P. K. Panigrahi

Estimation of convective heat transfer coefficient from transient liquid crystal data using an inverse technique

An estimation of the local Nusselt number distribution for a flat and a ribbed surface from transient liquid crystal images is presented. Liquid crystal thermography generates color images of the time-varying surface temperature field, when an initially heated surface is subjected to cooling in forced flow. The inverse technique compares the approximate numerical solution with the transient experimental temperature distribution, and enforces the applicable physical laws in such a way that a globally correct Nusselt number distribution is predicted. The related optimization problem has been solved by a conjugate gradient method, with a stabilization scheme based on additional experimental data. The partial differential equations arising at the intermediate stages have been solved numerically using the finite difference technique. Predictions of the local Nusselt number have been compared with the full numerical solution based on unsteady incompressible laminar flow, as well as the one-dimensional semi-infinite solid approximation applied to experimental data. Reynolds numbers considered in the study are 160 and 260, based on the rib height. Results show that the inverse technique is capable of resolving sharp as well as gradual changes in the heat transfer rates for the flat plate and the rib geometries. The peak in the Nusselt number distribution for flow past a rib is seen to fall at a location where the flow reattaches with the flat surface. The inverse technique is robust with respect to signal length, and within limits it is insensitive to noise in the experimental data.

Multi-Modal Forcing of the Turbulent Separated Shear Flow Past a Rib

Experiments have been conducted to study the development of flow behind a surface mounted rib under different phase controlled excitation. Single mode excitation and multi-mode excitation with different relative phases are studied. The results presented include the coherent and random components of the turbulent energy and shear stresses, the energy exchange with the mean flow and between the modes, and the phase decorrelation of the coherent components. The fundamental-subharmonic excitation does not provide any significant improvements in the shear layer growth over the fundamental excitation. The shear layer growth correlates with the subharmonic mode development. The the large scale structures are significant even after the reattachment region as evident from the magnitude of the coherent components of the turbulent energy and the shear stress. The binary exchange terms are significant in the near-field region whose sign is phase dependent, i.e., it reverses its sign based on the phase difference between the fundamental and 1st subharmonic mode

Liquid Crystal Heat Transfer Measurements in a Rectangular Channel with Solid and Slit Rib

AbsractAn experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/Dh)set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β=20%) is 61% in comparison to 40% for the solid rib at Re=32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.

Analysis of large scale structures in separated shear layers

Abstract This paper focuses on the characterization of large scale structures of a reattaching shear layer developing behind a surface mounted square rib with and without superimposed acoustic excitation. The pattern recognition technique has been compared with traditional ensemble-averaging/Fourier decomposition technique for determination of coherent structure magnitude. The coherent structure amplitude is underestimated by the traditional Fourier decomposition technique due to the presence of phase jitter in the signal. The pattern recognition technique is insensitive to phase jitter and successfully predicts the large scale structure magnitude. Pattern recognition technique has also been compared with HZFB method [J. Fluid Mech. 230 (1991) 319] for determination of phase jitter. The magnitude of the phase jitter in the near field is low indicating coherence of the flow structures. Phase decorrelation begins to occur about three rib heights from the rib and is attributed to the phase jitter in the subharmonics resulting from random vortex merging in the reattachment region. The pattern recognition technique results show that large scale structures are predominant in the reattachment region contrary to the results of Fourier method which indicate that mostly random turbulent structures are present after reattachment. The coherent contribution is primarily from the most unstable fundamental mode in the near field region which gets redistributed to other modes due to random vortex merging and nonlinear interaction in the reattachment region.

Depolarization characteristics of agglomerated particulates-reciprocity relations

Knowledge of the morphological characteristics of agglomerated structures in combustion systems is important for accurate prediction of radiative transfer in absorbing and scattering media, surface growth of particulates, and in quality control of material synthesis through the aerosol route. Especially nowadays when stringent governmental regulations are issued regarding permissible amounts of emitted particulate matter, accurate knowledge of size and morphology of particulates becomes increasingly important. This study focuses on depolarization properties of agglomerated particulates such as flame soot and other highly absorbing or dielectric particulates. Calculations of depolarization ratios for randomly oriented straight chains, randomly branched chains and clusters, composed of primary particles in the Rayleigh regime, show that the reciprocity relations are not satisfied for the agglomerated structures considered in this study. On the contrary, these ratios were found to be strongly dependent on the agglomerate parameters and the type of agglomerate considered.

Sensitivity of a Square Cylinder Wake to Forced Oscillations

The wake of a square cylinder at zero angle of incidence oscillating inline with the incoming stream has been experimentally studied. Measurement data are reported for Reynolds numbers of 170 and 355. The cylinder aspect ratio is set equal to 28 and a limited study at an aspect ratio of 16 has been carried out. The frequency of oscillation is varied around the Strouhal frequency of a stationary cylinder, and the amplitude of oscillation is 10–30% of the cylinder size. Spatial and temporal flow fields in the cylinder wake have been studied using particle image velocimetry and hot-wire anemometry, the former providing flow visualization images as well. A strong effect of forcing frequency is clearly seen in the near wake. With an increase in frequency, the recirculation length substantially reduces and diminishes the time-averaged drag coefficient. The timeaveraged vorticity contours show that the large-scale vortices move closer to the cylinder. The rms values of velocity fluctuations increase in magnitude and cluster around the cylinder as well. The production of turbulent kinetic energy shows a similar trend as that of spanwise vorticity with the former showing greater asymmetry at both sides of the cylinder centerline. The instantaneous vorticity contours show that the length of the shear layer at separation decreases with increasing frequency. The effect of amplitude of oscillation on the flow details has been studied when the forcing frequency is kept equal to the vortex-shedding frequency of the stationary cylinder. An increase in amplitude diminishes the time-averaged drag coefficient. The peak value of rms velocity increases, and its location moves upstream. The length of the recirculation bubble decreases with amplitude. The reduction in drag coefficient with frequency and amplitude is broadly reproduced in experiments with the cylinder of lower aspect ratio. DOI: 10.1115/1.2742736

DETAILED MEASUREMENT OF HEAT TRANSFER AND FLOW CHARACTERISTICS IN RECTANGULAR DUCT WITH RIB TURBULATORS MOUNTED ON THE BOTTOM SURFACE

ABSTRACT The present work is an experimental study to investigate the heat transfer and flow characteristics in the entrance region of a rectangular channel with a single and two solid square ribs mounted on the bottom surface of the channel. Hot wire anemometry (HWA) and resistance thermometry (RTD) have been utilized for the velocity and temperature measurements in the flow-field. Liquid crystal thermography (LCT) has been employed to map the surface temperature profiles and evaluation of the heat transfer coefficient. The effectiveness of one-rib and two-ribs is studied from flow modulation and heat transfer enhancement point of view. The Reynolds number (based on the hydraulic diameter of the channel) is set equal to 2.09 × 104. The rib pitch-to-height ratio set during the experiment is equal to 10. The Nusselt number variation on the bottom surface for the no-rib case is similar to what is observed from the law of the wall. The Nusselt number augmentation results from transient LCT closely follows the energy balance check. The turbulent boundary layer well past the ribs for both one-rib and two-rib case is different from that of the plane surface as evident from the higher Nusselt number observed in the far-field region. The stagnation region around the second rib is not as significant as that around the first rib. There is some degree of correlation between the velocity fluctuation and temperature fluctuation, the former being significant in the core of the shear layer while later is significant close to the wall. Overall, the thermal boundary layer, the hydrodynamic boundary layer, flow visualization and the Nusselt number results are observed to correlate with each other very well.

Heat Transfer and Flow Characteristics of a Rib With a Slit

The present investigation is an experimental study of convective heat transfer in the entrance region of a rectangular channel with a single surface mounted slit rib. The open area ratios of the slit rib set during the experiment are equal to 10, 20, 30, 40 and 50%. Hotwire anemometry (HWA) and resistance thermometry (RTD) have been used for velocity and temperature measurement respectively. Both mean and turbulent statistics of the velocity and temperature fluctuations have been reported. Smoke visualization has also been carried out to obtain a qualitative picture of the flow field behind the rib. The surface Nusselt number has been determined from liquid crystal thermography (LCT). The Reynolds number based on the hydraulic diameter of the channel has been set at Re = 32,100. The nature of the flow through the slit and its interaction with the shear layer from the top of the rib depend on the size of the slit. For the slit rib with higher open area ratio (β = 40 and 50%), the bottom part of the slit rib behaves like an independent small rib with its own reattachment region. At smaller open area ratio (β = 10, 20 and 30%), the flow through the slit manipulates the reattaching shear layer from the top of the rib. The size of the slit and its location from the bottom channel surface are the primary parameters responsible for the modification and manipulation of the flow behavior of a slit rib in comparison to the solid rib.Copyright

Numerical Prediction of Flow and Heat Transfer Past a Circular Tube With Annular Fins

A numerical investigation was carried out to study the flow and heat transfer behavior of a vertical circular tube, which is situated between two annular fins in cross-flow. The flow structure of the limiting streamlines on the surface of the circular tube and the annular fins was analysed. A finite volume method was employed to solve the Navier-Stokes and energy equations. The numerical results pertaining to heat transfer and flow characteristics were compared with the available experimental results. The following salient features were observed in this configuration. A horseshoe vortex system was formed at the junction of the stagnation line of the circular tube and the annular fin. The separation took place at the rear of the tube. The influence of the horseshoe vortices on local heat transfer was substantial. The ratio of the axial gap between two annular fins (L) to the radial protrusion length of the annular fin (LR ) was identified as an important parameter. The flow and heat transfer results were presented for different L/LR ratios for a Reynolds number of 1000.Copyright

Buoyancy-induced convection in differentially heated superposed two fluid layers in a rectangular cavity

Buoyancy-driven convection in a differentially heated rectangular cavity containing layers of air—water, air—silicone oil, and silicone oil—water has been experimentally studied using laser-interferometry. The cavity has dimensions 32 × 32 mm2 in the vertical plane and length 447 mm. The layer thicknesses were taken to be equal. Experiments are conducted with three cavity temperature differences of 10, 15, and 18 K. The flow field is mapped in the direction parallel to the cavity length using a Mach-Zehnder interferometer. The flow regimes (such as steady two-dimensional, time-dependent, etc.) established in the individual fluid layers were compared with the published regime diagrams of single fluid layer at different Rayleigh numbers. Quantities such as the interface temperature, average Nusselt number, and the temperature profiles were determined from the interferograms. The interface shapes are recorded in the form of shadowgraphs. These results were also looked upon in terms of the coupling mechanism established at the interface. The major conclusions arrived in this study are as follows. In the experiments involving air, the layers were found to be thermally coupled. The unsteadiness in water could however be transmitted to air in the mechanical coupling mode. The presence of silicone oil over water led to mechanical coupling in the sense that the convective field in water was visibly retarded. The interface temperature determined from the experiments matched those from correlations for a single fluid whenever the coupling was thermal in origin. The differences were higher during mechanical coupling. The interface deformation correlated well with the roll movement visible in the fringes.