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Gas Phase Distribution Effects on Heat Transfer in Upward Vertical Bubbly Channel Flows

Two-phase (non-boiling) flows have been shown to increase heat transfer in channel flows as compared with single-phase flows. The present work explores the effects of gas phase distribution such as volume fraction and bubble size on the heat transfer in upward vertical channel flows. A two-dimensional (2D) channel flow of 10 cm wide by 100 cm high is studied numerically. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. The bubble size is characterized by the Eotvos number. The volume fraction and the Eotvo number are varied parametrically to investigate their effects on Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition.Copyright

Numerical Simulations of Near-Nozzle Exit Characteristics for an Effervescent Atomizer at Low Gas to Liquid Mass Flow Ratio

Effervescent atomization is a process in which a bulk liquid is transformed into a spray of droplets by injecting a small amount of gas into the liquid before it is ejected from the atomizer. Advantages of using effervescent atomization method include larger exit orifices to reduce clogging issues, reduced injection pressures, and lower gas to liquid mass flow ratios (GLR) as compared to pressure or air-blast atomizers [1]. Effervescent atomization has been used in a number of applications including agricultural sprays, paint sprays, combustion for lowering pollutant emissions, spray cooling for gas turbine and medical applications, waste incineration, and process industry applications. In the present work, the near-nozzle exit characteristics of an air-water effervescent atomizer at gas to liquid mass flow ratio such as 0.25% are investigated numerically through two-fluid Eulerian-Eulerian ensemble-averaged modeling. The two-fluid model is solved through the finite volume method. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. The effects of effective (average) air bubble diameter size inside the atomizer, exit nozzle diameter, and injection pressures on the average liquid water jet width are presented. An optimal bubble size is observed for increasing the average liquid jet width which leads to enhanced jet breakup at downstream of the nozzle. The water volume fraction profiles within the sprays are also reported. The numerical results are compared with experimental visualizations and jet-width measurements to further the understanding of the spray characteristics of effervescent atomization for atomizer design.Copyright

Computational Modeling and Simulations of Isothermal Plane (Linear) Air Jet Velocity Profile for Slot Diffusers

In the present work, computational modeling and simulations of isothermal plane (linear) air jet velocity profile for slot diffusers are performed. Plane air jets are formed by linear slots or rectangular openings with a large aspect ratio. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. Three plane air jet flow simulations will be investigated such as free plane (linear) jets, attached jets, and air flow through a slot diffuser in a room setting. The purpose of simulating the free plane jet through a slot diffuser is to study the behavior of jet velocity profile that is not blocked by side walls or ceilings. The jet velocity profile is modified when obstructed by the walls and the air jet desires to attach to the surfaces along its path. For this reason, attached jet simulations through a slot diffuser will be conducted. The CFD study of plane air jet flows will eventually be extended to jet flows through a slot diffuser to a room to investigate the fluid flow behavior that enters a room under a ceiling. In addition, effects of two-equation turbulence models such as standard, renormalization group (RNG), and realizable k-e on the CFD simulations will be investigated. Predicted velocity profiles and decays of free plane jet through a slot diffuser will be validated with a semi-empirical model [1]. Predicted velocity profiles of attached jet simulations will also be compared with a semi-empirical expression [2]. The slot diffuser air flow simulations will be compared with experimental data by the work of Chen and Srebric [3]. All simulations will be conducted at a specified inlet air velocity. The effects of grid resolution are also examined. It is established that the standard k-e turbulence model best simulates attached and free jet flows. The standard k-e turbulence model is applied to a room setting under isothermal conditions. The results are compared with non-isothermal experimental data [3]. It is shown that temperature which is a passive scalar has less influence on the flow pattern at a high air velocity than at a low air velocity in a room setting.© 2014 ASME