Thermochromic Liquid Crystal Thermography Research Articles

An Experimental Investigation of Constrained Melting Heat Transfer of Nano-Enhanced Phase Change Materials in a Horizontal Cylindrical Capsule Using Thermochromic Liquid Crystal Thermography

Abstract The constrained melting of nano-enhanced phase change materials (NePCM) in a horizontal cylindrical capsule was investigated via the utilization of thermochromic liquid crystal (TLC) technique for tracking the invisible phase interfaces. A parametric study was carried out by varying both the loading of NePCM (i.e., 0 wt %, 1 wt %, and 3 wt %) and the wall superheat (at 10 °C and 30 °C), leading to a total of six cases. Numerical simulations, based on the enthalpy-porosity method, were also performed to reveal the evolutions of temperature and convective flow fields during melting. It was first shown that the numerically predicted melting front evolutions are in good agreement with the TLC imaging results. A comparison among the six cases indicated that there is a similar melting pattern that heat conduction dominates the initial stage of melting and natural convection then takes over to play a more important role when melting proceeds. With the TLC-assisted reconstruction of the melting fronts, the instantaneous melting and heat storage rates were estimated, and melting was clearly found to slow down with increasing the loading of NePCM, as a result of the dramatically increased viscosity that deteriorates the contribution of natural convection to melting heat transfer.

A Novel Indirect Visualization Method for Studying the Melting Heat Transfer of Nano-Enhanced Phase Change Materials Using Thermochromic Liquid Crystal Thermography

Abstract In order to give more sights into the melting (and solidification) heat transfer processes of nano-enhanced phase change material (NePCM) with invisible phase interfaces, a novel indirect method for tracking the phase interface by thermochromic liquid crystal (TLC) thermography is proposed. As an example case to demonstrate the applicability of the proposed method, the classical problem of melting heat transfer in a differentially heated rectangular cavity was revisited in the presence of NePCM of various loadings. A narrowband TLC was selected and calibrated carefully to build the hue–temperature relationship prior to being applied in the melting experiments. For validation purpose, the case of an unloaded NePCM, with a clear visible phase interface, was tested via combined direct and indirect observations. It was shown that this TLC method can easily and accurately capture the dynamic motions of the phase interface during melting. Based on the shape evolutions of the phase interface, it was concluded that for the NePCM sample with a higher loading (and hence a much greater viscosity), heat conduction becomes the dominant mode of heat transfer during melting as a result of the significantly deteriorated natural convection effect. This gives an intuitive confirmation of the hypothesis made in previous studies that were conducted using volume-average-based indirect methods.

Film cooling of cylindrical holes on turbine blade suction side near leading edge

The influences of blowing ratio, rotational speed, Reynolds number, and injection angle on film cooling were experimentally studied on turbine blade suction side near leading edge. One row of round holes is located at streamwise location of 13.5% on suction side. Three injection angles, 30°, 45°, and 60°, were tested. Investigations were conducted with three rotational speeds, 300 rpm, 400 rpm, and 500 rpm with blowing ratio varying from 0.5 to 3.0. Reynolds numbers corresponding to rotational speeds are 20,300, 27,200, and 33,800, respectively. Steady-state Thermochromic Liquid Crystal thermography was adopted to measure the adiabatic film cooling effectiveness. The pressure distributions and the second flow of the whole fluid domain were numerical simulated. Results showed that the blade mid-span has the best film coverage. Film cooling effectiveness descends with the increasing blowing ratio. Rotation can deflect the film trajectories toward the tip of the blade. The corresponding Reynolds number increases with the increase of rotational speed, which improves the film cooling performance. The coolant ejects with a smaller injection angle and a smaller blowing ratio can produce weaker inward film defections and so a better film coverage.

Quantifying ageing effects in thermochromic liquid crystal thermography as applied to transient convective heat transfer experiments

Thermochromic liquid crystal (TLC) thermography is used in transient heat transfer experiments to determine distributions of convective heat transfer coefficient (HTC) inside models of internally cooled gas turbine engine components. As these components become more geometrically complex, the application of TLC thermography becomes increasingly challenging and additional sources of experimental uncertainty grow to be significant. The present work quantifies the uncertainties introduced by TLC ageing using a state-of-the-art imaging system and a new post-processing methodology that are optimised for the intensity-based method of analysing TLC data. A coating comprising multiple TLCs with different active temperature ranges is considered and subject to 33 repeated thermal cycles. These repeated cycles are shown to increase the random and systematic uncertainties in the TLC measurements, resulting in consequent increases in the uncertainties associated with calculated HTCs. Increases in systematic uncertainty are caused by reflectance in the measured wavelength band moving to different temperatures, while increases in random uncertainty are related to changes in individual crystals or crystal clusters with ageing. Approaches to calibrating out increases in systematic uncertainty are proposed and recommended, but increases in random uncertainty will always persist unless the TLC coating is removed and reapplied.

Turbulent spot flow topology and mechanisms for surface heat transfer

The properties of artificially initiated turbulent spots over a heated plate were investigated in a water channel. The instantaneous velocity field and surface Stanton number were simultaneously established using a technique that combines particle image velocimetry and thermochromic liquid crystal thermography. Several characteristics of a spot are found to be similar to those of a turbulent boundary layer. The spacing of the surface heat transfer streak patterns within the middle or ‘body’ of a turbulent spot are comparable to the low-speed streak spacing within a turbulent boundary layer. Additionally, the surface shear stress in the same region of a spot is also found to be comparable to a turbulent boundary layer. However, despite these similarities, the heat transfer within the spot body is found to be markedly less than the heat transfer for a turbulent boundary layer. In fact, the highest surface heat transfer occurs at the trailing or calmed region of a turbulent spot, regardless of maturity. Using a modified set of similarity coordinates, instantaneous two-dimensional streamlines suggest that turbulent spots entrain and subsequently recirculate warm surface fluid, thereby reducing the effective heat transfer within the majority of the spot. It is proposed that energetic vortices next to the wall, near the trailing edge of the spot body, are able to generate the highest surface heat transfer because they have the nearest access to cooler free-stream fluid.

On the application of liquid‐crystal thermography for the nondestructive detection of delamination in composite structures

AbstractTo prevent the catastrophic failure of composite structures, several nondestructive testing methods have been developed. In this study, a new technique to evaluate these structures by using thermochromic liquid crystal (TLC) thermography is presented. Temperature gradients produced by sensitive liquid‐crystals are used to detect delamination in composite specimens. The effects of constituent materials and delamination size/location are investigated. The results from TLC thermography are compared to those from infrared thermography. Finally, advantages/disadvantages of the new method are discussed. On the basis of the results from this study, it could be concluded that the TLC thermography can be used as an inexpensive nondestructive testing method in inspection of composite structures. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers

Angular effects on thermochromic liquid crystal thermography

This paper directly discusses the effects of lighting and viewing angles on liquid crystal thermography. This is because although thermochromic liquid crystals (TLCs) are a widely-used and accepted tool in heat transfer research, little effort has been directed to analytically describing these effects. Such insight is invaluable for the development of effective mitigation strategies. Using analytical relationships that describe the perceived color shift, a systematic manner of improving the performance of a TLC system is presented. This is particularly relevant for applications where significant variations in lighting and/or viewing angles are expected (such as a highly curved surface). This discussion includes an examination of the importance of the definition of the hue angle used to calibrate the color of a TLC-painted surface. The theoretical basis of the validated high-accuracy calibration approach reported by Kodzwa et al. (Exp Fluids s00348-007-0310-6, 2007) is presented.

Reconstruction of time-dependent boundary heat flux by a BEM-based inverse algorithm

The objective of this study is to reconstruct an unknown time-dependent heat flux distribution at a surface whose temperature history is provided by a broad-band thermochromic liquid crystal (TLC) thermographic technique. The information given for this inverse problem is the surface temperature history. Although this is not an inverse problem, it is solved as such in order to filter the errors in input temperatures which are reflected in errors in heat fluxes. We minimize a quadratic functional which measures the sum of the squares of the deviation of estimated (computed) temperatures relative to measured temperatures provided by the TLC thermography. The objective function is minimized using the Levenberg–Marquardt method, and we develop an explicit scheme to compute the required sensitivity coefficients. The unknown flux is allowed to vary in space and time. Results are presented for a simulation in which a spatially varying and time-dependent flux is reconstructed over an airfoil.

Temperature measurement near the triple line during phase change using thermochromic liquid crystal thermography

The temperature generated by the evaporation of a volatile liquid in a confined space (tube ∅=1,630 μm) was mapped onto the tube surface with the use of unsealed thermochromic liquid crystals (TLCs). The strong evaporative cooling effect located near the meniscus triple line generates a temperature dip. Despite the thermal diffusion through the tube’s thickness and its geometry, the TLC thickness and the inherent difficulties of working with unsealed TLCs, the present technique has revealed to be a suitable tool for accurate temperature measurement at the microscale size. The evaporation flux is deduced from the profile of temperature and comparison with the experimental measurement shows a very good agreement. The role of the nature and thickness of the tube wall material on the diffusion of the temperature profile from the inside to the outside is also investigated.

Heat Transfer, Fluid Flow, and Pressure Measurements Inside a Rotating Two-Pass Duct With Detached 90-Deg Ribs

Transient thermochromic liquid crystal thermography, a laser-Doppler velocimeter, and pressure transducers have been used to measure the local heat transfer, velocity, and wall static-pressure distributions, respectively, in a rotating two-pass square duct with 90-deg ribs detached from the leading and trailing walls. The ribs were square in cross-section and their detached-distance/height ratio was 0.38. The rib-height/duct-height ratio and the pitch/rib-height ratio were 0.136 and 10, respectively. The duct Reynolds number was 1×104 and rotation number ranged from 0 to 0.2. Results are compared with attached rib cases in terms of regional averaged Nusselt number, transverse mean velocity component, pressure coefficient distributions and variation of friction factor with rotation number. The competition between convection effect of the wall jet and downwash effect of the rib-top separated shear layer on the heat transfer augmentation is addressed in detail. Discussion on local Nusselt number distribution, mean velocity components, and turbulent kinetic energy is included. Simple expressions are obtained to correlate friction factor with rotation number. Rib detachment is found to enhance heat transfer on the leading wall of the first outward pass and on the trailing wall of the second inward pass over as compared to the attached rib case. The trend is reversed on the other two walls. Nevertheless, detached ribs create more uniform heat transfer distributions on the leading and trailing walls than attached ribs.

Multi-dimensional heat flux reconstruction using narrow-band thermochromic liquid crystal thermography*

An inverse algorithm is developed to reconstruct multi-dimensional surface heat flux using transient surface temperature measurements provided by narrow-band thermal liquid crystals (TLC) or phase-change material (PCM). In this thermographic technique, the information provided is the time it took a certain location to reach the characteristic temperature of the TLC or PCM. A quadratic functional to be minimized is formulated. The Levenberg-Marquardt method and genetic algorithms are used to minimize the functional. It is shown that GA's can be used successfully to retrieve surface heat flux distributions with significant error in input times to arrival at the TLC or PCM critical temperature.

Transient temperature measurements in a convectively cooled droplet

The transient cooling of an evaporating water droplet, suspended in a jet of dry air, was experimentally investigated in this study using thermochromic liquid crystal thermography. Microencapsulated beads of thermochromic liquid crystals, suspended in the water droplets, enabled the visualization of the transient temperature fields within the droplets. Digital movies of the convectively cooled droplets reveal spatial and temporal temperature gradients resolved down to length scales of ∼100 μm and time scales of ∼0.03 s. The transient temperature measurements were analyzed to yield total droplet convective heat transfer rates. Droplet heat transfer rates determined from a heat balance on the droplets compare favorably to previously published measurements.

Measurements of heat transfer in a separated and reattaching flow with spatially varying thermal boundary conditions

Abstract The effect of highly varying thermal boundary conditions on the convective heat transfer in a backward-facing step flow was investigated in a two-dimensional (2D) boundary-layer tunnel in air. A modified form of the transient heat transfer technique was used that allowed variable surface temperature distributions to be imposed by heating the test surface with a radiative lamp prior to cooling. The experiments were restricted to axial variations in surface temperature. Surface temperatures were measured with both cement-on foil thermocouples and thermochromic liquid crystal thermography. A semianalytic superposition technique was developed that was capable of predicting the heat transfer for any arbitrary axial surface temperature distribution, given the general solution or “Green's” function. The Green's function was measured to a limited spatial resolution by solving a simultaneous set of equations using data from 30 experiments. The techniques were developed and tested in a 2-D flat-plate boundary layer with Re δ2 of 3500 at the leading edge and qualified against a well-verified boundary-layer code. The Green's function and inverse Green's function were then measured in a turbulent backward-facing step flow with Re based on step height of 26,000. In the separated flow, it was found that the effect of localized heating extended only a short distance upstream and downstream, and that the heat transfer was less sensitive to temperature boundary conditions than would be expected from attached flow behavior. This corroborates the conclusion of previous workers that the primary resistance to the heat transfer is localized near the wall. This localized behavior, and the spatial insensitivity of the Green's function, suggest that with proper scaling, the results could be extended to other separated flows.