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The effect of curvature and torsion on the flow in a helical pipe of circular cross-section is studied numerically by the spectral method. The calculations are carried out for 0 < s <0.6, 0 < f30 < 1.4 and 500 < o; <2000, where {) is the non-dimensional curvature, f30 the ratio of torsion to square root of curvature, and D n the Dean number. The results obtained indicate large effects of torsion on the flow: The conventional two-vortex secondary flow is distorted to become almost one single recirculating cell when f30 ;::: 0.8. The flux through the pipe at the given Dean number and curvature first decreases from that of the toroidally curved pipe as f30 increases from zero, reaches a minimum at f30 ~0.8, and then increases to values larger than that of the toroidally curved pipe. The minimum value decreases as {) increases.

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The effects of acid and hydrothermal pretreatments and the addition of polar compounds on the production of ashless-coal (HyperCoal) from subbituminous coals using cost-effective industrial solvents were investigated. The extraction yield of Wyodak subbituminous coal (C%, 75.0%) using crude methylnaphthalene oil (CMNO) at 360 °C was increased significantly by 19% following acid pretreatment; it was 41.3% for the raw coal and 60.5% for the acid-treated coal. The addition of strongly polar compounds, such as N-methyl-2-pyrrolidinone (NMP), also increased the extraction yields. For Pasir subbituminous coal (C%, 73.0%) the yield increased by 10% from 54.3% for the raw coal to 64.2% when 20% NMP was added to CMNO. The highest extraction yield of 72.2% was obtained for acid-treated Wyodak coal using CMNO with 20% NMP added. The ash content in HyperCoal tended to decrease following acid pretreatment and was less than 200 ppm in some coals. Hydrothermal pretreatment had a negative effect on the thermal extraction at 360 °C, but increased the yield at extraction temperatures below 200 °C.