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Turbulent Flows with Chemical Reaction
Published in Bart Merci, Tarek Beji, Fluid Mechanics Aspects of Fire and Smoke Dynamics in Enclosures, 2023
As mentioned in Section 2.8, the momentum and heat transfers strongly increase in turbulent motions, as compared to laminar flow conditions, because momentum and heat are transferred on a larger scale through the turbulent eddies. As a result, the surface friction and heat transfer increase and the boundary layer becomes thicker. It can be shown that: δturb=0.37xRex−1/5;Cf,x,turb=0.0592Rex−1/5.
External Forced Convection
Published in Je-Chin Han, Lesley M. Wright, Analytical Heat Transfer, 2022
The similarity method can be used to determine velocity and temperature distributions inside the laminar boundary layer formed on a surface exposed to an accelerating or decelerating mainstream flow. The acceleration or deceleration of the fluid results from a pressure gradient along the surface. With a “constant” pressure gradient, the mainstream velocity can be represented as U=cxm. Similarly, with a constant temperature gradient along the surface, the temperature difference is expressed as Tw−T∞=cxn. Figure 7.9 shows several flow configurations that can be considered as laminar boundary layer flow with a constant pressure gradient: flow over a wedge with acceleration (m > 0), flow in a nozzle with acceleration (m > 0), flow in a diffuser with deceleration (m < 0), flow across a cylinder with a stagnation point (m = 1), flow impingement on a wall with stagnation, etc. Note that flow over a flat plate with a constant velocity is a special case of m = 0. The boundary layer will be thinner with increased shear and heat transfer in the presence of flow acceleration and thicker with reduced shear and heat transfer with a decelerating fluid.
Viscous fluid flow – boundary layer
Published in Amithirigala Widhanelage Jayawardena, Fluid Mechanics, Hydraulics, Hydrology and Water Resources for Civil Engineers, 2021
Amithirigala Widhanelage Jayawardena
When the pressure gradient is negative, it is called a favourable pressure gradient. When it is so, there is a resultant force in the direction of flow due to pressure. This pressure acts against the boundary shear, thereby reducing the retarding action of the shear. Therefore, the boundary-layer thickness increases more slowly compared with the case when the pressure gradient is zero.
Simulation of the laminar thermal boundary layer problem over a flat plate by exploring the local non-similarity technique
Published in International Journal of Modelling and Simulation, 2023
Matthew O. Lawal, Yusuf O. Tijani, Suraju O. Ajadi, Kazeem B. Kasali
Boundary layer refers to a thin layer of fluid that forms near a surface that is in contact with a fluid flow. In this instance, this layer experiences a different velocity than the bulk of the fluid flowing over it, resulting in variations in the flow properties, such as velocity, temperature, and concentration of chemical species [1]. Boundary layers can be classified based on their thickness, which can be laminar or turbulent, depending on the flow conditions. This phenomenon plays an important role in many applications, including aerodynamics, hydrodynamics, and heat transfer. Understanding the boundary layer properties can help improve the performance and efficiency of various engineering systems and machines. Fluid tends to exhibit some behaviour such as diffusion, rotation, and dispersion to mention but few. Fluid particles will move in response to a shear force, resulting in a permanent change to their relative positions, even after the force has been removed [2].
A method for roughness height prediction by particle deposition and its effect on flow and heat transfer in a turbine cascade
Published in Numerical Heat Transfer, Part A: Applications, 2023
Hong Wang, Jialong Li, Haopeng Guo
In this study, the particle deposition is predicted by the critical viscosity criterion which is briefly described above. However, the surface condition, such as surface roughness, would be changed as the particle deposits on the solid surface. The surface roughness has a great impact on heat transfer, skin friction, and boundary layer turbulent transition. Therefore, the prediction of surface roughness is becoming essential to more accurately model the flow and heat transfer characteristics in a turbine cascade. They are closely concerned with sand roughness or equivalent sand-roughness ksproposed by Stalder [31] based on Nikuradse’s data [29]. Various correlations [12] were used to convert surface roughness parameters, such as center-line average roughness Ra and average peak to valley roughness Rz, to equivalent sandgrain roughness ksin the study of roughness effect in a turbine. In this study, a correlation of referring to the study [31] is used, in which center-line average roughness Ra is the parameter used to describe surface roughness, seen in Figure 1 and calculated as: where l is the sampling length. The function f(x) is the distance from the roughness height to the center-line of the profile.
Sisko fluid modeling and numerical convective heat transport analysis over-stretching device with radiation and heat dissipation
Published in Numerical Heat Transfer, Part A: Applications, 2023
Torikul Islam, M. Ferdows, MD. Shamshuddin, Marei Saeed Alqarni
The thin layer that covers the surface where a fluid boundary occurs is known as the boundary layer. BLF is the flow field region that significantly affects the wall. The investigation of BLFs and heat transmission on stretchable surfaces has received the researcher’s daily interest. This happened because the boundary layer behavior of over-stretching surfaces has important roles in engineering sectors such as glass fiber production, wire drawing, polymer extrusion, continuous stretching of plastic films, paper production, metal extrusion, and spinning, etc. The theoretical studies of the momentum energy transfer happening upon the boundary layer surrounding a surface moving were first introduced by Sakiadis [1]. Crane [2] first investigated boundary layer flow across the stretching sheet. Basically, Crane enhanced the work of Sakiadis [1]. After that, many researchers [3–6] work under different physical conditions of boundary layer flows upon stretchable surfaces. The flow and energy transmission upon an exponentially stretchable sheet is considered by Magyari and Keller [7]. Schowalter [8] found the similarity solutions to the power-law fluid flow within the boundary layer. Andersson [9] studied the magnetohydrodynamics (MHD) flow of a power law fluid on a stretchable surface. Cortell [10] studied energy transmission in a fluid flow upon a nonlinearly stretchable sheet. Many researchers investigate energy transfer in fluid flow upon an exponentially stretchable sheet with different physical conditions and get better output. All of the above-studied scientists were worried about the BLF.