Numerical and experimental studies on heat transfer in circulating fluidized bed boilers | |
| Author | Anusorn Chinsuwan |
| Call Number | AIT Diss. no.ET-08-05 |
| Subject(s) | Boilers--Fuel Heat--Transmission |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering in Energy Technology, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ET-08-05 |
| Abstract | The objective of this work was to study the heat transfer from bed to water wall tubes of commercial circulating fluidized bed (CFB) boilers and enhancement of the heat transfer made possible by adding longitudinal fins on the tube surfaces. The studies conducted were theoretical, numerical and experimental. Available data from the literature was used to develop an empirical model for predicting cross sectional average suspension density in CFB boilers. The model was developed as a function of dimensionless parameters: suspension to solid density ratio (p / ps), Froude number (Frt), the local height from secondary air ports to the total height from the ports to furnace exit ratio (z / H) and the ratio of particle to furnace diameter (dp / D). The model has an accuracy of ±25% when comparing the predicted values to measured values from commercial CFB boilers having capacity of 12MWth -135MWe. The obtained model was used to simulate the heat transfer behavior of water wall tubes at various operating conditions. Throughout this work, the commercial computational codes, FLUENTTM and GAMBITTM, were used as solver and grid generator respectively. The numerical results were used to develop models for predicting the dimensionless temperature and dimensionless heat flux profiles. The models were proposed as functions of the same parameters used to develop the model for predicting the suspension density. By comparing the predicted temperature with the measured temperature from commercial CFB boilers having capacity of 12 -165MWth, it is found that the model has an accuracy within +24.80% and -12.08%. The limits of tube material based on maximum allowable stress and oxidation were developed using the temperature profile. The limits of common tubes used as water walls of CFB boilers are presented in tabular form. In addition, the model for predicting the steam quality in the tubes has been proposed and the tendency for tube burnout under various operating conditions is presented in graphical form. The results show that CFB boilers are safe from tube burnout if their operating pressures are not greater than 20MPa. Experiments were conducted in a cold model circulating fluidized bed to investigate the heat transfer behavior of three tube types: a membrane tube, a membrane tube with a longitudinal fin at the tube crest and a membrane tube with two longitudinal fins at 45" on both sides of the tube crest. The results show that adding longitudinal fins on the tube surface can improve the heat transfer rate but decrease the heat transfer coefficient. There is no significant effect from the fin orientation on heat transfer rate. However, the fin orientation has a strong effect on the heat transfer coefficient of the longitudinal fin portion and the combination of tube and membrane fins portion. In addition, the experimental results show that the heat transfer and heat transfer coefficient ratio of membrane tubes with longitudinal fins to membrane tubes can be estimated as 1.25 and 0.8 respectively. The difference in the heat transfer coefficient estimated by the heat transfer ratio and the measured values from a CFB boiler is within ±10%. Finally, numerical simulations were conducted to compare the heat transfer behavior of the three tube types. It is found that the membrane tube with two longitudinal fins has the flattest profiles for both temperature and heat flux |
| Year | 2008 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ET-08-05 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Kumar, Sivanappan ;Dutta, Animesh (Co-Chairperson); |
| Examination Committee(s) | Jayasuriya, Hemantha P. ;Weerakorn Ongsakul; |
| Scholarship Donor(s) | Faculty of Engineering, Khon Kaen University, Thailand; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2008 |