Advanced evaluation of transient heat transfer experiments using thermochromic liquid crystals

Abstract

An advanced evaluation method for transient heat transfer experiments using thermochromic liquid crystals (TLCs) combining the advantages of standard hue and maximum intensity methods is presented. In order to obtain a global evaluation of locally correct heat transfer coefficients by using the one-dimensional solution of Fourier's equation, assuming heat conduction in a semi-infinite medium with a convective boundary condition, local input values have to be identified from measurements of the fluid and surface temperatures. For that reason, two different approaches have emerged. First, a two-dimensional numerical method has been adapted to evaluate the transient fluid temperature distributions in multi-pass systems from a few local measurements. Additionally, on the basis of latest calibration and indication experience of TLCs, especially in complex passages, an innovative temporal indication analysis method using a neural network has been implemented in the process of heat transfer evaluation.

Optimum Jet-to-Plate Spacing of Inline Impingement Heat Transfer for Different Crossflow Schemes

“…This, in turn, was needed for precise calculation of heat transfer coefficients. Based on the temperature data for certain locations temporally, which are recorded with time by the thermocouples, a numerical method is applied to the experimental data in order to calculate the complete fluid temperature distribution during a transient experiment [20]. A two-dimensional interpolation scheme for an unknown pixel at (x, y) is based on the assumption that, for each time step, the fluid temperature distribution T (x, y, t) can be described by the Laplace equation.…”

“…When the reference temperature at any pixel of the geometry has been determined, the heat transfer coefficient is determined using Eq. (3) [20].…”

Rotating Heat Transfer Measurements on a Multi-Pass Internal Cooling Channel: I — Rig Development

“…The validated time matrices are finally de-warped and merged together in order to provide a final map of activation times on the whole geometry. The Tb distribution map at each time interval is computed with the finite element method based on the Laplace diffusion equation [9]. The Tb computation starts from the loading of the thermocouples readings obtained thanks to the Ni-cRIO.…”

Rotating Heat Transfer Measurements on a Multi-Pass Internal Cooling Channel: II — Experimental Tests

“…Heat transfer measurements on the test section are carried out by means of thermochromic liquid crystals with the well-known transient technique. The fundamentals of the technique can be found in [24][25][26], while the specific methodology used in this work is reported in [23]. A mix (50% in volume) of two narrow banded encapsulated liquid crystals supplied by Hallcrest has been used, namely R2C1W and R12C1W (in the following named LC2 and LC1, respectively).…”

“…A mix (50% in volume) of two narrow banded encapsulated liquid crystals supplied by Hallcrest has been used, namely R2C1W and R12C1W (in the following named LC2 and LC1, respectively). The selection of these activation temperatures (2°C and 12°C) is the result of a tradeoff between different requirements: -liquid crystal activation must occur inside all the passage legs; -keep the activation time below the maximum allowable test duration to ensure the validity of the 1D thermal diffusion hypothesis inside the model wall, which is at the basis the transient technique [24][25][26]. Moreover, another requirement for LC1 and LC2 identification was the value of the experimental parameter Θ, the "non dimensional temperature".…”