Phillip M. Ligrani

Flow structure and local Nusselt number variations in a channel with dimples and protrusions on opposite walls

Abstract Flow structure characteristics are presented for a channel with a dimpled surface on one wall, both with and without protrusions (with the same shapes as the dimples) on the opposite wall. Channel aspect ratio is 16, ratio of channel height to dimple print diameter is 0.5, and Reynolds numbers based on channel height range from 380 to 30,000. Instantaneous flow visualization images and surveys of time-averaged flow structure show that the protrusions result in added vortical, secondary flow structures and flow mixing. As a result, local friction factors and local Nusselt numbers are augmented compared to a channel with no protrusions on the top wall. For Nusselt numbers, such augmentations are present near the downstream edges of dimple rims mostly near dimple diagonals, as well as on flat surfaces immediately downstream of dimple rims.

Local Heat Transfer and Flow Structure on and Above a Dimpled Surface in a Channel

Experimental results, measured on and above a dimpled test surface placed on one wall of a channel, are given for Reynolds numbers from 1250 to 61,500 and ratios of air inlet stagnation temperature to surface temperature ranging from 0.68 to 0.94. These include flow visualizations, surveys of time-averaged total pressure and streamwise velocity, and spatially resolved local Nusselt numbers, which are measured using infrared thermography, used in conjunction with energy balances, thermocouples, and in situ calibration procedures. The ratio of channel height to dimple print diameter is 0.5. Flow visualizations show vortical fluid and vortex pairs shed from the dimples, including a large upwash region and packets of fluid emanating from the central regions of each dimple, as well as vortex pairs and vortical fluid that form near dimple diagonals. These vortex structures augment local Nusselt numbers near the downstream rims of each dimple, both slightly within each depression, and especially on the flat surface just downstream of each dimple. Such augmentations are spread over larger surface areas and become more pronounced as the ratio of inlet stagnation temperature to local surface temperature decreases. As a result, local and spatially averaged heat transfer augmentations become larger as this temperature ratio decreases. This is due to the actions of vortical fluid in advecting cool fluid from the central parts of the channel to regions close to the hotter dimpled surface.

NUMERICAL PREDICTIONS OF HEAT TRANSFER AND FLUID FLOW CHARACTERISTICS FOR SEVEN DIFFERENT DIMPLED SURFACES IN A CHANNEL

Heat transfer and fluid flow characteristics for seven different dimpled surfaces on one surface of a channel are predicted numerically using version 6.1.18 of FLUENT. The turbulent model employed is a realizable κ–ϵ model without a wall function. The different dimples investigated are spherical dimples, tilted cylinder dimples, cylinder dimples, in-line triangular dimples, reverse in-line triangular dimples, staggered triangular dimples, and reverse staggered triangular dimples. Results show the existence of a centrally located vortex pair and vortex pairs near the spanwise edges of each dimple for the three circular dimple types, which augment local magnitudes of eddy diffusivity for momentum and eddy diffusivity for heat. Advection of reattaching and recirculating flows from locations within the spherical-type dimple cavities, as well as strong instantaneous secondary flows and mixing within the vortex pairs, are especially apparent. For the four triangular types of dimples, only one primary flow circulation zone generally is present within individual dimples. In all cases, regions of augmented streamwise vorticity show approximate correspondence to locations where eddy diffusivities for momentum and heat are increased. Overall, the highest heat transfer augmentations, and the most significant local and overall increases to eddy diffusivity for momentum and eddy diffusivity for heat, are produced by the spherical dimples and the tilted cylinder dimples.

Comparisons of flow structure above dimpled surfaces with different dimple depths in a channel

Flow structural characteristics over dimple surfaces located on one wall of a rectangular channel with three different dimple depths (δ∕D=0.1, 0.2, and 0.3) are studied experimentally. Reynolds number based on channel height ReH ranges from 2100 to 20 000, and the ratio of channel height to dimple print diameter H∕D is 1.0. Presented are instantaneous flow visualization images, spectra of longitudinal velocity fluctuations, vortex pair frequency information, and time-averaged surveys and profiles of different quantities. Regardless of dimple depth, primary vortex pairs are periodically ejected from the central parts of each dimple and exist in conjunction with edge vortex pairs present near the spanwise edges of staggered dimples. As dimple depth increases, larger deficits of total pressure and streamwise velocity are present, along with higher magnitudes of time-averaged streamwise vorticity, vortex circulation, and longitudinal Reynolds normal stress. Bigger and stronger vortices with increased turbulen...

Effects of Dean vortex pairs on surface heat transfer in curved channel flow

Heat transfer in transitional curved channel flow is investigated over a range of Dean numbers less than 300. The channel aspect ratio is 40, radius ratio is 0.979 and the ratio of boundary layer thickness to channel width is 0.011. Forced convection Nusselt numbers show the influences of Dean vortex pairs and other transitional phenomena. In particular, Nusselt numbers on the concave surface become higher than ones measured at the same streamwise location on the convex surface just after the start of curvature. Important Nusselt number increases also occur due to the twisting secondary instability as the Dean number increases from 150 to 200.

Bulk flow pulsations and film cooling—I. Injectant behavior

Abstract Experimental results are presented which describe the effects of bulk flow pulsations on film cooling from a single row of simple angle film cooling holes. The pulsations are in the form of sinusoidal variations of static pressure and streamwise velocity. Visualizations of film cooling distributions and trajectories illustrate dramatic alterations which occur as the pulsations are imposed on the film cooled boundary layer. In particular, significant changes occur as the coolant Strouhal number becomes greater than 1–2 and the film changes from quasi-steady behavior to non-quasi-steady behavior. Data from these two regimes are presented and discussed along with time-averaged surveys of injectant distributions at different streamwise locations, both with and without pulsations. The results provide clear evidence of the dramatic impact of bulk flow pulsations on film cooling heat transfer.

Transonic Turbine Blade Tip Aerothermal Performance With Different Tip Gaps—Part I: Tip Heat Transfer

A closely combined experimental and computational fluid dynamics (CFD) study on a transonic blade tip aerothermal performance at engine representative Mach and Reynolds numbers (Mexit=1,Reexit=1.27×106) is presented here and its companion paper (Part II). The present paper considers surface heat-transfer distributions on tip surfaces and on suction and pressure-side surfaces (near-tip region). Spatially resolved surface heat-transfer data are measured using infrared thermography and transient techniques within the Oxford University high speed linear cascade research facility. The Rolls-Royce PLC HYDRA suite is employed for numerical predictions for the same tip configuration and flow conditions. The CFD results are generally in good agreement with experimental data and show that the flow over a large portion of the blade tip is supersonic for all three tip gaps investigated. Mach numbers within the tip gap become lower as the tip gap decreases. For the flow regions near the leading edge of the tip gap, surface Nusselt numbers decrease as the tip gap decreases. Opposite trends are observed for the trailing edge region. Several “hot spot” features on blade tip surfaces are attributed to enhanced turbulence thermal diffusion in local regions. Other surface heat-transfer variations are attributed to flow variations induced by shock waves. Flow structure and surface heat-transfer variations are also investigated numerically when a moving casing is present. The inclusion of moving casing leads to notable changes to flow structural characteristics and associated surface heat-transfer variations. However, significant portions of the tip leakage flow remain transonic with clearly identifiable shock wave structures.

Numerical Predictions of Heat Transfer and Flow Characteristics of Heat Sinks with Ribbed and Dimpled Surfaces in Laminar Flow

Steady flow characteristics and surface Nusselt number distributions of heat sinks with smooth, ribbed, and dimpled surfaces are investigated in laminar flow using the numerical code FLUENT 6.2.16. Presented are local and spatially averaged surface Nusselt numbers, as well as distributions of numerically predicted flow characteristics within the heat sink passages. These include distributions of static pressure, velocity, and streamwise vorticity in flow cross-sectional planes. These results are valuable because of the design information provided to optimize heat sink thermal performance, without the use of costly and time-consuming experiments.

Heat transfer in a swirl chamber at different temperature ratios and Reynolds numbers

Abstract Nusselt numbers are presented for a swirl chamber with multiple inlets and helical flow for Reynolds numbers from 6100 to 19,200, and ratios of inlet temperature to wall temperature from 0.60 to 0.95. Local Nusselt numbers, measured over the entire interior surface of the swirl chamber using infrared thermography, are highest near the inlets where important influences from Gortler vortex paris are especially evident. Axially-averaged, circumferentially-averaged, and globally-averaged Nusselt numbers, determined from local values, generally increase as Reynolds number increases and as the temperature ratio decreases.

Film cooling subject to bulk flow pulsations: effects of blowing ratio, freestream velocity, and pulsation frequency

Abstract Bulk flow pulsations, in the form of sinusoidal variations of static pressure and streamwise velocity, are investigated as they affect film cooling from round, simple angle holes in a turbulent boundary layer. Such pulsations are important to turbine airfoils and end walls in gas turbine engines because similar pulsations are induced by potential flow interactions and passing families of shock waves. Distributions of adiabatic film cooling effectiveness, iso-energetic Stanton number ratio, and film cooling performance parameter are presented for different pulsation frequencies, blowing ratios, freestream velocities, and hole length-to-diameter ratios. A correlation is given for the onset of protection reduction as it depends upon these parameters. The most important reductions to film cooling protection result as the pulsation frequency increases at the smallest length-to-diameter ratio and smallest blowing ratio tested.