Afficher dans l'index de A à Z
Nombre de vues :
1419

DETAILED 2-D SURFACE HEAT TRANSFER MEASUREMENTS USING THERMOCHROMIC LIQUID CRYSTALS

Srinath V. Ekkad
North Carolina State University

Highly localized thermal measurements can provide details of heat transfer behavior that can help overall thermal management of cooling of turbine blades, electronic chip cooling, and any other cooling behavior in complex geometries The experimental technique involves using a thermochromic liquid crystal coating on the test surface. The colour change time of the coating at every pixel location on the heat transfer surface during a transient test is measured using an image processing system. The heat transfer coefficients are calculated from the measured time responses of these thermochromic coatings. This technique has been used for complex turbine blade internal coolant passage heat transfer measurements as well as turbine blade film cooling heat transfer measurements. Results can be obtained on complex geometry surfaces if visually accessible. The technique has been used for heat transfer results for experiments with jet impingement, internal cooling channels with ribs, flow over simulated TBC spallation, flat plate film cooling, cylindrical leading edge and turbine blade film cooling, rotating channel heat transfer, and also for electronic chip cooling.

The time of the liquid crystal to reach a prescribed temperature at every pixel location during a transient test is determined using the image processing system. Initially, the surface is at a uniform temperature all over the surface and is suddenly exposed to the oncoming air. Each pixel location will reach the prescribed temperature depending upon the local heat transfer coefficient. This indicates that the image processing system basically tracks the iso-heat transfer coefficient lines on the test surface. This technique was extended to a rotating channel by Lamont et al. (2012) for the first time. More details are explained on the technique and examples are shown in Ekkad and Han (2000) and Ekkad and Singh (2021).

The local heat transfer coefficient (\(h\)) over a surface coated with liquid crystals can be obtained by using a 1D semi-infinite solid assumption for the test surface. To apply this assumption on the test surface, the test section has to be manufactured using a low thermal conductivity, low thermal diffusivity material. Most of the experiments using this technique have used Plexiglas as the material for test section fabrication. The 1D transient conduction equation, the initial condition, and the convective boundary condition on the liquid crystal coated surface are:

\[\label{eq1} k\dfrac{\partial^2 T}{\partial x^2}=\rho C_p\dfrac{\partial T}{\partial t} \tag{1}\]

boundary conditions:

at \(t=\) 0, \(T=T_{i}\);

at \(x=\) 0, \(-k\dfrac{\partial T}{\partial x}=h(T_{w}-T_{m})\);

at \(x\to\infty\), \(T=T_{i}\).

Solving Eq. (1) with prescribed initial and boundary conditions, one obtains the non-dimensional temperature at the convective boundary surface (at \(x=\) 0):

\[\label{eq2} \dfrac{T_{w}-T_{i}}{T_{m}-T_{i}}=1-\exp\left(\dfrac{h^2\alpha t}{k^2}\right)\,\mathrm{erfc}\left(\dfrac{h\sqrt{\alpha t}}{k}\right) \tag{2}\]

The colour change temperature or the prescribed wall temperature \(T_{w}\) is known prior to the test based on factory setting or in situ calibration. The initial temperature (\(T_{i}\)) of the test surface and the mainstream temperature (\(T_{m}\)) are measured before and during the test respectively. The time of colour change of the liquid crystal coating at each pixel (\(t\)) is determined using the image processing system as earlier reported. The local heat transfer coefficient (\(h\)) can be calculated from Eq. (2). Test conditions are set such that the time of colour change on the surface is between 10 and 80 seconds. This enables the validity of the semi-infinite solid assumption on the test surface, as the test duration does not allow for temperature penetration across the test wall (depends on wall thickness). The heat transfer should not penetrate across the test wall to maintain the validity of the assumption. An analysis of the validity of the 2-D technique to a 3-D surface was presented by Ahmed et al. (2020).

An example for cooling concept with ribbed turbulator channel feeding an impingement cooling surface is shown in Fig. 1.

Detailed heat transfer measurements for a complex flow channel using liquid crystal measurements

Figure 1: Detailed heat transfer measurements for a complex flow channel using liquid crystal measurements

REFERENCES

Ekkad, S.V. and Han, J.-C. (2000) A Transient Liquid Crystal Thermography Technique for Gas Turbine Heat Transfer Measurements, Meas. Sci. Technol., 11(7), 957–968. DOI:10.1088/0957-0233/11/7/312

Lamont, J., Ekkad, S.V., and Alvin, M.A. (2012) Detailed Heat Transfer Measurements Inside Rotating Ribbed Channels Using the Transient Liquid Crystal Technique, J. Thermal Sci. Eng. Appl, 4(1), 011002. DOI:10.1115/1.4005604

Ahmed, S., Singh, P., and Ekkad, S.V. (2020) Three-Dimensional Transient Heat Conduction Equation Solution for Accurate Determination of Heat Transfer Coefficient, ASME. J. Heat Mass Transf., 142(5), 051302. DOI:0.1115/1.4044678

Ekkad, S.V. and Singh, P. (2021) Liquid Crystal Thermography in Gas Turbine Heat Transfer: A Review on Measurement Techniques and Recent Investigations, Crystals, 11(11), 1332. DOI:10.3390/cryst11111332

Les références

  1. Ekkad, S.V. and Han, J.-C. (2000) A Transient Liquid Crystal Thermography Technique for Gas Turbine Heat Transfer Measurements, Meas. Sci. Technol., 11(7), 957–968. DOI:10.1088/0957-0233/11/7/312
  2. Lamont, J., Ekkad, S.V., and Alvin, M.A. (2012) Detailed Heat Transfer Measurements Inside Rotating Ribbed Channels Using the Transient Liquid Crystal Technique, J. Thermal Sci. Eng. Appl, 4(1), 011002. DOI:10.1115/1.4005604
  3. Ahmed, S., Singh, P., and Ekkad, S.V. (2020) Three-Dimensional Transient Heat Conduction Equation Solution for Accurate Determination of Heat Transfer Coefficient, ASME. J. Heat Mass Transf., 142(5), 051302. DOI:0.1115/1.4044678
  4. Ekkad, S.V. and Singh, P. (2021) Liquid Crystal Thermography in Gas Turbine Heat Transfer: A Review on Measurement Techniques and Recent Investigations, Crystals, 11(11), 1332. DOI:10.3390/cryst11111332
Retour en haut de page © Copyright 2008-2024