Shock Compression

 What Is It?
 Flight Applications of Shock Compression
 The Rampressor - Ramgen's Compression Technology
 The Rampressor Competitive Advantages
 Ramgen's Technology Development Focus
 RAM 1 Proof of Concept
 RAM 2 CFD Validation

RAM 1 Proof of Concept

Ramgen has successfully demonstrated the proof-of-concept of its Rampressor supersonic compressor in a 12-month testing program conducted at the Boeing Company’s Nozzle Test Facility, located in Seattle, WA. Figure 1 shows the installation of the first Rampressor test rig “RP-1”.

These test results, combined with Ramgen’s performance prediction tools, confirmed expectations and validated the preliminary performance and operational claims of the Rampressor technology.

The principal goal of this test was to demonstrate the successful adaptation of supersonic flight inlet designs and performance methods to the rotating environment found on a compressor disc. A 2.5:1 pressure ratio Rampressor rotor was extensively instrumented to measure the performance and operational characteristics of the test rotor. This relatively low pressure ratio was chosen to allow construction of a lower-cost one-piece machined rotor, while still generating meaningful test results. Analytical performance prediction tools were also constructed and refined with the experimental results.

Ramgen’s team of scientists and engineers has an extensive and varied experience in the development and testing of turbo compressors, and led the design of experiment and the build efforts on both the test article and the test apparatus. Ramgen’s team of 20 people began the design of the RP-1 test rig in September of 2002, and completed extensive testing in April of 2004. Approximately $1,800,000 of equipment and facility expenses were required to complete the program.

Figure 1  Rampressor-1 Test Rig Installed in Test Cell

Figure 2 is a representation of the compressor rotor flowpath. This “unwrapped” view of the rotor, together with the pre-swirl cascade, was used to establish the rotor inflow field in the RP-1 test.

The design rotor speed in the RP-1 program was 21,100 rpm. Figure 2 defines the basic flow-path stations surrounding the rotor (shown in gray in the inset). The direction of rotation of the rim shown in Figure 2 is right to left. The combination of the rotor speed and the pre-swirl from the upstream cascade of pre-swirl nozzles create a rotor inflow that is supersonic relative to the moving rotor. Some details of the flow-field are summarized in Table 1.

Figure 2.  Rotor Station Numbering Convention

Ramgen conducted over 300 hours of spin testing during a 7 month test sequence at the Boeing Nozzle Test Facility. Boeing’s world-class support ensured consistent and repeatable test results and provided additional oversight to Ramgen’s industry-standard approach to conducting the test.

The experiment successfully demonstrated that supersonic flight inlet design methods and performance predictions can be adapted to a rotating disc and that the operational characteristics of the technology are similar in nature to conventional compressor technologies. Figure 3 shows data from a typical day of testing. Discharge throttling, which changes the back-pressure on the compressor, rotational speed and tip clearance were all varied to determine their impact on compressor performance and operating characteristics.

Effect of Clearance, Bleed and Throttling

Figure 3 is an example of a test sequence. The data progression starts in the lower left corner of the figure and was recorded as the rotor accelerated to the specified test speed. At 80% speed, the compressor variables were adjusted to achieve supersonic flow – referred to as “starting” in supersonic inlet technology jargon. When started, the compressor immediately moved up and to the right on the figure, as it was able to process more airflow when operating supersonically. The discharge pressure was then raised by throttling the discharge, followed by a tip clearance adjustment. These physical parameters were then varied as part of the test sequence to explore the range of efficiencies and the operating envelope.

Figure 4, indicates the compressor characteristics at speeds ranging from 30% to 110%, and the expected decrease in mass flow with increasing pressure. These characteristics were generated with relatively tight tip clearances. This original test was not designed to produce a compressor map per se, and the lower speed lines were extrapolated from limited data. Many other sensitivities and trade studies were performed.

The “star” on Figure 4 corresponds to the peak adiabatic compression efficiency observed in the test program. This peak efficiency value was ~83% and was viewed as good performance, particularly considering that there had been minimal effort to optimize the flow-path efficiency. Further flow-path geometry refinements would certainly have resulted in further increases. Of particular interest is that the efficiency of the rotor did not drop-off significantly with decreased speed and/or flow.

Several important discoveries were made during testing that advanced understanding of the Rampressor operating characteristics. Perhaps the most significant is that the compressor is self-aspirating. That is, it does not require an impeller or blower to feed air to the rotor. During the course of the test program the air supply system was disconnected to allow the compressor to draw air directly from the room to verify this behavior. Elimination of an impeller or blower stage significantly decreases the cost and complexity of the future Rampressor product.

An important measure of compressor stability is the resistance to “surge,” or the tendency for flow to suddenly (often violently) reverse through the compressor. Although Ramgen did establish the surge line in test, it did not experience a sudden surge event as do conventional turbo compressors. The unique nature of the shock wave compression process and the rotor geometry led to a gradual and benign surge that was also easily corrected. This is a substantial improvement over traditional, highly loaded turbo-compressors.

For clarity, the individual data points in this figure have been reduced to a series of solid lines represented measured speed lines and a series of hollow diamonds for extrapolated data; each series represents the behavior of the compressor at different rotational speed (referred to as a speed line).

The RP-1 Test also confirmed that the Rampressor does maintain its efficiency across a range of airflow rates, as shown in Figure 5. This counteracts conventional “wisdom” which holds that such acoustically-based technologies can only operate at their design point. This is an important feature for any compressor design to load follow.

The peak efficiency achieved during this test was approximately ~83%. There is a large assortment of techniques available to increase the efficiency level to that required for subsequent testing and any future commercial offering. These techniques and know how are the result of Ramgen’s developing performance prediction tools and include, but are not limited to, improvements in basic flowpath geometry, rotor tip clearance and bleed optimization.

5.  Rotor Efficiency Versus Airflow

Ramgen has compared the experimental data to the predicted performance, and, overall, the data agreed well with predictions. Several data disparities were discovered as a direct result of the test data. The code has been calibrated and is now considered reliable in predicting performance of the compressor.

Extensive Computational Fluid Dynamics simulations were performed to determine the usefulness of this approach to future modeling. The exact results of this comparison are confidential and Ramgen is highly confident in the design tools at its disposal in its ongoing development efforts.

Ramgen has presented a comprehensive review of the test program results to the scientific staff of the DoE, who have given their approval and continued support to the program.