A series of experimental investigations by using the transient liquid crystal method on the studies related to the convective heat transfer in rectangular channels with different wall material types have been successfully performed. The results show that the local and average Nusselt numbers on the test channel surface for wall material type (II) are higher than those for wall material type (I). The effect of the steady-state air preheating temperature on average Nusselt numbers is not significant, while the average Nusselt numbers increase with increasing Reynolds number in a power of 0.627. Furthermore, two empirical (Nu) over bar correlations for channels with wall material type (I) and type (II) are respectively proposed in this study.