Industrial chimneys are essential components to exhaust combustion gases out of a building. Those gases typically come from fireplaces or other consuming devices. Thus, they are generally exposed to very high temperatures. When a chimney is not correctly cleaned, chimney fire can initiate and rise inside this conduct and temperatures can climb to extreme values for several minutes. For that purpose, the chimneys also have to resist to this type of phenomenon.
Canadian and American standards require chimney manufacturer to test their products with an enclosure made of plywood, to replicate typical condition of installation and usage in a building. When testing, if the temperatures of the wooden enclosure are too high according to standards, the chimney could not be homologated. The manufacturer has to modify his product to obtain a successful design; this could lead to a long process of prototyping and testing. Thus, to avoid such “back and forth” development technique, it is necessary to understand what is controlling heat transfer from the chimneys to the enclosure and its surrounding.
Hence, the main goal of this project is to develop a 1D theoretical model that calculates the average heat transfer (and average temperatures) and use it to predict the behavior of chimney setups. This model can be built fairly easily in a programming software with all existing heat transfer formulas. Unfortunately, for the specific heat conditions and geometrical aspect of the cavity made by the industrial chimney and wooden enclosure, it seems there is no formula that has been developed to calculate the Nusselt numbers for the natural convection happening in this setup.
In order to solve this precise problem, a dimensional analysis of the considered case is initially done. Then, commercial software using finite volumes method (Flow Simulation (FS) that comes with Solidworks CAD software) is used to virtually replicate the aerothermal phenomena. FS can be used to generate data to develop a correlation, which dépends on dimensional parameters found by the mentioned dimensional analysis.
The usage of the finite volume method needed to be validated before it is used intensively. To do so, two chimneys setups, with and without wooden enclosure, were tested to produce experimental data. The temperatures were evaluated at specific places on the setups and data (mainly temperature and CO2) from combustion gases was collected. The need to have those two setups is to isolate different aero-thermal phenomenon happening when chimneys are heated, thus facilitating validation of the numerical approach. According to the results obtained by FS in comparison to experimental data, it seems that the forced flow inside the chimney is particular (since it comes from a burner) and tough to adequately replicate with this numerical tool. A method is described to aids FS to agree with experimental results. When the results from the numerical simulations correspond with experimental data, it is now possible to use this software to analyze the natural convection taking place in the cavity, i.e. the flow in between the wooded enclosure and the chimney, and finally, generate data to develop a formula to calculate the Nusselt numbers for the considered case.
The developed correlation is incorporated to the 1D model and this theoretical model can be used and compared with the previously collected physical experimental data. Again, like the usage of FS, the same conclusion is drawn: the internal forced flow in chimneys seems to be special (exact reasons could not be stated) and existing equations show difficulties to calculate adequate Nusselt numbers in this region. A second correlation is thus developed with physical experimental data to replace the existing formula. Then, the 1D model is re-run and new results from it now show a good agreement with experimental data.
The usage of the mentioned numerical tools requires to evaluate thermo-physical properties, such emissivity and thermal conductivity, of the materials that are used to fabricate industrial chimneys. Therefore, two test benches are developed to measure each of the considered properties for different temperatures ranges. They appear to be simple and reliable when their results are compared to some available data found in literature.
Finally, the numerical tools seem to work well and they could be used in the steps needed to improve or design industrial chimneys products.
Henrichon, C. (Author),
Lamarche (Supervisor) &
Soulaïmani (Co-supervisor),
12 Jan 2012Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering