A Study of Spontaneous Condensation in an LP Test Turbine

Abstract

Recent advances in computational fluid dynamics (CFD) offer the possibility to predict condensing flows in 3D LP steam turbine geometries. Correct analysis of wetness losses, droplet deposition and other two-phase effects in LP steam turbines requires accurate prediction of the non-equilibrium flow field and droplet sizes.The paper compares numerical results from a 3D, polydispersed, condensing flow CFD code to experimental data measured in a scaled model LP turbine for a range of operating conditions. In order to compare the computed efficiencies with the measured values, a method for averaging non-equilibrium flow fields has been developed. Comparisons are made between computational and experimental results for a series of inlet temperature variation tests where the inlet and exit pressures were kept constant. The steady calculations accurately predict the temperature that the primary nucleation zone moves to an upstream row. Furthermore, the mechanism of condensation as nucleation changes rows is explored and it is shown that initially a significant degree of subcooling is maintained in the inter-blade section and, as a result, nucleation occurs at a relatively low rate in a zone that extends far downstream of the blade's trailing edge. This produces relatively large droplets compared to when nucleation occurs predominantly within the blade passage and is clearly visible in the measured module efficiencies and local flow angles, static pressures and light extinction. The measured variation of efficiency and specific work with inlet temperature is predicted accurately by the computations.It is concluded that steady condensing flow wet-steam * Address all correspondence to this author.calculations are able to predict the location of nucleation and the variation of flow dynamics and performance with inlet temperature accurately. A description of the condensation process as nucleation moves between rows has been given and is consistent with the numerical and experimental results.

Nucleation and wake-chopping in low pressure steam turbines

“…Steady calculations do not predict this type of droplet spectrum and in the present case can underpredict the level of thermodynamic loss in the nucleating stage by up to 40%. The current findings are also consistent with the measurements reported by Chandler et al 31 in a five-stage model turbine. Chandler et al measured the overall turbine efficiency and the attenuation of two frequencies of light within the two-phase flow prior to the last turbine stage.…”

“…Steady calculations do not predict this type of droplet spectrum and in the present case can underpredict the level of thermodynamic loss in the nucleating stage by up to 40%. The current findings are also consistent with the measurements reported by Chandler et al 31 Although only a specific case has been analysed, the effects shown are common to all condensing flows in turbines. Understanding the relation between steam turbine pressure profiles, wake-chopping, thermodynamic wetness loss, and the resulting droplet spectra is necessary to make design changes to minimise total wetness loss.…”

Efficiency calculation on 10 MW experimental steam turbine

“…This turbine configuration is often used when the last stage or group of stages is placed on a special rotor with a brake for defining only the torque of this last group of stages, e.g. [3,4]. The design of flow path efficiency for the situation when wet steam is found in the last or the last four out of five stages is shown in [5].…”

Numerical Investigation of Boundary Layers in Wet Steam Nozzles

“…[1][2][3]) and several three-dimensional CFD codes are now emerging (e.g. [4][5][6][7]), steam turbine designers have not fully incorporated wet steam flow calculations into their design process. There are several reasons why this might be so, but it is sometimes argued that the computational methods have not yet been comprehensively proven.…”