T.-M. Liou

Heat Transfer and Fluid Flow in a Square Duct With 12 Different Shaped Vortex Generators

Detailed local Nusselt number distributions in the first pass of a sharp turning two-pass square channel with various configurations of longitudinal vortex generator arranged on one wall were measured using transient liquid crystal thermography. Flow patterns and friction factors were measured by the use of laser-Doppler velocimeter and pressure transducer, respectively. The Reynolds number, based on channel hydraulic diameter and bulk mean velocity, was fixed at 1.2 × 10 4 . The vortex generator height-to-hydraulic diameter ratio and pitch-to-height ratio were 0.12 and 10, respectively. Comparisons in terms of heat transfer augmentation and uniformity and friction loss are first performed on 12 configurations of single longitudinal vortex generator. The fluid dynamic mechanisms and wall confinement relevant to heat transfer enhancement are then documented for three-selected vortex generator models. In addition, the differences in fluid flow and heat transfer characteristics between a single vortex generator and a vortex generator array are addressed for the delta wing I and 45 deg V (with tips facing upstream) models which provide better thermal performance among the 12 configurations examined

TLCT and LDV measurements of heat transfer and fluid flow in a rotating sharp turning duct

Heat transfer measurements using a transient liquid crystal thermometry (TLCT) are presented to study the eAect of rotation on the local heat transfer distributions around a sharp 180∞ turn of a two-pass smooth square duct for the first time in open literature. The duct has a dimensionless divider wall thickness of 0.25. Detailed local Nusselt number distributions on the leading and trailing walls are given for rotation numbers ranging from Roa 0 to 0.20 at a Reynolds number of 1:0 10 4 . The results show informative spanwise variations of heat transfer enhancement or abatement resulting from rotation. A critical range of Ro is identified, below which the regional averaged Nusselt number ratios of the regions after the 180∞ sharp turn in a rotating two-pass cooling passage are rather insensitive to the rotation eAect. Laser‐Doppler velocimetry (LDV) measurements within the turn are further performed to explain the local heat transfer distributions. Under rotating conditions, the curvature induced symmetric Dean vortices in the turn for the stationary case change greatly and are dominated by a single vortex most of which impinges directly on the leading wall’s outer part, resulting in a high heat transfer enhancement on the leading wall in the turn. ” 2001 Elsevier Science Ltd. All rights reserved.

Heat transfer in a rotating two-pass smooth passage with a 180° rectangular turn

Abstract Measurements are presented of the distributions of heat transfer coefficients along the leading and trailing walls of a rotating two-pass rectangular channel with through flow. The channel has a 180° rectangular turn and a cross-sectional aspect ratio of 1.25. The Reynolds number and rotational number were varied from 5.0×103 to 5.0×104 and 0 to 0.44, respectively. Complementary flow visualization and laser-Dopper velocimetry measurements were also performed to facilitate interpretation of the measured heat transfer distributions. The results show that the secondary flow induced by the curvature and turbulence enhancement associated with unsteadiness of separation bubble downstream of the sharp turn are responsible for the significant heat transfer augmentation attained in the first part of the second pass. Moreover, the quantitative effects of the Coriolis force on the skewness of streamwise mean velocity profiles and in turn different local and average heat transfer features in the radially outward and inward flows are demonstrated and addressed.

Non-intrusive measurements of near-wall fluid flow and surface heat transfer in a serpentine passage

Abstract Flow visualization using a transient liquid crystal method is presented to study the effect of divider thickness on the local heat transfer distributions around a sharp 180-deg turn of a two-pass smooth square duct. Detailed local Nusselt number distributions on the bottom, blade-tip, and side walls are given for three divider thicknesses, W ∗ d =0.10 , 0.25, and 0.50, at a Reynolds number of 1.2×104. Complementary pressure loss measurements are presented in terms of variation of friction factor with W ∗ d . The thermal performance investigation shows that W ∗ d =0.25 provides the highest total averaged Nusselt number ratios, both at constant flow rate and at constant pumping power conditions, and a moderate friction factor. Near-wall laser-Doppler velocimetry measurements are further performed to explain the liquid crystal measured surface heat transfer distributions. The results show that the direction and strength of the secondary flow with respect to the wall are most important fluid dynamic factors affecting the heat transfer distributions, followed by the convective mean velocity, and then the turbulent kinetic energy.

Heat Transfer in High-Speed Rotating Trapezoidal Duct With Rib-Roughened Surfaces and Air Bleeds From the Wall on the Apical Side

An experimental study of heat transfer in a radially rotating trapezoidal duct with two opposite walls roughened by 45 deg staggered ribs and bleed from the apical side wall is performed. Centerline heat transfer variations on two rib-roughened surfaces are measured for radially outward flows with and without bleeds at test conditions of Reynolds number (Re), rotation number (Ro), and density ratio (Δρ/ρ) in the ranges of 15,000-30,000, 0-0.8, and 0.04-0.31, respectively. Geometrical configurations and rotation numbers tested have considerably extended the previous experiences that offer practical applications to the trailing edge cooling of a gas turbine rotor blade. A selection of experimental data illustrates the individual and interactive influences of Re, Ro, and buoyancy number (Bu) on local heat transfer with and without bleeds. Local heat transfer results are generated with the influences of bleeds on the apical side examined to establish heat transfer correlations with Re, Ro, and Bu as the controlling flow parameters for design applications. The rotation of present trapezoidal duct with rib-roughened surfaces and air bleeds on the apical side worsens the impairing heat transfer impacts due to bleeds. Within the Ro range of 0.1-0.8, bleeds on the apical side of the rotating channel respectively produce 25-50% and 25-40% of heat transfer reductions from the rotational no-bleed references along the leading and trailing centerlines. Such heat transfer reductions due to the combined bleeds and Ro-Bu impacts need design precautions for turbine rotor blades.

Effects of momentum ratio on turbulent nonreacting and reacting flows in a ducted rocket combustor

Abstract A numerical study of turbulent nonreacting and reacting flows in a ducted rocket combustor with an ASM turbulence model and a finite-rate combustion model is reported. In addition, detailed measurements of flow velocities and turbulence parameters have been conducted by use of a four-beam two-color LDV system. Three different values of the ratio of fuel momentum to air momentum were selected to investigate its effects on the turbulent flow structure, mixing and combustion characteristics. It is found that the momentum ratio has strong effects on the number, size and rotational direction of dome region recirculation zones, reattachment length, axial fuel-jet spreading rate and penetration ability, flame temperature distributions and total pressure loss in the ducted rocket combustor. A useful correlation between the reattachment length and momentum ratio is derived. Moreover, a moderate value of the momentum ratio is recommended for better combustion performance and lower total pressure loss. The reported data are believed to provide valuable guidelines for practical design of combustors.

High Rotation Number Heat Transfer of Rotating Trapezoidal Duct With 45-deg Staggered Ribs and Bleeds From Apical Side Wall

An experimental study of heat transfer in a radially rotating trapezoidal duct with two opposite walls roughened by 45° staggered ribs and mid-rib bleeds from the apical side wall is performed. Centerline heat transfer variations on two rib-roughened surfaces are measured for radially outward flows with and without bleeds at test conditions of Reynolds number (Re), rotation number (Ro) and density ratio (Δρ/ρ) in the ranges of 15000–30000, 0–0.8 and 0.04–0.31, respectively. Geometrical configurations and rotation numbers tested have considerably extended the previous experiences that offer practical applications to the trail edge cooling of a gas turbine rotor blade. A selection of experimental data illustrates the individual and interactive influences of Re, Ro and buoyancy number (Bu) on local heat transfer with and without bleeds. Local heat transfer results are generated with the influences of sidewall bleeds examined to establish heat transfer correlations with Re, Ro and Bu as the controlling flow parameters for design applications.Copyright

Heat Transfer in a Reciprocating Thermosyphon Fitted with Staggered Transverse Ribs

This paper describes an experimental investigation of heat transfer in a reciprocating antigravity open thermosyphon. The thermosyphon tube is square in cross section, and two opposite walls are roughened with staggered transverse ribs. This flow/heat-transfer system has relevance, as a fundamental study, to the shaker-cooling system for the pistons of reciprocating engines. A series of experiments are undertaken to demonstrate the complex interaction that exists between inertial, reciprocating, and buoyancy forces on the performance of this form of cooling system. It is shown that reciprocation of the system significantly improves heat transfer in comparison with the stationary antigravity thermosyphon performance. The buoyancy effects from gravity and reciprocation improve heat transfer. Reductions in the effectiveness of buoyancy forces are produced when the relative strengths of inertial or/and pulsating forces increase. In the extrapolated case of zero buoyancy, it was found that the local heat transfer along the thermosyphon tube was reduced in comparison to the static case at relatively low levels of reciprocation frequency. As the reciprocation frequency was increased, this effect was reversed and heat transfer improved in comparison with the stationary case.

Heat transfer and pressure drop measurements of rotating rib-roughened parallelogram channel

This experimental study examines the pressure-drop coefficients f and the detailed Nusselt number Nu distributions over two opposite leading and trailing walls roughened by 45 deg ribs for a rotating parallelogram channel with radially outward flow. For the first time, the isolated effects of ReynoldsRe, rotationRo, and buoyancy Bu numbers on local and area-averaged Nusselt numbers Nu and Nu measured from the infrared thermography method were successfully examined at the parametric conditions of 5000 Re 15; 000, 0 Ro 0:3, and 0:001 Bu 0:23 for the single-pass parallelogram channel. A set of selected heat transfer data illustrates the Coriolis and rotating-buoyancy effects on the detailed Nusselt number distributions and the area-averaged heat transfer performances of the rotating parallelogram channel. With the consideration of the f data generated at the isothermal conditions, the thermal performance factors for this radially rotating channel were evaluated. The Nusselt numbers obtained from the leading and trailing walls of the rotating test channel fall in the respective ranges of 0.78–1.34 and 1.09–1.38 times the stationary levels, whereas the factors are in the range of 0.979–1.575 for the present test conditions.

Heat Transfer in a Rotating Rectangular Channel With Two Opposite Walls Roughened With Spherical Protrusions at High Rotation Numbers

Detailed heat transfer distributions over two opposite leading and trailing walls roughened by spherical protrusions were measured from a rotating rectangular channel at rotation number up to 0.6 to examine the effects of Reynolds (Re), rotation (Ro) and buoyancy (Bu) numbers on local and area averaged Nusselt numbers (Nu and Nu ) using the infrared thermography. A set of selected heat transfer data illustrates the Coriolis and rotating-buoyancy effects on the detailed Nu distributions and the area-averaged heat transfer performances of the rotating channel. The Nu for the developed flow region on the leading and trailing walls are parametrically analyzed to devise the empirical heat transfer correlations that permit the evaluation of the interdependent and individual Re, Ro and Bu effect on Nu .Copyright