The turbulent flow fields of a parallel plate or channel with spatially periodic condition have been widely investigated by many researchers. However the rectangular or square curved duct flow has not been fundamentally scrutinized in spite of its engineering significance, especially for cooling device. Hence, in the present study large eddy simulation is applied to the turbulent flow and heat transfer in a rectangular duct with 180° curved angle varying its aspect ratio. The turbulent flow and the thermal fields are calculated and the representative vortical motions generated by the secondary flow are investigated. From the results, the secondary flow has a great effect on the heat and momentum transport in the flow. With changing the aspect ratio, the effect of the geometrical variation to the secondary flow and its influence on the turbulent characteristics of the flow and heat transfer are studied.

An experimental study of disperse composition of an air-droplet flow under atomization of water superheated relative to the saturation temperature is conducted. Atomization was accomplished with the use of centrifugal (with a nozzle diameter of 0.6 mm) and once-through jet (with a nozzle diameter of 0.3 mm) atomizers. Water pressure at the inlet to the atomizer was 4 and 8 MPa, with a temperature being varied from 20 to 270°C. The disperse composition of the atomized water was determined from a distribution of the intensity of the scattered laser beam with a wavelength of 532 nm over a wide range of the scattering angles. This technique was combined with measurements of light extinction, filming of the droplet spray, and temperature measurements in the flow to determine a degree of droplets evaporation. The experiments showed that at a water temperature of more than 170°C flashing of water discharging from the atomizers takes place, due to which considerably finer atomization of water is obtained as compared to that with the use of mechanical and pneumatic atomizers. At a temperature of injected water of 220–240°C, 70% of droplets (by mass) have a value of d 32 diameter of 0.2 to 3 μm. While atomizing superheated water, the size distribution of droplets, has a bimodal pattern, i.e., along with the existence of fine droplets, certain amount of droplets of 6–15 μm in diameter (up to 30%) is retained in the flow. When atomizing superheated water, centrifugal and jet atomizers provide approximately the same distribution of droplet sizes. This attests that flashing plays the determining role in the atomization process. Saturation of water to be atomized by air or carbon dioxide does not noticeably change disperse composition of the air-droplet mixture, while atomizing water at a temperature of more than 170°C. Due to the impact of the air flow, fine droplets form a spatially localized spray extended along the flow with a semi-angle of expansion of 10–12 deg. This fact should be accounted for while arranging water injection systems. The fine atomization achieved significantly increases a rate of droplet evaporation in the carrying flow, prevents their deposition on the articles of equipment, and provides more efficient cooling of the working medium.

A primary mechanism of heat transfer in spray cooling is the impingement of numerous droplets onto a heated surface. This mechanism is isolated in the present and ongoing work by numerically simulating the impact of a single train of FC-72 droplets employing an implicit free surface capturing methodology. The droplet frequency and velocity ranges from 2000–4000 Hz, and 0.5–2 m/s, respectively, with a fixed drop size of 239 μm. This gives a corresponding Weber and Reynolds range of 10–170 and 330–1300, respectively. Results show that the impingement zone is largely free of phase change effects due to the efficient suppression of the local temperature field well below the saturated value. Due in part to the relatively high value of the Prandtl number and the compression of the boundary layer from the impingement flow, a cell size on the order of 1 μm is necessary to adequately capture the heat transfer dynamics. It is shown that the cooling behavior increases in relation to increasing frequency and impact velocity, but is most sensitive to velocity. In fact, for sufficiently low velocities the calculations show that the momentum imparted on the film is insufficient to maintain a near stationary liquid crown. The consequence is a noticeable penalty on the cooling behavior.

This paper focuses on the numerical simulation of pressure-driven gas flow in long microchannels, with uniform heat flux wall boundary conditions. The flow is assumed to be two-dimensional and the momentum and energy equations are solved, considering variable properties, rarefaction effects, including velocity slip, thermal creep and temperature jump, compressibility effects and viscous dissipation. A combined serial-parallel algorithm is employed to simulate the flow in long microchannels. The numerical solution is found to be much more involved than that for the isothermal boundary conditions and the convergence of the scheme to be much slower, as expected. The numerical results are also quite different and, in some cases, quite unexpected. The thermal and hydraulic characteristics are carefully examined and analyzed. It is found that a nonlinear temperature profile arises along the microchannel due to the combined effects of pressure work and viscous dissipation. Similarly, compressibility effects lead to a nonlinear centerline pressure profile. The ratio of pressure work to viscous dissipation is investigated as a function of the Knudsen number and is found to increase with the Knudsen number. The rarefaction effects are found to increase the Nusselt number near the outlet and to decrease it near the inlet. An increase in the inlet/outlet pressure ratio is seen to significantly enhance microchannel cooling.

The current study investigates the flow of nanofluids in microchannels experimentally and numerically. For this purpose, two microchannels of hydraulic diameters of 211 and 300 μm are used with alumina(45nm)-water nanofluids. The nanofluids with the concentrations 0.25, 0.50 and 1 vol% are used to observe the effect of volume fraction in the present analysis. With regard to the numerical simulation of nanofluids in microchannels, two approaches have been chosen in the current work. First one considers the nanofluids as single phase fluid and applies the mixture rule for evaluating properties for the simulation. The second type of modeling is done using the discrete phase approach which involves Eulerian-Lagrangian considerations. The fluid phase is assumed to be continuous and governed by Navier-Stokes equation. The movement of discrete nanoparticles is determined by the Newton’s second law which takes into account the body force, hydrodynamic forces, the Brownian and thermophoresis forces. The predictions are validated against experimental results obtained for nanofluid flow in a chemically etched silicon wafer channel. It is found that the discrete phase modeling is more accurate with regard to the prediction of nanofluids behavior in microchannels, as compared to the single phase model. The results also show the non-uniformity of nanoparticle distribution across the channel cross-section. This non-uniformity in distribution can be attributed to the shear induced particle migration. This can also be the reason for the difference in pressure drop and heat transfer from the single phase model. The pressure drop with 0.25 and 0.5 vol% of alumina is more or less same as that of water and thus, makes it a suitable cooling liquid. However, an enhancement in heat transfer is observed from the computational predictions.

Dropwise cooling represents a major subject of interest for both academic and industrial researches. The present work is focused on investigating the thermal transient occurring as two water droplets are gently released ( We < 30) onto a heated solid surface. This latter has been kept at initial temperature lower than 373.15 K to analyze the single-phase-evaporation regime. To the purpose, both an experimental and a numerical approach have conveniently been employed. Infrared thermography has been used to evaluate the temperature trend at the solid-liquid interface: an experimental facility has been built to carry out measurements from below, thus realizing a fully non-intrusive approach. A transparent-crystal disk has been inserted to serve as the solid substrate; its upper surface has been painted by a black coating, thus providing a black-body surface as the solid-liquid interface. The infrared thermocamera has been placed below and perpendicular to that surface; temperature has been thereby measured, being emissivity a known parameter. A numerical code has been developed to predict the involved physical phenomena: temperature trend, evaporation time and evaporated flux result from discretizing the three-dimensional energy-diffusion equation by the finite-volume method. Moreover, the model is based on structured non-uniform mesh to adapt to the occurring temperature gradients. Very good agreement is shown between experimental and numerical outcomes in terms of thermal transient and recovery.

Spray cooling is a very effective means of heat removal from hot surfaces. Its efficiency can be further improved using structured wall surfaces. One of the fundamental processes governing spray cooling is an impact of a single cold droplet onto a heated wetted wall. The hydrodynamics of drop impact governs the transient heat transport in the film and in the wall. We study hydrodynamics and heat transfer during impact of a single drop onto heated smooth and structured heated plates. The temperature distribution in the heated plates has been measured with seven thermocouples. The splash dynamics and the evolution of interface temperature distribution have been visualized using high-speed infrared thermography. The film thickness evolution in the inner region has been measured using chromatic confocal imaging technique. Initial film thickness and drop impact parameters have been varied in the experiments. The evolution of the temperature distribution at the liquid-gas interface and the instationary temperature distribution in the heated plate depend on the initial film thickness, impact parameters and the plate topography. A self-similar analytical solution of the full Navier-Stokes equations and of the energy equation is obtained for the velocity and temperature fields in the spreading film. The theory allows prediction of the contact temperature and the residual film thickness.

Cooling of a hot metal by a spray occurs in various situations. Such is the case for a loss of coolant accident in a nuclear reactor, where a generated spray impacts the fuel rod assemblies. Design of an experimental characterization setup for cooling a hot (600°C) disk shape Nickel sample by a stream of monodisperse droplets is presented here. Non-invasive excitation/measurement techniques have been used in order to implement an inverse technique for quantitative estimation of both wall heat flux and temperature: heating is made by induction and infrared thermography is used for rear face temperature measurement. Control and calibration of the losses are key points here: their level is of the same order of magnitude as the flux removed by the droplets. Examples of inversion are presented.

An experimental investigation has been done to elucidate the effects of mass flux G , degree of subcooling ΔT sub and initial solid temperature T b0 on transient spray cooling of a downward facing φ89 mm hot block surface. The spray impact diameter was adjusted to φ110mm and φ36mm which simulate uniform and non-uniform spray cooling of the surface. The block made of copper, brass and carbon steel at an initial temperature of 200–500 °C was cooled with subcooled water and ethanol spray. The subcooling was from 10 to 80 K and the mass flux was from 1 to 72 kg/m 2 s. Surface temperature and surface heat flux were evaluated with an axisymmetric 2D inverse heat conduction analysis. A transient transition regime was characterized with a wetting temperature and a quenching temperature. The wetting and quenching temperatures were correlated fairly with GΔT sub . Effects of G , ΔT sub , T b0 and a thermal inertia of the solid ( ρcλ ) s on a maximum heat flux are evaluated.

Spray quenching is widely used in industrial applications. In atomized spray quenching (ASQ), water and air are supplied to the nozzle at a certain flow rate and pressure to produce a full cone spray consisting of discrete droplets. Impingement density of spray i.e. coolant mass flow per unit area per second is considered to be the most influential parameter for heat transfer. Impingement density varies with radius and so as the heat flux. Water quality is altered by adding five different salts i.e. NaCl, Na 2 SO 4 , NaHCO 3 , Na 2 CO 3 , and MgSO 4 in de-ionized water with various concentrations. On the other hand, a surfactant Ethoxylated ester, which is commonly added in cooling water in cast houses of metals, is added to pure water in different concentrations i.e 50, 100, 200 and 500ppm. A circular disc made of Nickel of thickness 2mm is heated to 600°C and sprayed on one side by atomized spray and the temperature distribution with respect to time is measured using Infrared camera on the other side of the disc. By this IR thermography, transient temperature measurement can be done within the window of 320×80 pixels with a minimum pixel real distance of 1mm on the sheet surface. Frequency of measurement is 150Hz. Since the temperature measurement and cooling sides are opposite at 2mm thickness apart, inverse heat conduction problem is solved by applying finite element method for calculating temperature and heat flux on the quenched side of metal sheet with respect to space and time. It has been observed that increasing the concentration of salts increase the leidenfrost point and shortens the film boiling regime. While addition of surfactants decrease the leidenfrost point and prolong the film boiling regime. Maximum heat flux position is considered as the wetting front position. There is an abrupt variation of heat flux at wetting front position due to the change of boiling phenomenon. Wetting front velocity has been compared for salt solutions, surfactant and de-ionized or pure water.

Study of fluid flow characteristics at microscale level is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in biomedical industry, MEMS, electronic chip cooling, heat exchangers, and microfluidic devices. Also, due to short lengths employed in microchannels, entrance header effects can be significant and needs to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (α = a/b) ranging from 0.1 to 10, are numerically simulated using CFD software, FLUENT. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of non-dimensional thermal entrance length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions were performed for four wall, three wall and two wall cases. Large numerical data sets, generated in this work for rectangular cross sectional microchannels for three walls and two walls H2 boundary condition, can provide better understanding and insight into the transport processes in the microchannel.

Spray cooling is one of the most promising technologies in applications which require large heat removal capacity in very small areas. Previous experimental studies have suggested that one of the main mechanisms of heat removal in spray cooling is forced convection with strong mixing due to droplet impingement. These mechanisms have not been completely understood mainly due to the large number of physical variables, and the inability to modulate and control variables such as droplet frequency and size. Our approach consists of minimizing the number of experimental variables by controlling variables such as droplet direction, velocity and diameter. An experimental study of single and multiple droplet impingements using HFE 7100 as the cooling fluid under constant heat flux conditions is presented. A monosized droplet train is produced using a piezoelectric droplet generator with the ability to adjust droplet frequency, diameter and velocity. In this study, heaters consisting of a layer of Indium Tin Oxide (ITO) as heating element, and silicon substrates are used. Film morphology was characterized using a Laser Induced Fluorescence (LIF) technique with a focus on the droplet impact zone by measuring variables such as film thickness and diameter of the impact zone. Infrared thermography was used to measure surface temperature at the liquid-solid interface. The IR thermography technique was also used to characterize temperature gradients at the droplet impact zone. The results and effects of droplet frequency, fluid flow rate, and fluid temperature on heat flux are also presented.

Computational modeling is performed for liquid sprays and thermal mixing in desuperheating applications, in which water sprays are injected into a high-temperature steam crossflow (H 2 O l -H 2 O g system) to reduce the enthalpy. The basis of the model is the standard RANS approach coupled to the Lagrangian treatment for statistical groups (parcels) representing the physical droplet population. A vaporization/boiling model is developed for the treatment of phase change under a single chemical species environment. Additionally, the best performing hybrid breakup models depicting the hydrodynamic breakup process governed by the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, are used. The calculations are run for both air-H 2 O l and H 2 O l -H 2 O g systems. Under crossflow conditions, the calculations show that liquid sprays undergo sudden atomization near the injector tip, and have a much smaller characteristic droplet size compared to sprays injected into quiescent environments. This is attributed to the larger gas-droplet relative velocities drastically enhancing the Weber number. Additionally, a blockage effect of the spray on the crossflow is observed in the near-nozzle field, and the typical entrainment of small droplets into the wake is captured in the predictions. The region of maximum cooling coincides with the generation of the highest values of interfacial area produced by the sudden atomization in the near field.

Water spray cooling is an important technology which has been used in a variety of engineering applications for cooling of materials from high-temperature nominally up to 900°C, especially in steelmaking processes and heat treatment in hot metals. The effects of cooling water temperature on spray cooling are significant for hot steel plate cooling applications. The local heat flux measurements are introduced by a novel experimental technique in which test block assemblies with cartridge heaters and thermocouples are used to measure the heat flux distribution on the surface of hot steel plate as a function of heat flux gauge. The spray is produced from a fullcone nozzle and experiments are performed at fixed water impact density of G and fixed nozzle-to-target spacing. The results show that effects of water temperature on forced boiling heat transfer characteristics are presented for five different water temperatures between 5 to 45°C. The local heat flux curves and heat transfer coefficients are also provided to a benchmark data for the actual spray cooling of hot steel plate cooling applications.

With the advancement in microfabrication technologies the size of electronic devises keeps decreasing. At the same time high density of circuits and the directly related heat generation requires simultaneous increase in the performance of cooling systems to keep the electronics operating at optimum temperatures. Liquid cooling incorporating microchannel networks as the heat sinks is known to be a promising approach to this heat transfer problem because of its plausible manufacturability at the chip level. However, even the upper range of heat fluxes achieved in such microchannel heat sinks will not meet the increasing demands of the electronics industry. We present herein a novel experimental study of convective heat transfer in serpentine microchannels with segmented liquid-liquid emulsions. It is demonstrated that this concept yields significant heat transfer enhancement in micro channel heat sinks compared to traditional single phase liquid cooling. The flow of water and mineral oil droplets is examined experimentally in microchannels with cross section 100 by 100 μm and flow rates up to 130 μl/min. The study is focused on the investigation of the local temperature of the liquid and on the influence of the oil droplets on the temperature distribution and heat transfer. Laser Induced Fluorescence (LIF) is employed to measure the temperature of the flow fields with and without droplets, and PIV measurements are conducted to determine the velocity field. The range of flow conditions which led to a stable segmented flow with enhancement of heat transport was identified. The increase of the oil fraction was accompanied by an increase of the recirculation zone in the transverse direction to the flow and by higher velocity gradients inside the zones thus resulting in enhanced mixing of water across the channel. The LIF measurements indicate that for the same total flow rate, the local temperature of the water region separating the droplets will be higher than in the case of single phase liquid flow. For segmented flow, up to a four-fold increase of the Nusselt number compared to pure water flow was observed.

During a thermal spraying process, the heat flux reaches a few hundreds M W/m 2 while the spreading duration is extremely short, being about a microsecond. The interfacial phenomena of thermal deposit/substrate constriction play an extremely important role since they control the coating cooling rate, and as a consequence, the thermo-mechanical properties of the deposited layer. We present in this work an analytical solution intended for the computation of the thermal constriction resistance during the spreading. This solution is derived by using the Hankel finite transform and the Duhamel theorem leading to a simple expression as a series expansion with fast convergence. The effect of the spreading velocity on the thermal constriction resistance has been studied. Results show the existence of a critical threshold of spreading velocity for which the transient problem has to be considered. On the other hand (below the threshold) the steady state regime should be sufficient for the constriction estimate.

In this study we present a preliminary investigation into the effect of hydrophobicity on the heat transfer rate due to the impingement of cold water droplets on heated flat surfaces. Two extreme contact angles were compared; hydrophilic (∼20°) and superhydrophobic (∼160°) using different surface coatings on a thin metal substrates. Images of the droplet impingement were simultaneously recorded by a high speed camera and a high speed, high resolution infrared camera so we could correlate droplet shape and dynamics to the heat transfer rate. There is a large effect on both the droplet fluid dynamics and heat transfer between hydrophilic and superhydrophobic surfaces. The heat transfer difference between the superhydrophobic and hydrophilic cases is a complex interplay between the increased droplet contact line velocity due to induced slip and the insulating properties of the air gap. Overall we have shown significant reductions in both the instantaneous heat transfer rates and the overall cooling effect of droplets impinging on superhydrophobic surfaces relative to those for hydrophilic surfaces. In the range of droplet velocities varied (We = 50 to 190) there was little dependency of the heat transfer or fluid flow with impact velocity, due to the dominance of inertial forces.

A three-dimensional mathematical model of unsteady heat and mass transfer in porous hydrogen-absorbing media, accounting for presence of “passive” gas admixtures, is developed. New technique for evaluation of effective thermal conductivity of porous medium, which consists of microparticles, is suggested. Effect of “passive” gas admixtures on heat and mass transfer and sorption rate in metal hydride reactor is analyzed. It is shown that decrease of effective thermal conductivity and partial hydrogen pressure under decrease of hydrogen concentration effect on the hydrogen sorption rate considerably. It is disclosed that an intensive 3D natural convection takes place in a gas volume of reactor under certain conditions. Numerical analysis of heat and mass transfer in metal-hydride reactor of hydrogen accumulation systems was done. Sorption of hydrogen in cylindrical reactors with external cooling and central supply of hydrogen are analyzed including reactors with finned active volume and tube-shell reactor with external and internal cooling cartridge matrix. Unsteady three dimensional temperature and concentration fields in solid phase are presented. Integral curves representing the dynamic of sorption and desorption are calculated. Data on efficiency of considered reactors are presented and compared.

Heat generation and temperature rise in electronic devices is a technical problem with increasing importance, since the number of transistors per surface area on integrated circuitries is rapidly increasing. If the heat cannot effectively be carried away damage in the circuitry may occur. Therefore enhanced and integrated cooling is needed. A promising technique is evaporative microchannel cooling. However, a major problem in modeling such micro-device is that the continuum approach starts to fail in the vapor phase and more detailed modeling becomes necessary. Since on these small scales the boundary and interface conditions are very important for the overall performance of the device, we choose the approach in which we start with understanding the essential physical phenomena at a molecular level. In this paper a detailed particle-based model is derived for these interactions: local interactions between the three phases are studied by molecular dynamics (MD) simulations in a detailed way. In this way physically and thermodynamically correct interface and boundary conditions (e.g. slip velocities and temperature jumps) are ensured. Finally, the enhanced heat transfer in the evaporative zone (Argon on a Calcium surface) is simulated by our molecular model and is compared to the results obtained by the continuum microregion model developed by P. Stephan et al. (Int. J. Heat Mass Transfer, 35, pp. 383–391, 1992).

Multiple microchannel heat sinks for potential use for electronic chip cooling are studied experimentally and numerically to characterize their thermal performance. The numerical simulation is driven by experimental data, which are obtained concurrently, to obtain realistic, accurate and validated numerical models. The ultimate goal is to design and optimize thermal systems. The experimental setup was established and liquid flow in the multiple microchannels was studied under different flow rates and heat influx. The temperature variation versus time was recorded by thermocouples, from which the time needed to reach steady state was determined. Temperature variations under steady state conditions were compared with three-dimensional steady state numerical simulation for the same boundary and initial conditions. The experimental data served as input parameters for the validation of the numerical model. In case of discrepancy, the numerical model was improved. A fairly good agreement between the experimental and simulation results was obtained. The numerical model also served to provide input that could be employed to improve and modify the experimental arrangement.