Development of an adjacent three-point MEMS heat flux sensor for internal combustion engines

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 μm on a 900 μm 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/m2, which is small enough to detect instantaneous heat flux. The instantaneous heat flux had oscillation with the amplitude of a few hundred kW/m2. 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.