| As the exhaustion of fossil fuel, the necessity of energy saving and emission reduction becomes more important. Compared with subcritical boiler, the supercritical once-through boiler with high efficiency is more popular. However, for the supercritical boiler, the phenomenon of hydrodynamic instability is more likely to happen during the starting process. This phenomenon may cause uneven distribution of the flow in the water wall tube, resulting in the overtemperature of the water wall. So, to calculate the hydrodynamic characteristics during the starting up process of the supercritical boiler water wall and to master the influence law of various influencing factors on the hydrodynamic stability are very necessary. Based on the results, we can put forward adjusting methods and technical proposal to improve the hydrodynamic stability of the boiler. This study has important significance to the improvement of our county’s supercritical boiler operation safety during the low load condition.In this paper, based on the hydraulic calculation method and the nature of the water and water vapor in the international industry standard IAPWS-IF97, the pressure drop calculating model of a single heating water wall tube is established.Moreover, we respectively compile Matlab programs for both the constant pressure operation process and the variable pressure operation process. On this basis, we calculate and study the hydrodynamic characteristics of the1000MW once-through boiler during the start-up operation condition.(1)Choose21different operating conditions and calculate the hydrodynamic characteristics of the water wall when the flow rate fraction is between the5%and35%conditions. Based on the calculation, we analyze the results and come to the conclusion as follows:During the constant pressure operation stage, within the calculating range, the improvement of the pressure, inlet temperature and thermal flux are beneficial to the stability of the hydrodynamic characteristics. The higher they are, the more stable it is. Under the chosen operating conditions, the boiler’s hydrodynamic characteristic curves increase monotonically. However, when the pressure, temperature and heat flux are8MPa、250℃、17kW/m2,8MPa、270℃、17kW/m2and8MPa、290℃.17kW/m2,the hydrodynamic stability is bad during the10%-15%flow fraction, the minimum transfer dc load is higher than20%MCR.If the pressure is too high, the outlet temperature of the water wall tube may be too high, causing the tube to burst; The increase of the inlet fluid’s temperature and boiler’s thermal flux ask for higher start-up flow rate. So, keep them in a suitable range may prevent the overtemperature and hydrodynamic instability at the same time. The influence of the heat flux is obvious. For the given pressure and temperature range, when the heat flux is57kW/m, the minimum transfer dc load are all beyond30%MCR.When the boiler work in the constant pressure condition, for this type of water wall tube, the pressure of the vertical water wall contribute to the hydrodynamic stability. However, the spiral water wall is likely to appear instability.(2) Selected two groups of pressure conditions, three groups of inlet temperature conditions and four heat flux value to calculate the hydrodynamic characteristics of the once-through boiler’s water wall. Also, we analyze the results through three aspects including pressure, temperature and heat flux. The results are as follows: Increasing operating pressure can improve the hydrodynamic stability of the supercritical boiler under variable pressure condition. But when the boiler is working under the selected conditions in the paper, the hydrodynamic line all appear a drop stage. The hydrodynamic stability is lower than that in the constant pressure condition. The smallest transforming load is all beyond30%MCR; Under the several conditions, improving the temperature is not effective enough to improve the hydrodynamic characteristics. The start-up flow rate need to be higher, thus causing energy wasting; Increasing of the heat flux may improve the hydrodynamic stability, but in the selected operating conditions, only when the starting pressure is5MPa,the starting temperature is250℃and the heat flux is17kW/m, the minimum transfer dc load is30%MCR.For other conditions, no suitable heat flux exists to keep the line rising steadily and monotonically; During variable-pressure process, both the spiral water wall and the vertical water wall make a difference in the hydrodynamic characteristics. The effect of the vertical water wall to improve the stability is not obvious at all; The variable-pressure operating mode requires higher fluid flow rate. When the boiler works under the low-load operation, the phenomenon of hydrodynamic multiple values is more easily to occur. So, this mode is not suitable for the start-up stage of the supercritical pressure boiler.(3)According to the actual changes of the boiler’s operating parameters, we calculate the pressure changes by simulating the changing law of the temperature and heat flux. We analyze the results and come to some conclusions. For this1000MW once-through boiler, the fluid mass velocity should keep more than646.3kg/m2.s to assure the hydrodynamic stability of the boiler water wall. The vertical water’s pressure drop contribute to the whole pressure drop. The spiral water wall has the possibility to occur instability before25%flow rate fraction condition. The flow resistance occupies a large part of the whole pressure drop while the weight pressure drop decreases with the increase of flow rate. The acceleration pressure drop takes a very small portion of the whole pressure drop, so we often ignore it during calculation.(4)Select two operating conditions and check the pulsating characteristics at the25%flow rate fraction of the spiral water wall. The results are as follows. When the pressure,temperature and heat flux is11MPa、290℃、57kW/m2,8MPa.270℃37kW/m2and9.7MPa、247.5℃、47.75kW/m2,the pressure drop type pulsation,the density wave type pulsation and the thermal type pulsation won’t happen. |
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