A transient method is used to determine the distribution of the local heat transfer coefficient on a Perspex model with five parallel cooling channels at Reynolds number 20000. The rectangular channels with an aspect ratio of 5:2 and a length to diameter ratio of 43.75 are featured by one-sided oblique ribs with 25% blockage. Due to the high length and the local tabulators, the problem arises of evaluating the heat transfer in one experiment within an acceptable error range. While the wall near the ribs heats up very fast, the region at the end of the channel takes too long to reach the desired evaluation temperature. In the available Perspex model with parallel channels, the three inner channels can be assumed to have same flow and thermal conditions. The advantage is that these three channels can be evaluated separately within one experiment.

Stationary numerical RANS simulations using the SST turbulence model are performed to approve the assumption of equal mass flow rates and flow conditions for all the investigated channels. A block-structured mesh with 19 million cells was generated such that the first wall adjacent cell matches y_1\^+\<1 for all regions of interest.  In the simulations, it was shown that the mass flow rates of the different channels alter by much less than one percent from each. To validate that the numerical situation reflects the experiments well, the pressure loss of both is compared.

In the experiments, the transient Thermochromic Liquid Chrystal method is applied. The relevant channels of the Perspex model are sprayed with narrowband Thermochromic Liquid Crystals (TLC) with the indication temperatures of 31°C, 38°C and 45°C respectively. The fluid temperature is measured by six thermocouples in each channels centerline as well as before and after the channels. It is shown that the measured temperatures at the same downstream positions in the respective parallel channels do not differ significantly. In order to calculate the heat transfer distribution in all channels with the same temperature field, the thermocouple data are averaged over all three channels.

The fact of equal flow and thermal conditions in the investigated channels give the opportunity to evaluate the heat transfer coefficients in dependency of the used TLCs in the respective channel. Multiple experiments were conducted. All start at ambient conditions before the fluid is heated up to varying temperature levels between 45°C and 75°C. It can be shown, that the calculated heat transfer coefficients and therefore the dimensionless Nusselt number ratios strongly depend on the temporal uncertainties and temperature uncertainties. A key result is that the different Thermochromic Liquid Crystals provide the same heat transfer coefficients within the measurement uncertainties as long as a suitable temperature step is chosen.

In addition, the accuracy in the evaluation of the heat transfer coefficients is improved by superimposing the results of the individual TLCs and weighting them depending on their local time and temperature uncertainties. Within one experiment, the distribution of the local heat transfer coefficient in the investigated channels can be calculated with high accuracy.


Heat Transfer Measurements Using Multiple Thermochromic Liquid Crystals in Symmetric Cooling Channels

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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