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.     

Influence of the optical configuration on temperature measurements with fluid-dispersed TLCs

An accurate temperature calibration of fluid-dispersed thermochromic liquid crystal (TLC) particles is an important prerequisite for quantitative liquid crystal thermometry (LCT) measurements in flows. Encapsulated TLCs are subjected to uniform and linear temperature fields and are illuminated with a sheet of white light. A digital camera records the color distribution reflected by the particles. For the first time, a telecentric objective is used to eliminate the angular dependence of the color within the image plane. The paper systematically assesses how the temperature calibration is affected by the angle between the camera axis and the light-sheet plane, and by the properties of the working fluid. The obtained results provide design criteria for quantitative LCT measurements in situations where small spatial variations of the fluid temperature need to be resolved, namely for turbulent heat transfer problems in wall-bounded flows. Received: 22 January 2001/Accepted: 16 October 2001     

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.     

Supercritical nitrogen free jet investigated by spontaneous Raman scattering

The injection of fuel and oxidizer in high pressure rocket combustion chambers is simulated in a model experiment dedicated to investigate the phenomenology of coaxial injection at supercritical pressures. Cryogenic nitrogen is injected in a pressurized reservoir of N₂ at ambient temperature at 4 Mpa and 6 Mpa, above the critical pressure for nitrogen of 3.4 Mpa. The radial density distribution of the N₂ is determined by spontaneous Raman-scattering, from the measured densities temperature distributions are derived. The radial density and temperature pro les are analyzed as a function of the distance from the injector for x/D = 1 to 30. The influence of density gradients, internal field effects and interference effects on the performance of the Raman technique when applied to turbulent high density flows is discussed. Received: 5 August 1998/Accepted: 10 January 1999     

PLIF imaging of mean temperature and pressure in a supersonic bluff wake

 Planar laser-induced fluorescence (PLIF) of seeded nitric oxide was used to obtain mean 2-D temperature and pressure fields in the near-wake region of a thick flat plate in a Mach 3 flow. A two-line ratio technique was used to obtain the temperature field, while an image obtained at the limit of low quenching rate was used to infer the pressure field. An analysis shows that these time-average measurements can suffer from significant weighted averaging bias errors in regions where there are large temperature fluctuations; however, these bias errors can be minimized by judicious selection of the absorption lines used. The resulting temperature field reveals the warm upstream boundary layer, the temperature jump across the recompression shocks and the expected minimum and maximum temperatures in the expansion and recirculation regions, respectively. The pressure measurements indicate a uniform low pressure in the base region, a rapid increase near reattachment, followed by a gradual approach to the free stream value farther downstream. Received: 25 July 1996 / Accepted: 11 September 1997     

T型槽端面密封气膜热弹流润滑动态稳定性

对T型槽端面密封气膜热弹流润滑动态稳定进行了分析. 考虑端面热变形和弹性变形以及辅助密封的阻尼特性,数值分析了不同振动频率下密封气膜动态压力分布和温度分布规律,并利用小扰动方法分析了外界扰动频率对气膜刚度、阻尼和振幅的影响规律. 结果表明:高压和高速条件下,密封端面的弹性变形和热变形产生发散间隙,导致密封气膜厚度显著降低;外界扰动产生附加压力和温度分布,刚度随扰动频率的增加而迅速增加,阻尼随扰动频率的增加而迅速下降;一定扰动频率范围内,轴向振幅与扰动频率成对数线性关系增加,辅助密封阻尼使得密封气膜的振幅显著上升.      

Synthesis of thermal effects in misaligned hydrodynamic journal bearings

The analysis presented herein deals with the evaluation of pressure and temperature fields which are generated in thin fluid films of varying thickness. The particular problem of a misaligned journal bearing has been studied by solving simultaneously the Reynolds and energy equations, which also include the effects of viscous dissipation and the variation of fluid viscosity with temperature. The method has been used to predict pressure and temperature fields as well as global performance parameters for a typical journal bearing operation.     

Wall pressure fluctuations and topology in separated flows over a forward-facing step

Laboratory measurements of wall pressure fluctuations and aerodynamic fields were made in separated flows over a forward facing step (h = 30, 40 and 50 mm with U _e = 15–40 m/s). An array of 16 off-set pressure probes extending in the streamwise and the spanwise directions was especially developed for sensing the wall pressure fluctuations. The flow field was also investigated by wall flow visualizations and PIV to analyze the flow topology in an open section wind tunnel. The results show a different behavior of the flow depending on the aspect ratio l/h and δ/h for high Reynolds numbers. The space time correlations between the wall pressure and the velocity fields were highlighted. The results show that high levels of these correlations are located at the top of the recirculation bubble, mainly in the shear layer and are extended downstream of the re-attachment point. Indeed, the results indicate that the flapping motion at the separation is important in the flow organization at the re-attachment point.     

High speed rotor wake total temperature measurements using a hot-film anemometer

 To develop a quantitative understanding of unsteady and interacting turbomachine flow fields, it is necessary to quantify the instantaneous efficiency of high speed turbomachines. This requires the measurement of both the unsteady velocity and total temperature variation in the exit flow of a high speed rotor. In this paper, techniques to utilize a single slant-film anemometer to measure unsteady total temperature are developed and evaluated. Then a series of preliminary experiments are performed in a high speed axial fan facility to quantify the instantaneous rotor efficiency. This is accomplished by utilizing these single slant-film methods to measure the total temperature in the rotor wakes. Results show that measurements at multiple overheats and several probe orientations are required. The simplest method proves to be useful for determining parameters used in other methods. An analysis based on King’s law gives good results even when measurements are outside the calibration range. Within the calibration range, a polynomial representation of the wire response to mass flux and total temperature yields good total temperature fluctuation results. A model analysis technique is also assessed. Received: 13 November 1997/Accepted: 16 February 1998     

Digital particle image velocimetry/thermometry and application to the wake of a heated circular cylinder

 Digital particle image velocimetry/thermometry (DPIV/T) is a technique whereby the velocity and temperature fields are obtained using thermochromic liquid crystal (TLC) seeding particles in water. In this paper, the uncertainty levels associated with temperature and velocity measurements using DPIV/T are studied. The study shows that large uncertainties are encountered when the temperature is measured from individual TLC particles. Therefore, an averaging procedure is presented which can reduce the temperature uncertainties. The uncertainty is reduced by computing the average temperature of the particles within the common specified sampling window used for standard DPIV. Using this procedure, the velocity and temperature distributions of an unsteady wake behind a heated circular cylinder are measured experimentally at Re=610. The instantaneous DPIV/T measurements are shown to be useful for computing statistical flow quantities, such as mean and velocity-temperature correlations. Received: 3 January 2000/Accepted: 26 June 2000     

A finite element method for analysis of fluid flow,heat transfer and free interfaces in Czochralski crystal growth

A finite element algorithm is presented for simultaneous calculation of the steady state, axisymmetric flows and the crystal, melt/crystal and melt/ambient interface shapes in the Czochralski technique for crystal growth from the melt. The analysis is based on mixed Lagrangian finite element approximations to the velocity, temperature and pressure fields and isoparametric approximations to the interface shape. Galerkin's method is used to reduce the problem to a non-linear algebraic set, which is solved by Newton's method. Sample solutions are reported for the thermophysical properties appropriate for silicon, a low-Prandtl-number semiconductor, and for GGG, a high–Prandtl–number oxide material. The algorithm is capable of computing solutions for both materials at realistic values of the Grashof number, and the calculations are convergent with mesh refinement. Flow transitions and interface shapes are calculated as a function of increasing flow intensity and compared for the two material systems. The flow pattern near the melt/gas/crystal tri-junction has the asymptotic form predicted by an inertialess analysis assuming the meniscus and solidification interfaces are fixed.     

Experimental study of thermoacoustic effects on a single plate Part I: Temperature fields

 The thermal interaction between a heated solid plate and the acoustically driven working fluid was investigated by visualizing and quantifying the temperature fields in the neighbourhood of the solid plate. A combination of holographic interferometry and high-speed cinematography was applied in the measurements. A better knowledge of these temperature fields is essential to develop systematic design methodologies for heat exchangers in oscillatory flows. The difference between heat transfer in oscillatory flows with zero mean velocity and steady-state flows is demonstrated in the paper. Instead of heat transfer from a heated solid surface to the colder bulk fluid, the visualized temperature fields indicated that heat was transferred from the working fluid into the stack plate at the edge of the plate. In the experiments, the thermoacoustic effect was visualized through the temperature measurements. A novel evaluation procedure that accounts for the influence of the acoustic pressure variations on the refractive index was applied to accurately reconstruct the high-speed, two-dimensional oscillating temperature distributions. Received on 22 March 1999     

Color-based image processing to measure local temperature distributions by wide-band liquid crystal thermography

This study presents a color-image-processing procedure for non-intrusive local temperature measurements by thermochromic liquid crystals (TLCs). The image evaluation software is completely independent of the color detection and acquisition hardware. This allows to use a wide variety of hardware solutions. An easy reproducible calibration of camera and light source is presented. The dependence of the detected hue values on intensity is investigated and further the hueversus temperature relation is studied.Sprayable TLC formulations and TLC-coated polyester sheets are studied and compared with regard to their signal-to-noise ratio and the dependence of their hue values on illumination and viewing angle. Furthermore, a method to investigate the hue resolution is presented. The relation between the resolution of hue values and the illumination intensity and its influence on signal noise is discussed for the first time for TLC applications. Different techniques of signal noise reduction are implemented in the image processing system. Their effects on the signal noise level are discussed. As an example the two dimensional temperature distribution caused by wing-type vortex generators in a channel flow is given.     

A New Cell for Electrical Conductivity Measurement on Saturated Samples at Upper Crust Conditions

Electrical resistivity soundings are used by geophysicists to determine the structure and composition of the Earth’s crust and mantle and to explore natural resources (ore, oil, gas, water). Their interpretations in terms of composition and in-situ physical conditions depend mainly on laboratory measurements of electrical conductivity of rocks at simulated crustal conditions of temperature, pressure, saturation and pore pressures. These measurements present a numbers of limitations, in particular, in the case where conductive pore fluids are present, as in the case of deep reservoir conditions, where temperature exceeds 250 °C. Here, we present a new cell capable of measuring electrical conductivity of large saturated samples at confining pressure up to 200 MPa, pore pressure up to 50 MPa, and temperature up to 500 °C. The measurement cell has been developed in a commercial, internally heated, gas pressure apparatus (Paterson press). It is based on the concept of “guard ring” electrode, which is adapted to samples that are jacketed by a very conductive, metallic material. Numerical modeling of the current flow in the electrical cell allowed defining the optimal cell geometry. Calibration tests have been performed on Fontainebleau sandstones saturated with electrolytes of different conductivities, up to 350 °C. The resulting electrical formation factor and temperature dependence of electrical conductivity are in very good agreement with previous studies. This new cell will improve the exploration and exploitation of deep fluid reservoirs, as in unconventional, high enthalpy geothermal fields. In particular, the investigations address possible effects of fluid-rock interactions on electrical resistivity of a reservoir host rock.     

Experimental and numerical studies of thermo-hydrous transfers in concrete exposed to high temperature

The compressive strength of concrete can be as high as 80 MPa at 28 days. High strength concrete (HSC) can be obtained by decreasing porosity and lowering permeability. Concrete, especially HSC, performs poorly when subjected to fire. This is attributed to high thermal stresses and water vapour pressure. High thermal gradient induces high thermo-mechanical stresses in the concrete system. Low permeability prevents water from escaping and induces high water vapour pressure causing cracking and spalling. The aim of this study is both experimentally and numerically study the coupled heat and mass transfers in concrete exposed to elevated temperature. Five concrete mixtures with various cement contents and water cement ratios of a constant aggregate content were studied before and after heating–cooling cycles. The concrete cylindrical specimens were subjected to several tests: compression and splitting tensile tests, measurement of modulus of elasticity, heating–cooling cycles, thermal field and mass loss during the heating–cooling cycles, and permeability tests. Comparisons between the numerical and experimental results on the thermo-hydrous behaviour were reported. Parametric analyses were carried out in order to underline main parameters involved in concrete behaviour at high temperature. The numerical and experimental results included thermal gradient, water vapour pressure, relative humidity, concrete mass losses due to dehydration, and water content for concrete elements heated from 20 to 600°C. The results show the degrees of damage due to the concrete chemical transformations at high temperature.     

Experimental study of the temperature field generated during orthogonal machining of an aluminum alloy

During the machining of metals, plastic deformation and friction lead to the generation of heat in the workpiece, which results in thermomechanically coupled deformation. Recently, several numerical models of this highly coupled process have been produced in response to increased interest in high speed machining. It is important to characterize the thermal field in the cutting zone in order to completely verify these models of high speed machining and to direct further advancement in this area. In this work, HgCdTe infrared detectors are used to experimentally measure the temperature distribution at the surface of a workpiece during orthogonal cutting. From these temperature measurements, the heat generated in the primary shear zone and the friction zone can be examined and characterized. A modified Hopkinson bar technique has been developed to perform orthogonal machining at speeds ranging from 10 to 100 m/s. In the present work, a cutting velocity of 15 m/s is employed in all the tests in order to demonstrate the capability of the apparatus and characterize thermal fields during low speed machining. Temperature fields are obtained during the orthogonal cutting of aluminum as a function of depth of cut. It is seen that depth of cut can vary both the maximum temperature as well as the distribution of the temperature field in the aluminum workpiece. the maximum temperature increased with depth of cut (238°C for 1.5 mm cut, 207°C for 1.0 mm cut and 138°C for 0.5 mm cut) and the temperature field extended further beneath the cut surface with decreasing depth of cut.     

Tomographic particle-image velocimetry and thermography in Rayleigh-Bénard convection using suspended thermochromic liquid crystals and digital image processing

Steady-state flow and temperature fields in shallow rectangular enclosures heated from below were visualized and quantitatively characterized by using glycerol as the working fluid and suspended thermochromic liquid crystals as tracers. Couples of photographs taken on 120 transparency film for two orthogonal sets of vertical plane sections were digitized by a 1,200-dpi flatbed scanner and split into HSL (hue-saturation-lightness) components by using commercial general-purpose image processing software. Two-dimensional velocity fields were obtained from the lightness component by a two-frame cross-correlation technique using a commercial particle-image velocimetry (PIV) package. Temperature fields were obtained from the hue component on the basis of an in situ calibration procedure, conducted under conditions of stable thermal stratification. Finally, 2D flow and temperature distributions were interpolated by a purpose-written Fortran program to give 3D flow and thermal fields in the enclosure. Results are presented here for the case of a 1:2:4 aspect ratio cavity at a Rayleigh number of ∼ 14,500, for which a complex 3D flow and temperature distribution was observed. Published online: 7 January 2003     

Two-phase flow instabilities in a silicon microchannels heat sink

Two-phase flow instabilities are highly undesirable in microchannels-based heat sinks as they can lead to temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations. This work is an experimental study of boiling instabilities in a microchannel silicon heat sink with 40 parallel rectangular microchannels, having a length of 15 mm and a hydraulic diameter of 194 μm. A series of experiments have been carried out to investigate pressure and temperature oscillations during the flow boiling instabilities under uniform heating, using water as a cooling liquid. Thin nickel film thermometers, integrated on the back side of a heat sink with microchannels, were used in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities. Flow regime maps are presented for two inlet water temperatures, showing stable and unstable flow regimes. It was observed that boiling leads to asymmetrical flow distribution within microchannels that result in high temperature non-uniformity and the simultaneously existence of different flow regimes along the transverse direction. Two types of two-phase flow instabilities with appreciable pressure and temperature fluctuations were observed, that depended on the heat to mass flux ratio and inlet water temperature. These were high amplitude/low frequency and low amplitude/high frequency instabilities. High speed camera imaging, performed simultaneously with pressure and temperature measurements, showed that inlet/outlet pressure and the temperature fluctuations existed due to alternation between liquid/two-phase/vapour flows. It was also determined that the inlet water subcooling condition affects the magnitudes of the temperature oscillations in two-phase flow instabilities and flow distribution within the microchannels.     

Effect of film cooling on the aerodynamic performance of an airfoil

The effect of film cooling on the aerodynamic performance of turbine blades is becoming increasingly important as the gas turbine operating temperature is being increased in order to increase the performance. The current paper investigates the effect of blowing ratio on the aerodynamic losses of a symmetric airfoil by pressure measurements and Particle Image Velocimetry (PIV). The test model features 4 rows of holes located on the suction side at 5%, 10%, 15% and 50% of the chord length. The Reynolds number based on the airfoil chord is 1.2 × 10⁵. Experiments are performed by varying the location of air injection, the angle of attack, and the mainstream velocity. The coolant air is injected at ambient temperature and the blowing ratio is varied from 0 to 1.91. It is observed that the losses due to film cooling increase with blowing ratio of 0 to 0.48, and the wake is shifted towards the suction side. Conversely, the aerodynamic losses decrease when the blowing ratio is increased further from 0.64 to 1.91. This trend has been observed for all the experimental configurations. The effect of blowing ratio on flow separation is investigated with the time-averaged velocity fields obtained from PIV measurements. It is observed that low blowing ratios, the separation point shifts upstream and at high blowing ratios the ejected coolant energizes the flow and delays separation. The pressure field around the airfoil is reconstructed from the integration of the Poisson equation based on the PIV velocity fields. The experimental results can be used for validation of numerical models for predicting losses due to film cooling.     

Flow field characterization of a jet and vortex actuator

 An actuator, which produces several different flow fields that may be used for active flow control, is characterized in still air using flow visualization and velocity measurements. The primary actuator-induced flow fields are: free jets, wall jets, and vortex flows. The non-dimensional parameters governing these actuator-induced flows are developed. For the vortex-flow regime, the operational range of the actuator increases as it’s size decreases without a significant decrease in either the actuator induced velocity or vortex core size. The velocity scaling is developed for the vortex flow and suggests that the optimum actuator efficiency occurs at a Stokes number of approximately 7.9 for the range of parameters surveyed. In a turbulent, zero pressure gradient boundary layer, measurements made just downstream of the actuator (when operated in the vortex mode) indicate a vortical disturbance is generated in the boundary layer. Received: 2 September 1998/Accepted: 9 January 1999