High Heat Flux Boiling Heat Transfer for Laser Diode Arrays

The principle limit for achieving higher brightness of laser diode arrays is thermal management. State of the art laser diodes generate heat at fluxes in excess of 1 kW cm−2 on a plane parallel to the light emitting edge. As the laser diode bars are packed closer together, it becomes increasingly difficult to remove large amounts of heat in the diminishing space between neighboring diode bars. Thermal management of these diode arrays using conduction and natural convection is practically impossible, and, therefore, some form of forced convective cooling must be utilized. Cooling large arrays of laser diodes using single-phase convection heat transfer has been investigated for more than two decades by multiple investigators. Unfortunately, either large fluid temperature increases or very high flow velocities must be utilized to reject heat to a single phase fluid, and the practical threshold for single phase convective cooling of laser diodes appears to have been reached. In contrast, liquid-vapor phase change heat transport can occur with a negligible increase in temperature and, due to a high enthalpy of vaporization, at comparatively low mass flow rates. However, there have been no prior investigations at the conditions required for high brightness edge emitting laser diode arrays: >1 kW cm−2 and >10 kW cm−3. In the current investigation, flow boiling heat transfer at heat fluxes up to 1.1 kW cm−2 was studied in a microchannel heat sink with plurality of very small channels (45 × 200 microns) using R134a as the phase change fluid. The high aspect ratio channels (4.4:1) were manufactured using MEMS fabrication techniques, which yielded a large heat transfer surface area to volume ratio in the vicinity of the laser diode. To characterize the heat transfer performance, a test facility was constructed that enabled testing over a range of fluid saturation temperatures (15°C to 25°C). Due to the very small geometric features, significant heat spreading was observed, necessitating numerical methods to determine the average heat transfer coefficient from test data. This technique is crucial to accurately calculate the heat transfer coefficients for the current investigation, and it is shown that the analytical approach used by many previous investigations requires assumptions that are inadequate for the very small dimensions and heat fluxes observed in the present study. During the tests, the calculated outlet vapor quality exceeded 0.6 and the base heat flux reached a maximum of 1.1 kW cm−2. The resulting experimental heat transfer coefficients are found to be as large a 58.1 kW m−2 K−1 with an average uncertainty of ±11.1%, which includes uncertainty from all measured and calculated values, required assumptions, and geometric discretization error from meshing.

On the Impact of Liquid Drops on Immiscible Liquids

The release of liquid hydrocarbons into the water is one of the environmental issues that have attracted more attention after deepwater horizon oil spill in Gulf of Mexico. The understanding of the interaction between liquid droplets impacting on an immiscible fluid is important for cleaning up oil spills as well as the demulsification process. Here we study the impact of low-viscosity liquid drops on high-viscosity liquid pools, e.g. water and ethanol droplets on a silicone oil 10cSt bath. We use an ultrafast camera and image processing to provide a detailed description of the impact phenomenon. Our observations suggest that viscosity and density ratio of the two media play a major role in the post-impact behavior. When the droplet density is larger than that of the pool, additional cavity is generated inside the pool. However, if the density of the droplet is lower than the pool, droplet momentary penetration may be facilitated by high impact velocities. In crown splash regime, the pool properties as well as drop properties play an important role. In addition, the appearance of the central jet is highly affected by the properties of the impacting droplet. In general, the size of generated daughter droplets as well as the thickness of the jet is reduced compared to the impact of droplets with the pool of an identical fluid.

Hydrodynamic and Thermal Characteristics of the Flow Inside a Rectangular Microchannel With Different Cylindrical Micro Pin Fins

This article presents a computational study to investigate the hydrodynamic and thermal characteristics of the flow inside a rectangular microchannel with the dimensions of 5000 × 1500 × 100 μm3 (l × w × h’) with different inline arrangements of cylindrical micro pin fins. A parametric study is performed on the effect of different geometrical specifications of micro pin fins on the wake-pin fin interaction. Three values of (50, 100 and 200 μm) are considered for the pin fin diameters (D) while the overall height (H) of the system is set to be constant (100 μm). For the first two cases, two longitudinal and vertical pitch ratios (SL/D and ST/D) of 1.5 and 3 are considered while for H/D ratio of 0.5, only ST/D ratio of 1.5 and SL/D ratios of 1.5 and 3 are considered. As a result, a total number of ten different geometries are analyzed in five different Reynolds numbers of 20, 40, 80, 120 and 160. A constant heat flux is applied through the bottom surface of the microchannel as well as the micro pin fins surfaces. All other surfaces are assumed to be thermally isolated. Thermodynamic properties of water are set to vary with temperature and it is assumed that the working flow remains in the liquid form in all operating conditions. ANSYS commercial package v14.5 with an academic license is utilized to generate the 3D models, applying the appropriate grid networks and simulating the flow fields for each configuration. Results show major dependencies of pressure drops, friction factors, Nusselt numbers and Thermal Performance Index values on ST/D ratio and Reynolds number while minor dependencies of these parameters on SL/D and H/D ratios are observed.

Comparative Studies on Water Self-Diffusivity Confined in Graphene Nanogap: Molecular Dynamics Simulation

Fundamental understanding of the water in graphene is crucial to optimally design and operate the sustainable energy, water desalination, and bio-medical systems. A numerous atomic-scale studies have been reported, primarily articulating the surface interactions (interatomic potentials) between the water and graphene. However, a systematic comparative study among the various interatomic potentials is rare, especially for the water transport confined in the graphene nanostructure. In this study, the effects of different interatomic potentials and gap sizes on water self-diffusivity are investigated using the molecular dynamics simulation at T = 300 K. The water is confined in the rigid graphene nanogap with the various gap sizes Lz = 0.7 to 4.17 nm, using SPC/E and TIP3P water models. The water self-diffusivity is calculated using the mean squared displacement approach. It is found that the water self-diffusivity in the confined region is lower than that of the bulk water, and it decreases as the gap size decreases and the surface energy increases. Also, the water self-diffusivity nearly linearly decreases with the increasing surface energy to reach the bulk water self-diffusivity at zero surface energy. The obtained results provide a roadmap to fundamentally understand the water transport properties in the graphene geometries and surface interactions.

Effect of Open Micro-Channels on External Condensation Heat Transfer

This research experimentally investigates the heat transfer performance of open-micro channels under filmwise condensation conditions. Filmwise condensation is an important factor in the design of steam condensers used in thermoelectric power generation, desalination, and other industrial applications. Filmwise condensation averages five times lower heat transfer coefficients than those present in dropwise condensation, and filmwise condensation is the dominant condensation regime in the steam condensers due to a lack of a durable dropwise condensation surface. Film thickness is also of concern because it is directly proportional to the condenser’s overall thermal resistance. This research focuses on optimizing the channel size to inhibit the creation of a water film and/or to reduce its overall thickness in order to maximize the heat transfer coefficient of the surface. Condensation heat transfer was measured in three square channels and a plane surface as a control. The sizes of the square fins were 0.25 mm; 0.5 mm; and 1 mm, and tests were done at a constant pressure of 6.2 kPa. At lower heat fluxes, the 0.25mm fins perform better, whereas at larger heat fluxes a smooth surface offers better performance. At lower heat fluxes, droplets are swept away by gravity before the channels are flooded. Whereas, at higher heat fluxes, the channels are flooded increasing the total film thickness, thereby reducing the heat transfer coefficient.

On the Heat Transfer Characteristics of a Single Bubble Growth and Departure During Pool Boiling

In the present study, bubble growth and departure characteristics during saturated pool boiling were investigated numerically, and a comprehensive model was proposed and developed to study the heat transfer during growth and departure of a bubble as well as bubble growth rate and departure time. Two-phase characteristics of the boiling phenomena can be captured by well-known Volume of Fluid (VOF) method. However, the VOF method is susceptible to parasitic currents because of approximate interface curvature estimations. Thus, sharp surface formula (SSF) method was employed to effectively eliminate the presence of the parasitic currents. VOF method is a volume capturing method and hence, may be subject to interface diffusion, due to the fact that interface is smeared through some number of computational cells. Interface compression scheme is applied to prevent the plausible interface diffusion of the VOF method. To avoid unrealistic temperature profiles at the solid-liquid surface, a conjugate heat transfer model was used to calculate the heat flux going into the liquid region from the heater through the solution of conduction equation in solids. Phase change at the interface was incorporated based on Hardt and Wondra’s model in which source terms are derived from a physical relationship for the evaporation mass flux. Furthermore, effects of micro region heat transfer on the departure time of the bubble was investigated. Micro region heat transfer was included in the model by solving a temporal evolution equation and incorporating the resulting heat flux in the tri-phase contact line. In this study, OpenFOAM package was used to investigate the characteristics of the bubble growth and departure as well as the wall heat flux. The model was benchmarked by comparing the simulation results to available experimental and numerical literatures, as well as analytical solutions.

Nonlinear Study of Convective Heat Transfer in Tightly Curved Rectangular Microchannels

To address the effects of curvature, initial conditions and disturbances, a numerical study is made on the fully-developed bifurcation structure and stability of the forced convection in tightly curved rectangular microchannels of aspect ratio 10 and curvature ratio 0.5 at Prandtl number 7.0. Eleven solution branches (seven symmetric and four asymmetric) are found with 10 bifurcation points and 27 limit points. The flows on these branches are with 2, 4, 6, 7, 8, 9 or 10-cell structures. The flow structures change along the branch because of the flow instability. The average friction factor and Nusselt Number are different on different solution branches. It is found that more than 22.33% increase in Nu can be achieved with less than 9.34% increase in fRe at Dk of 2000. As Dean number increases, finite random disturbances lead the flows from a stable steady state to another stable steady state, a periodic oscillation, an intermittent oscillation, another periodic oscillation and a chaotic oscillation. The mean friction factor and mean Nusselt Number are obtained for all physically realizable flows. A significant enhancement of heat transfer can be obtained at the expense of a slightly increase of flow friction in tightly coiled rectangular ducts.

Design of an Experimental Setup to Investigate an Oscillating and Evaporating Meniscus Using a Feedback Control Loop

Nucleate boiling is one of the most efficient methods to dissipate heat. However, the complex physics of heat transfer near the contact line is not well understood. Due to the difficulty in measuring and analyzing heat transfer around a bubble at high heat fluxes, novel approaches must be taken. This paper focuses on the design of an experimental setup used to simulate heat transfer at the contact line by studying an oscillating meniscus on a heated surface. A preliminary design of the experimental test setup is described in this paper. The experimental test setup will be composed of a liquid injection system with a needle, an oscillator, a heated surface, and a sensor to measure the meniscus volume. A feedback loop will be used to control the liquid injection system and prevent dry out or flooding during evaporation. Furthermore, a conic speaker will be used to induce oscillations at a range of 10–200 Hz. These oscillations simulate liquid displacement during bubble nucleation, growth, and bubble departure. Finally, a sensor that measures the volume of the liquid will be connected to the heated plate and the needle in order to measure the volume of the meniscus while oscillating. A fundamental understanding of the heat transfer in the contact line region is expected.

Thermal Performance Analysis of Hybrid Jet Impingement/Microchannel Cooling for Concentrated Photovoltaic (CPV) Cells

The increase in the CPV temperature significantly reduces the efficiency of CPV system. To maintain the CPV temperature under a permissible limit and to utilize the unused heat from the CPVs, an efficient cooling and transportation of coolant is necessary in the system. The present study proposes a new design of hybrid jet impingements/microchannels heat sink with pillars for cooling densely packed PV cells under high concentration. A three-dimensional numerical model was constructed to investigate the thermal performance under steady state, incompressible and laminar flow. A constant heat flux was applied at the base of the substrate to imitate heated CPV surface. The effect of two dimensionless variables, i.e., ratios of standoff (distance from the nozzle exit to impingement surface) to jet diameter and jet pitch to jet diameter was investigated at several flow conditions. The performance of hybrid heat sink was investigated in terms of heat transfer coefficient, pressure-drop, overall thermal resistance and pumping power. The characteristic relationship between the overall thermal resistance and the pumping power was presented which showed an optimum design corresponding to S/Dj = 12 having lower overall thermal resistance and lower pumping power.

Three-Dimensional VOF Simulations of Laminar Fluid Flows in Micro-Pipes Containing Superhydrophobic Walls With Micro-Posts and Micro-Ridges

This paper investigates water flowfield characteristics inside micro-pipes containing superhydrophobic walls under laminar flow conditions. It also investigates the effects of solid fraction, wall pattern, and Reynolds number on both skin friction drag and flow field characteristics. A transient, incompressible, three-dimensional, volume-of-fluid (VOF) methodology has been employed to continuously track the air–water interface and to visualize the dynamic behavior of the complex flows inside micro-pipes containing different superhydrophobic wall features (square micro-posts and longitudinal micro-ridges). The results of the present simulations show that micro-pipes containing superhydrophobic walls with longitudinal micro-ridges features have a better frictional performance than those having square posts features. The predicted results also show that the frictional performance of micro-pipes is a monotonically decreasing function of Reynolds number for both patterns examined in the present study. In addition, as the solid fraction decreases, the flow enhancement of superhydrophobic micro-pipes increases and it seems, based on the studied cases, to reach an asymptotic value. However, a further study is needed to confirm this latter issue.