Patent Application
20170336431
The present application relates to supersonic propulsion systems, and more specifically, to a system and method for measuring exhaust flow velocity of supersonic nozzles.
Several known methods exist to obtain a flow velocity measurement. Intrusive methods include hotwire anemometry, Pitot-static probes, and turbines among others. Due to material constraints, these are likely not viable for high temperature and corrosive flows characteristic of rocket nozzles. Non-intrusive (or minimally so) methods rely on optical techniques often involving high speed cameras and powerful lasers. Examples include: Schlieren imaging where gradients in refractive index can be visualized by passing a collimated beam through the flow, Doppler where the spectral shift from a stimulated flame emission or an illuminated particle is recorded, and visual particle tracking techniques such as Particle Image Velocimetry (PIV) where a seed material is added to the flow and illuminated.
A key distinction in the non-intrusive methods is the requirement of known flow composition. Shock waves observed by Schlieren can provide Mach number which requires molecular composition and temperature information to convert to velocity. Doppler methods using laser excitation techniques such as Raman spectroscopy are referenced to emission wavelengths of specific species in the exhaust. In contrast, seeded particle techniques require less stringent knowledge of flow properties making them more ideal for investigation of wholly new propellant formulations. Particles must simply be sized such that the velocity lag relative to length and time scales in the flow of interest is sufficiently small. Provided that background flame emissions can be filtered out from the seed signal, seeded techniques can be applied in the exact same manner irrespective of the exhaust plume composition.
While non-seeded techniques require more information than seeded techniques, some, such as Doppler, become more resolvable as velocity increases. Conversely, velocities of seeded flows have an upper limit based on the system. In PIV, the maximum velocity is governed by the window length and the time step between frames. A conventional metric for PIV to avoid aliasing is that the particle displacement across one frame pair must be at least four times smaller than the interrogation windows. Pulsed illumination sources can easily reach down to sub-nanosecond separation time, but the shutter speed of commercially available, cutting-edge cameras used to image such flows is typically limited to between 100 and 500 ns. The measured velocity is averaged across an entire window meaning that constant or linearly accelerating flows produce accurate mean values at the window center location. However, this is not valid for flow fields with high velocities and non-linear acceleration in the direction of the flow; both of which are characteristic of rocket exhaust flows. At time scales of 100 to 500 ns, the interrogation window size required to accurately resolve the magnitude and position of velocity vectors without aliasing becomes prohibitively small. Therefore, improvements are needed in the field.
The present disclosure provides a system for measuring exhaust flow velocity of supersonic nozzles. The system comprises a seeding module, an optics module, and a streak camera. The seeding module injects seed particles into a nozzle flow. The streak camera comprises a photocathode, a sweep module, a micro-channel plate, a phosphor screen, and a charged coupled imaging device. On a single frame, a signal on the y-axis is swept across the x-axis at a controlled rate and the time step is governed by the dwell time on each pixel using the streak camera.
In the following description and drawings, identical reference numerals have been used, where possible, to designate identical features that are common to the drawings.
The attached drawings are for purposes of illustration and are not necessarily to scale.
In the following description, some aspects will be described in terms that would ordinarily be implemented as software programs. Those skilled in the art will readily recognize that the equivalent of such software can also be constructed in hardware, firmware, or micro-code. Because data-manipulation algorithms and systems are well known, the present description will be directed in particular to algorithms and systems forming part of, or cooperating more directly with, systems and methods described herein. Other aspects of such algorithms and systems, and hardware or software for producing and otherwise processing the signals involved therewith, not specifically shown or described herein, are selected from such systems, algorithms, components, and elements known in the art. Given the systems and methods as described herein, software not specifically shown, suggested, or described herein that is useful for implementation of any aspect is conventional and within the ordinary skill in such arts.
The measurement apparatus 30 includes a laser light source 1 (e.g., a laser diode), a focusing lens 2, a beam dump 3, a positive spherical lens 4, a micrometer slit 5, a prism 6, a macro lens 7, a spectrometer 8, and the streak camera 9. The laser light source 1 is oriented with the linearly polarized axis vertically oriented. The output of the laser light source 1 is formed into an expanding sheet using a collimating lens and the cylindrical lens 2. The horizontal waist is located at the nozzle center line with the sheet height being approximately 5 mm in one example and positioned at the nozzle exit plane. The spherical lens 4 and micrometer slit 5 act as a field stop to limit the width of the axial line being imaged. This allows further control of the number of particles being imaged and the total light intensity. The dovetail prism rotates the image 90° to align with the streak camera 9 photocathode orientation. A 200 mm f/4 macro Nikon lens is used in one example to collect the image. In one example, the spectrometer is a UV-Vis spectrometer, but for the purposes of this experiment, this is bypassed by setting the diffraction grating to a zero-order reflection which will act as a plane mirror directing the signal into the streak camera 9.
The streak camera comprises a photocathode 40, a sweep module 42, a micro-channel plate 44, a phosphor screen 46, and a charged coupled imaging device (CCD) 48 as shown in
The elimination of the shutter allows the system 30 to operate with a continuous light source as opposed to the frame straddling of dual pulse lasers used in prior art systems. An example system of the type described herein uses a 1.6 Watt, 445 nm laser diode with a 2 Amp switching DC power supply. The micro channel plate 44 integrated into the streak camera 9 provides a lower detection threshold than CCD or CMOS detectors alone.
The invention is inclusive of combinations of the aspects described herein. References to “a particular aspect” and the like refer to features that are present in at least one aspect of the invention. Separate references to “an aspect” (or “embodiment”) or “particular aspects” or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to “method” or “methods” and the like is not limiting. The word “or” is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted.
The invention has been described in detail with particular reference to certain preferred aspects thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.
The present patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 62/338,567, filed May 19, 2016, the contents of which is hereby incorporated by reference in its entirety into the present disclosure.
| Number | Date | Country | |
|---|---|---|---|
| 62338567 | May 2016 | US |
