The stability of stratified layers within ventilated enclosures
Author
Pikos, Konstantinos
Attention
2299/14351
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.
Publication date
2006Published version
https://doi.org/10.18745/th.14351https://doi.org/10.18745/th.14351