Studies On Hybrid Ventilation Combined With Thermal And Mechanical Ventilation

Mechanical ventilation is being used in many buildings; while natural ventilation is often ignored. The problems of high energy, poor indoor air quality in conventional air conditioning systems are often complained by occupants. The hybrid ventilation, combined thermal with mechanical ventilation is considered, can make lull use of natural ventilation to realize energy saving. Up to now, the basic models of hybrid ventilation are not very clear; generally, natural ventilation strength is small, the influence of mechanical ventilation on natural ventilation may result in: enhancement or inhibition; in other words, there should be a turning point or turning zone from enhancement to inhibition when thermal and mechanical ventilation coexist. This is an important and essential problem, which is the basic point for hybrid ventilation design.Most of previous literature are focused on single model, which are not directly applied to hybrid ventilation systems, while the studies on hybrid ventilation model are very little. So, this paper we carry out some studies on hybrid ventilation model. The hybrid ventilation model is combined thermal with mechanical ventilation, including two parts : mechanical supply air and thermal ventilation (model 1); exhaust fan and thermal ventilation (model 2). Those two models are considered separately. Supply air velocity, heat power, the location of inlet vent and heat source; temperature efficiency and thermal stratification height are investigation to predict the ventilation effectiveness. In addition, the turning point or turning zone parameter is also important, based on previous studies, a new parameteror Fr = is suggested to describe the ventilation effectiveness for turningpoint from enhancement to inhibition, when thermal and mechanical ventilation coexist. Frparameter is defined the relative strength of mechanical and thermal ventilation, which can be used to describe the turning point from enhancement to inhibition.Ventilation in office buildings are investigated by CFD simulation and we obtained the influence of Fr parameter, the location of ventilation inlet and heat source on the air temperature distribution for double-vent ventilation systems in model 1. The results reveal that thermal stratification exist when mechanical ventilation strength is small, room air flow is dominated by thermal ventilation, on the other hand, room air flow is dominated by mechanical ventilation, air temperature distribution apting to uniform. The turning point are changing from enhancement to inhibition, when thermal and mechanical ventilation coexist is Fr =1.96~2.83. In addition, for three-vent ventilation systems, a turning point the course of supply air velocity increasing ranges is Fr = 1.13 ~ 1. 45. For model 2, the influence of A P on temperature distribution is similar to model 1. However, with the increasing of AP, a turning point Fr =2.31 is achieved, which results in the transition from thermal ventilation to mechanical ventilation.