Influence of non-uniform flow distribution on overall heat transfer in convective bundle of circulating fluidized bed boiler

In the paper the results of comparative investigations on heat transfer performance of boiler convective bundle and its additional surface, i.e. membrane water wall are presented. For this purpose the non-uniform flow field was modelled in an isothermal test stand operated in self-modeling mode. Then the heat transfer characteristics of such arrangement were estimated by means of naphthalene heat/mass transfer analogy technique. The bundle samples in the shape of round bars (rods) were cast with naphthalene and placed in 27 locations in the bundle while water-wall-modeling samples were coated with naphthalene by painting. Both types of samples were exposed to cold air flow. The results were then compared to the mean heat transfer performance of the same bundle exposed to uniform flow field. The differences of approximately 10% were noticed. Moreover, the heat transfer coefficients for additional surface were even three times lower than those of the bundle. In view of results obtained in the work, the commonly made assumption of equality of heat transfer coefficients for both the bundle and its additional surface may lead to certain errors in heat transfer calculations and discrepancies between the calculated values of heating surfaces area and later operational needs of steam generator.