Multiphase flow
In fluid mechanics, multiphase flow refers to the flow of a mixture of several phases .
In normal fluid mechanics, only one fluid (e.g. water) is considered; in multiphase flows, flows made up of different substances, e.g. B. water and oil, or in general several different fluids or combinations of gases, liquids and solids .
A two-phase flow consists of a mixture of two phases. This is mostly a liquid and a gaseous phase. A basic distinction is made between a one-component two-phase flow , an important application here would be the flow of a mixture of boiling water and water vapor in energy technology and in evaporative cooling , and a multi-component two-phase flow , for example water and air . In general, both phases do not flow in the pipe at the same speed.
Multiphase flows are divided into
- separate (separated) multiphase flows (e.g. film flow ) The two phases are not mixed.
- discontinuous multiphase flows (e.g. plug flow ) similarly large volume fractions of the individual phases.
- disperse multiphase flows (e.g. spray flow ) consisting of a continuous phase with a high volume fraction and disperse phases in particle form .
Types of multiphase flows
Depending on the speed and mass flow ratio of the phases, different characteristic distributions of the phases appear over the cross section of the flow channel, which are also called flow forms. The resulting flow results from the forces prevailing in the flow and acting on each of the phases.
Gas - liquid
Such currents occur when generating steam from water in power plants or during chemical reactions. These gas-liquid mixtures are common in industry and have therefore been well studied.
Heinz Brauer has shown corresponding general phase distribution states for the existing flow directions. In the bubble flow , the gas phase is uniformly dispersed in the continuous liquid phase. It can be assumed that the speed of both phases is the same (slip = 1). If there is an increase in the proportion of gas, so more and more bubbles grow together and form a gas piston, one observes a Kolbenblasen- or plug flow . A further increase in the proportion of gas in the horizontal pipe ultimately leads to stratified flow . In this case, gas and liquid flow separately through the tube according to their density, whereby small bubbles can occur in the liquid. This form of flow is not observed in the vertical pipe. This also applies to wave and slug flow, in which the gas flow transfers a shear stress to the liquid, which causes waves if a certain value is assumed. From a certain gas throughput, individual wave crests reach such a great height that they are pushed through the pipe as a surge. The plug flow in turn occurs in the vertical as well as in the horizontal pipe. Plugs of gas push through the pipe, the liquid surrounds them and covers the pipe wall. At very high gas throughputs, a film flow , also known as an annular flow , forms . The liquid only forms a film on the pipe wall. Usually the gas flows faster than the liquid (slip> 1). The outer limit of two-phase flow is represented by mist or spray flow. The liquid droplets are homogeneously distributed in the gas phase .
As a rule, four main groups can be distinguished for a pipe with an upward flow, namely bubble flow , piston flow , annular flow and spray flow .
Gas - solid
Such currents occur during the pneumatic conveying of granulates, grain, powder or dust removal . In process engineering, this flow is called the entrained flow .
Liquid - solid
Such currents occur during the hydraulic conveyance of sand, gravel, mud or also when separating waste.
Three and multiphase flows
Such flows occur in gas-liquid reactions that require a solid catalyst or in gas-oil-water mixtures.
Numerical simulation
Numerical fluid mechanics has developed different models to calculate multiphase flows . Models for disperse multiphase flows:
- Euler-Euler model (each phase is viewed as a continuum )
- Euler-Lagrange model (single particle observation in the flow)
- Algebraic slip model
- Euler-Granular Model
Models for separated multiphase flows:
literature
- Heinz Brauer: Basics of single-phase and multi-phase flow. In: Verlag Sauerländer , 1971.
- Lutz Friedel: Model law for the friction pressure loss in two-phase flow. In: VDI research booklet. 572, 1975.
- Numerical models for two-phase turbulent flows, CTCrowe, TRTroutt, and J, N, Chung, Ann.Rev.Fluid Mech. 28, 1-45 (1996)
- Volume of fluid (VOF) method for the dynamics of free boundaries, CW Hirt, BD Nichols, Journal of Computational Physics, Vol. 39, pp. 201-225, (1981)
- Level Set Methods and Dynamic Implicit Surfaces, SJ Osher, R. Fedkiw, Springer, New York, 2003. ISBN 0-387-95482-1
- International Journal of Multiphase Flow
Individual evidence
- ↑ a b Dominik Surek, Silke Stempin: Technical fluid mechanics: For studies, exams and practice. Springer-Verlag, 2017, ISBN 3658187573 , p. 550.