ABSTRACT
Experiments were conducted for pool boiling on the outside of 8x3 (eight rows and three columns) plain and coated tube (Ra = 8.279 μm) bundles for three different pitch distances with the distinct objective to study the behavior and the enhancement of boiling heat transfer in horizontal staggered tube bundles (of plain and coated tubes for different equilateral triangular arrangements) with heat flux values ranging from ~12 to 45kW/m2.
The maximum enhancement in the local tube heat transfer coefficient compared to the heat transfer coefficient on the lowermost tube was found to be equal to 5.83 at the seventh tube from bottom in case of plain tube bundle of minimum pitch and corresponding to a heat flux of 14.1 kW/m2, while for the coated tube bundle it was equal to 5.0 corresponding to the seventh tube of the minimum pitch bundle (p/d = 1.4) at a heat flux of 14.2 kW/m2.
The maximum enhancements in pool boiling bundle average heat transfer coefficient as compared to the heat transfer coefficient on the lowermost tube for the plain tube bundle was found to be 3.7 at a heat flux of 14.1 kW/m2 with minimum pitch distance (p/d=1.4), while for the coated tube bundle the same was equal to 3.0 corresponding for the minimum pitch bundle (p/d=1.4) at a heat flux of 14.2 kW/m2.
At higher heat fluxes, coated and plain tube bundles had almost similar bundle average heat transfer coefficients at a given pitch distance while at lower heat fluxes, the coated tube bundles have higher bundle average heat transfer coefficient as compared to that of the plain tube bundle. The coated tube bundles with the minimum pitch (p/d=1.4) exhibited the maximum bundle average heat transfer coefficients. The present study concludes that the bundle factor needs to be considered in the design of flooded evaporators.
