Stratified flow in the built environment
Iial-Awad, Ahmad Salmeh
Stratified flow in an environmental chamber has been investigated. The chamber of dimensions (7.5m long, 5.6m wide and 3.0m) at the University of Hertfordshire has been used. Sets of experiments investigating the effect of the major flow parameters such as airflow rate, jet momentum, flow conditions and height of the air supply device have been conducted. Results have been obtained to evaluate the flow characteristics and thermal stratification mechanism. The study has demonstrated the validity of using smoke visualization to evaluate the stratified flow characteristics such as interface level height, stratified layer thickness, and degree of stratification. The effects of both hot and cold airflow rates in the ranges of (0.0 to 8.0 m3 /min) were investigated. The flow characteristics vary depending on the flow parameters and the experimental conditions. The effect of supply terminal and extract terminal at various airflow rates on the flow characteristics is experimentally investigated. It has been found that relative influence of inertia and buoyancy forces resolves the stratified flow characteristics. The stratification interface level height and the ventilation flow rates are two main factors in the design of natural ventilation system. The results can be used to obtain a good estimation of the effectiveness of a ventilation system at design stage. Experimental work was carried out using ceiling jet to supply hot and cold air to a confined space, to investigate the effect of jet momentum in breaking and mixing the stratified layer. The flow of high momentum was supplied downward from the ceiling. The magnitude of momentum needed depends on the degree of stratification, stratified layer interface level height and the stratification conditions. It can be seen that the jet momentum has significant influence on the mixing of the stratified flow characteristics. The results indicated that once the momentum was initiated a mixed flow grew in the occupied zone above the floor. The height of this zone depends on the stratified flow characteristics, and the temperature and momentum of the ceiling jet. Another area of experimentation was the inversion of input airflow supplies. In this case, the flow of high buoyancy was supplied upward, whilst the flow of high momentum was supplied downward from the ceiling. The stratified layer lost its stability and broke down due to the drag and tearing of cold air penetrated downward from higher levels. The compound effect of these two conditions will circulate the air in the whole space and disturb the stability of the stratified layer to reach fully mixed flow A comprehensive definition of the degree of stratification was formulated. Analytical solutions were developed for the stratified layer thickness and location as a function of temperature gradient and airflow ratios. These expressions were calibrated using the experimental results. The critical momentum needed to breakdown the stratified layer also evaluated. Comparisons with previous studies where also carried out. It was found that the stratified layer interface level height is dependent on the ratio of airflow rate and geometrical effects. If mixed flow is desired then the cold inflow aperture should be located higher than the hot inflow aperture, whiles the interface level height is not located at the exhaust aperture height. The critical vertical momentum necessary in order to break down a stratified layer has been found to depend on the stratified layer interface level height. A semi-empirical formula based on the present experimental results has been developed to predict the critical vertical momentum for given stratified conditions. Based on the present experimental results, the effect of momentum is greater than the effect of buoyancy and the time needed to break down the stratified layer is considerable less than the time it takes to stratify. Experimental data also demonstrate a ventilation method for increasing the occupied zone height without breaking down the stratified layer.