Fast and accurate 4‐node quadrilateral

An accurate and variationally consistent 4‐node quadrilateral element is introduced where high coarse mesh accuracy and low mesh distortion sensitivity are characteristic qualities, even when incompressibility is approached for plane strain. One‐point quadrature integration procedure is adopted and a new improved stabilization technique is developed. Orthogonality conditions are utilized so that the patch test is satisfied for arbitrary quadrilaterals. Several numerical examples including a convergence rate study are presented which confirm the excellent performance of this element. Copyright © 2004 John Wiley & Sons, Ltd.     

On the off-design of a natural gas-fired combined cycle with CO2 capture

During the last 15 years cycles with CO₂ capture have been in focus, due to the growing concern over our climate. Often, a natural gas fired combined cycle with a chemical absorption plant for CO₂ capture from the flue gases have been used as a reference in comparisons between cycles. Neither the integration of the steam production for regeneration of amines in the combined cycle nor the off-design behaviour of such a plant has been extensively studied before.     

On-line H NMR spectroscopic investigation of hydrogen bonding in supercritical and near critical CO2–methanol up to 35 MPa and 403 K

On-line NMR spectroscopy can beneficially be applied to studies of supercritical and near-critical fluids as an alternative to optical spectroscopy. Up to now high pressure NMR experiments are predominantly accomplished using custom made NMR batch reactors. The authors present a novel high pressure cell with displacement plunger for on-line NMR experiments on compressible fluids which can be used in conjunction with commercially available SCF NMR flow probes. The on-line technique offers advantages compared to stopped flow techniques such as enhanced control of mixture composition and reaction parameters as well as the facility of engagement into the reaction. The new apparatus is used for NMR studies on hydrogen bonding of methanol in near critical and supercritical carbon dioxide up to 403 K and 35 MPa for which data on the chemical shift of the hydroxyl group and methyl group are reported and interpreted.     

Numerical investigation of cyclic variations in gasoline engines using a hybrid URANS/LES modeling approach

Cycle to cycle variations are an important aspect in the development and optimization process of internal combustion engines. In this study the feasibility of using a detached eddy simulation (DES) SST model, which is a hybrid URANS/LES model, to predict cycle to cycle variations is investigated. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS shear-stress transport (SST) model is used. In the other regions with higher grid resolution an LES model is applied. First, the numerical requirements associated with the hybrid URANS/LES and the employed solver are studied in detail. The numerical dissipation of the spatial scheme and the choice of the temporal scheme including the step size are evaluated. In addition, the accuracy of the solver for moving meshes, which are required for engine calculations, is assessed. The modeling constant linking the grid size to the DES filter length scale is determined by calculating a decaying homogeneous isotropic turbulence test case for different grid resolutions. The final applications of the model are two different engine cases with increasing complexity. The first case is the statistically stationary flow through an engine intake port. The time resolved flow structure predicted by the DES SST model is analyzed and the resulting time-averaged velocity fields are compared to experimental data at different locations. The second application is a motored multi-cycle simulation of a series production engine. The instantaneous flow development during the intake and compression stroke of one single cycle is studied and the ensemble-averaged and the instantaneous velocity fields as well as the resolved velocity fluctuations are compared to optical measurements. Special emphasis is placed on the cyclic differences of the velocity fluctuations at the time of ignition in the vicinity of the spark plug and the expected influence on the combustion process.     

Prediction of Transport Properties of Liquid Ammonia and Its Binary Mixture with Methanol by Molecular Simulation

Transport properties of ammonia and of the binary mixture ammonia + methanol are predicted for a broad range of liquid states by molecular dynamics (MD) simulation on the basis of rigid, non-polarizable molecular models of the united-atom type. These models were parameterized in preceding work using only experimental vapor-liquid equilibrium data. The self- and the Maxwell-Stefan (MS) diffusion coefficients as well as the shear viscosity are obtained by equilibrium MD and the Green-Kubo formalism. Non-equilibrium MD is used for the thermal conductivity. The transport properties of liquid ammonia are predicted for temperatures between 223 K and 473 K up to pressures of 200 MPa and are compared to experimental data and correlations thereof. Generally, good agreement is achieved. The predicted self-diffusion coefficient as well as the shear viscosity deviates on average by less than 15 % from the experiment and the thermal conductivity by less than 6 %. Furthermore, the self- and the MS transport diffusion coefficients as well as the shear viscosity of the liquid mixture ammonia + methanol are studied at different compositions and compared to the available experimental data.     

Sub-zero cooling: A novel strategy for high performance cutting

In this paper, a novel cooling strategy for high performance cutting is presented. A metalworking fluid, composed of water and polyhydric alcohols is applied at temperatures below 0 °C, but above the temperatures used for cryogenic cutting. This cooling strategy is applied when rough turning Ti-6Al-4V. An analysis of temperatures, forces, tool wear and chip formation is carried out. The results are compared with those obtained using emulsion, CO₂, LN₂, and dry turning. It can be shown that when tempering the sub-zero metalworking fluid down to ?30° C, tool temperatures as well as tool wear are reduced, and favourable chips are produced.