Abstract
This paper studies experimentally the forced convection heat transfer of turbulent flow in a cylindrical pebble bed channel with internal heat generation. Exergy and entropy generation analyses are performed to optimize energy conversion in the system identify the destruction of exergy in the pebble bed channel. Stainless steel spheres are used in stacked pebble bed channel. Internal heating is generated uniformly by electromagnetic induction heating method in metallic spheres. Dry air is used as the working fluid in the process of cooling of the heated spheres. The experiment is performed for turbulent flow regimes with Reynolds (Red) number (based on the diameter of the spheres) in the range of 920–2570, which is equal to Reynolds (Re) number, based on channel diameter, in the range of 4500–10,000. The effects of different parameters, including spheres diameter (d=5.5, 6.5 and 7.5mm), inlet volumetric flow rate (V̇) and internal heat generation (Q) on the forced convection heat transfer, exergy transfer and entropy generation are studied. For second law and exergy analyses, mean exergy transfer Nusselt number (Nue) and entropy generation number (Ns) are investigated. Results show that for a fixed d and Q, the mean exergy transfer Nusselt number (Nue) decreases with the increase of Red number until it becomes zero for a critical Red number. This critical Red number found to be about 1450, 1800 and 2300 for d=5.5, 6.5 and 7.5mm, respectively. Further increase in the Red number, decreases Nue to negative values. It is found for spheres with diameter of d=5.5mm and for a fixed Q, as Red increases, the entropy generation number Ns increases monotonically. While, for d>5.5mm and fixed Q, the entropy generation number (Ns) decreases with the increase of Red number up to a critical Red value that makes Ns to be minimum. Further increase in Red number, increases Ns. It is also found that for Red>1800, among the sphere diameters studied in this work, balls with highest diameters yield the minimum entropy generation in the system.
Highlights
Nuclear energy is an important part of the energy mix, which generates roughly 10% of the world’s electricity, making up around one-third of the world’s low-carbon electricity supply [1]
We extend the work of Meng et al [36] by performing both heat and exergy transfer analyses for a cylindrical pebble bed channel with internal heat generation, using air as the cooling working fluid
For exergy analysis performed in this work we considered few parameters including entropy generation number (Ns), merit function (MF), mean exergy transfer Nusselt number (Nue), lost exergy transfer Nusselt number due to pressure drop (NueP), and exergy transfer Nusselt number due to temperature difference (NueT)
Summary
Nuclear energy is an important part of the energy mix, which generates roughly 10% of the world’s electricity, making up around one-third of the world’s low-carbon electricity supply [1]. Single phase forced convection heat transfer of a water-cooled pebble bed reactor was investigated experimentally by Meng et al [36] for a wide range of Reynolds number from 1000 to 8000, and stainless steel spheres with diameters of 3 and 8 mm They used electromagnetic induction heating to generate heat in the porous channel filled with metal spheres. A number of industrial applications involve internal heat generation such as nuclear reactor, agricultural product storage, electronic cooling, or a solar air heater packed with a porous medium, where the packed material provides the heat transfer enhancement and acts as an absorbing media for the solar radiation [47] It has been well demonstrated analytically by Yang and Vafai [12,48,49] and Vafai and Yang [50] that porous systems with internal heat generation can feature temperature gradient bifurcation. We study the problem of cooling a pebble bed cylindrical channel using air as the working fluid
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