Accurate and inexpensive thermal time-of-flight sensor for measuring refrigerant flow in minichannels

Abstract

Thermal flow meters are good candidates for measuring flow rates in small diameter channels because they can be integrated into compact geometries and consist of only a few simple components. A new thermal time-of-flight (TOF) flow sensor is developed here for measuring liquid refrigerant velocities in minichannels. The sensor consists of a small heater and two downstream thermocouples. The heater is pulsed to induce a temperature rise in the fluid, and the time for the temperature pulse to reach the downstream thermocouples is measured. Insights from the data and a simplified one-dimensional transient model are used to calibrate the device. The calibration relates the time of flight to both the average refrigerant velocity and local temperature gradients in the fluid. The calibrated TOF device can detect refrigerant velocities between 1 and 20 mm s−1, measuring 98.7% of the data to within ±3% of the full scale (±0.6 mm s−1).