A unique supersonic compressor rotor with high pressure ratio, termed the Rampressor, is presented by Ramgen Power Systems, Inc. (RPS). In order to obtain the excitation characteristic and performance of Rampressor inlet flow field under external excitation, compression inlet flow of Rampressor is studied with considering Rampressor rotor whirling. Flow excitation characteristics and performance of Rampressor inlet are analyzed under different frequency and amplitude of Rampressor rotor whirling. The results indicate that the rotor whirling has a significant effect for flow excitation characteristics and performance of Rampressor inlet. The effect of rotor whirling on the different inlet location excitation has a definite phase difference. Inlet excitation becomes more complex along with the inlet flow path. More frequency components appear in the excitation spectrum of Rampressor inlet with considering Rampressor rotor whirling. The main frequency component is the fundamental frequency, which is caused by the rotor whirling. Besides the fundamental frequency, the double frequency components are generated due to the coupling between inlet compression flow of Rampressor rotor and rotor whirling, especially in the subsonic diffuser of Rampressor rotor inlet. With the increment of rotor whirling frequency and whirling amplitude, the complexity of Rampressor inlet excitation increases, and the stability of Rampressor inlet performance deteriorates.
Ramgen engine with a proofofconcept version of a new type of compression system has been proposed by American Ramgen Power Systems, Inc. [
Ramgen Power Systems, Inc. has developed the related numerical simulation of Rampressor inlet, which provided the advantageous validation for the design of inlet flowpath structure and supersonic shock wave compression system [
Operational condition of Rampressor rotor is determined by the inlet excitation characteristic that causes the rotor whirl, simultaneously inlet flow distribution and excitation characteristic are affected by rotor whirl. However, most of the previous researches emphasized on Rampressor inlet structural design based on aerodynamic performance, and Rampressor inlet excitation characteristics had not been reported in the references. Therefore, it is necessary to study the effect of rotor whirl on flow excitation characteristics and performance of Rampressor inlet.
Models of Rampressor rotor and inlet are established in this study, and the effects of exit back pressure on the shock wave structure, flow field distribution, and flow performance of Rampressor inlet are numerically studied. Then this paper emphatically analyzes the inlet flow field considering Rampressor rotor whirling. The variations of inlet excitation characteristic and performance under rotor whirling are obtained and discussed in this paper.
The Rampressor rotor impeller can be developed with two, three, four, or more inlets according to the Rampressor flow. The Rampressor rotor model in this study is established with three inlet flow paths as shown in Figure
Model of Rampressor rotor.
The three inlets of the designed Rampressor are the symmetric periodic layout on the rotor. If Rampressor rotor does not whirl, the flows of the three inlets are of central symmetry, so one of the three inlets can be used for numerical study of the flow field. Beside this, the rotor whirl is generally generated in the radial direction, but the difference between threedimensional and twodimensional models is in the axial direction. Because the rotor whirling almost has no effect on the axial structure of threedimensional model, the axial flow gradient of the inlet can be ignored. Therefore, it is accepted to use the twodimensional model for studying the excitation characteristic and performance of Rampressor inlet with considering Rampressor rotor whirling.
The twodimensional simplified model of Rampressor inlet is established for numerical study of flow excitation and performance as shown in Figure
Twodimensional simplified model of Rampressor inlet.
Solution of the compressible form of the Euler equations for the simulations presented herein is conducted using a finitevolume and density based scheme in the fluent simulation of this study. So the twodimensional simplified model does not take into account the boundary layer developed upward, but the centrifugal force is taken into consideration in the calculation. The calculation formulation is implicit, and the convection flux type is Roe averaged flux difference splitting (RoeFDS). Figure
Inflow boundary condition: the Mach number is 0.348, the total pressure is
Wall boundary condition: the noslip and adiabatic wall boundary conditions are placed on the wall surfaces. The engine case is the stationary adiabatic wall. The rotor rim is also adiabatic wall, and the direction of rotation of the rim shown in Figure
Subsonic outlet boundary condition: the exit condition is set to the pressure outlet in order to generate normal shock waves within internal inlet flow field.
Specific boundary conditions of Rampressor inlet.
The numerical method of this paper is validated by comparing the numerical results of Rampressor inlet flow field of American Ramgen Power Systems, Inc. [
Comparison of relative centerline Mach number versus normalized streamwise distance (S) between result in this paper and result in Ramgen.
In this paper, high quality grid of twodimensional simplified model of Rampressor inlet is calculated by using the structured grid technology. The computational grid density of Rampressor inlet model should be examined. Mach number distributions of Rampressor inlet are computed in different grid sizes. Comparison of Mach number contour of Rampressor inlet in different grid sizes is given in Figure
Comparison of Mach number contour of Rampressor inlet in different grid sizes.
Gird size = 11,636
Gird size = 20,249
Gird size = 45,439
Gird size = 84,590
Figure
Comparison of pressure distribution of stationary engine case versus normalized streamwise distance in different grid sizes.
The computational model of grid size 45,439 is chosen for the latter Rampressor inlet simulation in comprehensive consideration with computational accuracy and computational complexity. The grid employed 781 nodes (maximum number) in the streamwise direction and 88 (maximum number) in the radial.
The following parameters are defined to analyze the performances of Rampressor inlet flow path for different operating conditions [
Static pressure ratio of flow path can be obtained as follows.
Totalpressure recovery coefficient of flow path:
Pressurization ratio in flow path:
Loss coefficient in flow path:
Kinetic energy efficiency in flow path:
Nondimensional total pressure distortion of flowpath exit is defined as
In order to study the excitation characteristic of Rampressor inlet well, pressure pulsation of key points in Rampressor inlet should be measured. The arrangement of key points is shown in Figure
Schematic diagram of key point in Rampressor inlet.
The equation
Flow distribution of Rampressor inlet in different
A series of oblique shock waves is generated by the compression ramp of inlet flow path to achieve airflow compression, and the airflow pressure after the shock wave increases abruptly as shown in Figure
Pressure distributions along stationary engine case and rotor rim surface of Rampressor inlet are given in Figure
Pressure distribution of Rampressor inlet in different
Stationary engine case
Rotor rim surface
The pressure distribution curves of the stationary engine case and rotor rim surface are completely overlapped before normal shock wave in the different
The consequences of Rampressor inlet flow performance in different pressure ratios are shown in Table
Flow performance parameters in the different







9  0.7649  11.48  0.0116  0.9292  36.73 
10  0.8294  12.44  0.0075  0.9512  43.39 
10.6  0.8483  12.73  0.0063  0.9572  25.46 
Rampressor inlet flow may be affected by Rampressor rotor whirl in the work process. When the inlet pressure regularly changes, which is caused by rotor whirl, Rampressor rotor bears the inconstant pressure load and then vibrates.
Structure schematic diagram of inlet flow path under Rampressor rotor whirl is illustrated in Figure
Structure schematic diagram of inlet flow path on Rampressor rotor.
Because the three inlets of the designed Rampressor are the symmetric periodic layout on the rotor, the flow excitation characteristics and flow performance of inlet flow path 1 are studied under Rampressor rotor periodic whirl in this paper. Expression of rotor periodic whirl is given as follows:
Result of steady flow is taken as the initial result in the unsteady calculation of this paper. Time step size is set to 1.478 × 10^{−5} s in the design rotor speed. The unsteady flow of Rampressor inlet under rotor whirl is studied when
Pressure pulsation time history and spectrogram on every key point of Rampressor rotor inlet are shown in Figure
Pressure pulsation time history and spectrogram on every key point of Rampressor rotor inlet.
Point A
Point B
Point C
Point D
Figure
Time history of nondimensional excitation in a pulsation cycle is given (as shown in Figure
Time history of nondimensional excitation.
Pressure distributions along stationary engine case and rotor rim surface of Rampressor inlet in a whirling motion cycle are shown in Figure
Pressure distributions along stationary engine case and rotor rim surface in a whirling motion cycle.
Stationary engine case
Rotor rim surface
Figure
Partial enlarged drawing of pressure distributions along stationary engine case.
Point A
Point B
The curves of flow performance parameters of Rampressor inlet in a whirling motion cycle are shown in Figure
Flow performance of Rampressor inlet in a whirling motion cycle.
Totalpressure recovery coefficient
Pressurization ratio
Kinetic energy efficiency
Pressure pulsation spectrograms of key point D (shown in Figure
Calculation results of point D in different rotor whirling frequencies.
Figure
Calculation results of airflow exciting force on the rotor rim surface of Rampressor inlet in different rotor whirling frequencies.
Figure
The curves of flow performance parameters of Rampressor inlet in a whirling motion cycle are, respectively, obtained in different whirl frequencies such as
Flow performance in different whirling frequencies during a whirling motion cycle.
Totalpressure recovery coefficient
Pressurization ratio
Kinetic energy efficiency
Excitation characteristics of Rampressor inlet are analyzed in different rotor whirling amplitudes such as
Calculation results of point D in different rotor whirling amplitudes.
The spectrograms of airflow exciting force on Rampressor rotor rim surface are, respectively, obtained in different rotor whirling amplitudes such as
Calculation results of airflow exciting force on the rotor rim surface of Rampressor inlet in different rotor whirling amplitudes.
As shown in Figure
Flow performance of Rampressor inlet is studied in different rotor whirling amplitudes such as
Flow performance in different rotor whirling amplitudes during a whirling motion cycle.
Totalpressure recovery coefficient
Pressurization ratio
Kinetic energy efficiency
Based on Rampressor rotor model and inlet flow model, the compression inlet flow field of Rampressor rotor is numerically studied with consideration of Rampressor rotor whirling. Flow excitation characteristics and performance of Rampressor inlet are analyzed and discussed under the different frequencies and amplitudes of Rampressor rotor whirling. The following conclusions are obtained.
Along with the increment of
More frequency components appear in the excitation spectrum of Rampressor inlet with considering Rampressor rotor whirling. The main frequency component is the fundamental frequency, which is caused by the rotor whirling. Besides the fundamental frequency, the double frequency components emerge because of the coupling between inlet compression flow of Rampressor rotor and rotor whirling, especially in the subsonic diffuser of Rampressor rotor inlet. The effect of rotor whirling on the excitation of Rampressor inlet wall has a definite phase difference. Inlet excitation becomes more complex along with inlet flow path. With the increase of rotor whirling frequency and whirling amplitude, the complexity of Rampressor inlet excitation increases.
With the increase of rotor whirling amplitude, wave amplitudes of totalpressure recovery coefficient, pressurization ratio, and kinetic energy efficiency of Rampressor inlet gradually enlarge, and the stability of inlet performance reduces. But wave amplitudes of totalpressure recovery coefficient, pressurization ratio and kinetic energy efficiency of Rampressor inlet are constant with the increment of rotor whirling frequency, and only wave frequency of inlet flow performance parameters increases. Stability of inlet performance is better in the practical engineering when Rampressor rotor whirling frequency and amplitude are all less.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research presented here was supported by the National Natural Science Foundation of China (Grant no. 51106035). The authors are grateful for the support provided. The authors would like to thank Dr. Guanghui Zhang and M.S. Jianhua Lu for their constructive suggestions and/or assistant provided.