Numerical study of spacer-induced heat transfer impairment in mixed and free convection heat transfer of water upward flow

Abstract

Spacer effects on mixed and free convection heat transfer of water vertical upward flow in a circular channel are studied numerically. The analyses indicate that the effects of spacer on downstream heat transfer significantly depend on the Bo* number at low flow rates. In particular, under the upward section of heat transfer impairment (HTI) regimes and heat transfer enhancement regimes, the spacer-induced HTI phenomenon appears. The HTC evolution in the spacer downstream is damped oscillation and lower than the smooth channel in some regions. By analyzing the downstream developments of axial velocity and eddy viscosity distributions in detail, it is found that the spacer-induced damped oscillations in the recovery of the downstream boundary layer result in the spacer-induced HTI. The effects of spacer radial structure, blockage ratio, and number are also investigated. The impairment phenomenon exists under both the orifice and disk shape spacer conditions. The maximum impairment degree of the spacer-induced HTI increases in the blockage ratio. Extra spacers significantly delay the HTC ratio valley but only suppress it slightly. All three methods can not eliminate the spacer-induced HTI phenomenon.