Study of Radial Wall Jets from Ceiling Diffusers at Variable Air Volume

Maria Hurnik,Zbigniew Popiolek,Jan Kaczmarczyk

doi:10.3390/en14010240

Abstract

The knowledge of the air velocity distribution in the supply jets is essential when designing ventilation and air conditioning systems. In this study, we tested and analyzed the velocity distributions in the radial wall jets—these jets are commonly used in ventilated rooms. Tests included jets from two ceiling diffusers of different constructions, at three airflow rates. The mean air speed distributions were measured with a 16-channel hot-sphere anemometer both in the self-similarity zone and in the terminal zone. A specially developed method of converting the mean speed to mean velocity was used. The measurement results show that the spread coefficients of the jets from both diffusers were the same, but the positions of the virtual origin were different. Due to the friction of the jet with the ceiling and the transfer of momentum to the recirculating flows, the momentum flux in the self-similarity zone decreased by up to 50%. An improved method for calculating velocity distributions in radial wall jets was developed and validated. This method takes into account the decrease of momentum, non-zero position of the jet origin, and faster velocity decrease in the terminal zone. A reliable method of predicting air velocity distribution in radial wall jets (RWJs) from ceiling diffusers may allow to properly select the diffuser size, its location, and the range of flow rate changes. The design process for variable air volume systems can be facilitated.

Highlights

The air distribution in ventilated rooms depends on the number, type, size, and location of the diffusers, and the velocity, direction, and turbulence intensity of the supplied air
A method for determining the mean velocity distribution in radial wall jets (RWJs) was validated at Reynolds number from 24,000 to 77,800
This method takes into account the decrease of momentum, non-zero position of the jet origin, and faster mentum flux M decreases to approx. 50% of the value of momentum flux at the diffuser outlet M

Summary

Introduction

The air distribution in ventilated rooms depends on the number, type, size, and location of the diffusers, and the velocity, direction, and turbulence intensity of the supplied air. A significant decrease in the supply airflow decreases the throw length of the jets, which affects air distribution in the room. In cooling mode, this may cause cold air dumping, i.e., the jet detachment from the ceiling and flowing of cold air directly to the occupied zone. This may cause cold air dumping, i.e., the jet detachment from the ceiling and flowing of cold air directly to the occupied zone In heating mode, this may deteriorate the air mixing in the room, decrease the air change effectiveness, and increase the thermal stratification. The review showed that, with a supply air temperature

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