On the effects of internal heat sources upon forced convection in porous channels with asymmetric thick walls

Chemical Engineering and Processing - Process Intensification

Torabi

Govone

et al. 2018

Self Cite

Transport of heat and mass and the thermodynamics of porous microreactors with thermal diffusion and radiation effects are investigated analytically. The examined configuration includes an axisymmetric, thickwall microchannel with an iso-flux thermal boundary condition imposed on the external surfaces. The microchannel is filled with porous materials and accommodates a zeroth order homogenous chemical reaction. Internal radiative heat transfer is modelled in addition to heat convection and conduction, while the local thermal non-equilibrium approach is taken within the porous section of the system. The transport of species is coupled with that of heat via the inclusion of thermodiffusion or Soret effect. Two-dimensional heat and mass transfer differential equations are solved analytically. The results are subsequently used to predict the thermodynamic irreversibilities inside the reactor and a thorough analysis of local and total entropy generation rates is performed. Also, the changes in Nusselt number, calculated on the internal walls of the microreactor, versus various parameters are reported. It is shown that the radiation effects can impact the temperature of the solid phase of the porous medium and lead to alteration of Nusselt number. It is further observed that the transfer of mass is the main source of irreversibility in the system. The findings are of particular use for the design and analysis of the microreactors with homogenous chemical reactions and can be also used for the validation of computational models.

mentioning

confidence: 99%

“…(35)Finally, substituting Eqs (31). and(35) into Eq (30). gives the dimensionless, two-dimensional concentration field: analytical expressions for the constants appearing in the above equations.…”

mentioning

confidence: 99%

Two-dimensional heat and mass transfer and thermodynamic analyses of porous microreactors with Soret and thermal radiation effects—An analytical approach

Chemical Engineering and Processing - Process Intensification

Torabi

Govone

et al. 2018

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“…The authors separated the contributions of local entropy generation from different sources and compared the irreversibilities from each source. In keeping with other authors [18,27,39,40], Ting et al emphasised the necessity to consider LTNE when there are heat source/sinks in the system. The investigations of Ting et al [37,38] did not include thick walls.…”

Section: Introductionmentioning

confidence: 70%

“…Porous inserts have been repeatedly used for heat transfer enhancement in microchannels, rendering them an essential part of thermal analysis of many microreactors. In general, low heat exchanges between the fluid and porous solid phase [24], internal heat generations [25,26] and short lengths of flow conduits [27] can cause deviation from local thermal equilibrium (LTE) and necessitate local thermal non-equilibrium (LTNE) analysis.…”

Section: Introductionmentioning

confidence: 99%

The effects of exothermic catalytic reactions upon combined transport of heat and mass in porous microreactors

International Journal of Heat and Mass Transfer

Karimi

Yadollahi

et al. 2019

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Microreactors for chemical synthesis and combustion have already attracted significant attention. Exothermic catalytic activity features heavily in these devices and thus advective-diffusive transport is of key importance in their analyses. Yet, thermal modelling of the heat generated by catalytic reactions on the internal surfaces of porous microreactors has remained as an important unresolved issue. To address this, the diffusion of heat of catalytic reactions into three phases including fluid, porous solid and solid walls is investigated by extending an existing interface model of porous media under local thermal non-equilibrium. This is applied to a microchannel fully filled with a porous material, subject to a heat flux generated by a catalytic layer coated on the porous-wall boundary. The finite wall thickness and viscous dissipation of the flow kinetic energy are considered, and a twodimensional analytical model is developed, examining the combined heat and mass transfer and thermodynamic irreversibilities of the system. The analytical solution is validated against the existing theoretical studies on simpler configurations as well as a computational model of the microreactor in the limit of very large porosity. In keeping with the recent findings, the wall thickness is shown to strongly influence the heat and mass transport within the system. This remains unchanged when the symmetricity of the microchannel is broken through placing walls of unequal thicknesses, while deviation from local thermal equilibrium is significantly intensified in this case. Importantly, the Nusselt number is shown to have a singular point, which remains fixed under various conditions. NomenclatureInterfacial area per unit volume of porous media, (m -1 ) Prandtl number Biot number Wall heat flux ratio ′ Modified Brinkman number ′′ Total catalytic heat flux (W m -2 ) Mass species concentration (kg m -3 ) 1 ′′ Lower wall heat flux (W m -2 ) 0 Inlet concentration (kg m -3 ) 2 ′′ Upper wall heat flux (W m -2 ) Specific heat capacity (J K -1 kg -1 ) Reynolds number Mass diffusion coefficient (m 2 s -1 ) Specific gas constant (J K -1 kg -1 ) Darcy number Shape factor of the porous medium Coefficient of thermal mass diffusion (m K -1 kg -1 s -1 ) ̇′ ′′ Volumetric entropy generation due to mass diffusion (W K -1 m -3 ) ℎ 1 Half-thickness of the microchannel to lower wall (m) ′ ′′ Volumetric entropy generation due to fluid friction (W K -1 m -3 ) ℎ 2 Half-thickness of the microchannel to upper wall (m) ̇′ ′′ Volumetric entropy generation in the fluid (W K -1 m -3 ) ℎ 3 Half-height of microchannel (m) ′ ′′ Volumetric entropy generation in the porous solid (W K -1 m -3 ) ℎ Interstitial heat transfer coefficient (W K -1 m -2 ) ̇1 ′′′ Volumetric entropy generation rate from lower wall (W K -1 m -3 )

“…The validity of this assumption has been shown in a number of recent investigations of the systems with similar configurations to that of Figure 1 [19,[35][36][37][38][39], and hence is not further discussed here. In particular, this assumption is very realisable in the limit of low thermal Peclet number, which is frequently reached in microreactors [3].…”

Section: Governing Equationsmentioning

confidence: 93%

Non-Equilibrium Thermodynamic Analysis of Double Diffusive, Nanofluid Forced Convection in Catalytic Microreactors with Radiation Effects

Govone

Torabi

et al. 2017

Entropy

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This paper presents a theoretical investigation of the second law performance of double diffusive forced convection in microreactors with the inclusion of nanofluid and radiation effects. The investigated microreactors consist of a single microchannel, fully filled by a porous medium. The transport of heat and mass are analysed by including the thick walls and a first order, catalytic chemical reaction on the internal surfaces of the microchannel. Two sets of thermal boundary conditions are considered on the external surfaces of the microchannel; (1) constant temperature and (2) constant heat flux boundary condition on the lower wall and convective boundary condition on the upper wall. The local thermal non-equilibrium approach is taken to thermally analyse the porous section of the system. The mass dispersion equation is coupled with the transport of heat in the nanofluid flow through consideration of Soret effect. The problem is analytically solved and illustrations of the temperature fields, Nusselt number, total entropy generation rate and performance evaluation criterion (PEC) are provided. It is shown that the radiation effect tends to modify the thermal behaviour within the porous section of the system. The radiation parameter also reduces the overall temperature of the system. It is further demonstrated that, expectedly, the nanoparticles reduce the temperature of the system and increase the Nusselt number. The total entropy generation rate and consequently PEC shows a strong relation with radiation parameter and volumetric concentration of nanoparticles.