Search results

The purpose of this study is to examine the physical processes experienced by a particle-laden gas due to various types of collisions, different heat transfer modalities and jet axis switching. Here, attention is focused on a particle-laden gas subjected to jet axis switching while experiencing fluid flow and heat transfer.

The methodology used to model and solve these complex problems is numerical simulation treated here as a two-phase turbulent flow in which the gas and the particles keep their separate identities. For the turbulent flow model, validation was achieved by comparisons with appropriate experimental data. The considered interactions between the fluid and the particles include one-way fluid–particle interactions, two-way fluid–particle interactions and particle–particle interactions.

For the fluid flow portion of the work, emphasis was placed on the particle collection efficiency and on independent variables that affect this quantity and the trajectories of the fluid and of the particles as they traverse the space between the jet orifice and the impingement plate. The extent of the effect depended on four factors: particle size, particle density, number of particles and the velocity of the fluid flow. The major effect on the heat transferred to the impingement plate occurred when direct heat transfer between the impinging particles and the plate was taken into account.

This paper deals with issues never before dealt with in the published literature: the effect of jet axis switching on the fluid mechanics of gas-particle flows without heat transfer and the effect of jet axis switching and the presence of particles on jet impingement heat transfer. The overall focus of the work is on the impact of jet axis switching on particle-laden fluid flow and heat transfer.

The purpose of this paper is to present two different methods for the imposition of thermal boundary conditions (BCs) in the framework of two-phase flows: an immersed boundary method (IBM) and a Ghost cell method. Both methods are able to handle Dirichlet as well as Neumann BCs.

Direct numerical simulations of two-phase flows are performed where the thermal BCs at the phase boundary is accounted for with two different approaches.

Both methods are validated with the results obtained on a body-fitted mesh. Simulations of the three-dimensional flow and temperature field around a sphere demonstrate versatility and accuracy of both methods.

This is the first time Neumann BCs are imposed by means of an IBM with a direct heating approach employing regularized delta functions. The test cases considered may also serve as benchmarks for other studies.

The purpose of this study is to explore the mist-air film cooling performance on a three-dimensional (3-D) flat plate. In mist-air film cooling technique, a small amount of water droplets is injected along with the coolant air. The objective is to study the influence of shape of the coolant hole and operating conditions on the cooling effectiveness.

In this study, 3-D numerical simulations are performed. To simulate the mist-air film cooling over a flat plate, air is considered as a continuous phase and mist is considered as a discrete phase. Turbulence in the flow is accounted using Reynolds averaged Navier–Stokes equation and is modeled using k–e model with enhanced wall treatment.

The results of this study show that, for cylindrical coolant hole, coolant with 5% mist concentration is not effective for mainstream temperatures above 600 K, whereas for fan-shaped hole, even 2% mist concentration has shown significant impact on cooling effectiveness for temperatures up to 1,000 K. For given mist-air coolant flow conditions, different trend in effectiveness is observed for cylindrical and fan-shaped coolant hole with respect to main stream temperature.

This study is limited to a flat plate geometry with single coolant hole.

The motivation of this study comes from the requirement of high efficiency cooling techniques for cooling of gas turbine blades. This study aims to study the performance of mist-air film cooling at different geometric and operating conditions.

The originality of this study lies in studying the effect of parameters such as mist concentration, droplet size and blowing ratio on cooling performance, particularly at high mainstream temperatures. In addition, a systematic performance comparison is presented between the cylindrical and fan-shaped cooling hole geometries.

The purpose of this paper is to investigate the effects of gravity on the heat transfer behavior of the two-phase flow of water undergoing phase change. Most of the earlier studies of convective boiling considered systems where the gravity is neglected. In contrast, the authors investigated systems where the gravity is considered. The heat transfer characteristics of water during its evaporation in microchannel heat sink are studied for different channel inclinations.

Computational fluid dynamics software ANSYS Fluent is used for the computational study. The volume of fluids multiphase method available in the package is used to capture the vapor–liquid interface. Heat transfer studies are carried out for a rectangular microchannel having a characteristic dimension of 825 µm at different inclinations, which varied from −90° (vertically downward) to 90° (vertically upward). During each simulation, the vapor quality is set at the inlet. Uniform heat flux of 250 kW/m² is applied at the bottom wall of the channel in all orientations of the channel, keeping the upper wall insulated.

As compared to horizontal configuration, a significant increase in the values of heat transfer coefficient during the fluid flow in inclined microchannels is noticed. It is observed that the Nusselt number for the vertically upward (+90°) and horizontal (0°) configuration are similar and that for the 45° upward configuration exceeds other configurations. It is also observed that the heat transfer performance becomes lower in downward configurations; nearly 40-50 per cent drop in average Nusselt number is observed for a mass flux of 250 kg m^-2 s^-1 with respect to 45° inclined microchannel. This behavior can be attributed to the gravitational effect on the two-phase flow because of which the vapor phase being less dense moves away from the heated wall, whereas the primary phase being heavier moves towards the heated wall of the channel. Also, the conductivity of the liquid being higher than the vapor phase, as well as the aperture of the liquid being small during this process, its velocity increases resulting in the augmentation of heat transfer.

User-defined-functions for the mass and energy source terms have been written in C code and hooked in ANSYS Fluent to incorporate the phase change mechanism during the evaporation of water.

This paper seeks to discuss a mechanistic modeling concept for local phenomena governing two‐ and multi‐phase flows and heat transfer.

An overview is given of selected issues concerning the formulation of multidimensional models of two‐phase flow and heat transfer. A complete computational multiphase fluid dynamics (CMFD) model of two‐phase flow is presented, including local constitutive models applicable to two‐phase flows in heated channels. Results are shown of model testing and validation.

It has been demonstrated that the overall model is capable of capturing various local flow and heat transfer phenomena in general, and the onset of temperature excursion (CHF) in low quality forced‐convection boiling, in particular.

Whereas the multiphase model formulation is applicable to a large class of problems, geometries and operating conditions, the closure laws and results are focused on forced‐convection boiling in heated channels.

The proposed approach can be used to predict multidimensional velocity field and phase distribution in two‐phase flow devices and components used in thermal power plants, nuclear power plants and chemical processing plants.

A complete mechanistic multidimensional model of forced‐convection boiling in heated channels is given. The potential of a CMFD approach is demonstrated to perform virtual experiments that can be used in system design and optimization, and in safety analysis.

This paper investigates the multi‐phase behaviour of droplets injected into a nozzle at two separate wall locations. The physical features of the droplets (rate of mass, density and radius) at each injector location are identical. This system can be described by a two‐phase Eulerian—Eulerian approach that yields classical systems of equations: three for the gaseous phase and three for the dispersed droplet phase. An underlying assumption in the two phase model is that no interaction occurs between droplets. The numerical solution of the model (using the MacCormack scheme) indicates however that the opposite jets do interact to form one jet. This inconsistency is overcome in the current paper by associating the droplets from a given injection location with a separate phase and subsequently solving equations describing a multiphase system (here, three‐phase system). Comparison of numerical predications between the two‐phase and the multiphase model shows significantly different results. In particular the multiphase model shows no jet interaction.

To find the optimal number of channels of rocket engine thrust chamber, it was found that the optimal channel number is 335, at which the cooling effect of the thrust chamber cooling channel reaches the best, which can be helpful to design rocket engine thrust chamber.

The commercial computational fluid dynamics (CFD) software FLUENT with standard k‐ε turbulent model was used. The CFD method was validated via comparing with the available experimental data.

It was found that both the highest temperature and the maximal heat flux through the wall on the hot‐gas side occurs about the throat region at the symmetrical center of the cooling channel. Owing to the strong curvature of the cooling channel geometry, the secondary flow reached its strongest level around the throat region. The typical values of pressure drop and temperature difference between the inlet and exit of cooling channel were 2.7 MPa and 67.38 K (standard case), respectively. Besides an optimal number of channels exist, and it is approximately 335, which can make the effect of heat transfer of cooling channels best with acceptable pressure drop. As a whole, the present study gives some useful information to the thermal design of liquid rocket engine thrust chamber.

More detailed computation and optimization should be performed for the fluid flow and heat transfer of cooling channel.

A very useful optimization on heat transfer and fluid flow in cooling channel of liquid rocket engine thrust chamber.

This paper provides the performance of optimization on heat transfer and fluid flow in cooling channel of liquid rocket engine thrust chamber, which can make the effect of heat transfer of cooling channels best with acceptable pressure drop. As a whole, the present study gives some useful information to the thermal design of liquid rocket engine thrust chamber.

This paper aims to derive a reduced-order model for the heat transfer across the interface between a millimetric thermocapillary liquid bridge from silicone oil and the surrounding ambient gas.

Numerical solutions for the two-fluid model are computed covering a wide parametric space, making a total of 2,800 numerical flow simulations. Based on the computed data, a reduced single-fluid model for the liquid phase is devised, in which the heat transfer between the liquid and the gas is modeled by Newton’s heat transfer law, albeit with a space-dependent Biot function Bi(z), instead of a constant Biot number Bi.

An explicit robust fit of Bi(z) is obtained covering the whole range of parameters considered. The single-fluid model together with the Biot function derived yields very accurate results at much lesser computational cost than the corresponding two-phase fully-coupled simulation required for the two-fluid model.

Using this novel Biot function approach instead of a constant Biot number, the critical Reynolds number can be predicted much more accurately within single-phase linear stability solvers.

The Biot function for thermocapillary liquid bridges is derived from the full multiphase problem by a robust multi-stage fit procedure. The derived Biot function reproduces very well the theoretical boundary layer scalings.

The purpose of this paper is as follows: to significantly reduce the computation time (by a factor of 1,000 and more) compared to known numerical techniques for real-world problems with complex interfaces; and to simplify the solution by using trivial unfitted Cartesian meshes (no need in complicated mesh generators for complex geometry).

This study extends the recently developed optimal local truncation error method (OLTEM) for the Poisson equation with constant coefficients to a much more general case of discontinuous coefficients that can be applied to domains with different material properties (e.g. different inclusions, multi-material structural components, etc.). This study develops OLTEM using compact 9-point and 25-point stencils that are similar to those for linear and quadratic finite elements. In contrast to finite elements and other known numerical techniques for interface problems with conformed and unfitted meshes, OLTEM with 9-point and 25-point stencils and unfitted Cartesian meshes provides the 3-rd and 11-th order of accuracy for irregular interfaces, respectively; i.e. a huge increase in accuracy by eight orders for the new 'quadratic' elements compared to known techniques at similar computational costs. There are no unknowns on interfaces between different materials; the structure of the global discrete system is the same for homogeneous and heterogeneous materials (the difference in the values of the stencil coefficients). The calculation of the unknown stencil coefficients is based on the minimization of the local truncation error of the stencil equations and yields the optimal order of accuracy of OLTEM at a given stencil width. The numerical results with irregular interfaces show that at the same number of degrees of freedom, OLTEM with the 9-points stencils is even more accurate than the 4-th order finite elements; OLTEM with the 25-points stencils is much more accurate than the 7-th order finite elements with much wider stencils and conformed meshes.

The significant increase in accuracy for OLTEM by one order for 'linear' elements and by 8 orders for 'quadratic' elements compared to that for known techniques. This will lead to a huge reduction in the computation time for the problems with complex irregular interfaces. The use of trivial unfitted Cartesian meshes significantly simplifies the solution and reduces the time for the data preparation (no need in complicated mesh generators for complex geometry).

It has been never seen in the literature such a huge increase in accuracy for the proposed technique compared to existing methods. Due to a high accuracy, the proposed technique will allow the direct solution of multiscale problems without the scale separation.