Effect of wall thermal properties on the mean temperature profile in near-wall turbulence (original) (raw)
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WALL PROPERTIES AND HEAT TRANSFER IN NEAR-WALL TURBULENT FLOW
Numerical Heat Transfer, Part A: Applications, 2004
Direct numerical simulation of a passive scalar in fully developed turbulent channel flow is used to show that Nusselt number is not only a function of Reynolds and Prandtl number, but also depends on properties of a heating wall. Variable thickness of the heating wall and variable heater properties, combined in a fluid-solid thermal activity ratio
International Journal of Heat and Mass Transfer, 1993
Measurements in a fully developed turbulent channel flow with one wall heated at constant temperature and the opposite wall at approximately ambient temperature are compared with available direct numerical simulations. The consequences of the different thermal boundary conditions used in the experiment and the simulations are explored, especially with regard to distributions of the turbulent heat flux and the average production of temperature variance. In the near-wall region, the measured mean and r.m.s. temperature distributions are in good agreement with the simulations. Outside this region, differences exist mainly due to differences in the thermal boundary conditions at the opposite wall.
On the calculation of heat transfer rates in fully turbulent wall flows
Applied Mathematical Modelling, 1987
A wall function for heat transfer is derived from the heat and mass transfer laws developed by Kader and Yaglom for turbulent wall flows. The wall function is used as a component of a prediction procedure to compute heat transfer rates in boundary layers, pipes, and wall jets. The results are generally in good agreement with the experimental data, but under most conditions the new function gives only a relatively minor improvement over existing functions. However, significant improvement is obtained for very large molecular Prandtl numbers.
ON THE CALCULATION OF HEAT TRANSFER RATES IN TURBULENT WALL FLOWS
A wall function for heat transfer is derived from the heat and mass transfer laws developed by Kader and Yaglom for turbulent wall flows. The wall function is used as a component of a prediction procedure to compute heat transfer rates in boundary layers, pipes, and wall jets. The results are generally in good agreement with the experimental data, but under most conditions the new function gives only a relatively minor improvement over existing functions. However, significant improvement is obtained for very large molecular Prandtl numbers. Table 7 Proposed forms of the sublayer resistance function Originator P-function Reynolds"* P=O PrandW P= 5.8(a, -1) Taylor'* P=12.53(0,-1) Von Karman6* P=5(0,-l)+ln [l +%((I,-1)l Hofmann'* P = 7.54o;"'yq -1) Jayatilleke' P = 9.24w3'4l] [l + 0.28 exp(-0.007&l Launder and Spalding2 P = 9.0(/3 -1 )&"
Side wall effects in turbulent thermal convection
J. Fluid Mech. 741, 1 (2014)., 2014
We investigate the influence of the temperature boundary conditions at the sidewall on the heat transport in Rayleigh–Bénard (RB) convection using direct numerical simulations. For relatively low Rayleigh numbers Ra the heat transport is higher when the sidewall is isothermal, kept at a temperature Tc C 1=2 (where 1 is the temperature difference between the horizontal plates and Tc the temperature of the cold plate), than when the sidewall is adiabatic. The reason is that in the former case part of the heat current avoids the thermal resistance of the fluid layer by escaping through the sidewall that acts as a short-circuit. For higher Ra the bulk becomes more isothermal and this reduces the heat current through the sidewall. Therefore the heat flux in a cell with an isothermal sidewall converges to the value obtained with an adiabatic sidewall for high enough Ra ('1010). However, when the sidewall temperature deviates from Tc C 1=2 the heat transport at the bottom and top plates is different from the value obtained using an adiabatic sidewall. In this case the difference does not decrease with increasing Ra thus indicating that the ambient temperature of the experimental apparatus can influence the heat transfer. A similar behaviour is observed when only a very small sidewall region close to the horizontal plates is kept isothermal, while the rest of the sidewall is adiabatic. The reason is that in the region closest to the horizontal plates the temperature difference between the fluid and the sidewall is highest. This suggests that one should be careful with the placement of thermal shields outside the fluid sample to minimize spurious heat currents. When the physical sidewall properties (thickness, thermal conductivity and heat capacity) are considered the problem becomes one of conjugate heat transfer and different behaviours are possible depending on the sidewall properties and the temperature boundary condition on the ‘dry’ side. The problem becomes even more complicated when the sidewall is shielded with additional insulation or temperature-controlled surfaces; some particular examples are illustrated and discussed. It has been observed that the sidewall temperature dynamics not only affects the heat transfer but can also trigger a different mean flow state or change the temperature fluctuations in the flow and this could explain some of the observed differences between similar but not fully identical experiments.
International Journal of Recent advances in Mechanical Engineering, 2014
This article investigates the effects of radiation and blowing from a wall on a turbulent heat transfer in vertical channels with asymmetrical heating. The equations involved were numerically solved with three turbulent models including Spalart Allmaras, R-N-G k-ε with "Standard Wall Function" wall nearby model, R-N-G k-ε with "Enhanced Wall Treatment" wall nearby model and "Ray Tracing" radiation techniques. The results were compares with experimental data and appropriate methods were selected for turbulent modeling. The problem of Rayleigh number, Reynolds, radiation parameters and Prandtl were solved and the effects of these parameters on the flow lines, lines of constant temperature, radiation, convection, heat transfer caused by blowing and the total heat transfer were determined.
International Journal of Thermal Sciences, 2014
An experimental work is reported that studied the effect of mixed convection on the mean and turbulent flow structure in the near wall region inside a horizontal square channel heated from below at low Reynolds numbers (Re) and high Grashof numbers (Gr). The Gr/Re 2 values ranged from 9 to 106 indicating that natural convection was dominant over forced convection for all studied cases. Velocity fields were measured using particle image velocimetry (PIV) in the vertical mid plane and two horizontal planes close to the heated wall. The results show that the bottom wall heating altered the mean velocity field and induced turbulence. Both mean and turbulent velocity magnitudes showed partial dependency on the Gr/Re 2 ratio. In the higher range of Gr/Re 2 , mean streamwise velocity showed larger magnitude whereas, in the lower range of Gr/Re 2 , streamwise and spanwise turbulent velocities have larger magnitudes. The streamwise and spanwise turbulent velocity magnitudes were also found to be largest close to the heated wall. It was observed that in the vicinity of the heated wall, the warm fluid converges along the streaks which initiate the rising plumes while the falling parcels of cooler fluid disperse in the spanwise plane.
The thermal entrance region in fully developed turbulent flow
AIChE Journal, 1960
The temperature profile and the local rate of heat transfer from the wall were measured a t 0.453, 1.13, 4.12, and 9.97 tube diameters downstream from a step increase in wall temperature for air in fully developed turbulent flow a t Reynolds numbers of 15,000 and 65,000 in a 1.52-in. tube. The velocity profile and the pressure were also measured a t these lengths.
Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity
Journal of Fluid Mechanics, 2013
Direct numerical simulations (DNS) of turbulent boundary layers over isothermally heated walls were performed, and the effect of viscosity stratification on the turbulence statistics and skin friction were investigated. An empirical relation for temperaturedependent viscosity for water was adopted. Based on the free-stream temperature (30 • C), two wall temperatures (70 • C and 99 • C) were selected. In the heated flows, the turbulence energy diminishes in the buffer layer, but increases near the wall. The reduction in turbulence kinetic energy in the buffer layer is accompanied by smaller levels of Reynolds shear stresses and, hence, weaker turbulence production. The enhanced turbulence energy near the wall is attributed to enhanced transfer of energy via additional diffusion-like terms due to the viscosity stratification. Despite the lower fluid viscosity near the wall, dissipation is also increased owing to the augmented nearwall fine-scale motion. Wall heating results in reduction in the skin-friction coefficient by up to 26 %. An evaluation of the different contributions to the skin friction demonstrates that drag reduction is primarily due to the changes in the Reynolds shear stresses across the boundary layer. Quadrant and octant analyses showed that ejections (Q2) and sweeps (Q4) are significantly reduced, a result further supported by an examination of outer vortical structures from linear stochastic estimation of the ejection events and spanwise vortices.