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     

Laminar natural convection in a fully partitioned enclosure containing fluid with nonlinear thermophysical properties

The effects of a heat conducting partition on the laminar natural convection heat transfer and fluid flow were obtained by comparing the numerical and experimental results for a cubic enclosure without and with a partition. The two opposite vertical walls of the enclosure were isothermal at different temperatures. The working fluid was glycerol. The complete vertical partition, made of Plexiglass, was positioned in the middle of the enclosure. The visualizations of the velocity and temperature fields were obtained by using respectively, Plexiglass and liquid crystal particles as tracers. A middle plane perpendicular to the partition was numerically modeled. The steady two-dimensional model accounted for the variable thermophysical properties of the fluid. The finite volume method based on the finite difference approach was applied. The convective terms were approximated using a deferred correction central difference scheme. The velocity and temperature fields and the distribution of the local and average Nusselt numbers were found as a function of the Rayleigh (38 000 <Ra <369 000) and Prandtl (2700 < Pr < 7000) numbers.     

Investigation of a particulate flow containing spherical particles subjected to microwave heating

Microwave heating of a liquid and large spherical particles that it carries while continuously flowing in a circular applicator pipe is investigated. A three-dimensional model that includes coupled Maxwell, continuity, Navier–Stokes, and energy equations is developed to describe transient temperature, electromagnetic, and fluid velocity fields. The hydrodynamic interaction between the solid particles and the carrier liquid is simulated by the force-coupling method (FCM). Computational results are presented for the microwave power absorption, temperature distribution inside the liquid and the particles, as well as the velocity distribution in the applicator pipe and trajectories of particles. The effect of the time interval between consecutive injections of two groups of particles on power absorption in particles is studied. The influence of the position of the applicator pipe in the microwave cavity on the power absorption and temperature distribution inside the liquid and the particles is investigated as well.     

Natural convection in a bottom-heated top-cooled cubic cavity with a baffle at the median height: experiment and model validation

This paper presents an experimental and numerical investigation on the natural convection flow and heat transfer in an enclosure with a single-hole baffle at the median height. The temperature in the fluid is quantified by means of temperature sensitive thermo-chromic liquid crystal (TLC) particles. The fluid flow velocity is measured non-intrusively with a full field particle tracking technique. The three-dimensional numerical model, developed and validated with experimental data, provides a computational tool for further investigation of mass and energy transport through the baffle openings in these types of enclosures. The experimentally visualized and numerically simulated flow structures show a pair of streams across the baffle-hole. The two chambers communicate through this pair of streams which carry the fluid exchange and heat transfer between the two chambers. At the baffle opening, the two streams are aligned in a diagonal direction across of the enclosure. The streams are accelerated and form jet-like flows that drive the whole circulation in the chambers. The jet-like flows leave the baffle opening, approach the vertical centerline of the cavity, and finally impinge on the top/bottom walls.     

Non-isothermal flow diagnostics using microencapsulated cholesteric particles

The temperature and the flow field of thermo-convective liquid flows are visualized using cholesteric liquid crystal material as tracer particles. This type of tracers offers the scientifically valuable feature of measuring the flow and the temperature field simultaneously. Three thermoconvective flow configurations have been investigated successfully using liquid crystals. The results are discussed in some detail. It turns out that the liquid crystal technique is a valuable tool for thermo-convective liquid flow analysis.     

半浮区液桥热毛细振荡流

采用非定常、三维直接数值模拟方法研究大Pr数半浮区液桥热毛细对流从定常流向振荡流的过渡过程．文中详细描述了热毛细振荡流的起振和振荡特征,给出了液桥横截面上振荡流的流场和温度分布．在地面引力场条件下计算的结果与地面实验的结果进行比较,得出液桥水平截面上的流场和温度分布图样以一定的速度旋转,自由表面固定点处流体的环向流速正、负交替变化的一致结论．     

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.     

Combined two-dimensional velocity and temperature measurements of natural convection using a high-speed camera and temperature-sensitive particles

This paper proposes a combined method for two-dimensional temperature and velocity measurements in liquid and gas flows using temperature-sensitive particles (TSPs), a pulsed ultraviolet laser, and a high-speed camera. TSPs respond to temperature changes in the flow and can also serve as tracers for the velocity field. The luminescence from the TSPs was recorded at 15,000 frames per second as sequential images for a lifetime-based temperature analysis. These images were also used for the particle image velocimetry calculations. The temperature field was estimated using several images, based on the lifetime method. The decay curves for various temperature conditions fit well to exponential functions, and from these the decay constants at each temperature were obtained. The proposed technique was applied to measure the temperature and velocity fields in natural convection driven by a Marangoni force and buoyancy in a rectangular tank. The accuracy of the temperature measurement of the proposed technique was ±0.35–0.40°C.     

A numerical investigation of flow past a bluff body using three-state anemometry

An application of a new flow measurement technique is described which allows for the non-intrusive simultaneous measurement of flow velocity, density, and viscosity. The viscosity information can be used to derive the flow field temperature. The combination of the three measured variables and the perfect-gas law then leads to an estimate of the flow field thermodynamic pressure. Thus, the instantaneous state of a flow field can be completely described. Three-state anemometry (3SA), a derivative of particle image velocimetry (PIV), which uses a combination of three monodisperse sizes of styrene seeding particles is proposed. A marker seeding is chosen to follow the flow as closely as possible, while intermediate and large seeding populations provide two supplementary velocity fields, which are also dependent on fluid density and viscosity. A simplified particle motion equation, aimed at turbomachinery applications, is then solved over the whole field to provide both density and viscosity data. The three velocity fields can be separated in a number of ways. The simplest and that proposed in this paper is to dye the different populations and view the region of interest through interferometric filters. The two critical aspects needed to enable the implementation of such a technique are a suitable selection of the diameters of the particle populations, and the separation of the velocity fields. There has been extensive work on the seeding particle behaviour which allows an estimate of the suitable particle diameters to be made. A technique is described in this paper to allow the separation of particles in a range of micrometer sized velocity fields through fluorescence (separation through intensity also being possible). Some preliminary results by direct numerical simulation (DNS) of a 3SA image are also presented. The particle sizes chosen were 1 μm and 5 μm, tested on the near-wake flow past a cylinder to investigate viscosity only, assuming uniform flow density. The accuracy of the technique, derived from simulations of swirling flows, is estimated as 0.5% RMS for velocity, 2% RMS for the density and viscosity, and 4% RMS for the temperature estimate. © 1998 John Wiley & Sons, Ltd.     

Natural convection from a thin heated wire situated on the surface of a liquid

The temperature and velocity fields associated with the free convection of a liquid near a thin heated wire situated close to the horizontal surface of the liquid were studied experimentally. The temperature field was analyzed by the shadow method using a Svil' 80 instrument, and the velocity field by observing the motion of light-scattering particles. Universal profiles of the horizontal velocity and vertical temperature gradient were derived by making scale transformations of the spatial profiles measured in various cross sections of the heated zone for several values of the power developed by the wire.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 139–142, November–December, 1970.     

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     

Velocity field measurement on natural convection inside an automotive headlamp using time-resolved stereoscopic particle image velocimetry

Automotive headlamps undergo environmental changes such as radiation from outside and heat from the engine. Thus, the internal flow characteristics have an unsteady flow regime due to heat transfer depending on the lighting state of the internal bulb. In this study, we measured the quantitative 2D3C velocity vector field in a headlamp with a complex shape using stereoscopic particle image velocimetry (SPIV) to determine the area that is vulnerable to condensation. In order to obtain the 3D velocity component and calibration for the image distortion, the calibration function was obtained using a calibration target. An olive oil aerosol was used as PIV-tracking particles. The particles were injected by a Laskin nozzle through the bent hole of a headlamp model. The SPIV measurements showed that the flow inside the headlamp has a strong 3D velocity component. It was found that two or more vortex components formed in a direction perpendicular to the main flow based on the natural convection. The Reynolds stresses were analyzed using a statistical method based on the instantaneous velocity components, and most of the flow fields had laminar flow characteristics. However, in the case of a bulb-type headlamp, turbulence was locally generated due to a thermal plume induced by the high temperature from the bulb surface and complex internal structures in the headlamp, such as the reflector.     

Numerical modeling of time-dependent three-dimensional flows and heat and mass transfer in a cylindrical liquid bridge in the absence of gravity

Numerical modeling of the temperature distribution, the velocity fields, and the dopant (Ge:Si) concentration during germanium crystal growth by the floating zone method under microgravity is carried out. The time-dependent three-dimensional problem is solved using the difference method. The deformations of the free surface of a liquid bridge and of crystallization and melting fronts are neglected. The relaxation of axisymmetric flow regimes to a steady state is observed, whereas the unsteadiness of three-dimensional flows develops with time.     

Freezing of Water in a Differentially Heated Cubic Cavity

An experimental and numerical study has been made of transient natural convection of water freezing in a cube-shaped cavity. The effect of the heat transfer through the side walls is studied in two configurations: with the cavity surrounded by air and with the cavity immersed in an external water bath of constant temperature. The experimental data for the velocity and temperature fields are obtained using liquid crystal tracers. The transient development of the ice/water interface is measured. The collected data are used as an experimental benchmark and compared with numerical results obtained from a Finite-difference code with boundary fitted grid generation. The computational model has been adopted to simulate as closely as possible the physical experiment. Hence, fully variable fluid properties are implemented in the code, and, to improve modelling of the thermal boundary conditions, the energy equation is also solved inside the bounding walls. Although the general behaviour of the calculated ice front and its volume matches observations, several details of the flow structure do not. Observed discrepancies between experimental and numerical results indicate the necessity of verifying and improving the usual assumptions for modelling ice formation.     

Ultrasound thermometry in transparent and opaque fluids

We have exploited the temperature dependence of sound velocity to measure the thermal fields in transparent and opaque fluids. A chamber containing glycerol undergoing Rayleigh–Bénard convection was probed with an ultrasound transducer operating in the pulse-echo mode. The times-of-flight for the ultrasound pulse to traverse the fluid at several transducer locations were converted into a temperature profile that is in qualitative agreement with simultaneous thermochromic liquid crystal visualization of the flow pattern. Temperature profiles in a mercury-filled stainless steel chamber have also been obtained, both for quiescent and turbulent flows, thereby validating the ultrasound thermometry concept for opaque fluids as well.     

Temperature fields in a liquid due to the thermocapillary motion of bubbles and drops

 Experiments were performed on the motion of isolated air bubbles and drops of Fluorinert FC-75 moving in a Dow-Corning silicone oil under the action of an applied temperature gradient in a reduced gravity environment aboard the Space Shuttle in orbit. The disturbance of the imposed temperature field due to the motion of the objects was studied optically using a shearing interferometer with a Wollaston prism and the results of a typical bubble run were compared with theoretical predictions. Also, the liquid velocity field surrounding the bubbles and drops has been qualitatively investigated in a few runs by the observation of tracer particles dispersed in the continuous phase fluid. The measurement techniques are described, and the results for the temperature and flow fields are presented and discussed. Received: 27 June 2000/Accepted: 17 November 2000     

Noncontact manipulation of microflow by photothermal control of viscous force

In this paper, we investigate a potential of local control of the viscous force in a microfluidic device for a noncontact microflow manipulation method. Photothermal effect and temperature dependence of the liquid viscosity play a key role to induce an inhomogeneous viscosity distribution in the flow field in a microchannel. Absorption of focused laser beam generates the local change in the viscosity of liquid corresponding to the temperature change. The velocity and temperature fields are measured by the micron-resolution particle image velocimetry and laser-induced fluorescence, respectively. Measurement results indicate that the local reduction of the fluid viscosity due to the temperature rise can cause the change of the flow structure in the microchannel. At the focused area of heating laser beam, namely high temperature area, the flow velocity was increased. The accompanying fluid behavior around the heated region was also recognized. In addition, the agreement between the experimental results and numerical simulation clarifies that the primary factor for the change of the microflow structure is the locally controlled viscous force.     

Convection experiments in an inclined narrow cavity

The liquid flow behaviour in a small vertical gap with a heated and a cooled sidewall was studied experimentally in a former work as far as heat and mass transfer are concerned [Heiland et al. in Heat Mass Transf 43:863–870, 2007]. Following this, the study of thermal convection in a narrow cavity with variable inclination angle has been performed with liquid crystal techniques. Velocity and temperature fields of the flow have been measured. The results show that the strongest convection intensity arises in a vertical cavity.     

Probing the velocity fields of gas and liquid phase simultaneously in a two-phase flow

The feasibility of simultaneous measurements of the instantaneous velocity fields of gaseous and liquid phase is demonstrated in a laminar, unsteady two-phase flow. Thus, the instantaneous relative velocity field can be measured in such media. This is achieved by combining Particle Image Velocimetry (PIV) and a gas-phase velocimetry technique, which is based on laser-induced fluorescence (LIF) from a gaseous tracer. The wavelength shift of LIF is exploited to separate it from Mie scattering from the liquid phase. The new technique and the PIV measurement system work independently in this approach. Thus, the measurement accuracy and precision of the new technique can be validated by comparing it to the PIV results in regions of the flow field where the relative velocity vanishes. Received: 18 October 1998/Accepted: 16 October 1999     

Thermally induced velocity gradients in electroosmotic microchannel flows: the cooling influence of optical infrastructure

An axially non-uniform temperature distribution is shown to induce a disturbance to the electroosmotic flow field in microchannels, causing a significant deviation from the ideal plug-like velocity profile. Such axial temperature gradients are shown to be induced passively by the increased dissipation of Joule heat through the optical infrastructure of a viewing window. A combination of caged-dye-based molecular tagging velocimetry (to determine the cross-stream velocity profiles), fluorescence-based thermometry (to determine the in-channel fluid temperatures), and electrical current measurements are employed. The temperature visualization experiments demonstrate that the fluid is locally cooled in the viewed region, resulting in a local increase in the electric field strength. When large fields are applied, measurements indicate that the fluids temperature in the viewed region can be as much as 30°C less than in the remainder of the capillary. Despite an increase in viscosity, this local cooling results in a locally increased electroosmotic wall velocity which induces a concave velocity profile in the viewed portion and a convex velocity profile elsewhere. Experimentally determined profiles exhibit a variation in velocity across the channel of up to 5%. The cause of this velocity profile curvature is confirmed by comparing the velocity profiles obtained at a range of fields to an analytical solution that includes the effects of temperature on the liquid conductivity and viscosity.