Convective Heat Transfer in a Moving Bed with Special Reference to Melting

Abstract

Previous workers have been unable to correlate data on forced
convection melting of metals or organic solids in packed beds in
the accepted manner of heat transfer factor or Nusselt number
against Reynolds number. it was recognised that melting in a tower
furnace was restricted to a short zone at the bottom so, to
simplify the problem, Akers (2) simulated the upper zone under
moving bed conditions. His heat transfer coefficients and those of
Norton 26) were as much as an order of magnitude less than those
obtained by other workers using fixed beds. It was considered that
this discrepancy had to be explained before proceeding to the
analysis of melting beds. The present work has shown that it
stemmed from two main causes: dislocations of the bed near the
walls permitted bypassing of the fluid in the regions of high
voidage; with long beds the terminal temperature differences tended
to zero so measurement errors became unusually significant. By
operation with vessel to particle diameter ratios not less than
13 and by selection of conditions to prevent small terminal
differences, data have been obtained for 6, 9 and 12 mm diameter
glass ballotini heated by air which are in close agreement with
published fixed bed results. They are correlated by:
Nu = 0.183 x Re0.834 for 95<Re<1662.

A previous model of adiabatic moving bed heat transfer with
intra-particle temperature distribution has been extended to the
non-adiabatic case. <A new digital computer approach to solution
of the model has showm that a previous hybrid-analogue computer
method was unstable. Digital numerical integration has been used
for analysis of experimental results and for prediction of general
parameters which will greatly simplify the design of moving beds.

Date of Award	1972
Original language	English