Thermal Dispersion Study

Abstract

Cooling water outfalls of power plant is one of engineering applications of discharge that mostly found in coastal areas. It contains buoyant turbulent jets phenomenon that depends on geometry configuration, discharge momentum, and its temperature. Further, thermal dispersion is one of the most critical phenomenon that has to be analyzed thoroughly because it directly affect the marine ecosystem and power plant products. With its wide range application in research and engineering problems, computational fluid dynamics (CFD) simulation is done to provide more detailed analysis of thermal dispersion. Moreover, CFD simulations of discharge are considered to be particularly challenging to perform. The momentum flux, buoyancy flux, and outfall geometry that strongly influence the buoyant jet interaction required fine mesh to compute. Not only accuracy but also time cost is affected by the mesh. It follows that by optimizing the mesh configuration, an accurate and most economically simulation can be obtained. A three-dimensional model of turbulent jets discharge into stationary ambient sea water is presented in this paper to study certain mesh grading variation that have not been employed before. The simulation is limited only to near-field area. It is found that mesh with its grading is centered near the outfall could increase the accuracy of the simulation without increasing the number of mesh elements.

Introduction

Cooling Water Outfalls

Thermal or chemical pollutants often discharges to the ocean through outfalls of power plant stations. This activity leads to several changes of condition in the ambient, one of them is the rising of seawater temperature. Consequently, this phenomenon could potentially endanger the seawater ecosystem as well as disturbing the production activity of power plant stations due to the increasing temperature of intake seawater. To overcome that problems, a careful assessment must be done to provide thorough analysis. The thermal dispersion of discharge occurs in a wide range area. The region close to the outfalls, called by near-field region, that can be characterized as turbulent buoyant jets (Veld, 2013) which the momentum and buoyancy effect are governed the mixing process (Robinson, Wood, Piggott, & Gorman, 2015). Moreover, the discharge in this region form a jet-like flow due to its momentum dominance, however it become plume-like flow when as initial momentum is dissipated and buoyancy dominated.

Theory of Jets and Plumes

The theory about jets-like and plume-like behavior are summarized briefly by Fischer (Fischer, 1979) and Lee and Chu (Lee & Chu, 2003) research. According to Fischer, a pure jet is typically considered as a flow from an orifice with a pure momentum source. He formulated three basic dimensional numbers such as the volume flux, momentum flux, and buoyancy flux to define the influence of momentum and buoyancy force.