Investigation for the optimal thermohydraulic performance of artificially roughened solar air heaters

Artificially roughened solar air heaters perform better than the plane ones under the same operating conditions. However, artificial roughness leads t o even more fluid pressure thereby increasing the pumping power. Roughness and flow parameters viz. relative roughness pitch p/e, relative roughness h eight e/D and flow Reynolds number Re have a combined effect on the heat tr ansfer as well as fluid pressure (friction factor). Investigation for the o ptimal thermohydraulic performance (i.e. maximum heat transfer for minimum friction loss) of artificially roughened solar air heaters has been carried out. An optimisation parameter known as roughness Reynolds number which co mbines the roughness and flow effect and is expressed as e(+) = e/D root f( r)/2 Re has been considered. Thermohydraulic performance has been defined b y the equation n(thermo) = (St(r)/St(s))(3)/(f(r)/f(s)). It has been found that e(opt)(+) similar or equal to 24 gives the optimal thermohydraulic per formance in such collectors and therefore the optimal thermohydraulic perfo rmance curves [3], for designing such collectors for practical applications are suitable. The Value of optimal thermohydraulic performance has been fo und to be about 71% corresponding to e(opt)(+) = 24. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.