The present paper concerns the development and validation of an Eulerian multiphase boiling model to predict boiling and critical heat flux within the general-purpose computational fluid dynamics (CFD) solver FLUENT. The governing equations solved are generalized phase continuity, momentum and energy equations. Turbulence effects are accounted for using mixture, dispersed or per-phase multiphase turbulence models. Wall boiling phenomena are modeled using the baseline mechanistic nucleate boiling model, developed in Rensselaer Polytechnic Institute (RPI). Modifications have been introduced to the quenching heat flux model to achieve mesh-independent solutions. The influences of boiling model parameters have also been systematically investigated. To model non-equilibrium boiling and critical heat flux, the PRI model is extended to the departure from nucleate boiling (DNB) by partitioning wall heat flux to both liquid and vapor phases and considering the existence of thin liquid wall film. Topological functions are introduced to consider the wall boiling regime transition from the nucleate boiling to critical heat flux (CHF), and the corresponding flow regime change from bubbly flows to mist flows. A range of sub-models are implemented to model the interfacial momentum, mass and heat transfer and turbulence-bubble interactions. To validate the Eulerian multiphase boiling model, it has been used to predict nucleating boiling and critical heat flux in a range of 2D and 3D boiling flows. The examples presented in the paper include: (1). Nucleate boiling of sub-cooled water in an upward heated pipe; (2) R113 liquid flows through a vertical annulus with internal heated walls; (3). 3D boiling flows in a rectangular-sectioned duct; and (4). Critical heat flux and post dryout in vertical pipes. The results demonstrate that the model is able to predict reasonably well the distributions of wall temperature, the bulk fluid sub-cooling temperature and cross-sectional averaged vapor volume fraction in the vertical pipe. The computed profiles of the vapor volume fraction, liquid temperature, and the liquid and vapor velocity profiles are generally in good agreement with available experiments in the 2D annular case. In the 3D rectangular duct, the cross-sectional averaged vapor volume fractions are well captured in all the ten cases under investigation. In the case of critical heat flux and post dryout, the model is also able to predict reasonably well the location and the temperature rise under critical heat flux conditions. The computed wall temperature distributions along the pipes are in overall good agreement with available experiments.
An air nozzle having a axial angle of 50 • and inner diameter of 2.2 mm was placed below the front roller nip in a ring frame, at various distances. Simulation of the airflow pattern inside the nozzle provides some useful insight into the actual mechanism of hairiness reduction. A CFD (computational fluid dynamics) model has been developed to simulate the airflow pattern inside the nozzle using Fluent 6.1 software, to solve the three-dimensional flow field. To create a swirling effect, four air holes of 0.4 mm diameter are made tangential to the inner walls of the nozzle. Airflow directions viz., against and along the direction of yarn movement are studied by changing the nozzle position, and the best results are obtained for the former case. Thirty tex Z-twisted ring spun yarns were produced with and without nozzle and tested for hairiness, tensile, and evenness properties. The total number of hairs equal to or exceeding 3 mm (i.e., the S3 values) for yarn spun with nozzle (NozzleRing yarn) is nearly 36-58% less than that of ring spun yarns (without placing nozzle), while both the yarn types show little difference in evenness and tensile properties. Hairs are wrapped along the direction of twist in the NozzleRing yarns. It is observed that air pressure, distance of the nozzle from the nip of the front roller, and direction of airflow affects the hairiness. An air pressure of 0.5 kgf/cm 2 (gauge) is found sufficient to reduce S3 values. Finally, based on the airflow simulation inside the nozzle, a mechanism of hairiness reduction has been proposed.
Open window buses without air-conditioning are a major mode of urban and inter-city transport in most countries. High occupancy combined with hot and humid conditions makes travel in these buses quite uncomfortable. In this study air flow through a bus has been studied that could be the basis for low cost and ecofriendly methods of increasing passenger comfort and possibly reduce drag. The aerodynamics of such a road vehicle has not been studied as previous investigations have been confined to vehicles with closed windows that present a smooth exterior to air flow. Using a 1:25 scaled Perspex model of an urban bus in Delhi, flow visualization was performed in a water channel. The Reynolds numbers were onetenth of a real bus moving at 10 m/s. Smoke and tuft visualizations were also performed on an urban bus at 40 km/h. Numerical simulations were performed at the actual Reynolds number. Even though there were Reynolds number differences, the broad features were similar. Air enters the bus from the rear windows, moves to the front (relative to the bus) and exits from the front windows. Inside air velocity relative to the bus is about one-tenth of the free-stream velocity. The flow is highly three-dimensional and unsteady.
In this present work we report the influence of nozzle parameters viz., axial angle and diameter of the nozzle in reducing yarn hairiness during NozzleRing spinning. Three nozzles having varying nozzle axial angles of 40 • , 45 • , and 50 • and a constant nozzle diameter of 2.2 mm, and another nozzle having axial angle of 40 • with a nozzle diameter of 2.6 mm were placed at a fixed distance of 10 cm below the front roller nip at ring frame for reducing yarn hairiness. Thirty tex carded cotton yarns were produced with and without nozzles. A CFD (computational fluid dynamics) modeling of airflow inside the nozzle indicates that a nozzle axial angle of 45 • and 2.2 mm diameter gives best results in terms of reduction of S3 values. Swirling intensity plays a major role in reduction of yarn hairiness. The change of nozzle diameter plays a lesser role in reducing the S3 values. The difference in yarn diameter and bending rigidity values of NozzleRing yarns and ring yarns spun without nozzle are not statistically significant. Fabrics made from NozzleRing yarns show lesser pilling tendency as compared to those produced from yarns spun without nozzles.
There has been a change in the thermal management of IC engines where engineers now like to harness the superior heat transfer rates available when limited and controlled nucleate boiling is used to remove heat from high temperature zones. Any flaws in the design of such systems, such as uncontrolled boiling that leads to Dry Out situation, can have an adverse effect on the cooling performance. A detailed engineering model of this process would allow engineers to weed out flawed designs early in the design process. In this paper, we proposed and validated a CFD model for this process. A CFD model is built using the commercial CFD solver ANSYS FLUENT. The mixture multiphase model is used to study subcooled nucleate boiling in IC engine cooling jackets. The departure of bubbles enhances heat transfer at walls, which is captured using the empirical correlation. Volumetric mass transfer is modeled using the inbuilt evaporation-condensation model. Results obtained from heat transfer in channels are compared with experimental results available in the literature for a range of operating pressures, different inlet sub-cooling and different inlet flow velocities. The predicted heat fluxes are in good agreement with experimental data. Results from a typical I.C. engine cooling jacket geometry are also presented.
In modern cooling systems the requirement of higher performance demands highest possible heat transfer rates, which can be achieved by controlled nucleate boiling. Boiling based cooling systems are gaining attention in several engineering applications as a potential replacement of conventional single-phase cooling system. Although the controlled nucleate boiling enhances the heat transfer, uncontrolled boiling may lead to Dry Out situation, adversely affecting the cooling performance and may also cause mechanical damage due to high thermal stresses. Designing boiling based cooling systems requires a modeling approach based on detailed fundamental understanding of this complex two-phase heat and mass transfer phenomenon. Such models can help analyze different cooling systems, detect potential design flaws and carry out design optimization. In the present work a new semi-mechanistic wall boiling model is developed within commercial CFD solver ANSYS FLUENT. A phase change mechanism and wall heat transfer augmentation due to nucleate boiling are implemented in mixture multiphase flow framework. The phase change phenomenon is modeled using mechanistic evaporation-condensation model. Enhancement of wall heat transfer due to nucleate boiling is captured using 1D empirical correlation, modified for 3D CFD environment. A new method is proposed to calculate the local suppression of nucleate boiling based on the flow velocity, and hence this model can be applied to any complex shaped coolant passage. For different wall superheat, the wall heat fluxes predicted by the present model are validated against experimental data, in which 50-50 volume mixture of aqueous ethylene glycol (a typical anti-freeze coolant mixture) is used as working fluid. The validation study is performed in ducts of different sizes and shapes with different inlet velocities, inlet sub-cooling and operating pressures. The results are in good agreement with the experiments. This model is applied to a typical automobile Exhaust Gas Recirculation (EGR) system to study boiling heat transfer phenomenon and the results are presented.
The growing awareness of pollutant emissions from gas turbines has made it very important to study fuel atomization system, the spray wall interaction and hydrodynamic of film formed on engine walls. A precise fuel spray spatial distribution and efficient fuel air mixing plays important role in improving combustion performance. Cross-flow injection and film atomization technique has been studied extensively for gas turbine engines to achieve efficient combustion. Air blast atomizer is one of these kind of systems used in gas turbine engines which involves shear driven prefilmer secondary atomization. In addition to gas turbine combustor shear driven liquid wall film can be seen in IC engines, rocket nozzles, heat exchangers and also on steam turbine blades. In our work we have used Eulerian Wall Film (EWF) [1] model to simulate the experiment performed by Arienti et al. [2]. In the Arienti’s experiment liquid jet is injected from a nozzle from the top of the chamber. Droplets shed from the jet surface due to primary and later secondary atomization in the presence of high shearing cross flowing air. Further liquid fuel particles hit the wall to form film, film moves subjected to shear from the gas phase. Liquid film can reatomizes due to subgrid processes like stripping, splashing and film breakup. In current study we have validated Arienti et al. [2] experimental data by modeling complex & coupled physics of spray, film and continuous phase and by accounting complex subgrid processes.
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