Aerodynamic study of a Hyperloop pod equipped with compressor to overcome the Kantrowitz limit

This paper describes an investigation of the aerodynamic performance of a Hyperloop pod equipped with an axial compressor using CFD simulation. The compressor is expected to reduce the drag if the operational speed of the pod exceeds the Kantrowitz Limit (KL). To evaluate the effectiveness of the compressor, two types of pod have been designed: those with and those without a compressor. A validation study was undertaken for supersonic flow around a cylindrical body in a wind tunnel. Several combinations of pod Mach number and blockage ratios were examined. The axial compressor was modeled with the fan interface model and our original method, which we call the source terms model. Results are similar, but our model has the advantage that it can be applied to higher pressure ratios without inducing numerical instabilities. The power consumption was reduced owing to the compressor by decreasing the pressure accumulated in front of the pod. The effectiveness of the compressor was shown to be greater for conditions well above the KL, with a maximum power reduction of 47.5%. Simulations performed half-scale, and at a different system pressure, showed that the effectiveness of the compressor was not greatly influenced by these parameters.

Highly loaded compressor with boundary layer suction

Klaus Hubrich

An experimental and numerical investigation of boundary layer suction (BLS) on a transonic compressor blade has been conducted. The objective of the present work is to identify the possible benefit of boundary layer suction via a slot in the region of the shock/boundary layer interaction (SBLI). The study has two major parts: First, an investigation of the flow on an isolated airfoil without and with BLS on the suction side (SS). This investigation is intended to optimise the suction slot geometry in terms of location and width. Second, a study on a transonic cascade with supersonic inlet flow has been conducted with the goal to show the influence of the BLS on the pressure loss coefficient and the surface Mach number distribution. An initial literature study revealed that only few work has been carried out in the field of BLS in transonic compressors and no data close to the suction slot was available. The main steps of the present study are: The design of an isolated airfoil for testing in a Laval nozzle, representative for the flow at a compressor root section, with enough space inside to evacuate the aspirated air. A numerical study of the flow on the isolated airfoil without and with BLS on the SS . An experimental investigation of the isolated airfoil without and with BLS on the SS using static surface pressure taps, PSP and the Schlieren method for data acquisition. Design and numerical investigation of a suitable transonic compressor cascade with enough space inside to evacuate the suction mass flow. An experimental investigation of the designed transonic cascade with an inlet Mach number of 1.23 with two sets of blades in the non-rotating annular cascade test-rig; one without and one with BLS at 40% chord. The main results are: The numerical study of the flow on the isolated airfoil without and with BLS on the SS shows, that the onset of shock-induced separation could be shifted to an increased inlet Mach number. Steady 2D-NS simulations show that the reference configuration exhibits an attached flow behind the shock up to an inlet Mach number of up to 0.70 for an inlet flow angle of +4°. The configuration with BLS shows attached flow behind the shock for an an inlet Mach number of 0.725. Unsteady 2D-NS simulations show for an inlet flow angle of +4° at an inlet Mach number of 0.725 for the reference configuration a shock, which moves periodically on the SS due to a separated boundary layer created by the shock/boundary layer interaction. The same configuration with BLS shows a stable shock position. The experimental investigation of the isolated airfoil without and with BLS on the SS shows that BLS at 15% chord upstream of the initial shock location for an inlet Mach number of 0.725 at a flow angle of +4° leads to a shock location 10% chord downstream of the initial position. Boundary layer suction suppresses flow separation and stabilises the shock. Due to the suction a stagnation point is created at the downstream edge of the suction slot. Downstream of this stagnation point, the flow is re-accelerated to the isentropic Mach number level of the reference configuration without BLS. The design and 3D-numerical investigation of a suitable transonic compressor cascade shows, that BLS with a suction mass flow of 2% can increase the maximum pressure ratio and diffusion factor of approximately 10%. This is the consequence of the stabilisation of the shock in the blade passage due to the suction. The experimental investigation of the designed transonic cascade with an inlet Mach number of 1.23 with 2% suction mass flow shows, that BLS acts locally and that suction leads to an increased isentropic surface Mach number in front of the suction slot and due to the suction related stagnation point, to a locally decreased velocity immediately downstream of the suction slot. Then the flow re-accelerates to the same level as the non-suction configuration attains. The pressure loss coefficient is decreased by up to 3.6% for the most positive incidences. The results of this investigation confirm that boundary layer suction on the suction side of an airfoil is a suitable way to increase the working range of a compressor as the shock position can be stabilised (shock-trapping) and the increase of the pressure loss coefficient can be shifted to higher incidences.

EPFL2005

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

The modelling of natural gas combustion with detailed as well as reduced kinetic mechanisms has been subject to many publications already. Most of the mechanisms are validated at higher temperatures and ambient pressures. Only recently - in parallel to a gain of interest in the homogeneous charge compression ignition (HCCI) engine concept - there are more efforts trying to reproduce the low temperature chemistry, and in particular autoignition of natural gas under engine-like conditions. This work is part of a project on stationary natural gas engines equipped with unscavanged prechambers. It has been demonstrated that it is possible to reduce the emissions below the Swiss limits both with natural gas and biogas with the spark ignition prechamber, while keeping high efficiencies of around 38 %. We are now aiming at converting the prechamber from spark ignition to autoignition leading to longer maintenance intervals in the case of stationary natural gas engines and to a probable cleaner combustion - HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front - compared to prechamber spark ignition. In order to be able to model the fluid dynamics and the chemistry inside the cylinder, a good chemical model is one of the most important aspects. To obtain an appropriate mechanism - by validating and/or optimising existing mechanisms - experiments have been conducted in the rapid compression machine facility at the University of Lille with different mixtures of methane, ethane and propane. The relative air-to-fuel ratio λ varied from 1 to 1.6, the temperature and pressure at the end of compression varied from 850 to 925 K and 13 to 21 bar respectively. The compression ratio was constantly kept at 9.2 for all experiments and the final temperature and pressure were defined by varying the charge and the composition of the inert gases (Ar/N2). The compression time was between 50 and 60 ms for all experiments. For modelling the ignition delay a zero-dimensional model assuming homogeneity was used, based on the measured pressure and calculated core temperature at the end of compression of each experiment. The chemical solver package used for the simulations is CHEMKIN. Different mechanisms (PRF, GRI3.0, Konnov, …) have been tested. Most of the mechanisms are mainly validated at higher temperature ranges and therefore their applicability to ignition delay prediction in our conditions is questionable. Several attempts to adjust the GRI mechanism (version 1.2) to the lower temperature regime exist, whereof we chose the UBCMech and Petersen mechanism (GRI+RAMEC) to test on our simulations. Both mechanisms initially were developed based on pure methane mixtures while the UBCMech recently was extended to model natural gas as well. The experimentally obtained ignition delays vary from 40 ms at high pressures and temperatures to more than 300 ms for the lower regime of the experimental space. For the ignition delays up to 60 ms the best fit was obtained by a reduced version of PRF accounting only for the C1 to C3 compounds. Konnov’s mechanism and UBCMech are under predicting the experimental ignition delays while the GRI3.0 mechanism (with and without RAMEC sub module) over predicts the ignition delay for the natural gas mixtures. It has to be mentioned that the GRI3.0+RAMEC mechanism as well as the initial mechanism of Petersen (GRI1.2+RAMEC) on the other hand under predict the experimental ignition delay for pure methane mixtures. This indicates that the low temperature chemistry for methane that has been added in the RAMEC sub module is enhancing ignition too strong. Replacing part of methane in pure methane mixtures by equivalent amounts of ethane and propane - know as ignition promoters - lead to longer ignition delays in the simulations. But as the GRI3.0+RAMEC mechanism (without nitrogen chemistry) is the smallest of all tested mechanisms considering number of species and reactions, it still is an interesting option for coupling it to a computational fluid dynamics code as we are planning to do. We though performed a sensitivity analysis to obtain a set of the most dominating reactions and used a multi-objective optimisation algorithm to optimize the reaction rates within their limits of uncertainty. The results show a good fit for the short ignition delays of natural gas mixtures but still an under prediction of the methane delays. For the longer ignition delays the assumption of a constant core temperature during the post compression phase becomes questionable and the under prediction of all mechanisms of the experimental delays is an obvious consequence. A heat loss model predicting the temperature evolution has to be implemented in the simulations to properly simulate the ignition delays.

2006

Preliminary design of the pressure relief system of the EDIPO 2 helium vessel

Pierluigi Bruzzone, Roberto Guarino, Kamil Sedlák, Evgeny Solodko

The European Dipole (EDIPO) test facility is undergoing a major upgrade of its magnet assembly, which will provide ground for testing both fusion and high-energy physics superconducting samples at variable temperature and with a background field of 15 T. In view of a cooling of the superconducting magnet based on a saturated helium bath, a helium vessel has been designed and procured. In this work, we present a preliminary design of the pressure relief system of the helium vessel. After having introduced the main requirements of the cryogenic circuit, we describe a staged pressure protection concept. A bursting disc, chosen as safety pressure relief device, is then sized following the appropriate European norms and according to credible accidental scenarios. Finally, we introduce the need of pressure relief and helium recovery following a quench of the superconducting magnet, after having determined the actual heat load transferred to the helium bath. Numerical simulations are also employed to study the dynamics of the pressure and temperature evolutions, and ideas for future studies are briefly discussed.