Posted by Nikunj Patel on December 15th, 2014
Venturi scrubbers are economical pollution-control devices, employed in industrial plants to remove harmful solid particles from exhaust gas streams before being exposed to atmosphere. The contaminated gas is filtered by the stream of water droplets that capture the particles and absorb them from the gas stream. It consists of three main sections: a convergent, throat and a diffuser. The gas is made to enter through the convergent section of the scrubber, which increases the gas velocity.
However, upon reaching the throat section, its velocity attains uniformity and begins to reduce once the gas flows through the diffuser. The scrubbing liquid is introduced usually at the convergent or throat section, where the gas velocity is much higher. As such, the performance of the venture scrubber relies on the collection and absorption efficiency along with operating conditions such as injection system and fluid velocities.
The fraction of liquid inside the scrubber is an important parameter to decide the performance as liquid-free regions will leave the flue gases as it is without absorbing the particles. In order to evaluate the venture system, experimental investigations are often costly and requires complex test bench to capture accurate physics.
Additionally, it is quite difficult to obtain information on the fluid flow and mixing characteristics inside the scrubber unit. As such, use of simulation tools has emerged as powerful tools, allowing designers to improve the design and physical phenomenon inside the scrubber.
To simulate the flow process inside the scrubber requires accurate understanding of flow parameters of gas as well as the scrubbing liquid. The foremost requirement is to build a computational domain that must include all the necessary details that otherwise would affect the flow field. Defining the boundary conditions shall include specifying the mass flow rate and pressure conditions of the gas and scrubbing liquid at the inlet.
For the outlet, pressure boundary conditions must be applied and at the walls, no-slip conditions shall be imposed for both liquid and gas. It is also required to select a suitable turbulence model. Since the gas behaves as a continuous fluid and liquid as dispersed, the two phase flow can be simulated by using k-epsilon turbulence model for the gas and dispersed phase zero equation model for the liquid.
It is also important to ensure mesh independence to ensure meaningful results from CFD simulations. This requires having fine meshed domain, despite the consumption of extra simulation time, in order to ensure much closer results to that of the experimental tests.
The k-epsilon turbulence model will provide insights on the effect of turbulence on liquid fraction and required mass flow rate, to ensure sufficient diameter of gas and liquid droplets for maximum absorption. Velocity streamlines will also help in determining the liquid-gas contact regions more comprehensively.
Additionally, it is possible to evaluate the exit spray angle from the nozzle to ensure maximum reach of the gas stream in the mixing chamber. As such, the CFD results will provide enough room for the designers to optimize the scrubber geometry, in order to obtain maximum performance. Once the design is optimized, a prototype test can be followed to validate the significance of CFD results.
About Author: Nikunj Patel is a design engineer working with Hi-Tech CADD Services for the past 4 years. He loves designing specialized industrial equipments and can always be found in the lab discussing, brainstorming & tweaking designs. He has also worked on architectural projects taking interest in every aspect of design & analysis.
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