Influence of flow baffles on mixing of two air streams flowing in a mixing chamber with dissimilar temperatures: A computational study

One of the most important components of heating, ventilation, and air conditioning (HVAC) is an air handling unit (AHU) which is used to condition and mix air that arrives from outside the building (OA) and return air (RA) that exits from the indoor space. Poor mixing between the warm return air and the cold outside air often leads to air stratification in AHU, which results in various problems, such as, nuisance trips of low temperature safety thermostats, freezing of chilled or hot water coils in a stratified subfreezing air stream and loss of control when sensors cannot read the true temperature of the air stream. A static air mixer such as dampers and baffles comprise one method that may be used to overcome the stratification and its associated problems. In this study, baffles were used as a solution to the stratification problem and to obtain a good thermal mixing effectiveness which expresses how well the baffles mix two air streams. Computational fluid dynamics (CFD) has been used to simulate a specific air handling unit with outside air passing straight through the mixing chamber and the outdoor air entering the mixing chamber perpendicular to the outside air. Mixing characteristics such as thermal statistic and range mixing effectiveness of a single baffle (return or outside air baffle) and two baffles have been investigated. The influence of the baffle size and angle on these characteristics was studied and compared to the results of a case with no baffle. The results show that for a single baffle an increase in the mixing characteristics occurred with an increase in the size of the return air baffle, and the influence of the return air baffle angle depend on the size of the return air baffle. The outside air baffle when installed in the mixing chamber, also provided a better mixing for outside air baffle angles greater than 15⁰. When both the Outside Air Baffle and the Return Air Baffle are used results show no progress in mixing characteristics when the angle of the baffles equal 0⁰. However, increasing the angles of the return and outside air baffles resulted in an improvement in the mixing characteristics. The relationship between the pressure drop and mixing effectiveness was plotted for all cases in order to determine the best case that leads to good mixing characteristics with a small pressure drop --Abstract, page iii