A method and system of monitoring fluid flow in a wellbore. One or more space-distributed temperature input pulses are induced into at least a portion of a fluid in a housing in the wellbore. The housing has at least one cavity and one or more flow ports. A temperature response pulse, caused by the temperature input pulses, are sensed at one or more locations downstream of the cavity and a flow rate of the fluid through the cavity is determined. The flow rate may be determined based on one or more characteristics of the temperature response pulse, and a Retention Time Distribution (RTD) of the cavity.