Numerical and experimental investigation of unsteady natural convection in a non-uniformly heated vertical open-ended channel

Experimental and three-dimensional numerical investigations have been undertaken to improve the understanding of the flow and heat transfer phenomena in narrow, open-ended channels similar to those formed by the double skin fa\ue7ades with PV panels. Non-uniform heating configurations in which heat sources alternated with unheated zones on both walls were studied to simulate opaque PV arrays and glazed panes/windows of the building. Heat transfer and flow measurements were obtained for periodicity 1/15 of the overall height of the heated and unheated zones with a heat input of 220 W/m2. Largeeddy simulations were utilized to capture the flow behaviour, heat transfer rates and possible transition to turbulence. A separation of the thermal plumes from the heated walls of the channel has been observed both experimentally and numerically. In the case of the non-uniformly, both sides heated channel these plumes move across the channel and turn into travelling waves leading to significant mixing thereby allowing increases in the heat transfer rates comparing with one side uniformly heated channel. In addition, the staggered position of the heated and unheated zones on the opposite walls of the channel results in a recirculation zone occurred in the unheated area of one of the walls at the channel entrance. This separation contributes to the periodicity of flow within the channel and the lower average temperatures compared with uniformly heated channel. The improved mixing as the result of the staggered heating on both sides of the channel was confirmed by the parameter termed degree of mixing which was significantly higher in the channel heated from both sides. The results of this study reveal that staggered arrangement of PV panels will produce higher mass flow rate of air entrained into the channel which together with enhanced mixing would further reduce the PV surface temperatures and prolong their life span