NUMERICAL INVESTIGATION ON TWO-PHASE FLOW CHARACTERISTIC IN THE SEPARATED STRUCTURE SHELL-AND-TUBE WASTE HEAT BOILER

摘要

The shell-and-tube waste heat boiler is a common facility to recover and utilize the energy of flue gas in industries. To improve the ability and efficiency of the boiler, a steam dome is configured above the drum so as to arrange more heat exchange tubes. Simulation and analysis of vapor-liquid two-phase flow across tube bundles arranged in the drum are of vital importance to design and safety operation. Numerical simulation of boiling two-phase flow across tube bundles in the drum was carried out to analyze the shell side thermal-hydraulics. Commercial software ANSYS FLUENT 14.5 was adopted for modeling and computational calculations. The applied modeling approach was validated against experimental results with a good agreement. In order to analyze the vapor-liquid two-phase flow performance under various working conditions, the inlet velocity of downcomer tubes of 3m·s~(-1), 4m·s~(-1) as well 5m·s~(-1) for saturated water were simulated, respectively. The pressure field, flow characteristic, void fraction distribution and heat transfer characteristic were analyzed to have a good knowledge of the boiler operation. The following conclusions have been drawn through analyzing simulation results. (1)The total pressure drop on shell side increased with increasing the inlet velocity of downcomer tubes of saturated water. (2)The velocity of saturated water decreased after flowing into the drum less than z=0.1m as the flow area increasing, and then increased rapidly as the volume of the mixture two-phase flow increasing. (3)The integral average void fraction of the drum decreased as the mass flow rate of inlet saturated water increasing. (4)The HTC (heat transfer coefficient) of the heat exchange tubes varied with the flow direction, which is related to the vapor-water void fraction. The conclusions obtained above can be used as a reference for the design of the separated structure shell-and-tube waste heat recovery boiler.