| Sammanfattning: | As a civil gas turbine cools down, asymmetric natural convective heat transfer causes the bottom sector of the rotor to cool
faster than the top; this circumferential thermal gradient can potentially cause the shaft to deflect – a phenomenon called thermal
or rotor bow. Rotor bow is tremendously difficult to predict due
to its dependence on a number of engine design parameters, in
addition to the complex nature of natural convective flows.
A novel experimental facility has been developed to gain further understanding into shutdown cooling of a gas turbine. The
scope of this paper is to quantify the effect of basic design features on natural convective cooling in an engine annulus during
shut-down; specifically the influence of the thermal boundary
wall conditions and the annular diameter ratio. In addition to
this, a low-cost, robust thermocouple probe has been developed
and validated, which allows for accurate temperature measurements in a natural convective boundary layer.
An extensive experimental campaign has been completed.
The key finding is that the local radial wall temperature difference was found to be the most influential parameter on the local heat transfer. Non-isothermal walls did not alter the overall
distribution of the inner wall equivalent conductivity. This was
true for both cylindrical and conical sections. An appropriate
characteristic length for use in the Rayleigh number definition
for both the concentric cylinder and conical sections have been
validated. The conical inner section (5
◦ hade angle) did not affect the overall heat transfer in the range of conditions tested.
Therefore, the mean surface heat transfer for non-isothermal inner and outer profiles, within the range −0.4 < ∆Ra/RaLc < 0.4,
where the thermal gradient is negative in the clockwise from topdead-centre, can be predicted using isothermal correlations for
RaLc < 5.0×105 and Dr <= 1.5.
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