Air distribution numerical simulating of isothermal jet with interference parameters in large space

Abstract

It is difficult for the traditional ventilation, such as general ventilation or local ventilation, to remove dusts effectively when dust source is changeable in a workshop, especially in a large space. This paper uses the software of PHOENICS, simulating stratified air distribution of three-dimension turbulence in a large space with many kinds of interference parameters. It adopts the k–ε model of standard turbulence, applies staggered grid and finite volume approach discrete equation analysis, and simulates for predicting indoor air distribution of isothermal jet in the large space. The comparison and analysis between the results of simulation and empirical formula show that the simulation is availab1e to predictive the indoor air distribution. The results are useful for the optimizing the design of complex air distribution in large spaces.

Introduction

Environmental quality is directly related to people’s health. In the metal-processing industry, a large amount of fume, smoke and dust, even CO₂, HF, O₃, are produced during the processing. These contaminants can do great harm to the workers. The steel construction workshop of a bridge girder plant is a typical large workshop, in which the main work is to eliminate rust and polish the surface of steel bridge workpieces. During the process of burnishing, it produces large quantity of dust which can reduce visibility, impede operations, seriously endanger the health of all those around and pollute the environment. In reality, the dust is not easily controlled in large spaces, especially when the dust source is changing and the local exhaust hood can not work effectively. The contaminants may fill the whole space if the distribution of air in building is poor. According to many industry standards, the usual form of ventilation and dust removal in such buildings is general ventilation. However, general ventilation is expensive and may not meet the requirements for environmental protection. The larger building, the greater the volume of air that must be moved for ventilation and the more difficult it is to control dust. So the large workshop with a changeable dust source is paid more and more attention.

The energy-saving and economic method of dust control in large spaces is to reduce the area that needs dust control. An isothermal horizontal jet is applied to divide the space into three parts as shown in Fig. 1. The middle zone is the airflow zone in which the airflow has high velocity and can prevent dust from moving up freely. The lower zone is the control zone which is filled with a great deal of dust that should be exhausted by the ventilation system or using the local exhaust hood. The upper zone is the non-controlled zone, which has little dust if the airflow zone can effectively prevent the dust from moving up.

Multi-parallel jets are different from a single jet because it is the synthesis of several air supplies which leads to the phenomenon of streamline coincidence. Each jet can follow the law of free turbulent jet in the beginning. Soon each jet will be impacted by an adjacent jet, the flow rate will increase and the expansion degree of cross-section will slow down. Then, jet fluid can join with adjacent jet streamlines as is shown in Fig. 2 [1]. Multi-parallel jets can form an air curtain in space and impede contamination.

The characteristics of indoor air distribution are essential to the design of a ventilation system. Efforts have been made to improve numerical prediction techniques for the airflow in rooms. The k–ε turbulence model has been used in the simulation of room airflow with a certain degree of accuracy [2], [3].

This paper takes the steel constructions workshop of a bridge girder plant as an example. Fig. 3 shows a schematic layout of the workshop, which are 63.0 m long, 33.0 m wide and 17.0 m high. The air supply outlets and exhaust outlets were set in the two side walls. The shapes of the workpieces are box, strip, flat plate, etc. The maximum one is 3 × 3 × 40 m.