Combined convection and other effects in heat transfer in horizontal flows
For many internal flow situations the effect of buoyancy is to cause significant modifications of the internal flow field and heat transfer rate. The present study focuses on combined forced and free convection under laminar flow conditions in horizontal cylindrical ducts. There, the superposition of secondary, buoyancy induced circulations to the basic forced flow, and the presence of peripheral and axial conduction in the duct wall, give rise to a three-dimensional conjugate heat transfer problem. Such a combined convection feature had not been investigated previously. A novel parallel predictive and experimental study of combined convection for laminar flow in cylindrical ducts is carried out here. The finite volume code FLOW3D from Harwell is used to treat the fully elliptic three-dimensional thermal-flow problem. The choice derives from a complete and detailed survey of the numerical techniques used in the context of combined convection. The predictive work relates specifically to a new experimental study, which has the object of obtaining fresh data for combined convection in horizontal duct flow. A 3 m long, 16 mm I.D. copper pipe is used, with uniform peripheral electrical heating. Wall temperature measurements are taken at twelve axial positions. The experiment covers the range of stable mixed convection and strictly laminar flow conditions, with the Reynolds number ranging from 500 to 1000, and the modified Rayleigh number, Raq, from 1x10⁵ to 5x10⁶. The scope of the experiment is to provide data for comparison with numerical predictions. These, in turn, are designed to model the experimental conditions very accurately, including, in particular, the effects of peripheral and axial wall conduction. The study is complemented with various analyses intended to understanding properly both pure forced and natural convection modes. These are investigated separately in the first part of the work. Overall, this work provides fresh experimental and predictive evidence on various features relevant to the onset and the development of buoyancy induced secondary flows in round ducts under heating conditions. More specifically, the effects, of conduction in the duct wall are highlighted, and demonstrated to have a definite influence on wall temperature, and Nusselt number distributions, even in the case of long, thin-walled ducts.