Experimental and numerical study of heat and hydraulic characteristics of low head natural circulation loops with single-phase flow of working liquid

A 2D numerical simulation of velocity and temperature fields for laminar low flow regimes have been carried out for laboratory experimental natural circulation loop with vertical electrically heated circular tube as flow up section. Calculations have been done for the case of full-length heating of flow up section at constant heat flux density on the heated wall. The variant of loop design with negligibly small hydraulic losses due to local drag reduction and friction on down comer compared with hydraulic losses due to friction in flow up tube has been considered. Laminar flow regime as the regime of most complex friction factor behaviour in buoyancy driven flows was the subject of the analysis. On the basis of calculated velocity and temperature fields in heated zone the longitudinal change of friction factor and heat transfer coefficients have been determined. In general, according to 2D numerical simulation the wall shear stresses are mainly affected by the change of wall velocity gradient due to practically continuous velocity profiles deformation along the whole heated zone. The form of velocity profiles and the extent of their deformation in its turn depend upon the wall heat flux density and the hydraulic diameter. It is shown that in single-phase natural circulation loop where fluid flow is governed exclusively by buoyancy forces wall shear stresses change along the heated zone in a complex way and friction factor for use in 1D calculations can not be described by simple correlations in the form of ξ = a /Re b . In all calculated regimes including the lowest considered wall heat flux density the Nusselt numbers exceeds that for stabilized forced flow with constant thermophysical properties. After decreasing with the distance from the inlet to the heated section Nusselt numbers achieve minimum values and then start to increase.