Pulsed operation of high-power light emitting diodes (LED) for flow diagnostics

Abstract

The proposed contribution investigates high powered light emitting diodes (LED) for possible uses as light sources in flow diagnostics, in particular, as an alternative to laser-based illumination in flow velocimetry. While earlier generations of LEDs were primarily used as indicators in every-day electronics, recent developments have resulted in mass-produced devices whose light output rivals that of conventional light sources such as incandescent light bulbs and now find an increased range of applications ranging from architectural lighting to projection system and automotive head lights. In the framework of flow diagnostics LEDs offer an number of attractive advantages in comparison to lasers which deserves closer investigation. Aside from the dramatically reduced cost of procurement, an LED provides incoherent light over a considerable wavelength range covering tens of nanometers, which alleviates many issues related to speckle in laser based illumination. Of particular interest is the fact that LEDs can be operated in pulsed mode at currents significantly beyond their standard damage threshold. With rise times on the order of tens of nanoseconds short but intense light pulses can be generated. So far this light source has been used for high-frame rate (100 kHz) kerosene spray investigations using a shadowgraphy configuration. High speed measurements of a simple water flow have been demonstrated at frame rates up to 4 kHz using volume illumination. First PIV measurements of air flows seeded with micrometer sized aerosol and illumination pulses of 1-2 µs have been shown to be successful on small field of views ( < 3 cm2). Generally being a non-collimated light source is one the more critical drawbacks of the LED. This makes it difficult to provide light sheet illumination commonly found in macroscopic PIV applications. On the other hand it is viable for LEDs to provide volume illumination for microscopic PIV or photogrammetric particle imaging methods such as tomographic PIV or 3-D particle tracking. The proposed contribution intends to assess the possible utilization of high power LEDs operated in pulsed mode in these and related flow measurement techniques.