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An experimental and computational study of laminar combined natural and forced convection in vertical ducts

Lewis, J.S. (1992). An experimental and computational study of laminar combined natural and forced convection in vertical ducts. (Unpublished Doctoral thesis, City, University of London)

Abstract

Buoyancy forces can exert significant influences on convective heat transfer for internal flows. This study considers some aspects of combined natural and forced convection for laminar flows in vertical ducts. Both computational and experimental approaches are employed.

A common framework is developed for the parabolic equations governing hydrodynamically and thermally developing duct flows exhibiting planar or axisymmetric two—dimensionality. A variable—property, quasi—incompressible fluid is assumed. Although primarily developed for the purpose of studying combined convection heat transfer in vertical ducts, the theoretical development is also applicable to the corresponding natural convection problem and to pure forced flows with or without heat transfer. The analysis caters for numerous possible combinations of duct geometry (circular, concentric annular and parallel plates), thermal boundary conditions (uniform wall temperature or uniform heat flux) and inlet flow profile (uniform, partially developed or fully-developed). A marching procedure for obtaining numerical solutions of the fully—implicit, finite—difference versions of the governing equations is described.

Local heat transfer results are reported from an accompanying experimental investigation of water flowing upward in a uniformly heated vertical tube almost 160 diameters long. The data presented cover a range of inlet Reynolds number from 75 to 1180 and values of the buoyancy parameter Grq/Re from 71 to 2070 (based on inlet bulk properties). The experimental local Nusselt numbers shown exhibit an increase with Grq/Re which is more marked at longer axial distances. In many tests, the variation of the local Nusselt number is characterised by a minimum value at some intermediate axial position followed by a rise in the downstream portion of the tube. Examples are included of transient records obtained of wall temperature fluctuations which have also been observed in previous studies using similar apparatus. The fluctuations, which are generally believed to indicate departure from steady laminar flow, increase in magnitude with both flow rate and heat flux, and appear to be responsible for a marked improvement in the local heat transfer.

Numerical predictions of local Nusselt number and wall temperature are compared with the experimental results for selected conditions. The predictions are restricted to regions where the flow remains unidirectional. The agreement seen is generally good, except in regions where either strong wall temperature fluctuations or upstream axial wall conduction is evident in the measured data, thus precluding a strict comparison with the numerical model which assumes steady laminar flow and uniform heating. The prediction of the Nusselt number minima is particularly pleasing.

The experimental data collected in this study are also compared with published correlations for estimating the convection regime and the Nusselt number.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
T Technology > TJ Mechanical engineering and machinery
Departments: School of Science & Technology > Engineering
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