Convection refers to the flow of a single or multiphase fluid. This flow occurs because of the heterogeneity in the material properties and the body forces acting on the fluid. The most common of these forces is gravity. The forces that are responsible for the flow of fluids are density and gravity.
Natural convection
Natural convection is an important mechanism of heat transfer. It occurs without any external force and is driven by density gradients in the atmosphere. This is an important concept for many industrial applications, as it also plays a key role in many natural processes. To keep electronic components from overheating, other heat transfer methods are necessary.
Natural convection works when hot fluid rises and cool air replaces it. The rate at which heat is lost is directly related to the temperature of the object. Therefore, the colder the object, the faster the fluid is going to lose heat. This can be seen in the case of ice cubes. While one cube will only remove a small portion of heat from a drink, more cubes will result in faster chilling.
During the past two decades, researchers have studied natural convection in porous media. The field has many engineering applications, including the design of geothermal systems, storage of nuclear waste, and thermal insulation. Electronic cooling systems are used in petroleum reservoir modeling. Burying drums containing heat-generating chemicals in the earth is also considered natural convection.
This phenomenon can be observed in a wide variety of natural environments. For instance, in the outermost regions of stars, convection serves as a heat transfer mechanism. It also occurs in plate tectonics and oceanic currents. In addition to weather systems, convection is also visible in fires and rising plumes of hot air. Flows of liquid around heat-dissipation fins, solar ponds, and even solar cells are all examples of convection.
Natural convection can be studied as a process in which the density differences between a hot fluid and a cold fluid cause convection currents. It can be used to heat water, for example, for solar applications. Researchers have also studied the process of natural convection in nanofluids using different geometries.
Other local atmospheric phenomena are caused by convection, such as foehn wind (down-slope wind). Foehn wind is a natural phenomenon caused by adiabatic warming. Its temperature is influenced by different adiabatic lapse rates. This process contributes to the development of weather and climate in coastal areas.
Natural convection can be studied using numerical simulation methods. COMSOL Multiphysics is a powerful tool for this task. The pressure-wave-free option in the physics suite allows for larger time steps and shorter solution times. It is also possible to model natural convection using a weakly compressible flow option.
Free convection has two kinds: turbulent and laminar. The former occurs when fluid flows past a solid surface that is at a higher temperature than the surrounding air. This is the most common type of free convection. Its characteristic examples are shown in Figures 1 and 2. The initial heating of a vertical surface produces a laminar boundary layer. This layer grows along the flow direction and becomes unstable at some distance.
Gravitational convection
Gravitational convection is the flow of a fluid from a source in a given gravitational field. The flow pattern of a water-solute mixture depends on the source strength, the effective mass diffusion coefficient of the solute, and the hydraulic conductivity of the soil. For the most part, gravitational convection is insignificant in fine-textured soils.
In addition to heat, gravitational convection also occurs due to differences in buoyancy within a fluid. These differences in buoyancy may arise for other reasons than temperature, such as variations in the composition of the fluid itself. Regardless of the source, however, gravitational convection is a type of fluid motion that requires the presence of proper acceleration to occur.
A two-dimensional moving buoyant cloud was studied on a slope angle of 5 to 90 degrees. Thermal theory can be applied to this flow. For a given slope angle, the spatial growth rate of the cloud in height and length is a linear function of slope angle. In addition, the length-to-height ratio of the inclined starting plume and the inclined thermal cloud is identical. Furthermore, the shape of the cloud is a half-ellipse.
This flow of fluid is a natural form of convection. The heavier a fluid is, the more likely it is to be pushed or pulled by gravity. This movement of fluids is also possible on a large scale. This can occur in oceans and stars. It may even happen in accretion disks of black holes.
Multiphase convection
Multiphase convection is a process that involves mixing of different gases or fluids at the same time. This is a common phenomenon that occurs in atmospheric clouds and has several industrial applications. However, most studies on the subject of convection have focused on single-phase flow. Recent experiments on multiphase Rayleigh-Benard convection have shown that dispersed phases can enhance the heat transfer properties of a fluid by interacting with the primary phase mechanically and thermally.
This paper proposes an enhanced version of a traditional VOF model for simulation of complex multiphase flows. The model is further developed to include a novel concept of flux-limiter function based on the TVD concept. A novel pressure-based scheme combining two-step projection algorithms is also introduced. The method is then computer programmed to incorporate the PLIC-ELVIRA technique for reconstruction of the interface.
The drifting solid phase is the dominant solid phase in the multiphase convection process. This solid phase is more stable than the single-phase convection model. However, the drifting solid phase may create channels within the growing solid mush. The drifting solid phase can have a significant influence on the formation of freckles.
Multiphase convection in the Earth’s mantle is an important process in geothermal energy production and ore formation. It is important to understand fluid flow in such environments. To do this, numerical simulations are needed. The control volume finite element method is a popular tool for thermohaline convection. The method is flexible in that it can handle unstructured meshes, sharp thermal fronts, and variable compressibility.
Numerical validation has proven the accuracy of the Eulerian-Lagrangian model. Furthermore, the method’s robustness has been demonstrated in two analytical heat transfer problems. The method is also applicable to multiphase convection-diffusion problems. It was also tested with heated air blast atomization to verify its robustness.
Multiphase convection has a wide range of applications. It is a versatile process, with applications in geophysical processes, groundwater contamination, and thermal energy storage. It is also an important tool in metal casting and separation processes in the chemical industry. Furthermore, it has been the subject of increasing research interest in the past two decades.
Simulations of pedestrian flows in public areas are important tools for planning and operation of public spaces. Several studies have shown that the multiphase convection-diffusion approach can be used to simulate intersections of two different pedestrian flows. In the case of two intersecting flows, the convection corresponds to the movement in a strategic direction, while diffusion corresponds to the tactical movement aimed at avoiding jams. The different phases represent different populations, and numerical experiments have shown that the simulation model accurately reproduces the observed qualitative behaviour.
