Gail Duursma

Experimental Studies of Nanofluid Droplets in Spray Cooling

Spray cooling is used in cooling of electronic devices to remove large heat fluxes. Heat transfer to droplets impinging on a heated surface and boiling off has been studied. Most work is on a well-controlled system of a single drop falling onto a horizontal heated plate from a fixed height. These have revealed the droplet impingement mechanics to be a function largely of Weber number and excess temperature, and a range of regimes is observed similar to those in pool boiling, with a clearly identifiable critical heat flux. Nanofluids exhibit enhanced boiling heat transfer in pool boiling. The effect of nanoparticles on droplet boil-off was studied in this work. Nanofluid drops were let fall onto a surface at temperature greater than the saturation temperature, and behavior and heat flux were recorded and contrasted to that of a pure fluid. The working fluids used were pure water, ethanol, and dimethyl sulfoxide (DMSO) and ethanol– or DMSO–nanoparticle solutions (the nanoparticles were aluminum, with concentrations of up to 0.1% by weight in DMSO and 3.2% by weight in ethanol). High-speed photographic images of droplet evolution in time were obtained and indicate that there are differences in the behavior of nanofluid droplets as they boil off the surface, compared to pure fluids. Increasing nanoparticle concentration decreases the receding droplet breakup on rebound after impingement and appears to reduce the maximum spreading of a droplet as well. Maximum recoil height is reduced with increasing nanoparticle concentration. Experimental measurements of the heat fluxes associated with the pure and nanofluid droplets did not show significant enhancement, though there was noticeable improvement in the DMSO nanofluids.

PIV investigations of flow structures in the fluidised bed freeboard region

The whole field velocimetric technique particle image velocimetry (PIV) has been used to produce vector maps of the gas-phase flow in the freeboard region of fluidised beds of square and round cross-sections. A characteristic toroidal vortex is generated as a single bubble erupts. In freely bubbling beds, there is a highly chaotic region immediately above the bed surface. At low fluidising velocities, the flow higher above the bed surface is characterised by a high velocity wall region and a near-stagnant core. The velocity profile slowly becomes more uniform at greater heights above the bed. At higher fluidising velocities, the velocity profile is more uniform from the outset. An expansion in freeboard bed area delays the onset of this pattern by concentrating flow in the core region.

Insights into the instantaneous freeboard flow above a bubbling fluidised bed

The freeboard of a fluidised bed reactor, that is, the space above the bed material where the fluidising gas disengages from solids before leaving the fluidisation vessel, can be a place where unexpected additional reaction can take place. In this study particle image velocimetry (PIV), implemented by image shifting, has been used to provide instantaneous gas velocity vector maps of the central vertical plane of a bubbling fluidised bed freeboard in order to study the flow pattern in a cold model of a fluidised bed used for polymer recycling. The velocity vector maps obtained show that the freeboard gas flow is characterised by the presence of eddies and vortices generated by bubble eruption at the bed surface.

Study of the flow pattern above an erupting bubble in an incipiently fluidised bed using image shifting

Abstract The gas flow in the freeboard of a fluidised bed is strongly dependent on bubble eruption at the bed surface. Here, the gas flow above an erupting bubble has been studied by the injection of single bubbles in an incipiently fluidised bed. Gas velocity vector maps of the vertical central plane above the bed surface, i.e. a plane parallel to the direction of the overall flow, have been determined using image shifting (an implementation of particle image velocimetry). These vector maps are presented and discussed. A mechanism has been proposed to describe the process of single-bubble eruption.

Displacement of horizontal layers by bubbles injected into fluidised beds

Abstract Mixing in bubbling gas-fluidised beds comes primarily from bubble motion through the particulate phase and this has previously been studied experimentally by examining the displacement of a marked layer of particles by rising bubbles. The reported experimental profiles show considerable variation in the distortion of the interface. In this work, the distortion of the interface by a two-dimensional bubble of initial radius a , rising inviscidly in a semi-infinite channel of width b , is examined theoretically and the results applied to the interpretation of experimental data from fluidised beds, assuming that the particulate phase may be modelled as an inviscid fluid. Two new contributions are considered: (i) the contribution from the injection of the bubble, and (ii) the effect of initial finite separation, z 0 , of the bubble and marked layer. Numerical calculations and analytic expressions show that the distortion of the interface depends on z 0 b and not z 0 a . When z 0 b , the distortion of the interface is shown to be critically dependent on the initial separation of bubble and layer. The area between the final and initial position of the layer, the partial drift area (Eames et al. , 1994, J. Fluid Mech. 275 , 201–223), is equal to the area of the bubbles primary wake. A review of experimental results shows that many of the results were taken for z 0 b , even though z 0 a was large. Qualitative comparisons are made between theory and experiment which show some agreement.

Flow and Heat Transfer of Single-and Two-Phase Boiling of Nanofluids in Microchannels

Experiments using nanofluids in horizontal, rectangular, high-aspect-ratio microchannels were performed where heat was provided electrically to the microchannel wall to bring about heating and phase change while recording temperature (via infrared camera) and channel pressure drop. High-speed video captured images of boiling two-phase flow through the transparent microchannel wall. Nanofluids used were solutions of aluminium oxide in ethanol with particle concentrations from 0.01% to 0.1%, with pure ethanol as reference. Fluid mass flux ranged from Reynolds numbers of 2.3 to 18.1 and heat fluxes from 1.5 to 9 kW m−2. Friction factors for the nanofluids were evaluated. Single-phase fluid pressure drop did not vary significantly with nanoparticle concentration. When flow boiling occurred, the two-phase flow pressure drop was unstable and fluctuating. Inlet pressures had greater amplitude of oscillation but similar frequency to outlet pressures. Heat transfer increased with the presence of nanoparticles compared with pure ethanol. Moreover, evaporation from the meniscus was studied. There is a sudden evaporation phenomenon where the meniscus rapidly forms. Infrared data demonstrate the effect of heat flux on temperature distribution near the three-phase contact line. Nanoparticles enhance boiling heat transfer coefficients, peaking at a concentration of 0.05% with significant enhancement over pure ethanol.

Obstacle-induced voids in two-dimensional gas fluidized beds

Abstract Novel observations of two-dimensional obstacle-induced voids in marginally fluidized beds are presented. For obstacles in the form of horizontal circular cylinders, these voids are thin and can evolve symmetrically and periodically beneath the cylinder. A mathematical model is developed and its predictions are compared with observations.

Leidenfrost droplets on microstructured surfaces

The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary between structured sections, the droplet becomes asymmetric and may move or spin. An axisymmetric computational fluid dynamics (CFD) model is also presented that shows qualitative agreement with experimental observations.

Effect of micropillar spacing and temperature of substrate on contact angle dynamics

ABSTRACT Contact angle dynamics of droplets deposited on a structured surface were studied in this work and the effects of substrate microstructure and temperature were investigated. Microstructures consisting of uniformly-sized, cubic micropillars with varying pillar spacings were constructed by microfabrication. Droplets (of the order of tens of microlitres in volume) were deposited on these surfaces and dynamic contact angles were observed using various techniques. Advancing and receding contact angles were measured using tilting of the surfaces or by injection and aspiration of fluid from a horizontal droplet by syringe. Droplets on these surfaces appeared to be mainly in the Wenzel state. Contact angle hysteresis was obtained as a function of pillar spacing or, equivalently, surface roughness. Depinning force was deduced and a linear dependence on maximal three phase contact line was found. The techniques of tilting the surface on which the droplet was deposited and uniformly increasing and reducing the volume of the droplet via the syringe both gave the same contact angle hysteresis for a given micropillar spacing. The effect of temperature was then assessed using a heated tilting plate. Contact angle hysteresis was found to increase with temperature. Further work to elucidate mechanisms governing this dependence will be undertaken.

Void Fraction Measurement of Gas-Liquid Two-Phase Flow Based on Empirical Mode Decomposition and Artificial Neural Networks

ABSTRACT A new void fraction estimation method for gas–liquid two-phase flow combining two differential pressure (DP) signals acquired from a single Venturi tube and based on Empirical Mode Decomposition (EMD) and Artificial Neural Networks (ANN) was experimentally investigated. In order to study gas–liquid distribution in horizontal pipes, two DP signals from the top and bottom sections of the Venturi tube are acquired and EMD is adopted to extract stable and fluctuating components of the DP signals. Experimental data revealed that fluctuating index increases nearly linearly with increasing void fraction when void fraction is less than 0.4. When void fraction is larger than 0.4, this near-linearity ceases. A combination of ANN method and the fluctuating index of DP signals is developed to estimate void fraction. Experimental results show that void fraction based on DP signal from top section of Venturi tube is overestimated because of clustering of bubbles and the scarcity of liquid information when gas–liquid mixture velocity is low. Void fraction is underestimated when the mixture velocity is high. A high gas–liquid slip ratio results in void fraction underestimation. Void fraction prediction performance is satisfactory when void fraction is less than 0.4 and fluctuating index of DP signal less than 1.2.