Heattransfer from an oscillated vertical annular fluid column through a porous domain: A thermodynamic analysis of the experimental results

Heat transfer in an oscillating vertical annular fluid column flowing through a porous domain in the single phase or bubbly flow two-phase regime (sub-cooled or saturated nucleate flow boiling) are investigated experimentally and theoretically, in quasi-steady state conditions. Forced oscillations are applied to water via a frequency controlled dc motor and a piston-cylinder device. Heat transfer is from the stationary concentric tubular electric heating element outer surface to the reciprocating flow. The heat transfer in an oscillating vertical annular fluid column flowing in the single phase or in the bubbly flow regime is altered by using stainless steel wool porous medium. For the single phase region of flow, it is understood that, the effective heat transfer mechanism is enhanced and it is due to the hydrodynamic boundary layer which can not follow the core flow. Bubbly (nucleate) flow boiling in oscillating flow is also investigated experimentally and theoretically using a simplified thermodynamic analysis. The onset of boiling temperature is distinctly dropped compared to the pool and flow boiling experiments on polished surfaces due the finned surface effect of the steel porous domain, due to the enhanced mixing of the boundary layer flow and core flow; due to the improvement of apparent surface roughness and due to the alteration of ebullition cycle (bubbles are limited by the cell volume here). The developed correlation predicted cycle-space averaged Nusselt number is shown to be in good agreement with the experimental data. The present investigation has possible applications in moderate sized wicked heat pipes, boilers, compact heat exchangers and steam generators.