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.

Optoelectrokinetic effects were effectively used for rapid concentration of particles in microfluidics. In this study, we clarified detail mechanism of particle aggregation by numerical simulation using COMSOL v4.2a multiphysics software. A 3D simulation was conducted with axisymmetric boundary conditions. AC voltage was applied to the two parallel electrodes in a microchannel to generate temperature gradient in the fluids. In addition to the AC electrothermal (ACET) effect, local heating by a laser illumination was also considered. Numerical simulations were carried out for dielectric fluids. A toroidal microvortex induced by the optoelectrokinetic effect shows that fluid motions in the middle of bottom boundary are cancelled out by flows in opposite directions and consequently producing stagnation. It is expected that micro/nano particles can be deposited in the bottom electrode. Local heating by the laser illumination enhanced the intensity of microvortex substantially. It is confirmed that the dominant driving force for the microvortex is natural convection by the laser illumination, however AC voltage is necessary for particle aggregation in the spot area.

The present paper is concerned with the study of flow and heat transfer characteristics in the steady state free convective flow of Al 2 O 3 -waternanofluids in a square enclosure in the presence of magnetic field. Attention is given to the temperature variation of the electrical conductivity and its effect on the electromagnetic force induced by the motion of the nanofluid. A new experimental correlation recently presented in the literature was used for this aim. In all the earlier studies in this area the electrical conductivity variation of nanofluid with temperature was neglected. The fluid viscosity and thermal conductivity are assumed to vary as a function of temperature and this variation is modeled using the available experimental correlations. The governing differential equations are solved numerically using finite element method. The features of fluid flow and heat transfer characteristics are analyzed for various strengths of the magnetic field and different nanoparticle volume fractions. The results show that when the inclusion of the variation of the electrical conductivity with temperature in the numerical model noticeably affects the natural convection heat transfer in the studied rectangular cavity. The variations of Nusselt number for natural convection of Al 2 O 3 -water nanofluid with nanoparticle volume fractions are presented at various Rayleigh and Hartmann numbers.

A theoretical answer to the controversial issue on the anomalous convective heat transfer in nanofluids has been provided, exploiting the Buongiorno model for convective heat transfer in nanofluids with modifications to fully account for the effects of nanoparticle volume fraction distributions on the continuity, momentum and energy equations. Firstly, a set of exact solutions have been obtained for hydrodynamically and thermally fully developed laminar nanofluid forced convection flows in channels and tubes, subject to constant heat flux. From the solutions, it has been concluded that the anomalous heat transfer rate, exceeding the rate expected from the increase in thermal conductivity, is possible in such cases as titania-water nanofluids in a channel, alumina-water nanofluids in a tube and also titania-water nanofluids in a tube. Moreover, the maximum Nusselt number based on the bulk mean nanofluid thermal conductivity is captured when the ratio of Brownian and thermophoretic diffusivities is around 0.5, which can be exploited for designing nanoparticles for high-energy carriers. Secondly, another set of exact solutions have been obtained for free convection in a vertical channel filled with a nanofluid, exploiting the Buongiorno model with nanoparticle volume fraction modifications. The effects of the bulk mean volume fraction of nanoparticles, the ratio of Brownian and thermophoretic diffusivities and the buoyancy ratio on both velocity and temperature profiles has been investigated in depth for the first time. The volume fraction of nanoparticles increases exponentially towards the cold wall, which makes the velocity and temperature gradients steeper near the hot wall than those near the cold wall. Unlike the case of forced convection, no anomalous heat transfer enhancement has been observed in this case of free convection, so that the Nussult number based on the thermal conductivity of nanofluid stays close to unity as in pure base fluid free convection.

The Constrained Vapor Bubble (CVB) is a prototype for a wickless heat pipe and was developed into an experiment that was run in the microgravity environment of the International Space Station during 2010. Since the CVB is transparent, we can visualize the flow processes within the device in a way not possible before. Results from the experiment indicate that the CVB operates at higher pressures and temperatures in microgravity, a consequence of radiation being the only mechanism for removing heat from the device. The temperature profile data along the heat pipe and corresponding heat transfer calculations indicate that CVB performance is enhanced in the microgravity environment due to increased capillary flow even though heat transfer to the external environment is diminished by the absence of natural convection. Image data of the liquid profile in the grooves of the heat pipe indicate that the curvature gradient is considerably different from that on Earth and supports the conclusion that capillary flow and internal heat transfer is increased. Operations with the 20 mm version of the device allowed us to view explosive nucleation within the CVB upon device start-up. In this scenario, bubble nucleation occurred spontaneously and periodically at the hot end of the device. The nucleation process sent a shock wave through the pipe that collapsed the original bubble as a new vapor space was generated. The newly formed bubble returned to its original size, shape and location as heat loss from the CVB reestablished the original, pseudo-steady-state temperature and pressure profiles.

The lack of rapid, affordable, and easy to use medical diagnostic technologies is a critical issue confronting global public health. A major challenge to these efforts lies in the design of instrumentation used to perform a key step in the analysis. This step, the polymerase chain reaction (PCR), involves a sequence of thermally activated biochemical processes that selectively replicate well-defined sub regions within a longer DNA strand. Although PCR is generally considered to be a mature technology from a biochemical standpoint, many limitations are still imposed by the highly inefficient design of conventional PCR thermocycling hardware that is slow, expensive, and consumes considerable electrical power to repeatedly heat and cool the reagent mixture. Here we describe an alternative thermocycling approach that has the potential to addresses these needs by harnessing thermally driven natural convection to perform rapid DNA amplification via the PCR. A buoyancy driven instability is induced within a confined volume of fluid by imposing a spatial temperature gradient. Under the right conditions (fluid properties, geometry, temperature gradient, etc.) a stable circulatory flow pattern can be established that will repeatedly transport PCR reagents through temperature zones associated with each stage of the reaction. The inherently simple design (similar in principle to a lava lamp) and minimal electrical power consumption make this approach well-suited for use in portable applications. We also describe our computational and experimental studies of the flow fields established within convective thermocycling reactors, revealing a rich complexity not found in most steady laminar flows. These complexities arise because, under the thermal conditions associated with PCR, the nature of the buoyancy driven instabilities that initiate and sustain motion make it necessary to operate in a transition regime associated with the onset of convective turbulence. These unique characteristics can be harnessed to guide the design of new devices capable of generating optimal conditions for ultra-rapid PCR replication.

In the present work, Molecular Dynamics technique is performed to study free convection in the square cavity. Simulated system is a 3-dimensional box containing fluid atoms. The walls of box are solid and formed in FCC. Fluid and solid atoms are chosen as argon and platinum respectively. The pair potential between fluid-fluid and fluid-wall atoms is governed by truncated Lennard-Jones (6–12) potential as these molecules have well-established molecular potential interaction. The periodic boundary condition (PBC) is applied in only one direction being perpendicular to the simulation surface. It helps ignore the effects of wall in this dimension. Half of the box is remained in a constant temperature (T1) and the temperature of opposite half is set to different value (T2). The velocity field is obtained and its dependency to the temperature ratio (T1/T2) is investigated. Also, we are eager to obtain the effect of gravity force on the velocity fields.

We consider the two-dimensional problem of steady natural convection in a narrow (micro) horizontal cylindrical annulus filled with porous medium due with linear volumetric heat generation. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but pade approximation leads our results to be good even for much higher value of the similarity parameter.

A unique way for maximizing turbulent free convection from heated vertical plates to cold gases is studied in this paper. The central idea is to examine the attributes that binary gas mixtures having helium as the principal gas and xenon, nitrogen, oxygen, carbon dioxide, methane, tetrafluoromethane and sulfur hexafluoride as secondary gases may bring forward. From fluid physics, it is known that the thermo-physical properties affecting free convection with binary gas mixtures are viscosity η mix , thermal conductivity λ mix , density ρ mix , and heat capacity at constant pressure. The quartet η mix , λ mix , ρ mix , and C p,mix is represented by triple-valued functions of the film temperature the pressure P , and the molar gas composition w . The viscosity is obtained from the Kinetic Theory of Gases conjoined with the Chapman-Enskog solution of the Boltzmann Transport Equation. The thermal conductivity is computed from the Kinetic Theory of Gases. The density is determined with a truncated virial equation of state. The heat capacity at constant pressure is calculated from Statistical Thermodynamics merged with the standard mixing rule. Using the similarity variable method, the descriptive Navier-Stokes and energy equations for turbulent Grashof numbers Gr x > 10 9 are transformed into a system of two nonlinear ordinary differential equations, which is solved by the shooting method and the efficient fourth-order Runge-Kutta-Fehlberg algorithm. The numerical temperature fields T ( x, y ) for the five binary gas mixtures He-Xe, He-N 2 , He-O 2 , He-CO 2 , He-CH 4 , He-CF 4 and He-SF 6 are channeled through the allied mean convection coefficient hmix/B varying with the molar gas composition w in proper w-domain [0, 1]. For the seven binary gas mixtures utilized, the allied mean convection coefficient h mix / B versus the molar gas composition w is graphed in congruous diagrams. At a low film temperature T f = 300 K, the global maximum allied mean convection coefficient h mix,max / B = 85 is furnished by the He-SF 6 gas mixture at an optimal molar gas composition wopt = 0.93. The global maximum allied mean convection coefficient h mix,max / B = 57 is supplied by pure methane gas SF 6 ( w = 1) at a high film temperature T f = 1000 K instead of the He-SF 6 gas mixture.

Developing natural convection gaseous flows in an open-ended parallel plate vertical microchannel with isothermal wall conditions are numerically investigated to analyze the rarefaction effects on heat transfer and flow characteristics in slip flow regime. The Navier-Stokes and energy equations are solve by a control volume technique subject to higher-order temperature jump and velocity slip conditions including thermal creep effects. The flow and thermal fields in the entrance and fully developed regions along with the axial variations of velocity slip, temperature jump, and heat transfer rates are examined in detail. It is found that rarefaction effects significantly influence the flow and thermal fields such that mass flow and heat transfer rates are increased considerably as compared to the continuum regime. Furthermore, thermal creep contribution to the velocity slip is found to be dominant close to the channel inlet and vanishes in the fully developed region, while velocity slip approaches a finite value there. Both Mass flow rate and thermal entrance length increase with increasing Knudsen number in slip flow regime.

We consider the two-dimensional problem of steady natural convection in a narrow horizontal cylindrical annulus filled with porous medium due to a constant temperature variation on the outer and adiabatic conditions at the inner boundaries with constant volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but Pade approximation leads our results to be good even for much higher value of the similarity parameter.

We have developed a model capable of predicting the performance characteristics of a wiretype Joule-Thomson microcooler intended for use within a cryosurgical probe. Our objective was to be able to predict evaporator temperature, temperature distribution and cooling power using only inlet gas properties as input variables. To achieve this, the model incorporated changing gas properties due to heat transfer within the heat exchanger and isenthalpic expansion within the capillary. In consideration of inefficiencies, heat in-leak from free convection and radiation was also considered and the use of a 2D axisymmetric finite difference code allowed simulation of axial conduction. Two types of microcoolers differing in inner tube material, poly-ether-ether-ketone (PEEK) and stainless steel, were tested and simulated. CO 2 was used as the coolant gas in the calculations and experimental trials for inlet pressures from 0.5 MPa to 2.0 MPa. Heat load trials of up to 550 mW along with unloaded trials were conducted. Comparisons to experiments show that the model was successfully able to obtain a good degree of accuracy. For the all PEEK microcooler in a vacuum using 2.0 MPa inlet pressure, the calculations predicted a temperature drop of 57 K and mass flow rate of 19.5 mg/s compared to measured values of 63 K and 19.4 mg/s therefore showing that conventional macroscale correlations can hold well for turbulent microscale flow and heat transfer as long as the validity of the assumptions is verified.

A numerical study on flow and heat transfer of de-ionized water over in-line and staggered micro-cylinder-groups had been performed with Reynolds number varying in the range from 0 to 150. A 3-D incompressible numerical model was employed to investigate the vortex distributions and the influences of the vortexes on the flow and heat transfer characteristics at low Re numbers in micro-cylinder-groups with different geometrical parameters, including micro-cylinder diameters (100μm, 250μm and 500μm), ratios of pitch to micro-cylinder diameter (1.5 2 and 2.5) and ratios of micro-cylinder height to diameter (0.5, 1, 1.5 and 2), etc. The vortex distributions, the flow and temperature fields, and the relationships among them were investigated by solving the numerical model with the finite volume method. It was found that the vortex number became larger with the increase of pitch ratio, and the change of flow rate distribution affected the heat transfer characteristics apparently. The appearance of vortexes in micro-cylinder-group increased the differential pressure resistance; as a result the total flow resistance in micro-cylinder-groups correspondingly increased. Meanwhile, the local heat transfer coefficients nearby the locations of vortexes greatly increased due to the boundary layer separation, which further enhanced the heat transfer in micro-cylinder-groups. The new correlations which could predict Nusselt number of de-ionized water over micro-cylinders with Re number varying from 0–150 had been proposed considering the differential pressure resistance and the natural convection based on numerical calculations in this paper.

We consider the two-dimensional problem of steady natural convection in a narrow (Micro size) Horizontal Cylindrical annulus filled with viscous fluid and periodic volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the Gap ratio to the power of four, and Rayleigh number and the series extended by means of symbolic calculation up to 16 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is almost zero but Pade approximation lead our result to be good even for much higher value of the similarity parameter.

The heat transfer enhancement of natural convection, using electrohydrodynamic technique inside a horizontal enclosure heated from below, is studied numerically. The interactions between electric field, flow field, and temperature field are investigated by CFD methods. The flow and temperature fields are affected by voltage applied at the wire electrodes. For different voltages and number of electrodes, it is noticed that the Nusselt number increased in all cases and the best enhancement is obtained at lower Rayleigh numbers. It is also shown that increasing the number of electrodes doesn’t always cause an increase in the heat transfer enhancement. Actually, when the number of electrodes is equal to the number of Be´nard cells, the best heat transfer enhancement is obtained.

VOCs natural convective flow driven by thermo, solute and humidity buoyancy forces from a porous wall to indoor room was studied numerically in this paper, including coupled diffusion effects by three gradients interactions. The physical model for the fluid flow made use of Brinkman-Forchheimer extended Darcy equation in the porous wall and Navier-Stokes equation in the clear region. Finite element method was used to solve governing equations. Effect of interface at the interface on flow, heat and mass transfer was studied, which varied with stress jump coefficient β and permeability. The results showed that flow velocity varied greatly with increase of coefficient β in the boundary. Decrease in the value of dimensionless permeability Da also had influence on velocity variation at the interface. However, there are little influence of interface on the distribution of temperature and concentration in the whole region.

We consider the two-dimensional problem of steady natural convection in a narrow (Micro size) horizontal cylindrical annulus filled with viscous fluid and constant volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of four, and Grashof number and the series extended by means of symbolic calculation up to 14 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is almost zero but Pade approximation lead our result to be good even for much higher value of the similarity parameter.

This paper presents the numerical study of internal free convection in a vertical annulus. To predict the characteristics of nanofluids in heat transfer regime, thermal conductivity, density, and viscosity of clear fluid need to be modified. Numerous models have been disclosed to calculate these properties but recent models for viscosity and thermal conductivity which are in excellent agreement with experiments have been used. These models are thermal conductivity model of Jang and Choi and viscosity correlation of Nguyen et al. for 36nm Al 2 O 3 particles. Inner and outer vertical walls are in constant temperature while horizontal walls are adiabatic. The continuity, Navier-Stokes and energy equation were solved numerically. Effect of nanofluids on buoyancy-driven heat transfer is investigated as a function of geometrical and physical parameters and various particle concentrations for aspect ratio of 0.2< H/L <5, Grashof number of 10 3 < Gr <10 5 and concentration of 0<φ<0.12. Finally, one correlation is developed to demonstrate the effect of using atom-size particles on Nusselt number.

Fluid flow in micro/nano-scales channels has received much research interest due to its seemingly endless applications. By entering technology to the area of micro and nano scales, the design and fabrication of miniaturized instruments, such as microelectronics devices, MEMS, NEMS and ..., become very desirable. In such small sizes, the performance of the devices can be affected by some undesired small scale world phenomena. One of them is the generated heat in miniaturized devices that has suicidal effect on the performance of the devices and must be removed. The traditional cooling methods, such as free convection of air, are not enough efficient for cooling of these devices, therefore, other effective methods must be considered. Using nano- and micro-channels as heat sinks is one of the most effective methods for cooling the miniaturized devices. Increasing cooling performance and decreasing pressure drop along the channels are two important factors in designing nanochannels as a heat sink. Optimized design of cooling systems has significant effect on improving cooling performance and energy management of the devices. In this study, using a Genetic Algorithm technique, it is desired to find the optimized design for the cooling performance of transitional nano-channel heat sink.

In the present paper, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in four kinds of multi-port extruded aluminum tubes of about 1 mm in hydraulic diameter. Two tubes are composed of plane rectangular channels, while remaining two tubes are composed of rectangular channels with straight micro-fins. The experimental data of frictional pressure drop (FPD) and heat transfer coefficient (HTC) in plane tubes are compared with previous correlations, most of which are proposed for the condensation of pure refrigerant in a relatively large diameter tube. It is confirmed that parameters such as tube diameter, surface tension, free convection in FPD and HTC correlations should be taken into account more precisely. Considering the effects of surface tension and kinematic viscosity, new correlation of FPD is developed based on the Mishima-Hibiki correlation. New correlation of HTC is also developed modifying the effect of diameter in the correlation of Haraguchi et al. Both new correlations are compared with experimental data for tubes with micro-fins. Satisfactory agreement between experimental and predicted results is obtained. This means that the micro-fin effect is taken into account by using hydraulic diameter and the heat transfer enhancement effect of micro-fins is mainly due to the enlargement of heat transfer area.