Heat transfer characteristics of air heater heating elements.
Date
1998
Authors
Kumar, K. Pradeep.
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Abstract
This project is a part of a research programme by Eskom to develop
power station rotary regenerative air heaters that are more compatible with
South African coal properties. Large costs are involved in the replacement of air
heater surfaces due to the erosion caused by the abrasive South African coal ash.
The performance of an air heater L governed by numerous parameters, some of
which (such as erosion) are unpredictable. A laborar: 'v testing facility which
can produce a real-situation environment for air heater research is not only
impractical but also expensive. Hence it has been decided to generate a
computer simulation model of a power station air heater. The various
alternatives for the improvement of existing air heaters will be assessed using
this computer model. Extensive information regarding the influence of various
parameters such as thermal performance, erosion, flow resistance, corrosion,
etc. is necessary as input to the simulation model. Various test facilities have
been commissioned to obtain this information.
This project is an experimental study on the thermal performance of the
regenerative air heater heating elements using a thermal test facility situated at
the Eskom Technology Research and Investigations centre in Johannesburg. The
facility uses the single-blow transient technique to establish the heat transfer
coefficients of various air heater heating elements. Ten different types of heating
elements were tested and the results were analysed .. These test results and
recommendations give useful indications for power station application even
before the final simulation model is available.
The primary objective of this project was to find the heat transfer
coefficient and also to make correlations between Colburn j factor, Fanning
friction factor and Reynolds number. Packs were tested for various air tlow
rates, ranging from Reynolds number 1200 to 6000. The test results were
analysed and the correlations were made. A detailed uncertainty analysis was
done and found that the results are showing less than 7 % error which is
acceptable. The consistency of the test results was tested by a repeatability test
and the results were quite satisfactory. The single blow method used in this
project considered the longitudinal conduction within the material and it can
accommodate any arbitrary inlet fluid temperature history.
A comparative study of the various packs was done and traced certain
trends. The high density packs gave better heat transfer and high pressure drop.
The packs with alternate corrugated and flat plates had lower heat transfer
performance, due to the fact that the tlat plates do not create such turbulence to
the flow to reduce the boundary layer thickness to enhance the heat transfer. In
short the flat plates only add weight to the pack, making it heavier than other
packs.
Based on the correlations and trends obtained from the analysis, some
recommendations are also made. A further modification of the test facility was
recommended to include a wider range of flow from very low Reynolds number
to very high. This is necessary to find whether the packs with flat plates really
perform better for highly turbulent tlow. The non - adiabatic nature of the side
wall has to be considered for better reliability of the results. Some other
recommendations are made to make the testing of packs more convenient.
Description
A project report submitted to the Faculty of Engineering, University of the Witwatersrand,
Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering.
Keywords
Heat -- Transmission.