Solar Engineering of Thermal Processes, 4th Edition

6.21 Flow Distribution in Collectors

Performance calculations of collectors are based on an implicit assumption of uniform flow distribution in all of the risers in single- or multiple-collector units. If flow is not uniform, the parts of the collectors with low flow through risers will have lower than parts with higher flow rates. Thus the design of both headers and risers is important in obtaining good collector performance. This problem has been studied analytically and experimentally by Dunkle and Davey (1970). It is of particular significance in large forced-circulation systems; natural-circulation systems tend to be self-correcting and the problem is not as critical. Weitbrecht et al. (2002) reviewed some earlier studies and presented an analytical approach to the flow distribution problem with experimental verification.

Dunkle and Davey (1970) calculated the pressure drop along the headers for the common situation of water entering the bottom header at one side of the collector and leaving the top header at the other side. Their analysis is based on the assumptions that flow is turbulent in headers and laminar in risers (assumptions logical for Australian and many other water heaters). An example of calculated pressure distributions in top and bottom headers is shown in Figure 6.21.1. The implications of these pressure distributions are obvious; the pressure drops from bottom ...