Study On The Technologies Of Heat Transfer Enhancement Of The Vertical Gas-fired Boiler

AbThis subject comes from the Zhengzhou City Cooperative Project. Considering a series of problems in the engineering practice of the vertical gas-fired boiler, such as the short gas flue flow, the low convective heat transfer coefficient of the pipe, the higher exhaust gas temperature and the lower boiler efficiency, series of studies are carried out, the specific studies can be summarized in the following four aspects.Firstly, this paper talks about the development of the new strengthened gas-fired boiler smoke pipe. To improve the flue gas flow, increase the convective heat transfer coefficient and heat transfer area of boilers, a new type of enhanced heat transfer smoke pipe is developed in the way that a blind tube winded helical fin is fitted in the new type of pipe and longitudinal corrugated fin is added to the outside. This new type of pipe is studied by numerical simulation and theoretical analysis.Secondly, it is about the numerical simulation research and field synergy theory analysis about the inner pipe. The numerical simulation and theoretical analysis research have been made on the smoke pipe with built-in blind pipe and spiral wrapping outside. The research results are as follows: a spiral in a smoke pipe can change the direction of gas flow, so as to increase effective process of flue gas, reduce the angle between the flue gas velocity field and temperature gradient field, increase the degree of field synergy, and significantly reduce the temperature of boiler exhaust. In addition, when t, the pitch of spiral, is 126 mm, pipe PEC reaches its highest value to 1.46. A blind tube inserted in a smoke pipe occupy the center of the pipe flow area, which force the flue gas flow near the pipe wall, and improve flue gas velocity under the same flow. Under the same number of Re, Nu number of smoke tube and PEC value is 1.86 to2.2.33 times and 1.27 to 1.46 times of light tube respectively, and when the tube diameter is 12 mm, the PEC value maximum of this new pipe is 1.46,, Tube structure is analyzed through field synergy theory. Affected by spiral piece of pitch, the synergy Angle of smooth circular tube is changing within the range of 84.2 °to 87.3 and the average Angle is 89.9°.Additionally, this paper performs Numerical Simulation Study and theoretical analysis of heat transfer enhancement on the smoke pipe outside. The angle, height, pitch and the structural forms of longitudinal fins are researched. The result shows that finned pipe works best when the angle is 0°,the height is 9mm and the pitch is 7.44 mm. When the height reaches 36 mm, total heat exchange amount of finned tube stop increasing. Compared the heat transfer effect between the ordinary longitudinal fins and the corrugated longitudinal fins, the result indicated that the total heat of ordinary longitudinal fins is 2.02 to2.09 times higher than the corrugated longitudinal fins. When this two kind of fins occupy the same heat transfer area, the total heat of ordinary longitudinal fins is 1.29-1.32 times higher than the corrugated longitudinal fins when they occupy the same space. Overall, the heat transfer performance of ordinary longitudinal fins is better than the corrugated longitudinal fins.Finally, the research for whole structure of the vertical gas- fired boiler has been researched. The new strengthened smoke pipes are used in gas-fired boiler. Calculations show that the quantity of smoke pipes are only two fifths to the original, and the boiler volume is decreased, the water flow rate is 2.3 times than original at the same mass flow rate and the heat forms turns natural convection into hybrid heat which contains natural convection and forced convection, and the convective heat transfer coefficient increased to 2.15 times. In the gas side, combined the effect of blind pipe and spiral pieces, the convective heat transfer coefficient of gas increased to 2.42 times and flue gas flow increased to 1.26 times. However, the growth rate of the flue gas flow lower than flue gas velocity. A dual returned vertical gas- fired boiler is developed based on the traditional, which aimed to increased to the path of flue gas. The calculation results show that the path of flue gas in the new type of boiler is 2.68 times to the traditional. Convection heat transfer and radiation heat transfer are enhanced.The research has a better effect on improving the efficiency of vertical gas-fired boiler and significantly reducing boiler exhaust gas temperature, and laying a good foundation for the future practical application of engineering.