Performances and Optimization of Impinging Jet Ventilation System Under Thermoneutral Conditions

Abstract

Thermal comfort and indoor air quality are of great importance to human comfort and well-being. People spend about 90% of their time indoors in modern society. An environment of thermal discomfort or poor air quality decreases the productivity of occupants and even harms their health. Furthermore, the pandemic of COVID-19 raises concerns about indoor air quality more than ever because indoor transmission far outstrips outdoor transmission. Meanwhile, the basic reproduction number of the pandemic variant increases rapidly over time. Therefore, it remains crucial to improve ventilation performance in the post-pandemic period or after the reopening of society. Additionally, ventilation is vital to reduce the risk of various airborne transmission viruses, e.g., influenza virus, respiratory syncytial virus, and MERS-COV.

Among existing types of ventilation systems, impinging jet ones aroused much attention for their advantages in energy saving, contaminant removal, and suitability for heating and cooling modes. The impinging jet provides the discharged airflow with sufficient momentum to sweep over the floor and form a thin layer of air. This flow pattern supplies fresh air directly to the occupied zone and raises the heated and contaminated air upwards due to buoyancy effects; therefore, the impinging jet ventilation (IJV) systems save energy and simultaneously provide occupants with good air quality. The layouts of the supply diffuser, exhaust diffuser, and return vents have been intensively investigated for the IJV systems and other stratified air distribution systems. Previous research optimized return vent height under a fixed supply condition for IJV systems and underfloor air distribution (UFAD) systems and suggested locating the exhaust near sources for displacement ventilation systems. Furthermore, it is reported that an IJV system removes contaminants more efficiently than traditional mixing ventilation systems. However, certain missing gaps still exist and need to be addressed urgently. Firstly, the emergence of demand response systems may adjust supply parameters to maintain a thermally comfortable indoor environment. Secondly, the performance of IJV systems for densely occupied spaces, which is important during the pandemic of COVID-19, has not been reported. Thirdly, the ceiling exhaust location has not been studied for IJV systems.

This thesis aims to optimize the return vent height for IJV systems under thermoneutral conditions, apply the IJV system to densely occupied spaces, and optimize the ceiling exhaust location of IJV systems. The dissertation consists of three parts: a) optimization of return vent height for IJV systems under the thermoneutral condition; b) investigation of the performance of the IJV system to densely occupied spaces; c) optimization of exhaust layout for IJV systems with few heat sources and/or dense occupancy.

The main results are as follows: a) the raw evaluation indices (e.g., CO₂ mass fraction and cooling coil load) are suggested for optimization of return vent height instead of normalized ones; b) the optimized return vent height for IJV systems is near the ceiling under the condition of thermoneutral occupied zone and perform better than the previously suggested mid-level return vents in terms of energy-saving and indoor air quality; c) the optimized return vent height for a UFAD system is near the ceiling as well; d) compared with traditional mixing ventilation systems, the IJV system improves indoor air quality and reduces cross-infection probability in a densely occupied space; e) the ceiling exhaust is suggested to be located near thermal plumes for a room with strong heat sources, or be located at the same side as the supply diffuser for a densely occupied space. These results indicate that, firstly, a thermoneutral occupied zone is necessary for the optimization of ventilation systems; secondly, the IJV system reduces infection probability significantly; thirdly, ceiling exhausts should be located according to the flow pattern rather than evenly distributed or at the ceiling center. In general, this study will shed light on ventilation systems design and further insight into the performance of IJV systems.

Date of Award	24 Mar 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	W. Z. LU (Supervisor)