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Cooled EGR is recognized as a solution for the reduction of NOx in diesel exhaust gas. The key component of cooled EGR is a tubular type heat exchanger, however the performance drops because of particulate deposition whose characteristics and mechanism are not clrealy understood. Therefore the requirements for a EGR cooler is not only higher heat exchange performance and lower pressure loss, but also less performance deterioration due to DEP (Diesel Exhaust Particulate) deposition known as fouling. In order to improve heat transfer performance of an EGR cooler, it is considered that the surface with riblets aligned in the flow direction is preferable for the heat transfer surface because of its less skin friction and heat transfer enhancement. To apply riblet design to EGR coolers, however, it is necessary to investigate the performance with fouling from diesel particulate.

In this study, the temperature at the sliding interface between paper disc and the sapphire plate in oil was measured experimentally. Theoretical model of heat transfer at the sliding interface was introduced and temperature distribution was calculated as a function of sliding verocity and torque. Theoreticaly estimated value of temperature agreed well with experimental values. The relationship between the maximum surface temperature and the critical cycles of engagement was introduced from those theoretical and experimental results, and “the degradation curve” for wet friction paper was proposed.