On the Nusselt number definition adapted to natural convection in parallelogrammic cavities

This paper revises the definition to be adopted for the Nusselt number when treating natural convection in closed cavities of parallelogram section. The two-dimensional cavities, filled with air, are formed by two active, vertical and isothermal walls, while the remaining walls are adiabatic. In most works treating this geometry, the Nusselt number employed is based either on the height of the active walls or on the distance between them. Although this definition correctly accounts for the heat exchange phenomena in cavities of rectangular section, we show that it is insufficient for the case of the cavities studied in the present work. Following a numerical 2D analysis based on the boundary element method, that makes use of measurements at the walls, we propose an adapted definition of the Nusselt number in order to maintain its physical sense for the parallelogrammic enclosures.