Non-Intrusive Fluid Flow Measurement Method for Loop Heat Pipes

Abstract

The ability to accurately calculate the rate of fluid flow in a Loop Heat Pipe (LHP) is essential in the analytical modeling of the loop performance. The first step in the process of validating a LHP model is, therefore, to verify the predicted flow rate against the measured value in the actual loop operation. Since the LHP operational characteristics are sensitive to the system pressure drop, any mechanical measuring device utilized as part of the fluid loop will certainly affect the thermal performance, most likely, in a bad way. Accordingly, the U.S. Naval Research Laboratory (NRL) developed a non-intrusive method of determining the mass flow rate in a single-phase section of the LHP transport lines simply by measuring the time rate of change of its wall temperature. Hence, the LHP flow rate can be determined at any time during operation, whether under a steady state or transient condition. A proofof-concept test program was carried out to demonstrate the accuracy and responsiveness of the NRL flow measurement method. Different algorithms were employed to deduce the flow rate from the measured temperatures. Preliminary data assessment showed that the method produced excellent results at various flow settings for either gasor liquid-phase fluid flow.