Tatsurou Yashiki

Dynamic Heat-Exchanger Model for Any Combination of Water and Steam States

The purpose of this study is to propose a dynamic heat transfer model for predicting transient heat recovery steam generator (HRSG) behaviors involving phase changes in heat exchanger tubes. The model deals with any combination of phase states by switching the equations for heat transfer coefficient, specific volume, and friction factor corresponding to their physical characteristics. The model also constrains the change of mass flow calculated by momentum balance to satisfy thermodynamic relationships which are neglected by conventional models. The simulation results show that the proposed model predicts the transient pressure drop, outlet mass flow changes, and the reduction in heat transfer coefficient caused by dryout during heating or evaporating processes. In addition, the model improves the accuracy of mass flow transients compared to those obtained by conventional models.

Casting Defects Analysis Making the Best Use of High-Energy X-Ray CT

A novel method of analyzing casting defects such as shrinkage cavities by utilizing high-energy x-ray computed tomography (CT) has been developed. The method was realized as the module of three-dimensional (3-D) bitmap CAD software StereoCooker for utilizing x-ray CT images. The cavity measurement method was based on the defocusing feature of an x-ray CT image. The authors assumed the defocusing to Gaussian, and therefore the diameter of the cavity could be calculated from the summation of the voxel value profile of the cavity and the circumference. The Gaussian defocusing hypothesis was confirmed by the CT reconstruction simulation with an accuracy higher than 10% for the conditions of the actual x-ray CT apparatus. The cavity measurement method was verified by applying it to cavity test pieces. The test pieces had cavity diameters of 0.5, 0.6, 0.7, 0.8 and 1.0mm. Using the rotate-only mode of the 6MeV x-ray CT, cavities of 0.6mm or greater in diameter could be detected. The accuracy of the cavity size is higher than 0.1mm for diameters of 0.7mm and greater. The verification of simulation by a comparison of the defects predicted by a simulation and those measured by this method has been proposed.Copyright