This paper investigates the combined effect of the kiln or brick surface roughness and the brick lattice setting density on the fluid flow and heat transfer characteristics in tunnel kilns. The flow uniformity, pressure drop, convective heat transfer coefficient (CHTC), and pumping power are studied. A high-density setting (HDS), which comprises 768 bricks, and a low-density setting (LDS), which comprises 512 bricks, are tested for kiln boundaries and brick surface roughness levels of 0, 1, 2, 3, and 4 mm. The investigation is conducted using a 3D-CFD model with the k-ω turbulence model. The surface roughness changes from 0 to 4 mm for either kiln walls or bricks while fixing the other. The results show that increasing the tunnel kiln surface roughness from 0 to 4 mm increases the pressure drop of both the HDS and LDS by about 13.5%. It also increases the established CHTC value of the LDS more than the HDS by about 23% for all tested roughness levels. Changing the brick surface roughness from 0 to 4 mm increases the pressure drop and CHTC value for the LDS more than for the HDS by about 10% and 12%, respectively. Additionally, the total heat transfer rate-to-pumping power ratio for the LDS is larger than for the HDS by 17.4% for smooth bricks and 23.1% for the brick roughness of 2 mm, i.e., the brick roughness provides a greater advantage to the LDS. The results confirm that the LDS for rough and smooth bricks loaded in tunnel kilns attains a better brick quality, a higher heat transfer rate, and a lower pumping power than the HDS.

This paper proposes correlation equations of heat transfer and pressure drop for the design and operation of tunnel kilns loaded with lattice brick settings of different geometrical parameters in the form of Nusselt number and friction factor. The developed correlation equations considered parameters that were not investigated in previous studies, such as the relative roughness of the bricks and the stack channels, and they also extended the Reynolds numbers to a practical range that was not covered before in a simple, practical form. The correlation equations are valid for Reynolds number between 125 and 10,200, Prandtl number between 0.68 and 0.73, brick’s relative roughness between 0.23 and 0.93, voidage fraction between 0.48 and 0.653, and the geometrical parameters of the tested lattice brick settings. The achieved correlations of the Nusselt number and the friction factor are well compared with the available correlations in the literature in their valid range of parameters. It is found that Nusselt numbers and the friction factors for low-density are higher than those of high-density settings for all considered parameters except the voidage fraction. The effect of the considered parameters confirms that Nusselt numbers increase and the friction factors decrease substantially with the Reynolds number and slightly with the Prandtl number. At a constant Reynolds number, both the Nusselt number and the friction factor increase as the brick’s relative roughness is increased. Moreover, as the stack channel spacing is increased, the Nusselt number decreases, and the friction factor increases. The voidage fraction of the setting has a monotonic effect on both Nusselt numbers and friction factors. Nusselt numbers for high-density are higher than those for low-density settings as the voidage fraction varies.

The heat transfer performances of ionic liquids [C₄mpyrr][NTf₂] and ionanofluids with Al₂O₃ nanoparticles under a laminar flow regime, and with constant heat flux on the tube wall is numerically modeled and analyzed for three values of initial/inlet temperature and for two Reynolds numbers. Heat transfer characteristics were considered by analyzing the temperature distribution along the upper wall, as well as by analyzing the Nusselt number and heat transfer coefficient. The results obtained numerically were validated using Shah’s equation for ionic liquid. Thermophysical properties were temperature-dependent, and obtained by curve-fitting the experimental values of the thermophysical properties. Furthermore, the same set of results was calculated for the ionic liquid and ionanofluids with constant thermophysical properties. It is concluded that the assumption that thermophysical properties are constant has a significant influence on the heat transfer performance parameters of both ionic liquid and ionanofluids, and therefore such assumptions should not be made in research.

This paper presents the effect of brick and kiln wall roughness on the fluid flow, pressure drop, and convection and radiation heat transfer in tunnel kilns. The surface roughness of 0–4 mm is investigated for bricks and tunnel boundary. Another wall roughness of 10 mm is considered to explore the effect of significant defects in the tunnel boundary. The study is conducted using a three-dimensional computational fluid dynamics (CFD) model based on the finite volume method with the k – ω turbulence model. The convective heat transfer coefficients enhance by 45% and 97%, and the pressure drop increases by 25.1% and 80.4% as the brick roughness is increased from 0 to 1 mm and 0 to 4 mm, respectively. The ratio of heat transfer rate to pumping power reaches its maximum at a brick roughness of 2 mm. These results provide essential knowledge about the acceptable range of brick roughness for manufacturers. As the tunnel boundary roughness is increased from 0 to 1 and 0 to 10 mm, the heat transfer rates increase by 1.34% and 3.88%, while the pressure drops increase by 7.5% and 18.2%, respectively. These results are supportive of scheduling the maintenance of tunnel kilns’ interior structure. Moreover, the enhancement of the radiation heat transfer depends on the brick emissivity and the area ratio of rough to smooth surfaces.