Donald M. McEligot

Temperature, velocity and mean turbulence structure in strongly heated internal gas flows: Comparison of numerical predictions with data

The main objective of the present study is to examine whether “simple” turbulence models (i.e., models requiring two partial differential equations or less for turbulent transport) are suitable for use under conditions of forced flow of gas at low Reynolds numbers in tubes with intense heating, leading to large variations of fluid properties and considerable modification of turbulence. Eleven representative models are considered. The ability of such models to handle such flows was assessed by means of computational simulations of the carefully designed experiments of Shehata and McEligot (IJHMT 41 (1998) 4297) at heating rates of q+in≈0.0018, 0.0035 and 0.0045, yielding flows ranging from essentially turbulent to laminarized. The resulting comparisons of computational results with experiments showed that the model by Launder and Sharma (Lett. Heat Transfer 1 (1974) 131) performed best in predicting axial wall temperature profiles. Overall, agreement between the measured velocity and temperature distributions and those calculated using the Launder–Sharma model is good, which gives confidence in the values forecast for the turbulence quantities produced. These have been used to assist in arriving at a better understanding of the influences of intense heating, and hence strong variation of fluid properties, on turbulent flow in tubes.

Turbulent and laminar heat transfer to gases with varying properties in the entry region of circular ducts

Abstract A numerical method is developed to solve the coupled boundary layer equations for turbulent flow of a gas with large property variations in a circular tube. Uniform and fully developed entering velocity profiles are treated. Axial variation of the heating rate is permitted and is used for comparison with experiments. Cases treated include: (1) constant fluid properties, laminar flow with variation of hydrodynamic entrance length; (2) variable, idealized air properties with uniform laminar entering velocity profile; (3) constant properties, fully developed turbulent flow in the immediate thermal entry region; (4) constant fluid properties, turbulent flow, with uniform entering velocity profile; (5) variable, idealized air properties and real gas properties, turbulent flow at heating rates to ( q w Gc p,i T i ) = 0·02 . Predictions for several turbulent transport models are compared to an experiment with a peak wall temperature ratio ( T w T i ) of about 12, and the model yielding the closest agreement in the thermal entry region, a version of van Driests mixing-length model, is used for further predictions.

Effect of Large Temperature Gradients on Convective Heat Transfer: The Downstream Region

Large temperature gradient effect on convective heat transfer in turbulent downstream region of heated tube

Mean structure in the viscous layer of strongly-heated internal gas flows. Measurements

Abstract Experiments for air flowing upward in a vertical circular tube were conducted for heating rates causing significant property variation in primarily forced convection. Two entry Reynolds numbers were employed, concentrating on three heating rates of qi+ = qw⧹Gcp,in ≈Tin 0.0018, 0.0035 and 0.0045, to give conditions considered to be ‘turbulent’, ‘sub-turbulent’ and ‘laminarizing’. In addition to variation of integral parameters, results include the mean velocity and temperature distributions-needed to guide development of advanced turbulence models for this situation. However, comparison to a simple approach-a modified version of the van Driest model-shows useful agreement. 1998 Published by Elsevier Science Ltd.

Direct numerical simulation for laminarization of turbulent forced gas flows in circular tubes with strong heating

The direct numerical simulation (DNS) of turbulent transport for a gas with variable properties has been conducted to grasp and understand the laminarization phenomena caused by strong heating. In this study, the inlet Reynolds number based on a bulk velocity and pipe diameter was taken as Re=4300 as in the experiments by Shehata and McEligot (1998). The measured wall temperature distribution was applied as a thermal boundary condition. The number of computational nodes used in the heated region was 768×64×128 in the z-, r- and φ-directions, respectively. Turbulent quantities, such as the mean flow, temperature fluctuations, turbulent stresses and the turbulent statistics, were obtained via DNS. Predicted mean velocity and temperature distributions and integral parameters agreed well with the experiments. The Reynolds shear stress, indicating turbulent transport of momentum, decreases along the streamwise direction. The cause of this reduction can be considered to be that the fluid behavior changes drastically in the near wall region due to strong heating which induces significant variations of the gas properties and, in turn, acceleration and buoyancy effects. In a visualization of the results, one sees that the vortical structures are primarily suppressed within the first section of the heated region (z/D=0–5) and are not regenerated further downstream.

Quadrant analysis of a transitional boundary layer subject to free-stream turbulence

This paper presents analyses of particle image velocimetry measurements from a boundary layer on a flat plate subject to grid-generated free-stream turbulence. The pre-transition region and early stages of breakdown to turbulent spots are explored by means of quadrant analysis and quadrant hole analysis. By isolating the contributors to the Reynolds shear stresses, it is possible to identify coherent structures within the flow that are responsible for the production of TKE. It is found that so called ‘ejection’ events are the most significant form of disturbance, exhibiting the largest amplitude behaviour with increased negative spanwise vorticity. Sweep events become increasingly large close to the wall with increased Reynolds number and intermittency.

Calculation of Turbulent Flow and Heat Transfer in Channels With Streamwise-Periodic Flow

A computational method for the calculation of the flow and heat transfer in a channel, with elements of various heights inducing a streamwise-periodic flow, is presented and evaluated. The time-averaged conservation equations of mass, momentum, and energy were solved together using a finite-control-volume method. Reynolds stresses were obtained using a two-equation model, which solves the time-averaged equations of the turbulence kinetic energy and its dissipation rate. The calculated flow field is shown to be in satisfactory agreement with the experimental data. The results indicate that the local and overall heat loss parameters increase with increasing Reynolds and Prandtl numbers and element height and with decreasing spacing.

Laterally converging flow. Part 1. Mean flow

Laterally converging flow occurs between two parallel surfaces with an exit hole formed in one. The present study examines the flow at a distance from the exit as a means of investigating an accelerating radial internal flow induced by the lateral convergence and satisfying the boundary-layer approximations. The measurements range from laminar to turbulent conditions, including the intermediate stage referred to by some investigators as laminarizing or laminarescent. The acceleration parameter K v = (ν/ V 2 ) dV / dr ranges from 2·6 × 10 −8 to 2·2 × 10 ×4 and the local Reynolds number varies from 210 to 6·8 × 10 4 for the data reported; the relation between the Reynolds number and the acceleration parameter was varied by adjusting the convergence angle or the plate spacing. For the main experiment the accelerating region is 86 plate spacings in length. Comparison with numerical predictions for laminar and turbulent flow leads to identification of flow regimes in terms of popular acceleration parameters K v , K p = (ν/ρ u 3 * ) dp / dr and K τ = (ν/ρ u 3 * ) (∂τ/∂ z ) w . Results demonstrate that a potentially turbulent entry flow subjected to accleration due to lateral convergence shows features common to laminarization in accelerating turbulent boundary layers in other geometries. Application of the function A + ( K p ) for a modified van Driest wall-region model is examined briefly for the intermediate regime.

Thermal entry for low Reynolds number turbulent flow.

Thermal entry problem solution for low Reynolds number turbulent gas flow based on Reynolds number dependent velocity profile

AN ASSESSMENT OF k–ω AND v 2–f TURBULENCE MODELS FOR STRONGLY HEATED INTERNAL GAS FLOWS

Both k –ω and v 2– f turbulence models are used to model an axisymmetric, strongly heated, low-Mach-number gas flowing upward within a vertical tube in which forced convection is dominant. The heating rates are sufficiently high, so that fluid properties vary significantly in both the axial and radial directions; consequently, fully developed mean flow profiles do not evolve. Comparisons between computational results and experimental results, which exist in the literature, reveal that the v 2– f model performs quite well in predicting axial wall temperatures, and mean velocity and temperature profiles. This may be contrasted with the k –ω model results, in which the wall heat transfer rates and near-wall velocities are significantly overpredicted.