In this work, an investigation of the heat sink performance employing sinusoidal microchannels embedded with pin fins was conducted. The effect of the sine wave frequency, the pin fins’ diameter, and the hydraulic diameter of the microchannel are studied. The results are quantified in terms of thermal resistance and pressure drop. The study was done using Reynolds numbers varying from 250 to 2000. As Reynolds number increases, the heat sink’s thermal resistance decreased while the pressure drop increased accordingly for all scenarios. The sinusoidal microchannels showed better performance — lower thermal resistance — but with the cost of higher pressure drop compared to the straight microchannel heat sink. The heat sink’s performance was improved by increasing the frequency, diameter of pin fins, and hydraulic diameter; however, this reduction in thermal resistance was associated with an increase in pressure drop. The reduction in thermal resistance of the different configurations of the sinusoidal microchannels was between 17% and 69% compared to the straight microchannel heat sink.

Ferrofluid channel flows have been used for many non-invasive flow manipulation applications, including drug-delivery, heat transfer enhancement, mixing enhancement, etc. Heat transfer enhancement is one of the most coveted outcomes from novel cooling systems employed for electronic cooling. While using Ferrofluids for heat transfer enhancement, the external magnetic field usually induces Kelvin Body Force, which causes the ferrofluid to swirl or ‘mix’. This mixing process causes extra convection over what is induced through fluid inertia and is responsible for heat transfer enhancement. In order to understand the phenomenon of heat transfer enhancement, it would be logical to view it from the perspective of mixing enhancement. Moreover, channel flows are most common in liquid cooling of electronics equipment, and hence such a fundamental understanding of synergies between mixing and heat transfer enhancement can help pose design rules for advanced cooling configuration for electronics cooling. In this work, a Ferrofluid channel flow is analyzed in the presence of an external magnetic field. A 2-D 90° bend channel ferrofluid flow is considered, with a significant length scale of 0.01 m, where two external current-carrying wires provide an external magnetic field. An external inward heat flux of 1000 W/m 2 is applied on the walls of the channel. The channel flow is studied numerically by varying different parameters relating to the external magnetic field and flow conditions. The ferrofluid used is considered magnetite based on water as the carrier fluid, and the properties of which are modeled using appropriate mixture models for nanofluids. The mixing induced in the flow is characterized by using two different mixing numbers based on the flow velocity. This type of characterization is analogous to characterizing flow turbulence. The heat transfer enhancement is characterized using Nusselt numbers. These non-dimensional numbers (mixing) are studied in congruence with the Nusselt number to understand the relationship between the mixing and heat transfer and draw comparative inferences with flow conditions without heat transfer enhancement. Finally, conclusions are drawn between the mixing & heat transfer intensification at local and global levels and choosing the apposite mixing numbers to characterize heat transfer enhancement.

Extruded radial fin heat sinks are typically used in power electronics for temperature management. The goal of this study is to compare the temperature predictions of LPTN (Lumped Parameter Thermal Network), Finite element based thermal analysis (using ANSYS Mechanical) and Finite volume based CFD (Computational Fluid Dynamics) & thermal analysis in power electronics hardware. The power electronics hardware under study is used for control of compressors / heaters used in a medical thermal management device. The hardware consists of 2 SCRs (Silicon Controlled Rectifiers) and 2 TRIACs (TRIodes for Alternating Current) arranged in straight in-line configuration with the TRIACs having a higher internal heat generation (1.643E7 W/m 3 ) than the SCRs (4.53E6 W/m 3 ). Each of these four devices is bolted to a separate anodized aluminum heat sink using a TIM (Thermal Interface Material). It was found that CFD based Conjugate Heat Transfer (CHT) analysis with S2S (Surface to Surface) Radiation transfer gave the most accurate thermal predictions (within 6°C). These were comparable to the predictions from 3-D Solid energy equation with uniform convection and S2S radiation. Depending on the availability of computational time and resources, either full CHT study or 3-D Solid energy equation based thermal analysis with uniform convection and S2S radiation is recommended for initial design study.

Recently, the new research field of room temperature liquid metal cooling for extreme heat flux electronic devices has gradually attracted much attention. And many investigations claimed that liquid metal cooling has a better heat transfer performance than water cooling. However, such a conclusion was just applicable to the special geometry sizes. This paper is dedicated to deeply interpret and systematically compare the laminar heat transfer in a mini/micro pipe with water and liquid metal as heat transfer fluid. Theoretical analysis for the laminar fully developed segment and numerical investigation for the laminar developing segment were conducted. The results indicated that liquid metal galinstan shows a better heat transfer performance in mini-pipe. And the shorter the pipe, the better performance the liquid metal cooling has. However, when the velocity is small, the galinstan cooling is worse than water cooling in micro-pipe. Moreover, based on thermal resistance analysis, the general criterion equation for laminar developed segment was derived to judge which cooling method possesses a better heat transfer performance. Overall, this comparative study provides a comparison method and criterion about liquid metal cooling and water cooling, which will attribute to the coolant selection and heat sink design.

Due to the rapid growth of the power density in data centers, the thermal management of the data center has become more and more important. Computational Fluid Dynamics has been proven to be one of the most effective tools in the design and analysis in data centers due to its ease of use and fast prediction in data centers. Due to the large size of a data center facility, one of the most challenging tasks in the data center CFD simulation is to establish simplified models to provide a fast but accurate prediction for both the temperature and flow field in data centers. In the past few years, people have proposed many tile models specifically to simulate the jet effect of the perforated tile. However, other aspects of the simulation were still in the early stage, e.g. rack models and the effect of buoyancy forces. This paper mainly studied the effect of the three rack models, which were Black Box Model, Modified Black Box Model, and Volumetric Heat Model. The effects of the buoyancy force were also studied. The result showed that buoyancy force had a huge impact on the simulation result, while the three rack models investigated had little difference.

With the recent developments in miniaturization techniques of electronic chips, the power density of these chips has risen drastically. Available thermal management technologies like air cooled heat sink and liquid cooled heat sink are unable to keep up with the demand. However, thermal management technologies using flow boiling in microchannel heat sink can dissipate higher heat fluxes. Flow boiling technologies in micro channel heat sinks are not commercially established due to issues such as reliability, flow reversal, dry out, critical heat flux, limited knowledge of bubble dynamics, correlations, etc. In this study, performance of flow boiling in a diverging microchannel with uniform heat flux condition has been investigated. Simulations have been performed on ANSYS Fluent using Volume of Fluid (VOF). VOF is used to track the interface between different phases. The impact of angle on the bubble dynamics of the coolant and flow patterns has been studied. The simulated numerical results are compiled and presented. The results provide a clear understanding of the impact of angle on the bubble dynamics in flow boiling microchannel heat sink.

The understanding of the liquid-gas flow and heat transfer in the high-aspect-ratio microchannel is very important to realize the high-efficiency phase change chip cooling. In this work, a novel ultra-high-aspect-ratio microchannel with capillary-structured wall was developed to enhance the evaporation heat transfer in microchannel, in which the capillary grooves on the side walls (capillary-structured wall) were designed to avoid the dryout phenomenon. A three-dimensional VOF model was established to predict the immiscible gas-liquid flow in microchannel. The influences of wettability of capillary grooves on the gas-liquid two-phase flow behavior in microchannel were investigated based on the numerical predictions. The slug bubble can be observed for different inlet flow conditions. Variation of pressure loss between inlet and outlet of microchannel with time were studied for different flow rates and gas-liquid ratios. The results show that the existence of capillary structured wall has a significant influence on the liquid-gas two-phase flow behavior in the microchannel. The liquid flow in microgrooves is driven by the capillary force, which can supply more liquid to the side wall to promote the evaporation heat transfer process. The design of capillary-structured wall for ultra-high-aspect-ratio microchannel in this work provides a new approach to improve the performance of the chip cooling technique with microchannels.

In this paper, the performance of a new low-GWP refrigerant R513a was experimentally investigated, during spray cooling. A spray cooling system was designed to work as a sub-system within a closed-loop refrigeration system. The influence of chamber pressure on heat flux and heat transfer coefficient were experimentally investigated. A smooth plain copper surface heated by a cartridge heater was cooled by the refrigerant (R513a) while flowing through a nozzle in the spray chamber. The results showed that chamber pressure has a significant impact on the overall thermal performance of the spray cooling operation. It was also determined that higher chamber pressures resulted in higher thermal performance. The highest chamber pressure attained in this study was 0.6 MPa. Furthermore, the surface temperature of the heated surface increased due to the increase of the saturation temperature of the liquid over the surface.

Data centers in recent years have grown so fast so that their energy consumptions become a big issue in the industrial sector. One of the strategies to make better use of energy in data centers is to improve the efficiency in cooling. As the load density in data centers increases dramatically over the years, the number of computer room air handlers (CRAHs) are also increased to accommodate the high cooling demands. However, the number of CRAH units and their layouts really affect the air flow through the perforated tiles. Non-uniform airflow distributions in the perorated tiles in the cold aisles cause inefficient cooling of all the servers mounted in racks in data centers. Application of necessary strategies to minimize airflow non-uniformity is therefore very important because of its direct impact on the power density capacity. In this paper, a simulation study to examine how computer room air handler (CRAH) positions, the number of operating units, and tile types affect the airflow uniformity in selected data center models. Also, the placement of mixed tiles in cold aisles to regulate the airflow through the perforated tiles to accommodate greater heat loads from server racks is evaluated.

This work experimentally studied the convective heat transfer characteristics of a novel nanostructured heat transfer fluid: “Ethanol/Polyalphaolefin(PAO) nanoemulsion fluids” flowing through a heat exchanger made of twelve circular minichannels. Ethanol/PAO nanoemulsion fluid is a thermodynamically stable system formed by dispersing ethanol into a mixture of PAO and surfactants, in which the ethanol added inside forms self-assembled nanodroplets of tens of nanometers in diameter. These ethanol nanodroplets can serve as pre-seed boiling nuclei at elevated temperature. The Reynolds number was varied between 140 and 1200 to maintain the entire range of flow regime remained at laminar flow for both single- and two-phase convective heat transfer experiments. Pure PAO was also tested under same conditions and used as baseline data for comparison. It is found that: for single phase flow, there is no significant increase in Nusselt number of Ethanol/PAO nanoemulsion compared to that of PAO fluid in laminar flow regime. However, when the nucleation of ethanol nanodroplets inside the nanoemulsion fluid was initiated, it showed a substantial increase in heat transfer coefficient compared to that of PAO fluid: a 75% enhancement can be achieved under current test conditions. While its mechanism is not completely clear yet, it is believed that such an effect is likely related to the latent heat carried by ethanol bubbles, as well as the increased turbulence and mixing generated during the two-phase flow of nanoemulsion which can increase the heat transfer rate.

This study uses CFD to consider the effects of obstructions (bosses) on the fluid flow and heat transfer in finned heat sinks used for cooling electronic components. In particular, the effect of bosses, used for mounting components, on the fluid flow distribution and temperature distribution in the heat sink are evaluated. A typical heat sink has fins sandwiched between top and bottom plates, with electronic components mounted on the plates. The top and bottom plates spread the heat generated in the components to reduce the local heat flux. The fins substantially increase the heat transfer area, reducing the temperature rise from the coolant to the top and bottom plates. In this case a uniform distribution of flow across the heat sink can be achieved and there will be no localized hot spots. Ideally there are no protrusions into the finned portion of the heat sink which would cause disruptions in the uniform flow through the heat sink. However, a boss may be needed to bolt a component to the heat sink. The presence of the boss has three effects on the heat sink performance. The boss disrupts the flow in its immediate vicinity, increasing the thermal resistance. This will cause an increase in operating temperature at that location. In addition, the boss will change the flow distribution in the heat sink. Locations upstream and downstream of the boss may see reduced flow due to the obstruction, which in turn will cause an increase in operating temperature for these areas of the heat sink. Finally, the change in flow distribution may increase the pressure drop through the entire heat sink, increasing the power required to operate the system. The purpose of this study is to numerically evaluate the clearance requirements around circular bosses. Comparisons between an unobstructed heat sink and a heat sink with an obstruction are made for the maximum component temperature rise, the pressure drop and the flow distribution. Clearance ratios, diameter of the fin cut out to boss diameter, were varied from 1.1 to 3.3. The Reynolds number for the flow was varied from roughly 3000 to 70,000 based on the hydraulic diameter of flow passage.

With increasing heat fluxes in microelectronics, thermal management of these devices will soon no longer be attainable through current methods. One thermal management technology that could be integrated into the design of microelectronics is jet impingement cooling. Past research has primarily focused on evenly spaced, equal-sized, circular or slot jets perpendicular to the surface. A significant problem associated with this technology, especially as the surface to be cooled increases in size, is crossflow. This is the interaction of the transverse flow from the spent inner jet fluid with the jets closer to the outer edge of the surface. In an attempt to attenuate the crossflow effects, the heat transfer performance of jet arrays with non-uniform jet diameter and jet spacing were investigated. The testing apparatus housed a 3D-printed jet array nozzle that could be easily exchanged to accommodate many tests. The use of advanced manufacturing techniques allows for array geometries that may have previously been difficult to create. The impingement surface was a circular, polished, oxygen-free copper surface with a diameter of 25.4 mm. Heat transfer rates nearing 400 W could be delivered to the surface, for a heat flux of more than 75 W/cm 2 . The working fluid was single phase water, and the heat transfer rate was measured for each jet array over a range of flow rates. Experimental data was compared to simulation data obtained through CFD analysis. CFD modeling was used to predict the most promising geometries, which were then validated through experiment. Out of the nozzles tested, it was determined that the nozzle with larger diameters toward the edge of the surface attained the highest heat transfer rate of h = 38,822 W/m 2 -K. The nozzle with closer jet spacing at the outside of the array was found to have the highest experimental Nusselt number with Nu D = 88.8. It was determined that angled confining walls do not have a definitive association with improved heat transfer. The simulation data was found to predict the heat transfer performance of the various geometries with an average percent difference in heat transfer coefficient of 11%.

Constructal principles are used to investigate the optimization of material utilization in a metal matrix heat sink that maximizes the total heat transfer rate through the base of heat sink. This approach utilizes a two-dimensional geometry to examine spatial heat flow and optimal material distribution in a metal matrix in the plane perpendicular to the coolant flow direction. The matrix is composed of multiple layers of conductive tees built up from the smallest constituent, the unit T-cell. The unit cell consists of a conductive tee-shaped geometry with the two rectangular void regions each making up half of a cooling channel. The horizontal boundaries must match the temperature and heat flux at the boundaries of the neighboring unit cells as this is a conjugate conduction/convection problem. The geometry of the unit cell is characterized by aspect ratios of channel width to height, overall cell width to height, and channel height to cell height. The matrix structure is assembled by stacking unit cells into multiple layers where the number of cells in each layer is an integer multiple of the cells contained in the lower layer. The solution is obtained for optimal T-cell geometric parameters under a set of predetermined constraints including overall volume, solid fill fraction, and number of layers. When a large number of stacked unit cells are considered, the results describe the ideal spatial distribution of porosity and pore sizes for two dimensional functionally graded metal-matrix heat sink. These results will lead to a better understanding of the role played by the porosity distribution in a metal-matrix heat sink and may be applied to the analysis, optimization, and design of more effective heat sinks.

This paper presents an experimental investigation to study the thermal and material characterization of an array of composite copper-carbon nanotubes (CNT) micropillars for applications in passive two-phase cooling systems. These novel micropillar structures have a larger spacing at the base of the micropillars to provide a higher liquid permeability and mushroom-like structures on the top surface of the micropillars with a smaller spacing to provide a greater capillary pressure. First, composite copper-CNT micropillars are fabricated by an electrodeposition method on a patterned copper template. Then, cauliflower-like nanostructures are grown on the top surface of the micropillars using chronoamperometry technique to improve the capillary pressure and thermal performance of the micropillars. Finally, a series of tests are conducted to quantify the thermal performance of the fabricated micropillars. The results indicate that the performance of mushroom-like composite copper-CNT micropillars is significantly higher than those of copper micropillar arrays.

3D thermoreflectance (TR) imaging utilizing light field camera is proposed and developed in this study. With the capability of resolving light direction and intensity simultaneously, we are able to measure directional TR signal from tilted or curved surfaces through the light field camera. Once the directional TR coefficients of the target surface is collected, we can then estimate the temperature distributions of tilted or curved surfaces based on the measured directional TR signal. This new 3D TR imaging approach has the potential to measure temperature of surfaces with different configurations as long as the TR signal is strong enough. In this technical paper, we first present basic principles of 3D TR imaging techniques based on light field theory. The construction and calibration procedures of the resulting 3D TR imaging system are then presented. At the end of the technical paper we present steady-state surface temperature measurements of microscale electronic devices including an infrared laser diode and a gold wire with the current 3D TR imaging system. The obtained temperature distributions of the microscale samples showing high temperature and spatial resolutions in real-time temperature measurement on the 3D surfaces that cannot be achieved with traditional 2D TR imaging techniques.

The exponential growth in electronic power has brought amazing technology but with it also comes a burden in high heat production that threatens the safety of the product since electronics’ failure rate increases by its operating temperature. As such, cooling techniques have a key role to keep the temperature of electronics devices, such as processors, memory and graphics chips, below a maximum operating temperature. In this work, a porous filled heat exchanger has been numerically modeled to investigate the cooling effectiveness and temperature distribution on the base of the heat exchanger subjected to high heat flux leaving these devices. The effects of different nanofluid coolants (0.75% double walled carbon nanotube in water (DWCNT), 1% alumina in water, and 1% diamond in 20:80 ethylene glycol/water), porous materials (copper and annealed pyrolytic graphite (APG)), and porosity values are investigated. The coolant enters from an inlet channel normal to the base, moves through the porous field, and then leaves the heat exchanger through two opposite exit channels parallel to the base. The study is performed for two and three dimensional geometries in which two different designs are studied for 3D cases. One of the designs has a rectangular cross sectional inlet channel (along transverse direction) and the other design has square one. The results indicate that utilizing APG porous matrix, for all studied coolants of pure water and water based nano fluidics, improves substantially the cooling of the base of the heat exchanger in 2D and 3D with rectangular inlet. The results also show that utilizing carbon nanofluids (DWCNT) as coolant for high porosity structures, in both copper and APG porous matrices, improves cooling efficiency and temperature uniformity over the base, for all 2D and 3D cases. The effect of inlet channel geometry, square and rectangular, is also investigated for either similar velocity or similar mass flow rate at the inlet channel entrances.

This paper presents an experimental and numerical investigation of the thermal behavior of thin phase change material (PCM) packages as a solution to thermal management in portable electronic devices. The thin packages are made of encapsulated PCM in aluminized laminated film. The experimental setup is designed to include the most fundamental aspects of a portable electronic system while also being simple enough to be easily simulated using the finite element method; it is rectangular in nature. Two different types of PCM are used for the experimental work; the commercially available PT-37 and n -eicosane. It was determined that the use of a thin PCM thermal energy storage package significantly improved the temperature behavior of the experimental setup, by reducing the rate of temperature increase at the heater and the back cover. The time needed to reach a critical cover temperature of 45°C was increased by 12 to 18% while the time for the heater temperature to reach 70°C was increased by up to 66%. Numerical simulations of the system were in good agreement with the experimental data.

AlGaN/GaN based high electron mobility transistors (HEMTs) have been intensively used due to their high-efficiency power switching and large current handling capabilities. However, the high power densities and localized heating in these devices form small, high temperature regions called hotspots. Analysis of heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HEMT devices. For accurate analysis of heat transfer using thermal simulations in such devices with heat transfer occurring at different length scales, a roadmap is needed. For this purpose relative importance of different heat transfer modes in removing heat from devices with different substrate materials, operating at different power densities while different boundary conditions are analyzed using two and three-dimensional COMSOL Multiphysics simulations. Results give the relative importance of different parameters on the heat removal mechanism from devices and provide a roadmap for building simpler yet still accurate thermal models for AlGaN/GaN HEMTs and similar devices.

Lasers are widely used as high-accuracy tools for material processing. Different types of lasers such as CO 2 , Nd:YAG, and excimer lasers are used in different operating modes such as continuous wave, pulsed or Q-switched. Volumes of materials and their composition, structure, and properties can be controlled or modified by varying laser pulses. In this research, by using laser as a material processing tool, an experimental method was developed for laser induced implantation doping of glass substrates with conductive metals. Experiments were performed on glass samples using Q-switched Nd:YAG lasers. Gold, silver, and copper were used as conductive dopant materials. Initial experiments were performed using nickel as a catalyst. Effect of the catalyst on the composition of implanted dopant material was observed using Large Area Rapid Imaging Analytical Tool (LARIAT). Through further experiment, the effect of several parameters such as beam fluence, scanning speed, pulse repetition frequency, wavelength, substrate temperature, dopant material, and glass substrate material on the morphology of heat affected zones were investigated by optical microscopy (OM). Depth of penetration in doped glass samples was measured for different substrate temperatures by means of a laser displacement sensor. The effect of beam fluence and glass substrate thickness on depth of penetration was investigated. The results of these non-destructive measurements were verified using scanning electron microscopy (SEM). Based on optical observations, morphological characteristics of the heat affected zone were assessed in order to obtain the best parameter settings in different experiments. These settings were defined by factors such as the number and size of cracks in glass substrates, and the quality of the distribution of dopant metal over the scanned pattern. While using a catalyst with substrates at room temperature, the best parameter settings were obtained at wavelength of 532 nm, pulse repetition frequency of 6 kHz, beam fluence of 0.36 J/cm 2 , and scanning speed of 0.10 m/s. By removing the catalyst, these settings were changed to 355 nm, 10 kHz, 0.09 J/cm 2 , and 0.01 m/s for gold sputtered soda-lime glass substrate at 500 °C. For beam fluence values ranging from 0.06 J/cm 2 to 0.38 J/cm 2 , the obtained values for average depth of penetration were 255 μm and 187 μm in 1 mm and 3 mm thick soda-lime glass substrates respectively. Further development of this implantation method could lead to implantation of electronic circuits in transparent substrates, inspiring the evolution of transparent electronic devices such as transparent smart phones, smart windows and displays, and lighting products in the future.

This paper presents a study of thermal management of tablet computers (tablet PCs) using phase change materials (PCMs) encapsulated in aluminized laminated film under continuous operation. The experimental setup consists of original tablet PC parts and a simplified dummy printed circuit board (PCB) with a thermal response similar to the original PCB. Two PCMs were used in the experiments, n-eicosane and PT-37 (a commercial PCM from PureTemp). These PCMs have similar melting temperatures (n-eicosane – 35.6 °C; PT-37 – 36.3 °C) but different latent heats of fusion (n-eicosane – 236 kJ/kg; PT-37 – 206 kJ/kg). Two encapsulations with different sizes (6″ × 2.6″, 7″ × 1.5″) but the same thickness (0.0792″ (2 mm)) were used in this study. The effects of inclination and power input level on the thermal behavior of the tablet were investigated. Experiments showed that PCM encapsulated in laminate film led to lower back cover temperature for constant heat flux applications. As much as a 20 °C temperature reduction of the back cover hotspot was achieved with encapsulated PCM. It was also observed that better thermal behavior was achieved both by the melting of PCMs and heat spreading through the laminate film. It was found that the rate of PCM melting is directly related to the power input. No significant effect on PCM melting and temperature history was observed in relation to the system inclination.