1. Introduction
The design and optimization of structured reactors’ performance are crucial for environmental catalytic processes such as the catalytic combustion of volatile organic compounds (VOCs) or the selective catalytic reduction (SCR) of nitrogen oxides (NOx). It is highly desirable to ensure a large surface-area-to-volume ratio and high heat and mass transfer rates to maximize the catalyst consumption and, at the same time, low flow resistance to reduce pumping costs [
1,
2]. Although significant progress in catalysis has been made, there is still a need to intensify the heat/mass transfer for catalytic reactors to ensure the overall process rate of faster reaction kinetics is not limited.
An effective technique to improve the rate of heat transfer to fluid flowing in a duct, as well as the rate of mass transfer of reagents to the catalyst surface, is the enhancement of fluid mixing. It can be obtained by applying a certain type of catalyst support, which provides some disarray in the flow. However, it would result in an increase in pressure drop. In order to keep the flow resistance at a minimum level, the mixing flow should exist in a region very close to the wall surface of the catalyst carrier.
Many different reactor fillings have been proposed to achieve the above phenomenon. Short-channel structures, which are short monoliths, are the simplest example due to the possibility of modifying their geometry (the ratio of the channel diameter to its length), as well as their arrangement in the reactor (alternating and/or with gaps) [
3,
4,
5,
6]. Solid foams ensure mixing flow, thus enhancing heat/mass transfer and low flow resistance, due to their stochastic structure [
7,
8,
9,
10,
11,
12,
13]. The geometry of periodic open cellular structures is, in some sense, the combination of a foam matrix and a monolith structure, hence they are expected to enable fluid mixing [
14,
15].
Meshes, called also screens or grids, have been used for years to control fluid motion, thus it is possible to influence the direction, speed, or turbulence intensity of the flow [
16,
17]. Kołodziej et al. [
18] investigated knitted and woven wire gauzes because of their wire arrangement. It was proven that gauzes enhance heat/mass transfer with a slight increase in flow resistance. In comparison with other available catalyst supports, gauze seems to be more efficient [
7,
19,
20,
21,
22].
The expanded metal mesh (EMM) is a metal foil that is simultaneously slit and stretched longitudinally into a network of diamond- or hexagonal-shaped holes of uniform size, shape, and regularity [
16,
17,
23]. It comes in four basic types: Raised (or standard), flattened, Gridwalk, and architectural (or decorative) meshes. EMM’s applications include energy absorption, construction, protection–decoration, flow control, filtration, biomechanics, and electrochemical applications [
17,
24]. Expanded metal meshes are produced from a solid sheet or a plate. Carbon, aluminum, or stainless steel and different alloys including copper, nickel, silver, titanium, etc., can be used as materials. No metal is lost in the expanding process. After the process of cutting and plastic deformation, the area of the EMM is up to 12 times larger and its weight is reduced by 80% per square meter in comparison with the original sheet or plate [
17,
25]. The final product is much stiffer and more durable and has solid joints and no seams or welds. Because it is made from a solid sheet of metal, it can never untangle. Even if cut at one or more points, the remaining strand intersections continue to hold.
The flow behavior and its impact on heat transfer and flow resistance around expanded meshes have been studied by several researchers. Saini and Saini [
26] experimentally investigated the turbulent flow in an artificially roughened rectangular duct with a large aspect ratio with an expanded metal mesh as the roughness element. The authors concluded that the use of an expanded metal mesh on the absorber plate of a solar air-heater duct brings the enhancement of heat transfer, depending on the system and operating parameters. The correlations for the Nusselt number and friction factor for the system were also developed. Oshinowo and Kuhn [
17] performed an experimental study of the turbulent flow in a low-turbulence wind tunnel with expanded metal screens. The characteristics of mean velocity, pressure drop, and turbulence of expanded metal sheets were shown. Mustaffar et al. [
24] experimentally and numerically tested the melting of the phase-change material (PCM) via a raised aluminum expanded metal mesh. It was shown that the presence of expanded metal meshes increased the PCM’s effective thermal conductivity, resulting in a reduction in melting time. Mallick and Thombre [
27] evaluated the performance of the passive direct methanol fuel cell (DMFC) and compared it with different combinations of supporting plates and an expanded metal mesh current collector (EMCC). It was found that better results were achieved for the passive DMFC with EMCC, which also facilitates a better distribution of the fuel on the anode catalyst layer and increases the operating temperature. Lafmejani et al. [
25] experimentally and numerically studied the application of expanded metal meshes as flow plates in polymer electrolyte membrane (PEM) water electrolysis cells. It was shown, in particular, that the expanded metal grid behaves as a porous medium although there is a local flow of mixing. The authors measured the pressure loss under different orientations and velocities and found the dependence of measured parameters on the flow direction and size of the pores.
In this work, the internal fluid flow and heat transfer performance of raised expanded metal meshes differing in mesh openings was investigated experimentally and numerically. The effect of mesh geometry and dimensions on flow friction and heat-transfer characteristics in expanded metal grids was analyzed. New empirical equations of Fanning friction factors and Nusselt numbers were developed based on the experimental data. The results achieved for expanded metal screens were compared with packed bed, monolith, and woven wire gauzes in relation to the heat/mass transfer and flow resistance.
4. Conclusions
New formulae to evaluate the heat transfer and flow resistance characteristics of expanded metal meshes were developed. Fanning friction factors and Nusselt numbers were described by three equations dependent on the mesh type. The dimensions of meshes have an impact on the flow and transport characteristics; mesh type B seems to have the most desirable geometry.
Simple CFD models of expanded metal meshes were used to simulate the pressure drop and heat transfer coefficients. Numerical results agree well with experimental data. CFD modelling could be employed as a rapid and effective tool to predict the flow and transport properties of EMMs.
The comparison of expanded metal meshes with the monolith, packed bed, and woven-wire gauzes showed that their flow and transport characteristics are higher than other reactor internals. The Performance Efficiency Criterion (PEC) indicated that EMM type B and wire gauzes can be applied as catalyst carriers for methane oxidation.