INFLUENCE OF THE FLOW CONDITION (LAMINAR / TURBULENT) IN THE FLUID TUBE ON THE COLLECTOR EFFICIENCY FACTOR OF A FIN ABSORBER

It is generally accepted that the heat transfer from the fluid tube of a fin absorber to the fluid itself, depends to a great extent on the flow condition. Laminar flow, as compared to turbulent flow, can result in a considerable loss in efficiency which could often be avoided by the appropriate design of the absorber and the absorber arrangement in co-ordination with the actual use. According to the current theory, by optimising the degree of efficiency of a collector, by selecting the appropriate absorber tube, absorber arrangement and the flow rate, the collector output should be improved by several percent, as compared to operation under non-optimal conditions. In general, as an increase in the total friction pressure loss of the collector occurs, limits are set by the practical pump capacity. This study examines the statement derived from the theory of an improvement in the collector efficiency with turbulent flow and the additional necessary pump power is calculated. 1 THEORY AND CONSEQUENCES FOR WORK IN PRACTICE According to [1] the thermal output of a collector with a fin absorber is directly in proportion to the collector efficiency factor F’ and this in turn depends yet again on the heat transfer coefficient fluid – tube hfi. The influence of hfi on F’ is even greater the higher the heat transfer coefficient absorber plate – ambient UL . The heat transfer coefficient fluid – tube hfi depends quite considerably on the flow condition in the tube. hfi can be calculated according to [2], with due consideration to the physical properties of the fluid and the geometry of the tube. hfi –values in the laminar area of about 200 500 W/m2K and of about 1000 7000 W/m2K in the transition area laminar turbulent are attained with corresponding change-overs in-between. Common operating conditions for collectors are in the laminar area (Reynolds’ number Re < 2320) respectively in the transition area laminar turbulent (2320 ≤ Re < 10000). In rarer cases a fully turbulent flow is achieved (Re ≥ 10000). With normal antifreezes the Reynolds’ numbers drop compared to water to about 20 % to 50 % of the values. A rough calculation for commonly available absorbers shows that the collector efficiency factor F’ and, therefore, the collector efficiency η can reveal a dependence on the flow condition of several percent (about 3 8 % rel.). From this it follows that a collector which was measured in the laminar condition and comes more into the turbulent area when in use, produces better yields than corresponds to the test certificate (figure 1); but the reverse is also possible (e. g.: test with water, operating with antifreeze mixture).