RESEARCH AREAS

Dimple Arrays on Surfaces of Channels

  • Heat transfer and flow characteristics
  • Fundamental and applied investigations
  • Heat transfer augmentation with minimal pressure drop penalties
  • 3-D, unsteady, complex, elliptic flow fields
  • Numerous applications including internal cooling of turbine airfoils used in gas turbine engines, heat exchangers, electronics cooling, biomedical devices
  • Dimples in combination with other heat transfer augmentation devices
  • Experimental results and numerical predictions of heat transfer and flow structure

Investigations of Confined, Millimeter-Scale, Unsteady Laminar Impinging Slot Jets

  • As miniaturization and power requirements continue to progress, millimeter-scale and microscale electronic circuit components must be cooled effectively, not only with lower temperature levels, but with lower local temperature gradients. One means to accomplish this cooling is with impinging jets, because of the relatively high heat transfer rates which are produced at and near their stagnation points, as well as the concentration of cooling effects over a relatively narrow area.
  • The effects of slot width for confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements of spatially resolved distributions of local Nusselt numbers measured on a constant heat flux surface.
  • The present investigation provides particular attention given to the effects of slot width for confined, laminar impinging slot jets of millimeter-scale.
  • The effects of Reynolds number, nozzle-to-plate distance, and dimensional slot width on the local Nusselt number are investigated for slot nozzle width B values of 0.5 mm, 1.0 mm, and 1.5 mm.
  • Reynolds numbers Re range from 120 to 200, nozzle-to-plate distances H/B vary from 0.75 to 12.5, and the nozzle aspect ratio y/B is 50.
  • Observed are different stagnation point Nusselt number variations with Re, H/B, and B, where the onset of unsteadiness, and the intermittent flapping motion of the jet column are both associated with important variationsto local, stagnation region Nusselt numbers, as experimental configuration and condition change.
  • The variations of these stagnation-point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/B, for different dimensional values of B.

Aerodynamic Losses and Mixing Losses from Turbine Airfoils

  • Applicable to turbine components of gas turbine engines
  • Effects of surface roughness, turbulence intensity, Mach number, Reynolds number
  • Aerodynamic losses, mixing losses, shock wave losses
  • Wake distributions of total pressure coefficients, Mach numbers, kinetic energy
  • IAL – Integrated Aerodynamic Losses

Impingement Cooling

  • Applicable to internal cooling of turbine airfoils for gas turbine engines
  • Independent effects of Mach number and Reynolds number
  • Effects of relative values of surface temperature, impingement temperature, cross-flow temperature
  • Surface heat transfer coefficients and surface impingement effectiveness

Internal Cooling – Surface Heat Transfer Augmentation

  • Applicable to internal cooling of turbine airfoils for gas turbine engines
  • Many additional applications including heat exchangers
  • Rib turbulators, pin fins, dimples, surface roughness, swirl chambers
  • Effects of variable properties due to large differences between surface temperature and gas temperature
  • Heat transfer and flow characteristics

Miniature and Micro-Scale Pumps

  • RSP – Rotating Shaft Pump
  • Viscous disk pumps
  • Osmotic pumps
  • Pressure and temperature compensation

Surface Roughness

  • Three-dimensional, non-uniform, irregular surface roughness
  • Determination of equivalent sandgrain roughness size entirely from roughness geometry
  • Internal channel surfaces and surfaces of turbine airfoils used in gas turbine engines
  • Reynolds analogy equations for real component surface roughness

Electronics Cooling

  • Improved heat sinks with larger heat transfer augmentations
  • Use of dimpled surfaces along with other heat transfer augmentation devices
  • Use of dimples on the surfaces of micro-scale passages
  • Jet impingement cooling of a chip equipped with a single cylindrical pedestal profile fin
  • Jet impingement cooling of chips equipped with multiple cylindrical pedestal profile fins

Transitional Flows in Curved Channels

  • Diversity of practical applications
  • Heat transfer and flow characteristics
  • Applicable to cooling of surfaces of nozzles in rocket engines
  • Dean numbers from 50 to 1100
  • Laminar, transitional, turbulent flows
  • Dean vortex pairs along with secondary instabilities such as splitting and merging, undulating vortex pairs, twisting vortex pairs
  • New procedure to deduce forced convection Nusselt numbers from mixed convection measurements

Film Cooling

  • Applicable to external cooling of turbine airfoils for gas turbine engines
  • Heat transfer and flow characteristics
  • Effects of bulk flow pulsations
  • Hole shape, hole angle, staggered and single rows of holes, supply plenum arrangement
  • Effects of longitudinal vortices
  • Effects of shock waves, and transonic freestream and film flows
  • Effects of compressibility and variable properties

Flow And Heat Transfer On And Near A Transonic Turbine Blade Tip

  • The transonic flow pattern and its influence on heat transfer on a high-pressure turbine blade tip are investigated using experimental and computational methods.
  • Spatially-resolved heat transfer data are obtained at engine representative conditions (Mexit=1.0, Re=1.27×106) using the transient infrared thermography technique within the Oxford High Speed Linear Cascade research facility.
  • CFD predictions are conducted using the Rolls-Royce HYDRA/PADRAM suite.
  • The CFD solver is able to capture most of the spatial heat flux variations and gives prediction results which compare well with the experimental data.
  • The results show that the majority of the blade tip experiences a supersonic flow with a local peak Mach numbers reaching 1.8.
  • Unlike other low speed data in the open literature, the turbine blade tip heat transfer is greatly influenced by the shock wave structure inside the tip gap.
  • Oblique shock waves are initiated near the pressure side edge of the tip, prior to being reflected multiple times between the casing and the tip.
  • Supersonic flow within the tip gap is generally terminated by a normal shock near the exit of the gap.
  • Both measured and calculated heat transfer spatial distributions illustrate very clear stripes as the signature of the multiple shock structure.
  • Overall, the supersonic part of tip experiences noticeably a lower heat transfer than that near the leading-edge where the flow inside the tip gap remains subsonic.

Slip Phenomina in Micro-Fluidic Devices
Improvements in manufacturing technology and micro-fabrication have led to the miniaturization of devices and sensors such as heat exchangers, micro-sensors, micro-pumps, biological reactors, selective membranes, and other devices. The ability to predict the fluid motion in and around these devices is essential for design and optimization of small scale devices like micro-pumps. As the length scales of these devices decrease, effects such as viscous dissipation, slip flow, roughness effects, non-Newtonian fluid behavior, and variations of fluid properties become significant for liquid flows because of micro-scale dimensions.

Buoyancy-Driven Continuous SPLITT Fractionation: A New Technique for Separation of Microspheres

A new method of Continuous Fractionation using a SPLITT cell is conceived, developed, and tested, and is demonstrated to be useful for separation of collections of particles with different sizes and densities. With this previously uninvestigated mode of operation, this is accomplished for particles by buoyancy-driven separation. Some of the capabilities of this system are illustrated by successful separations of different-sized fluorescent polymer microspheres, with different carrier densities. The resulting experimentally-measured fraction recovery variations are then in good agreement with theoretical calculation from buoyancy-driven SPLITT theory.

Investigations of Full Coverage Film Cooling

  • Full coverage film cooling provides a layer of protective coolant fluid over surfaces exposed to gas at elevated temperatures.
  • The coolant fluid layer acts as a heat sink and an insulator for the protected surface, and in doing so, reduces the heat load transferred to the surface.
  • Film cooling is currently employed in a variety of high temperature applications, including turbine blades and combustor liners of gas turbine engines.
  • The present investigation considers appropriate distributions of streamwise static pressure gradient and varying blowing ratio along the length of the contraction passage which contains the cooling hole arrangement.
  • Presented are local, line-averaged, and spatially-averaged adiabatic film effectiveness, and local, line-averaged, and spatially-averaged iso-energetic heat transfer coefficients.
  • A flow stream heater mesh is used to provide a near instantaneous step-change in mainstream temperature, as an infrared camera is employed to measure time-varying and spatially-resolved distributions of test surface temperature.

Shock Wave Boundary Layer Interactions (SWBLI)

  • Shock waves interact with the boundary layer
  • Low-momentum flow cannot overcome adverse flow conditions and the steep pressure gradient produced by the shock
  • Boundary layer thickens and separates
  • Flow stagnates and reverses resulting in areas of separation

Double Wall Cooling

  • Experimental results are presented for a double wall cooling arrangement which simulates a portion of a combustor liner of a gas turbine engine.
  • The results are collected using a new experimental facility designed to test full coverage film cooling and impingement cooling effectiveness using either cross flow, impingement, or a combination of both to supply the film cooling flow.
  • New heat transfer data are provided for both the surfaces of the full coverage effusion cooling plate within the double wall cooling test facility.
  • Also utilized are unique mainstream mesh heaters, which provide transient thermal boundary conditions, after mainstream flow conditions are established.
  • For the effusion cooled surface (or hot-side of the full coverage effusion cooling plate), presented are spatially-resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients, which are measured using infrared thermography.
  • For the coolant side (or hot-side of the full coverage effusion cooling plate), presented are spatially-resolved distributions of surface Nusselt numbers, which are measured using liquid crystal thermography.
  • Of particular interest are the effects of streamwise development, blowing ratio, and Reynolds number.

Elastic Turbulence

  • New and innovative methods to enhance mixing and transport in small-scale environments
  • Direct application to miniature heat exchangers, and electronic component cooling.
  • Micro-scale phenomena will be induced using surface roughness and sub-millimeter-scale dimensions within microfluidic devices with elastic turbulence in liquids.
  • Use of polymer additives, such as polyacrylamide, to augment thermal transport is the unique ingredient
  • Associated with highly non-linear, non-Newtonian behavior.

Surface Roughness Effects on Impingement Jet Array Surface Heat Transfer

  • The focus of the effort is fundamental understanding of thermal transport, and heat transfer phenomena, as altered by impingement array jets as they impact upon different target surface roughness arrangements.
  • Applications are varied, and include electronic cooling, electronic component chip cooling, heat exchangers, utility gas turbine engines employed for power generation, micro-fluidic devices which are employed within electronic components, as well as a variety of other heat transfer augmentation devices.

Dean Flow Dynamics in Low-Aspect Ratio Spiral Microchannels

  • A wide range of microfluidic cell-sorting devices has emerged in recent years, based on both passive and active methods of separation.
  • Curvilinear channel geometries are often used in these systems, due to presence of secondary flows, which can provide high throughput and sorting efficiency.
  • Most of these devices have been designed on the assumption that there are two counter rotating Dean vortices, present in the curved rectangular channels, which exist in the state of steady rotation and amplitude.
  • In this work, we investigate these secondary flows in low aspect ratio spiral rectangular microchannels and define their development with respect to the channel aspect ratio and Dean number.
  • This work is the first to experimentally and numerically investigate Dean flows in microchannels for Re 100, and show presence of secondary Dean vortices beyond a critical Dean number.
  • We further demonstrate the impact of these multiple vortices (>2) on particle and cell focusing.
  • Ultimately, this work offers new insights into secondary flow instabilities for low-aspect ratio, spiral microchannels, with improved flow models for design of more precise and efficient microfluidic devices for applications such as cell sorting and micromixing.
Copyright Trademarks Ligrani Research Group 2009