The stability of stratified layers within ventilated enclosures

Abstract

The project consists of experimental and numerical investigation of buoyancy-dominated flow leading to thermal stratification in ventilated spaces for the range of Re from 4,000 to 50,000 and Rio below 40. In the evolution of turbulence in stratified shear flows, solving for the primitive variables both in time and space is most important. Steady-state Reynolds Averaged Navier Stokes (RANS) simulations have shown in the past to model adequately the flows that become eventually steady-state. Therefore, three-dimensional steady-state CFD models were used to simulate the flow. The experimental set-up used an Environmental Test Chamber to investigate the stratification in buildings. Temperature differential was created by introducing cold air through a terminal at floor level and hot air through a diffuser pointing vertically downwards at the ceiling level. An extract point was located opposite to the inlets at the rear wall. Different combinations of inlet velocities/flow rates, temperatures as well as different arrangements of the exhaust height were studied to evaluate its effect on velocity and temperature distribution and the effect of jet flow interaction with the stratified layers. The experimental data obtained in this work were also used to validate the predictions from the numerical simulations so that CFD was used to perform more parametric simulations. To model turbulence in the flow field, the appropriateness of several eddy-viscosity based turbulence models was evaluated. The turbulence models used were the standard k-c model, the k-e Chen-Kim modified model, a Low- Re modification and the RNG k -c model. All models are in close agreement with each other. The buoyancy extended standard k-e model gives a sharper prediction of mixing in the interface. The physics of the flow are well predicted. The hot jet from the ceiling is buoyed up when it reaches the height of the interface where buoyancy forces dominate gravitational forces marking its appearance on the temperature gradient. From the simulations made at this stage, the characteristics of stratified layers are revealed. It is also shown that the interface height is proportional to the exhaust height for certain room temperatures and velocities. In a comparison between experimental results and numerical simulation, the differences are attributed on the non-adiabatic effects such as heat losses and radiation. A correlation is found between inlet parameters and stratified flow in buildings obtained both experimentally and computationally.