of the MSV, the operation range of the bypass witnesses a significant shrink, thereby narrowing the comprehensive operation zone of the compression system (by more than 60% in the present case).The influence of the MSV is mainly on the right boundary of the bypass operation zone, i.e., the flow capability of the outer bypass, whereas the bypass regulation range in the feasible zone is much wider than that of the components.Since MSV is either fully open or closed (AMSV= 0 or AMSV= 1) at the steady state of the engine,the results in Fig.13 indicate that the bypass in the current compression system is well-designed to eliminate extra restrictions.The results also imply that the mode transition of the compression system(0

It should be mentioned that the aerodynamic range of the bypass system is related to the operating state of the components.When the matching condition of the components changes, the adjustment range of the bypass and the corresponding restrictive factors will also change.The example given in this paper is only to illustrate the interaction between the compressor component and the complex bypass, whereas the method proposed in the present work is universal for all kinds of VCE compression systems.

4.2.Variations of system operating range: Single bypass mode

To start with, consider the component operation zone in Fig.14.Under the single bypass mode,the MSV is fully closed,thus the line at Bout= 0 represents the operating range of the compressor components.In other words, at the component level,the single bypass mode is an exceptional case of the double bypass mode,where the operation range of the components degenerates from a two-dimensional domain into a straight line.According to the former analysis, the variation range of the inner bypass ratio depends on the mass flow range of the high pressure compressor.

From the perspective of the bypass system, with the matching point of the fan and the CDFS determined (i.e.,Bout= Const), the inflow condition of the inner bypass duct is uniquely determined.Consequently, at each FVABI condition, the regulation range of the outer bypass ratio is limited by the geometric flow capacity of the bypass duct and the backpressure poutat the total bypass outlet.Given that the present work is focused on the compression system,the restriction on poutis not considered in this study, thus the minimum value of the bypass ratio is zero (with a high enough pout).Meanwhile,the maximum bypass ratio corresponds to the case when the Mach number of the inner bypass reaches 1.0.Fig.15 presents the variation law of the maximum bypass ratio with the variation of AFVABI.Results show that the maximum value of the bypass ratio increases with the increase of AFVABI.The increment comes directly from the increase in the through-flow area of the inner bypass duct.

To figure out the coupling effect of the bypass and component on the system operation range, the upper and lower boundaries of the inner bypass ratio for the case in Fig.14 are depicted in Fig.15, dividing the FVABI condition into 3 types: (A) In Type I, the through-flow area of the FVABI is large enough to achieve the maximum bypass ratio in Fig.14, so the bypass configuration will not restrict the regulation range of the system; (B) In Type II, the maximum bypass ratio that can be achieved by the bypass system is between the upper and lower limits of Binin Fig.14.As a result, the maximum flow capacity of the FVABI becomes the limiting factor of the system operation range; (C) In Type III,the through-flow area of the FVABI is too small to achieve even the lower limit of the bypass ratio in Fig.14, thus the compression system cannot be matched in this area.

According to the former analysis, under the single bypass mode,the factors that restrict the regulation range of the compression system include the stall margin of the high pressure compressor and the flow capacity of the FVABI.By plotting the attainable bypass ratio at different FVABI conditions,the operation region for the single bypass mode can be obtained, as shown in Fig.16.In Fig.16 (a), the compression system could manage to work when the FVABI opening is larger than 0.377, and the conditions with AFVABI< 0.433 falls into the matching Type II.At AFVABI>0.433,the flow capability of the FVABI is high enough to allow the maximum bypass ratio,thus reaching matching Type I.With the decrease of the fan rotating speed,see Fig.16(b),the bypass will always have a restriction on the compression system, hence the whole operation zone is under Type II.Note that the operation zone in Fig.16 is derived based on specific fan and CDFS working states.The shape and range of the operation zone will change with the variation of the system matching state, but the method to determine the operation zone is universal.

Fig.13 Effect of MSV geometry on operation range of compression system.

Fig.14 Component operation range of compression system for single bypass mode(nFancor=1.0,SMFan=10%,nCDFScor=0.9,αVIGV = 0°).

Fig.15 Variation of maximum bypass ratio with FVABI opening condition under single bypass mode.

5.Conclusions

This paper focuses on the compression system of the double bypass engine, and developed a multi-dimensional method to investigate the coupling effect between the complex bypass and the compressor component.The working mechanism of the compression system is revealed through thorough analysis,and the operation range of the compression system is proposed.The main conclusions are as follows:

Fig.16 Operation zone of compression system for single bypass mode.

(1) The operation range of the DBE compression system is constrained by both compressor components and bypass configurations.The operation range of the compressor components depends on their stall and choke boundaries, while the regulation range of the bypass is determined by the geometric boundary,the aerodynamic boundary, and the backflow boundary together.

(2) In the double bypass mode, there exists a minimum MSV area that ensures a feasible bypass flow when the matching state of the fan and the CDFS are determined.Under the specific MSV geometry, there exists a minimum CDFS stall margin that ensures a feasible bypass flow.The minimum MSV area is in one-to-one correspondence to the minimum CDFS stall margin.

(3) The limiting factor for the bypass system diverse in the double bypass mode: the maximum value of the inner bypass ratio is restricted by the upper limit of AFVABIand the maximum Mach number in the total bypass,while the minimum value of the inner bypass ratio depends on the lower limit of AFVABIand the system recirculation margin.

(4) The substantial operation range of the compression system is the overlapped part of the component operation zone and the bypass operation zone.The state of the MSV turns out to reduce the comprehensive operation zone by more than 60%,so the mode transition process should be carefully designed.

(5) The single bypass mode is a unique condition of the double bypass mode, where the operation zone of the compressor component degenerates from a twodimensional surface to a straight line.Under the given fan and the CDFS condition, there are multiple bypass states available by the compression system,whereas the regulation range of the bypass ratio is jointly determined by the stall margin of the high pressure compressor and the aerodynamic boundary of the FVABI.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was co-supported by the National Natural Science Foundation of China (No.52206038), the National Science and Technology Major Project, China (No.Y2022-II-0003),and the Science Center for Gas Turbine Project, China (Nos.P2022-A-II-001-001 and P2022-B-II-002-001).