Research On Underground Car Park Ventilation

In the large cities, the scarce space and the expensive land have led to most underground car parks being built in basements of the high-rise or the multi-storey buildings by most estate agents. Vehicle activities when starting or stopping result in the emission of a number of harmful substances such as CO, NOx, SO₂, VOCs and lead particles etc. Among them the carbon monoxide (CO) which affects human beings most severely is the main one. The enclosed nature in the underground garages makes the contaminants emitted from cars being difficult to diffuse. So the mechanical ventilation systems are installed in almost all underground garages. These systems not only consume the power but also consume much ventilation heat load, especially in the severe cold zones.For enclosed car parks, except for the air contaminants and ventilation, fire safety and smoke extraction attract more and more attentions. An excellent ventilation or smoke extraction system provides not only a safe and comfortable environment, but also a possibility to save energy and reduce construction costs and operation charges.In the underground car parks the ventilation rate is the most important factor that influences the air quality and the energy consumption. The ventilation requirement is mainly determined by the indoor criterion of carbon monoxide concentration limit and its emission from cars. Vehicle is operating in the cold start mode, when it leaves the parks; and operating in the idle mode, when it enters the parks. The CO emission rate in these two operation modes is significantly higher than it in highway driving. Especially it is significantly higher during the cold start mode than in the idle mode. And further, the CO emission rate for the cold start and the idle is closely related to the environment air temperature. In"Code for building design of garage", the air changes rate of 6h-1 is given. The mathematical model for calculating the ventilation rate and the formulas for the different air temperatures in the underground car parks were presented, and that the different ventilation strategies for the underground garages in different seasons or different areas were discussed.Based on the survey and measure of the three typical underground car parks for commercial building in Changchun, Shenyang and Beijing, no matter when it was in winter or in summer, mechanical ventilation system are not used for saving operation charges, even if the garages are highly utilized in peak time. But the CO concentrations do not exceed the standard limit. The reason is the great natural ventilation rate due to the"chimney effect"from the buoyancy force. In the paper the CO emitted from cars was taken as the tracer gas to calculate the natural ventilation rate. In summer the natural ventilation rate generated by buoyancy driven flow is significantly lower than in winter, but the hot environment air makes the CO emission rate much lower. Hence, in general, the natural ventilation rate can meet ventilation requirement basically in the underground car parks of high-rise or multi-storey buildings when they are not highly utilized.The heat transfer for underground buildings was analyzed and the fact that heat transfer between enclosed structure and soil can be neglected was presented. The main energy consumption for underground car parks is the ventilation heat load and floor heat transfer load between the underground and the upper building. In most time the car park is not highly utilized and the CO emitted from cars is random, which shows that the significant energy can be saved if the variable air volume and an adjustable ventilation fan are implemented.The CFD method was applied, including discrete governing equations in finite volumes, choice of turbulence model, meshing of calculating zone and definition of boundary condition. Several kinds of mechanical ventilation cases, a natural ventilation mode and two smoke extraction cases were simulated by the CFD.The underground garage model of Changchun Chartering Shopping Center has been simplified to reflect the main characteristics. The simulation was performed according to stable process after inputting corresponding boundary conditions, which is in terms of the feature of the indoor heat rate achieved in summer or in winter. And the average temperatures measured in the actual time interval were adopted as the outdoor environment temperature. The calculating results showed that the natural ventilation rate by buoyancy is much higher in winter than in summer and the air flow direction in winter is opposite to that in summer, which is in accordance with the measured result. The output of the CO concentration and temperature distribution of all measuring points are compared with the measured average values. The results showed that the simulation results are close to the measured ones.The ventilation effects, performance features and adaptabilities of all kinds of ventilation and smoke extraction schemes were presented through simulation and calculation."The mechanical exhaust with natural supply"can save energy, and its ventilation effect is not bad than"combined mechanical exhaust with mechanical supply". Based on the simulation and comparison for the two smoke extraction schemes defined in the regulation, it was found that the"hang wall"can not obviously separate the hot smoke and obstruct its diffusion, and that the scheme of extracting smoke in less than 2000m2 area which is regulated in"Code for fire protection design of car garages, repairing car garages and parks"is better than that in less than 500m2 area regulated in"Code for fire protection design of tall civil buildings".In underground car parks it is found that the CO concentration is same in different vertical height by the experiment and simulation with CFD software. The regulation that"exhaust for underground car park should be arranged beneath ceiling and under floor, the upper area takes 1/3～1/2 total, the lower area takes 1/2～2/3 total"in"Code for building design of garage"is not fit for the truth, which leads inconvenience in design and application.It is the misunderstanding that inducing ventilation system without duct is used extensively in underground garage. According to the cost analyses and the CFD simulation, it was found that inducing ventilation system without duct has not clear advantages in ventilation effect, energy saving and investment and it should not be popularized strongly in underground garages. The ventilation scheme should be selected by concrete considerations.It has been pointed out innovatively that the natural ventilation system can be recommended to use in the underground garages. The natural ventilation rate generated by buoyancy can meet the requirement of extracting the contaminants whenever in winter or in summer in most time (the running rate of the garages is not more than 1). A method to calculate the ventilation rate was presented and emphasizing that the environment temperature has big influence to it. Variable ventilation rate should be adopted according to different seasons and climate areas in our country. Simultaneously a few misunderstandings about the air flow distribution in the underground garages were presented in the paper. These conclusions and results are all presented firstly. They contain the ideas of saving energy and protecting environment, provide the foundation for the engineer to design ventilation systems and favor the project designs effectively. And they have important meaning in keeping with the strategy of protecting environment, saving energy and the sustainable development of our country.