Numerical Study of Flow In Mechanically Ventilated Rooms Under Non-Isothermal Conditions

A computational procedure was conducted in (3x3x3 m) rooms and in a full – scale model building to study the effect of supply temperature on the room flow patterns in mechanically ventilated spaces. The procedure is based on the solution in finite-volume form of 3D equation for the conservation of mass, momentum, energy, kinetic energy and dissipation rate. Effect of buoyancy on the turbulence model has been included. The results are shown to be in reasonable agreement with experimental data.


Introduction
The major aim of ventilating spaces for human occupancy is find cleaner, safer homes that are cost effective and energy wise efficient [1].Recent studies indicate that 80% of our life is present indoors and that about one third of the world's energy is utilized for heating and cooling [2].Predication of the distribution of velocity, temperature in a ventilated space is important for designing air ventilation system, by studies of building thermal analysis, indoor air 535 quality and thermal comfort [1].CFD provides a cost effective method to predict flow field in buildings.Developments of CFD for building simulations were done by Awbi et al. [3,4], Haghighat [1], Davidson [5] and others.Awbi simulated flows in two and three-dimensional office spaces for turbulent flows using the standard (k-ε) method.The studies show the importance of understanding effects of buoyant flows where instabilities may occur.Haghighat [1] simulated the pattern of isothermal flow caused by infiltration and ventilation in three-dimensional multi-zones by numerical simulation and that zones were separated by a partition with a door opening.The result shows that the location of the door not only guides the direction of the air movement but also affects the strength of the air circulation.Batzanas and Kittas [6] studied the effect of different ventilation types (sizes and position) on ventilation rates, air flow temperature distribution in tunnel greenhouse and conclude that the highest ventilation rates are not always the best solution to cool the green house.In this paper, the study of the effect of supply temperature on airflow patterns inside a room was performed using (k-ε) turbulent model.Comparison of code performance with available measurements is presented.

MATHMATICAL MODEL
The mathematical description of airflow is based on the fundamental laws of physics, conservation of mass, momentum and energy.These equation can be expressed in the form ( ) where φ is the dependent variable.
Table (1) gives the expression for the source terms φ S for each variable that is likely to be needed in solving problems.The constants for the (k-ε) method are the standard constant proposed by Launder and Spalding [7].
The turbulent viscosity and turbulence production terms are, The simple (Semi Implicit Method for Pressure Linked Equation) scheme proposed by Patankar [8] is applied.This finite volume formulation applies staggered grid for the pressure variable to avoid pressure instabilities .The iterative sequence was carried out using a point-by-point technique, with correction of the velocity components, inflow velocity, turbulence and dissipation levels, temperature and pressure are specified.Along the boundaries the standard wall-function is used [9]. )

CODE VALIDATION
To validate the code program for turbulent non-isothermal flow, flow through a rectangular shaped room was used.Figure (1) shows velocity vectors for fine grid systems, it is also show the comparison of the present results with that obtained by Weather [10], the figure shows acceptable agreement.

RESULTS
A room (3 x 3x 3 m), having an inlet and outlet area equal to (0.5 x 0.5 m) with two windows is used.Effect of supply temperature is studied, one is a supply of warm air and the other is a supply of cold air.

Supply of warm air
For ventilation rate 15 air changes per hour (ACH), the supply temperature is fixed at o 30 C and the room temperature was initially fixed at o 20 C respectively.Figure 2 shows the velocity vectors and temperature contours for that case.respectively.Figures 3 and 4 show the velocity vectors and temperature contours for that case.

DISCUSSION
For the supply of warm air it is possible that the air will only circulate in the upper part of the room which leads to stratified flow.Shows Figure 2, an upward plum, with large recirculation vortex.This vortex mixes the air in the room.This is an example of an inefficient form of heating, where the heating energy is lost with out effective heating to the room.The effect of buoyancy is great forcing the flow to deflect upwards sharply, with a large primary vortex, circulating in the space.We can conclude that that effect of velocity can not be taken alone to state whether mixing flow pattern is present or not.For the supply of cold air it is possible for the jet to drop into the occupied and caused unacceptable high velocities.As seen in Figure 3 the effect of the cold air is to deflect the flow downwards decreasing the "jet throw".This may lead to thermal discomfort for the occupants under the jet.It is seen from Figure 4, that