Experimental and Numerical Similitude Study Using a Novel Turbocompressor Test Facility Operating With Helium-Neon Gas Mixtures

A novel turbocompressor test facility has been designed for helium-neon gas mixtures and its specific features are presented. To account for the heat transfer originating from the motor coolant, a surrogate model has been derived. By combining these results with additional numerical ones, a similitude study is conducted quantifying the individual effects and contributions on efficiency of the Reynolds number, tip Mach number and specific heat ratio. Decoupling the effect of the different parameters shows that their respective contribution on efficiency variation is highly correlated to the Reynolds number actual value. The negative contribution of the tip Mach number and the positive effect of Reynolds number can be used to explain the efficiency variation with increasing tip Mach number. Specific heat ratio variation leads to minor changes in polytropic efficiency except at low tip Mach numbers.

Investigation of the Effects of Different Working Fluids on Compressor Cascade Performance

The compressor of closed Brayton cycle (CBC) plant operating with working fluid other than air is a vital element of the energy conversion unit. However, due to insufficient understanding of the influence of the physical properties of working fluids on the performance of the compressor, the actual working conditions and design conditions of the compressor’s performance deviate greatly. In this paper, the objective is to analyze the influence mechanism of the physical properties on the performance of the cascade of compressor (static pressure ratio and total pressure loss coefficient). Therefore, the impact of a specific heat ratio on the performance of the compressor cascade is studied utilizing carbon dioxide (γ = 1.29), air and carbon monoxide (γ = 1.4), argon and helium (γ = 1.667). Moreover, the relationships of static pressure ratio and total pressure loss coefficient with physical properties of the working fluids are analyzed in the compressor cascade. It is established that a higher specific heat ratio fluid gives a higher coefficient of total pressure loss and static pressure ratio in contrast to smaller specific heat ratio at matching inlet Reynolds number and Mach number.

THE PROPERTIES AND STABILITY OF SELF-GRAVITATING, POLYTROPIC SPHERES WITH γ = 1 TO 1.4 SPECIFIC HEAT RATIOS

We study self-gravitating, hydrostatic spheres with a polytropic equation of state P ∝ ρ^γ (where γ is the specific heat ratio of the gas), considering structures with γ ≈ 1 as a model for molecular cloud cores with small departures from isother- mality. We derive the properties (i.e., mass, radius and center to edge density ratio) as a function of γ for the maximal stable sphere through an application of “Bonnor’s stability criterion”. We find that in the γ = 1 → 4/3 range the mass of the maximal sphere (for a given central temperature) is almost constant, and that its radius and center to edge density ratio are growing functions of γ. We therefore have maximal stable, self-gravitating spheres with similar masses, but with increasing center to edge density contrasts for increasing departures from isothermality.