6. Optimization of air flow distribution in a closed industrial space
6.8. Conclusion
A conveyor bridge tempered by ventilation was simulated with CFD. The transient simulations were performed with the RNG k-ε model. Three ventilation system designs were investigated (Design I: the airduct is under the conveyor and grilles are installed into the sidewall of it, the air leaves on the sides of the duct; Design II: nozzle like perforations are in the duct wall and the air is blown directly to the conveyor belt from under; Design III: the airduct is next to the conveyor and the air is blown sideward to the conveyor). The best design was chosen on the basis of the minimal air flow rate needed to maintain relative humidity around the transported hygroscopic material under the specific critical value. The minimal flow rate was determined for each design and it was concluded, that the best design is Design II. As the thermal resistances of the building materials usually applied are low, a few percent reduction in the usually high supply flow rates will result in considerable energy savings, so with a well-chosen geometrical setup the supply flow rates can be decreased to reach an improved energy efficiency level.
In addition, possible duct geometries were determined for three different sets of input variables, from which a duct geometry providing constant static pressure and uniform outflow distribution, can be selected.
OPTIMIZATION OF AIR FLOW DISTRIBUTION IN A CLOSED INDUSTRIAL SPACE
SUMMARY
SUMMARY
The aim of this thesis was to develop a new sizing method for long supply ducts. After extensively studying the available sources, it was concluded, that 1D models can be used for this purpose, even though the fluid flow is complex in these ducts, if experimentally determined coefficients are available to account for the impact of the different phenomena.
A 1D continuous model was developed by which any duct geometry can be determined to maintain constant static pressure, so that the duct distributes the inflowing air uniformly.
The general equation was used as the basis to develop two equations for specific geometries, one for circular cross sections and another for rectangular cross section with constant height and variable width. By using the equations derived, the sensitivity of the duct geometry on different dimensionless parameters was determined. As the circular geometries with varying cross section cannot be manufactured, and therefore a rectangular duct was designed and placed in the ventilation laboratory of the department. The measurements on the duct confirmed, that the outflow is uniform (within the acceptable error limits).
Moreover computational fluid dynamical (CFD) simulations were performed. The simulations for the constant static pressure design (CSPD) confirmed the uniform outflow distribution and the CSPD is better than a duct with constant cross section. Furthermore the features of the flow were also analysed, both for the CSDP and constant cross section ducts.
One of the empirical coefficients usually used in the calculation of the air flow rates at the outlet ports is the discharge coefficient, which can be used to predict the fluid distribution performance of supply ducts. For this coefficient by using experimental results published by other authors a new equation was developed. This equation (with the auxiliary ones) covers a wider range than the equations previously published in the literature.
In addition CFD simulations were performed to determine the sensitivity of the fluid distribution performance on different inlet profiles. It was found, that although the outflow distribution of the duct is influence to a certain extent by the inlet profile, but this can be neglected for practical applications.
The last Chapter of the thesis presented a study on the optimisation of the placement of these long ducts in a conveyor bridge. In this industrial application the air supplied is used to temper the conveyor bridge which is transporting fertilizer. If the parameters of the inside air does not meet with the requirements, due to the hygroscopicity of the material, the quality of the final product deteriorates. It was found by a new method, that an optimal placement of these ducts can result in significant energy saving.
SUMMARY
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