TED-AJ03-522 THERMAL CONVECTION IN A POROUS NATURAL CIRCULATION LOOP

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Natural circulation loops have many applications in solar water systems, geothermal systems and nuclear reactors. In the last decades the stability of the thermal convection in the loop has been a major research topic. The research is evoked by an experimental observation by Creveling et al. [1]. In a Newtonian flow they found a Lorenz-like chaos. Because this unstable flow is undesirable in engineering, many methods were tried to avoid it. As a promising choice to obtain stable flow, the feasibility of filling the loop with porous media are opening for comprehensive examination. This article studies the natural convection and its stability in a porous loop in circular geometry (see Fig.A-1). The porous flow differs much from the Newtonian flow due to the changes of the thermal and hydraulic properties of the fluid. By selecting three variables, that temperature differences T_3-T_9,T_6-T_<12> and flow velocity w, and substituting them in the governing equations, we proposed a one-dimensional model to formulate the flow stability. The model suggests that the flow stability depend largely on an equivalent Prandtl number Pr'. The global flow can probably be chaotic only if the Pr' is finite. As an instance of infinite Pr', the Darcy flow should be globally stable. The flow behavior was experimentally studied afterwards. The apparatus shown in Fig. A-1 was fabricated by a glass tube filled with water saturated porous matrixes. To investigate the effects of Pr' two kinds of porous media were tried; one is made of glass spheres and the other of copper spheres. The shape and the boundary condition are the same as Creveling et al. [1]. To examine the feasibility of one-dimensional formulation, the cross-sectional profiles of fluid temperature were firstly measured. The profiles argue that the flow is approximately one-dimensional. Under the experimental limitation, the porous flow was found always stable in contrast to the Newtonian flow. The damping process of disturbance to flow field was direct at small heating power and fluctuant at large one. The results agreed with the conclusions of the model. On flow in a porous medium with large thermal conductivity, the effects of axial heat conduction are considerable. Although it was stable, the porous flow was much slower than the Newtonian flow under the same condition. As a remarkable comment for future work aiming to find unstable flow, it is suggested that high pressure should be introduced into the fluid so as to obtain single-phase flow at much larger heating powers.
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TED-AJ03-522 THERMAL CONVECTION IN A POROUS NATURAL CIRCULATION LOOP

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