Huihe Qiu

Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling

We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7° wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are, up to respectively, 65% and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer.

Detailed measurements in a swirling particulate two-phase flow by a phase Doppler anemometer

Abstract The particle dispersion characteristics in a confined swirling flow with a swirl number of 0.47 have been studied in detail by applying the phase-Doppler technique, which allows the measurement of particle size and velocity. Furthermore, simultaneous measurement of the gas velocity was performed with some improvements in the data processing technique that allowed the gas velocity in presence of the dispersed phase to be obtained. From the particle velocity and size measurements the behavior of different sized classes out of the whole size spectrum ranging from about 20 to 80 μm could be studied, and the response of the particles to the mean flow and the fluid turbulence could be characterized. As a result of the complexity of the flow field and the very different time scales involved, the particle behavior was found to be a result of different phenomena. Due to the combined action of turbulent diffusion, radial particle transport, and centrifugal forces exerted on the particles, a separation of the particle phase was observed, resulting in a streamwise increasing particle mean number diameter in the core region of the flow.

Experimental studies of spray evaporation in turbulent flow

Abstract A number of processes in the chemical industry and in combustion science involve the evaporation of atomised liquids in a turbulent environment. To allow an optimisation of such processes and to provide data for the validation of numerical calculations, the spray evaporation in a heated turbulent air stream was studied experimentally. The flow configuration was a pipe expansion with an expansion ratio of 3, where heated air entered through an annulus with the hollow cone spray nozzle being mounted in the centre. In the experiments isopropyl alcohol was used as a liquid because of its high evaporation rate. Measurements were taken for different flow conditions, such as air flow rate, air temperature, and liquid flow rate in order to provide a set of reliable data. Phase-Doppler anemometry (PDA) was applied to obtain the spatial change of the droplet size spectrum in the flow field and to measure droplet size–velocity correlations. From these local measurements profiles of droplet mean velocities, velocity fluctuations and droplet mean diameters were obtained by averaging over all droplet size classes. Moreover, recent extensions of the PDA signal processing allowed for accurate determination of the droplet mass flux, from which also the global evaporation rates could also be determined. The data for the different flow conditions also include the inlet conditions for air flow and spray (i.e., for all three velocity components), inlet temperature, and wall temperature profiles. The latter were measured using a thermocouple with a special wall sensor.

Characterization of particle-laden, confined swirling flows by phase-doppler anemometry and numerical calculation

Abstract The particle dispersion characteristics in a confined swirling flow with a swirl number of approx. 0.5 were studied in detail by performing measurements using phase-Doppler anemometry (PDA) and numerical predictions. A mixture of gas and particles was injected without swirl into the test section, while the swirling airstream was provided through a co-flowing annular inlet. Two cases with different primary jet exit velocities were considered. For these flow conditions, a closed central recirculation bubble was established just downstream of the inlet. The PDA measurements allowed the correlation between particle size and velocity to be obtained and also the spatial change in the particle size distribution throughout the flow field. For these results, the behaviour of different size classes in the entire particle size spectrum, ranging from about 15 to 80 μm, could be studied, and the response of the particles to the mean flow and the gas turbulence could be characterized. Due to the response characteristics of particles with different diameters to the mean flow and the flow turbulence, a considerable separation of the particles was observed which resulted in a streamwise increase in the particle mean number diameter in the core region of the central recirculation bubble. For the lower particle inlet velocity (i.e. low primary jet exit velocity), this effect is more pronounced, since here the particles have more time to respond to the flow reversal and the swirl velocity component. This also gave a higher mass of recirculating particle material. The numerical predictions of the gas flow were performed by solving the time-averaged Navier-Stokes equations in connection with the well known kϵ turbulence model. Although this turbulence model is based on the assumption of isotropic turbulence, the agreement of the calculated mean velocity profiles compared to the measured gas velocities is very good. The gas-phase turbulent kinetic energy, however, is considerably underpredicted in the initial mixing region. The particle dispersion characteristics were calculated by using the Lagrangian approach, where the influence of the particulate phase on the gas flow could be neglected, since only very low mass loadings were considered. The calculated results for the particle mean velocity and the mass flux are also in good agreement with the experiments. Furthermore, the change in the particle mean diameter throughout the flow field was predicted approximately, which shows that the applied simple stochastic dispersion model also gives good results for such very complex flows. The variation of the gas and particle velocity in the primary inlet had a considerable impact on the particle dispersion behaviour in the swirling flow and the particle residence time in the central recirculation bubble, which could be determined from the numerical calculations. For the lower particle inlet velocity, the maximum particle size-dependence residence time within the recirculation region was considerably shifted towards larger particles.

A reliable method for determining the measurement volume size and particle mass fluxes using phase-Doppler anemometry

A reliable method for the calibration of the measurement volume cross-section has been developed, in order to correct the particle size distribution measured by a phase-Doppler anemometer (PDA) with respect to the counting bias in favour of the large particles. Furthermore, this method allows the measurement of particle concentration or mass flux with high accuracy in two-dimensional two-phase flows. A comparison of the mass flow rate obtained by the integration of the mass flux measured in a water spray by applying this method with the global mass balance showed a difference of about 5%.The basis of the present method is the detection of the amplitude of the filtered Doppler signal in connection with the particle size measured by the PDA. The detection process is performed using an electronic circuit which validates the Doppler burst and has additionally the advantage that the highest amplitude portion of the burst can be selected for digitizing, acquisition and subsequent processing. Therefore, this method has the great advantage that the processed part of the burst has the highest signal-to-noise ratio which results in high accuracies for frequency and phase estimation.

High-resolution data processing for phase-Doppler measurements in a complex two-phase flow

Measurements of the particle size and velocity in a gas-solid two-phase flow using the phase-Doppler technique based on transient recorder data processing and the cross-spectral density (CSD) function are improved in accuracy by introducing new complex interpolation functions to the discrete frequency lines. The improvements of the accuracy for size and velocity measurements are additionally based on the use of an external electronic unit, which is allowed to trigger the transient recorder near the peak of the envelope of the band-pass filtered bursts. This procedure could improve the signal-to-noise ratio of the processed part of the signal and hence the accuracy and data rate. A simultaneous measurement of the gas velocity is achieved by seeding the flow with small spherical glass beads which have the same refractive index as the dispersed phase particles. The identification of signals from these seeding particles-with a diameter less than 4 mu m-is based on the phase between the two signals.

Method of phase-Doppler anemometry free from the measurement-volume effect

A novel method is developed to improve the accuracy of particle sizing in laser phase-Doppler anemometry (PDA). In this method the vector sum of refractive and reflective rays is taken into consideration in describing a dual-mechanism-scattering model caused by a nonuniformly illuminated PDA measurement volume. The constraint of the single-mechanism-scattering model in the conventional PDA is removed. As a result the error caused by the measurement-volume effect, which consists of a Gaussian-beam defect and a slit effect, can be eliminated. This new method can be easily implemented with minimal modification of the conventional PDA system. The results of simulation based on the generalized Lorenz-Mie theory show that the new method can provide a PDA system free from the measurement-volume effect.

Interfacial thermal conductance in rapid contact solidification process

In this study, an interfacial temperature sensor, having a junction thickness of only 1 lm was developed utilizing micro-fabrication technique. As a simulation of rapid contact solidification process, a sudden falling experiment was conducted. The special sensor was employed to measure the rapid temperature changes at the substrate surface. A simple procedure was proposed for determining the interfacial thermal conductance during the rapid contact process of molten metal (Indalloy-158) with copper substrate. The influence of initial state of molten metal on thermal conductance was investigated. The agreement between the calculated thermal histories and measured ones indicated the validity of the present method. 2002 Elsevier Science Ltd. All rights reserved.

Fringe probing of liquid film thickness of a plug bubble in a micropipe

The liquid film thickness of a plug bubble in a micropipe is of interest in micro heat pipe, microfluidics and two-phase flows. A novel technique was developed to measure the liquid film thickness of a plug-like bubble in a capillary pipe utilizing a spatial fringe scattering method. The scattered fringes were measured by CCD camera and the calculated spatial frequencies were used to determine the film thickness between the plug and the wall. To demonstrate the capability of the newly developed technique, a validation experiment was conducted with water/air and water–honey-mixture/gas-plug flows. The velocity-dependent film thickness can also be observed using this measurement technique. This newly developed method is easy to implement and it will be a very useful technique for micro heat pipe research.

Measurements of interfacial film thickness for immiscible liquid–liquid slug/droplet flows

A novel method for measuring the interfacial liquid film thickness between immiscible liquids of a slug/droplet in a micropipe is proposed. This method is capable of measuring an oil slug/droplet in water with the relative refractive index (m) between the immiscible liquids very close to one in a capillary tube. Unlike the measurement configuration for an air slug in water, an optical oriental for optimizing the refracted fringes pattern by the liquid–liquid interface is introduced. Measurements of film thicknesses of a kerosene oil droplet/slug in a water wetted capillary tube (m > 1) and a water droplet/slug in a kerosene oil wetted capillary tube (m < 1) were demonstrated. This diagnostic method is easy to implement and it will have great potential for immiscible liquid flow research in microfluidic channels.