Jie Deng

Heat source boundary conditions study for numerical simulation of performance of flat-tube and multi-louvered fin heat exchangers

In the previous studies on heat transfer performance of the flat-tube and multi-louvered fin heat exchangers, most of the numerical studies generally select a heat transfer unit as the computational domain for the effective realization of CFD (Computational Fluid Dynamics). The majority of the numerical studies usually simply treated the flat-tube water side as fixed wall temperature or convective heat transfer boundary conditions without checking thermal equilibrium. The present study tries to demonstrate the differences of boundary condition settings based on thermal equilibrium between the flat-tube water side and the air side of the whole heat exchangers. Revised numerical method is presented with corresponding judging error and amended parameters for fixed wall temperature and convective heat transfer boundary conditions. And computations with two types of boundary conditions are also compared. The results indicate that the heat transfer performances are nearly the same for both boundary condition amendments. Nevertheless, it is necessary to modify the boundary condition parameters of the flat-tube water side based on thermal equilibrium of the whole multi-louvered fin heat exchangers, for accurately calculating heat transfer quantities of the heat exchangers.

Natural Ventilation of Removing Air Pollutants in Long Railway Tunnels with Diesel Traction on Lhasa-Rikaza Line of China

Diesel traction Locomotives are widely used in railway tunnels in the plateau of China. Harmful gases given out by internal combustion engines must be excluded from the tunnel for human safety on railway. In the present study, nitrogen dioxide (NO2) was taken into account as a representative harmful pollutant of the investigation object. Natural ventilation of diluting the concentration of NO2 pollutant in the long railway tunnels was discussed according to real engineering on Lhasa-Rikaze line of China. One-dimensional unsteady flow models of tunnel ventilation were set up. The transient differential equations of the air velocity in the tunnel were solved by Runge-Kutta method and discrete convection-diffusion equations of scalar NO2 concentration were discretized by controlling volume method and solved by first order upwind scheme. Cases with and without service gallery were discussed in terms of piston wind speed and elapsed time of diluting NO2 concentration. Both positive and negative winds were considered. Based on the feasibility analysis of natural ventilation conditions, it is suggested that natural ventilation can settle for diluting harmful gases in tunnels with tunnel length less than 3 km on Lhasa-Rikaze line. Whereas the tunnel length is larger than 3 km, piecemeal natural ventilation with service gallery isnt recommended because it cant efficiently shorten the time of removing pollutants.

Analysis on the Cross Pollution between Two Openings of the Long Tunnels on Lhasa-Rikaze Line

Among the diesel traction railway tunnels on Lhasa-Rikaze line of China, some of two tunnels are adjacent. When a train gets across a given tunnel, air pollutants extruded from the exit of the tunnel may diffuse towards the entrance of its neighboring tunnel and thus it results in thereof air pollution. The so-called cross pollution can affect the established ventilation scheme of excluding pollutants from the tunnels. Hence, in the present study, interactional influence of typical pollutant nitrogen dioxide in the long tunnel groups was concerned. The flow field and distribution of nitrogen dioxide concentration were considered to be transient. Three-dimensional computational fluid dynamics (CFD) models were established in the space between two tunnels using the state-of-the-art commercial CFD software package Fluent 6.2 and the models were validated. Cases of different distances L between two neighboring tunnel openings were studied and the effect of natural wind speed was also analyzed for the distributions of nitrogen dioxide concentration. It can be concluded that, both of short distance between two tunnel openings and side natural wind can lessen the cross pollution of two neighboring tunnels.