Abstract
ABSTRACT A series of experimental investigations with a stringent liquid crystal transient method on the studies related to the convective heat transfer in hollow and porous channels have been successfully performed. For the study on the heat transfer in a rectangular hollow channel, the local spanwise-averaged Nusselt number decreases along the streamwise direction. A symmetric distribution of heat transfer coefficients in the spanwise direction has been verified; and a maximum Nusselt number exists at the center location of the test channel inlet for all the experimental cases. The effect of the air preheating temperature on local and average Nusselt numbers is not significant; while the local and average Nusselt numbers increase with increasing Reynolds number in a power of 0.55. The local and average Nusselt numbers on the test channel surface for material Type (II) are higher than those for material Type (I). These Nusselt numbers for the cases of wall material Type (II) will approach those for the cases of material Type (I) when the Reynolds number increases. Additionally, two new empirical correlations of heat transfer in a hollow channel for wall material Type (I) and Type (II) are respectively proposed. For the study on the heat transfer in a rectangular porous channel, the local Nusselt number increases from the channel inlet to X=0.15-0.20 and then gradually decreases along the streamwise direction. A uniform distribution of local Nusselt numbers in spanwise direction at a specified streamwise location can be found. A maximum heat transfer is generally existed around X=0.15-0.20 for all the experimental cases. The effect of the air preheating temperature on local and average Nusselt numbers is not significant. The local and average Nusselt numbers increase with increasing Reynolds number or decreasing medium porosity. The peak of the distribution of local Nusselt number becomes more sharper when the Reynolds number increases. The medium porosity has a significant influence in the entrance region of the channel, i.e., X<0.5; while its effect is gradually diminished along the streamwise direction in the region of 0.5£X<1.0. No significant deviation on the local and average Nusselt numbers for material Type (I) and Type (II) . Besides, all the ratios of are much greater than unity. Two new correlations of in terms of Re, e and Da for wall material Type (I) and Type (II) are proposed. Furthermore, a concept of the amount of enhanced heat transfer is introduced. Based on all the experimental data in the experiments of porous channels, a new empirical correlation of j/f in terms of e and Re is proposed in the study.