Droplet separators for evaporative towers: efficiency estimation by PDA

Abstract A Phase Doppler Anemometry system has been set up to characterize the behaviour of different arrangements of static impactors that are used as drift eliminators to intercept and remove residual water droplets entrained in the hot air flow released by an evaporative tower. The investigated evaporative tower has a square section of 60x60 cm, that is the standard size for modular separation elements; it was tested while working with or without the droplet separators; for each test condition, the residual water droplets entrained by the air flow and expulsed by the tower are detected few centimetres above the tower exhaust and are characterized by the PDA system that can measure the velocity and diameter of each droplet. Many parameters describing the droplet population over the whole exit section, (number, mean velocity and diameter) can be obtained. The distribution of the droplets is calculated as a function of their diameter, and represented as a percentage of their total number and of their total volume. The velocity-diameter plot of the same droplets shows other aspect of the population. Droplets with same diameter show a spread velocity distribution due to the high turbulence of the exhaust air flow. Droplets with larger diameters have smaller mean velocity, since the gravitational downward force is not negligible compared to the aerodynamic upward drag from the air flow. Few very large droplets have even negative velocity: they are interpreted as droplets that, after the expulsion, are falling back downward. The use of an LDV-PDA system allows to detect such droplets and to discard them from the separation efficiency calculations. The efficiency can be calculated by direct comparison of the number of water droplets that are detected in the exhaust air flow, both globally or for any specific class of droplet size. Global results can be calculated on the basis of the number or of the volume of the droplets. The accuracy of the efficiency estimation is also studied. Two main aspects are considered. The first is that the PDA measurement volume dimension variation with the detected droplet size: it has negligible effect on the result per classes of diameters, but the effect on the global efficiency is present and will be discussed. The second aspect is the presence of droplets that are falling downward in the upward air flow: their presence should be considered and corrected for. The results of this paper are useful when comparing them to other measurements obtained with techniques that are not able to detect the droplet velocity

Effect of injection conditions on penetration and drop size of HCCI Diesel sprays

Abstract. The development of direct injection strategies in modern Diesel engines needs increasingly high injection pressures and better fuel distribution in the combustion chamber. The ongoing study of the HCCI concept will probably require early fuel injection in air at low pressure and density, and also fuel composition will probably require some modification to reach a perfect mixing and to match evaporation and ignition requirements. In this work different common rail nozzles, fed with fuel supplied at constant pressure in the range from 30 to 100 MPa, are used to produce sprays in air at ambient temperature and pressure ranging from 1 to 7 bar, to investigate the spray penetration as a function of air and fuel pressure. From experimental results a scale low is then deduced, which is able to account for different penetration curves in the various tests by a unique common behaviour: a linear penetration part, whose length is function of the air density and of the nozzle diameter, followed by a decrease of the tip velocity. For a reduced set of experimental conditions drop size and velocity are measured by phase Doppler anemometry; time averaged mean diameter is then computed and analysed as a function of the fuel injection pressure, and shows a clear reduction of the drop diameter with increasing injection pressure

Analysis of PDA measurements in double injection GDI sprays

Abstract A N-heptane spray from a GDI multi-hole injector operated in ambient air at fixed conditions and with double injection commands is studied with different experimental techniques to better understand the spray behaviors, focusing the analysis on the effect of different dwell times between the two pulses. Results from spray photographic analysis, fuel injected quantity, droplet velocity and sizing by Phase Doppler Anemometry are presented and compared. The peculiarities and usefulness of a complementary application of the different techniques is illustrated. The two spray pulses have the same time length, so that the first spray evolves in a nearly quiescent and clean ambient, while the second, nominally identical to the first one, evolves in its trailing edge. The direct comparison allows an immediate perception of the differences among the two sprays, at the different dwell times, where the shorter tested, 160 microseconds, was chosen as the one that shows the first appreciable effect with at least one of the used techniques; the differences are clearly evident in the PDA results, sufficiently visible from the injection rate, not appreciable in the imaging at short distance. The effect of the longer dwell times becomes more evident and is illustrate

Flash boiling effect on swirled injector spray angle

Abstract A swirled injector for gasoline direct injection was used to investigate the effect of fuel flash boiling on the initial angle of the spray. The hollow cone spray was injected into a constant pressure bomb filled with quiescent air. The fuel was fed at 7 MPa constant pressure to the injector. Three parameters were changed to study the effect of the injection conditions on the spray angle: fuel composition, fuel temperature and air pressure in the test bomb. The injector tip was heated up to 150°C to keep the fuel to be injected at the desired temperature. Different blends of iso-octane and n-pentane were used to obtain fuels with different bubble temperature at the same air pressure. In a reduced set of experiments, only with pure fuels, the ambient pressure was varied to change the bubble temperature independently from the fuel temperature. It was observed that, when the fuel conditions exceed the bubble point, the spray angle, measured close to the injector, becomes wider. This angle was chosen as an indicator of the flash boiling intensity. The experimental results show that the angle value is well fitted by a unique correlation if it is expressed as a function of the ratio P=Pb/ Pair between the fuel bubble pressure and the bomb pressur

Setting up a PDPA system for measurements in a Diesel spray

Abstract. A PDPA system was set up, optimised and used to measure the time resolved characteristics of the droplets inside a spray produced by a common-rail diesel fuel injection system. Some preliminary tests are performed with gas flows to optimise the optical set-up. Parametric studies are performed to gain an understanding of the particle density limits of the system, and their dependence on PDPA system parameters. Then the diesel spray produced by a single-hole injector is measured, with the fuel pressure ranging from 500 to 1300 bar, gas density in the test chamber ranging from ambient conditions to 40 kg/m3. Fuel and gas temperature were 25ºC. Beam waist size is reduced to the minimum value allowed by the optical stand-off of the spray enclosure. Receiver lens focal length is similarly reduced. Receiver slit width, which is found to have a dramatic effect on the detection of droplets during the injection period, was tested in the range from 100um to 25um. Tests performed with two different slit heights are tested, respectively 1mm and 50 μm, show that this parameter has minimal effect on performance. PMT voltage (gain) is held to a moderately low value between 400 and 500 volt and the laser power between 400 and 800 mW in the green line. Optimum burst threshold is found to obtain the best quality data regardless of background level, which varies greatly in high-density pulsed sprays

A Modular Rack for Shared Thermo-Fluid Dynamics Experiments in Reduced Gravity Environment

Abstract Parabolic flights represent an important tool for short space-related experiments under reduced gravity conditions. During the ballistic flight manoeuvres, the investigators have the possibility to operate their experiments, in a laboratory-like environment, where the level of gravity subjected to the experiments repetitively in a series of periods of reduced gravity, preceded and followed by periods of hypergravity. Aboard large aircraft, the duration of this phases varies from approximately 20 s for a 0g flight up to up to 32 s for a Martian g level. A parabolic flight rack able to host experiments concerning thermo-fluid dynamics, has been designed, realized and qualified during the ESA 66th Parabolic Flight Campaign. This microgravity research platform, is the first UK facility available for such investigations, providing a data acquisition system, cooling system and heating system compliant with Novespace requirements

Effect of the Ambient Temperature on the Start-Up of a Multi-Evaporator Loop Thermosyphon

Two-phase heat transfer devices are becoming fairly ubiquitous; the capability to transport heat at high rates over appreciable distances, without any external pumping device, the low cost, durability and relatively simpler modeling/design process, make this technology very attractive for many thermal management applications. Indeed, such devices have been investigated in plenty of fields such as: nuclear plants, energy systems, solar heat recovery, air conditioning, electronic cooling in avionics and in railway traction. As a consequence, they can operate under different environmental conditions that can affect their behavior. Nevertheless, it is difficult to find in literature something related to the effect of the ambient temperature on the thermal performance of such devices. The actual temperature, varying the thermo-fluid properties of the fluid inside the device, the condensation and the evaporation phenomena, could be an important parameter that can affect the performance. In this work a Multi-Evaporator loop thermosyphon is tested at different ambient temperatures, ranging from -20 °C up to 30 °C. The start-up behavior, as well as the thermal performance, are analyzed by means of temperature and pressure measurements and fluid flow visualization

SINGLE LOOP PULSATING HEAT PIPE WITH NON-UNIFORM HEATING PATTERNS: FLUID INFRARED VISUALIZATION AND PRESSURE MEASUREMENTS

Abstract. A novel Single Loop Pulsating Heat Pipe (SLPHP) filled at 60% filling ratio with pure ethanol, with an inner diameter of 2mm is tested in Bottom Heated mode varying the heating power. The system is designed with two sapphire tubes mounted between the evaporator and the condenser allowing simultaneous fluid flow high-speed visualizations and IR analysis. Furthermore, two highly accurate pressure transducers carry out local pressure measurements just at the ends of one of the sapphire inserts. Additionally, three heating elements are controlled independently, in such a way to heat up the device varying the distribution of the heating location at the evaporator. It is found that peculiar heating distributions promote the slug/plug flow motion in a preferential direction, increasing the overall performance of the device. Pressure measurements point out that the flow patterns are strictly related to the pressure drop between the evaporator and the condenser. Furthermore, the IR visualization highlights interesting phenomena related to the liquid film dynamics during the device operations, which represent a very useful information for future numerical modeling of Pulsating Heat Pipes

Flash boiling in a multihole G-DI injector – Effects of the fuel distillation curve

The effects of fuel temperature and chamber pressure on the spray of a multi-hole G-DI injector were analyzed in a quiescent test chamber. The analysis was focused on the behavior of the global spray angles both close and far from the injector. Three pure hydrocarbons (n-hexane, n-heptane, and isooctane), three gasolines of known distillation curve and a commercial 95 RON gasoline from a gas station were utilized. The tests were performed at four chamber pressures (atmospheric, 80 kPa, 60 kPa and 40 kPa) and the fuel temperature was varied from 30 °C to 110 °C. The results for n-hexane and gasolines were very similar, while n-heptane and isooctane showed a different behavior. The ratio between the fuel saturation pressure at the operating temperature and the air pressure (ps/pa) is confirmed as a fundamental parameter for spray angle data reduction. The near field spray angle data for pure hydrocarbon fuels merge to a unique curve when plotted in function of ps/pa. An approximated method to deduce the gasoline saturation pressure curves starting from the distillation curve is presented. Using the calculated saturation pressures for the reduction of near field spray angle data for the gasolines, a unique curve is obtained, coincident with that of the tested pure hydrocarbons. In alternative, from the results obtained for a fuel of known saturation pressure curve, it is possible to obtain a direct correlation between near field spray angle and saturation pressure. From this relationship, an approximated saturation pressure curve from the experimental angle measurements obtained on the same injector for an unknown fuel can be derived

Effect of Injection Strategy and Fuel Pressure on Diesel Spray Penetration: Application to HCCI Engines

ABSTRACT Direct injection strategies play an important role in modern Diesel engines and high-pressure common rail injection systems made easier to achieve better engine performances and lower fuel consumption. It is generally recognized that the design of the fuel injection strategy and in particular the performance of the injection system represent the key factor in meeting the increasing demand for improvement of engine performance and reduced exhaust emissions. The recent introduction of the homogeneous charge compression ignition (HCCI) combustion concept, as an attractive alternative for future IC engines, poses an increased demand for better fuel injection strategies and improvement of the mixture formation process. The HCCI concept requires early fuel injection in a gas environment at lower pressure and density, compared to the conventional Diesel engines, and thus the injection strategy needs modification to assure sufficient mixing and to match evaporation and ignition requirements, while avoiding fuel impingement on the combustion chamber surfaces. This paper describes spray penetration characteristics of a common-rail piezo-type injector operated at medium-high pressures. Optical diagnostics and CCD camera photography are currently applied to generate an extensive data base which is intended for validation of CFD simulations of HCCI engines. The preliminary results show that a proper selection of the injection strategy together with high injection pressures can fulfil the main requirements of HCCI engine conditions, still avoiding impingement of the spray on the wall. However, poor mixture formation is achieved and intense gas motion may be necessary inside the engine cylinder