Investigation On The Heat Transfer Enhancement Of Serrated Fin Tube Banks

Serrated fin tubes have the advantages of easy manufacturing, the larger finnedratio and the enhanced heat transfer in comparison with the conventional solid fintubes, and have been applied in various large scale heat exchangers with flue gasemissions, such as the combined cycle heat recovery steam generators (HRSGs), etc.However, the number of the published researches on the heat transfer and pressuredrop of serrated fin tube banks is very limited, and the experimental results of thedifferent researchers are mostly inconsistent on the effects of various structure factors.Such status quo hampers the efficient use and widespread application of serrated fintube. Based on the theoretically analyses of the principles of the heat transferenhancement of serrated fin tube banks, this paper performed a comprehensiveinvestigation on the thermal and hydraulic characteristics of serrated fin tube bankswith staggered layouts by means of a lot of experiments and numerical simulations.Twelve tube banks with different tube spacing and fin pitch were tested in a hightemperature circulating wind tunnel, resultingly obtained the respective effects oftransversal tube spacing, longitudinal tube spacing and fin pitch on the thermal andhydraulic characteristics of serrated fin tube banks in the range of Renolds numberfrom4000to30000. Based on the test data from this paper, a set of heat transfer andpressure drop correlations were presented for the related engineering design orcalculation. Then, ten testing banks were simulated using the Fluent software tovalidate the numerical methods of this paper by means of comparing the numericalresults of the heat transfer and pressure drop of tube banks, respectively, with thecorresponding experimental results. The comparison results show that the numericalresults coincided with the experimental results satisfactorily. Afterwards, twenty-eighttube banks were designed and simulated in the range of Renolds number from10000to40000, resultingly obtained the effects of almost all related structure factors on thethermal and hydraulic characteristics of serrated fin tube banks. Among which thesegment height, segment width and segment twist-angle are the particular structureparameters of serrated fin tube, this paper studied the effects of those structure factorson the thermal and hydraulic characteristics of tube banks for the first time. Moreover,this paper compared four types of helically finned tube banks by means of numericalsimulation, they were the solid fin, serrated I-foot fin, serrated L-foot fin and serratedtwist-fin tube banks, respectively. Based on the evaluation methodologies of the overall thermal-hydraulicperformances for the compact heat exchangers, this paper presented three evaluationmethodologies of the overall thermal-hydraulic performances for the heat exchangerswith crossflow tube banks, to compare the frontal flow areas, heat transfer areas andspatial volumes of tube banks, respectively, in a certain heat duty and transportingpower consumption of the tube outside gas. In this paper, the effects of structurefactors were compared and analyzed not only on the heat transfer and pressure drop oftube banks, but also on the overall thermal-hydraulic performances of tube banks,hence the conclusions drawn by this paper can directly guide the structureoptimization of serrated fin tube banks.From the experimental and numerical investigations of this paper, the effects ofvarious structure factors on the overall thermal-hydraulic performances of the serratedfin tube banks with staggered layouts were founded as follow:(1) In certain longitudinal tube spacing, a decrease in transversal tube spacingcould result in a decrease in the heat transfer areas and spatial volumes of tube banks,while would increase the frontal flow areas of tube banks simultaneously. In certaintransversal tube spacing, there would be an optimal longitudinal tube spacing tominimize the frontal flow areas and heat transfer areas of tube banks, while the spatialvolumes of tube banks always increased as the longitudinal tube spacing increased.(2) An equal propotional increase in the transversal and longitudinal tube spacingwould result in a moderate decrease and increase in the frontal flow areas and heattransfer areas of tube banks, respectively, but a significant increase in the spatialvolumes of tube banks. It means that the compact tube layouts is of benefit todecreasing the heat transfer areas and spatial volumes of tube banks, while the shapesof tube banks will tend to a pancake having a large frontal flow area and a short flowlength due to the accompanying increase in the frontal flow areas of tube banks.(3) In certain diagnol tube spacing, an increase in the ratios of transversal-diagnolfree flow areas of tube banks would result in an increase in the heat transfer areas andspatial volumes of tube banks, while the frontal flow areas of tube banks were firstlydecreased and then increased as the ratios of transversal-diagnol free flow areasincreased. So a small ratios of transversal-diagnol free flow areas is of benefit todecreasing the heat transfer areas and spatial volumes of tube banks, but it should beconcerned that the spatial volumes of tube banks will be increased dramatically as the ratio of transversal-diagnol free flow areas is very small.(4) An increase in fin height would result in an increase in the frontal flow areasand heat transfer areas of tube banks; there would be an optimal fin height tominimize the cost of finned tubes in view of the cost deffierence between fin and basetube. The spatial volumes of tube banks tended to decrease as the fin height increased,whereas the decrease extents were decreasing as the fin height increased. Thevariation of fin pitch had an insignificant effect on the frontal flow areas and heattransfer areas of tube banks, while the spatial volumes of tube banks would increasedistinctly with an increase in fin pitch.(5) An increase in segment height would result in an obvious increase in thefrontal flow areas and a slightly decrease in the heat transfer areas of tube banks.Within the allowable limits of the manufacturing process of serrated fin tube, themoderate decrease in segment height could make a further decrease in the spatialvolumes of tube banks. The variation of segment width had an insignificant effect onthe frontal flow areas, heat transfer areas and spatial volumes of tube banks.(6) In the range of segment twist-angle from0°to60°, an increase in segmenttwist-angle would result in an increase in the frontal flow areas of tube banks. In thepractical range of segment twist-angle from15°to30°, the frontal flow areas ofserrated twist-fin tube banks would be increased approximately8%~12%incomparison with the serrated fin tube banks without fin twisting. Both the heattransfer areas and spatial volumes of serrated twist-fin tube banks decreasedapproximately5%as the segment angles increased from0°to30°, and then basicallyremained unchanged as the the segment angles increased from30°to60°.(7) Compared with the solid fin tube bank in a certain tube spacing and thestructural parameters of finned tube, the frontal flow areas of serrated I-foot fin,serrated L-foot fin and serrated twist-fin tube banks increased approximately10%,23%and30%, respectively; and the heat transfer areas of those corresponding tubebanks decreased approximately9%,6%and12%, respectively. The spatial volumes ofthe solid fin, serrated I-foot fin and serrated twist-fin tube banks were almost same,while the spatial volumes of the serrated L-foot fin tube bank increased approximately18%relative to the solid fin tube bank.