Numerical Analysis On The Design Of Low-pressure Exhaust Hood Of High-power Steam Turbine Combined With Last Stage

Low-pressure exhaust hood is one component part of steam turbine, which locatesat the of end the turbine. It has two major parts assembled, namely spiral case anddiffuser. Besides, there are stiffening ribs and plates etc inside the low-pressure exhausthood for sustaining, which leads to a complex structure. There are two main purposesfor an exhaust hood, one is the connection between last stage cascade and condenser,which make the exhaust steam flows nearly along the axial direction turn90°and thendischarged into the condenser, leading to a possibility of a layered arrangement for thesteam turbine and condenser, and is propitious to save space; the other propose is toincrease the static pressure of exhaust steam through diffuser, aims to recovering theleaving kinetic energy to improve the efficiency. Since low pressure exhaust hood has agreat influence on steam turbine’s efficiency and performance, it is essential tounderstand its internal structure and flow condition properly for a preparation ofmodification design.In this article a certain type of steam turbine’s low pressure exhaust hood ismodeled and simplified by UG, and commercial software ANSYS CFX is applied toprocess the numerical calculation. To make this stimulation approach the real flowinside the low pressure exhaust hood as much as possible, in the calculation processfull-circle last stage cascade and low pressure exhaust hood are combined as an entirety,and proper wet steam model is applied as working fluid. The calculation result indicatesthat there is a great mutual influence between exhaust hood and last stage cascade,which leads to an uneven flow at inlet of the exhaust hood; the flow inside the exhausthood is complex, with vortexes produced mainly centralize where the steam flow turnsinside the volute and near the diffuser, in which the vortexes near the inner ring ofdiffuser are much more than obviously, these vortex effect the performance of lowpressure exhaust hood a lot and a huge amount of energy is lost as a result.To decrease the flow energy losses inside the exhaust hood, modification designsare processed for volute and diffuser respectively whose energy losses are the largest,while base on the analysis of original exhaust hood flow condition in this article. Forvolute modification designs, influence of front wall inclination on flow condition ismainly considered; while for diffuser modification design, it is one’s focus on the influence of radius size of exhaust hood inner ring’s arc part. Base on the calculationsafter modifications, it is known that in the nine cases including the archetype, the bestperformance on increasing pressure occurs in the original one, while considering totalefficiency and flow conditions at outlet, case9, which has a large radius of exhaust hoodinner ring’s arc part, shows a best performance, therefore when considering the wholeperformance of exhaust hood, case9is respected to be the most predominant one, whichwill provide some guidance on optimization for exhaust hood in practice.