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Thermal Characteristics of Tube Bundles in Ultra-Supercritical Boilers

In this study, flow and thermal characteristics of tube bundles in ultra-supercritical boilers were analyzed. The local heat transfer around the tube bundles was measured to predict the local temperature distribution and vulnerable positions of the superheated tube bundles. The maximally superheated tube bundles were simulated in the laboratory and local heat transfer was measured by using the naphthalene sublimation method. The experiment was conducted on three lines of tube bundles, all with in-line arrangements. Each line consist of six tubes. The distance in the streamwise direction ( S x /∅) was 1.99 and that in the spanwise direction ( S z /∅) was 5.45. The Reynolds number varied from 5000 to 30,000, which covers a range of different operating conditions. Thermal and stress analyses were conducted numerically, based on the experimental data. The results showed that the flow characteristic changes the local heat transfer of the tube bundles. The flow impinged on the stagnation point of Tube 1 and reattached at 60° of Tube 2. The high heat transfer occurred at those positions of the tube bundles. The temperature and stress distributions on the surface of each tube bundle also varied. The reattachment point on Tube 2 had the highest heat transfer and temperature distribution. That position on Tube 2 was subjected to the highest stress due to the large temperature gradient. This result indicates that Tube 2 of the ultra-supercritical (USC) boiler is the weakest of the tube bundles, changing the pitch of the streamwise direction of Tube 2 is one method to reduce the highest stress in superheater tube bundles in the USC boiler.