Energy 32 (2007) 1260–1270 Effect of flow maldistribution on thermal performance of a solar air heater array with subcollectors in parallel Rajendra Karwa a,Ã , Nitin Karwa b , Rohit Misra c , P.C. Agarwal c a Department of Mechanical Engineering, Faculty of Technology, Addis Ababa University, P.O. Box 385, Addis Ababa, Ethiopia b Department of Mechanical Engineering, Indian Institute of Technology, Delhi 110 016, India c Department of Mechanical Engineering, Faculty of Engineering, Jai Narain Vyas University, Jodhpur 342 011, India Received 11 October 2004 Abstract This paper presents results of a theoretical study carried out to investigate the effect of flow maldistribution, caused by the manufacturing imperfections and tolerances, on the thermal efficiency of a solar air heater array with subcollectors in parallel. The air mass flow rate, ambient temperature, solar insolation and wind heat transfer coefficient have been systematically varied to study the effect under a wide range of these parameters. The collector length, duct height, and plate emissivity were also changed to study their effect. It has been found that the maximum reduction in thermal efficiency due to flow maldistribution is less than about 3% for an array with a commercial grade finish of duct surfaces and 710% manufacturing tolerance for the duct height. r 2006 Elsevier Ltd. All rights reserved. Keywords: Solar air heater array; Subcollectors in parallel; Flow imbalance; Thermal performance 1. Introduction The collectors in a solar air heater array are often arranged so that the air flows through the collectors in series, parallel, or combined series and parallel mode. Yeh and Lin [1], and Garg and Adhikari [2] showed that the thermal efficiency (ratio of the useful heat gain and the incident solar radiation on the air heater plane) of an array can be improved by operating several subcollectors with identical collector aspect ratios (ratio of collector length to width, L/W) in a series in place of a single large collector with the same total area. A collector module in an array is termed as subcollector of the array. It is to be noted that N-subcollectors in a series basically constitute a high aspect ratio collector while the parallel arrangement of N-subcollectors is equivalent to a low aspect ratio collector. Thus, it is evident that the improvement in the thermal efficiency obtained [1,2] for a number of subcollectors in a series is the result of a decrease in the cross-sectional area of the air duct. This increases the velocity of air and, hence, increases the convective heat transfer rate from the absorber plate to the air flowing through the duct. However, these studies did not consider the effect of the configuration change on the pressure drop through the duct and the pumping power required for propelling the air. Recently, Karwa et al. [3] carried out a thermo- hydraulic performance evaluation of the solar air heater arrays (subcollectors arranged in a series, parallel or combined series and parallel mode) using a mathematical model. Based on the performance evaluation criterion of equal pumping power, they have shown that an array with subcollectors in parallel is about 3–33% better in thermal efficiency than the array with subcollectors in a series. Jones and Lior [4,5] studied the effect of the ratio of tube diameter to manifold diameter, the number of tubes, and the length of the tubes on flow maldistribution in solar water heaters with a vertical orientation of the tubes. In general, they found that the flow-starved tubes would become warmer than other tubes and this causes the onset of natural convection inside the flow-starved tubes (the buoyancy-driven flow). The buoyancy effect tends to diminish the flow maldistribution to some extent. The results of their study further showed that for the tube-to-manifold ARTICLE IN PRESS www.elsevier.com/locate/energy 0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.08.002 Ã Corresponding author. Tel.: +251 1 639016; fax: +251 1 239480. E-mail address: karwa_r@yahoo.com (R. Karwa).