Boundary layer flows such as film flows takes a major role in some industrial application. Film flow occurs on any technical process where the surface is fully covered by the fluid. Typical examples for this kind of flows can be found for e.g. on industrial coating process, film cooling of components or even cleaning processes. Simulations of this kind of flows can be very complex and time consuming. Even if simulation results can be produced, there is a lack of appropriate and accurate measurement technique for flows in this small scale for validation purpose. Conventional LDV Measurement is not capable for this small flow-field in the range of a few hundred microns.
Within the scope of a research-project with TU Dresden, PTB Braunschweig, ILA R&D GmbH and OPTOLUTION a new type of Sensor called LDV Profile Sensor was designed, which is capable to make LDV measurements with a very high spatial resolution and tested on a film flow set-up in the lab. The Profile Sensor works in backwards-detection-arrangement and is commercially available now. More details can be found on the datasheet:
The film flow was created in our lab by directing a small jet of water horizontally on a glass plate. The running off water was creating uniformly spread film flow with thickness between 0.2 mm and 1 mm over the glass plate, which was measured with the profile sensor from the backside through the glass. The measurements were taken with a xy-traverse system to take velocity profiles towards the drain. To avoid backreflexions from the oscillating water-surface to the laserhead a polarisation filter was used.
The measurement-results were similar to the analytically derived results introduced in the literature by (STEVENS/WEBB, 1993). The Measurement revealed the typical velocity profile of a flow film starting with a steep slope near to glass plate and ending with a flattened curve at the free end of the water surface. With the conventional LDV Sensor, it is not possible to take such kind of velocity profiles in this range of resolution. As the measuring volume of a conventional has a finite length of approx. 100 microns, the measurements represents spatially averaged values of 100 microns and don't allow to measure the flow structure within the measuring volume length.
These test-results have shown the ability of the profile sensor to capture the flow field in a small area of interest such as boundary layer, film flow or microchannel-flow.
Additionally to this particular survey of analyzing the film flow further case-studies were conducted.