Enhanced heat transfer in oscillatory flows within multiple-hole baffled tubes

Abstract

Compound enhancement techniques are considered to be the forefront of heat transfer enhancement. In this work, the combination of active and passive techniques in low-Reynolds number tube flows is explored by means of the superposition of a fully-reversing oscillatory flow into a baffled tube. This arrangement has been employed during the last twenty-years in the so called ‘oscillatory baffled reactors’, focusing on the achievement of plug flow. Little work has been done, however, on the experimental and numerical analysis of the enhanced convective heat transfer that follows the resulting chaotic flow. A standard single-hole baffle geometry has been previously characterized experimentally by some authors, whereas the potential of multiple-hole baffles has not been studied from the point of view of enhanced heat transfer. A numerical investigation has been undertaken to examine the heat transfer augmentation in multiple-hole baffled tubes with fully-reversing oscillatory flow. Different circular baffles with 1, 3, 7, 19 and 43 holes are analyzed, all of them releasing the same total cross sectional area. The flow across these baffles generates a beam of jets which extend downstream and upstream –according to the reversing flow - showing different swirl structures that promote intensive radial mixing and early onset of turbulence. As a consequence, heat transfer between the fluid and the tube wall is significantly enhanced. A circular tube of 25 mm inner diameter has been modeled with 10 baffles uniformly spaced. The simultaneously hydrodynamic and thermal developing flow has been simulated with uniform heat flux as boundary condition in the tube wall, using water as working fluid. The achievement of spatial and time periodicity is thoroughly analyzed prior to the data reduction for the computation of Nusselt number. The time-resolved and time-averaged heat transfer characteristics are presented for an oscillating frequency ranging from f=0.1 Hz to f=1Hz and oscillating amplitudes of x0=𝑑� , 2𝑑�/3 and 𝑑�/3 (where 𝑑� is the inner hole diameter for each baffle). The strong dependency of Nusselt number on the operating parameters of the oscillations is reported. Besides, the positive influence of an increasing number of baffle holes is demonstrated, and a description of the flow structures that induce this heat transfer augmentation is discussed.