Nasarek, Ralph (2010)
Temperature Field Measurements with High Spatial and Temporal Resolution Using Liquid Crystal Thermography and Laser Induced Fluorescence.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Temperature field measurements with high spatial and temporal resolution are necessary to understand microscale flow phenomena which occur in various applications like nucleate boiling, evaporation of menisci or heat transfer enhancement through micro-structured surfaces. These phenomena often cannot be fully described by theoretical or numerical models. The two techniques, which are probate for temperature measurements in macro regions, are liquid crystal thermography (LCT) and laser induced fluorescence (LIF). The attraction of the LCT technique is the possibility to simultaneously measure temperature and velocity fields, because the encapsulated TLCs (Thermochromic Liquid Crystals) can be applied as tracer particles for temperature and velocity measurements. If LCT is applicable for high spatial and temporal resolution measurements, it was not clarified at this point, though. In principal, the LIF method can be applied for high spatial and temporal resolution measurements due to its physical principle. On this account, the measurement methods LCT and LIF were characterized regarding their spatial and temporal resolutions and applicability. Moreover parameters which affect these measurement techniques were identified, and possibilities to minimize the error sources were presented. As the analysis of the measurement data plays an important role in the enhancement of the measurement accuracy, it was investigated in detail and new processing methods were developed. To determine the measurement accuracy of the LCT method, measurements of homogeneous temperature distributions and thermal induced convection were conducted in a cubic facility and compared with numerically calculated ones. The influence of the angle between the illumination and the camera axis was investigated and compared with results from literature. Due to diverse statements in literature concerning hysteresis effects caused by overheating of the TLCs, adequate measurements were conducted. It was found, that hysteresis occurs but it does not depend on the grade of overheating. Further, the hysteresis disappears, when the TLCs are cooled beyond the start temperature of the activity range. The main restriction of the temporal resolution is assumed in the thermal response time of the encapsulated TLCs. Therefore, measurements concerning the response characteristics of TLCs were performed and a thermal response time of less than 10 ms at high temperature gradients was observed. Several image analysis algorithms were developed and applied which leaded to an enhancement of the measurement accuracy. Volume illumination was employed for the first time in a channel flow, and it was shown that extraction of temperature fields by applying special image analysis methods is possible. Moreover, the optical setup was extended and scanning of the channel flow was realized so that 3D temperature fields could be achieved. Similar to the LCT method, the applicability of the LIF method is investigated regarding its accuracy and good results concerning the accuracy and spatial resolution were found. To overcome the distinctive dependency on the dye concentration and the illumination, two-color-LIF was employed. It was found that fluorescence dyes used in literature for two-color-LIF are not appropriate for the use with a Nd:YAG laser which again is useful for high spatial resolution measurements. For this reason, spectral investigations on eight different dyes were conducted. As a result, Pyridine 1 and Rhodamine 6G were found as suitable dyes for two-color-LIF which provide a good signal to noise ratio and therefore potential for high resolution measurements. Further, it was proved that the two-color-LIF method significantly reduces the influence of inhomogeneities in illumination and in concentration. When reckoning the TLC capsule diameter, the thermal response time and the measurement uncertainty, the limitation in the spatial and temporal resolution of LCT becomes clear. Nevertheless, due to the simplicity of the measurement setup and the possibility for simultaneous measurements of temperature and velocity fields, LCT is attractive for applications like static flows. A further advantage is the possibility to resolve small temperature differences of hundredth of Kelvin when applying TLCs with small measurement ranges. TLCs also offer an important advantage for applications in micro flows: volume illumination can be applied instead of a light sheet. The LIF technique exhibits the possibility of very high spatial and temporal resolution and the limitation is mainly caused by the optical equipment like the laser and the camera. Further, the basis for high temporal and spatial resolution measurements using two-color-LIF was established. Employing an Nd:YAG laser as a powerful illumination source in combination with Pyridine 1 and Rhodamine 6G enables high acquisition rates (up to 500 Hz with the employed equipment) and high spatial resolution.

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