Local and Average Heat Transfer and Pressure Drop Characteristics of Annularly Finned Tube Heat Exchangers

摘要

Finned tube heat exchangers are important because of their direct impact on energy andmaterial use in petrochemical, power, transportation, and HV AC/R systems. The design ofeffective heat exchangers has direct social, economic, and environmental ramifications. Designimprovements rely on a clear understanding of the flow and heat transfer interactions in thecomplex flowfields which exist in heat exchangers. In this thesis, results are presented fromexperiments conducted to develop a more complete understanding of heat exchangerperformance. Using a novel adaptation of the naphthalene sublimation method, opticallymeasured sublimation depths are used to determine local mass transfer coefficients with veryhigh spatial resolution. Local and average heat transfer data are inferred through the heat andmass analogy. These heat transfer data are also used to numerically compute true finefficiencies. These true efficiencies are compared to the analytical solution of Gardner, whichassumes a constant heat transfer coefficient over the entire fin surface. Pressure drop data arealso presented, and thus a complete measure of heat transfer and pumping power performance foreach finned tube arrangement is provided. Four different annularly finned tube banks areexamined in a Reynolds number range from 5,000 to 30,000: a conventional single-rowexchanger, a baffled single-row exchanger, a staggered two row exchanger and an inline two rowdevice. The heat transfer results are compare quite well to several existing bundle correlations.In contrast to earlier work, these experiments indicate that Gardner's solution is a very goodapproximation to the true fin efficiency over a wide range of conditions. Reasons for thisdiscrepancy between the current study and previous work are presented. The data for the inlineand staggered tube banks surprisingly indicate that the inline configuration may be superior tothe staggered arrangement for high profile fins. Justification for this conclusion is provided bycomparison to other studies and analysis of the local heat/mass transfer data. Finally, this studyprovides the first pressure drop data for gull-wing baffling. The results indicate that theunbaffled geometry provides nearly the same heat transfer with only 20 to 25% of the pressuredrop penalty in the Reynolds number range of interest.