S. Varoutis

Simulation of gas flow through tubes of finite length over the whole range of rarefaction for various pressure drop ratios

The rarefied gas flow through circular tubes of finite length has been investigated computationally by the direct simulation Monte Carlo method. The reduced flow rate and the flow field have been calculated as functions of the gas rarefaction, the length-to-radius ratio, and the pressure ratio along the tube. The gas rarefaction, which is inversely proportional to the Knudsen number, is varied from 0 to 2000, i.e., the free-molecular, transitional, and hydrodynamic regimes are embraced. A wide range of the length-to-radius ratio, namely, from 0 corresponding to the orifice flow up to 10 representing a sufficiently long tube, has been considered. Several values of the pressure ratio between 0 and 0.7 have been regarded. This pressure range covers both gas flow into vacuum and into a background gas. It has been found that the rarefaction parameter has the most significant effect on the flowfield characteristics and patterns, followed by the pressure ratio drop, while the length-to-radius ratio has a rather ...

Experimental results and numerical modeling of a high-performance large-scale cryopump. I. Test particle Monte Carlo simulation

For the torus of the nuclear fusion project ITER (originally the International Thermonuclear Experimental Reactor, but also Latin: the way), eight high-performance large-scale customized cryopumps must be designed and manufactured to accommodate the very high pumping speeds and throughputs of the fusion exhaust gas needed to maintain the plasma under stable vacuum conditions and comply with other criteria which cannot be met by standard commercial vacuum pumps. Under an earlier research and development program, a model pump of reduced scale based on active cryosorption on charcoal-coated panels at 4.5 K was manufactured and tested systematically. The present article focuses on the simulation of the true three-dimensional complex geometry of the model pump by the newly developed ProVac3D Monte Carlo code. It is shown for gas throughputs of up to 1000 sccm (∼1.69 Pa m3/s at T = 0° C) in the free molecular regime that the numerical simulation results are in good agreement with the pumping speeds measured. Me...

Effect of the Dome on the Collisional Neutral Gas Flow in the Demo Divertor

This paper presents the analyses of the dome effect on the pumping efficiency and screening of the neutrals for the European DEMO ITER-like divertor configuration of 2015. The effect of the dome on neutral compression in the private flux region is assessed by using the DIVGAS code based on the direct simulation Monte Carlo method. The numerical analysis includes the calculation of neutral density, temperature, and pressure in the divertor plenum and the overall conductance of the subdivertor structure, which consequently affects the estimation of the effective pumping speed. It is quantified how the presence of the dome impedes the reflux of neutrals toward the plasma. In addition, due to the intermolecular interactions, the dome does not strongly influence the macroscopic quantities near the pumping duct. A strong reflux of neutrals in the domeless case, as it was previously found in the free molecular flow case, is confirmed to be present also in the case of a collisional flow.

Initial studies of the divertor dome effect on pumping efficiency in DEMO

In this paper we analyze the gas dynamics effect on the pumping efficiency in the European DEMO 1 ITER-like divertor configuration with and without dome. The achievement of detachment at given upstream heat flux and upstream density under burning conditions depends on the level of neutral pressure in the divertor private flux region (PFR) and on the required throughput at actual pumping speed. The principle effect of the dome on neutral compression in the PFR is assessed by using the DIVGAS code based on direct Monte Carlo simulation [1]. The numerical analysis includes the calculation of neutral density in the PFR and the overall conductance of the sub-divertor structure, which consequently affects the estimation of the effective pumping speed and, ultimately, the achievement of detachment. It is shown that the configuration with dome requires a higher pumping efficiency and that the dome impedes the detachment onset. The dome position can be further optimized to ensure good pumping of impurities and ease detachment onset. DIVGAS is demonstrated to be a most suitable tool to do further parametric and detail studies.

Rarefied gas flow into vacuum through short tubes at variable wall temperatures

Rarefied gas flow into vacuum through short tubes at variable wall temperatures was studied numerically with the direct simulation Monte Carlo method. A wide range of tube wall temperatures varying from 120 to 1200 K was considered, as well as several rarefaction parameters and tube aspect ratios. The mass flow rate, specific impulse, thrust, and characteristics of the gas flow field were analyzed. The results showed that the tube wall temperature, as well as the aspect ratio and rarefaction parameter, had a significant influence on the characteristics of the tube flow. The mass flow rate was found to decrease with increasing wall temperature, while the specific impulse increased. This paper reveals the combinations of rarefaction parameter and aspect ratio whereby the wall temperature has the strongest influence on the flow field of the gas, as well as on the overall macroscopic parameters.

TRANSFLOW: An experimental facility for vacuum gas flows

The TRANSFLOW experimental facility represents a reliable tool for measuring the conductance of 1:1 scale components as typically used in vacuum systems in a wide range of the Knudsen number (e.g. 10−4≤Kn≤103). The main principle of this facility is the dynamic measurement of the pressure difference upstream and downstream of the duct by setting a constant mass flow rate through the test channel. Many experiments on fully developed and developing flows, based on long and short channels respectively, have been already completed and comparisons with corresponding numerical results have been successfully performed. It has been clearly proven that the TRANSFLOW experimental setup provides conductance results with overall uncertainty between 1 to 10% and it could be used as a benchmark facility for any new proposed scientific numerical method in rarefied gas dynamics and in the whole range of gas rarefaction.

Operation space of divertor detachment in DEMO

In the paper we consider the operation window of DEMO divertor detachment expected under DEMO I operation conditions for various external impurity levels, particle throughput and pumping conditions, envisaged for fusion reactor. The operation region is defined in terms of upstream heat flux and the upstream density and is bounded by the burn conditions and by the transition threshold between L and H mode. It is also restricted by the density and upstream heat fluxes required for detach operation in DEMO divertor. The transition from the attached to the detached state is simulated in the frame of a simple model which includes the pumping efficiency dependence on density, recombination at the divertor plate and particle throughput. The model was benchmarked with JET experiment and reproduced global features of the transition. The neutral pressure in the divertor chamber and the particle flux to the cryo-and diffusion pump are estimated for detached divertor conditions as well as the number of required pumps.