Experimental and numerical study of heat transfer in horizontal concentric annulus containing phase change material

Canadian Journal of Chemical Engineering, August, 2008 by Ritabrata Dutta, Arnab Atta, Kumar Tapas Dutta

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The heat flux for the natural convective heat transfer from the surface of the inner cylinder to the molten PCM changes along the surface from the bottom to the top point ([theta] = 0[degrees] -180[degrees]). The heat flux has the maximum value at the bottom and decreases to a minimum value at the top point, that is almost at 180[degrees] (e.g. Figures 11-14). In the case of concentric annulus, as there is virtually no net circulation, the variation of heat flux with position on the surface of the inner cylinder is almost symmetric about the vertical midline. It is observed that with the change in the vertical eccentricity the heat flux value reaches the maximum for [epsilon] = 0.5 in the direction of [phi] = -90[degrees]. On the other hand at the same value of the eccentricity in the direction of [phi] = 90[degrees] the heat flux at the bottom point on the surface of the inner cylinder reaches a very low value. The changes in the angle of inclination of the eccentricity the heat flux variation along the surface of the inner cylinder are not symmetric about the vertical midline due to the presence of net circulation. For a particular angle of inclination when the inner cylinder is in the first and the fourth quadrant the maximum thermal flux value are different. The maximum thermal flux is higher in the case of first quadrant than that in fourth quadrant. It is also observed that there is a shift of angular position for the occurrence of the minimum heat flux for the cases of same inclination angle but at different angle (Figures 12-14).

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Effects of Angle of Inclination of Eccentricity

The net circulation [[psi].sub.0], being the difference between the [[psi].sub.max] values of the "intrusion" and "expanded" zones of the annulus, changes with the inclination angle of the eccentricity. When the inclination angle is -[pi]/2 or [pi]/2, that is, the case of vertical eccentricity, the net circulation is absent or very small. The net circulation is also symmetric along the axis of [phi] = -30[degrees] and has a maximum value. This also indicates the imbalance of fluid convection between the two sides of the inner cylinder is the reason for the net circulation around the annulus. A new phenomenon identified by this study is that when the inclination angle of eccentricity is not vertical, a net circulation is identified around the inner cylinder. For a given [epsilon] the magnitude of net circulation is expected to approach maximum when the angle of inclination is horizontal. The strength of the net circulation reaches a maximum for a given cylindrical annuli with the eccentricity in the range of 0 < [epsilon] < 0.5, for the angle of eccentricity [phi] = -30[degrees] (Figure 15). This is due to the fact that the natural circulation of the molten phase creates an unmelted or solid phase front as an eccentric zone around the molten phase in addition to its eccentric position with respect to outer cylinder. The maximum value of the dimensionless heat flux changes with the eccentricity as well as the angle of inclination of eccentricity (Figure 16). It is observed that the maximum value of dimensionless heat flux reaches the highest value at [epsilon] = 0.5 and [phi] = 90[degrees].