A Semi Empirical Flow Model for Low-Velocity Air Supply in Displacement Ventilation

Similar to supply air jets in mixing ventilation this paper describes a comprehensive flow model for displacement ventilation derived from the integrated Navier-Stokes differential equations for boundary layers. A new test method for low velocity diffusers in displacement ventilation is developed based on this new flow model. Contrary to jet flow, it is shown that the only independent variable in the new model is the buoyancy flux. In addition to this variable the calculations need a single empirical constant, which is determined from a limited number of full-scale tests of a limited number of similar shaped dilsers of different size. There are made a number of tests to try out the new model. The results are promising. For plane (two-dimensional) flow the velocity accelerates to a constant value. For radial flow there is also an acceleration zone, after which the velocity decays. Both theoretical and empirical data predicts that for similar shaped difhsers the width of the near zone (distance from the centre of the diffuser to a chosen velocity depends only on the buoyancy flux, not the dimensions of the diffuser (radius and height). One consequence of this is, among others, that the width of near zone cannot, for a certain air flow rate, be shortened by choosing a larger radius and a lower height of the diffuser. The diffuser constant K for radial difhsers has, however, turned out to be more or less dependent on the difference in temperature between the supply air and the room air, probably due to that the outflow is not ideally radial, and the effect of the temperature difference is to make the flow become more radial. The new model also enables the designer to calculate the near zone for arbitrary airflow rate, supply air temperature and arbitrary supply difiser size of similar shaped diffusers. Practical benefits are, among other things, improved test standards and design methods for displacement ventilation.