What is an Expansion Tube?

An expansion tube is an impulse flow device for studying supersonic flow. The applications of supersonic flow research are primarily in the field of aeronautical engineering for military jets, space shuttles, and rockets; however, supersonic conditions also occur in some turbines.

Expansion tubes provide several advantages over other supersonic test facilities, such as supersonic wind tunnels and shock tubes. Expansion tubes can generate a wide range of mach numbers and stagnation conditions. Unlike supersonic wind tunnels, the test gas does not pass through these high temperature, high pressure stagnation; therefore, it is possible to perform combustion tests with the fuel mixed with the test gas. Most importantly, expansion tubes are safer test facilities and more appropriate for undergraduate use.  

The Lafayette College expansion tube will allow for test flows from approximately Mach 2 to Mach 8 for approximately 500 µs. When a test is run, shocks will propagate down the tube as depicted in the x-t diagram below. The test gas will start in the driven section and its flow will be observed in the end of the expansion section.  

The expansion tube is composed of four sections: the driver, the double diaphragm, the driven, and the expansion. The sections will be separated by thin sheets of mylar plastic that will be bolted in between the flanges to hold the gasses in their sections until the test commences. The driver will be pressurized to a maximum of 800 psi with helium, which is an inert gas that will be distinguishable from the test gas and not interact with it. The double diaphragm will be pressurized to a mid-range pressure of 400 psi and evacuated quickly to break the diaphragms. The driven section will contain the test gas at a low vacuum pressure at the start of the test. The expansion section, which includes the viewing section and dump tank, will be vacuumed down to 30 mTorr prior to the test. When the diaphragms rupture, the sudden expansion of gas from the driver creates a shock wave. Following this wave, gas will accelerate down the tube through the viewing section at the desired velocity for testing.

References:

Anderson, John David. Modern Compressible Flow: with Historical Perspective. 3rd ed. New York: McGraw-Hill, 2003. Print.

Ben-Yakar, Adela, and Ronald K. Hanson. "Characterization of Expansion Tube Flows for Hypervelocity Combustion Studies." Journal of Propulsion and Power 18.4 (2002): 943-952. Print.

Hanson, Ronald K. "Expansion Tube Theory", Internal Report, Stanford University, 2001. Print.

Parziale, N. J., J. Rabinovitch, G. Blanquart, H. G. Hornung, and J. E. Shepherd. "Proposed Vertical Expansion Tunnel." AIAA Journal 51.12 (2013): 2792-2799. Print.

 

Here is Hanson's description with diagrams from Stanford. 

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