Investigations on pool boiling outside horizontal tube bundles

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

As boiling outside horizontal tube bundle designs can result in a seemingly large number of combinations of heat flux, quality and bundle geometry, it needs to be studied as to how the various permutations might effect the flow path of liquid through a tube bundle in a boiling mode. Thus the probable effect on the heat transfer due to a disruption in the wetting (partial dry-out) of upper tubes in a bundle and the possible influence of a variation in the tube pitch so as to affect the wetting and dry-out characteristics of a tube needs to be established. Experiments conducted for pool boiling in 8 x 3 (eight rows and three columns) plain and coated tubes (Ra = 8.279 μm) bundles for three different inter-tube pitch distances (p/d=1.4,1.7 and 2.0) in an equilateral triangular arrangement using distilled water at atmospheric conditions revealed that the local heat transfer coefficient for a tube in a bundle increased with an increase in heat flux as well as with a decrease in the pitch distance. The coated tube bundles with the minimum pitch (p/d=1.4) exhibited the maximum bundle average heat transfer coefficients. The circumferential variation of heat transfer coefficient for the tubes suggest the lowest values for the upper surface of the tube periphery due to a coalescence of bubbles near the top surface while the highest values were observed nearer to the lower surface of the tube periphery due to the striking bubbles from the lower tubes. The experimental data is best fit to suggest a suitable correlation for the enhancement ratio in local pool boiling heat transfer coefficient (hnpb,local/hbottom tube) for the plain and coated tube bundles taking into account the local void fraction and the p/d ratio where the void fraction is calculated using an iterative procedure beginning with the homogeneous void fraction as the initial guess and iterated until the assumed and calculated values agree within a precision of 0.0001 and valid for heat flux ranging from ~ 12 to 45 kW/m2. The present study did not find any conclusive evidence of partial dry-out and deterioration of heat transfer in the upper tubes for an eight-row tube bundle in the heat flux range and pitch-diameter (p/d) ratio considered.