In fluid dynamics,turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow,which occurs when a fluid flows in parallel layers,with no disruption between those layers.
Thermofluids is a branch of science and engineering encompassing four intersecting fields:
In fluid dynamics,the Schmidt number of a fluid is a dimensionless number defined as the ratio of momentum diffusivity and mass diffusivity,and it is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes. It was named after German engineer Ernst Heinrich Wilhelm Schmidt (1892–1975).
In combustion,a diffusion flame is a flame in which the oxidizer and fuel are separated before burning. Contrary to its name,a diffusion flame involves both diffusion and convection processes. The name diffusion flame was first suggested by S.P. Burke and T.E.W. Schumann in 1928,to differentiate from premixed flame where fuel and oxidizer are premixed prior to burning. The diffusion flame is also referred to as nonpremixed flame. The burning rate is however still limited by the rate of diffusion. Diffusion flames tend to burn slower and to produce more soot than premixed flames because there may not be sufficient oxidizer for the reaction to go to completion,although there are some exceptions to the rule. The soot typically produced in a diffusion flame becomes incandescent from the heat of the flame and lends the flame its readily identifiable orange-yellow color. Diffusion flames tend to have a less-localized flame front than premixed flames.
A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughout a homogeneous stoichiometric premixed charge,the combustion process once initiated sustains itself by way of its own heat release. The majority of the chemical transformation in such a combustion process occurs primarily in a thin interfacial region which separates the unburned and the burned gases. The premixed flame interface propagates through the mixture until the entire charge is depleted. The propagation speed of a premixed flame is known as the flame speed which depends on the convection-diffusion-reaction balance within the flame,i.e. on its inner chemical structure. The premixed flame is characterised as laminar or turbulent depending on the velocity distribution in the unburned pre-mixture.
In fluid dynamics,an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle,followed by a short reverse flow of fluid behind the obstacle flowing upstream,toward the back of the obstacle. This phenomenon is naturally observed behind large emergent rocks in swift-flowing rivers.
Horizontal convective rolls,also known as horizontal roll vortices or cloud streets,are long rolls of counter-rotating air that are oriented approximately parallel to the ground in the planetary boundary layer. Although horizontal convective rolls,also known as cloud streets,have been clearly seen in satellite photographs for the last 30 years,their development is poorly understood,due to a lack of observational data. From the ground,they appear as rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind. Research has shown these eddies to be significant to the vertical transport of momentum,heat,moisture,and air pollutants within the boundary layer. Cloud streets are usually more or less straight;rarely,cloud streets assume paisley patterns when the wind driving the clouds encounters an obstacle. Those cloud formations are known as von Kármán vortex streets.
In combustion engineering and explosion studies,the Markstein number characterizes the effect of local heat release of a propagating flame on variations in the surface topology along the flame and the associated local flame front curvature. The dimensionless Markstein number is defined as:
The Sugden Award is an annual award for contributions to combustion research. The prize is awarded by the British Section of The Combustion Institute for the published paper with at least one British Section member as author,which makes the most significant contribution to combustion research. The prize is named after Sir Morris Sugden.
Micro-combustion is the sequence of exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species at micro level. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds in the gas,liquid or solid phase. The major problem of micro-combustion is the high surface to volume ratio. As the surface to volume ratio increases heat loss to walls of combustor increases which leads to flame quenching.
The eddy break-up model (EBU) is used in combustion engineering. Combustion modeling has a wide range of applications. In most of the combustion systems,fuel and oxygen are separately supplied in the combustion chamber. Due to this,chemical reaction and combustion occur simultaneously in the combustion chamber. However,the rate of the chemical reaction is faster than the rate of mixing fuel and oxygen. Therefore,that rate of combustion is controlled by rate of mixing. Such cases,where formation of pre-mixture is difficult,are called diffusion combustion or diffusion flames.
Combustion models for CFD refers to combustion models for computational fluid dynamics. Combustion is defined as a chemical reaction in which a hydrocarbon fuel reacts with an oxidant to form products,accompanied with the release of energy in the form of heat. Being the integral part of various engineering applications like:internal combustion engines,aircraft engines,rocket engines,furnaces,and power station combustors,combustion manifests itself as a wide domain during the design,analysis and performance characteristics stages of the above-mentioned applications. With the added complexity of chemical kinetics and achieving reacting flow mixture environment,proper modeling physics has to be incorporated during computational fluid dynamic (CFD) simulations of combustion. Hence the following discussion presents a general outline of the various adequate models incorporated with the Computational fluid dynamic code to model the process of combustion.
Chemical reaction models transform physical knowledge into a mathematical formulation that can be utilized in computational simulation of practical problems in chemical engineering. Computer simulation provides the flexibility to study chemical processes under a wide range of conditions. Modeling of a chemical reaction involves solving conservation equations describing convection,diffusion,and reaction source for each component species.
The laminar flamelet model is a mathematical method for modelling turbulent combustion. The laminar flamelet model is formulated specifically as a model for non-premixed combustion
Multiscale turbulence is a class of turbulent flows in which the chaotic motion of the fluid is forced at different length and/or time scales. This is usually achieved by immersing in a moving fluid a body with a multiscale,often fractal-like,arrangement of length scales. This arrangement of scales can be either passive or active
In continuum mechanics,an energy cascade involves the transfer of energy from large scales of motion to the small scales or a transfer of energy from the small scales to the large scales. This transfer of energy between different scales requires that the dynamics of the system is nonlinear. Strictly speaking,a cascade requires the energy transfer to be local in scale,evoking a cascading waterfall from pool to pool without long-range transfers across the scale domain.
In combustion,the Karlovitz number is defined as the ratio of chemical time scale to Kolmogorov time scale ,named after Béla Karlovitz. The number reads as
Paul Andrews Libby was a professor of mechanical and aerospace engineering at the University of California,San Diego,a specialist in the field of combustion and aerospace engineering.
Thierry Poinsot,is a French researcher,research director at the CNRS,researcher at the Institute of Fluid Mechanics in Toulouse,scientific advisor at CERFACS and senior research fellow at Stanford University. He has been a member of the French Academy of sciences since 2019.
Albert Alan Townsend was an Australian scientist specialized in fluid dynamics. He was the author of the textbook The Structure of Turbulent Shear Flow. The terms Townsend's eddies(or Townsend's wall-attached eddies),Batchelor–Howells–Townsend spectrum and Townsend–Perry constants in turbulence research are named after him. His PhD advisor was G. I. Taylor and he was a close collaborator of George Batchelor.
