Heat transfer enhancement in a vertical tube confining two immiscible falling co-flows

•Heat transfer in falling immiscible fluids inside a vertical tube is modeled.•Performance factors are obtained using a validated finite difference method.•The maximum enhancement factor for (water, mercury) pair is 1.029 fold.•The maximum enhancement factor for (mercury, water) pair is 4.58 fold.•Enhancement factor for (air, mercury) pair can be 2.82 fold under fully developed condition.

Heat transfer enhancement in falling two immiscible co-flows arranged concentrically inside a vertical tube is investigated in this work. The momentum and energy equations of both fluids are solved analytically and numerically. The numerical and the analytical results based on adiabatic and zero-shear-stress interfacial conditions are well matched. A parametric study including the influence of fluids relative densities, viscosities, thermal conductivities, specific heats and the flows relative radii is conducted for various reference Reynolds numbers. Different ranges of the fluids thermophysical properties that augment heat transfer are obtained and discussed. When the (inner, outer) fluids pair is given as (water, mercury), the maximum enhancement factor is found to be 1.029 fold relative to the case when the outer fluid filling the whole volume. Interchanging the fluids of that pair can increase the enhancement factor up to 4.58 fold due to flow rate amplification caused by the decrease in the friction force. The use of (air, mercury) pair can increase the enhancement ratio to above 2.82 fold if the flows are thermally fully developed due to significant reduction in effective viscosity. This work demonstrates that significant heat transfer enhancement is attainable when combining layering of immiscible fluids and flow rate amplification mechanisms.