Packed beds are widely used in industrial applications, such as catalytic reactors or nuclear reactors. Reducing the pressure drop and improving the heat transfer performance a packed bed is a common research aim. Traditionally, a randomly packed bed is utilized due to the low cost and ease of use. The pressure drop of randomly packed beds, which is calculated by Ergun empirical correlation [
1], is usually much higher than that of other packed beds, such as the structured packed bed. A novel type of structured catalytic reactor packing with a very low channel to particle diameter ratio (between 1.0 and 2.0) called composite structured packing (CSP) is reported by Strangio et al. [
2]. This is a feasible way to achieve the structured packing form. Calis et al. [
3] studied the flow characteristics of these CSP by computational fluid dynamics (CFD) and experiment. Five different channel to particle diameter ratio (
N) packing forms were studied, as shown in
Figure 1a. Their results showed that the pressure drop of these structured packed beds was lower than that of randomly packed beds. At the same time, they fitted a two-parameter pressure drop correlation for different
N by using the CFD results. Rokmes et al. [
4] numerically and experimentally investigated the heat transfer performance of these structured packed beds and gave correlations of Nusselt numbers by fitting the CFD results. Palle and Aliabadi [
5] used direct numerical simulation for a structured packed bed in simple cubic (SC) configuration. They studied the friction factor for both infinite and wall bounded structured packed beds and proposed correlations for modified friction factor. Lin et al. [
6] numerically studied two dimensional transient turbulent fluid flow and heat transfer in a packed sphere bed used in a regenerator furnace. In our group, Yang et al. [
7] investigated flow and forced convective heat transfer in structured packed beds with spherical or ellipsoidal particles with symmetry boundary conditions. Wang et al. [
8] designed a grille-sphere composite structured packed bed (GSCSPB), aiming to achieve the SC structured packing configuration easily, which is similar to
N = 1.00 packing form, as shown in
Figure 1b. Pressure drop and heat transfer performance of GSCSPB have been experimentally and numerically studied. The results showed that the GSCSPB can lower the pressure drop of the randomly packed bed and improve the heat transfer coefficient of the structured packed bed. Hu et al. [
9] numerically studied the GSCSPB design parameters by using Taguchi method. All these works focus on packed beds with smooth particles at low channel to particle diameter ratios.
With different aerodynamic characteristics, a single dimpled sphere has been investigated by many researchers. Some studies show that the dimples on the sphere surface can reduce drag significantly at high Reynolds number. Jin et al. [
10] measured the streamwise velocity above the dimpled surface to get a detailed mechanism of drag reduction by dimples on the surface. They considered that dimples caused local flow separation and large turbulence intensity. As a result, dimples delay the main separation and reduce drag significantly. Aoki et al. [
11] applied the oil film method and particle image velocimetry (PIV) technology to obtain the flow pattern of the stationary and rotating dimple balls. Their results showed that, as the number of dimples became larger and the depth became deeper, the critical region shifted toward the lower Reynolds number range. Smith et al. [
12] used direct numerical simulation to investigate the flow over a golf ball in the subcritical and supercritical regimes. Prediction of the drag coefficient was in reasonable agreement with measurements. At the same time, some researchers applied the dimpled structure to a rectangular channel [
13,
14] or shell and helically-coiled tube heat exchangers [
15]. They all concluded that the dimpled structure could enhance heat transfer, while Kim et al. [
15] found that the inline and staggered dimples showed the highest pressure drop.
Motivated by these researches, we wonder the influence of a series of dimpled spheres in wall bounded structured packed bed on flow and heat transfer performance, while the reference is rare. Since the dimpled surface can be approximately treated as roughness surface, we find an experimentally study about the influence of surface roughness on resistance to flow through randomly packed beds by Crawford and Plumb [
16]. In their work, glass microspheres have been glued to smooth surfaces to obtain specific roughness. The results showed that the pressure drop was substantially increased by the presence of surface roughness. In 2017, Yang et al. [
17] have initially studied the flow and heat transfer in infinite structured packed beds of dimple-particles with SC configuration by numerical simulation. The results showed that, the overall heat transfer efficiency can be improved by up to about 7% compared with the structured packed bed with smooth particles at the same inlet velocity condition.
In the present study, the flow and heat transfer characteristics in wall bounded structured packed beds with smooth or dimpled spheres are numerically investigated. Two different low channel to particle diameter ratios (N = 1.00 and N = 1.15) are considered to investigate the influence of different configuration. The pressure drop and the Nusselt number are obtained by CFD and the overall heat transfer performance of packed bed with smooth or dimpled spheres is compared.