Abdulaziz, Y. “Impact of Different H/D Ratio on Axial Gas Holdup Measured by Four-Tips Optical Fiber Probe in Slurry Bubble Column”. Journal of Engineering, Vol. 22, no. 2, Feb. 2016, pp. 42-55, http://joe.uobaghdad.edu.iq/index.php/main/article/view/256.