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     

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     

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.     

Dynamic simulation of free surfaces in capillaries with the finite element method

The mathematical formulation of the dynamics of free liquid surfaces including the effects of surface tension is governed by a non-linear system of elliptic differential equations. The major difficulty of getting unique closed solutions only in trivial cases is overcome by numerical methods. This paper considers transient simulations of liquid–gas menisci in vertical capillary tubes and gaps in the presence of gravity. Therefore the CFD code FIDAP 7.52 based on the Galerkin finite element method (FEM) is used. Calculations using the free surface model are presented for a variety of contact angles and cross-sections with experimental and theoretical verification. The liquid column oscillations are compared for numerical accuracy with a mechanical mathematical model, and the sensitivity with respect to the node density is investigated. The efficiency of the numerical treatment of geometric non-trivial problems is demonstrated by a prismatic capillary. Present restrictions limiting efficient transient simulations with irregularly shaped calculational domains are stated. © 1998 John Wiley & Sons, Ltd.     

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.     

Flooding in vertical two-phase flow

The phenomenon of flooding limits the stability of a liquid film falling along the walls of a tube in which a gas is flowing upward. This paper describes a series of experiments to observe and photograph the flooding process. Flooding in a vertical perspex tube was observed by viewing axially along the tube from the top, and flooding on the walls of a stainless steel rod supported centrally within a perspex tube was observed by viewing normally through the tube walls. The resulting observations show that flooding occurs as a result of the formation and motion of a large disturbance wave on the surface of the liquid film. A simple theoretical treatment of flooding is presented, based on the observations of the flooding disturbance. The resulting equations enable the calculation of the gas velocity and the liquid wave height at flooding, and the predictions are in good agreement with experimental observations reported here and elsewhere.     

A compact viewing configuration for stereoscopic micro-PIV utilizing mm-sized mirrors

When applying PIV to micrometer-resolution systems, the out-of-plane velocity component has shown to be much more difficult to obtain than the two in-plane velocity components. In this work, we present a novel stereoscopic viewing configuration for stereoscopic micro-PIV, utilizing mm-sized mirrors introduced in front of the microscope objective lens. Attractive applications, where the advantage of the mirror viewing configuration system could be utilized, include (on-chip) miniaturization and the potential to introduce more than two simultaneous viewing angles. Here, a first validation of our system is presented, including stereoscopic micro-PIV measurements in a micro-fabricated test device.     

Study of the impacts of droplets deposited from the gas core onto a gas-sheared liquid film

The results of an experimental study on droplet impactions in the flow of a gas-sheared liquid film are presented. In contrast to most similar studies, the impacting droplets were entrained from film surface by the gas stream. The measurements provide film thickness data, resolved in both longitudinal and transverse coordinates and in time together with the images of droplets above the interface and images of gas bubbles entrapped by liquid film. The parameters of impacting droplets were measured together with the local liquid film thickness. Two main scenarios of droplet-film interaction, based on type of film perturbation, are identified; the parameter identifying which scenario occurs is identified as the angle of impingement. At large angles an asymmetric crater appears on film surface; at shallow angles a long, narrow furrow appears. The most significant difference between the two scenarios is related to possible impact outcome: craters may lead to creation secondary droplets, whereas furrows are accompanied by entrapment of gas bubbles into the liquid film. In addition, occurrence of partial survival of impacting droplet is reported.     

A multiphase,micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.     

Effects of oxidation and surface roughness on contact angle

Contact angle is known to be a parameter that effects boiling. This study was undertaken to measure contact angle of high and low surface tension fluids on copper and aluminum surfaces.Data were taken for polished, oxidized, and rough surfaces. A simple, yet fairly accurate method of measuring the static equilibrium contact angle of a solid/liquid interface is presented. The principles of a line light source and tilting plate were modified and then combined in the design of this apparatus. The angles obtained and their variation with the solid surface properties were in good agreement with previously published data. The contact angle of distilled water o of the organic fluids and refrigerants tested were in the range of 2–5°. Roughness and oxidation reduce the contact angle. If the depth of the roughness is less than 0.5 μm contact angle. The apparatus is fairly simple in construction, is inexpensive, and has good reproductibity. The measured angles were then compared to those measured with the sessile drop method.     

Numerical Analysis of Coupled Heat and Mass Transfer Phenomena in Concrete at Elevated Temperatures

In this work, we present a novel methodology for incorporating the effect of fibre surface morphology on liquid water transport in polymer electrolyte membrane fuel cell gas diffusion layers (GDLs). Roughness features presented on the surface of the fibre are analysed using atomic force microscopy and are found to significantly impact the capillary pressure of liquid water pathways propagating through the GDL. A threshold capillary pressure was defined as the largest capillary pressure exhibited by the liquid water phase during the invasion of the throat. The threshold capillary pressures observed in the presence of roughness features are significantly greater than those in the absence of roughness features. Two-dimensional circumferential roughness models in cylindrical and converging-diverging throats are established, and an interfacial meniscus advancing algorithm is presented to determine the resulting threshold capillary pressures required for liquid water penetration. Revised Young–Laplace equations, which are particularly useful for pore network modeling, are suggested for calculating threshold capillary pressures that account for the effect of the roughness of throats with intrinsic contact angles greater than \(90^{\circ }\).     

Interfacial measurements in stratified types of flow. Part I: New optical measurement technique and dry angle measurements

The development of a new non-intrusive computerized image analysis and optical observation method for accurately detecting the vapor–liquid interface in stratified two-phase flows is presented. This technique is applied to a round horizontal sight glass tube using a monochromatic laser sheet for observing two-phase flow patterns and for measuring cross-sectional dry angles and void fractions in these types of flow. The cross-sectional image observed externally is distorted by refraction and is thus reconstructed by computer. From this image, the shape of the vapor–liquid interface is detected and the dry angle and void fraction are accurately determinable over a wide range of conditions for a glass tube of 13.6 mm diameter. Results for dry angle are reported here while the test facility and void fraction measurements are presented in Part II. R-22 and R-410A are the test fluids. Dry angles were quite close to values predicted for stratified flow and much larger than comparable values for air–water flows.     

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.     

An Approximate Lie Group Investigation into the Spreading of a Liquid Drop on a Slowly Dropping Flat Plane

The approximate Lie group method is used to investigate the evolutionof the free surface of a thin liquid drop on a slowly dropping flat plane. Surfacetension effects are ignored. A group classification is performed to determine the rateat which the plane drops. An approximate group invariant solution is then calculatedfor the free surface of an evolving liquid drop on the slowly dropping flat plane. Animportant parameter in the solution is the initial angle of the plane. For small anglesthere is no significant difference in the drop profile. For larger angles, changes in thedrop profile and rate of spreading are significant.     

Telecentric lenses for imaging in particle image velocimetry: a new stereoscopic approach

A new stereoscopic approach based on telecentric lenses is introduced. The method offers in-focus imaging at high viewing angles (highly tilted object planes) with diminishing systematic image distortion. In single-view particle image velocimetry (PIV) applications telecentric lenses can be used to eliminate projection errors. Normal and oblique viewing are tested using a commercial telecentric lens with particle image velocimetry.     

Experimental investigation of the onsets of gas and liquid entrainment from a small branch mounted on an inclined wall

The phenomena of the onsets of liquid entrainment and gas entrainment were investigated experimentally for the case of a flat plane with a circular outlet branch of diameter d (=6.35 mm) at the plane centre. This flat plane was situated in a large tank containing a stratified mixture of air and water under pressure (317 kPa for most experiments and 520 kPa for a few experiments) and at room temperature. The plane was inclined through various angles (θ) in increments of 30°, from the outlet branch orientation being vertically upward through the horizontal to vertically downward. For both onsets the vertical distance between the centre of the outlet branch and the undisturbed gas–liquid interface (h) was measured for various angles of inclination and Froude numbers. Both onsets were observed visually through a large viewing part of the test section. It was found that for both onsets there is a range of inclination angles where the onset h depended on θ and a range where the onset h essentially did not depend on θ. The data were correlated in terms of onset h/d, Froude number, and θ where there was dependence of onset h on the angle of inclination.     

Stability of a rivulet flowing in a microchannel

A novel microfluidic technique has been recently proposed to produce quasi-monodisperse collections of microbubbles with a controlled size. In this technique, a gaseous stream is injected through a T-junction into a microchannel transporting a liquid current. The gas adheres to a hydrophobic strip printed on the channel surface. When the gas and liquid flow rates are set appropriately, a gaseous rivulet flows over that strip. The rivulet breaks up downstream due to a capillary pearling instability, which leads to a quasi-monodisperse collection of microbubbles. Motivated by this application, we here analyze the stability of both gas and liquid rivulets coflowing with a current in a quadrangular microfluidic channel. The results essentially differ from those of cylindrical jets because the contact-line-anchorage condition affects fundamentally the rivulet’s instability nature. The temporal stability analysis shows that the rivulet becomes unstable not only for (unperturbed) contact angles larger than 90° (as can be expected) but also for values smaller than that angle. Interestingly enough, the maximum growth factor exhibits a non-monotonic dependence with respect to the Reynolds number (i.e., the viscosities). In fact, there are intervals of that parameter where the fluid system becomes unstable, while all the perturbations are damped outside that interval. The gaseous rivulet does not stabilize as the Reynolds number decreases, which means that it can be unstable even in the Stokes limit and for contact angles less than 90°. In addition, the stability of a flowing liquid rivulet is not determined by its contact angle exclusively (as occurs in the static case), but by the Reynolds number as well. Liquid rivulets with contact angles less than 90° can be unstable for sufficiently high Reynolds numbers.     

Heat transfer from an obliquely impinging circular, air jet to a flat plate

A series of experiments was conducted for the measurement of local convective heat transfer coefficients for an obliquely impinging circular air jet to a flat plate. In the experiments, the oblique angles selected were 90°, 75°, 60° and 45°, with 90° being a vertical jet. Two different Reynolds numbers of 10,000 and 23,000 were considered for the purpose of comparison with previous data available in the literature. Another parameter varied in the measurements was the dimensionless jet-to-plate distance, L/D. Four values of L/D(2, 4, 7, and 10) were considered in the experiments. The experiments were conducted using the preheated wall transient liquid-crystal technique. Liquid-crystal color changes were recorded with a video system. Local convective heat transfer coefficients were obtained through the surface transient temperatures that were related to the recorded color information. Detailed local heat transfer coefficients were presented and discussed in relation to the asymmetric wall jet upon impingement of the jet flow. Results of experiments show that, for a given flow situation, the point of maximum heat transfer shifts away from the geometrical impingement point toward the compression side of the wall jet on the axis of symmetry. The shift is more pronounced with a smaller oblique angle (larger jet inclination) and a smaller jet-to-plate distance. Comparisons of experimental results with existing heat transfer data for both obliquely impinging jets and vertical impinging jets are made. The effect of oblique angles on heat transfer was assessed.     

Numerical simulations of interfacial instabilities on a rotating miscible droplet in a time‐dependent gap Hele–Shaw cell with significant Coriolis effects

Interfacial instability of a rotating miscible droplet with significant Coriolis force in a Hele–Shaw cell is simulated numerically. The influences of the relevant control parameters are first discussed qualitatively by fingering patterns. More vigorous fingerings are found at higher rotational effects, a lower viscosity contrast and a weaker effective surface tension (Korteweg constant). For a time‐dependent gap Hele–Shaw cell, a higher cell lifting rate makes the rotating droplet bear an inward straining flow, which leads to fingering enhancement. On the contrary, a higher pressing rate provides more stable effects by additional squeezing outward flow. A quantitative analysis between the Coriolis effects and tilting angles of fingers is addressed. For arbitrary combinations of all relevant control parameters, the values of tilting angles follow a nearly linear relationship with the Coriolis effects. We estimate the correlation between the relevant control parameters (dimensionless Coriolis factor Re, viscosity parameter R, cell lifting rate a) and tilting angles (θ) of fingers that can be approximated as for significant Korteweg stresses. Copyright © 2005 John Wiley & Sons, Ltd.