The gas-solid two phase flow under different operating conditions is visualized in a 19 mm x 114 mm narrow rectangular riser at the fully developed region through a high-speed digital image camera. Based on a calibration between image grayscale and solids holdup, the mean solids holdups under different operating conditions can be calculated from the mean grayscale of the images, the results of which is consistent with previous studies. By using different solids holdup as thresholds to transform the grayscale images into the binary images, the solids holdup distribution under different operating conditions can be clearly visualized. To quantify the solids phase separation, the term "relative dense phase area" is introduced as a parameter to characterize the variation of dense phase. A critical solids holdup value of epsilon(sc) = 0.04 is chosen by carefully examining the variation profiles of the relative dense phase area with solids holdup thresholds, to demarcate the dilute and dense cluster phases. The cluster fraction is then obtained and ranges from 1% to 59% under the operating conditions of the present research. With images divided into three regions along the lateral direction, it is found that cluster fraction at the wall region is higher than that of the core and the middle regions. The cluster fraction is also found to increase with the mean solids holdup value. (C) 2014 Elsevier B.V. All rights reserved.