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On Parametric Effects of Wettability, Roughness and Porosity on Nucleate Pool Boiling Heat Transfer
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A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.
We demonstrate that smooth and flat surfaces combining hydrophilic and
hydrophobic patterns improve pool boiling performance. Compared to a
hydrophilic surface with 7 degree wetting angle, the measured critical heat
flux and heat transfer coefficients of the enhanced surfaces are up to
respectively 65 and 100% higher. Different networks combining hydrophilic and
hydrophobic regions are characterized. While all tested networks enhance the
heat transfer coefficient, large enhancements of critical heat flux are
typically found for hydrophilic networks featuring hydrophobic islands.
Hydrophilic networks indeed are shown to prevent the formation of an insulating
vapor layer.
The effect of surface roughness on pool boiling heat transfer is experimentally explored over a wide range of roughness values in water and Fluorinert (TM) FC-77 two fluids with different thermal properties and wetting characteristics. The test surfaces ranged from a polished surface (R-a between 0.027 mu m and 0.038 mu m) to electrical discharge machined (EDM) surfaces with a roughness (R-a) ranging from 1.08 mu m to 10.0 mu m. Different trends were observed in the heat transfer coefficient with respect to the surface roughness between the two fluids on the same set of surfaces. For FC-77, the heat transfer coefficient was found to continually increase with increasing roughness. For water on the other hand, EDM surfaces of intermediate roughness displayed similar heat transfer coefficients that were higher than for the polished surface, while the roughest surface showed the highest heat transfer coefficients. The heat transfer coefficients were more strongly influenced by surface roughness with FC-77 than with water For FC-77, the roughest surface produced 210% higher heat transfer coefficients than the polished surface while for water a more modest 100% enhancement was measured between the same set of surfaces. Although the results highlight the inadequacy of characterizing nucleate pool boiling data using Ra, the observed effect of roughness was correlated using h alpha R-a(m) as has been done in several prior studies. The experimental results were compared with predictions from several widely used correlations in the literature.
The effect of subcooling and length of hydrophobic-spot periphery on nucleate pool boiling heat transfer from TiO2-coated surface with and without PTFE (polyetatafluoroethylene) hydrophobic circle spots at intermediate heat flux has been examined. The experiments are performed with liquid subcooling ranging from 0-20 K and heat transfer block used were TiO2-coated copper block with a PTFE hydrophobic circle spot with various diameters with total area of PTFE being constant. Bubble nucleation and behavior were observed by using high-speed camera. The results showed that the heat transfer performance of surfaces with PTFE hydrophobic circle spot is better than superhydrophilic surface in overall condition. Furthermore, the heat transfer performance decreases under subcooled condition for all surfaces. Increase in peripheral length of hydrophobic-spot enhances the heat transfer performance.
Pool boiling heat transfer of metal foams with gradient pore densities has been investigated at atmospheric pressure in saturated deionized water. The gradient foams are made of two foam layers. The foam materials are copper and nickel. Pore densities of foam layers are from 5 PPI to 100 PPI, while the porosity remains the fixed value of 0.98. A parametric study is performed by varying the foam pore size and thickness. The effects of alumina nanoparticles and surfactant (SDS) on pool boiling heat transfer of gradient metal foams are also investigated in the present study. Images of foam fiber deposited by nanoparticles are captured by SEM. Compared to single-layer foams, gradient metal foams significantly enhance pool boiling heat transfer and the enhancement is heavily dependent on foam thickness and pore density gradient. The effects of SDS and nanoparticles on pool boiling heat transfer of gradient foams are dependent on concentrations.
This study reviews recent experimental investigations performed on pool and flow boiling over nano- and micro engineered structures for enhancements in boiling heat transfer, namely heat transfer coefficient (HTC) and critical heat flux (CHF). Modified surfaces having nano/micro porous features of mainly irregular shapes through anodic oxidation processes, coating of metallic and non-metallic layers, deposition of powder materials, and roughening for improving boiling heat transfer have been of research interests of many researchers. In addition, pool boiling and flow boiling studies on artificial structures, mainly fabricated on a plain surface, such as pins, pillar fins, grooves (in different shapes, i.e. rectangular, square, cylinder, etc) for increasing the heated surface area, or cavities created on substrates for increasing bubble nucleation sites were also considered for both micro and nano scale. The results reported in recent investigations on pool boiling and flow boiling from micro/nanostructured surfaces were included, and a comprehensive overview was provided.
With fast growing power consumption and device miniaturization, micro/minichannels are superior to macrochannels or conventional channels for high heat-flux dissipation due to their large surface area to volume ratios and high heat transfer coefficients. However, the associated large pressure drop penalty and flow boiling instability of micro/minichannels hinder their advancement in many practical applications. Therefore, enhancement techniques are required to stabilize the flow and further augment the heat transfer performance in micro/minichannels. This work first presents the classification of micro/minichannels for single-phase flow and flow boiling and gives a general statement of heat transfer enhancement. Then a state-of-the-art overview of the most recent enhancement techniques is specifically provided for further sing-phase flow and flow boiling enhancement in micro/minichannels. Two promising enhancement techniques, i.e., interrupted microfins and engineered fluids with additives are discussed for single-phase flow. For flow boiling, the focus is given on several selected enhancement approaches which can effectively mitigate flow boiling instability and another hot research topic, i.e., nanoscale surface modification. Besides, effects of wettability on bubble dynamics are presented, and a concept of flow-pattern based heat transfer enhancement is proposed. For both single-phase flow and flow boiling enhancement, a special emphasis is on those enhancement techniques with high thermal performance and relatively low pressure drop penalty.
We experimentally investigated surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces. Microstructured surfaces with a wide range of well-defined surface roughness were fabricated, and a maximum CHF of ∼208 W/cm2 was achieved with a surface roughness of ∼6. An analytical force-balance model was extended to explain the CHF enhancement. The excellent agreement found between the model and experimental data supports the idea that roughness-amplified capillary forces are responsible for the CHF enhancement on structured surfaces. The insights gained from this work suggest design guidelines for new surface technologies with high heat removal capability.
Experiments were conducted to study the effects of micro-pin-fins and submicron-scale roughness on the boiling heat transfer from a silicon chip immersed in a pool of degassed and gas-dissolved FC-72. Square pin-fins with fin dimensions of 50X50X60 mum(3) (widthXthicknessXheight) and submicron-scale roughness (RMS roughness of 25 to 32 nm) were fabricated on the surface of square silicon chip (10X10X0.5mm(3)) by use of microelectronic fabrication techniques. Experiments were conducted at the liquid subcoolings of 0, 3, 25, and 45 K. Both the micro-pin-finned chip and the chip with submicron-scale roughness showed a considerable heat transfer enhancement as compared to a smooth chip in the nucleate boiling region. The chip with submicron-scale roughness showed a higher heat transfer performance than the micro-pin-finned chip in the low-heat-flux region. The micro-pin-finned chip showed a steep increase in the heat flux with increasing wall superheat. This chip showed a higher heat transfer performance than the chip with submicron-scale roughness in the high-heat-flux region. The micro-pinfinned chip with submicron-scale roughness on it showed the highest heat transfer performance in the high-heat-flux region. While the wall superheat at boiling incipience was strongly dependent on the dissolved gas content, it was little affected by the liquid subcooling.
Anodizing technique has been recognized as an efficient way to grow the well-ordered oxide nano-structures on metal substrate. In the present experimental study, the nucleate pool boiling heat transfer coefficient and long-term performance of nano-porous surface fabricated by the cost-effective and simple anodizing technique were investigated with water. The incipient wall superheat of pool boiling in nano-porous surface was lower than that in non-coating surface. The nucleate boiling heat transfer coefficient of nano-porous coating surface appeared higher than that of non-coating surface particularly at the low heat flux condition. The higher coefficient remained throughout 500h of operation.
The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of a microporous enhanced and a plain (machine-roughened) reference surface. The test surfaces, 1-cm(2) flat copper blocks in the horizontal, upward facing orientation, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa, a liquid subcooling range of 0 (saturation) to 50 K, and both gas-saturated and pure subcooling conditions. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with the prevailing trends in the literature. For the present heater geometry, the effects of dissolved gas of the boiling performance were generally small, however; as the dissolved gas content increased (through either increased pressure or subcooling) more of the nucleate boiling curve was affected (enhanced). The enhancement of CHF from increased liquid subcooling was greater for the microporous surface than the plain surface. Correlations for both nucleate boiling and CHF were also presented.
Titanium Dioxide, TiO2, is a photocatalyst with a unique characteristic. A surface coated with TiO2 exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO2-coated and non-coated surfaces were used for comparison. In the case of the TiO2-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd.
Experiments were performed to highlight the influence of surface wettability on nucleate boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20° to 110° by modifying nanoscale surface topography and chemistry. The bubble growth was recorded by a high speed video camera to enable a better understanding of the surface wettability effects on nucleation mechanism. For hydrophilic (wetted) surfaces, it was found that a greater surface wettability increases the vapour bubble departure radius and reduces the bubble emission frequency. Moreover, lower superheat is required for the initial growth of bubbles on hydrophobic (unwetted) surfaces. However, the bubble in contact with the hydrophobic surface cannot detach from the wall and have a curvature radius increasing with time. At higher heat flux, the bubble spreads over the surface and coalesces with bubbles formed at other sites, causing a large area of the surface to become vapour blanketed. The best heat transfer coefficient is obtained with the surface which had a water contact angle close to either 0° or 90°. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer.
Boiling heat transfer from hybrid micro-nano structured thermal interfaces has been studied experimentally using PF5060 and deionized water as working fluids for the range of saturation temperatures Tsat between 35 °C and 60 °C. Various enhancement structures were fabricated using surface micromachining in silicon. The performance of the smooth and roughened silicon surfaces, bare and fully coated with carbon nanotubes (CNTs), the silicon-etched as well as CNTs-based pin-fin arrays, and the 3D microstructures have been studied in detail. The highest heat fluxes were obtained using the 3D microstructure without CNTs—these are 27 and 130 W/cm2 for PF5060 and water, respectively. Experimental results indicate that use of the CNT-enabled, purely nano-structured interfaces appear to improve boiling heat transfer only at very low superheats, as compared to the smooth surfaces.
Experimental investigation of pool boiling is conducted in stationary conditions over very smooth bronze surfaces covered by a very thin layer of gold presenting various surface treatments to isolate the role of wettability. We show that even with surfaces presenting mean roughness amplitudes below 10 nm the role of surface topography is of importance. The study shows also that wettability alone can trigger the boiling and that the boiling position on the surface can be controlled by chemical grafting using for instance alkanethiol. Moreover, boiling curves, that is, heat flux versus the surface superheat (which is the difference between the solid surface temperature and the liquid saturation temperature), are recorded and enabled to quantify, for this case, the significant reduction of the superheat at the onset of incipient boiling due to wettability.
The effect of copper (Cu) nanorod deposition on Cu substrate on the boiling performance for nanostructured Cu surfaces has been evaluated. Cu nanorods were deposited on a polished CU substrate by oblique-angle deposition, in which a flux of Cu atoms is incident on the substrate at a large incidence angle. The test facility used to evaluate the effect of nanorod deposition on the boiling performance consisted of a Cu block, liquid chamber, liquid supply system, and heating system. During the boiling tests, the power to the heater was increased and the process repeated until the critical heat flux (CHF) was achieved. Significant differences in the dynamics of bubble nucleation and release from the Cu nanorods, including higher bubble release frequencies and a considerable increase in the density of active bubble nucleation sites. Nanostructured Cu interfaces exhibited enhanced boiling performance at low superheated temperatures compared to untreated Cu, caused by such large increase in the density of active bubble nucleation sites.