A. Boiling outside single tubes in cross flow

(a) Flow patterns

This section deals with the case where the flow past the tube or cylinder is by forced rather than by natural convection. This latter situation is dealt with in Section 192. Photographs presented by Vliet and Leppert (1962a) show very clearly the flow patterns that occur when nearly saturated water flows upward across a uniformly heated cylindrical tube. At moderate heat fluxes, typically around 20% of the critical heat flux, a vapor cavity forms in the cylinder’s wake. Initially this cavity is not continuous along the length of the cylinder, but as the heat flux is increased, the increase in the length of the cavity in the direction of flow results in the formation of a very uniform vapor sheet. An increase of velocity from 0.4 to 1.5 m/s or of tube diameter from 0.254 mm to 4.8 mm also results in a large stable vapor cavity behind the cylinder. Under these circumstances the only liquid reaching the top half of the cylinder is that which is supplied between the vapor bubbles and the heater surface as the bubbles enter the cavity wake near the horizontal diameter. For low heat fluxes, more liquid is supplied than evaporated and the excess is removed by entrainment in the cavity. The critical heat flux is reached when the liquid supplied in this manner becomes insufficient to cool the upper half of the cylinder.

In a parallel study, Vliet and Leppert (1962b) extended their work to include the effect of subcooled water flowing across the heated rod. For low subcoolings (< 16 °C), the flow pattern observed was similar to that for water at the saturation temperature. For greater subcoolings there is insufficient vapor to form a cavity in the wake of the cylinder because of the rapid condensation.

(b) Boiling heat transfer at heat fluxes lower than the critical heat flux