Current Search:   Peles, Yoav (x)

View All Items

  • CSV Spreadsheet
(1 - 6 of 6)
Investigation of Novel Fin Structures Enhancing Micro Heat Sink Thermal Performance
Title
Investigation of Novel Fin Structures Enhancing Micro Heat Sink Thermal Performance.
Creator
Ismayilov, Fuad, Peles, Yoav, Kassab, Alain, Putnam, Shawn, Akturk, Ali, University of Central Florida
Abstract / Description
Operating temperature in electronics applications is continuously increasing. Therefore, for the past few decades, high heat flux removing micro heat sinks are investigated in terms of heat transfer effectiveness. This study generally concentrates on improving the passive heat transfer techniques. Micro heat sinks used in experiments are fabricated using MEMS techniques. Resistance temperature detectors, RTDs, were used for temperature measurements. The experimental data was obtained for...
Show moreOperating temperature in electronics applications is continuously increasing. Therefore, for the past few decades, high heat flux removing micro heat sinks are investigated in terms of heat transfer effectiveness. This study generally concentrates on improving the passive heat transfer techniques. Micro heat sinks used in experiments are fabricated using MEMS techniques. Resistance temperature detectors, RTDs, were used for temperature measurements. The experimental data was obtained for single and two phase flow regions; however, only single phase flow results were considered in numerical simulations. Numerical validations were performed on the micro heat sinks, including cylinder and hydrofoil shaped pin fins. Following the validation phase, optimization has been performed to further improve the hydraulic and thermal performance. DOE study showed that the chord length and leading edge size of the hydrofoil pin fin are significant contributors to the thermal performance. The ranges of geometrical variables were identified and fed into multi-objective optimization cycles implementing the multi-objective genetic algorithm. The optimization objectives were to minimize pumping requirements while enhancing the local and global heat transfer effectiveness over the surface of the heater in single phase flow environment. A broad range of geometries were obtained with an acceptable tradeoff between thermal and hydraulic performance for low Reynolds number. Additionally, tandem geometries were investigated and showed that higher heat transfer effectiveness could be obtained with acceptable pumping power requirements. The importance of such optimization studies before the experimental testing is highlighted, and novel geometries are presented for further experimental investigations. Thermal performance improvement of 28% was obtained via implementing proposed geometries with only a 12% pressure drop increase. Local heat transfer optimization, aiming to decrease the local temperatures were also performed using doublet pin fin configurations. Results showed that tandem hydrofoils could control the flow with minimum pressure drops while reaching the desired low local temperatures.
Show less
Date Issued
2019
Identifier
CFE0007821, ucf:52828
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007821
Controlled Bubble Dynamics Inside Micropillar Arrays
Title
Controlled Bubble Dynamics Inside Micropillar Arrays.
Creator
Khalil Arya, Faraz, Peles, Yoav, Gou, Jihua, Kassab, Alain, Abdolvand, Reza, University of Central Florida
Abstract / Description
Bubble dynamics inside micro domains was manipulated and used to pump liquid. Micropillars were formed inside two 1.5 mm wide and 220 (&)#181;m high microchannels with a length of 23 mm. One microchannel had three arrays of micropillars with diameters of 40 (&)#181;m and the other had a single array of micropillars with diameters of 30 (&)#181;m. An array of five 200 (&)#181;m by 200 (&)#181;m heaters was deposited inside these micropillar arrays and was used to control bubble size and...
Show moreBubble dynamics inside micro domains was manipulated and used to pump liquid. Micropillars were formed inside two 1.5 mm wide and 220 (&)#181;m high microchannels with a length of 23 mm. One microchannel had three arrays of micropillars with diameters of 40 (&)#181;m and the other had a single array of micropillars with diameters of 30 (&)#181;m. An array of five 200 (&)#181;m by 200 (&)#181;m heaters was deposited inside these micropillar arrays and was used to control bubble size and trajectory. A sequential power switching of the heaters was used to pump liquid in a desired direction with a flow rate of up to 133 (&)#181;l/min for the three arrays micropillars microchannel and up to 44.4 (&)#181;l/min for the single array micropillars device.
Show less
Date Issued
2018
Identifier
CFE0007757, ucf:52381
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007757
Cavitation and heat transfer over micro pin fins
Title
Cavitation and heat transfer over micro pin fins.
Creator
Nayebzadeh, Arash, Peles, Yoav, Chow, Louis, Kassab, Alain, Plawsky, Joel, University of Central Florida
Abstract / Description
With the dramatic increase in the usage of compact yet more powerful electronic devices, advanced cooling technologies are required to maintain delicate electronic components below their maximum allowable temperatures and prevent them from failure. One solution is to use innovative pin finned heat sinks. This research is centered on the evaluation of hydrodynamic cavitation properties downstream pin fins and extended toward single-phase heat transfer enhancement of array of pin fins in...
Show moreWith the dramatic increase in the usage of compact yet more powerful electronic devices, advanced cooling technologies are required to maintain delicate electronic components below their maximum allowable temperatures and prevent them from failure. One solution is to use innovative pin finned heat sinks. This research is centered on the evaluation of hydrodynamic cavitation properties downstream pin fins and extended toward single-phase heat transfer enhancement of array of pin fins in microchannel. In this work, transparent micro-devices capable of local wall temperature measurements were micro fabricated and tested. Various experimental methods, numerical modeling and advanced data processing techniques are presented. Careful study over cavitation phenomena and heat transfer measurement downstream pin fins were performed.Hydrodynamic cavitation downstream a range of micro pillar geometries entrenched in a microchannel were studied. Three modes of cavitation inception were observed and key parameters of cavitation processes, such as cavity length and angle of attachment, were compared among various micro pillar geometries. Cavity angle of attachments were predominantly related to the shape of the micro pillar. Fast Fourier transformation (FFT) analysis of the cavity image intensity revealed transverse cavity shedding frequencies in various geometries and provided an estimation for vortex shedding frequencies.Experimental and numerical heat transfer studies over array of pin fins were carried out to find out the influence of lateral interactions of fluid flow on the enhancement of heat transfer. Local temperature measurements combined with a conjugate fluid flow and heat transfer modeling revealed the underlying heat transfer mechanisms over pin fin arrays.
Show less
Date Issued
2019
Identifier
CFE0007690, ucf:52407
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007690
Electrospray and Superlens Effect of Microdroplets for Laser-Assisted Nanomanufacturing
Title
Electrospray and Superlens Effect of Microdroplets for Laser-Assisted Nanomanufacturing.
Creator
Castillo Orozco, Eduardo, Kumar, Ranganathan, Mansy, Hansen, Peles, Yoav, University of Central Florida
Abstract / Description
Nanoparticles of various materials are known to exhibit excellent mechanical, chemical, electrical, and optical properties. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and three-dimensional structures, such as thin films and discrete arrays. Electrospray technology and laser heating enable the deposition of these nanoparticles through the dual role of microdroplets as nanoparticle carriers and superlenses. The main goals of this dissertation are...
Show moreNanoparticles of various materials are known to exhibit excellent mechanical, chemical, electrical, and optical properties. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and three-dimensional structures, such as thin films and discrete arrays. Electrospray technology and laser heating enable the deposition of these nanoparticles through the dual role of microdroplets as nanoparticle carriers and superlenses. The main goals of this dissertation are to delineate the electrospray modes, to achieve subwavelength focusing, and to enable a process for the deposition of nanoparticles into microlayers and discrete nanodots (a nanodot is a cluster of nanoparticles) on rigid and flexible substrates. This additive manufacturing process is based on the electrospray generation of water microdroplets that carry nanoparticles onto a substrate and the laser sintering of these nanoparticles. The process involves injecting nanoparticles (contained inside electric field-driven water microdroplets) into a hollow laser beam. The laser beam heats the droplets, causing the water to evaporate and the nanoparticles to sinter and form deposit of material on the substrate.The electrohydrodynamic inkjet printing of nanoparticle suspensions has been accomplished by the operation of an electrospray in microdripping mode and it allows the deposition of monodisperse microdroplets containing nanoparticles into discrete nanodot arrays, narrow lines, and thin films. For flow rates with low Reynolds number, the mode changes from dripping to microdripping mode, and then to a planar oscillating microdripping mode as the electric capillary number, Cae increases. The microdripping mode which is important for depositing discrete array of nanodots is found to occur in a narrow range, 2 ? Cae ? 2.5. The effect of the physical properties on the droplet size and frequency of droplet formation is more precisely described by the relative influence of the electric, gravity, viscous, and capillary forces. A scaling analysis is derived from a fundamental force balance and has yielded a parameter based on the electric capillary number, capillary number, and Bond number. Results for different nanoparticle suspensions with a wide range of physical properties show that the normalized radius of droplet, can be correlated using this parameter in both dripping and microdripping modes. The same parameter also correlates the normalized frequency of droplet formation, Nd* as an increasing function in the microdripping mode. Viscosity affects the shape of the cone by resisting its deformation and thus promoting a stable microdripping mode. Reduction in surface tension decreases the droplet size in the electrospray modes. However, the capillary size and electrical conductivity have minimal effect on the size of the ejected droplets. Electrical conductivity affects the transition between microdripping and oscillating microdripping modes. Based on this analysis, it is possible to design the electrospray to produce uniform monodisperse droplets by manipulating the voltage at the electrode, for any desired nanoparticle concentration of a suspension to be sintered on a substrate. For the fabrication of nanodots, a laser beam of wavelength ? = 1064 nm was focused to a diameter smaller than its wavelength. When the microdroplets did not carry nanoparticles, the subwavelength focusing of the laser yielded nanoholes smaller than its wavelength. Results show that tiny features with high resolution can be created by loading microdroplets with nanoparticles and squeezing the laser beam to subwavelength regions. Nanodots of silicon and germanium with diameters between 100 - 500 nm have been deposited on a silicon substrate. This study demonstrates an interdisciplinary mechanism to achieve subwavelength focusing in a laser process. In this process, the microdroplets serve as both a nanoparticle carrier and a superlens that focuses a laser beam to subwavelength diameters up to ? /10, thus overcoming the diffraction limit. The microdroplets are generated from a suspension of nanoparticles using an electrospray technique and the superlens characteristic of these microdroplets is attributed to three optical phenomena such as Maxwell's fish eye lens or L(&)#252;neberg lens, evanescent waves by laser scattering, and evanescent waves by the total internal reflection principle. A microfluidic cooling effect can also contribute to creating subwavelength features. In summary, this work describes a new laser-assisted additive manufacturing process for the fabrication of nanodots and microlayers using nanoparticles of different materials. In this process, microdroplets from an electrospray are used as nanoparticle carriers and superlenses to focus the laser to a diameter smaller than its wavelength. While this process is demonstrated to produce subwavelength holes and nanodots, the process is scalable to produce narrow lines and thin films of semiconductor materials by an additive manufacturing technique. This process extends the application of infrared lasers to the production of nanostructures and nanofeatures, and, therefore, provides a novel technology for nanomanufacturing.
Show less
Date Issued
2018
Identifier
CFE0007563, ucf:52579
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007563
Droplet impact on deep liquid pools: secondary droplets formation from Rayleigh jet break-up and crown splash
Title
Droplet impact on deep liquid pools: secondary droplets formation from Rayleigh jet break-up and crown splash.
Creator
Castillo Orozco, Eduardo, Kumar, Ranganathan, Mansy, Hansen, Peles, Yoav, University of Central Florida
Abstract / Description
This work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet...
Show moreThis work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet impact, it is not clear how liquid properties can affect the instabilities leading to the Rayleigh jet breakup and the number of secondary drops formed after it pinches off. In this work, through systematic experiments, the droplet impact phenomena is investigated by varying viscosity and surface tension of liquids as well as impact speeds. Further, using a Volume-of-Fluid (VOF) method, it is shown that Rayleigh-Plateau instability is influenced by these parameters, and capillary timescale is the appropriate scale to normalize the breakup time. Increase in impact velocity increases the height of the thin column of fluid that emerges from the liquid pool. Under certain fluid conditions, the dissipation of this extra kinetic energy along with the surface tension forces produces instabilities at the neck of the jet. This could result in jet breakup and formation of secondary drops. In other words, both the formation of the jet and its breakup require a balance between viscous, capillary and surface tension forces. Based on Ohnesorge number (Oh) and impact Weber number (We), a regime map for no breakup, Rayleigh jet breakup, and crown splash is suggested for 0.0033 ? Oh ? 0.136. For Weber numbers beyond the critical value and Oh ? 0.091 the jet breakup occurs (Rayleigh jet breakup regime). While for Oh (>) 0.091, the jet breakup is suppressed regardless of the Weber number. In addition, high impact velocity initiates the crown formation and if further intensified it can disintegrate it into numerous secondary drops (crown splash) and it is observed to occur at all Ohnesorge numbers and high enough Weber numbers, however, at high Oh, a large portion of kinetic energy is dissipated, thus Rayleigh jet breakup is suppressed regardless of the magnitude of the impact velocity. Moreover, a correlation is proposed for normalized time with respect to the normalized maximum height of jet.
Show less
Date Issued
2015
Identifier
CFE0006278, ucf:51593
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006278
Local transient characterization of thermofluid heat transfer coefficient at solid-liquid nano-interfaces
Title
Local transient characterization of thermofluid heat transfer coefficient at solid-liquid nano-interfaces.
Creator
Mehrvand, Mehrdad, Putnam, Shawn, Peles, Yoav, Orlovskaya, Nina, Abdolvand, Reza, University of Central Florida
Abstract / Description
The demands for increasingly smaller, more capable, and higher power density technologies in microelectronics, energy, or aerospace systems have heightened the need for new methods to manage and characterize extreme heat fluxes (EHF). Microscale liquid cooling techniques are viewed as a promising solution for removing heat from high heat flux (HHF) systems. However, there have been challenges in physical understanding and predicting local thermal transport at the interface of micro and...
Show moreThe demands for increasingly smaller, more capable, and higher power density technologies in microelectronics, energy, or aerospace systems have heightened the need for new methods to manage and characterize extreme heat fluxes (EHF). Microscale liquid cooling techniques are viewed as a promising solution for removing heat from high heat flux (HHF) systems. However, there have been challenges in physical understanding and predicting local thermal transport at the interface of micro and nanoscale structures/devices due to ballistic effects and complex coupling of mass, momentum, and energy transport at the solid-liquid-vapor interfaces over multiple time and length scales. Moreover, it's challenging to experimentally validate new HHF models due to lack of high resolution techniques and measurements.This dissertation presents the use of a high spatiotemporal and temperature resolution measurement technique, called Time-domain Thermoreflectance (TDTR). TDTR is used to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and sub-cooled channel-flow configuration. Studies focused on room temperature, syringe-pumped single-and two-phase water flow in a ?480 ?m hydraulic diameter microchannel, where the TDTR pump heating laser induces local heat fluxes of ?0.5-2.5 KW/cm2 in the center of the microchannel on the surface of a 60-80 nm metal or alloy thin film transducer with hot-spot diameters of ?7-10 ?m. In the single-phase part, a differential measurement approach is developed by applying anisotropic version of the TDTR to predict local HTC using the measured voltage ratio parameter, and then fitting data to a thermal model for layered materials and interfaces. It's shown that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the local HTC, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ?1060(&)#177;190 kW/m2?K, where the well-established Nusselt number correlations predict ?900(&)#177;150 kW/m2?K.In the two-phase part, pump-probe beams are first used to construct the local pool and flow boiling curves at different heat fluxes and hot spot temperatures as a function of HTC enhancement. At a same heat flux level, it's observed that fluid flow enhances HTC by shifting heat transfer mechanism (or flow regime) from film boiling to nucleate boiling. Based on observations, it's hypothesized that beyond an EHF flow may reduce the bubble size and increase evaporation at the liquid-vapor interface on three-phase contact line, but it's unable to rewet and cool down the dry spot at the center due to the EHF. In the last part of two-phase experiments, transient measurements are performed at a specific heat flux to obtain thermal temporal fluctuations and HTC of a single bubble boiling and nucleation during its ebullition cycle. The total laser power is chosen to be between the minimum required to start subcooled nucleation and CHF of the pool boiling. This range is critical since within 10% change in heating flux, flow can have dramatic effect on HTC. Whenever the flow gets closer to the dry spot and passes through it (receding or advancing) HTC increases suddenly. This means that for very hot surfaces (or regions of wall dry-out), continuous and small bubbles on the order of thermal diffusion time and dry spot length scales respectively could be a reliable high heat flux cooling solution. This could be achieved by controlling the bubble size and frequency through geometry, surface structure and properties, and fluid's thermos-fluid properties.
Show less
Date Issued
2017
Identifier
CFE0006765, ucf:51832
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006765