Yogen Vishwas Utturkar

Assessment of Cooling Enhancement of Synthetic Jet in Conjunction With Forced Convection

Synthetic jets are meso or micro fluidic devices, which operate on the “zero-net-mass-flux” principle. They impart a positive net momentum flux to the external environment, and are able to produce the cooling effect of a fan sans its ducting, reliability issues, and oversized dimensions. As a result, recently their application as electronics cooling devices is gaining momentum. Traditionally, synthetic jets have been sought as a replacement to the fan in many electronic devices. However, in certain large applications, complete replacement of the fan is not feasible, because it is necessary to provide the basic level of cooling over a large area of a printed assembly board. Such applications often pose a question whether synthetic jet would be able to locally provide reasonable enhancement over the forced convection of the fan flow. In the present study, we present the cooling performance of synthetic jets complementing forced convection from a fan. Both experiments and CFD computations are performed to investigate the interaction of the jet flowfield with a cross flow from fan. The inlet velocity, jet disk amplitude, and channel height are varied in the computational simulations to evaluate the impact of these changes on the cooling properties. Overall, both studies show that a synthetic jet is able to pulse and disrupt the boundary layer caused from fan flow, and improve heat transfer up to 4× over forced convection.Copyright

Development of a High-Lumen Solid State Down Light Application

Light-emitting diode (LED)-based solid-state lighting (SSL) products have been exceeding the predicted performances especially at the chip and package levels. This has led to new SSL-based products for energy savings and long lifetimes. Large amounts of government funding and private investments have been made during the last decade to accelerate and guide the technology. This paper focuses on the development of an LED-based high-lumen luminaire technology. The critical subcomponents of the luminaire are the LED light engine (LED chips and optical system), thermal management, and driver electronics. Each of these subcomponents will be discussed in detail for a 100 W incandescent replacement technology. The paper addresses system integration of each of the subcomponents. While the design of new products evolve, the lack of reliability data poses a risk of premature failure of LED-based products. Premature failures would trigger customer rejection and may delay market penetration. Therefore, luminaire reliability is an important aspect of luminaire design. In cohort with this notion, finally, the luminaire reliability has been discussed.

Interaction of Synthetic Jet Cooling Performance With Gravity and Buoyancy Driven Flows

Meso scale cooling devices have been of interest for low form factor, tight space, and high COP thermal management problems. A candidate meso scale device, known as synthetic jets, operates with micro fluidic principles and disturbs the boundary layer causing significant heat transfer over conventional free convective heat transfer in air. Previous papers have dealt with the impingement and cross flow, but did not study mixed convection for synthetic jet with natural convection. In the present study, we discuss the results of an experimental study to investigate the interplay between jet orientations with respect to gravity, elevated temperature conditions, and synthetic jet heat dissipation capacity. Experiments were performed by placing synthetic at different positions around a square, 25.4mm heated flat surface. The flow physics behind the experimental findings is discussed. It is found that impingement heat transfer outperformed more than 30% compared to other orientations. The jet showed about 15% sensitivity to angular orientations.Copyright

Interaction of a synthetic jet with an actively cooled heat sink

In high power electronics applications, the most preferred way of thermal management is the use of a heat sink and a fan for active cooling. Though this thermal solution is fairly adequate for the current heat dissipation needs, it suffers from some serious limitations, which may tend to limit its use with time. In the present study, we quantify the limitations of a conventional fan-cooled heat sink with computational models, and propose a remedy to the problem in the form of synthetic jets. Synthetic jets are meso-scale devices operating at zero-net-mass-flux principle. These devices produce periodic jet streams, which may have velocities 10-20 times greater than the average fan velocities. As a result, positioning one or more of these jets closer to the fins can cause high velocity air currents in tightly spaced fin gaps and enhance the surface heat transfer. Results of a CFD based study have been presented. Different heat sink designs are considered, and the quantification of the observed heat transfer enhancement is provided along with the underlying flow physics. It is found that synthetic jets can enhance heat transfer in excess of 2.5 times over fan cooled tight-spaced aluminum heat sinks.

Immersion Cooling of Light Emitting Diodes

Light emitting diodes (LEDs) historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors has allowed them to penetrate specialty and general illumination applications. The increased electrical currents used to drive the LEDs have resulted in higher heat fluxes than those for average silicon CPUs. This has created a need to focus more attention on the thermal engineering of LED power packages. The output of a typical commercial high brightness 1mm2 LED has exceeded 100 lumens at drives levels exceeding 300 W/cm2.

Theoretical evaluation and experimental investigation of microencapsulated phase change materials (MPCM) in electronics cooling applications

In many liquid-cooled applications, the bulk temperature of the coolant, rather than the surface heat flux, limits the amount of cooling achieved. In such applications, the coolant flow path is often characterized by significant pressure losses, thus limiting the overall mass flow rate within the system. As a remedy, the idea of increasing the heat capacity of the cooling fluid by adding microencapsulated phase change material (MPCM) to the fluid has been lately explored. Firstly, this increase in the heat capacity comes at the cost of higher pressure-drop due to the increased viscosity of the slurry. Thus, the benefit of adding MPCMs to the coolant can only be realized if the increase in the latent heat capacity due to these materials is substantially more than the heat capacity lost due to the reduction in the mass flow rate. Secondly, it is expected that the conduction of heat through the plastic encapsulate from the coolant to the MPCM might tend to be a limiting factor at high flow rates. As a result, the latent capacity might get under-utilized at higher flow rates if the MPCMs do not completely fuse while the coolant undergoes heat absorption in the system to be cooled. In the present work, a theoretical performance metric is developed to assess the benefit margins obtainable with the use of MPCM slurries. The theoretical predictions on the effective specific heat of the slurry have been assessed against experimental measurements. The figures of merit proposed in this paper serve as design guidelines for liquid-cooled applications employing MPCM slurries.

Effect of Synthetic Jets Over Natural Convection Heat Sinks

In moderate power electronics applications, the most preferred way of thermal management is natural convection to air with or without heat sinks. Though the use of heat sinks is fairly adequate for modest heat dissipation needs, it suffers from some serious performance limitations. Firstly, a large volume of the heat sink is required to keep the junction temperature at an allowable limit. This need arises because of the low convective film coefficients due to close spacing. In the present computational and experimental study, we propose a synthetic jet embedded heat sink to enhance the performance levels beyond two times within the same volume of a regular passive heat sink. Synthetic jets are meso-scale devices producing high velocity periodic jet streams at high velocities. As a result, by carefully positioning of these jets in the thermal real estate, the heat transfer over the surfaces can be dramatically augmented. This increase in the heat transfer rate is able to compensate for the loss of fin area happening due to the embedding of the jet within the heat sink volume, thus causing an overall increase in the heat dissipation. Heat transfer enhancements of 2.2 times over baseline natural convection cooled heat sinks are measured. Thermal resistances are compared for a range of jet operating conditions and found to be less than 0.9 K/W. Local temperatures obtained from experimental and computational agreed within ± 5%.Copyright

Vortex Dynamics of Synthetic Jets: A Computational and Experimental Investigation

Seamless advancements in electronics industry resulted in high performance computing. These innovations lead to smaller electronics systems with higher heat fluxes than ever. However, shrinking nature of real estate for thermal management has created a need for more effective and compact cooling solutions. Novel cooling techniques have been of interest to solve the demand. One such technology that functions with the principle of creating vortex rings is called synthetic jets. The jets are meso-scale devices operating as zero-net-mass-flux principle by ingesting and ejection of high velocity working fluid from a single opening. These devices produce periodic jet streams, which may have peak velocities over 20 times greater than conventional, comparable size fan velocities. Those jets enhance heat transfer in both natural and forced convection significantly over bare and extended surfaces. Recognizing the heat transfer physics over surfaces require a fundamental understanding of the flow physics caused by micro fluid motion. A comprehensive computational and experimental study has been performed to understand the flow physics of a synthetic jet. Computational study has been performed via Fluent commercial software, while the experimental study has been performed by using Laser Doppler Anemometry. Since synthetic jets are typical sine-wave excited between 20 and 60 V range, they have an orifice peak velocity of over 60 m/s, resulting in a Reynolds number of 2000. CFD predictions on the vortex dipole location fall within 10% of the experimental measurement uncertainty band.Copyright

Reduced-Order Investigation of Synthetic Jet Cooling for Electronic Applications

Synthetic jets are meso-scale devices operating at zero-net-mass-flux principle. These devices produce periodic jet-like streams, which have local velocities 10–20 times greater than the average fan velocities. As a result, positioning one or more of these jets close to a heat sink causes high-velocity air currents in tightly spaced fin gaps and enhances the surface heat transfer. A reduced-order modeling (ROM) approach was followed in simulating the heat transfer analysis of commercial heat sink with synthetic jet. Unsteady state results are matched with steady state results as part of the ROM approach. The methodology is implemented on two problems (i.e canonical problem of jet impinging perpendicularly on a flat plate and jet blowing on a commercial heat sink). Results from ROM for two cases are validated against experimental results. It is found that, this approach provides 90% time saving within ±5% accuracy. Modeling via ROM is much faster and cheaper computationally; hence this approach can be used for studying the system-level convective heat transfer enhancement of heat sinks using synthetic jets.Copyright