The Influence Of Charge-Effect On The Heterogeneous Condensation In Wet Steam Flow

Charge-effect can strengthen mass transfer in steam flow at low degree of supersaturation, this viewpoint has been confirmed in the earliest Wilson cloud chamber experiment. As we know, The process of phase transition as well as mass transfer in the last or the second-last stage of steam turbine mainly takes place in the form of homogeneous condensation(non-equilibrium condensation). Because the nucleation occurs with a certain degree of supersaturation, homogeneous condensation generally causes evident additional thermodynamic loss and othe flow problems. The presence of impurities in steam flow will inccur the heterogeneous condensation, which means the impurities act as the nucleus when vapor molecules condense on them. This kind of mass transfer can reduce the degree of supersaturation when the nucleation process happens while this effect may be weakened as the impurity size decreases. Making the impurities charged could further strengthen the mass transfer in heterogeneous condensation, which may become a new method for decreasing the degree of supercooling and improving the efficiency in steam turbine.At present, the study about the heterogeneous nucleation based on the charged impurities is still less, establishing a flow model suitable for heterogeneous condensation with charge-effect and studying the influence mechanism of charge-effect on the homogeneous/heterogeneous condensation flow may have important theoretical guidance meaning and practical application value.At first, based on the coronary nucleation theory proposed by Fletcher, the free energy change during the heterogeneous nucleation process could be resulted from two changes, i.e. state change and change of phase interface. The computation model for each change has been discussed in this paper. According to the generalized definition of the free energy, the electric displacement as well as the electrostatic energy at different locations around the charged impurities has been studied. The nucleation model for the heterogenous nucleation with charge-effect has also been given subsequentlly. The discussion about the critical radius in the heterogeneous nucleation with charge-effect shows that this critical radius could be replaced with the Kelvin radius when the impurities have a relatively larger size or lower carried charge. Combined with the experimented results base on the flow cloud chamber test, the validation for the above models has been carried out. The calculated result for the hetergeneous nucleation with or without charge-effect both match well with the experiment, and the little difference then results from the nonuniformity of impurity size as well as carried charge.Secondly, based on the volume-averaged method, a specific derivation of the two-fluid model for the wet steam flow with conexistence of homogeneous heterogeneous nucleation has been implemented and a discussion for the calculation of thermodynamic parameters in wet steam flow has also been given. Based on the existing experimental results, the reliability verification for this two-fluid model on both of the homogeneous and heterogeneous condensation have been carried out.The calculation error for the homogeneous condensation is under 8%. In the simulation on the heterogeneous condensation, the pressure distribution match well with the experiment and the maximun error is 5.45%, which exists in the temperature(supercooling) distribution. This implies that the two-fluid model used in this paper has high accuracy.Then, numerical simulation for wet steam flow in Moses`s nozzle has been carried out with consideration of homogeneous condensation and heterogeneous condensation with or without charge-effect. The impurities concentration has decisive impact on the heterogeneous condensation when the impurities have not been charged. Higher concentration of impurities may strengthen the heat and mass transfer in heterogeneous condensation process and has stronger inhibitory effect on the subsequent homogeneous condensation process. The impurities size influences the heterogeneous condensation in two ways. On the one hand, larger size could increase the nucleation rate. On the other hand, smaller size of impurities surrounded by droplet will lead to a faster growth rate. The charge-effect could further increase the nucleation rate of droplets on the impurity surface. However, the charge-effect may also be influenced by the impurity size. Specifically, larger impurity size will weaken the charge-effect.Furthermore, a comparative analysis between homogeneous condensation and heterogeneous condensation with charge-effect has been carried out on the wet steam flow in cascade under different conditions. It could be found that the value of wetness fraction at the outlet of cascade in heterogeneous condensation with charge-effect is not larger than that in homogeneous condensation though the mass transfer occurs later in homogeneous condensation. The wetness fraction at the exit of Bakhtar’s cascade and White’s cascade are both lower in heterogeneous condensation with charge-effect compared with homogeneous condensation. Although the wetness fraction at the exit of VKI-1 turbine stage in two condensation modes are close to each other, the value of wetness fraction is still larger in homogeneous condensation.At the same time, obvious difference of supercooling distribution could also be found between the fields under two condensation modes. The degree of supercooling in the flow field with heterogeneous condesantion is generally lower than that in the flow field with only homogeneous condensation. This difference even reaches 20 K in cascade flow and the local degree of supercooling is generally lower 10 K in the flow field with heterogeneous condensation, which means that the state of the steam is close to equilibrium. In addition, the maximum entropy production under heterogeneous condensation is about 5%, 22.68%, 23.2% lower than that in homogeneous condensation in Bakhtar’s cascade, White’s cascade and VKI-1 turbine stage. Meanwhile, the heterogeneous condensation is favorable for flow deflection, which may improve the through-flow in turbine stage.At last, based on the analysis of the heterogeneous nucleation, a numerical research has been made to investigate ion-induced nucleation in the wet steam two-phase flow in stators of the last stage in a real steam turbine. According to the comparison of the flow field between the homogeneous and ion-induced condensation, the ion-induced condensation process changes the distribution of blade loading. Compared with the original one, the overall blade loading migrate from leading edge to trailing edge the blade. This load redistribution restrains the flow separation around the suction side near the trailing edge and upper end-wall. The ion-induced condensation could also inhibit the development of non-equilibrium state with no wetness fraction increase, while this effect seems to be weaker than the heterogeneous condensation with charge-effect. Additionally, the ion-induced condensation may also improve the flow characteristics and decrease the flow losses.