An unbiased method for PRF-shift temperature measurements in convective heat transfer systems with functional parts made of metal

Abstract

The purpose of this study is to provide a PRF-shift method for fluid temperature measurements in convective heat transfer systems, which contain functional parts made of metal. Such measurements are extremely useful to examine and design convective cooling systems for industrial devices. Metals like copper seem inevitable for such applications because no commonly available non-metallic material reaches the same level of thermal conductivity as for example copper. In MRI, electrically conductive parts embedded in the measured sample are known to produce various kinds of errors including errors in the image phase caused by eddy currents. Two methods are compared that can be used to remove the eddy currents induced phase: via the phase difference of two readouts at different echo times and via the sum of the phase of two acquisitions with reverse gradient polarity and same echo time. The latter method, termed Rot-Echo, provides a better measurement efficiency because of higher temperature sensitivity. Also, this technique is able to directly measure the eddy currents induced phase map. After verification in a stationary water experiment, the Rot-Echo method was applied to time-averaged temperature measurements in a Pin Fin heat exchanger, comprised of water flow that is heated by an array of copper pins. The temperature fields acquired in the Pin Fin heat exchanger under stable thermal-hydraulic conditions agreed well with data from temperature sensors. No eddy currents effects were visible in any of the data sets in the vicinity of the copper rods. In conclusion, this study shows that unbiased PRF-shift temperature measurements are feasible in complex flow systems even if metallic material is used for the functional parts.

Keywords: Convective heat transfer; Eddy currents errors; Fluid heat exchanger; Magnetic resonance thermometry; PRF-shift.