Srinath Ekkad - Academia.edu (original) (raw)

Papers by Srinath Ekkad

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.

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...

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...

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...

Powder Technology, 2011

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located ... more A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized ...

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...

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...

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...

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...

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...

Journal of Electronic Packaging, 2020

High porosity, high pore-density (pores per inch: PPI) metal foams are a popular choice in high h... more High porosity, high pore-density (pores per inch: PPI) metal foams are a popular choice in high heat flux cooling applications as they offer large heat transfer area over a given volume, however, accompanied by a concomitant increase in pumping power requirements. This experimental study aims toward developing a novel metal-foam based cooling configuration featuring thin copper foams (3 mm) subjected to orthogonal air jet array impingement. The foam configurations allowed strategic and selective placement of high pore-density (90 PPI) and high porosity (∼96%) copper foam on the heated surface with respect to the jet array in the form of foam stripes aiming to enhance heat transfer and reduce pressure drop penalty. The thermal-hydraulic performance was evaluated over range of Reynolds numbers, jet-to-jet (x/dj, y/dj) and jet-to-target (z/dj) spacings and compared with a baseline smooth surface. The effect of pore density was further analyzed by studying 40 PPI copper foam and compare...

Journal of Thermal Science and Engineering Applications, 2021

As stringent emissions controls are being placed on gas turbines, modern combustor design optimiz... more As stringent emissions controls are being placed on gas turbines, modern combustor design optimization is contingent on the accurate characterization of the combustor flame side heat loads. Power generation turbines are increasingly moving toward natural gas, biogas, and syngas, whose composition is highly dependent on the sourcing location. With fuel flexible nozzles, it is important to understand the heat load from various gas mixtures to optimize the cooling design to make sure the liner is not under/over cooled for some mixtures as this has a larger effect on NOx/CO emissions. In addition to knowing the heat load distribution, it is important to understand the peak heat load under start/stop transient conditions which tend to be much higher than steady-state/cruise altitude heat loads. The present work focuses on the experimental measurement of the transient heat load along a can combustor under reacting conditions for a swirl-stabilized premixed methane–air flame. Tests were ca...

Array jet impingement heat transfer onto thin metal foams of different pore densities has been ex... more Array jet impingement heat transfer onto thin metal foams of different pore densities has been experimentally investigated in the current study. Aluminum foams with high porosity (93%) and different pore densities of 5, 20 and 40 ppi are subjected to array jet impingement under an intermediate crossflow exit scheme. The jets are arranged such that the streamwise jet-to-jet spacing is x/dj = 8 and spanwise jet-to-jet spacing is y/dj = 4. Jet to target plate spacing was maintained at z/dj = 6 where ‘z’ is the distance between the jet plate and the target surface on which metal foams were installed. A steady state heat transfer technique has been used to obtain local heat transfer coefficients along the streamwise direction. It is observed that heat transfer enhancement levels increase as pore density increases. An enhancement of 50–100% over the baseline case of impingement onto smooth surface is obtained over the flow range tested (3000 < Redj < 12000). At a constant pumping po...

Journal of Thermal Science and Engineering Applications

High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are inst... more High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90,000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. The present study is targeted toward these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg, V-shaped, inverse V-shaped, W-shaped, and M-sha...

Journal of Turbomachinery

The combined action of Coriolis and centrifugal buoyancy forces results in nonuniform heat transf... more The combined action of Coriolis and centrifugal buoyancy forces results in nonuniform heat transfer coefficient on pressure and suction side internal walls, hence leading to nonuniform metal temperatures and increased thermal stresses. The present study addresses the problem of nonuniform heat transfer distribution due to rotation effect and proposes novel designs for serpentine cooling passages, which are arranged along the chord of the blade. The two configurations were four-passage and six-passage serpentine smooth channels. Detailed heat transfer coefficients were measured using transient liquid crystal thermography under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 12,294 to 85,000 under stationary conditions. Rotation experiments were carried out for the Rotation numbers of 0.05 and 0.11. Heat transfer enhancement levels of approximately two times the Dittus–Boelter correlation (for developed flow in smooth tu...

Journal of Engineering for Gas Turbines and Power

In this study, we provide detailed wall heat flux measurements and flow details for reacting flow... more In this study, we provide detailed wall heat flux measurements and flow details for reacting flow conditions in a model combustor. Heat transfer measurements inside a gas turbine combustor provide one of the most serious challenges for gas turbine researchers. Gas turbine combustor improvements require accurate measurement and prediction of reacting flows. Flow and heat transfer measurements inside combustors under reacting flow conditions remain a challenge. The mechanisms of thermal energy transfer must be investigated by studying the flow characteristics and associated heat load. This paper experimentally investigates the effects of combustor operating conditions on the reacting flow in an optical single can combustor. The swirling flow was generated by an industrial lean premixed, axial swirl fuel nozzle. Planar particle image velocimetry (PIV) data were analyzed to understand the characteristics of the flow field. Liner surface temperatures were measured in reacting condition w...

Journal of Turbomachinery, 2013

Detailed heat transfer coefficient distributions have been obtained for narrow diverging channels... more Detailed heat transfer coefficient distributions have been obtained for narrow diverging channels with and without enhancement features. The cooling configurations considered include rib turbulators and concavities (or dimples) on the main heat transfer surfaces. All of the measurements are presented at a representative Reynolds number of 28,000. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to the pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The model wall inner surfaces were sprayed with thermochromic liquid crystals and a transient test was used to obtain the local heat transfer coefficients from the measured color change. An analysis of the results shows that the choice of designs is limited by the available pressure drop, even if the design provides significantly higher heat transfer coe...