Narrow-band Thermochromic Liquid Crystal Research Articles

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.

Influence of obstacles on the wall heat transfer for 2D and 3D helically ribbed pipes

Obstacles placed on the inner walls of pipes lead to flow separation and subsequent reattachment with an overall beneficial effect on the local heat transfer. In this work, the effects of both a continuous and a discontinuous helicoidal turbulator on the local heat transfer are investigated and compared for Reynolds numbers in the range from 20,000 to 80,000. Both turbulators are cylindrical helixes with constant angle and fixed direction. The continuous geometry has a pitch-to-diameter-ratio (p/D) equal to 11, an inclination angle of 80° and a uniform obstacle height-to-diameter-ratio (e/D) equal to 3.6%. The discontinuous geometry is equivalent to the continuous one except that it has a varying obstacle height throughout the helix. Steady-state heat transfer measurements are carried out by means of Liquid Crystals Thermography (LCT). A recently developed calibration method for the narrow-band Thermochromic Liquid Crystals (TLCs) is successfully modified to lower the angular effects on the measurement uncertainty. The local wall heat transfer rates and the temperature distributions are strongly influenced by the flow reattachment and thus by the geometry of the investigated turbulators. Compared to a smooth pipe at the same Reynolds number, the continuous and the discontinuous turbulator increase the average Nusselt number by factors of approximately 1.8 and 1.5 respectively. These heat transfer measurements with respective uncertainties are meant to be a valuable dataset for the validation of Large Eddy Simulations (LES) of turbulent flows in pipes equipped with turbulators.

Detailed heat transfer investigation of an impingement jet array with large jet-to-jet distance

The heat transfer performance of a large jet-to-jet distance impingement configuration with a jet-to-jet spacing X/D = Y/D = 10 was experimentally studied for turbomachinery applications. Narrowband thermochromic liquid crystals (TLCs) were used to record the target wall temperature. Pressure taps were set along the experimental rig to measure the pressure loss. Improved measurement methods with multiple TLCs and a ramp heater were studied. The work was done for different Reynolds numbers (Re = 15,000–35,000), different jet to target plate distances (H/D = 3–5) and different cross-flow schemes. The results from the improved methods show good agreement with the basic measurement method. It confirms that the single TLC, step heater method is sufficient for the current experimental study. The improved methods are confirmed to be reliable and are suitable for complex applications with extremely large heat transfer variations over the measured surface, or extremely high local heat transfer coefficients. Experimental results indicate that a smaller jet to target plate distance and a smaller cross-flow scheme can provide a better heat transfer performance in the studied range. When compared to the impingement configuration with X/D = Y/D = 5, the results show that a larger jet-to-jet distance configuration provides a higher overall heat transfer when the mass flow rate is kept constant.

On correlated measurements in the transient thermochromic liquid crystals technique with multiple indicators

This work demonstrates that it is possible, and with relative ease, to determine unique characteristic temperatures for the red & green light intensity signals of a narrowband thermochromic liquid crystal mixture. It is proposed to utilize additional indicators in future transient thermochromic liquid crystals measurements to increase data throughput, improve robustness, and reduce uncertainty in the calculated heat transfer coefficient. Potential algorithmic implementations for a multi-color technique are discussed. Optimum conditions for minimizing the uncertainty in the calculated heat transfer coefficient are discussed and compared to that of the traditional single-color technique. The multi-color technique is also extendable to multi-band techniques.

Experimental and numerical investigation on the role of holes arrangement on the heat transfer in impingement/effusion cooling schemes

In the present work, two different impingement/effusion geometries have been investigated, both having staggered hole configuration and an equal number of impingement and effusion holes. The first geometry, which is designed in case of low coolant availability, has impingement hole pitch-to-diameter ratios of 10.5 in both orthogonal directions, a jet-to-target plate spacing of 6.5 hole diameters, with effusion holes inclined of 20° with respect to the target surface. The second geometry, which is designed in case of high coolant availability, has impingement hole pitch-to-diameter ratios of 3.0, a jet-to-target plate spacing of 2.5 diameters and normal effusion holes. For each geometry, two relative arrangements between the impingement and effusion holes have been investigated, as well as various Reynolds numbers for the sparser geometry. The experimental investigation has been performed by applying a transient technique, using narrow band thermochromic liquid crystals (TLCs) for surface temperature measurement. A CFD analysis has also been performed in order to support interpretation of the results. Results show unique heat transfer patterns for every investigated geometry. Weak jet-jet interactions have been recorded for the sparser array geometry, while intense secondary peaks and a complex heat transfer pattern are observed for the denser one, which is also strongly influenced by the presence and position of effusion holes. For both the geometries, effusion holes increase heat transfer with respect to impingement-only, which can be mainly attributed to a reduction in flow recirculation for the sparser geometry and to the suppression of spent coolant flow for the denser one.

Experimental and numerical heat transfer investigation of an impingement jet array with V-ribs on the target plate and on the impingement plate

The secondary vortex structure of an impingement jet system is enhanced by V-ribs on both the impingement and target plates. Numerical and experimental investigations are conducted to study the flow field and heat transfer resulting from V-rib turbulators in an impingement cooling configuration. Three different cases are tested: V-ribs on both the impingement and target plates (V-rib), V-ribs only on the impingement plate (V-rib-impingement) and V-ribs only on the target plate (V-rib-target). The experiment is carried out on a 9 by 9 inline impingement array test facility. For the transient measurements, narrow band thermochromic liquid crystals (TLC) and thermocouples are applied to obtain the local heat transfer distribution. Pressure taps are used to measure the pressure loss. The numerical simulation is carried out with ANSYS CFX 14, using a steady state Reynolds-Averaged Navier-Stokes (RANS) approach and the Shear Stress Transport (SST) turbulence model. All studies are done for a Reynolds number range of 15,000 to 35,000. There is a good overall agreement between the experimental and numerical results for the cases studied. The detailed flow field from the numerical simulation is used to understand and complement the phenomena observed in the experiment. The evaluation of the flow field confirms that the V-ribs enhance the secondary flow structure in the impingement system and induce a positive heat flux ratio compared to the baseline case. Both experimental and numerical results show a Nusselt number increase for the V-rib-impingement and V-rib configuration, with a highest Nusselt number ratio of 1.16. Notice that the experiment cannot take the rib part into account due to the invalid 1D semi-infinite wall assumption there, while the CFD simulation allows for the consideration of heat transfer on the rib surface and thus complements the heat flux data on the target plate. Depending on the configuration, the CFD simulation shows a heat flux ratio of 1.06–1.34. The pressure loss of the system is comparable to the case with a smooth impingement plate and a smooth target plate.

Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater

In the present work, an experimental investigation of convective heat transfer and pressure drop was carried out for the turning portion of a U-channel where the outer wall was equipped with ribs. The shape of the ribs was varied. The investigation aims to give guidelines for improving the thermo-hydraulic performance of a solar air heater at the turning portion of a U-channel. Both the U-channel and the ribs were made in acrylic material to allow optical access for measuring the surface temperature by using a high-resolution technique based on narrow band thermochromic liquid crystals (TLC R35C5 W) and a CCD camera placed to face the turning portion of the U-channel. The uncertainties were estimated to 5 and 7 % for the Nusselt number and friction factor, respectively. The pressure drop was approximately the same for all the considered shapes of the ribs while the dimpled rib case gave the highest heat transfer coefficient while the grooved rib presented the highest performance index.

Heat Transfer Measurements in a Leading Edge Geometry With Racetrack Holes and Film Cooling Extraction

An experimental survey on a state of the art leading edge cooling scheme was performed to evaluate heat transfer coefficients (HTC) on a large scale test facility simulating a high pressure turbine airfoil leading edge cavity. The test section includes a trapezoidal supply channel with three large racetrack impingement holes. On the internal surface of the leading edge, four big fins are placed in order to confine impingement jets. The coolant flow impacts the leading edge internal surface, and it is extracted from the leading edge cavity through 24 showerhead holes and 24 film cooling holes. The aim of the present study is to investigate the combined effects of jet impingement and mass flow extraction on the internal heat transfer of the leading edge. A nonuniform mass flow extraction was also imposed to reproduce the effects of the pressure side and suction side external pressure. Measurements were performed by means of a transient technique using narrow band thermochromic liquid crystals (TLCs). Jet Reynolds number and crossflow conditions into the supply channel were varied in order to cover the typical engine conditions of these cooling systems (Rej=10,000-40,000). Experiments were compared with a numerical analysis on the same test case in order to better understand flow interaction inside the cavity. Results are reported in terms of detailed 2D maps, radial-wise, and span-wise averaged values of Nusselt number.

Effect of Radial Location of Nozzles on Heat Transfer in Preswirl Cooling Systems

This paper investigates heat transfer in a rotating disk system using preswirled cooling air from nozzles at high and low radius. The experiments were conducted over a range of rotational speeds, flow rates, and preswirl ratios. Narrow-band thermochromic liquid crystal (TLC) was specifically calibrated for application to experiments on a disk, rotating at ∼5000 rpm and subsequently used to measure surface temperature in a transient experiment. The TLC was viewed through the transparent polycarbonate disk using a digital video camera and strobe light synchronized to the disk 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 was accounted for the exponential rise in the air temperature driving the heat transfer, and for the experimental uncertainties in the measured values of h. The experimental data was supported by “flow visualization,” determined from CFD. Two heat transfer regimes were revealed for the low-radius preswirl system: a viscous regime at relatively low coolant flow rates, and an inertial regime at higher flow rates. Both regimes featured regions of high heat transfer where thin, boundary layers replaced air exiting through receiver holes at high radius on the rotating disk. The heat transfer in the high-radius preswirl system was shown to be dominated by impingement under the flow conditions tested.

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.

Application of the inverse analysis for boundary condition retrieval

The technique of solving linear steady-state and transient inverse problems aimed at retrieving an unknown heat transfer coefficient (HTC) is proposed. The idea is to approximate the temperature field by a linear combination of known auxiliary temperature fields. Each of these fields is evaluated by assuming that the unknown boundary temperature (heat flux) is described by a compact support, low-order trial function. The unknown coefficients of this linear combination are determined from the least square fit to the measured temperatures employing the modified Levenberg–Marquardt (LM) method. Once the boundary temperature and heat flux are retrieved, the HTC is determined from its definition. The auxiliary fields are evaluated using an arbitrary numerical or analytical technique. The sensitivity coefficients of the problem can readily be obtained by sampling the auxiliary fields at sensor locations. The values of the sensitivity coefficients used in the LM procedure need to be determined only once, before entering the iterative least squares procedure. Piecewise linear or stepwise functions were used to approximate the spatial and temporal variation of the retrieved heat flux and temperatures. The technique has been applied to reconstruct HTC for so-called time-to-arrival measurements technique with narrow band thermochromic liquid crystal used as temperature sensors. The idea behind this approach is to monitor the time at which a measuring point reaches a characteristic temperature. The developed technique has been used to retrieve the HTC for impingement cooling of the simplified piston of a car engine.

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.

Internal Heat Transfer Characteristics of Lamilloy Configurations

A transient measurement technique by using narrow-band thermochromic liquid crystal (TLC) is employed to determine temperature and heat transfer coefficient (HTC) distribution on inner surfaces of the typical lamilloy configurations. With this technique, both local HTC distribution and average HTC distribution could be obtained. The experimental results indicate that the variation of the porosity ratio, the one that the area of impingement holes divided by that of the plate, has a great effect on the HTC distribution on the inner surfaces. Heat exchange of inner surfaces varies directly as the porosity ratio. The impingement Reynolds number ranges from 20 000 to 50 000. The average HTC of inner surfaces bears a linear relationship with the Reynolds number.

Transient heat transfer measurements using thermochromic liquid crystal. Part 1: An improved technique

It is common practice to employ thermochromic liquid crystal (TLC) to determine heat transfer coefficients, h, in transient experiments. The method relies on the solution of Fourier’s conduction equation, usually with the boundary condition of a step-change in air temperature. In practice a step-change can be difficult to achieve, and a more general solution to the one-dimensional conduction equation is presented here for a “slow transient,” where the rise in air temperature is represented by an exponential series. An experimental method, based on this technique, requires only a single measurement of surface temperature history, and this has the advantage that narrow-band TLC can be used. As an example, measurements of h are presented from an experiment modelling the internal flow of cooling air inside a gas turbine engine. The measurements are analysed using both the conventional step-change method and the exponential-series technique, and the results show that using the step-change method can give rise to significant errors in the calculated values of h. The new technique should be applicable to many other slow transient heat transfer measurements.

A Converging Slot-Hole Film-Cooling Geometry—Part 1: Low-Speed Flat-Plate Heat Transfer and Loss

This paper presents experimental measurements of the performance of a new film-cooling hole geometry—the con¯vergings¯lot-hole¯ or console. This novel, patented geometry has been designed to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems. The physical principles embodied in the new hole design are described, and a typical example of the console geometry is presented. The cooling performance of a single row of consoles was compared experimentally with that of typical 35-deg cylindrical and fan-shaped holes and a slot, on a large-scale, flat-plate model at engine representative Reynolds numbers in a low-speed tunnel with ambient temperature main flow. The hole throat area per unit width is matched for all four hole geometries. By independently varying the temperature of the heated coolant and the heat flux from an electrically heated, thermally insulated, constant heat flux surface, both the heat transfer coefficient and the adiabatic cooling effectiveness were deduced from digital photographs of the color play of narrow-band thermochromic liquid crystals on the model surface. A comparative measurement of the aerodynamic losses associated with each of the four film-cooling geometries was made by traversing the boundary layer at the downstream end of the flat plate. The promising heat transfer and aerodynamic performance of the console geometry have justified further experiments on an engine representative nozzle guide vane in a transonic annular cascade presented in Part 2 of this paper.

Uncertainties in transient heat transfer measurements with liquid crystal

Thermochromic liquid crystal (TLC) is used extensively as an experimental tool to determine heat transfer coefficients. In a transient experiment, if the time at which the TLC changes colour is known, then h, the heat transfer coefficient, can be found from the solution of the so-called semi-infinite-plate problem. If T aw, the adiabatic-wall temperature, is unknown, then two narrow-band TLCs can be used to determine both h and T aw. In this paper, an uncertainty analysis is used to calculate P h , the uncertainty in h, and P T aw , the uncertainty in T aw (when T aw is unknown), in terms of the random uncertainties in the measured temperatures. Computed values, obtained using a Monte Carlo method, are in good agreement with the uncertainties obtained from the analysis. It is also shown how the uncertainties P h and P T aw can be minimised by selecting the appropriate ranges of TLC. Conversely, a poor choice of TLC can result in large values of these uncertainties.

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.

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.

Application of Thermochromic Liquid Crystal to Rotating Surfaces

Encapsulated thermochromic liquid crystal (TLC) can be used to measure the surface temperature of stationary or rotating bodies. However, some research workers have reported a “rotational shift”: When the temperature of a rotating body is measured by thermocouples and TLC, there is a difference between the two sets of temperatures, and this difference increases with increasing rotational speed. Two research groups (Camci and Glezer in the USA, and Owen, Pilbrow, and Syson in the UK) have independently examined the effect of speed on TLC applied to the surfaces of rotating disks. The USA group used narrow-band TLC on a disk of 305 mm diameter rotating up to 7500 rpm measuring the surface temperature using an infrared (IR) sensor. The UK group used wide-band TLC on a disk of 580 mm diameter rotating up to 7000 rpm, measuring the temperature with an IR thermal imager. Both groups used the so-called hue technique to evaluate the temperature of the TLC and concluded that, even for centripetal accelerations in excess of 104 g, there is no significant effect of rotational speed on either narrow-band or wide-band TLC. It is suggested that the “rotational shift” observed by some researchers was probably caused by thermal-disturbance errors, which affected the thermocouples, rather than by changes in the TLC.

Effect of rotation on temperature response of thermochromic liquid crystal

Encapsulated thermochromic liquid crystal (TLC) can be used to determine the surface temperature of stationary or rotating bodies. For narrow-band TLC, with a colour change from red to blue over a bandwidth of 1°C, temperatures on stationary surfaces can be measured with an uncertainty of around ±0.1°C. For gas turbine applications, where centripetal accelerations in excess of 10 4 g are common, there is a belief held by many research workers that TLC is significantly affected by rotation: “rotational shifts” have been observed where temperature measurements made by thermocouples and TLC diverge with increasing rotational speed; shifts up to 7°C at around 10 4 g have been observed. In this paper, the surface temperature of a rotating disc is measured simultaneously by an infrared OR) thermal imager and wide-band TLC with an effective temperature range from 48 to 58°C. Within the uncertainty of the measurements, which was around 0.5°C, it was concluded that there was no significant rotational shift for accelerations up to 16,000 g.