Enhanced Flow Boiling Heat Transfer Research Articles

Enhanced flow boiling heat transfer characteristics of R134a on graphene-Cu nanocomposite coating on copper substrate

This study presents the flow boiling heat transfer characteristics of graphene-Cu nanocomposite coated copper surfaces. First, recently reported studies regarding efforts to enhance flow boiling heat transfer are discussed, then related studies on the impacts of using nanocomposite coated surfaces on flow boiling heat transfer characteristics are reviewed. Experimental investigations explore the effects of graphene concentration and feed mass flux of a refrigerant (R134a) on the heat flux and the heat transfer coefficient. The test chamber is a rectangular channel of 60 mm width, 3 mm height, and 460 mm length, and the dimensions of the graphene-Cu composite test plates are 10 mm × 10 mm. The feed mass fluxes used in the experiments are 240 and 480 kg/m2 s, while the wall heat flux are varied from 0.1 to 20 W/cm2. Based on the experimental analysis, the heat flux slightly increased before the onset of nucleating bubbles. Subsequently, the values drastically increased as the wall superheat continued to increase. Additionally, in experiments with higher graphene concentration, higher heat flux and heat transfer coefficient were demonstrated, and the increase in feed mass flux promoted flow boiling heat transfer.

Experimental study and dynamic simulation of the continuous two-phase instable boiling in multiple parallel microchannels

Continuous vapor liquid two-phase flow means that both subcooled single phase and saturated flow boiling always exist in microchannels with the moving interface between them. The instable boiling phenomena and mechanisms are very complicated and needed to be deeply investigated and well understood. In the present study, both experiments and dynamics simulation of the continuous two-phase instable boiling in multiple parallel microchannels were systematically conducted. In the experimental aspect, characteristics of the unstable flow boiling in the microchannels were experimentally investigated using acetone as the working fluid and flow boiling processes were visualized with a high speed video camera simultaneously. The test section includes 16 parallel microchannels having a hydraulic diameter of 104.3 µm. The test mass flux ranged from 437.2 to 868.1 kg/m2 s and the test heat flux ranged from 0 to 100 W/cm2. The measured flow boiling heat transfer coefficients and two-phase frictional pressure drops are analyzed according to the observed flow phenomena and mechanisms. They were further compared to the existing flow boiling heat transfer and two-phase pressure drop prediction methods. The top three flow boiling heat transfer methods predict more than 65% of the heat transfer coefficients within ±30% while the top two pressure drop methods only predicted 43% of the measured data within ±30%. It shows that the continuous two-phase instable boiling mainly occurs at the mass flux larger than 607.6 kg/m2 s and the heat flux larger than 30 W/cm2. The backflow causes instability of the liquid and the two-phase interface which change the thermal resistances of the fluid. In the theoretical aspect, a dynamic simulation model was developed by using the thermal network method and was used to simulate the dynamic process of the instable boiling process. The simulated results were compared to the experimental data in this study and from the literature and the relative errors are within ±24.4%. According to the simulation and analysis, the mechanisms are that the axial thermal conduction in the channel walls generates a negative differential zone in the heat power characteristic curves, where the wall temperature decreases with increasing the heat flux in the negative differential zone. In order to suppress the oscillations in the boiling process, it is essential to eliminate the negative differential zone. Enhancing single phase forced convection heat transfer by 100% or enhancing flow boiling heat transfer by 50% can suppress the continuous two-phase instable boiling in the microchannels. Elongating the microchannel length and enhancing the axis thermal conduction can reduce the amplitudes of the oscillations by 60–80% at the conditions of G = 700 kg/m2 s, qeff = 70 W/cm2 and Tin = 30 °C.

Enhanced boiling in microchannels due to recirculation induced by repeated saw-toothed cross-sectional geometry

A saw-toothed shaped microchannel heat sink is investigated for enhancing flow boiling heat transfer. Tests are conducted at mass fluxes of 444–1776 kg/m2 s and an inlet subcooling of 15 °C. The effects of channel geometry on boiling curves, flow patterns, pressure drops, and heat transfer coefficient are discussed in this letter. It is found that heat transfer performance is enhanced by up to 50% especially at heat flux levels associated with the current generation of microprocessors.

Subcooled flow boiling heat transfer enhancement using polyperfluorodecylacrylate (pPFDA) coated microtubes with different coating thicknesses

In this study, enhanced subcooled boiling heat transfer was achieved at high mass fluxes by applying a new surface enhancement method. In this method, polyperfluorodecylacrylate (pPFDA) was applied on the inner walls of the 4cm long stainless steel hypodermic microtubes with inner diameters of 889 and 600μm. Initiated chemical vapor deposition (iCVD) was employed for coating inner walls of the microtubes with different coating thicknesses of ∼50 and ∼160nm. iCVD could serve for a surface deposition method for closed geometries like microtubes and offered a uniform coating. The experiments were performed at high mass fluxes of 6000, 7000, and 8000kg/m2s with de-ionized (DI) water (as the coolant). The Joule heating method was used for applying heat to the test section, which was located at the end (the last 2cm) of the microtube. Temperature measurements were done at the very end of the microtubes. The experimental results indicated that pPFDA coated microtubes could significantly enhance flow boiling heat transfer. The largest heat transfer enhancement was achieved as ∼61% pertinent to the coated microtube of an inner diameter of 889μm with the coating thickness of 160nm, at G=8000kg/m2s, relative to its bare surface counterpart (at the same heat flux). The coatings were proven to be reliable and reproducible by analyzing the coated microtubes after performing boiling experiments with the Raman spectroscopy method.

Zastosowanie teksturowania laserowo-wibracyjnego do intensyfikacji procesu wrzenia w przepływie przez minikanał o przekroju prostokątnym

Zastosowanie teksturowania laserowo-wibracyjnego do intensyfikacji procesu wrzenia w przepływie przez minikanał o przekroju prostokątnym

Mechanistic Considerations for Enhancing Flow Boiling Heat Transfer in Microchannels

Research efforts on flow boiling in microchannels were focused on stabilizing the flow during the early part of the last decade. After achieving that goal through inlet restrictors and distributed nucleation sites, the focus has now shifted on improving its performance for high heat flux dissipation. The recent worldwide efforts described in this paper are aimed at increasing the critical heat flux (CHF) and reducing the pressure drop, with an implicit goal of dissipating 1 kW/cm2 for meeting the high-end target in electronics cooling application. The underlying mechanisms in these studies are identified and critically evaluated for their potential in meeting the high heat flux dissipation goals. Future need to simultaneously increase the CHF and the heat transfer coefficient (HTC) has been identified and hierarchical integration of nanoscale and microscale technologies is deemed necessary for developing integrated pathways toward meeting this objective.

Numerical simulation on single bubble behavior during Al2O3/H2O nanofluids flow boiling using Moving Particle Simi-implicit method

In this study, regression analysis on the thermal properties of Al2O3/H2O nanofluids was made firstly. The growth and departure of a single bubble behavior in Al2O3/H2O nanofluid and pure water flow boiling process were then numerically simulated by an improved Moving Particle Semi-implicit method in different flow boiling conditions. The results indicate that the bubble in Al2O3/H2O nanofluids grows faster and the bubble departure frequency of Al2O3/H2O nanofluids is greater than that in pure water. The flow boiling heat flux is also improved by dispersing nanoparticles of Al2O3/H2O in pure water. This work initially reveals that nanofluids can enhance flow boiling heat transfer from the point of view of bubble dynamics behavior. The effects of nanoparticle concentrations and diameters of Al2O3/H2O nanofluids on the bubble behavior were also investigated and compared under the same flow conditions. It is found that the increasing of nanoparticle volume concentration may increase the bubble departure frequency and departure diameter, while the increasing rates of departure frequency and departure diameter are lessened with the increasing of nanoparticle volume concentration. It is suggested that the suitable nanoparticle volume concentration of nanofluid for flow boiling heat transfer enhancement should not be too large, especially regarding the negative effect of flow resistance increase due to the increasing of nanoparticle volume concentration. The interesting finding is that in the same nanoparticle volume concentration condition, the bubble departure frequency for the nanofluid with nanoparticle diameter of 29 nm shows a maximum value. The increasing of nanoparticle diameter leads to the decreasing of bubble departure diameter. It is boldly to predict that an optimal nanoparticle diameter range between 20 and 38 nm should be beneficial to flow boiling heat transfer enhancement of Al2O3/H2O nanofluids.

Flow boiling phenomena in a single annular flow regime in microchannels (I): Characterization of flow boiling heat transfer

Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall microchannels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389kg/m2s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4–25.8% lower. The maximum HTC reached 125.4kW/m2K at a mass flux of 404kg/m2s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling.

Enhanced flow boiling in microchannels by self-sustained high frequency two-phase oscillations

Experimental study of flow boiling heat transfer in a microchannel array consisting of main channels connected to two auxiliary channels (each) was conducted. A microbubble-excited actuation mechanism, powered by high frequency vapor bubble growth and collapse, was established to create and sustain strong mixing in the microchannels. It was shown to significantly enhance flow boiling heat transfer in microchannels. Experimental studies were conducted at mass fluxes ranged from 150 to 480kg/m2s with de-ionized (DI) water as the working fluids. Compared with microchannels with inlet restrictors (IRs), the average two-phase heat transfer coefficient was improved by up to 149%. More importantly, a 71–90% reduction in pressure drop at moderate mass fluxes ranged from 400 to 1400kg/m2s was observed. Heat flux up to 552W/cm2 at a mass flux of 480kg/m2s was demonstrated. Flow and heat transfer mechanisms were studied and discussed.

Enhancing Flow Boiling Heat Transfer in Microchannels for Thermal Management with Monolithically-Integrated Silicon Nanowires

Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.

Enhanced flow boiling heat transfer with jet impingement on micro-pin-finned surfaces

Experiments were conducted to study the performance of subcooled flow boiling heat transfer with jet impingement of FC-72 over silicon chips (10 × 10 × 0.5 mm3). Four kinds of micro-pin-fins with dimensions of 30 × 60, 30 × 120, 50 × 60, 50 × 120 μm2 (thickness t× height h) were fabricated on the chip surfaces by using the dry etching technique. The experiments were made at two different liquid subcooling (25 °C and 35 °C), three different crossflow velocities Vc (0.5, 1, 1.5 m/s) and three different jet velocities Vj (0, 1, 2 m/s). A smooth surface was also tested for comparison. The results show that both the microstructure and impingement give a large enhancement on heat transfer. The maximum allowable heat flux increases with the velocity and liquid subcooling. For a fixed Vc, the enhancement degree increases with Vj especially for Vc = 0.5 m/s, Vj = 2 m/s. As Vc increases, the heat transfer enhancement of jet impingement weakens and the increase rate of CHF (the critical heat flux) also decreases. The largest value of qmax (the maximum allowable heat flux) can reach 167 W/cm2 for chip with the fin dimension of 50 × 120 μm2 at the condition of Vc = 1.5 m/s, Vj = 2 m/s and liquid subcooling of 35 °C.

Enhanced flow boiling heat transfer of FC-72 on micro-pin-finned surfaces

For the purpose of cooling electronic components with high heat flux efficiently, some experiments were conducted to study the flow boiling heat transfer performance of FC-72 on silicon chips. Micro-pin-fins were fabricated on the chip surface using a dry etching technique to enhance boiling heat transfer. Three different fluid velocities (0.5, 1 and 2 m/s) and three different liquid subcoolings (15, 25 and 35 K) were performed, respectively. A smooth chip (chip S) and four micro-pin-finned chips with the same fin thickness of 30 μm and different fin heights of 60 μm (chip PF30–60) and 120 μm (chip PF30–120), respectively, were tested. All the micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth one, and the critical heat flux increases in the order of chip S, PF30–60 and PF30–120. For a lower ratio of fin height to fin pitch and/or higher fluid velocity, the fluid velocity has a positive effect on the nucleate boiling curves for the micro-pin-finned surfaces. At the velocities lower than 1 m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat, and the wall temperature at the critical heat flux (CHF) is less than the upper limit, 85 °C, for the reliable operation of LSI chips. The CHF values for all surfaces increase with fluid velocity and subcooling. The maximum CHF can reach nearly 150 W/cm 2 for chip PF30–120 at the fluid velocity of 2 m/s and the liquid subcooling of 35 K.

Flow boiling heat transfer in a vertical spirally internally ribbed tube

Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube (φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected, respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube. Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data, correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.

Enhancement of flow boiling heat transfer with surfactant

The surfactant additive octadecylamine (ODA) was used to enhance the flow boiling heat transfer of water in vertical copper tube, and the effects of the aqueous solution properties, mass fraction of ODA, mass flux and heat flux etc. on flow boiling heat transfer were investigated. In order to analyze the mechanism of enhancement on boiling heat transfer with ODA, the copper surface was detected by XPS, and the diagram of binding energy was obtained. The results show that ODA can be adsorbed on the surface of the copper wall, and affects the properties of the heating surfaces and enhances the flow boiling heat transfer of water. Only in low heat flux and in a suitable range of concentration, can ODA aqueous solution enhance flow boiling heat transfer, and the suitable mass fraction of ODA is in the range of 1 × 10−5 −5 × 10−5. In addition, compared with water, ODA aqueous solution does not increase the flow drag under the same experimental conditions.