Monday, October 17, 2011

Exhibit Hall B (Minneapolis Convention Center)

Porous media forms an extensive area for heat transfer due to its high

volumetric heat transfer coefficient. This technology is used in solar

collectors to increase its efficiency in utilizing solar energy for drying

grain, home heating, etc., by natural convection. Based on the second law

of thermodynamics, energy efficiency in thermal processing can be

increased by reducing exergy losses due to irreversibilities, measured as

`entropy generation'. In this study, analysis of entropy generation during

natural convection in porous rhombic enclosures with various inclination

angles $\varphi$ have been carried for efficient thermal processing in

solar heating applications. Enclosure is bounded by adiabatic top wall,

cold side walls, isothermally (case 1) and non-isothermally (case 2)

heated bottom wall. Simulations are performed for the range of Darcy

number, $Da = 10^{-5}-10^{-3}$ and Rayleigh number, $Ra = 10^{3}-10^{6}$

for various $\varphi$s ($\varphi=30^\circ$, $45^\circ$ and $75^\circ$).

Entropy generation contours due to heat transfer ($S_{\theta}$) and fluid

friction irreversibility ($S_{\psi}$) are analyzed for both cases based on

their local distribution. At low $Da$ ($Da=10^{-5}$), the entropy

generation in the cavity is dominated by $S_{\theta}$ for all $\varphi$s.

On increase of $Da$ to $10^{-3}$, the fluid flow intensifies and the fluid

flow irreversibility, $S_{\psi}$ also increases for all $\varphi$s. The

maximum values of $S_{\theta}$ are found to occur near the hot-cold

junctions in case 1 whereas that occurs in various locations on the left

wall due to high flow for case 2. Significant $S_{\psi}$ is also observed

in the interior regions due to the friction between counter rotating

circulation cells for $\varphi=45^\circ$ and $75^\circ$ in case 1 and for

$\varphi=75^\circ$ in case 2. The total entropy generation rate

$S_{total}$ is found to increase with $Da$ for all $\varphi$s and the

average Bejan number $Be_{av}$ is found to be less than $0.5$, indicating

the dominance of $S_{\psi}$ at higher $Da$ for all $\varphi$s in both

cases. The total entropy generation ($S_{total}$) is found to be

significantly low for $\varphi=30^\circ$ and high for $\varphi=75^\circ$

at $Da=10^{-3}$ in both cases. It is found that, high heat transfer rate

($\overline{Nu_b}$) with minimum entropy generation ($S_{total}$)

occurs for $\varphi=30^\circ$ cavities at $Da=10^{-3}$ in case 1. The

non-isothermal heating strategy is energy efficient due to its less total

entropy generation ($S_{total}$) besides with low heat transfer rate

(Nusselt number, $\overline{Nu_b}$) due to its less heating effect than

case 1 for all $\varphi$s. Overall, rhombic cavities with

$\varphi=30^\circ$ may be the optimal geometrical design in solar heating

applications irrespective of heating strategy.

**Extended Abstract:**File Not Uploaded

See more of this Session: Poster Session: Thermodynamics and Transport Properties

See more of this Group/Topical: Engineering Sciences and Fundamentals

See more of this Group/Topical: Engineering Sciences and Fundamentals