A computer programme has been prepared in order to simplify studies of longitudinal air flows in tunnels, and the related problems of pollution concentrations in tunnels and emissions of vitiated air. The results are given as air velocity and pollution concentration along the tunnel. The programme calculates steady state conditions. The designer must give the relevant parameter values for the calculations. Unfortunately, values given in the literature are contradictory even for such fundamentals as friction factors, drag coefficients for cars and, for instance, the shortest acceptable distance between groups of jet fans. A short discussion of these problems is given in the paper. The programme has proved itself to be a useful tool when studying alternative ventilation concepts for a road tunnel, the relative influence of the uncertainty of parameter values and different traffic flows.

INTRODUCTION

Ventilation is a vital feature in the design of road tunnels because of the exhaust gases from cars and lorries. Congested traffic in particular makes large ventilation air flows necessary. Often a fairly large proportion of the total tunnel square area is used for air ducting, which makes ventilation rather expensive. The trend over the years has been to reduce the accepted level of pollution. Required ventilation air flows have remained about the same, although the emissions from cars have been greatly reduced during the last ten years. During this period there has also been growing concern about the pollution problems caused by vitiated air leaving the tunnel openings or ventilation shafts. It is perhaps possible that the desire to avoid vitiated air leaving a tunnel opening, especially in urban areas, will lead to the acceptance of higher pollution concentrations in the tunnel, at least when traffic is flowing freely and the time people are exposed is therefore short, in order to avoid high running costs for the exhaust air system. The modern part of the history of road tunnel ventilation started in the twenties with the Holland Tunnel in New York. This tunnel had a full transverse ventilation system, which means that fresh air is supplied and vitiated air exhausted by continuous ducting over the full length of the tunnel. This is naturally a very expensive system but the one most independent of wind forces, changes in the stock of cars and so on. The Mersey Tunnel in Liverpool, another river tunnel, is an example of half-transverse ventilation. The exhaust air ducting has been omitted and the exhaust air flow is concentrated to two points close to the river banks, but well within the tunnel. As a result of bad experience elsewhere of also having the supply air concentrated to certain points (in certain conditions stagnation can occur between a supply air shaft and an exhaust air shaft, resulting in no ventilation of that part of the tunnel), the supply air ducting is, however, continuous over the full length of the tunnel. The supply air is usually brought into the traffic space at a low level in order to give drivers and occupants air as fresh as possible at breathing level.

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