New insight into two-color LIF thermometry applied to temperature measurements of droplets

When laser-induced fluorescence of droplets is used for measurements such as droplet temperature, a new dependence of the droplet size on the spectral distribution of fluorescence has been highlighted. The two-color laser-induced fluorescence technique applied to droplet temperature measurement requires a single fluorescent tracer and two spectral bands of detection for which the temperature sensitivity is different. Generally, the ratio of the intensities measured on each of the spectral bands of detection is assumed to be only temperature dependent. However, droplet dependence on diameter is also likely to influence the intensities ratio. This study provides some illustrations of the phenomenon, first on sprays with different mean statistical diameters and secondly on single droplets, for two temperature-sensitive fluorescent tracers in their solvents: sulforhodamine B dissolved in water and pyrromethene 597-8C9 dissolved in n-decane.     

Scanning liquid-crystal thermometry and stereo velocimetry for simultaneous three-dimensional measurement of temperature and velocity field in a turbulent Rayleigh-Bérnard convection

In this paper, we report on an experimental technique for the simultaneous measurement of temperature and three components of velocity in a three-dimensional thermal flow using scanning liquid-crystal thermometry and stereo velocimetry. The temperature is measured by the color image analysis of the liquid-crystal particles suspended in a fluid, while the three velocity components are measured by stereo particle image velocimetry (stereo PIV) with the aid of tracer particles. The measurement is carried out by scanning the light-sheet plane while capturing the sequential color images of the liquid crystals and tracer particles. This measurement allows the reconstruction of the three-dimensional distribution of temperature and full velocity field simultaneously. The present experimental technique is applied to the horizontal fluid layer of a turbulent Rayleigh-Bérnard convection and the three-dimensional structures of thermal plumes are evaluated. The experimental results indicate that the structures of plumes are often correlated with the vertical velocity of the fluid, but they behave randomly in space, influenced by the large-scale turbulence evident in the middle of the fluid layer.     

Dual-tracer fluorescence thermometry measurements in a heated channel

The exponential growth of component density in microelectronics has renewed interest in compact and high heat flux thermal management technologies that can handle local heat fluxes exceeding 1 kW/cm². Accurate and spatially resolved thermometry techniques that can measure liquid-phase temperatures without disturbing the coolant flow are important in developing new heat exchangers employing forced-liquid and evaporative cooling. This paper describes water temperature measurements using dual-tracer fluorescence thermometry (DFT) with fluorescein and sulforhodamine B in laminar Poiseuille flow through polydimethyl siloxane-glass channels heated on one side. The major advantage of using the ratio of the signals from these two fluorophores is their temperature sensitivity of 4.0–12% per °C—a significant improvement over previous DFT studies at these spatial resolutions. For an in-plane spatial resolution of 30 μm, the average experimental uncertainties in the temperature data are estimated to be 0.3°C.     

Molecular tagging thermometry with adjustable temperature sensitivity

We report an extension to the technique of molecular tagging thermometry which allows for adjustable temperature sensitivity. The temperature dependence of laser-induced phosphorescence of the water-soluble phosphorescent triplex (1-BrNp•Mβ-CD•ROH) is used to conduct temperature measurements in aqueous flows. It is shown that the temperature sensitivity of phosphorescence intensity can be adjusted by changing the time delay between the laser excitation pulse and the start of the phosphorescence emission acquisition. For example, for a phosphorescence integration period of 1 ms, the temperature sensitivity of the measured phosphorescence intensity varies in the range 8.15–18.2% per °C at 25°C as the time delay changes from 1 to7 ms. This temperature sensitivity is much higher than that of most fluorescent dyes used for temperature measurements (e.g. less than about 2% per °C for Rhodamine B). The implementation and application of this new approach are demonstrated by conducting temperature measurements in the wake of a heated cylinder.     

Condensation with interfacial temperature drop

Using the boundary layer equations of momentum and energy, an asymptotic series has been developed to determine the velocity and temperature fields in a stationary metal vapor condensing over a vertical surface, with the film in laminar flow. Interfacial temperature drop has been taken into account, by using an approximate form of the developed byschrage to predict the condensate metal surface temperature. With two terms taken from the asymptotic series development, it is shown that the resulting values of heat transfer coefficients and average Nusselt numbers fall in the same range as the experimentally observed values for mercury vapor. These calculations have been carried out assuming a condensation coefficient of unity. The Nusselt number is seen to depend parametrically on both the saturation pressure and the length of the condensing surface. The calculated mass condensation rate is lower than that of the theory neglecting interfacial temperature drop. The results are all applicable for sufficiently long plates when the pressure is not too low. For very short plates and pressures below about 25 mm of mercury, more terms in the asymptotic expansion must be considered.

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Zusammenfassung Unter Benutzung der Grenzschichtgleichungen für Impuls und Energie wird eine asymptotische Eeihenentwicklung vorgenommen, um die Geschwindigkeits und Temperaturfelder in einem stationären Metalldampf, welcher an einer senkrechten Oberfläche kondensiert, zu bestimmen; der Film befindet sich dabei in laminarer Strömung. Das Temperaturgefälle an der Grenzfläche wurde berücksichtigt durch Benutzung einer Näherungsform der vonschrage entwickelten Gleichung, aus der sich die Oberflächentemperatur des kondensierenden Metalls ableiten läßt. Mit den beiden ersten Termen der asymptotischen Reihenentwicklung wird gezeigt, daß die resultierenden Werte der Wärmeübertragungskoeffizienten und der durchschnittlichen Nusselt-Zahl in denselben Bereich fallen wie die experimentell beobachteten Werte für Quecksilberdampf. Diese Berechnungen wurden unter der Annahme durchgeführt, daß der Kondensationskoeffizient gleich Eins ist. Es zeigt sich, daß die Nusselt-Zahl parametrisch vom Sättigungsdruck und von der Länge der kondensierenden Oberfläche abhängt. Die berechnete Massen-Kondensationsrate ist geringer, als sich aus einer das Temperaturgefälle an der Grenzschicht nicht berücksichtigenden Theorie ergibt. Die Resultate sind alle anwendbar für ausreichend lange Platten bei nicht zu geringem Druck. Bei kurzen Platten und Drucken unter 25 mm Hg müssen weitere Terme der asymptotischen Reihenentwicklung berücksichtigt werden.

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Extensional viscosity from entrance pressure drop measurements

Extensional rheological properties are important in characterization and processing of polymeric liquids. The use of entrance pressure drop to obtain extensional viscosity is particularly attractive because it can be applied to both low and high viscosity liquids using the Bagley correction obtained from a conventional capillary rheometer.Low density polyethylene of three different melt index values, including IUPAC-X (a different batch of IUPAC-A), and a high density polyethylene were tested using a commercial capillary rheometer. The entrance pressure drop (P _en ) was obtained with a zero-length orifice die with an abrupt contraction. The contraction ratio was 12:1. Predictions from several approximate analyses to calculate the uniaxial extensional viscosity _u (using an axisymmetric contraction) from P _en were compared. These comparisons are summarized in the appendices.Due to the transient nature of contraction flows, _u is also a function of the strain (). This was examined by comparing _u from P _en (Cogswell's analysis was chosen for convenience) with transient extensional viscosity ( _u ⁺) at different magnitudes of from fiber-windup technique (Padmanabhan et al., 1996). _u ⁺ at 3 was found to be close to _u from P _en (using Cogswell's analysis) for two LDPE samples that had fiber-windup data available. The magnitude of the strain in the contraction did not vary with strain rate.Dedicated to the memory of Tasos Papanastasiou     

Comparison of magnetic resonance concentration measurements in water to temperature measurements in compressible air flows

Magnetic resonance imaging (MRI) measurements in liquid flows provide highly detailed 3D mean velocity and concentration data in complex turbulent mixing flow applications. The scalar transport analogy is applied to infer the mean temperature distribution in high speed gas flows directly from the MRI concentration measurements in liquid. Compressibility effects on turbulent mixing are known to be weak for simple flows at high subsonic Mach number, and it was not known if this would hold in more complex flows characteristic of practical applications. Furthermore, the MRI measurements are often done at lower Reynolds number than the compressible application, although both are generally done in fully turbulent flows. The hypothesis is that the conclusions from MRI measurements performed in water are transferable to high subsonic Mach number applications. The present experiment is designed to compare stagnation temperature measurements in high speed airflow (M = 0.7) to concentration measurements in an identical water flow apparatus. The flow configuration was a low aspect ratio wall jet with a thick splitter plate producing a 3D complex downstream flow mixing the wall-jet fluid with the mainstream flow. The three-dimensional velocity field is documented using magnetic resonance velocimetry in the water experiment, and the mixing is quantified by measuring the mean concentration distribution of wall-jet fluid marked with dissolved copper sulfate. The airflow experiments are operated with a temperature difference between the main stream and the wall jet. Profiles of the stagnation temperature are measured with a shielded thermocouple probe. The results show excellent agreement between normalized temperature and concentration profiles after correction of the temperature measurements for the effects of energy separation. The agreement is within 1 % near the edges of the mixing layer, which suggests that the mixing characteristics of the large scale turbulence structures are the same in the two flows.     

Heat transfer and pressure drop measurements in rib-roughened rectangular ducts

In the present study, the thermal and hydraulic performance of three rib-roughened rectangular ducts is investigated. The aspect ratio of the ducts was 1 to 8, and the ribs were arranged staggered on the two wide walls. Three rib configurations were tested: parallel ribs and V-shaped ribs pointing upstream or downstream of the main flow direction. For all cases, the rib height-to-hydraulic diameter ratio was 0.06, with an attack angle of 60° and a pitch-to-height ratio of 10. The Reynolds number range was from 1000 to 6000. Liquid crystal thermography was employed in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on the ribbed surfaces. The secondary flows caused by the inclined ribs create a significant spanwise variation of the heat transfer coefficients on the rib-roughened wall with high heat transfer coefficient at one end of the rib and low value at the other. In the streamwise direction between two consecutive ribs, the temperature distribution shows a sawtooth fashion because of flow reattachment. Based on the local heat transfer coefficients, the average Nusselt numbers were estimated as weighted mean values. Isothermal pressure drop data were taken and presented as Fanning friction factors. The ducts are compared to each other by considering both heat transfer and friction factor performance.     

Pressure drop and film height measurements for annular gas-liquid flow

Measurements of the pressure drop and the film height averaged around the circumference are presented for air and water flowing in horizontal 2.54 and 5.08 cm pipelines. Film height measurements are interpreted using relations similar to those that have been developed for vertical flows. The frictional pressure loss is found to be primarily related to properties of the liquid film and to be approximately independent of the amount of entrained liquid.     

Cavity and flow measurements of reproducible bubble entrainment following drop impacts

High-speed motion pictures of air–water interface dynamics of drop impacts that reproducibly make bubbles are presented. The pictures show previously unobserved details of the phenomenon. Measurements are compared with available computational methods. Experimental and numerical results agree with each other on the overall shape of the interface and the occurrence of bubble detachment. Measurements, however, show that the cavity depth stagnates before bubble entrapment. This behavior is not predicted by simulation. Also discussed are the presence of a jet that strikes the new bubble after formation and the possible effect of droplet surface oscillations on bubble entrainment. Received: 25 April 2000 / Accepted: 26 April 2001     

Application of interconversions to viscosity measurements of polystyrene

Empirical and theoretically based numerical interconversions between viscoelastic material functions are used to compare results of two different measuring techniques for polymer melts. The methods compared are the magnetoviscometer and a dynamic-mechanical apparatus. For the case of two technical polystyrenes results are given which show the possibility of determining the zero shear viscosity and, using an empirical conversion, more of the flow curve from results of the magnetoviscometer. In general, the results of both techniques are in good accordance.     

Application of ultrasonic-pulse-spectroscopy measurements to experimental stress analysis

We apply the technique of ultrasonic pulse spectroscopy to measure the interference effects between two shear waves propagating in specimens loaded in uniaxial compression. We show that the power spectrum of an echo containing both fast and slow components of a shear wave will exhibit periodic minima. The periodicity exhibited in the spectrum is 1/Δτ, where Δτ is the difference in arrival time between the fast and slow waves. A change in the state of stress which produces a change in the two shear velocities results in a stress-dependent change in wave-arrival times. Because of this velocity change, the frequency at which a particular minimum occurs in the spectrum changes, and this can be used to indicate the state of stress in the material. Our results indicate that, if the spectrum minima frequencies could be resolved to within 10 kHz, the principal-stress differences within 36 psi (0.251 MPa) could be measured in specimens of aluminum 1 in. (2.54 cm) thick. Inherent in analyzing and measuring echo-interference effects is a single-echo requirement. Thus, transducer coupling effects are minimized and measurements in highly attenuating materials or at high frequencies in normal attenuating materials are possible. This technique shows considerable promise as a means of measuring and monitoring the applied stresses in materials.     

Effect of experimental parameters and image noise on the error levels in simultaneous velocity and temperature measurements using molecular tagging velocimetry/thermometry (MTV/T)

In this work, the effect of experimental parameters on the error levels associated with simultaneous measurement of displacement and temperature using Molecular Tagging Velocimetry/Thermometry (MTV/T) was quantified via simulated images. Images were simulated using Gaussian profile laser lines. Noise was added to the images using a uniform random distribution and a Gaussian random distribution to simulate electronic noise and shot noise, respectively. The results showed that the error levels in the displacement and temperature measurements were inversely related for most experimental parameters including the laser line thickness, fluid temperature and image delay times. It is concluded that the dynamic range of the technique depends on the flow speeds and temperatures and must be determined for each experiment individually. Error levels, for 95% confidence, were found to be better than 0.3°C for temperature and 0.2 pixels for displacement for typical real-world experimental parameters.     

Application of 2-D LIF temperature measurements in water using a Nd : YAG laser

 The application of Laser Induced Fluorescence (LIF) for temperature measurements in water using a Nd : YAG laser is investigated. A natural convection problem is used to test the applicability of LIF in the temperature range of 20–60°C. The measured temperature field is compared with numerical results and the influences of shadowgraph effects on the measured temperature field are investigated. An accuracy of 1.7°C is attained if shadowgraph effects can be neglected. This only holds if correction for photobleaching and variation of laser power output is applied. Received: 8 July 1998/Accepted: 3 February 1999     

Application of multivariate outlier detection to fluid velocity measurements

A statistical-based approach to detect outliers in fluid-based velocity measurements is proposed. Outliers are effectively detected from experimental unimodal distributions with the application of an existing multivariate outlier detection algorithm for asymmetric distributions (Hubert and Van der Veeken, J Chemom 22:235–246, 2008). This approach is an extension of previous methods that only apply to symmetric distributions. For fluid velocity measurements, rejection of statistical outliers, meaning erroneous as well as low probability data, via multivariate outlier rejection is compared to a traditional method based on univariate statistics. For particle image velocimetry data, both tests are conducted after application of the current de facto standard spatial filter, the universal outlier detection test (Westerweel and Scarano, Exp Fluids 39:1096–1100, 2005). By doing so, the utility of statistical outlier detection in addition to spatial filters is demonstrated, and further, the differences between multivariate and univariate outlier detection are discussed. Since the proposed technique for outlier detection is an independent process, statistical outlier detection is complementary to spatial outlier detection and can be used as an additional validation tool.     

Flow and temperature measurement of natural convection in a Hele-Shaw cell using a thermo-sensitive liquid-crystal tracer

The temperature and flow field of natural convection in a Hele-Shaw cell is visualized by using a liquid-crystal tracer. The tracer photographs obtained by this method are compared with the interferograms of previous experiments using the same experimental setup, and the applicability of the present methods is validated. Quantitative data of the temperature and velocity were obtained by applying a colour-image-processing technique to the visualized images.This investigation was partly carried out during the stay of M. Ozawa in the Institut für Angewandte Thermo- und Fluiddynamik, Kernforschungszentrum Karlsruhe with the support of the Alexander-von-Humboldt-Stiftung     

Topological structure bifurcation of temperature field of deformable drop in Marangoni migration

The unsteady processes of the Marangoni migration of deformable liquid drops are simulated numerically in a wider range of Marangoni number (up to Ma = 500) in the present work. A steady terminal state can always be reached, and the scaled terminal velocity is a monotonic function decreasing with increasing Marangoni number, which is generally in agreement with corresponding experimental data. The topological structure of flow field in the steady terminal state does not change as the Marangoni number increases, while bifurcation of the topological structure of temperature field occurs twice at two corresponding critical Marangoni numbers. A third critical value of Marangoni number also exists, beyond which the coldest point jumps from the rear stagnation to inside the drop though the topological structure of the temperature field does not change. It is found that the inner and outer thermal boundary layers may exist along the interface both inside and outside the drop if Ma > 70. But the thickness decreases with increasing Marangoni number more slowly than the prediction of potential flow at large Marangoni and Reynolds numbers.     

Pressure drop measurements for woven metal mesh screens used in electrical safety switchgears

The paper deals with an experimental investigation of the pressure drops and friction factors induced by sets of metal woven screens in the case of an incompressible fluid flow, namely water flow. These woven screens are of metallic type and are used in electrical safety devices, especially at the end of exhaust duct of low voltage circuit breakers. In a first step, the whole of the set-up is used to check some approximations dedicated to pressure drops due to sphere beds around 275 µm and 375 µm as diameters. Results are discussed in terms of Darcyan and non-Darcyan permeabilities compared with published data and are analyzed in terms of the Blake-type friction factor. In the second step, the pressure drops are measured for stacks composed of eight different woven screens (of plain dutch type) formed with millimetric wires. Various formulations of the pressure drops and friction factors published elsewhere are tested with a special care dedicated to the choice of the geometry for the flow pattern (hydraulic diameter, spherical diameter, cylindrical diameter) and to the consideration of laminar and turbulent contribution. We then give the formulation that characterizes the fluid flowing through stacks of woven screens used in electrical safety applications.