Buoyant jet in a ventilated room: Velocity field, temperature field and airflow patterns analysed with three different whole-field methods

doi:10.1016/j.buildenv.2008.02.009

Building and Environment

Volume 44, Issue 1, January 2009, Pages 137-145

https://doi.org/10.1016/j.buildenv.2008.02.009 Get rights and content

Abstract

The instantaneous velocity field and temperature field were measured and the airflow patterns visualised close to a diffuser for displacement ventilation. Since the low-velocity diffuser was located above the floor and the inlet air temperature was below the room temperature, the flow was governed by both momentum and buoyancy forces. The data were recorded with whole-field measuring techniques, particle streak velocimetry (PSV), particle image velocimetry (PIV) and infrared thermography (IR), in conjunction with a low thermal mass screen. The environment is very complex, supply of buoyant air with a commercial supply terminal with 20 nozzles pointing in different directions, which makes it difficult to use point-measuring techniques or computational fluid dynamics (CFD). The aim was twofold: (a) to explore what kind of information can be derived from whole-field measurement techniques in this context and (b) to investigate the trajectory of the flow discharged into the room and the entrainment of the flow.

Introduction

Displacement ventilation is a ventilation principle based on the properties of stratified flow. Air with a lower temperature than the room air temperature is supplied directly into the occupied zone. Because the air is supplied directly into the occupied zone, it is efficient ventilation. The problem is the risk of draught. To minimise the risk, the air is supplied at a low velocity. If the supply air terminal is located above the floor the flow generated is an inclined buoyant jet. When the buoyant jet hits the floor, it spreads on the floor as the gravity current. Due to the risk of draught it is important to monitor the air temperature and the velocity within the buoyant jet. Within the region of the air stream where the comfort criteria are not met, people cannot stay for a longer time. Therefore, the air penetration into the room must be monitored, which calls for determining the trajectory.

Fig. 1 displays the supply air terminal used in our study. It is a typical low-velocity terminal.

The terminal has 20 nozzles of diameter 8 cm located within an area of 44×38 cm². The orientation of the nozzles can be individually regulated. A perforated covering plate of dimensions 54×45 cm² and porosity 37% covers the nozzles.

The design of the terminal makes the flow from the supply terminal very complex. The air is always in transition between different states. The limited size of the room will limit the length of the trajectory. Therefore, it is not certain that the flow will reach an asymptotic state known as the zone of flow establishment (ZOF). The complex geometry of the terminal with nozzles oriented in different directions and the presence of buoyancy make computational fluid dynamics (CFD) predictions very difficult. A fine grid is required and the flow inside the terminal must be simulated, as was done by Cehlin [1]. To record the velocity within a large volume with a point-measuring technique, e.g., hot-wire anemometry, is an enormous task and in a field trial almost impossible. However, a whole-field method for measuring velocities and/or temperatures gives us the possibility to explore the whole buoyant jet within the room.

Section snippets

Properties of buoyant jets

The literature on buoyant jets is vast. Many studies concern problems with wastewater disposal. A recent review article on turbulent buoyant jets is by [2]. Some properties of jets and plumes are more easily derived by using the Lagrangian approach, and particle streak velocimetry (PSV) is basically a Lagrangian method. A book on jets and plumes using the Lagrangian approach is [3].

Fig. 2 shows a sketch of the main parameters.

In order to be able to get an expression we can make a simplification

Test room

The experiment took place in a climate chamber with a size of (L×W×H) 3.14×4.18×2.75 m³ and a displacement ventilation system. A flat diffuser, with a height of 0.45 m and a length of 0.54 m, was located at the centre of one of the walls. The diffuser had a face area, convex hull of face-plate holes, A=0.1995 m². For the characteristic length of the supply device, L=√A, which is equal to 0.45 m, is used.

Test conditions

In the room the mean air temperature was 21 °C. The air was supplied at a flow rate between 0.013

IR measurements

The results from the IR measuring method were images with different colours representing different temperatures of the air close to the diffuser. The IR images in Figs. 3a and b show the changes of the jet for two different thermal lengths, highest and lowest supply flow rate.

The trajectory of the jet on the corrected image coordinates was found to track the following power functions well, see Table 2 and Fig. 4.

The length of the trajectory when it reaches the floor is obtained from $L_{Traj} = \int_{0}^{x (y = H}$

Conclusion

In this study the trajectory of profile and the entrainment of air in a non-isothermal jet were studied with three different whole-field measuring methods. The first one, the IR method, shows that the trajectory was found to follow an x²-curve much better than the expected x³-curve. The main explanation for this deviation is that the flow is not fully established. An indication of this is that the ratio, L_Traj/L, between the length of the trajectory and the characteristic dimension of the

Acknowledgements

The help from Dr. Mirko Radic at the University of Gävle, Tomohiro Kobayashi at the University of Osaka and Dr. Jens Fransson at KTH is gratefully acknowledged.

References (9)

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