Enhancement of Direct-Contact Heat Transfer in Concentric Annuli

Abstract

Compared to conventional heat exchangers, direct contact heat exchangers have many advantages especially in an annular geometry; because, the unequal shear stresses exerted on the liquid drops located in the annulus by the inner and outer pipe surfaces cause the deformation of drops and the break-up of drops into droplets, thus the enhancement of heat transfer is favored. This paper is a study of direct contact heat exchange between two immiscible liquids (water and oil), without phase change, in co-current turbulent flow through a horizontal concentric annulus. A correlation exists among the Nusselt number, dispersed phase volume fraction, continuous phase Weber number, annular gap and the ratio of inner radius to outer radius, considering the effects of the physical properties of the phases, the operating conditions and the geometry of the system on the overall volumetric heat transfer coefficient. A discussion of the experimental parameters and a parametric study with a calculation scheme for estimating drop size, shear stress between a drop and the continuous phase, volumetric heat transfer coefficient and the difference between wall shear stresses exerted on a drop are presented in the paper.