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• PIV measurements in stratified pipe flow with controlled interfacial waves. • Characterization of base flow and changes due to waves. • Threshold for linear wave regime based on a nondimensional number • Quantification of additional drag induced by waves of different amplitude. • Correlation proposed to account for wave induced pressure loss. The work reports an experimental investigation on stratified gas-liquid pipe flow characteristics in the presence of controlled interfacial waves. Studies of this flow regime with controlled interfacial waves are scarce in the literature. Here, the disturbances are excited at the liquid interface by an oscillating paddle. The waves are synchronized with image acquisitions, enabling the utilization of phase-locked measurements and ensemble averaging techniques. Off-axis Particle Image Velocimetry (PIV) and Shadowgraph techniques were applied to provide information about mean and wave-induced modifications on the velocity fields. Results show that mean flow velocities in the liquid and gas phases close to the pipe walls adhere well to the single-phase flow log-law profile. In the liquid layer, this agreement was observed up to half of the water depth. Controlled disturbances enabled the estimation of the wave amplitude thresholds for the appearance of relevant nonlinear wave effects on the flow field. Results suggest that such a threshold can be fairly represented by a constant value of the non-dimensional parameter proposed in the work of Kirby (2008). Within the non-linear wave regimes investigated, noticeable changes in the flow field were observed close to the interface. However, near the wall the flow was weakly affected by the presence of waves, suggesting that interfacial wave effects are weakly coupled with near-wall disturbances and might be modelled independently. Moreover, contributions to interfacial shear stress due to the presence of waves were obtained experimentally. The results presented here are useful for validation and improvement of models used to predict flow characteristics in stratified flows. In addition, they contribute to shed further light on the physical mechanisms involved in the phenomenon. [ABSTRACT FROM AUTHOR]

Abstract: Experiments of air–water two phase heat transfer and frictional pressure drop without and with polyacrylamide (PAM) additive in a horizontal smooth circular tube were conducted. A PAM water solution with a concentration of 300ppm was used in the experiments. The test tube has an inside diameter of 40mm and an outside diameter of 48mm. Heat transfer tests were performed by cooling air–water flow inside the test tube through its wall using cooling water with a heat transfer length of 3m and two phase pressure drops were measured over a length of 3.1m. The liquid phase superficial velocity VLS =0.52, 1.02 and 1.46m/s and the gas phase superficial velocity VGS =1.66–4.9m/s. The average volume void faction β =0.532–0.904. The total mass flux G =5199–15,234kg/m2 s and the gas phase Reynolds number ReG =3892–12,025. It shows that the air–water two phase pressure drops can be reduced from 31.9% to 54.7% while the two phase heat transfer coefficients can be reduced from 36.8% to 70.3% with the PAM additive. Furthermore, the effects of the PAM additive on the air–water two phase heat transfer and pressure drop characteristics have been analyzed. The observed flow patterns are all slug flows. New physical mechanisms of the two phase pressure drop and heat transfer reductions have been proposed. [Copyright &y& Elsevier]

Abstract: A hydrodynamic study is carried out in a multistage-agitated contactor with cocurrent upflow of air/viscous fluids. The liquid phase backmixing is characterized through a residence time distribution study in various viscous fluids (up to 20mPas). The liquid Reynolds number with respect to the impeller applied in the investigation ranges from 1000 to 50000. A cascade of stirred tanks with back flow model satisfactorily fits all experimental residence time distribution curves in the present study. It is found that the increase in the liquid viscosity reduces the backmixing in the liquid phase due to its dampening effect on turbulence. The presence of the gas phase helps in reducing the backmixing by the straightening effect and entrainment effect in a co-current operation manner. The gas hold-up measured in the present study is comparable to the literature data. A new type of correlation taking into account the influence of the gas phase is proposed to predict the backmixing in a multistage-agitated contactor. [Copyright &y& Elsevier]

The use of acrylic cells and a CCTV camera for visually investigating the carbon dioxide gas evolution process inside an operating direct methanol fuel cell environment is demonstrated. Also, the effect of operating parameters on the system gas management, using a series of tests with different gas diffusion layer supporting materials, flow bed designs, cell sizes and exhaust manifold configurations, is studied. Carbon dioxide gas management is an important issue obstructing progress in viable direct methanol fuel cell systems development. Gas evolution mechanisms and gas management techniques are discussed and analysed with reference to several video picture and performance data. The data demonstrate that Toray carbon paper is not a suitable material for DMFCs due to its poor gas removal properties. ‘A’ type carbon cloth shows relatively good gas removal behaviour. Increasing the liquid phase inlet flow rate is beneficial for gas removal. Increasing the current density results in higher gas production and in the formation of gas slugs, especially at low flow rates, which can lead to blocking of the channels and hence deterioration in the cell performance. A new flow bed design, based on a heat exchanger concept, is affective for gas management and gives a more uniform flow distribution in the flow bed channels. Using the results of this study, and the modelling techniques developed by our group, will are able to determine suitable operating conditions for our prototype 0.5kW cell DMFC stack. [ABSTRACT FROM AUTHOR]