Flow boiling in microchannel heat sink offers an effective cooling solution for high power density micro devices. A three-dimensional numerical study based on volume of fraction model (VOF) coupled with evaporation condensation model accounting for the liquid-vapor phase change is undertaken to recreate vapor bubble formation in saturated flow boiling in wavy microchannel. Constant wall heat flux imposed at the bottom surface of the substrate while other faces are insulated. To understand the conjugate effects, simulations has been carried out for substrate thickness to channel depth ratio (δ sf ∼ 1–5), substrate wall to fluid thermal conductivity ratio (k sf ∼ 22–300) and waviness (γ ∼ 0.008–0.04). Bubble nucleation, growth, and departure of bubble plays a significant role in heat transfer and pressure drop characteristics in two-phase flow in wavy microchannel. The coolant (water) temperature at the inlet is taken to be 373 K while flow was at atmospheric pressure. This makes shorter waiting period of bubble nucleation, and the number density of bubbles on the solid surface increases. This results in enhancement of the boiling effect, and thus with the presence of bubbles, the mixing of laminar boundary layers improves and enhances the overall heat transfer coefficient. Channel amplitude play an important factor that can suitably reduce the friction factor and enhances the heat transfer coefficient.

Atmospheric condensation is important for multiple practical applications such as distillation/desalination of water, aerospace, dehumidification, and water harvesting etc. Graphene, an allotrope of carbon with two dimensional structure, has excellent thermal and electrical properties. Here we present condensation studies of water on plain copper and graphene oxide (GO) coated copper surface with different environmental conditions to explore the size distribution of the generated droplets and area coverage in order to characterize the surfaces for larger condensate harvesting. Later, droplet growth and size distributions were recorded for 41 minutes 20 seconds on the surfaces at 40% and 60% relative humidities with a surface temperature of 278 ± 0.5 K. The chamber was maintained at atmospheric pressure and 295 ± 0.5 K. The samples were observed via optical microscopy and videos of the condensation dynamics were captured. The droplet grew mainly by direct condensation and coalescence event. At later stages of condensation, surface coverage increased significantly compared to early stages for all the considered cases. Approximate 95% surface coverage was observed for GO coated copper surface which provides a great insight of this substrates for implementing it in the desired water harvesting applications. The pinning of droplets into the micro/nanostructures of the coated surfaces leads enough time for the first generation droplets to grow in larger size and made more preferential for subsequent coalescence events. Within the initial period of condensation, the number of droplets reduced according to power law decay. The contribution of coalescence mechanism in droplet growth was found larger for 60% RH than 40% RH. As droplet grew larger, direct growth became less significant compared to coalescence phenomenon.

Pool boiling experiments of water and ethanol-water binary mixtures were conducted on smooth and laser textured stainless steel foils. High-speed IR thermography was used to measure transient temperature field during boiling in order to determine nucleation frequencies, nucleation site densities, bubble activation temperatures, wall-temperature distributions and average superheats as well as heat transfer coefficients. Saturated pool boiling experiments were conducted at atmospheric pressure over a heat flux range of 5–250 kW m −2 for pure water and ethanol-water mixtures (1% and 10% m/m). For both mixtures and both types of surfaces we measured significant decrease in average heat transfer coefficient and increase in bubble activation temperatures in comparison to pure water. However, laser textured surface in average provided around 60% higher nucleation frequency and more than 100% higher nucleation site density compared to smooth surface for both of the tested binary mixtures. Consequentially, heat transfer coefficient was enhanced for more than 30%. Our results show that laser textured surfaces can improve boiling performance for water and ethanol-water mixtures, but at the same time the addition of ethanol reduces heat transfer coefficient despite the enhancement of nucleation site density and nucleation frequency. This is also in agreement with available experimental data and existing theoretical models.

This paper focuses on experimental studies of boiling heat transfer on surfaces with reentrant tunnels and pores. Three structured surface which have same tunnel width and height but different pore diameter, have been developed for enhancement boiling heat transfer. The experimental studies were carried out for the structured surfaces using distilled water at atmospheric pressure. The narrow reentrant tunnels are parallel to each other and have 3 mm width, 4 mm height. A number of pores whose diameter 1.5 and 2.0 mm were machined on lateral surfaces of tunnels. The surfaces were termed according to their geometric specifications as 3.0W-30-30, 1.5D-3.0W-30-30, 2.0D-3.0W-30-30. D and W capitals represent pore diameter and tunnel width, respectively. 30-30 part of name shows the dimension of square surface. The tunnels were used to increase area of heat transfer and active nucleation sites of vapor bubbles. In addition, sufficient amount of liquid must be supplied and vapor bubbles should be released fast from the boiling surface before they merge on the surfaces under conditions especially with high heat fluxes. Therefore, it was considered that pore structures would help for fluid transition hence the bubble frequency will increase. Pool boiling experiments were held to determine the performance of surfaces in different range of heat fluxes. Besides, high-speed visualization studies were conducted with high speed camera to observe behavior of nucleation of vapor bubbles. Amongst different geometry sizes the surface which has 1.5 mm of pore diameter (1.5D-3.0W-30-30) demonstrated the best nucleate boiling performance at high heat fluxes. However, the pored ones without pores has higher augmentation than pored structures at low heat fluxes. Thus, it is concluded that pored structures caused active nucleation sites to decrease under low heat fluxes.

As the need for efficient thermal management grows, pool boiling’s ability to dissipate high heat fluxes has gained significant interest. The objective of this work was to study the performance of pool boiling at atmospheric pressure using a dielectric fluid, HFE7000. Both plain and enhanced copper surfaces were tested, and these results were then compared to similar testing performed with water and FC-87. The enhanced surfaces utilized microchannels with porous coatings selectively located on different regions of the heat transfer surface. A maximum critical heat flux (CHF) of 41.7 W/cm 2 was achieved here, which translated to a 29% CHF increase in comparison to a plain chip. A maximum heat transfer coefficient (HTC) of 104.0 kW/m 2°C was also achieved, which translated to a 6-fold increase in HTC when compared to a plain copper chip. More notably, this HTC was achieved at a wall temperature of 38.4 °C. This HTC enhancement was greater than that of water and FC-87 when using the same enhanced surface. The effect of sintering location was found to have a similar effect on CHF with HFE7000 in comparison with water. The effect of microchannel size was shown to have similar effects on CHF when compared with FC-87 and water. From the results found here, it is concluded that the employment of selectively sintered open microchannels with HFE7000 has significant potential for enhanced heat dissipation in electronics cooling applications.

Microstructures on boiling surfaces had been adopted to study heat transfer enhancement mechanism because of its controllability of precise geometries by MEMS techniques. In addition, several bubble-inducing surfaces such as porous or hydrophobic surfaces were verified to be combined with microstructures for further enhancement of cooling efficiency. In this research, microstructures with porous and dual-wettability (Janus) were fabricated by soft-lithography with replicating molds. In saturated pool boiling at atmospheric pressure, the porous and Janus microstructures increased the heat transfer coefficient by 50% over that of plain-cylindrical microstructures. The increase occurred because the porous and wettability-controlled structures favored vigorous bubble generation, and supported high contact line density, which is strongly related to effective cooling through the near-bubble region.

Passive pool boiling enhancements offer attractive cooling possibilities to address the demand for effective thermal management in high powered electronic systems. Enhancements in pool boiling have been achieved through area augmentation, providing additional nucleation sites or by inducing liquid wettability changes. Graphene, a two-dimensional material, has garnered significant attention of researchers due to its excellent thermal properties. In this study, heat transfer surfaces are dip coated with an electrochemically generated solution consisting of graphene oxide (GO) and graphene and its pool boiling performance with distilled water at atmospheric pressure was obtained. The surfaces were characterized using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The static contact angles of the engineered surfaces are measured. The underlying wettability mechanisms are supported by high speed imaging of the nucleating bubbles. A maximum Critical Heat Flux (CHF) of 182 W/cm 2 and a Heat Transfer Coefficient (HTC) of 96 kW/m 2 °C was obtained with the thinnest coating which translated to an enhancement of 42% in CHF and 47% in HTC when compared to a plain uncoated surface.

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling levels, and mass fluxes are considered in this type of heat sink for use in applications with high fluxes up to 300 W/cm 2 . The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 μm and 1 cm radius with a single inlet and a single outlet. The channel is heated on one side through a copper conducting surface, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. Flow boiling heat transfer and pressure drop data were obtained for this heat sink device using water at near atmospheric pressure as the working fluid for inlet subcooling levels from 20 to 81°C and mean mass flux levels ranging from 184 to 716 kg/m 2 s. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface with water equilibrium contact angles in the range of 14–40°, typical of common metal surfaces. The other was a surface coated with zinc oxide nanostructures that are superhydrophilic with equilibrium contact angles measured below 10°. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter as indicated by reduced temperature oscillations during boiling and achieving higher maximum heat flux without dryout. Reducing inlet subcooling levels was also found to reduce the magnitude of oscillations in the oscillatory boiling regime. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm 2 with an average surface temperature of 134 °C and requiring a pumping power to heat rate ratio of 0.01%. The hydrophilic surface retained wettability after a series of boiling tests. Recommendations for use of this heat sink design in high flux applications is also discussed.

Thermal management in microelectronic devices involves development of high heat flux removal systems to meet the cooling requirements. Pool boiling addresses these demands by using latent heat transfer. In this study, heat transfer surfaces are fabricated by depositing porous coatings on an open microchannel surface. Screen printing and sintering are identified as techniques to deposit porous coatings and ensure substrate bonding respectively. Firstly, the effect of selective enhancement was studied by depositing porous coatings on (i) fin tops only (sintered-fin-tops), (ii) channels only (sintered-channels), and (iii) completely covering the boiling surface (sintered-throughout). The pool boiling performance with saturated distilled water at atmospheric pressure was obtained and a maximum critical heat flux (CHF) of 313 W/cm 2 at a wall superheat of 7.5 °C was reported here for a sintered-throughout surface. Furthermore, the effect of channel width on sintered-throughout surfaces was studied. The results indicated that channel width plays an important in improving the performance. High speed videos are taken to understand the underlying mechanism. Additional nucleation sites and separate liquid-vapor pathways are identified as contributing mechanisms for the enhancement in CHF and heat transfer coefficient (HTC).

Boiling is an efficient way to transfer heat due to the latent heat of vaporization. Many variables, such as surface properties, fluid properties, and system pressure, will affect the performance of pool boiling. Enhanced pool boiling has extensive applications in chemical, microelectronics, and power industries. Previous research has shown that micro- or nanostructured surfaces and coated surfaces will increase heat transfer coefficients up to one order of magnitude at atmospheric pressure. Graphene as a very good material with superb mechanical and electrical properties also has potential to enhance pool boiling performance. The purpose of this research is to investigate heat transfer enhancement on a graphene coated surface compared to a plane copper surface at atmospheric pressure and increased pressures with deionized water. The effect of the graphene coating on the critical heat flux is also investigated. To carry out the experiments, we designed and fabricated a special experimental facility that will withstand the high pressures (up to 20 bar) and high temperatures. Graphene is coated on a 1 cm 2 copper surface using spray coating. The boiling vessel is pressurized with nitrogen and the system pressure is controlled by a back pressure regulator. The test fluid is preheated to saturation temperature by two 500 W cartridge heaters. Multiple 150 W cartridge heaters are inserted in a copper cylinder to provide wall superheat for bubbles to nucleate on the studied surface. When the system reaches steady state, a process controller controls these cartridge heaters to increase the heat flux gradually from 0 kW/m 2 to the critical heat flux. The copper cylinder is insulated with PTFE to minimize heat loss from the side. The gap between the copper cylinder and the insulation surface is carefully sealed with high temperature epoxy to reduce undesired nucleation sites. The wall superheat corresponding to each heat flux is extrapolated using Fourier’s law from three thermocouple readings. The heat transfer coefficient can thus be calculated at each heat flux for the every test fluid at its corresponding pressure. A camera with 3.2 cm field of view at a working distance of 12 cm to 15 cm is used to visualize the bubble formation on the heated surface.

Boiling heat transfer enhancement via compound effect of Electro-Hydro-Dynamic (EHD) and contact angle has been experimentally and analytically investigated. A fluorinated dielectric liquid (Asahi Glass Co. Ltd, AE-3000) was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. In order to change the contact angle between the boiling surface and the dielectric liquid, the different materials Cu, Cr, NiB, Sn, and mixture of 5 and 1.5 micro meter diamond particles were electrically deposited on a boiling surface. The critical heat flux (CHF) for different contact angles showed 20.5 ∼ 26.9 W/cm 2 which was −7 ∼ 25 % of that for a non-coated Cu surface (21.5 W/cm 2 ). Upon application of a −5 kV/mm electric field to the micro structured surface (the mixture of 5 and 1.5 micro meter particles), a CHF of 99 W/cm 2 at a superheat of 33.5 K was obtained. The previous theoretical equation of pool boiling predicted the CHF with the electric field and without the electrode.

Air bubble injection experiments have been performed to obtain a better understanding and detailed data on bubble behavior and liquid velocity profiles to be used for validation of 3-D Interface Tracking Models and CFD models. Two test sections used were vertical rectangular minichannels with a width and gap of 20 mm × 5.1 mm and 20 mm × 1.9 mm, respectively. Subcooled water at near atmospheric pressure flowed upward under laminar and turbulent flow conditions accompanied by air bubbles injected from a small hole on one of the vertical walls. The experiments yielded data on bubble formation and departure, and interactions with laminar or turbulent water flow. Instantaneous and ensemble-average liquid velocity profiles have been obtained using a Particle Image Velocimetry technique and a high speed video camera.

Engineered colloidal suspensions of nano-sized particles (less than 100nm) dispersed in a base fluid (nanofluid), have shown potential for industrial cooling fluids due to their enhanced heat transfer characteristics. Understanding the rheological characteristics of these suspensions is vital while employing them for flow applications. The effect of temperature on the viscosity of nanofluids at atmospheric pressure is well documented in literatures; however, there are no available data for viscosity measurements of nanofluids at elevated pressure and temperature. In this work, rheological characteristics of oil based nanofluids at high pressures and temperatures, order of 100atm and 100 °C, respectively, are investigated. Nanofluid is prepared by dispersing commercially available SiO 2 nanoparticles (∼20nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO QATAR) which has wide applications for heat exchangers in oil industry. The rheological characteristics of both the base fluid and the nanofluid are measured using a High Pressure High Temperature (HPHT) viscometer. During experimentation, viscosity values are measured at pressures varying from 10MPa to 40MPa and temperatures ranging from 25°C to 170°C for nanofluid with mass concentrations of 3 percent. The viscosity values of nanofluids as well as base fluid are observed to increase with the increase in pressure. From the pressure coefficient values evaluated for basefluid and nanofluid, it is evident that the effect of pressure on nanofluid and basefluid was similar with no additional effect with respect to particle loading.

The growing trend in miniaturization has brought forth boiling as an important research topic for augmentation of heat transfer in electronic cooling application. Pool boiling has the ability to dissipate a large quantity of heat while maintaining a small temperature difference. The heat dissipation capacity of pool boiling is further augmented through the use of enhanced microchannel surfaces. The objective of this work is to investigate the pool boiling performance of various microchannel structures using ethanol as the fluid medium. Ethanol has higher heat of vaporization compared to refrigerants and lower saturation temperature compared to water. Hence it has the potential to be used as an alternative cooling medium. This investigation will focus on experimentally studying the effect of enhanced surface structures on pool boiling of ethanol at atmospheric pressure.

The Molecular Tagging Velocimetry (MTV) technique has been widely used for analyzing velocity fields in liquid mini- and microflows. Concerning gaseous flows, only few works describe the implementation of MTV at millimetric scale, and these studies are limited to the analysis of external flows, such as jet flows. The goal of the present work is to develop this technique for the analysis of internal gas flows in minichannels. It is a first step toward the visualization of velocity profiles in rarefied conditions, and direct measurement of velocity slip at the walls. A specific experimental setup has been designed. Its features are detailed. Velocity profiles are obtained in a pressure driven steady flow of argon through a long rectangular minichannel of 1.2 × 5 mm 2 cross-section and 15 cm length using acetone molecules as tracer. Experiments are carried out at atmospheric pressure, in a laminar continuum flow regime. The accuracy of the method is discussed by comparison between experimental and theoretical velocity profiles. The potential of the MTV technique for analyzing mini or micro gaseous internal flows is commented on. Perspectives of the work for discussing the validity of boundary conditions in the slip flow regime are presented.

The miniaturised online sensors that were developed in the laboratories were for atmospheric pressure and steady state flow, but in the water distribution network neither the pressure nor the flow is steady. Many of the state of the art drinking water quality monitoring sensors can be operated well below the drinking Water Distribution System (WDS) pressure. Moreover, each of the sensors requires different flow rates. This paper discusses simulation and design of an affordable constant flow and constant outlet pressure system and shows an easy way to provide different flow rates for different sensors. The other criterion which should be met is the flow rate of the water bled (leakage) from WDS which must also be low. To meet the above criteria a 2-D model was developed to represent the constant pressure constant flow system for online water quality monitoring (WQM) sensors. Different configuration of the system is considered and the optimum design includes 1.044 m/s flow velocity which is low enough for the flow to be steady.

Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m 2 s and the base heat flux varied from 30 to 80 kW/m 2 . Water enters the test channel under subcooled conditions. The samples are silicone oxide (SiOx), titanium (Ti), diamond-like carbon (DLC) and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26°, 49°, 63° and 103°, respectively. The results show significant impacts of surface wettability on heat transfer coefficient.

A concentric micro annular passage is a basic and important micro-geometry of micro-fluidic-systems from simple heat exchanger to the most complicated nuclear reactors. Therefore, heat transfer characteristics of gaseous flows in concentric micro annular tubes with constant heat flux whose value was positive or negative were numerically investigated. The slip velocity, temperature jump and shear stress work were considered on the slip boundary. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The computations were performed for two thermal cases. This is, the heat flux was constant at the inner wall and outer wall was adiabatic (Case 1) and the heat flux was constant at the outer wall and the inner wall was adiabatic (Case 2). Each constant heat flux of 10 4 Wm −2 for the positive value and −10 4 Wm −2 for the negative value was chosen. The outer tube radius ranged from 20 to 150 μm with the radius ratio 0.02, 0.05, 0.1, 0.25 and 0.5 and the ratio of length to hydraulic diameter was 100. The stagnation pressure was chosen in such a way that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure was fixed at the atmospheric pressure. The heat transfer characteristics in concentric micro annular tubes were obtained. The wall and bulk temperatures with positive heat flux are compared with those of negative heat flux cases and also compared with those of the simultaneously developing incompressible flow. The results show that the compressible slip flow Nusselt number is different from that of incompressible flow. And, the temperatures normalized by heat flux have different trends whether heat flux value is positive or negative. A correlation for the prediction of the heat transfer characteristics of gas slip flow in concentric micro annular tubes is proposed.

A concentric micro annular passage is a basic and important micro-geometry of micro-fluidic-systems from simple heat exchangers to the most complicated nuclear reactors. Therefore, the product of friction factor and Reynolds number ( f ·Re) for quasi-fully developed high speed and slip flow in a concentric micro annular tubes of Re<1000 and Ma<1.0 was obtained numerically. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The compressible momentum and energy equations with slip boundary conditions were solved for both isothermal flow and no-heat conduction flow conditions. The outer tube radius ranged from 5 to 40 μm with the radius ratios of 0.2, 0.5 and 0.8. The ratio of length to hydraulic diameter was 100. The stagnation pressure was chosen in such a way that the exit Mach number ranged from 0.1 to 1.0. The outlet pressure was fixed at the atmospheric pressure. The value of f ·Re for compressible slip flow in concentric micro annular tubes were obtained. The detail of the incompressible slip f ·Re is documented and its value defined as a function of r * and Kn is represented. The results show that in the case of fast flow, the values of f ·Re for compressible slip flow is higher than those for incompressible slip flow due to compressibility effects. Also, the f ·Re correlation for compressible slip flow is obtained from that of compressible no-slip flow and incompressible slip flow as a function of Mach and Knudsen numbers and radius ratio. In addition, a f ·Re correlation from the values of f ·Re obtained for micro-channel, micro-tube, and micro annular tube referred in author’s previous works that is applicable to micro-geometries whose cross-sections are parallel, rectangular, circular and annular, is proposed.

A decompression experiment of a water solution, saturated with methane gas at about 68 atm at room temperature, was done to investigate gas bubble nucleation under shear flow. A pressure reduction from 68 atm to atmospheric pressure is well below the decompression pressure required for spontaneous bubble nucleation of the methane gas, about 120 atm. The application of a shear flow from 5 minutes before to 1 minute after the decompression induced active bubble formation and the final gas content in the solution was reduced substantially, even with the application of low shear rate of 25/s.