Vishal A. Patil

Turbulent flow characteristics in a randomly packed porous bed based on particle image velocimetry measurements

An experimental study was undertaken to better understand the turbulent flow characteristics within a randomly packed porous bed. A relatively low aspect ratio bed (bed width to spherical solid phase particle diameter of 4.67) with the fluid phase refractive index matched to that of the solid phase was used to obtain time resolved particle image velocimetry data. Care was taken to assure that data were outside of the wall affected region, and results are based on detailed time dependent velocity vector maps obtained at selected pores. In particular, four pores were identified that display a range of very disparate mean flow conditions which resemble channel-like flow, impinging flow, recirculating flow, and jet like flow. Velocity data were used for a range of pore Reynolds numbers, Repore, from 418 to 3964 to determine the following turbulence measures: (i) turbulent kinetic energy components, (ii) turbulent shear production rate, (iii) integral Eulerian length and time scales, and (iv) energy spectra. T...

Spatially Resolved Heat Transfer Rates in an Impinging Circular Microscale Jet

An experimental study of a submerged 125-μm circular microscale jet impingement is presented. These jets flows are associated with low exit Reynolds number and a correspondingly high, subsonic Mach number. Detailed distributions of heated and adiabatic wall temperature, and local and average Nusselt number variations are presented for five laminar exit Reynolds numbers in the range of 690 to 1770 at three nozzle-to-surface spacings of 2, 4, and 6 times the nozzle diameter. The corresponding jet exit Mach numbers range between 0.26 and 0.63. An infrared radiometer is used in conjunction with a heated-thin-foil technique to measure detailed surface temperatures. Results indicate that the adiabatic surface temperature distribution is relatively insensitive to nozzle-to-surface spacing within the parameter range studied. With an increase in Reynolds number, the adiabatic surface temperature decreases significantly near the stagnation point. The average Nusselt numbers are higher compared to Martins correlation [1] for large Reynolds numbers. A similar observation has been reported previously in a numerical study on microscale jet impingement [5] and attributed to compressibility of the flow and the possible existence of a slip flow.

Application of heated-thin-foil thermography technique to external convective microscale flows

A heated-thin-foil thermography technique is applied to an external convective microscale flow. The experimental technique and analysis are developed in the context of an external jet impingement flow. Results indicate that lateral conduction along the heated impingement surface is significant and should be included in calculating the heat transfer coefficient. Detailed profiles of heated and adiabatic wall temperature, and local Nusselt number variations are presented for a single 125 µm diameter air jet.

Measurement of Near-Wall Liquid Temperatures in Single-Phase Flows Through Silicon Microchannels

A technique for quantitative temperature visualization of single-phase liquid flows in silicon (Si) microchannels using infrared thermography is presented. This technique offers a new way to measure, non-intrusively, local variations in wall temperature, or fluid temperature at the fluid-wall interface, in a microchannel fabricated entirely of silicon. The experimental setup and measurement procedure required to obtain a high desired signal-to-noise ratio is elaborated. A single 13-mm long and 50 μm wide by 135 μm deep Si microchannel is used in this study. Experiments were performed with a constant electrical heat input to the heat sink surface for four fluid flow rates between 0.6 g/min and 1.2 g/min, corresponding to a Reynolds number range from 200 to 300. Temperature profiles of water in contact with the visualized wall of the microchannel indicate a monotonically increasing trend from the channel inlet for all cases, which is expected of a hydrodynamically and thermally developing flow. The estimated experimental fully developed Nusselt number matches the solution provided in literature for laminar flows. Measurements of the heat sink surface temperature are performed to determine axial variation in heat flux to the visualized channel wall. Results indicate that axial non-uniformity can be significant for the larger Peclet number flows.Copyright

Flow structures and their contribution to turbulent dispersion in a randomly packed porous bed based on particle image velocimetry measurements

An experimental study was undertaken to explore the evolution of flow structures and their characteristics within a randomly packed porous bed with particular attention to evaluating turbulent scalar dispersion. A low aspect ratio bed of 4.67 (bed width to spherical solid phase particle diameter) with fluid phase refractive index matched to that of the solid phase was used in order to obtain time resolved two component particle image velocimetry data. Results are based on detailed velocity vector maps obtained at selected pores near the bed center. Pore, or large scale, regions that are associated with the mean flow were identified based on Reynolds decomposed velocity fields, while smaller scale structures embedded within pore scale regions were identified and quantified by combining large eddy scale decomposition and swirling strength analysis. The velocity maps collected in distinctive pore geometries showed presence of three types of flow regions that display very different mean flow conditions, descr...

Flow Structure Identification in Unsteady Flow in Porous Media

This study is an experimental investigation of the turbulent flow structure in randomly packed porous bed made with uniform sized spheres. Results are based on time resolved, two component PIV measurements in individual pore spaces of the bed. Data are presented for pore Reynolds number range of 54–3964. Three different flow regimes are identified, steady laminar, and unsteady transitional and turbulent flows. Small scale coherent vortical structures are visualized, by performing large eddy scale decomposition, for pore Reynolds number of greater than 1000. Quantative analysis of vortical coherent structures was performed using swirl strength analysis. The number density of vortical structures is found to monotonically increase gradually with pore Reynolds number. The rotation rate of these vortical structures is found to increase linearly with pore Reynolds number. The stretching rate (linear deformation) of the eddies were calculated using continuity to determine the out of plane stretching. The ratio of stretching rate to swirl strength (rotation rate) shows a normal distribution which collapsed onto a single curve. The convective velocities of the structures show a symmetric distribution with a peak value close to 0.8 times the average pore velocity.Copyright

Refractive Index Matching With Distortion Measurements in a Bed of Irregularly Packed Spheres

Experimental flow visualization in porous media is often conducted using optical techniques such as PIV and PTV for velocity field estimation and LIF for concentration field measurements. The porous bed is made optically accessible to laser light and imaging by matching refractive indices of the liquid phase to that of the solid matrix, including the channel walls. The methods most commonly employed to match the refractive indices have been to maximize the transmitted intensity through the bed or to rely on refractometers for measurement of the liquid and solid phases. Refractometers with sensitivity of 0.001 could still cause refraction problems in a porous bed, while accuracy and sensitivity of transmission based methods are limited by the camera resolution and noise scattered by impurities and stray light caused by reflections at interfaces. Both these methods fail to provide uncertainty estimates for particle position determination due to slight refractive index mismatching. This work presents a method for assessing the matching of refractive indices that relies on measuring distortion of a target when imaged through a porous bed. The target used is a grid of 250 μm dots irradiated with light at the necessary wavelength at which refractive indices are to be matched. Two principle types of distortion are quantified, distortion of the image centroid due to interface refraction and intensity distortion within the image for index mismatching as low as 0.0005.Copyright

EXPERIMENTAL VERSUS COMPUTATIONAL METHODS IN THE STUDY OF FLOW IN POROUS MEDIA

Both experimental and computational methods applied to the study of porous media flows are challenging due to the complex multi-phase geometry and ability to resolve scales over a reasonably large domain. This study compares experimentally obtained results based on refractive index matching of detailed velocity field vectors with those obtained using DNS to evaluate both methods for consistency. Data were obtained in a randomly packed bed using uniformly sized spherical particles. Experimental challenges including refractive index matching errors, magnification uncertainties, and the identification of the proper geometry as well as, the arduousness, of matching the geometry, grid resolution particularly near solid contact points, and proper boundary conditions DNS are presented. Detailed comparison of the numerical simulation with PIV measurements are presented by attention paid to the statistical distribution of velocities, and their deviation from DNS estimations from the measured values. There is reasonable matching the velocity fields except for some regions of constricted flow. The axial velocity results are within 12 percent and the normal velocity within 9%. Streamline details show that both methods agree well. It is found that inlet conditions play a significant role in being able to match results. NOMENCLATURE

Predictions of the High Reynolds Number Dispersion Asymptotic Regime Based on Turbulence Measurements in Porous Media

Turbulent porous media flows are encountered in man-made systems such as catalytic bed reactors and advanced heat exchangers. Dispersion experienced by a scalar in these flows play an essential role in the overall efficiency and performance of these systems. In an effort to understand turbulent flows in such complex geometry, time resolved PIV measurements in a refractive index matched porous bed were made. Pore Reynolds numbers, Repore, based on hydraulic diameter, DH, and pore average velocity, Vint, were varied from 839–3964. Pore scale structures such as high momentum jet-like regions and recirculation regions are documented to exist within individual pores. The existence and distribution of these structures is found to vary from pore to pore. The heterogeneity in transport properties introduced due to these structures can be accounted for in estimating transport properties using the method of volume averaging. Eddy viscosity maps and mean velocity field maps, both obtained from PIV measurements, along with the method of volume averaging were used to predict the asymptotic longitudinal dispersion coefficient versus Reynolds number. Asymptotic values of longitudinal dispersion compare well to existing correlations. The role of molecular diffusion was explored by varying the Schmidt number in volume averaged governing equations for tracer transport and it was found that the dispersion coefficient was dictated by the interaction of wandering tracer with recirculation regions.Copyright

Flow Regime Characteristics in Porous Media Flows at High Reynolds Numbers

An experimental study on the turbulent flow characteristics in a randomly packed porous bed is presented and discussed. Time resolved PIV measurements, taken in specific pore spaces are used to evaluate transitional and developed turbulent flow statistics for pore Reynolds numbers from 54 to 3964. Three different regimes of steady laminar, transitional and turbulent flow are presented. Small scale coherent vortical structures are examined, using large eddy scale (LES) decomposition, for pore Reynolds number of greater than 1000. Integral length scales were found to reach asymptotic values of approximately 0.1 times the hydraulic diameter of the bed. The integral Eulerian time scales are found to reach an asymptotic value of approximately 0.3 times the convective time scale in the bed. Mean velocity vector maps show flattening of the velocity distribution due to increased momentum mixing. Turbulent stresses show increasing level of homogeneity at higher pore Reynolds numbers.Copyright