Simple room flow

Underlying Flow Regime 4-11 © copyright ERCOFTAC 2004

Description

Introduction

The pattern of air flow within a building or within individual zones or rooms underlies many practical applications of CFD. Optimising internal ventilation design is an important problem, since the ventilation air flow influences the propagation of airborne pollutants, the thermal environment, and occupant comfort conditions. The ventilation pattern in a room and its influence on thermal and pollutant transport is a complex function of many factors, such as the location of supply and extract terminals, room layout, the location and strength of heat sources, and air leakage openings.

This UFR focuses on modelling forced ventilation flow patterns generated in simple rooms or enclosures, by modern air terminal devices. These typically involve a high velocity and turbulence jet near the air supply terminal. This jet forces a room-size re-circulation vortex with high velocities near the ceiling and the wall facing the jet, but elsewhere within the occupied zone of the room mean velocities and turbulence levels are low.

This UFR is of special relevance to AC 4-03, Air flows in an open plan air conditioned office.

Review of UFR studies and choice of test case

There are several examples of numerical model applications to internal ventilation problems, ranging from simple zone models, to detailed three-dimensional CFD simulations. Zone modelling and early CFD work were reviewed in a report commissioned by the International Energy Agency (Liddament, 1991). This report identified the potential of CFD as a practical modelling tool for internal ventilation problems and highlighted the need for evaluating the quality and trustworthiness of CFD simulations.

In response to this need, an extensive series of tests were subsequently commissioned as part of the International Energy Agency's collaborative project 'Annex 20, Air Flow Patterns within Buildings' (Lemaire, 1992). The experiments were designed for the purpose of CFD validation, and were conducted by a number of different participants for a common test room configuration (see fig2-01.gif). The room or enclosure considered was an orthogonal box configuration, ventilated by forced flow through a diffuser mounted on one of the walls, just below the ceiling. The extract was positioned on the same wall, below the diffuser. The ventilation flow pattern generated was typical of modern air terminal device design. A contaminant source, heat sink (window) and heat source (radiator), and an additional low velocity diffuser were also added to create a variety of room configurations.

In total, six different experimental programmes were conducted, on:

Forced convection (isothermal)

Free convection

Mixed convection

Contaminant releases

Displacement ventilation

This document focuses on the results of the forced convection experimental programme. This is selected for the following reasons:

it is a well constructed test case, carried out in the context of a recognised international comparison exercise for the purpose of CFD validation;

both full scale and model scale experiments were performed, and both mean flow and turbulence measurements were taken;

experiments were carried out using state-of-the-art techniques. Since various experimenters were involved, there was inter-comparison of results;

well-documented CFD simulation results are available, which include sensitivity runs designed to help assess various CFD modelling options (mesh dependence, sensitivity to boundary conditions, different turbulence and wall modelling approaches)