Experimental study of thermally stratified unsteady flow by NMR-CT

Thermally stratified unsteady flow caused by two-dimensional surface discharge of warm water into a rectangular reservoir is investigated. Experimental study is focused on the rapidly developing thermal diffusion at small Richardson number.The basic objectives are to develop a measurement system for the unsteady flow phenomena and to study the interfacial mixing between a flowing layer of warm water and the underlying body of cold water.Mean velocity field measurement is carried out by using NMR-CT (Nuclear Magnetic Resonance — Computerized Tomography). It detects a quantitative flow image of any desired section in any direction of flow. Transient mean temperature profiles are obtained by fine thermocouple arrays and a microcomputer-based data acquisition system.Results show that the warm layer penetrates more rapidly into the cold layer at smaller Richardson number because of decrease instability. This is clearly verified by flow visualization using thymol blue solution. It is found that the transport of heat across the interface is more vigorous than that of momentum.     

Measurements of gravity and gravity-capillary waves in horizontal gas-liquid pipe flow using PIV in both phases

An experimental study was performed in stratified wavy flow of air and water through a horizontal pipe. The velocity fields in both phases were measured simultaneously using PIV and the interfacial shape was resolved using a profile capturing technique. The objective of the study was to investigate the interfacial characteristics and the velocities of the liquid and gas phases in two wave patterns: ‘3D small amplitude’ and ‘2D large amplitude’ waves. The wave patterns were shown to consist of gravity and gravity-capillary waves, respectively, with substantial differences in the wave characteristics and liquid velocities. Contrary to this, the effect of the waves on the gas velocities was rather similar in both wave regimes, with both wave regimes causing an increase in the velocity fluctuations close to the interface. The current measurements also produced a valuable dataset that can be used to further improve the numerical modeling of the stratified flow pattern.     

Experiments with a Wire-Mesh Sensor for stratified and dispersed oil-brine pipe flow

Two-phase oil–water flow was studied in a 15 m long horizontal steel pipe, with 8.28 cm internal diameter, using mineral oil (having 830 kg/m³ density and 7.5 mPa s viscosity) and brine (1073 kg/m³ density and of 0.8 mPa s viscosity). Measurements of the holdup and of the cross-sectional phase fraction distribution were obtained for stratified flow and for highly dispersed oil–water flows, applying a capacitive Wire-Mesh Sensor specially designed for that purpose. The applicability of this measurement technique, which uses a circuit for capacitive measurements that is adapted to conductive measurements, where one of the fluids is water with high salinity (mimicking sea water), was assessed. Values for the phase fraction values were derived from the raw data obtained by the Wire-Mesh Sensor using several mixture permittivity models. Two gamma-ray densitometers allowed the accurate measurement of the holdups, which was used to validate the data acquired with the capacitive Wire-Mesh Sensor. The measured time-averaged distribution of the phase fraction over the cross-sectional area was used to investigate the details of the observed two-phase flow patterns, including the interface shape and water height. The experiments were conducted in the multiphase-flow test facility of Shell Global International B.V. in the Netherlands.     

Stability of multiple solutions in inclined gas/shear-thinning fluid stratified pipe flow

This work provides an investigation on multiple solutions in gas/shear-thinning fluid inclined stratified pipe flows. Multiple solution operative conditions are studied investigating the effect of the interfacial shear stress modeling and the rheology of the shear-thinning fluid. The modeling of the interfacial shear stress in counter-current has a strong influence of multiple solutions regions. The stability of multiple hold-up solutions is studied considering the structural stability, the interfacial stability, and the minimization of the dissipation approaches. The results of the three different approaches are commented both for concurrent and counter-current flows, giving the same conclusions only for upward inclined flows.     

Natural convection flow from a continuously moving vertical surface immersed in a thermally stratified medium

 Natural convection boundary layer flow over a continuously moving isothermal vertical surface immersed in a thermally stratified medium has been investigated here. The non-linear coupled partial differential equations governing the non-similar flow have been solved numerically using an implicit finite difference scheme. For small values of the streamwise distance the partial differential equations are solved by using a perturbation expansion procedure and also using the Shanks transformation. The results indicate that the thermal stratification significantly affects both the surface shear stress and the surface heat transfer. The buoyancy parameter and the Prandtl number increase significantly, both the surface shear stress and heat transfer. Also the buoyancy force gives rise to an overshoot in the velocity profile. Received on 1 February 2000     

Measurements of cross-sectional instantaneous phase distribution in gas–liquid pipe flow

Two novel complementing methods that enable experimental study of gas and liquid phases distribution in two-phase pipe flow are considered. The first measuring technique uses a wire-mesh sensor that, in addition to providing data on instantaneous phase distribution in the pipe cross-section, also allows measuring instantaneous propagation velocities of the phase interface. A novel algorithm for processing the wire-mesh sensor data is suggested to determine the instantaneous boundaries of gas–liquid interface. The second method applied here takes advantage of the existence of sharp visible boundaries between the two phases. This optical instrument is based on a borescope that is connected to a digital video camera. Laser light sheet illumination makes it possible to obtain images in the illuminated pipe cross-section only. It is demonstrated that the wire-mesh-derived results based on application of the new algorithm improve the effective spatial resolution of the instrument and are in agreement with those obtained using the borescope. Advantages and limitations of both measuring techniques for the investigations of cross-sectional instantaneous phase distribution in two-phase pipe flows are discussed.     

Measurements of thermally induced stress waves in a thin rod using birefringent coatings

When an elastic body is heated rapidly, significant inertial stresses are developed if the imposed heating rates cause a substantial temperature change in times which are short relative to the mechanical response time of the body. This work describes a method for inducing and measuring the thermally induced elastic stress waves in an unrestrained thin rod. Rapid heating was accomplished electrically by discharging a low-inductance capacitor bank (0.1 μH, 2800 J) through the rod. Utilizing the frozen-stress technique, an initial fringe pattern was introduced into thin strips of a birefringent material securely bonded to each side of the rod. The longitudinal strain oscillations were measured by direct observation of the movement of the fringe patterns with a high-speed framing camera. Interpretation of these measurements required a dynamic calibration and application of a dynamic correction factor for the reinforcing effect of the coating. Oscillations with periods from 35 to 250 μsec and stress amplitudes up to 900 psi were measured and compared with the uncoupled thermoelastic theory. Good agreement was obtained, and it was concluded that this technique is suitable for transient measurements in the presence of large magnetic fields which normally restrict the use of electronic methods.     

Determination of flow sub-regimes in stratified air–water channel flow using LDV spectra

In gas–liquid stratified flows, pressure drop and transport across the interface are strongly influenced by the interfacial wave structure, making the determination of interfacial topography in this kind of flows very important. An objective way of characterizing the wave pattern present in the interface is proposed here. The method consists in analysing the spectra of the signal obtained from Laser Doppler Velocimetry (LDV) measurements of fluctuations occurring close to the air-sheared interface. Transitions are defined by the appearance and disappearance of peaks in the frequency spectra. The method was applied to study the transition regimes of a stratified air–water flow in a square-cross section channel. A flow pattern map for air–water channel flow is presented and compared with the maps available from the literature.     

Measurements of two-phase mass flow rate: A comparison of different techniques

The development of practical and accurate methods to measure two-phase mass flow rates is of prime interest to applied nuclear reactor safety research. This article summarizes a comparison and evaluation of four commonly used mass flow rate devices. The particular systems investigated include (a) the true mass flow meter (TMFM), (b) the radionuclide technique, (c) the combination of a free field drag disk-turbine meter-transducer (DTT) and a gamma densitometer, and (d) the combination of a venturi nozzle and a full flow turbine meter. The experiments were performed under similar conditions in steady-state steam-water flow. The flow direction upstream of the instruments was horizontal except for the last method. The pressures varied between 3 and 9 MPa, and the highest values of the mass flow rate, the quality were 5 kg/s and 90 per cent respectively. The test matrix included wave-, slug- and annular flow. The measuring techniques are described briefly and a classification is proposed, which is based on the different ways of mass flow rate evaluation. The experimental results show that the accuracy of some methods is distinctively dependent on phase distribution (flow regime). Simple calibration correlations were developed to account for these effects.     

An investigation of thermally stratified turbulent channel flow with temperature oscillation on the bottom wall by large eddy simulation

Thermally stratified shear turbulent channel flow with temperature oscillation on the bottom wall of the channel is calculated to investigate the behavior of turbulent flow and heat transfer by use of large eddy simulation (LES) approach coupled with dynamic subgrid-scale (SGS) models. The objective of this study is to deal with the effect of the temperature oscillation on turbulent behavior of thermally stratified turbulent channel flow and to examine the effectiveness of the LES technique for predicting statistically unsteady turbulent flow driven by time-varying buoyancy force. To validate the present calculation, thermally stratified shear turbulent channel flow is computed and compared with available data obtained by direct numerical simulation (DNS), which confirm that the present approach can be used to predict thermally stratified turbulent channel flow satisfactorily. Further, to illustrate the effect of the temperature oscillation with different Richardson numbers and periods of the oscillation on turbulence characteristics, the phase-averaged mean value and fluctuation of the resolved velocities and temperature, and instantaneous velocity fluctuation structures are analyzed.     

Heterogeneous drag reduction in turbulent pipe flows using various injection techniques

The results of an experimental study of the injection of concentrated polymer solutions into the near-wall region of a turbulent pipe flow are reported. The injection experiments described here show drag reduction that was significantly larger than that obtained for homogeneous polymer solutions of the same average concentration. Local drag reduction and friction behavior was obtained by measuring pressure differences over a test section of 13 m in length. Furthermore the flow behaviour of the injected polymer solution was investigated by flow visualization experiments. Velocity profile measurements elucidate in case of near-wall injection that the turbulent structure could be altered in the near-wall and also in the core region of the pipe flow, indicating that the polymer lumps and threads created by the near-wall injection are able to influence a much wider spectrum of turbulent eddies in comparison to centreline injection or, all the more, to homogeneous drag reduction.     

Large-eddy simulation of stratified channel flow

An investigation of large-eddy simulation (LES) for turbulent channel flow with buoyancy effects was performed by solving the resolved incompressible Navier-Stokes equations under the Boussinesq approximation. The Smagorinsky eddy-viscosity model and Yoshizawa eddy-viscosity model were used to describe the unresolved subgrid scale (SGS) fluctuations respectively. After some numerical testing, the latter was further simplified so that it can be used in the dynamic model closure. A LES code was developed for parallel computations by using the parallel technique, and was run on the Dawn-1000 parallel computer. To demonstrate the viability and accuracy of the code, our results are compared with and found in good agreement with available LES results. The project supported by the National Natural Science Foundation of China and by the Youngster Funding of Academia Sinica     

Measurement of laminar,transitional and turbulent pipe flow using Stereoscopic-PIV

Stereoscopic particle image velocimetry (SPIV) is applied to measure the instantaneous three component velocity field of pipe flow over the full circular cross-section of the pipe. The light sheet is oriented perpendicular to the main flow direction, and therefore the flow structures are advected through the measurement plane by the mean flow. Applying Taylor’s hypothesis, the 3D flow field is reconstructed from the sequence of recorded vector fields. The large out-of-plane motion in this configuration puts a strong constraint on the recorded particle displacements, which limits the measurement accuracy. The light sheet thickness becomes an important parameter that determines the balance between the spatial resolution and signal to noise ratio. It is further demonstrated that so-called registration errors, which result from a small misalignment between the laser light sheet and the calibration target, easily become the predominant error in SPIV measurements. Measurements in laminar and turbulent pipe flow are compared to well established direct numerical simulations, and the accuracy of the instantaneous velocity vectors is found to be better than 1% of the mean axial velocity. This is sufficient to resolve the secondary flow patterns in transitional pipe flow, which are an order of magnitude smaller than the mean flow.     

Interfacial measurements in stratified types of flow. Part II: Measurements for R-22 and R-410A

A new optical void fraction measurement system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube. A series of evaporation tests have been run for two refrigerants. R-22 was tested under mass velocity conditions of 70, 100, 150 and 200 kg/m² s and R-410A for 70, 150, 200 and 300 kg/m² s in a 13.6 mm diameter glass tube. Using our newly developed image processing system, about 227 000 images have been analyzed in this study to provide the same number of dynamic void fraction measurements. From these images, 238 time-averaged void fraction values have been obtained for vapor qualities from 0.01 to 0.95. These experimental points show very good agreement with the horizontal version of the Rouhani–Axelsson drift flux void fraction model.     

Turbulence measurements in pipe flow using a nano-scale thermal anemometry probe

A new nano-scale thermal anemometry probe (NSTAP) has been developed using a novel procedure based on deep reactive ion etching. The performance of the new probe is shown to be superior to that of the previous design by Bailey (J Fluid Mech 663:160–179, 2010). It is then used to measure the streamwise velocity component of fully developed turbulent pipe flow, and the results are compared with data obtained using conventional hot-wire probes. The NSTAP agrees well with the hot-wire at low Reynolds numbers, but it is shown that it has better spatial resolution and frequency response. The data demonstrate that significant spatial filtering effects can be seen in the hot-wire data for probes as small as 7 viscous units in length.     

Advanced numerical method for a thermally induced slug flow: application to a capillary closed loop pulsating heat pipe

An advanced hybrid lumped parameter code for the simulation of Pulsating Heat Pipes is developed. Being able to simulate transient operative conditions and removing common physical simplified assumptions, it represents a step forward with respect to the present models of passive two‐phase systems. Mass, momentum and energy balances account for the thermal and fluid‐dynamics phenomena. Heterogeneous and homogeneous phase changes are directly integrated. In addition, a fitting correlation for the wall/vapour heat transfer coefficient is implemented and tuned against experimental data in order to evaluate the influence of the liquid film on conjugate heat transfer. The resulting numerical tool have been validated against experimental data achieved testing a copper pulsating heat pipe during the 58th ESA Parabolic Flight Campaign in several operative conditions and transient gravity levels. The predicted results show very good matching with the actual thermo‐physical behaviour of the system. Copyright © 2016 John Wiley & Sons, Ltd.