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Abstract
A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-ϵ turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results showed that this method can improve the heat transfer at the expense of higher pressure drop. Evaluating the method showed that using porous media inside the shell, with particular dimension and porosity can increase the heat transfer rate better than pressure drop. Using this method inside the tubes leads to two diverse results: In the first configuration, pressure loss prevails over the heat transfer augmentation and it causes energy loss, whereas in the second configuration a great performance enhancement is observed.
NOMENCLATURE
| B | = | central baffle spacing, m |
| Bc | = | baffle cut,% |
| cp | = | specific heat capacity, J kg−1 K−1 |
| C | = | inertial resistance factor, m−1 |
| Cμ, Cϵ1, Cϵ2 | = | turbulent model constants |
| df | = | fiber diameter, m |
| dp | = | pore size, m |
| dt | = | tube inner diameter, m |
| Dp | = | porous layer diameter, m |
| Ds | = | shell inner diameter, m |
| Da | = | Darcy number |
| e | = | ratio of porous radius to tube inner radius, rp / rt |
| f | = | friction factor, 2 ΔP.dt/ρ.L.um2 |
| h | = | convective heat transfer coefficient, W m−2 K−1 |
| k | = | thermal conductivity, W m−1 K−1 |
| k | = | turbulent kinetic energy |
| K | = | permeability, m2 |
| L | = | tube length, m |
| = | mass flow rate, kg s−1 | |
| Nb | = | number of baffles |
| Nt | = | number of tubes |
| Nu | = | average Nusselt number |
| NuD | = | average Nussselt number |
| Nux | = | local Nusselt number |
| P | = | pressure, Pa |
| Pr | = | Prandtl number |
| q | = | surface heat flux, W m−2 |
| r | = | distance from the center of the tube, m |
| rp | = | porous layer radius, m |
| rs | = | shell inner radius, m |
| rc | = | nonporous region radius, m |
| rt | = | tube inner radius, m |
| R | = | dimensionless distance from the center of the tube, r/rt |
| Rf | = | friction factor ratio, fp/ff |
| RNu | = | Nusselt number ratio, Nup/Nuf |
| Rp | = | porous radius ratio, rp/rs |
| Rc | = | nonporous radius ratio, rc/rt |
| RΔP | = | pressure drop ratio, ΔPp/ΔPf |
| Re | = | Reynolds number, ρ.um.dt/μ |
| Si | = | source term for i-th momentum equation, Pa m−1 |
| T | = | temperature, K |
| Tb | = | bulk temperature, K |
| Tc1 | = | tube-side inlet temperature, K |
| Th1 | = | shell-side inlet temperature, K |
| Tin | = | inlet temperature, K |
| Twall | = | wall temperature, K |
| |u| | = | velocity magnitude, m s−1 |
| u | = | velocity, m s−1 |
| = | mean velocity in i-direction, m s−1 | |
| uin | = | inlet velocity, m s−1 |
| um | = | mean velocity, m s−1 |
| U | = | dimensionless velocity, u/uin |
| V | = | velocity, m s−1 |
| Vsu | = | superficial velocity, m s−1 |
| x | = | axial coordinate, m |
| X | = | dimensionless axial coordinate, x/rt |
| ΔP | = | total pressure drop, Pa |
Greek Symbols
| ϵ | = | porosity |
| ϵ | = | dissipation rate of turbulent energy |
| ζ | = | shell-side heat transfer performance ratio, RNu/RΔP |
| η | = | tube-side heat transfer performance ratio, RNu/Rf1/3 |
| θ | = | dimensionless temperature |
| θm | = | dimensionless bulk temperature |
| μ | = | dynamic viscosity of fluid, Pa s |
| ν | = | kinematic viscosity, m2/s |
| νT | = | turbulent viscosity, m2/s |
| ρ | = | density, kg m−3 |
| ρr | = | relative density, ratio of foam density to solid matrix density |
| σk, σϵ | = | turbulent model constants |
Subscripts
| b | = | bulk |
| c | = | cold-side |
| e | = | effective |
| f | = | fluid |
| h | = | hot-side |
| i | = | in i-direction |
| in | = | inner |
| out | = | outer |
| p | = | porous |
| s | = | shell |
| su | = | superficial |
| t | = | tube |
| wall | = | wall |
Additional information
Notes on contributors
Sepideh Esmaeili Rad
Sepideh Esmaeili Rad is a Ph.D. student in the School of Mechanical Engineering at Sharif University of Technology, Tehran, Iran. She worked on performance enhancement of shell-and-tube heat exchangers for her master's thesis, and received her M.S. in 2012. She obtained her bachelor's degree in 2010 from Sharif University of Technology, where she investigated the air handling units’ energy label.
Hossein Afshin
Hossein Afshin is an assistant professor of mechanical engineering at Sharif University of Technology, Tehran, Iran. He received his M.S. and Ph.D. degrees from Sharif University of Technology. He has been teaching at Sharif University since 2010. He directs the research group of process integration in the Center of Excellence in Energy Conversion. His main research interests include heat exchangers, renewable energy, computational fluid dynamics, and two-phase flow. He is currently working on enhanced heat transfer in heat exchangers. He has co-authored more than 20 journal papers.
Bijan Farhanieh
Bijan Farhanieh is professor of mechanical engineering at Sharif University of Technology, Tehran, Iran. He earned his B.S. from Sussex University at Brighton, England, in 1981. He worked as a project engineer for about 4 years in power generation industries. He received his Ph.D. from Chalmers University of Technology in Sweden in 1991. He directs the research group of process integration in the Center of Excellence in Energy Conversion. His main research interests are thermofluid dynamics, computational fluid dynamics thermal systems, and stochastic processes. He has co-authored more than 80 journal papers. He has also authored three books in physics of turbulence, dynamics of heat fluxes, and viscous fluid flow.