Srinath Ekkad

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Papers by Srinath Ekkad

Effect of channel aspect ratio and fin geometry on fluid flow and heat transfer performance of sectional oblique fin microchannels

2021 20th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (iTherm)

Single-pass micro-channel heat sinks (MCHS) with water as the working fluid have been shown to de... more Single-pass micro-channel heat sinks (MCHS) with water as the working fluid have been shown to deliver large heat removal rates with marginal pressure drop penalty. MCHS, because of their high surface area to volume ratio, provide large heat transfer coefficients (HTC) not found in other macro-scale channels and have found applications in power electronic devices and aerospace heat exchangers. Micro-channels with sectional oblique fins in the same direction augment heat transfer by reinitializing the boundary layer at the leading edge of each fin, causing the flow to be in a developing state, resulting in improved thermal performance. A detailed numerical study has been carried out on oblique fin microchannel heat sinks (OMC) to investigate the impact of channel aspect ratio (AR) and fin-pitch to oblique channel width ratio (BETA) on heat transfer and fluid flow characteristics. OMC with an aspect ratio ranging from 0.25 to 1, followed by four BETA values between 4 and 8, have been tested. Results reveal that OMC with AR of 0.6 outperforms the other configurations. A trade-off has been established where taller fins, although providing higher heat transfer area, suffer from poor fin efficiency, significantly impacting Nusselt number. OMC with BETA up to 6 has been shown to have similar hydro-thermal performance. The existing interplay between increasing mean channel velocity and decreasing heat transfer area has been identified at large hydraulic diameters.

Effect Of Uniform And Stepped Pin-Fin Profiles On Heat Transfer And Frictional Losses In A Narrow Channel With Ar 4:1

Heat transfer and flow characteristics of different staggered pin - fin array configurations in a... more Heat transfer and flow characteristics of different staggered pin - fin array configurations in a channel of aspect ratio 4:1(W:H) have been numerically studied. Three different pin - fin shapes, viz. cylinder, diamond and triangle were modelled. The spanwise and streamwise separation between consecutive pins was 2.5 times the pin diameter across all configurations. Stepped cases ha d three coaxial cylinders, where the ones on either ends ha d same cross - sectional area and height, and the cylinder in middle ha d a reduced cross - sectional area and variable height. The ratio of pin diameter of inner part of stepped geometry to outer part was kept at 0.7. Ratio of height of stepped part to overall pin height was varied from 0.25 to 0.75. Reynolds number based on channel hydraulic diameter was varied f rom 10000 to 50000. Heat transfer results of uniform pin - fin cases have been compared with stepped cases using row averaged and array averaged normalized Nusselt number ratio. Overa...

format_quoteOblong pin fins yielded 20% higher heat transfer than cylindrical fins but at a cost of 100% increase in pressure drop.format_quote

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Effect of Pin Fins on Jet Impingement Heat Transfer on A Rotating Disk

Volume 5B: Heat Transfer — General Interest; Internal Air Systems; Internal Cooling, 2021

Heat transfer on rotating surfaces is a predominant phenomenon in rotating machinery as in the ca... more Heat transfer on rotating surfaces is a predominant phenomenon in rotating machinery as in the case of the gas turbine disk. The gas turbine disk needs to be cooled as well as protected from the ingress of hot turbine gases in the stator-rotor cavity. In the current study, an experimental investigation of the heat transfer of an impinging air jet on a surface rotating at low rotational Reynolds number has been carried out. Addition of pin-fins on the disk surface is an effective way to enhance the heat transfer between the disk and the jet of cooling air. The effect of addition of an inline array of square pin fins on the rotating disk heat transfer has been investigated in this study. Steady state measurements have been carried out using thermocouples embedded at different locations in an aluminum disk with an array of square pin-fins rotating in a large space. Experiments have been conducted at rotational Reynolds numbers (ReR) of 5,487–12,803 based on the disk diameter (D) and je...

Effect of Spanwise Hole to Hole Spacing on Overall Cooling Effectiveness of Effusion Cooled Combustor Liners for a Swirl Stabilized Can Combustor

Volume 5A: Heat Transfer — Combustors; Film Cooling, 2021

One of the most effective ways to cool the combustor liner is through effusion cooling. Effusion ... more One of the most effective ways to cool the combustor liner is through effusion cooling. Effusion cooling (also known as full coverage effusion cooling) involves uniformly spaced holes distributed throughout the combustor liner wall. Effusion cooling configurations are preferred for their high effectiveness, low-pressure penalty, and ease of manufacturing. In this paper, experimental results are presented for effusion cooling configurations for a realistic swirl driven can combustor under reacting (flame) conditions. The can-combustor was equipped with an industrial engine swirler and gaseous fuel (methane), subjecting the liner walls to engine representative flow and combustion conditions. In this study, three different effusion cooling liners with spanwise spacings of r/d = 6, 8, and 10 and streamwise spacing of z/d = 10 were tested for four coolant-to-main airflow ratios. The experiments were carried out at a constant main flow Reynolds number (based on combustor diameter) of 12,5...

format_quoteSome soft EOSs ruled out as they do not predict sufficient mass for observed neutron stars, refining EOS models.format_quote

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Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis

Powder Technology, 2011

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located ... more

A Transient Infrared Thermography Method for Simultaneous Film Cooling Effectiveness and Heat Transfer Coefficient Measurements From a Single Test

Journal of Turbomachinery, 2004

In film cooling situations, there is a need to determine both local adiabatic wall temperature an... more In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to dete...

Effect of Jet Pulsation and Duty Cycle on Film Cooling From a Single Jet on a Leading Edge Model

Journal of Turbomachinery, 2006

The effect of jet pulsation and duty cycle on film effectiveness and heat transfer was investigat... more The effect of jet pulsation and duty cycle on film effectiveness and heat transfer was investigated on a film hole located on the circular leading edge of a blunt body. A transient infrared technique was used to measure both heat transfer coefficients and film effectiveness from a single test. Detailed Frossling number and film effectiveness distributions were obtained for all flow conditions. Jet pulsing frequencies of 5 Hz, 10 Hz, and 20 Hz have been studied. The effect of duty cycle created by the valve opening and closing times was also set at different levels of 10%, 25%, 50%, and 75% of designated 100% fully open condition for different blowing ratios from 0.25 to 2.0. The combination of pulse frequency and duty cycle was investigated for different blowing ratios on a single leading edge hole located at 22 deg from geometric leading edge. Results indicate that higher effectiveness and lower heat transfer coefficients are obtained at the reduced blowing ratios, which result fro...

A Novel Antivortex Turbine Film-Cooling Hole Concept

Journal of Turbomachinery, 2008

A novel turbine film-cooling hole shape has been conceived and designed at NASA Glenn Research Ce... more A novel turbine film-cooling hole shape has been conceived and designed at NASA Glenn Research Center. This “antivortex” design is unique in that it requires only easily machinable round holes, unlike shaped film-cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film-cooling holes. This vorticity typically entrains hot freestream gas and is associated with jet separation from the turbine blade surface. The antivortex film-cooling hole concept has been modeled computationally for a single row of 30 deg angled holes on a flat surface using the 3D Navier–Stokes solver GLENN-HT. A blowing ratio of 1.0 and density ratios of 1.05 and 2.0 are studied. Both film effectiveness and heat transfer coefficient values are computed and compared to standard round hole cases for the same blowing rates. A net heat flux reduction is also determined using both the film effectiveness and heat tra...

Experimental study on solid circulation in a multiple jet fluidized bed

AIChE Journal, 2011

Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets locat... more Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two-dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle-entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. V V

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Film Cooling From a Row of Holes Supplemented With Antivortex Holes

Journal of Turbomachinery, 2009

The primary focus of this paper is to study the film cooling performance for a row of cylindrical... more The primary focus of this paper is to study the film cooling performance for a row of cylindrical holes each supplemented with two symmetrical antivortex holes, which branch out from the main holes. The antivortex design was originally developed at NASA-Glenn Research Center by James Heidmann, coauthor of this paper. This “antivortex” design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. The geometry and orientation of the antivortex holes greatly affect the cooling performance downstream, which is thoroughly investigated. By performing experiments at a single mainstream Reynolds number of 9683 based on the freestream velocity and film hole diameter at four different coolant-to-mainstream blowing ratios of 0.5, 1, 1.5, and 2 and using the transient IR thermography techn...

format_quoteMainstream Reynolds number of 11000 indicates fully turbulent flow, critical for validating test conditions in cooling effectiveness assessments.format_quote

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Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

Experiments and numerical computations are performed to investigate the convective heat transfer ... more Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared to fully-developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the fl...