The experimental and numerical results on the flow structure and heat transfer in a bubbly polydispersed upward duct flow in a backward-facing step are presented. Measurements of the carrier fluid phase velocity and gas bubbles motion are carried out using the PIV/PLIF system. The set of RANS equations is used for modeling the two-phase bubbly flow. Turbulence of the carrier fluid phase is predicted using the Reynolds stress model. The effect of bubble addition on the mean and turbulent flow structure is taken into account. The motion and heat transfer in a dispersed phase is modeled using the Eulerian approach taking into account bubble break-up and coalescence. The method of delta-functions is employed for simulation of distributions of polydispersed gas bubbles. Small bubbles are presented over the entire duct cross-section and the larger bubbles mainly observed in the shear mixing layer and flow core. The recirculation length in the two-phase bubbly flow is up to two times shorter than in the single-phase flow. The position of the heat transfer maximum is located after the reattachment point. The effect of the gas volumetric flow rate ratios on the flow patterns and maximal value of heat transfer in the two-phase flow is studied numerically. The addition of air bubbles results in a significant increase in heat transfer (up to 75%).
Characteristics of movement of gas bubbles behind a sudden channel expansion were studied experimentally. The data on the distribution of the gas phase and speed of movement of bubbles were obtained. It is shown that at a distance from the sudden channel expansion the bubbles slow down, which relates to the slowing down of liquid due to an increase in the flow cross section. After the reattachment point, the bubbles move along a more curved path than upstream. It is shown that bubble clusters can form in the flow recovery zone. The velocity distribution in the channel was studied by means of laser Doppler anemometry (LDA).
The experimental study of the flow structure behind the sudden expansion of the flat channel is performed. The cross section of the channel is 20×200 mm and the step height is 12 mm. Visualization of the flow structure and measurements by means of particle image velocimetry (PIV) is performed using high power green laser sheet and CCD camera. Liquid velocity profiles are measured by means of laser Doppler anemometry (LDA). Automatic position system is used. The structure of liquid flow distribution evolution behind the sudden channel expansion is shown. The data about pressure drop is presented for different liquid flow rates.
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