| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系 === 87 === Placing the power cable underground is an engineering tendency in the present days, especially in city areas and industrial zones. The constant heat flux is generated from the electrical resistance of the power cable which lies on the bottom of the conduit, while the concrete wall is adiabatic. This configuration does not permit of steady-state solution of a two-dimensional case. Many theoretical and experimental studies on natural convection in horizontal, eccentric annuli have been carried out.
In most of these studies, a two-dimensional model was used in which the annuli were assumed to be coupled with thermal boundary conditions on the cylinder surfaces specified as either with two constant wall temperatures or one with constant wall temperature while the other with constant wall heat flux (including adiabatic surface). A comprehensive literature survey revealed that published work is largely nonexistent on the three-dimensional eccentric annuli between two horizontal cylinders, where their geometric configurations possess an open end. The existed three-dimensional studies on natural convection were limited to the cavity flow problem.
The boundary conditions for this problem are as follows. The adiabatic condition is given on the outer cylinder (concrete conduit) surface, while a constant heat flux which is specified with the heat dissipated from the power cable is given on the inner cylinder (power cable). Due to the symmetric natural convection of the flow field with respect to the two free ends and to a vertical plane crossing the center of the cylinders, zero-gradient conditions are given there. Thus, the free end consists of inflow (fresh air) and outflow (heated air) at the same plane. This flow configuration leads to that there is no thermal fully developed field at the open end. Instead, the computational domain has to be extended into the outside environment so that the proper boundary condition can be specified. The "zonal grid" method is used to tread numerically this two-zone approach.
In addition to natural convection, mixed convection is studied in this work. In mixed convection, the fluid enters the annuli at one free open end and the other one becomes the outlet plane. For a long enough axial distance in the flow field, the "fully developed" boundary conditions can be reasonably specified at the outlet plane. It is economic to use the "fully developed" boundary conditions at outlet plane as compared to the two-zone approach.
The present work investigates that under what conditions (in terms of Gr/Re2 which represents the ratio of buoyant force to inertia force), the fully-developed outlet boundary conditions cab be properly used in the modeling. Effects of the eccentricity on the surface temperature distribution of the inner cylinder are also investigated in this work.
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