Investigation on transient temperature of a reciprocating compressor based on a two-thermocouple probe

Measuring transient pressure and temperature simultaneously inside a compressor can lead to a thorough characterization of the thermodynamic process in the compressor. However, the rapid changes in airflow temperature make it difficult to measure these conditions precisely, partly due to large thermal inertia of common thermometers such as thermistors and thermocouples. Measurement of rapid temperature changes requires thermometers that have a time constant of a few microseconds or less. Therefore, in this study, two fine wire thermocouples 18 and 35 μm in diameter, respectively, were prepared in advance. A two-thermocouple probe consisting of these two fine wire thermocouples was then used to measure the transient temperature of airflow in the working chamber of a reciprocating compressor. The temperatures detected by the two-thermocouple probe were used to reconstruct the true temperatures. Mean and fluctuating time constants were both evaluated and used to compensate for the raw temperatures. The estimated mean time constants of the two thermocouples were 9.98 and 29.97 ms at 420 rpm, respectively. The maximum difference between the two reconstructed temperatures was 9.7 °C, with an average value of 2.4 °C. Cycle-by-cycle variations of the transient temperatures were also measured.