Flow and Temperature Fields in Cooling Devices with Embedded Serpentine Tubes

The turbulent flow (Re = 5124) and conjugate heat transfer in heat-sink designs of the tube-on-plate type are numerically investigated. The cooling configurations employ a serpentine tube partially (or fully) embedded inside the plate. Two-and four-pass configurations are investigated. A constant heat flux is applied at the bottom surface of the heat-sink plate. The SST k-ω model is used for the prediction of the turbulent flow and heat transfer. Two pairs of longitudinal vortices, as well as secondary flow separation, have been found to set in at the tube curved section. The combined secondary flow pattern enhances heat transfer at the tube sections over a considerable distance downstream of the 180° bends. In the last part of the analysis, the overall performance of the two configurations is compared using a number of evaluation criteria suitable for heat exchanging devices. The four-pass configuration with fully embedded tubing exhibits the best thermal (energetic) and exergetic performance

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency

A general predictive methodology for fuel-mixture properties up to supercritical conditions

A predictive thermodynamic model is utilized for the calculation of fuel properties of oxymethylene dimethyl ethers (OME3–4), surrogates for gasoline, diesel and aviation fuel, as well as alcohol blends with gasoline and diesel. The alcohols used for these blends are methanol, ethanol, propanol, butanol and pentanol; their mixing ratio ranges from 10 to 50% by volume. The model is based on the Perturbed-Chain Statistical Association Fluid Theory (PC-SAFT) equation of state (EoS) and Vapor Liquid Equilibrium (VLE) calculations at constant temperature, density and composition. The model includes the association term, with the assumption of two association sites (2B scheme), to enable the modeling of alcohols. The pure-component parameters are estimated based on the Group Contribution (GC) method of various sources, as well as a parametrization model specifically designed for the case of OME3–4. The results of the computational model for the density, vapor pressure and distillation curves at various conditions, including high-pressure, high-temperature (HPHT), are compared to experimental and computational data available in the literature. In the cases where no measurements are available for the surrogates, experimental data for the corresponding target fuel are used, taking into consideration the inherent deviation in properties between real and surrogate fuel. Overall, the results are in good agreement with the data from the literature, with the average deviation not exceeding 12% for temperature (Kelvin) on the distillation curves, 10% for density and 46% for vapor pressure and the general trend being captured successfully. The use of different pure component parameter estimation techniques can further improve the prediction quality in the cases of OME3–4 and the aviation fuel surrogate, especially for the vapor pressure, leading to an average deviation lower than 18%. These results demonstrate the predictive capabilities of the model, which extend to a wide range of fuel types and pressure/temperature conditions. Through this investigation, the present work aims to establish the limits of applicability of this thermodynamic property prediction methodology

Assessment of injector-flow characteristics of additised and renewable diesel blends through high-speed imaging

The influence of additives inducing viscoelasticity in diesel fuel, on the in-nozzle cavitation evolution and the expelled spray morphology has been quantified by high-speed, diffused back-light and schlieren imaging applied to two single-hole true-scale transparent injectors of straight and tapered orifice layouts (so-called Spray C and D of the engine Combustion Network), as well as a five-hole configuration (Spray M). More specifically, the in-nozzle cavitating flow and its effect on near-nozzle spray formation of a non-Newtonian diesel fuel sample treated with Quaternary Ammonium Salt (QAS) additives and exhibiting viscoelastic effects, as well as biodiesel (FAME), are compared against conventional diesel fuel for the first time. The operating conditions corresponded to injection and ambient pressures in the range of 500–900 bar and 1–20 bar, respectively. It was found that viscoelasticity has an overall suppressing effect on wall-attached, or so-called geometrical, cavitation. Furthermore, the investigation revealed that the action of viscoelastic additives has the capability to enhance the magnitude of well-established longitudinal vortices, with the subsequent after-effect of leading to increased cone angles of the expelled spray. On the contrary, it tends to suppress turbulence-induced transient instabilities in a manner similar to turbulence suppression

Hypogammaglobulinemia: A contributing factor to multiple sclerosis fatigue?

OBJECTIVE Fatigue is one of the most disabling and difficult to treat symptoms of autoimmune diseases and frequently presents in people with multiple sclerosis (PwMS). Hypogammaglobulinemia for immunoglobulin G (IgG) affects approximately 8-25% of PwMS. We performed a retrospective analysis to investigate the association of MS-fatigue and IgG hypogammaglobulinemia. METHODS PwMS, treated at Eginition University Hospital Athens or at the University Hospital Bern, were included (n = 134 patients (Bern n = 99; Athens n = 35)). Mann Whitney U-test (MWT), ANOVA test, Chi2 test and multivariable linear regression models were run. RESULTS 97/134 (72.4%) PwMS reported fatigue. In the multivariable linear regression analysis, IgG serum concentration (-1.6, 95%CI -2.7 - -0.5, p = 0.006), daytime sleepiness (0.8, 95%CI 0.2-1.4, p = 0.009), and a depressive mood (1.1, 95%CI 0.8-1.4, p < 0.001) were significantly associated with fatigue. The impact of IgG serum concentration (-2.9 95%CI -4.7 - -1.1, p = 0.002) remained significant also in the subcohort of PwMS without depressive symptoms or daytime sleepiness. CONCLUSIONS We found an association between IgG hypogammaglobulinemia and fatigue in PwMS (Level of Evidence IV), which might be translated to other autoimmune diseases. It bears a potential therapeutic consequence considering IgG supplementation strategies, if our finding can be validated prospectively

Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging

The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0–7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice

Multiphase phenomena in Diesel fuel injection systems

Fuel Injection Equipment (FIE) are an integral component of modern Internal Combustion Engines (ICE), since they play a crucial role in the fuel atomization process and in the formation of a fuel/air combustible mixture, consequently affecting efficiency and pollutant formation. Advancements and improvements of FIE systems are determined by the complexity of the physical mechanisms taking place; the spatial scales are in the order of millimetres, flow may become locally highly supersonic, leading to very small temporal scales of microseconds or less. The operation of these devices is highly unsteady, involving moving geometries such as needle valves. Additionally, extreme pressure changes imply that many assumptions of traditional fluid mechanics, such as incompressibility, are no longer valid. Furthermore, the description of the fuel properties becomes an issue, since fuel databases are scarce or limited to pure components, whereas actual fuels are commonly hydrocarbon mixtures. Last but not least, complicated phenomena such as phase change or transition from subcritical to transcritical/supercritical state of matter further pose complications in the understanding of the operation of these devices

Flow visualisation in real-size optical injectors of conventional, additised, and renewable gasoline blends

Research on renewable and alternative fuels is crucial for improving the energy and environmental efficiency of modern gasoline internal combustion engines. To highlight the influence of fuel rheological and thermodynamic properties on phase change and atomisation processes, three types of gasoline blends were tested. More specifically, the campaign comprised a reference gasoline, an ethanol/gasoline blend (10% v/v) representative of renewable fuels, and an additised gasoline sample treated with viscoelasticity-inducing agents. High-speed imaging of the transient two-phase flow field arising in the internal geometry and the near-nozzle spray region of gasoline injectors was performed employing Diffuse Backlight Illumination. The metallic body of a commercial injector was modified to fit transparent tips realising two nozzle layouts, namely a two-hole real size model resembling the Engine Combustion Network spray G injector and an enraged replica with an offset hole. Experiments were conducted at realistic operating conditions comprising an injection pressure of 100 bar and ambient pressures in the range of 0.1–6.0 bar to cover the entire range of chamber pressures prevailing in Gasoline Direct Injection engines. The action of viscoelastic additives was verified to have a suppressive effect on in-nozzle cavitation (6% reduction in cavitation extent) , while also enhancing spray atomisation at flash-boing conditions, in a manner resembling the more volatile gasoline/ethanol blends. Finally, persisting liquid ligaments were found to form after the end of injection for the additised sample, owing to the surfactant nature of the additives