Cordierite thermal conductivity

Catalysis support Preparation of cordierite, thermal conductivity, surface control Normal/... 

The specific heat and thermal conductivity of extruded cordierite substrates are relatively insensitive to wall porosity and substrate temperature. Their average values are [22] ... 

Kiln furniture must be made of materials resistant to considerable temperature cycling in particular, during fast firing the changes in temperature are quite rapid (heating rate up to 30 — 40 ""C min ). A low thermal expansion coefficient and/or high thermal conductivity are therefore required. These requirements are best met by cordierite-mullite materials up to 1300 ""C and SiC-based materials for higher temperatures. 

We consider shaping in Chapter 23, but mention some additional features here. The method used depends on the material. Either brute force or a plasticizer should be used. The classic example is pottery—we mold the clay. Then we have the alumina thermal conduction module (TCM), cordierite honeycombs, Si3N4 fishhooks, and carbide blades for kitchen knives. 

Heat management in monoUth reactors via external heating or cooling is not as effective as in PBRs due to lack of convective heat transport in the radial direction. At this point, the material of construction of the monolithic structure affects the overall performance. Monolith reactors can be made of metals or ceramics. In case of nonadiabatic reactions, metallic monoliths are preferred due to their higher thermal conductivity which partially eliminates the lacking convective contribution. Ceramic monoliths, on the other hand, have very low thermal conductivities (e.g., 3 W/m.K for cordierite [11]) and are suitable for use in adiabatic operations. 

A glass-ceramic material was produced with the formation of the primary crystal phase of cordierite. This glass-ceramic material is distinguished for its high fracture toughness of approximately 2.2 MPa m -, a high degree of hardness (Knoop hardness 700), and thermal conductivity of 37.7 W/(m K). The linear thermal expansion coefficient is 45 x 10" K ... 

In the ensuing calculations, power outputs were computed and eombustion efficieneies were estimated. The wall thermal conductivity was set to — 2.0 or 14 W/mK, thus simulating a eeramie and a metalhc reactor (those values roughly correspond to cordierite and FeCr alloy materials, respectively). Mixture compression was fixed at p = 2.5 bar, the equivalence ratios were q> — 0.40, 0.35 and 0.33, while the channel wall thickness was (5 = 0.1 mm. The fortheoming diagrams were constructed by one-parameter continuation of the mixture inlet velocity Uw, which was increased until a critical value was reached with a marked drop in reactor performance. The main objective was to construct power eurves for a given set of reactor geometry, properties, and inlet conditions. An example of the results obtained can be seen in Fig. 5.3, where temperature and fuel distributions... 

Solid thermal conductivity ks (W/mK) 2 (cordierite), 16 (FeCr alloy)... 

Reactor material, inlet pressure p (bar), equivalence ratio (p, inlet velocity (/in (m/s), surface emissivity e, inlet/outlet enclosure emissivity Easr/eour. ignition time tjg (s) and steady-state time 1st (s). Cases 1-19 pertain to simulations with surface reactions only. Case 17 cordierite denotes a material with thermal conductivity ks = 5 W/mK Cases 20-25 simulations with catalytic and gas-phase chemistry... 

The more favorable start-up times for cordierite are mainly attributed to its lower thermal conductivity. Before ignition, axial heat conduction in the solid is less pronounced for the ceramic material, due to its lower k. Heat generated on the surface cannot diffuse away from the reaction front located near the channel exit at a fast enough rate this leads to the formation of a spatially confined reaction zone (see in Fig. 8.9a the more pronounced hot spot at the reactor rear-end), which in turn promotes faster fuel consumption and leads to faster light-off. This faster light-off is also attributed to the fact that less heat is accumulated in the cordierite compared to the FeCr alloy. In Fig. 8.10, thermal power generated by surface... 

For tigheat transfer mechanism in the channel, enhancing the upstream propagation of the reaction front. A numerical experiment was conducted to quantify the effect of radiation on the apparent thermal conductivity of the solid wall, by recomputing Case 15 and assuming an artificial material (cordierite ) for the microreactor wall, which had essentially the same properties with cordierite in Table 8.1 except for a higher thermal conductivity the value of was increased at 0.5 W/mK steps, until fig and fit assumed values close to the ones of Case 16. For = 5.5 W/mK, ignition and steady-state times for a microreactor without surface radiation, became essentially identical to the characteristic times of a microreactor with — 2.0 W/mK and a surface emissivity of 8 = 0.6 (see Case 17 in Table 8.2). This apparent increase in the thermal conductivity of the solid, however, should not be confused with numerical models employing effective wall thermal conductivity for catalytic monoliths [23]. 

This shift is manifested by a high temperature core in the gas phase, seen in Fig. 8.17b att — 17.5 s the maximum reactor temperatures are further retained in the gas phase till steady state (see Fig. 8.17b at r = 26.4 s). Because the thermal conductivity of the hot gases is approximately two orders of magnimde lower than the thermal conductivity of cordierite, both heat accumulation in the wall and upstream propagation of the high temperature front zone of the solid are hindered by the additional thermal resistanee of the gas phase, resulting in elongated characteristic steady-state times. The lower wall temperatures at the two time instances t = 15 and 17.5 s of Case 21 compared to those of Case 5 (Fig. 8.18) attest to this phenomenon. 

Cordierite. This material possesses a rare combination of properties, i.e., relatively high strength and hardness, low thermal expansion coefficient, high heat conductivity and plasticity it conserves mechanical strength under thermal shocks. Therefore, its application areas are varied. 

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