Abstracts:We report the results of a comprehensive series of coreflood experiments carried out at three different levels of gas/oil IFT namely, ultra-low, intermediate, and high gas/oil IFT values of 0.04, 0.15, and 2.70 mN m⁻¹ in mixed-wet rocks. Coreflood experiments included waterflooding (WF), gas injection (GI) and two WAG injection scenarios at each IFT value in the first series of WAG experiments, fluid injection started with water injection (I) followed by gas injection (D), and this cyclic injection of water and gas was repeated in four cycles (WAG-IDIDIDID). In the second series of WAG experiments, the test started with gas injection (D) followed by water injection (I), and this cyclic injection of water and gas was repeated four times (WAG-DIDIDIDI). In addition to these experiments, for the high and ultra-low gas/oil IFT systems, SWAG injection experiments have also been performed with SWAG ratio of unity (Qg/Qw = 1).

Abstracts:Atomic emission spectroscopy can well characterize atom release properties and flame temperature. This work aims to investigate the spectroscopic characteristics of alkalis atoms and measurement method of flame temperature in diesel impinging flames, based on a bench-scale opposed multi-burner (OMB) gasifier. A fiber-optic spectrometer and a CCD camera coupled with multiple bandpass filters are used to obtain the spectral emission lines and two-dimensional emission distributions of Na∗ and K∗, respectively. The results indicate that alkalis atoms can be released and excited in impinging region and four jet regions. The emission intensities of Na∗ and K∗ under different O/C are the strongest in the impinging region, suggesting that the atom release concentration and flame temperature are the highest. Moreover, the thermal excitation degree is related to the type of atom, and Na atom is much easier to be excited than K atom. It is confirmed that alkalis atom content and flame temperature were the two main influence factors on the emission intensity, and emission intensity can be used to qualitatively characterize flame temperature under the same O/C. In addition, atomic emission spectroscopy (AES) was given to quantitatively calculate the flame temperature. It is verified that the AES method is and feasible to derive diesel flame temperature and monitor the temperature changes of OMB gasifiers in real time.

Abstracts:The characteristics of backward-inclined non-premixed jet flames in a uniform crossflow were studied in a wind tunnel. Time-averaged photography techniques were used to study flame behavior. The flow field was captured by short exposure photography and Mie-scattering techniques. Flame temperatures were probed with a fine-wire R-type thermocouple. In the domain of jet-to-crossflow momentum flux ratio R and backward-inclination angle θ, the flames were categorized into three characteristic modes. The first mode consisted of crossflow dominated flames characterized by a down-wash recirculation flame in the wake of the burner tube. The second mode consisted of transitional flames characterized by a yellowish recirculation flame and a tail flame. The third mode consisted of jet dominated flames characterized by a blue flame base and an absence of the down-wash flame. The down-wash recirculation flames were observed for a backward inclination angle of θ < 40°. The ability of the flames to resist blow off when increasing the jet-to-crossflow momentum flux ratio decreased as θ increased. Coherent vortices were observed on the seeded fuel jet, whose type was dependent on R and θ. In the upstream region, the fuel appeared above the flame. However, in the downstream region, the fuel became engulfed within the flame. For a fixed θ, the Strouhal number of the upwind shear layer vortices on the fuel jet was observed to decrease asymptotically as R increased. In the near-field at x/d ≈ 5, the crossflow dominated flames presented temperature profiles characterized by a broad dual-hump peak profile in the symmetry plane, while in the near-field at x/d ≈ 5, the jet dominated flames presented a single peak profile in the symmetry plane.

Abstracts:Post-impact drop vibration on a hydrophilic surface has been studied experimentally in terms of the oscillation of spread and thickness factors. Water droplets were dropped from a piezoelectric droplet generator onto a smooth aluminum surface. Three different regimes featuring different oscillatory behaviors were classified based on Weber number. Quantitative characterization of the drop vibration was conducted for Regime I with $\mathit{We}\<30$. Mobile contact line (MCL) and pinned contact line (PCL) vibrations were differentiated for the first time in analyzing the post-impact drop vibration. A time-frequency analysis shows that the transition from MCL vibration to PCL vibration features a tiny shift to a higher oscillation frequency. The normalized oscillation frequency can be well scaled by $\mathrm{\Omega }\sim {\mathit{We}}^{-1/2}$ as shown from the classical models. A new, empirical, unifying model was developed to account for the contact line movement in order to incorporate both MCL and PCL vibrations. The hysteresis-induced force at the three-phase contact line was found to alternate the equilibrium position of the vibrating drop with time, and thus making the oscillatory behavior nonlinear. It can be derived from the unifying model that the oscillation frequency should also scale as $\mathrm{\Omega }\sim {\mathit{We}}^{-1/2}$ and the damping factor should scale as $\zeta \sim {\mathit{Re}}^{-1}$, which are validated by the presented experimental results. Finally, the good scaling of the damping factor indicates that the major damping mechanism, for both MCL and PCL vibrations, should originate from the viscous dissipation.

Abstracts:Flow boiling heat transfer performances of Cu-TiO2 nanocomposite coated copper surfaces have been studied experimentally in this work for its potential use in heat transfer applications. Experimental studies are performed in a bottom surface heated minichannel with DI water as the coolant. Thin Cu-TiO2 nanocomposite coated copper surfaces are developed using electrocodeposition technique. Coated surfaces developed through this technique have varied surface properties such as wettability, porosity, crystallinity, etc. The developed coated surfaces are characterized with respect to porosity, mean pore diameter, roughness, contact angle, and coating thickness. Flow boiling heat transfer experiments are performed at different mass fluxes in an experimental setup developed for this purpose. The results obtained with bare copper surface are used as the reference data for comparing the performances of the coated surfaces. The Cu-TiO2 coated copper surfaces are found to augment single-phase heat transfer coefficient slightly, whereas the enhancement in the two-phase region is up to 94% depending on the mass flow rate of coolant and heater surface temperature. The critical heat flux (CHF) also augments for the nanocomposite coated surfaces up to 92%. The augmentation in CHF and heat transfer coefficient of Cu-TiO2 coated surfaces is due to the enhanced surface roughness, wettability improvement, and presence of high-density active nucleate site on the surface. In order to have appreciation of the results obtained in present work, the boiling curves and CHF against mass flux for the bare and coated surfaces are compared with previously published experimental results. Thus, the arrangement of the minichannel and porous surface offers a capable choice for compact small size cooling devices due to decreased wall superheat temperature, high CHF, and high heat transfer coefficient.

Abstracts:The dynamics of bubble coalescence are of importance for a number of industrial processes, in which the size inequality of the parent bubbles plays a significant role in mass transport, topological change and overall motion. In this study, coalescence of unequal-sized microbubbles captive on a solid substrate was observed from cross-section view using synchrotron high-speed imaging technique and a microfluidic gas generation device. The bridging neck growth and surface wave propagation at the early stage of coalescence were investigated by experimental and numerical methods. The results show that theoretical half-power-law of neck growth rate is still valid when viscous effect is neglected. However, the inertial-capillary time scale is associated with the initial radius of the smaller parent microbubble. The surface wave propagation rate on the larger parent microbubble is proportional to the inertial-capillary time scale.

Abstracts:A multi-gate cross-correlation technique is applied to analyze the dynamics of a two-phase flow. The water and air concurrent flows are measured in a 5 mm diameter circular channel inclined at an angle of 45 degrees and set vertically, with different gas-to-liquid volume rates. The Experimental results are put in series with a constant water flow rate. Several variables are measured along with capturing video frames by a fast digital camera. Time-series data are extracted from the video data by setting seven conversion gates in the camera’s field of view. By calculation of the cross-correlation coefficients between the first and subsequent gates, the characteristic parameters of spatial correlation decay are measured for each flow. A comparison of the correlation decay in the modeled and experimental flows reveals some interesting aspects of the flow dynamics.

Abstracts:This paper reports an experimental investigation on the wake of a 1/50th-scale high-speed train (HST) with a slenderness ratio, L/W, of 15.7 (L and W are the length and width of the train model, respectively). Hot-wire anemometry, flow visualization and particle image velocimetry (PIV) measurements are conducted in a close-loop low-speed wind tunnel at a Reynolds number of 1.3 × 10⁵, based on the free stream oncoming flow velocity U ∞ and W. The results of both the frequency spectrum analysis and flow visualization suggest that the instantaneous near wake of a slender HST is dominated by a pair of large-scale counter-rotating streamwise vortices, which are shed alternatingly. Utilizing the proper orthogonal decomposition (POD) analysis for the PIV measurement results, the dynamic characteristics of the near wake are clarified. The first six POD modes, corresponding to the dominant coherent structures, are described in more detail. Each of the large-scale streamwise vortices presents increase/decrease alternatingly in size along with oscillating behaviour in both lateral and vertical direction, which is ascribed to the tilted vortex shedding from the bogie section of the trailing carriage. Moreover, the interaction between the streamwise vortices and the ground is also discussed.

Abstracts:This work presents and analyses the results of experimental activities aimed at a preliminary characterization of foamy flows for pipeline dewatering, in order to assess whether the addition of surfactants may effectively reduce the liquid holdup in horizontal pipelines. Static tests were run to compare the foam cycle (generation and decay) for three commercial surfactants and to choose the most suitable one. Dynamic tests with the selected product were performed in a 20 m long, 60 mm i.d. Plexiglas® pipe, where a 0.3% wt. solution of surfactant in tap water was pumped after mixing with an air flow at nearly atmospheric pressure and temperature. Superficial velocities ranged between 0.03 m/s and 0.05 m/s for water and between 1.5 m/s and 11.5 m/s for air, which would determine stratified/stratified wavy flows in the case of pure water-air flow, i.e. the benchmark case. Due to the presence of the surfactant, foam formed in the mixing section, which implied a significant change in the flow patterns that were photographically recorded and classified into three main types: plug, stratified wavy and stratified with foam entrainment, as far as the air superficial velocity was increased at constant water superficial velocity. The associated pressure drop, linearly distributed along the pipeline, resulted greater than the benchmark value in all the operating conditions, with a dramatic increase (even more than 100%) for plug flows. On the other hand, the percentage relative difference was found to lower with increasing the air superficial velocity, apart for stratified wavy flows where it seemed to keep constant at about 3.3%. Finally, a theoretical model for stratified flows was used to relate the pressure drop to the void fraction in order to get at least an approximate indication of the liquid load reduction due to the surfactant addition, which ranged between 6% and 39%.

Abstracts:The R245fa condensation heat transfer is investigated in a phase separation shell-tube condenser with mesh tubes inserted. The condenser has three copper tubes which have an inner diameter of 14.70 mm and a heat transfer length of 1600 mm. Four mesh tubes with mesh pore width of 15 μm are arranged in each copper tube. The comparative experiments are conducted for condensers with or without mesh tubes inserted. The flow pattern visualization is investigated to explore heat transfer enhancement mechanisms. The results showed that, with vapor mass qualities and mass fluxes increased, condensation heat transfer coefficients and friction pressure drops are increased, but heat transfer enhancement ratios and performance evaluation parameters are decreased. At low mass fluxes, the heat transfer coefficients behave a slow decrease trend with increase of inclination angles. The pressure difference between annular region and core region drives liquid flow towards core region, reducing liquid film thickness on tube bottom and increasing vapor void fractions near tube wall. With increase of inclination angles, the pressure difference is reduced but liquid film thickness on tube bottom is increased. For horizontal flow, there is an optimal match between pressure difference of annular region and core region and liquid-mesh tube contact area. Under such circumstance, the heat transfer enhancement ratios reach maximum.