A method of measuring a flow rate of a fluid flowing in a space between an outer conduit and an inner element in the outer conduit is provided. Ultrasonic waves are transmitted and received through the space between multiple pairs of ultrasonic transducers along multiple propagation paths, respectively. A mean line velocity of the fluid is calculated based on data from each pair of ultrasonic transducers. As such, multiple mean line velocities across the space are obtained. The flow rate of the fluid is calculated based on the multiple mean line velocities.