First visualization of temperature fields in liquids at high pressure using thermochromic liquid crystals

A first application of encapsulated thermochromic liquid crystals (TLCs) for visualizing temperature fields in pressurized liquids was studied experimentally. By means of a tempered high-pressure optical cell, investigations were performed in a wide temperature range and at pressures up to 7000 bar. The measured calibration curves of isochromes in the pressure/temperature domain as well as photographically documented temperature fields at high pressure are presented and discussed. The results found illustrate that TLCs provide an efficient instrument for investigating thermofluiddynamical processes even at high pressure. Received: 13 February 1998/Accepted: 9 December 1999     

Temperature sensing with thermochromic liquid crystals

A review of the most recent developments in the application of thermochromic liquid crystals to fluid flow temperature measurement is presented. The experimental aspects including application, illumination, recording, and calibration of liquid crystals on solid surfaces, as well as in fluid suspensions, are discussed. Because of the anisotropic optical properties of liquid crystals, on-axis lighting/viewing arrangements, combined with in-situ calibration techniques, generally provide the most accurate temperature assessments. However, where on-axis viewing is not possible, calibration techniques can be employed, which reduce the uncertainty associated with off-axis viewing and lighting arrangements. It has been determined that the use of hue definitions that display a linear trend across the color spectrum yield the most accurate correlation with temperature. The uncertainty of both wide-band and narrow-band thermochromic liquid crystal calibration techniques can be increased due to hysteresis effects, which occur when the temperature of the liquid crystals exceeds their maximum activation temperature. Although liquid crystals are commonly used to provide time-mean temperature measurements, techniques are available which allow the monitoring of temporal changes. Selected examples illustrating the use of thermochromic liquid crystals are shown, and a survey of reported temperature measurement uncertainties is presented. Received: 3 February 1999/Accepted: 30 March 2000     

Computer-based areal surface temperature and local heat transfer measurements with thermochromic liquid crystals (TLC)

The experimental technique presented is designed to obtain detailed local heat transfer data on both stationary as well as rotating disc-cavity surfaces applicable to gas turbines. The method employed utilizes thin coatings of thermochromic liquid crystals (TLC) as surface temperature indicators under aerodynamically steady but thermally transient experimental conditions. The color display of the liquid crystals is monitored by a video camera. The video signals are captured in real time by a computer-based color recognition system to extract areal temperature and heat transfer information. Some typical results are presented and compared with-literature data to illustrate the potential of the system.

List of symbols

Symbols Unit Physical property a m²/s thermal diffusivity - B - blue color signal - G - green color signal - G - rotor/stator spacing ratio z/r _o - Nu _ro - Nusselt number r _o/ - r m radial location - r _o m disc radius - R - red color signal - Re _m - mass flow Reynolds number V/2zv - Re _ro - rotational Reynolds number r _o ²/v - t s time - T _o K initial temperature - T _ref K convecting fluid temperature - T _s K disc surface temperature - U - color difference signal - V - color difference signal - Y - luminance signal - z m rotor/stator spacing - - spectral weight factor - W/m² K local heat transfer coefficient - 1/K volumetric expansion coefficient - - spectral weight factor - - scaling factor - _ij - Kronecker-Delta - - scaling factor - - spectral weight factor - W/m K thermal conductivity - v m²/s fluid kinematic viscosity - kg/m³ fluid density     

Influence of hue origin on the hue-temperature calibration of thermochromic liquid crystals

Liquid crystal thermography incorporates a process by which the liquid crystal color, bearing temperature information, is analyzed using image processing techniques to quantify complex temperature distributions on heat transfer surfaces. Previous researches have found that hue can be used to represent the liquid crystal color since there exists a monotonic relation between the hue and temperature of liquid crystals. It is found in this study that the white-light point, which is usually adopted as the hue reference point, does not generally lie at the center of the liquid crystal colors defined on a chromaticity diagram. This results that hue does not have uniform resolution to all liquid crystal colors and a biased error is produced subsequently in the temperature measurement. The non-uniform hue can be corrected by shifting the hue origin to the center of the colors or by a color calibration scheme so that the liquid crystal colors distribute evenly around the white-light point. Received on 15 November 1996     

Accurate heat transfer measurements using thermochromic liquid crystal. Part 1: Calibration and characteristics of crystals

Encapsulated thermochromic liquid crystal (TLC) can accurately measure surface temperature in a variety of heat transfer and fluid flow experiments. Narrow-band TLC, where the colour changes over a temperature range of 1 °C, can be used to determine surface temperature within an uncertainty of 0.1 °C. Wide-band TLC, typically active over 5–20 °C, allow the possibility of mapping surface temperature distributions. In part 1 of this two-part paper, an extensive set of calibrations for narrow-band and wide-band TLC is reported. This generic study provides insight into the importance and influence of the various factors governing the colour–temperature relationship. These governing effects include the variation in optical path, the spectrum of the illumination source, the lighting and viewing angles, the differences between cooling or heating cycles (hysteresis), the variation with the number of heating or cooling cycles (aging) and how this varies with TLC film thickness. Two narrow-band crystals are also specifically calibrated for application to experiments on a transparent disc rotating at high speed (5000 rpm). Part 2 of this paper describes how these accurately-calibrated crystals were used to measure the transient surface temperature on, and heat transfer to, a rotating disc.     

A high-accuracy calibration technique for thermochromic liquid crystal temperature measurements

There are a variety of phenomena which may impact the accuracy of wide-band thermochromic liquid crystal temperature measurements, including: irregularities in liquid crystal and black paint layers, reflective components from light sources, and variations in the lighting/viewing angle across the surface. A wide-band calibration technique has been developed which inherently accounts for these and other sources of uncertainty by employing a point-wise calibration of the entire test surface. Both on and off-axis lighting arrangements are assessed for ease of implementation and accuracy of color displayed under uniform temperature conditions. The technique employs a series of uniform-temperature images to construct calibration curves relating the local hue component to temperature in a point-wise manner for the entire test surface. An off-axis lighting/viewing arrangement is found to be most practical for typical experimental setups. Hysteresis effects are quantified for excursions beyond both the lower and upper clearing point of the liquid crystals. Finally, the total uncertainty of the measured temperature is determined to vary from ±1.2% to ±7.2% across the bandwidth of the liquid crystals. Received: 21 October 1998/Accepted: 7 August 1999     

Flow and temperature field measurements of thermal convection in a small vertical gap using liquid crystals

Thermal convection in a small vertical gap is studied experimentally applying digital particle image velocimetry/thermometry. This optical method enables the simultaneous measurement of two-dimensional flow and temperature fields in a liquid. The principle is based on seeding the liquid flow medium with thermochromic liquid crystal particles. The temperature is measured by the crystal particles which change their reflected colour as function of temperature. The flow velocity is measured by using the same particles as flow tracers. The investigation shall contribute to the understanding of the fluid mechanical behaviour of biological liquids within micro reactor systems. However, the problem is also of fundamental interest as far as heat and mass transfer is concerned. Measured temperature and flow velocity fields are presented and discussed. Presented in part at the 4th Chemnitz/Hamburger Colloquium (CHC) on Microflows, Hamburg, Germany, November 2004.     

The orientation of liquid crystals by temperature gradients

Summary It has been known for some time that the molecular orientation of liquid crystals can be affected by the presence of a temperature gradient.Stewart reported in 1936 that, whenp-azoxyanisol was placed in a vertical temperature gradient, the orientation adopted was vertical when the higher temperature was at the bottom and horizontal when the higher temperature was at the top. The continuum theory for nematic liquid crystals, proposed byLeslie, is used to explain this phenomenon. An examination of the stability of the static orientation adopted by the director in the presence of a vertical temperature gradient shows that the stable orientations are either vertical or horizontal, as found experimentally.

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Zusammenfassung Es ist längst bekannt, daß die Molekularorientierung kristalliner Flüssigkeiten durch die Anwesenheit einer Temperatursteigung beeinflußt werden kann.Stewart zeigte 1936, daß die angenommene Orientierung vonp-Azoxyanisol in einer vertikalen Temperatursteigung bei höherer Temperatur unten vertikal war, sowie horizontal bei höherer Temperatur oben. Die Kontinuumstheorie für nematische Kristallinflüssigkeiten — wie vonLeslie vorgeschlagen — wird benutzt, um dieses Phänomen zu erklären. Eine Untersuchung der Stabilität der statischen Orientierung, die der Direktor in Anwesenheit einer vertikalen Temperatursteigung annimmt, zeigt, wie auch das Experiment feststellt, daß die stabilen Orientierungen entweder vertikal oder horizontal sind.

     

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 m) 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 tubes 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.     

Visualization of temperature fields and double-diffusive convection using liquid crystals in an aqueous solution crystallizing along a vertical wall

This article presents a simple technique for temperature visualization using liquid crystals in an aqueous solution during the process of cooling and solidification. This method provides a clear picture of the role of double-diffusive convection in producing vertical compositional and density stratification in an initially homogeneous liquid during solidification.     

Film thickness effects on calibrations of a narrowband thermochromic liquid crystal

Thermochromic liquid crystals (TLCs) have been widely employed by researchers in heat transfer and fluid flow communities as a reliable and non-intrusive temperature measurement tool due to their unique optical properties such as birefringence, optical activity, circular dichroism and selective reflection of colours in the visible spectrum as function of temperature. The use of narrowband TLCs are attractive for temperature and heat transfer measurements due to their higher precision in temperature measurements and due to the fact that narrowband TLCs are less affected by variations in illumination-viewing angles and illumination disturbances. Narrowband TLCs have been used with full intensity-matching methods to provide robust image processing for measurements of thermal parameters in transient heat transfer tests. Calibration of narrowband TLCs is necessary in order to obtain the intensity-temperature relationship of the TLCs. Film thickness is one of the factors which affects calibrations of TLCs. In this research, film thicknesses of 10, 20, 30, 40 and 50 μm were investigated on green intensity-based calibrations of R35C1W TLC during heating and cooling. Results showed an increase in magnitude of peak green intensity with increasing film thickness, with a percentage increase of nearly 18% when film thickness increased from 10 to 50 μm. Results also showed an inconsistent shift in temperature at which peak green intensity occurs, with a maximum shift of 0.40 °C, suggesting that film thickness effects may be insignificant for narrowband TLCs compared with wideband TLCs. A theoretical method for estimating the volume of TLC coating required to achieve a desired film thickness has also been described in this paper, based on the surface area and dry solids content of the TLC. The method is easily implemented and applicable for sprayable TLC coatings.     

Accurate heat transfer measurements using thermochromic liquid crystal. Part 2: Application to a rotating disc

Encapsulated thermochromic liquid crystal (TLC) can accurately measure surface temperature in a variety of heat transfer and fluid-flow experiments. In Part 1 of this two-part paper, two narrow-band liquid crystals were specifically calibrated for application to experiments on a disc rotating at high speed (5000 rpm). Part 2 describes how these crystals were used to measure the surface temperature on the disc in a transient experiment that models the flow of internal cooling air in a gas turbine. The TLC was viewed through the transparent polycarbonate disc using a digital video camera and strobe light synchronised to the disc frequency. The convective heat transfer coefficient, h, was subsequently calculated from the one-dimensional solution of Fourier’s conduction equation for a semi-infinite wall. The analysis accounted for the exponential rise in the air temperature driving the heat transfer, and for experimental uncertainties in the measured values of h. The paper focuses on the method used, and sample experimental results are provided to demonstrate the accuracy and potency of the technique.     

Maximum thermal conductance for a micro-channel,utilising Newtonian and non-Newtonian fluid

This paper investigates the thermal behaviour of two micro-channel elements cooled by Newtonian and non-Newtonian fluids, with the objective to maximise thermal conductance subject to constraints. This is done firstly for a two-dimensional duct micro-channel and secondly for a three-dimensional complex micro-channel. A numerical model is used to solve the governing equations relating to flow and temperature fields for both cases. The geometric configuration of each cooling channel is optimised for Newtonian and non-Newtonian fluid at a fixed inlet velocity and heat flux. In addition, the effect of porosity on thermal conductance is investigated. It was found, in both cases, that the non-Newtonian fluid characteristics result in a significant variation in thermal conductance as inlet velocity is increased. The characteristics of a dilatant fluid greatly reduce thermal conductance on account of shear thickening on the boundary surface. In contrast, a pseudoplastic fluid shows increased thermal conductance. A comparison of the complex micro-channel and the duct micro-channel shows the improved thermal conductance resulting from greater flow access to the conductive area, achieved by the complex micro-channel.     

Detection of flow separation and reattachment using shear-sensitive liquid crystals

Coatings of pure chiral nematic liquid crystals are known to change colour under different levels of surface shear stress. In this study, the liquid crystal was used to provide information about flow separation and reattachment on both a two-dimensional aerofoil and a delta wing. The tests were carried out at a free-stream velocity of 28 m/s and a number of incidence angles. The Reynolds numbers based on the central chord length of the models were 200,000 and 270,000 for the aerofoil and delta wing models, respectively. The study showed that locations of boundary layer separation and reattachment can be identified from spatial variations in the surface colour; the agreement between the results and those obtained using surface oil flow was good. Issues relating to interpretation of the crystal colour pattern and the limitation of this technique in detection of flow separation were also discussed.     

Motion of a liquid droplet in a vibrating fluid

The motion of a liquid droplet in a liquid with density different from that of the liquid composing the droplet and subjected to harmonic excitations is investigated. Nonlinear equations are obtained that describe the translational motion of the droplet and its oscillations. It is shown by numerical means that, under the essential resonance conditions, the droplet ascends by a cascade method if the value of the load coefficient is small.S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 31, No. 7, pp. 78–83, July, 1995.     

Image shifting for PIV using birefringent and ferroelectric liquid crystals

This paper discusses a new implementation of electrooptical image shifting, used to resolve directional ambiguity in particle image velocimetry (PIV) measurements. The setup uses a ferroelectric liquid crystal (FLC) as a polarization rotator and a birefringent calcite plate as a shifter. The system can be used with non-polarized light sources and fluorescent particles. The minimum shifting time (pulse separation) is approximately 100 s. This compact electrooptical device is usually positioned in front of the camera lens, though it has also been mounted inside the lens body. Sample vector maps from a turbulent multidirectional flow are included.This work is supported by ARPA, Gary Jones, Program Manager. Funding is provided under ONR Contract N° N00014-92-1634. The authors would like to thank Shewen Liu for the use of his facility and data.     

Incompressibility conditions in liquid crystals

Received January 2, 2002 / Published online May 21, 2002     

Three-dimensional analysis of fluid flow and heat transfer in single- and two-layered micro-channel heat sinks

A three-dimensional numerical analysis of laminar fluid flow and conjugate heat transfer has been conducted for single- and two-layered micro-channel heat sinks. The validity of the numerical model has been confirmed by comparison with available experimental data. Results for the overall thermal resistance, pumping power, the maximum temperature difference on the heat-sink surface where the heat flux is applied, and an overall performance parameter were obtained for single- and two-layered sinks. The effects of Reynolds number, inlet velocity profile, and flow arrangement in the channels (parallel and counter) on these results are presented and discussed. A special emphasis was placed on the comparison between the thermal performances of the parallel and counter flow arrangements and further results were obtained in order to quantify and explain the relative performance under these flow arrangements.     

On the stress tensor in nematic liquid crystals

Summary This paper presents an analysis on the stress tensor in non-heat conducting nematic liquid crystals. Essentially it is shown that the Lee-Eringen theory possesses two inconsistent versions for the stress tensor. The first version appears to be incapable of describing the flow problems of nematic liquid crystals and the second version is shown to be similar to the stress tensor as proposed in the Ericksen-Leslie theory.

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Zusammenfassung Diese Arbeit beinhaltet eine Untersuchung des Spannungstensors in nicht-wärmeleitenden nematischen flüssigen Kristallen. Es wird vor allem gezeigt, daß die Theorie vonLee undEringen zwei nicht miteinander verträgliche Darstellungen des Spannungstensors enthält. Die erste scheint nicht imstande zu sein, die Strömungsverhältnisse bei nematischen flüssigen Kristallen richtig zu beschreiben. Von der zweiten wird dagegen gezeigt, daß sie einen ähnlichen Ausdruck für den Spannungstensor liefert wie die Theorie vonEricksen undLeslie.

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