A heat flux sensor was developed with micro-electro-mechanical systems (MEMS) technologies for investigating turbulent heat transfer characteristics in engines. The sensor has three thin-film resistance temperature detectors (RTDs) of a square 315 µm on a side on a 900 µm diameter circle in rotational symmetry. The performances of the MEMS systems sensor were tested in an open combustion chamber and a laboratory engine. In the open chamber tests, it was revealed that the MEMS sensor can measure the wall heat fluxes reflecting flow states of gas phase. In addition, the noise was evaluated as 3.8 kW/m2 with the standard deviation against the wall heat flux of a few hundred kW/m2. From these results, it was proved that the MEMS sensor has the potential to observe turbulent heat transfer on the order over 10 kW/m2 in the engine. In the laboratory engine test, the wall heat flux for continuous 200 cycles was measured with a good signal-to-noise ratio. The noise was evaluated as 13.4 kW/m2 with the standard deviation despite the noisy environment. Furthermore, it was proved that the MEMS sensor has the comparable scale with the turbulence in the engine because the three adjacent detectors measured similar but different phase oscillations in the local instantaneous heat fluxes. In addition, a heat flux vector reflecting the state of the local instantaneous heat transfer was visualized by the adjacent three-point measurement. It is expected that the three-point MEMS sensor will be a useful tool for the engine heat transfer research.

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It is necessary to understand wall heat transfer mechanisms in order to mitigate cooling losses in an internal combustion engine. To investigate the turbulent heat transfer on the engine wall, a heat flux sensor has to have a low noise and multi measurement points on comparable scale of gas turbulence. Therefore, the authors have developed a new heat flux sensor with three measurement points by using MEMS (Micro-Electro-Mechanical Systems) technologies. The MEMS sensor has three thin film RTDs (Resistance Temperature Detector) with the size of 315 m on a 900 m diameter circle in rotational symmetry. Measurement tests were conducted in a laboratory engine. The noise of the MEMS sensor was evaluated as 13.8 kW/m 2 , which is small enough to detect instantaneous heat flux. The instantaneous heat flux had oscillation with the amplitude of a few hundred kW/m 2 . Since the amplitude of the oscillation was much larger than the noise, it was supposed that the oscillation was a meaningful signal reflecting the disturbance of a velocity or temperature field in the gas phase. By a cross-correlation analysis between the three RTDs, it was found that the instantaneous heat fluxes had a moderate correlation with a certain delay time. That can be interpreted as the traveling of a turbulent vortex structure from one RTD to another RTD with the time. Therefore, it can be expected that the turbulent characteristics will be extracted from the instantaneous heat flux data measured with the three RTDs.

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In this study, it was attempted to estimate the flow characteristics in the vicinity of an engine inner wall from the instantaneous local heat fluxes measured using a micro-electro-mechanical systems sensor. As the sensor has three resistance temperature detectors with a size of 315 µm fabricated on a circumference with a diameter of 900 µm in rotational symmetry, it can measure local heat flux on the equivalent scale of the turbulence of in-cylinder flow. The advective velocity and turbulent eddy scale were estimated from heat flux fluctuations using a cross-correlation analysis, and these were compared with results of particle image velocimetry performed under motored operation conditions. As a result, it was found that the micro-electro-mechanical systems sensor has the potential to detect the gas side information such as the wall parallel flow velocity. Although further verification of the physical meanings of the estimated characteristics is necessary, the micro-electro-mechanical systems sensor will become a powerful tool for engine diagnostics.