Convective heat transfer from different brick arrangements in tunnel kilns

A laboratory-scale experiment, simulating a section of tunnel kiln, by scale 1:4, has been designed and constructed. Six different settings of brick arrangement have been experimentally examined for Reynolds numbers from 6000 to 33 000 and with constant heat flux. The measurements were carried out with bricks in the longitudinal direction of flow and transverse to it. Results are reported for local and average convection heat transfer coefficients and for pressure drop across the settings. The results show that the pressure drop and convective heat transfer coefficient strongly depend on the setting pattern. Enhancement of 56% in Nu is obtained as the spacing between bricks increases from 5 to 26 mm for lattice settings at Re=15 000 whereas the column spacing has a moderate effect providing that the roof gap is kept unchanged. Kilns using lattice setting, with equal spacing between columns and between bricks, achieve the largest convection heat transfer coefficient, the shortest production time and provide high measure of energy saving. On the other side, kilns using lattice setting of small column spacing and large number of bricks provide the largest productivity. Correlations obtained for use in calculating pressure drops and convective heat transfer rates in the firing of refractory materials are reported in the form of dimensionless groups. Nusselt numbers for all data are correlated either with Reynolds number and some other geometrical groups or with Reynolds number and friction factor as follows:Nu=0.088 Re0.625sa−2.30ε•sb1.837with correlation coefficient of 0.98Nu=0.081 f1.31 Re with correlation coefficient of 0.86