Effects Of Inlet Distortion Research Articles

Effect of inlet distortion on internal flow and performance of a contra-rotating fan operating with single-stage impeller

To establish a theoretical foundation for the rational selection of contra-rotating fans operating with a single-stage impeller under inlet distortion. Unsteady numerical simulations were conducted using the SST k-ω turbulence model to analyze the airflow fields, respectively, for the single-stage operation of front impeller (SSOFI) and the single-stage operation of rear impeller (SSORI) of the fan. The internal flow characteristics with single-stage operation under both distorted inlet condition (DIC) and uniform inlet condition (UIC) were studied. Furthermore, a comparison of the performance parameters of the fan, including total pressure and total pressure efficiency, was conducted. Focusing on the SSOFI with satisfactory operational performance of the contra-rotating fan, this study aims to reveal the propagation of inlet distortion inside the fan and reveal the influence mechanisms of inlet distortions on the performance of the fan. The results indicate that the impact of inlet distortion on the flow field is primarily concentrated in the upstream area of the front-stage impeller for SSOFI. However, the flow disturbance from the exit of the front-stage impeller to the diffuser is relatively minimal. For SSORI, the inlet distortion exacerbates the adverse effect of the front stage impeller as the front guide vane on the flow inside the rear stage impeller. The entropy loss in the two-stage impellers is significantly higher than that under the uniform inlet condition, and the turbulence level in the diffuser is intensified. While the efficiency of the fan is significantly influenced by the inlet condition and operating stages, their impact on the total pressure is small. Specifically, under uniform inlet conditions, the total pressure efficiency for SSOFI exceeds that of SSORI by 4.9 %, whereas the difference rises to 11 % under inlet distortion condition.

Combustion enhancement subjected to the inlet distortion in a cavity-based supersonic combustor

The present study experimentally investigates the effect of inlet distortion on the combustion process in a direct-connect test environment. A blockage by introducing physical intrusions with three heights (H = 2.5 mm, 3.2 mm, and 4 mm) was mounted at the entrance of the isolator to mimic the inlet distortion. Cases without blockage were used as a comparison. Schlieren visualization synchronized with high-speed CH* chemiluminescence imaging and pressure measurements were employed to characterize the flowfields. The results show that the blockage mounted at the entrance of the isolator could reasonably mimic distortion generated at the inlet. The combustion enhancement is achieved in all cases with blockage, and the lean blowoff limits could be widened in the combustor with blockage (H = 2.5 mm and 3.2 mm). In the cavity stabilized combustion mode and transition mode (Φ=0.159∼0.689), the stronger oblique shock wave emanating from the cavity leading edge and the additional background shock wave created by the inlet distortion are the chief causes of combustion enhancement in cases with blockage. The combustion enhancement gradually recedes with the increase of blockage height. It could be speculated that the relatively concentrated impinging of the incoming shock waves induces a more remarkable combustion enhancement. In the jet-wake stabilized combustion mode (Φ=0.848∼1.325), the effect of blockage height on combustion enhancement is significantly suppressed. As the equivalence ratio rises to 1.06, the regularity that the effect of blockage height on combustion enhancement is completely reversed. That is, the combustion enhancement attenuates with the decrease in blockage height. It could be inferred that in these cases, the dominance of combustion enhancement is the incoming flow velocity upstream of the reaction zone instead of the impinging sparsity of the incoming shock waves.

Mitigation of Boundary Layer Ingestion Circumferential Distortion Using Nonaxisymmetric Fan Exit Guide Vanes

Abstract This paper presents an assessment of the effect of nonaxisymmetric fan exit guide vanes (FEGVs) to mitigate, or even reduce to zero, the impact of circumferential flow distortion—such as created by boundary layer ingestion (BLI)—on fan rotor efficiency and vibratory structural forcing. The fan response to circumferential nonuniformities is characterized, for two-dimensional flow, in terms of the variations in incidence angle and unsteady rotor forces. FEGV designs with nonaxisymmetric exit flow angle perturbations can eliminate either of these. More importantly, nonaxisymmetric stator row designs can reduce both nonuniformities relative to the influence of an axisymmetric stator row. This suggests that design for distortion need not involve a tradeoff between performance and aeromechanics; a nonaxisymmetric aerodynamic design approach can improve both simultaneously. Analysis of the impact of stage design parameters has been carried out to determine FEGV exit flow angle variations that shield the rotor from the adverse effects of circumferential distortions representative of BLI. The results indicate that inlet distortion effects can be mitigated with realizable nonaxisymmetric FEGVs. The magnitude of the geometry variations required decreases with decreased axial rotor–stator spacing and increased flow coefficient.

Effect of inlet distortion with crosswind on aircraft-engine intake aerodynamic performance

Effect of inlet distortion with crosswind on aircraft-engine intake aerodynamic performance

Prediction and Evaluation of Waterjet Pump Performance under Nonuniform Inflow Using Parallel Compressor Theory

Parallel compressor theory (PCT) is commonly used to estimate effects of inlet distortion on compressor performance. As well as compressor, the actual inflow to pump is also nonuniform and unfavorable for performances. Nowadays, insufficient understanding of nonuniform inflow effects on pump performance restricts its development. Therefore, this paper applies PCT to predict external characteristics and evaluate internal flow instability of waterjet pump under nonuniform inflow. According to features of nonuniform inflow, the traditional PCT is modified and makes waterjet pump sub-divided into two circumferential tubes owning same performances but with different inlet velocity (representing nonuniform inflow). Above all, numerical simulation has been conducted to validated the applicability and accuracy of PCT in head prediction of waterjet pump under nonuniform inflow, since area-weighted sum of each tube head (i.e., theoretical pump head) is highly consistent with simulated result. Moreover, based on identifications of when and which tube occurs stall, PCT evaluates four stall behaviors of waterjet pump: partial deep stall, partial stall, pre-stall and full stall. Furthermore, different stall behavior generates different interactions between head variation of each tube, resulting in a multi-segment head curve under nonuniform inflow. The modified PCT with associated physical interpretations are expected to provide a sufficient understanding of nonuniform inflow effects on pump performances.

A study of fan-distortion interaction within the fan rotor

Boundary-layer-ingesting fans and compressors in the next-generation turbofan engines require high-performance operations under distorted inflow. The aim of this work is to study the effects of inlet distortions including inlet stagnation pressure and temperature distortion, on the aerodynamic performance of a transonic axial fan. Firstly, the validated full-annulus, unsteady, three-dimensional computational fluid dynamic code in conjunction with detached Eddy simulation approach is used here to simulate the fan flows assembly with individual inlet stagnation pressure/temperature distortion. Then, the propagation process of the inlet distortion waves is analyzed to understand how the aerodynamic performance degradation is triggered. The simulation results show that the fan performance is remarkably degraded when the inlet distortion is introduced. The leading-edge spillage, the trailing edge back flow and the “tornado vortex” occur when parts of fan blades encounter the incoming distorted flows. Finally, the responses of fan to the combined inlet stagnation distortion effects are discussed in this paper. It is found that the combined distortion effects can be predicted based on the sum of the performance responses to the individual constituent distortions. Furthermore, the relative location of the constituent distortions shows a non-ignorable influence on the overall fan performance, especially for the intensified inlet distortion.

The effect of inlet distortion on low bypass ratio turbofan engines

Inlet flow non-uniformity, commonly known as inflow distortion, has been a long-standing problem in the history of gas turbine engines. Distortion can be present in the form of total pressure, total temperature or inflow incidence or any combinations of these. The search for better and robust performance requires engines that can sustain a large amount of inlet distortion without considerable loss in the thrust. In the present paper, the effect of total pressure distortion on a single-stage compressor and low bypass ratio fans are studied. Distortion near hub and tip in the form of step radial total pressure profiles is imposed at far upstream of the rotor leading edge. A systematic approach to qualitatively predict the performance maps in the presence of these distortions is discussed. Further, two extents of total pressure distortion are explored for constant inlet distortion intensity. Hub distortion is found to increase the stability margin, whereas tip distortion reduces it. On extending the distortion extent, hub distortion drastically reduces the stability margin, whereas a comparatively lower reduction in stability margin with tip distortion is observed. The critical distortion limit is observed by varying the inlet distortion extent. Also, it is found that downstream ducts in the bypass axial fan do not interact with the upstream fan. This can be exploited to perform independent simulations of the core engine from low bypass ratio fans. Hub distortion is found to drastically affect the duct performance owing to the presence of thicker upstream inlet boundary layer.

Influence of the inlet distortion on fan stall margin at different rotational speeds

The aim of this paper is to develop a reliable and accurate numerical strategy that can be used to study the effects of inlet distortions on the aerodynamic stability of fan blades in aero-engines. As an initial step towards achieving this goal, three-dimensional unsteady Reynolds-Averaged-Navier-Stokes (URANS) simulations were carried out to predict the influence of total pressure distortion on the loss of stall margin of a fan blade. NASA rotor 67, for which a significant amount of measured steady and unsteady data is available, was used for this study. It was observed that the size of the time step has a significant effect on the solution near stall and hence the stall margin of the blade.In the second part of this work, unsteady simulations were carried out to study the effects of rotational speed on the stall margin and stalled operating point of the blade. The results showed that for the same level and pattern of inlet distortion, the stall margin of the blade decreases as the corrected speed decreases. However, the in-stall total pressure losses decrease as the speed decreases. Finally, in our research it has become apparent that there is a big lack of public domain measured data for the cases with inlet distortion, and therefore, validated CFD results can be very helpful to other researchers in the field.

Inlet Distortion Effects on Fuel Distribution and Ignition in Scramjet Cavity Flameholder

The effects of inlet distortion and shockwave interaction on fuel-air distribution and ignition in a cavity-based flameholder with supersonic flow were investigated using nanosecond-gated laser-induced breakdown spectroscopy (n-LIBS). Experiments were conducted in a Mach-3 freestream direct-connect-type wind tunnel with direct cavity fueling. Ethylene, , was used as the fuel for all testing. Inlet distortion was created using a wedge on the top wall of the tunnel, which formed an oblique shock. The distortion wedge was positioned at two distinct locations such that the shock intersected 1) the boundary layer just upstream of the cavity or 2) the shear layer above the cavity flameholder. N-LIBS with direct spectrum matching was implemented for quantitative measurements of gas density and fuel mole fraction throughout the cavity in both pre-ignited and ignited conditions for three different cavity flow patterns (two distortion cases and no distortion). For all flow patterns fuel mole fraction was seen to increase in the cavity between the nonignited case and the ignited case, due to changes in air entrainment from the freestream. Distortion was seen to affect the cavity shear layer and thus the fuel distribution within the cavity, as well as local gas density.

A Parametric Study of the Effects of Inlet Distortion on Fan Aerodynamic Stability

The performance and aerodynamic stability of fan blades operating in a circumferentially nonuniform inlet flow is a key concern in the design of turbofan engines. With the recent trends in the design of civil engines with shorter inlet ducts (such as low-speed fans), or boundary layer ingesting engines, quick and reliable modeling of rotor/distortion interactions is becoming very important. The aim of this paper is to study the effects of inlet distortions on the aerodynamic stability of a fan blade and to identify the parameters that have a major impact on the stability of the blade. NASA rotor 67, for which a significant amount of measured data is available, was used for this study. In the first part of this study, the behavior of the fan with inlet distortion at near stall (NS) condition is analyzed, and it is shown how rotating stall is triggered. In the second part of this study, unsteady simulations with inlet distortion were performed to study how exit duct length influences the stall margin of the blade.

Investigation of inlet distortion effects on axial compressor performance based on streamline curvature method

In this paper, the effects of inlet flow distortion on performance maps and details of the flow field are investigated using the Streamline Curvature Method. The results are presented for both design and off-design conditions and compared with experimental data, which shows good agreement. The effects of inlet flow distortion are investigated by inlet total pressure variation in three different cases in the way that the average total pressure remains constant and equal to the design value. The results show that a relative increase in the total pressure at tip causes an increase in the pressure ratio and efficiency as well as a better performance in the choking region. Alternatively, a relative increase in the total pressure in hub causes opposite behavior and a better performance at the surging region.

Effect of Inlet Distortion Features on Transonic Fan Rotor Stall

The effect of inlet distortion from curved intake ducts on jet engine fan stability is an important consideration for next-generation passenger aircraft such as the boundary layer ingestion (BLI) “silent aircraft.” Highly complex inlet flows which occur can significantly affect fan stability. Future aircraft designs are likely to feature more severe inlet distortion, pressing the need to understand the important factors influencing design. This paper presents the findings from a large computational fluid dynamics (CFD) investigation into which aspects of inlet distortion cause the most significant reductions in stall margin and, therefore, which flow patterns should be targeted by mitigating technology. The study considers the following aspects of distortion commonly observed in intakes: steady vortical distortion due to secondary flow, unsteady vortical distortion due to vortex shedding and mixing, static pressure distortion due to curved streamlines, and low momentum endwall flow due to thickened boundary layers or separation. Unsteady CFD was used to determine the stall points of a multipassage transonic rotor geometry with each of the inlet distortion patterns applied. Interesting new evidence is provided, which suggests that low momentum flow in the tip region, rather than distortion in the main body of the flow, leads to damaging instability.

Analysis of Fan Stage Conceptual Design Attributes for Boundary Layer Ingestion

This paper describes a new conceptual framework for three-dimensional turbomachinery flow analysis and its use to assess fan stage attributes for mitigating adverse effects of inlet distortion due to boundary layer ingestion (BLI). A nonaxisymmetric throughflow analysis has been developed to define fan flow with inlet distortion. The turbomachinery is modeled using momentum and energy source distributions that are determined as a function of local flow conditions and specified blade camber surface geometry. Comparison with higher-fidelity computational and experimental results shows the analysis captures the principal flow redistribution and distortion transfer effects associated with BLI. Distortion response is assessed for a range of (i) design flow and stagnation enthalpy rise coefficients, (ii) rotor spanwise work profiles, (iii) rotor–stator spacings, and (iv) nonaxisymmetric stator geometries. Of the approaches examined, nonaxisymmetric stator geometry and increased stage flow and stagnation enthalpy rise coefficients provide the greatest reductions in rotor flow nonuniformity, and may offer the most potential for mitigating performance loss due to BLI inlet distortion.

Effect of inlet distortion on the performance of axial transonic contra-rotating compressor

The present paper numerically conducted full-annulus investigation on the effects of circumferential total pressure inlet distortion on the performance and flow field of the axial transonic counter-rotating compressor. Results reveal that the inlet distortion both deteriorates the performance of the upstream and downstream rotors resulting in reduction of total pressure ratio, efficiency and stall margin of the transonic contra-rotating compressor. Regarding the development of distortion inside compressor, the downstream rotor reinforces the air-flow mixing effects and, thus, attenuates the distortion intensity significantly. Under the distorted inflow conditions, the detached shockwave at the leading edge of downstream rotor interacts with the tip leakage flow and causes the blockage of the blades passage, which is one important reason for the transonic contra-rotating compressor stall.

Comparison of a Direct-Connect and Freejet Dual-Mode Scramjet

Testing of identical dual-mode scramjet flowpath geometries in the freejet and direct-connect configurations was conducted in two separate facilities. These tests enabled a comparison study between the two configurations, which was directed toward the determination of the effects of inlet distortion and backpressure on the performance and operability of a dual-mode scramjet. Bulk flow conditions were matched between the two facilities at the isolator entrance plane to simulate Mach 4.8 flight, and a series of metrics were established to quantify similarities and differences. The effects of flowpath backpressure in the direct-connect case were seen to be isolated to regions close to the exhaust. An approximate 10% decrease in combustor pressure rise and consequently integrated pressure force were observed in the freejet configuration; shock train length, however, remained the same. Mode transition was delayed from an equivalence ratio of in the direct-connect case to in the freejet configuration. Further, ignition difficulties were experienced in the freejet tests which were not encountered in those of direct-connect tests, limiting the scope of comparable test points. This work represents the first attempt at a quantitative comparison in the literature of an identical direct-connect and freejet dual-mode scramjet.

Numerical investigation on hydrogen combustion in a scramjet with 3D sidewall compression inlet

Numerical simulations were employed to analyze the flowfield of a scramjet with three-dimensional (3D) sidewall compression inlet, and the effect of inlet distortion on the mixing and combustion process was examined. The numerical approach solved the compressible Reynolds Averaged Navier–Stokes (RANS) equations supplemented with a finite rate chemical reacting model for the combustion of hydrogen fuel and air. Turbulence closure was achieved using Menter shear-stress transport (SST) model. To verify the accuracy of the simulation, the computed wall pressure was compared with the experimental data of the direct-connect combustor test. The metrics employed in the simulations included qualitative assessments related to flow structure as well as quantitative values of fuel mixing efficiency, combustion efficiency and static pressure distribution. Intake sidewalls were found to strongly affect the inlet flow structure, which became more complex in the nonuniform flowfield. The shock train system affected the combustion region located upstream of the injection and led to pairs of asymmetric separation bubbles. Nevertheless, the shock train system dissipated due to the reactions, the combustion patterns of each fuel jets in downstream region were nearly identical, and the degree of improvement of mixing and combustion efficiency near the downstream injectors was less than that near the upstream injectors.

Studies of Inlet Distortion in a Direct-Connect Axisymmetric Scramjet Isolator

Direct-connect wind-tunnel facilities that produce uniform flow entering the test section are generally used for scramjet-component (isolator/combustor) studies. In freejet experiments and flight tests, however, air enters the engine through an inlet, and flow entering the isolator and combustor is typically distorted. The distortion effects can include nonuniform boundary-layer thicknesses on the walls and relatively strong oblique shock waves. This research focuses on the effects of inlet distortion on a round scramjet isolator. A numerical study was performed using various distortion devices, including ramps, injector ports, and injector slots, that were placed downstream of the direct-connect facility nozzle to simulate a distorted flowfield from a prescribed inlet. The computational results provided a methodology for simulating flow distortion in direct-connect testing. Based on the numerical findings, experimental testing was conducted in the supersonic wind-tunnel facility to validate the numerical results and determine the impact of flow distortion. In addition, this paper focuses on design, fabrication, and execution of flow-distortion experiments. Air injection in the distortion section was used to create the flow distortion, and shock angle increases with the injection flow. Comparisons of wall pressure, exit flow profiles, and shock-holding capability are made from the experimental and numerical results. Good agreement between these data on wall pressures and flow profiles was found.

Single Neuron PID Control of a Turbofan Engine with Inlet Pressure Distortion

In this paper, a preliminary exploration and research about the adaptive control law has been carried on for the turbofan engine with inlet pressure distortion. First, a real-time dynamic component-based model which contains the inlet distortion effects was established. Then, we select a single neuron PID controller with adaptive parameters to complete the dynamic acceleration process. The results illustrate that the single neuron control algorithm with gain adaptive has strong robustness, self-learning and anti-jamming capability and the performance of a turbofan engine can be effectively improved by using the turbine expansion ratio adaptive control law.

Experimental and numerical analysis on the effect of inlet distortion on the performance of a centrifugal fan with a mixing chamber

Inlet flow distortions, which are caused by fluid mixing, cause a significant deterioration in fan performance. An experimental test rig for an industrial fan with dual inlets and a mixing chamber was constructed. The flow fields in the mixing chamber of the fan were numerically investigated. Consequently, impact parameters, including the length of the mixing chamber (100, 200, and 300 mm) and the mass flow rate ratio (1 to 10), as well as their effects on fan performance, were discussed. A generalized formula considering the Reynolds number, hydraulic diameter, and mixing length was proposed to predict the pressure drop in dual inlets. Results show that the efficiency of and pressure in the fan decreased by 6.5% and 203 Pa, respectively, under mixing inlet condition. Optimum fan performance is achieved at a flow rate ratio of 5 under the same mass flow rate. The increase in the flow rate ratio kept the fan performance almost constant. At the design stage, fan performance and pressure decrease by an average of 2% and 70 Pa in increments of 100 mm mixing length, respectively. The results presented in this paper provide a basis in the design optimization of mixing structures.

Correlation Measure-Based Stall Margin Estimation for a Single-Stage Axial Compressor

This paper presents a method for estimating compressor stall margin and the results of applying the estimation technique to an axial compressor rig. Stall margin estimation is accomplished through the use of a compressor stability detection parameter called the “correlation measure.” The correlation measure captures the periodicity of the pressure in the rotor tip region of the compressor. The downcrossing frequency of the correlation measure across some preset threshold is measured while operating the compressor rig at various steady-state points along the design speed characteristic line. These measurements are used to generate a relationship with stall margin as a function of downcrossing frequency. The estimation technique is evaluated by applying it while dynamically ramping the operating point of the compressor up the design speed line towards surge. A brief investigation on the effects of inlet distortions on the correlation measure-based estimation system is also given.