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Viscous Flow and Boundary Layer
Published in Rose G. Davies, Aerodynamics Principles for Air Transport Pilots, 2020
The expression for viscous friction in a laminar boundary layer is relatively simple, and it is proportional to the speed change in the direction perpendicular to the surface. The expression for viscous friction in a turbulent boundary layer is relatively complex. A laminar sublayer exists next to the surface within the turbulent boundary layer, where the expression for viscous friction for laminar flow can be applied. However, to analyze the dynamic features, for example, velocity change and forces throughout the turbulent boundary layer, a turbulent model is required. This model is a mathematical expression, or a set of equations. Those model equations should indicate that the viscous friction is not in a linear relationship with the change of speed in the direction perpendicular to the surface. There would be more terms of second-order cross derivatives to describe the friction stresses in different directions around a fluid particle, as shown in the terms in the brackets of Equations (3.5) and (3.6). It is difficult to solve those equations for a turbulent boundary layer analytically, and it can be solved numerically with a standard PDE solving program. The details of the turbulent model and the principle of solving the equations of boundary layer can be found in Boundary-Layer Theory (Schlichting, 1978).
An Alternative Model for Flow Boiling Heat Transfer
Published in John C. Chen, Yasunobu Fujita, Franz Mayinger, Ralph Nelson, Convective Flow Boiling, 2019
The suppression of the convective component due to the presence of nucleate boiling is a simple concept. A variety of nucleate boiling heat transfer mechanisms that arise from bubble related activities, such as bubble growth and bubble departure, occur within or very close to the laminar sublayer. Thus nucleate boiling can be viewed as a heat transfer mechanism internal to the laminar sublayer. In contrast to this, convective heat transfer arising from flow can be viewed as a heat transfer mechanism which is external to the laminar sublayer. For example, increasing flow velocity will have an effect of thinning of the laminar sublayer or increase in the number of turbulent bursts acting on the sublayer. It can, therefore, be argued that wherever the internal heat transfer mechanism of nucleate boiling is present, the externally imposed mechanism of convective heat transfer is likely to have little or no effect. The concept of suppression of the convective component, thus described, is expressed as: () q˙T=(1−Anb)q˙c+q˙nb
Experimental Evaluation of Cleaning Techniques
Published in R. P. Donovan, Particle Control for Semiconductor Manufacturing, 2018
Spray Flow: This type of fluid flow has been used for many years for cleaning surfaces (Stowers, 1978; McVey, Campuzano, and Fowler, 1981). Silicon wafer rinsers/dryers and most scrubbers use pressurized liquid sprays to remove contaminants from surfaces. When the spray impinges a surface, the fluid changes direction and flows parallel to the surface (see Figure 22-4). When a fluid passes over a surface, the velocity approaches zero immediately at the surface but increases to a maximum, known as the free stream velocity (Us), at some distance above the surface. Fluid molecules at the solid surface are brought to rest, and those for a short distance above the surface are slowed because of viscous shear in the fluid. This region of retarded flow is called the “boundary layer” and for practical purposes extends to the point at which fluid velocity equals 99 percent of free stream velocity. For high-velocity spray applications, the flow within this boundary layer is usually turbulent over the entire surface. A thin laminar sublayer will exist between the solid surface and the turbulent portion of the boundary layer. Within this sublayer, velocities decrease rapidly to zero (Musselman and Yarbrough, 1987).
An Experimental Study of Thermohydraulic Performance of Solar Air Heater Having Multiple Open Trapezoidal Rib Roughnesses
Published in Experimental Heat Transfer, 2022
Naveen Kumar Gupta, Tabish Alam, Himanshu Singh
Figures 8 and 9 show the effect of Re on Nu and the friction factor due to smooth duct and multiple open trapezoidal ribs having the following fixed parameters: e/Dh = 0.043, L/w = 0.8, p/e = 10, α = 60°, and W/w = 6. The value of Nu increases with the increase in Re in the case of smooth duct and multiple open trapezoidal ribs, which are expected. However, enhancement in Nu is much higher in the case of roughened duct with multiple open trapezoidal ribs as comparison to the smooth duct case. Higher Nu has been observed in roughened duct due to the fact that ribs create secondary flow, which mixed with main flow after reattachment, leading to higher turbulence close to the roughness. The laminar sublayer is disturbed by turbulence in the flow near the heat transfer surface, which reduces the thermal resistance and increases the heat transfer coefficient. This higher turbulence leads to a higher convection coefficient. Also, the friction factor decreases as the value of Re increases in both cases of smooth duct and roughened duct with multiple open trapezoidal ribs. A higher friction factor has been observed in the case of roughened duct as that of smooth duct, which is because of higher turbulence creation due to roughness and contributes to the higher friction factor.
Assessment of Mixture and Eulerian Multiphase Models in Predicting the Thermo-Fluidic Transport Characteristics for Fly Ash-Water Slurry Flow in Straight Horizontal Pipeline
Published in Heat Transfer Engineering, 2019
Bibhuti Bhusan Nayak, Dipankar Chatterjee
Figure 3 shows the axial velocity distribution at a mid-horizontal cross sectional plane along which the flow occurs from top towards the bottom side of the pipe, and at the pipe outlet for mean inflow velocities of Vm = 1, 3, and 5 m/s at different solid volume fractions of Cvf= 20% and 30%. For the mixture model, the velocity is more at the core of the pipe at lower inflow velocities and concentrations. At higher velocities and concentrations, the velocity at the core is reduced. This is due to the stronger viscous shear stress in the laminar sublayer region. For the Eulerian model, the velocity of the solid particles is more towards the outlet core at higher concentrations and inflow velocities and less at the lower concentrations and inflow velocities. The velocity is more towards the outlet core of the pipe at higher inflow velocities and concentrations. At lower concentrations, the velocity distributions are symmetrical in nature; however at higher concentrations it becomes asymmetric due to increase of the viscous shear force. For both the models, the velocity distributions shows symmetry at higher velocities and concentrations due to complete mixing of the fluid and solid particles because of the turbulence. As we move radially towards the center of the pipeline, we observe that the axial velocity becomes higher than the mean velocity.
Experimental Investigation on Heat Transfer Enhancement of Artificially Roughened Solar Air Heater
Published in Heat Transfer Engineering, 2022
Anil Singh Yadav, Abhishek Sharma
The variation between Nusselt number and Reynolds number is represented in Figure 7 for given value of relative roughness height (e/Dh). Nusselt number increased with Reynolds number. This is because of turbulence caused by air passing through the duct. The turbulence depends on air velocity within the duct. Greater air velocity causes increase in Nusselt number. Results indicate that, as the Reynolds number rises, the flow undergoes intense turbulent mixing, which reduces the influence of laminar sublayer thickness and leads to improvements in heating rate and high Nusselt number.