Application of the thermochromic liquid crystal heat transfer measurement technique to rotor-stator systems

The ability to measure temperature, heat transfer, and heat transfer coefficients is vital to the design process of any kind of turbomachinery. This information is needed, for example, to calculate and predict thermal stresses inside an aircraft engine. In turn, knowledge of the thermal stresses is needed to ensure the engine operates efficiently and that the sealing characteristics between the various airpaths of the engine are optimal. Furthermore, heat transfer measurements can reveal insights into not only the temperature distributions that are present inside an engine, but also into the flow fields that drive the heat transfer in the first place.

The work in this thesis focuses on the application of the thermochromic liquid crystal temperature measurement technique, to measure local heat transfer coefficients at high spatial resolution on rotating surfaces. The technique, not previously available at the Thermo-Fluid Mechanics Research Centre at the University of Sussex, was implemented and developed from the ground up, and then applied to measure heat transfer on a shrouded rotor- stator test rig in the presence throughflow, introduced at the axis of rotation. The rig existed previously, but it was modified heavily to suit the purposes of the work. The rig, in its modified form, has a fully enclosed rotor with a radius of 224mm, and a movable stator that can achieve rotor-stator gap ratios anywhere between 0.02-0.08.

Experiments were conducted for rotational Reynolds numbers, 𝑅𝑅𝑒𝑒∅, between 2.1 - 4.9 x 10⁵ and for Throughflow Reynolds Numbers, CW, between 0.3 – 2.48 x 10⁴. The resultant turbulent flow parameter values, λT, ranged between 0.084 - 1.365, thus covering conditions where the flow fields are likely either throughflow or rotation dominated. These experiments were conducted for a smooth rotor, as well as a rotor that had 18 equispaced hexagonal bolts on it at a nondimensional radius of 0.9. Results were acquired for two rotor-stator gap ratios, 0.08 and 0.0625. Several liquid crystal calibration experiments were also conducted. A new intensity-based method depending on the ratio of the red and the green signal intensities was found to deliver a surprisingly good overall calibration for the entire region of interest inside the test rig. Local Nusselt numbers were measured approximately on a 50-degree segment of the rotor. Both Nusselt number vs. radius and full surface heat transfer coefficient distributions were consequently plotted and analysed.

Although not many measurements of this kind exist in the literature, the results were compared with the closest available data in terms of geometry and non-dimensional conditions, to validate the developed measurement technique. Good agreement was found in regions where this was to be expected, especially in terms of the overall trends in the data. It was also found that under certain conditions, the introduction of the bolts appear to alter the heat transfer along the entire radius of the rotor, not just in the localized region of the bolts. Asymmetry in the heat transfer coefficient values was also observed under certain non-dimensional conditions where this was not expected.

On the technology development side of the work, a high temporal accuracy, ultra-bright, and strobed LED lighting system was developed from the ground up. This custom-made solution was found to work well in delivering more than the required light inside confined spaces, such as those that are often found in turbomachinery experiments. It is thought that a lighting system like this will be useful to many other researchers’ work, if such work involves using camera shutter timings in the range of 1-20 microseconds to eliminate motion blur, and/or if the geometry of the test rig requires complex non-standard illumination patterns in confined spaces (i.e. where commercially available and often restrictively bulky strobe lights with simple illumination patterns cannot be used).

A new non-contact rotor heating method based on permanent magnets and induced eddy currents was also developed as part of this work. Although not ultimately used for various reasons, it is thought that this technique could prove useful to other researchers for certain types of experiments – offering some potential advantages over the film and infrared heating techniques in particular.