The torrefied solid biofuels have come to attract attention as promising alternative fuels to coal in coal-fired power plants and industrial boilers. The torrefaction is a thermal treatment performed at mild pyrolysis temperatures to reform biomass and includes two distinct technologies, dry torrefaction (DT) and wet torrefaction (WT). The DT is the conventional thermal treatment conducted in the absence of oxygen under atmospheric pressure in a temperature range of 200-300 ℃, while the WT is a thermal treatment in hydrothermal media or hot compressed water in a temperature range of 180-260 ℃. Both torrefaction treatments can produce energy dense solid biofuels with better chemical, physical properties than original biomass feedstock, but the external energy input is required for the torrefaction treatment. Especially, when producing the torrefied solid biofuels from biomass feedstock with high moisture content, the energy consumption for drying process may have a negative impact on the sustainability of the torrefied biofuel. In this article, properties of thermally treated biomass by DT and WT are summarized, and then the net energy content of torrefied biofuels is presented to evaluate the sustainability. The net energy content is expressed as a function of mass yield, and the optimum torrefaction condition with high net energy content can be evaluated by the relation for the net energy content.

In order to promote the use of biomass, we derived an equation for estimating the calorific value of biomass solid fuel with varying water content, taking into account the presence or absence of ash and its amount. The usefulness of the derived estimation equation of calorific value was verified using sugi sawdust and fermented poultry manure samples. As a result, the estimated gross calorific value obtained from the estimation equation of calorific value derived using the measurement results of sawdust and poultry manure in the initial condition agrees very well with the gross calorific value of the samples with and without ash, whose water content was changed by drying sawdust or pelletizing poultry manure. From the above results, the usefulness of the derived estimation equation of calorific value was demonstrated, and using this equation, it was clarified that the gross calorific value of the biomass solid fuel at any water content could be estimated. In addition, by using the derived estimation equation of calorific value for the gross calorific value of the sawdust sample on a dry/ash-free basis and the ash content ratio of the poultry manure sample on a dry basis, the gross calorific value of the fermented poultry manure at any water content could be estimated to some extent.

This paper aims to clarify dynamical threshold in experimental, numerical, and theoretical manners for cavitation in water under a finite duration of tension. A once-through, non-recirculating, cavitation tunnel is constructed to run the tests. Within the test section, a cylinder is installed, behind which cavitation occurs, and once cavitated, water cannot return to the monitoring section. Cavitation threshold experiments are conducted in conditions controlled for flow velocity, pressure, and filtration degree of water. Conditions suitable for threshold tests are found in relation to Reynolds number and cavitation number. In parallel, LES-based CFD computations with three dimensional turbulent models are made to simulate water flows in the visualization section under experimental conditions, by which velocity distribution at the throat, pressure distribution before and after the throat, flow separation, and vortex formation downstream of the separation point are clarified. As an alternative approach, bubble dynamics analysis using Rayleigh-Plesset equation is conducted for conditions under which bubble nuclei grow according to CFD-based pressure histories, revealing that nuclei with O(1μm)-radius suddenly start to appear and grow at the separation point. Finally, dynamical cavitation threshold theory is successfully applied to predict nuclei growth, resulting in agreement with the bubble dynamics analysis.

We report a unique technique in forming an entangled carbon fibers without any glue material. We describe here a phenomenon where spraying aerosol of short carbon fibers realizes a spontaneous formation of a clump of entangled carbon fibers in few seconds. We named this method as a fiber aerosol deposition (FAD) method. We investigated the origin of this phenomena with comparing the length of the raw carbon fiber materials. We found that a range of the length distribution from 85 to 111 µm of the fibers is necessary to reproduce the phenomenon. Also found that the appearance of an intermediate state layer in initial formation stage enhances the fast development of the entangled carbon fibers. We point out that a clump of glue less short carbon fibers may be applicable for the recycling process of the wasted carbon fibers that are increasing in modern industry which utilizes the carbon fiber technology.

Experimental investigation has been carried out on natural convection induced around a heated horizontal square cylinder. Main concerns were the flow and heat transfer characteristics of high-Rayleigh number flows. Air was utilized as the test fluid. The square cylinders having side length from 30 to 300 mm were utilized as a test cylinder, and they were heated with uniform heat fluxes. The modified Rayleigh numbers based on the side length ranged as; Rad^* = 10⁵- 3×10⁹. The flow fields around the cylinder were visualized with smoke. The results showed that the ascending flows along the side surface begin to separate three-dimensionally from the upper surface of the square cylinder when the modified Rayleigh numbers, Rad^*, exceed Rad^* = 2×10⁶. The local heat transfer coefficients were also measured. The results showed that the coefficients of the upper surface are increased, in particular, in the region behind the threedimensional separation.

In this study, the microscopic visualization of the pyrolysis of woody biomass was conducted using the BL20B2 beamline at “SPring-8,” a large synchrotron radiation facility. Changes in the shape and internal structure of the woody biomass were visualized using ultra-high-speed computerized tomography (CT). The sample was a Japanese cypress wood with a height of 5 mm and diameter of 5 mm. We used radiation as the heat source to achieve a high heat flux. When the heat flux is high, the sample expands. Gaps were observed in the sample. Compared with the experimental results under the low heat flux obtained in the previous study ¹⁾, we observed a completely different aspect. In a nitrogen or air atmosphere, the internal structure of the wood during transient pyrolysis was visualized using ultra-high-speed X ray CT. In addition, the schlieren method was used to study the pyrolysis behavior of the woody biomass and the behavior of the pyrolysis gases.

The bed stabilization effect of boulders in rivers was studied by a wind tunnel experiment. Boulders were modeled by metal spheres with diameter D = 2.0 cm and were set in a sand layer, and the boulder occupancy ratio αdefined as (πD²/4)/ℓ² (ℓ: boulder interval) in a square array was 0.014 - 0.087. The bed erosion velocity clearly decreased in the presence of boulders, and then, changed by the parameter α and would be the minimum around α = 0.056. In conclusion, it was found that boulders in rivers significantly stabilize bed variation, and that their effect will be great in the medium occupancy ratio.

The authors have been working on improving the Schmidt theory used in the design and analysis of Stirling engines in line with the actual phenomenon. In this study, we focus on a friction loss between a piston and cylinder of a Stirling engine, which is not considered in the conventional Schmidt theory. We attempted to derive a relational expression for the friction loss between a glass piston and cylinder that is widely used as a piston-cylinder for a model Stirling engine for education and research. The viscous friction coefficient C_v and the average Coulomb friction torque T_c were obtained experimentally because they are considered to change depending on the shape, material of the piston-cylinder and so on. As a result, we clarified that the viscous friction coefficient C_v is expressed as a relational expression of DS^5/3, which is the product of the piston outer diameter D and the stroke S to the 5/3 power.