I am simulating the transient thermal response of a steam turbine rotor, results of which will be used for subsequent structural analysis for thermal stress calculation. I am not sure how much detail I should model this in.
Right now I have decided that I will model axisymmetric, will model blade root and groove on rotor and heat transfer coefficients (HTC) will vary linearly from inlet to exit. As it is an axisymmetric model I modeled only root of blade and same value of HTC of rotor was applied to the face of root and thermal couplings are used at contact locations between them (figure attached).
My problem arose as I found that blade root is becoming hotter than groove during transient and differential thermal expansion is causing addition load on root, even it is changing the stress pattern of blade groove. I searched the literature on the net, no one models root in such analysis. I also feel same, if I have to design I would not like the blade root to load my rotor groove.
I want to know how much should I model, which way and what boundary conditions should I apply in such a simulation?
Read some text on Heat conduction transfer to get the boundary conditions.
It is not easy to identify the boundary of the disc from your grid. If there is no HTC coefficient applied to the disc then it is clear that the blade root will get hotter since heat flows from the HTC boundary condition to it and from there to the disc.
A completely axisymmetric model is a very strong simplification of reality here, since the blade will block some of the convective heat flux. Also you seem to ignore the weight of the blade in your analysis.
I suggest you do a very simple 3-D model where the blade is modelled like a box of right length, surface area and weight and the platform is present. Then you can apply a uniform HTC on all surfaces which are washed by hot gas.
Dear Michal
Greetings and thanks for your reply.
I feel I did not give the configuration detail in the original post…It is a HP steam turbine reaction rotor…not disc type construction….grooves are machined on a 700 mm diameter cylindrical forging and blades are assembled on it…in the figure only the portion of blade to groove assembly is shown…SAME HEAT TRANSFER COEFFICIENTs (HTC) ARE APPLIED TO THE END PORTION OF BLADE (as I modeled the blade till the bottom of profile only) AND ROTOR SURFACE ALSO.
I agree with your observation that analysis is too much simplified for a complex phenomenon…but during the beginning of design stage I want to do some hand calculations and some simple analysis...once configuration is finalized I will do the detailed analysis….blade pulls are applied in terms of nodal forces .
As you are suggesting for 3D analysis with uniform heat transfer coefficient in the whole zone that are washed by hot steam…in that case there will be only convective resistance (1/hA) will act at unbladed location and convective (to blade) + conductive (of blade) resistance will be there at bladed location and hence definitely the temperature of blade root will be lesser than blade groove (rotor) during the transient…actually I want to assure the same thing only….
You have pointed out correctly that blade will block some convective heat flux…but if HTC at blade profile are higher than the rotor surface there is a possibility that during transient there will be a temperature difference between blade root and rotor…I want to quantify this differenced during transient …so that I can decide my tolerances accordingly and no load come at rotor groove if blade root gets hotter…
Also I did not get any information from literature if steam enters in the clearances of blade root or not…how to simulate here thermal coupling/insulation or something else? please share if some information is available...
With sincere regards
Naveen
Hello,
the question whether steam can enter the clearance depends on the design. Something must be inside the clearances, so steam will enter however if the mass exchange is low, the steam will assume the metal temperature. So the answer to your question depends on whether there is some pressure sink drawing steam from the clearance or not. If there are tight gaps between rotor and blade I would assume that there is low mass exchange.
Hello Michael,
what will be the HTCs in blade groove and blade root if low flow is there...will entrapped steam will work as insulator between them....and can I assume that almost no heat transfer between rotor groove and blade root will be there wherever there is a gap filled with steam between them..
Hello Naveen,
if the steam is trapped I would choose free convection, but with gravity replaced by the centrifugal force field. In any case there will be pure heat conduction and you may compare the heat flux from pure conduction to the one calculated with free convection. If you use a HTC, you also need to calculate the temperature steam cavity (assumed spatially constant) from the sum of all heat fluxes to calculate the steam temperature using first law of thermodynamics. If you model the steam in the cavity as pure conducting medium, this is not necessary of course.
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