On course to maximization and optimization of a two-phase closed thermosyphon, this paper expounds on multiple impactful factors in an experimental and numerical setting. Among various fluids, ...1-Butanol with 6% concentration, due to having the largest difference in surface tension with water, engendered the most favorable outcome. Response Surface Method has been employed to develop two correlations for efficiency and the thermal resistance of the thermosyphon paving the way for more accurate quantifications without necessarily running costly experiments. Other studied factors based on heat transfer coefficients of evaporator and condenser have been analyzed. The resultant numbers suggest an increase up to roughly 40% and 9% by changing power input and fluid, respectively. Also, enhancements falling within the range of 16 to 20% with the employment of 6% water-Butanol were observed by manipulation of filling ratio. However, coolant mass flow rate influence was limited to less than 10% when exposed to a 40% jump in flow rate if the working fluid was the same, implying its poor dominance in thermal performance of thermosyphon. As 1-Butanol proved superior among enhancing determinants, two numerical cases based on this factor were analyzed, confirming experimental findings while concurrently revealing distinctive phase-change patterns arising from the self-rewetting fluid.
•Effect of different self–rewetting fluids on TPCT performance has been investigated.•The 1-Butanol, 1-Pentanol, and 1-Hexanol have been selected as self–rewetting fluids.•Effect of heat input, FR and coolant flow rate on TPCT performance have been studied.•Water/1-Butanol 6% has augmented the he by 8.6% at 250 W comparing with pure water.•Numerical simulation has been done for water and Water/1-Butanol 6% charged TPCT.
•New analytical solution for thermocapillary convection in self-rewetting fluids (SRF) layers in a microchannel.•Interface surface tension equation of state modeled with parabolic dependence on ...temperature.•Lattice Boltzmann (LB) method based on central moments for simulation of SRFs developed and validated.•Thermocapillary flow patterns in SRF layers shown to be markedly different from normal fluid layers.•Effect of surface tension sensitivity parameters, fluid thickness ratio, thermal conductivity and viscosity ratios studied.
Self-rewetting fluids (SRFs), such as aqueous solutions of long-chain alcohols, exhibit anomalous quadratic dependence of surface tension on temperature having a minimum and with a positive gradient. When compared to the normal fluids (NFs) that have negative gradient of surface tension on temperature, the SRFs can be associated with significantly modified interfacial dynamics, which have recently been exploited to enhance flow and thermal transport in various applications. In this work, first, we develop a new analytical solution of thermocapillary convection in superimposed two SRF layers confined within a microchannel that is sinusoidally heated on one side and maintained at a uniform temperature on the other side. Then, a robust central moment lattice Boltzmann method using a phase-field model involving the Allen-Cahn equation for interface tracking, two-fluid motion, and the energy transport for numerical simulations of SRFs is constructed. The analytical and computational techniques are generally shown to be in good quantitative agreement with one another. Moreover, the effect of the various characteristic parameters on the magnitude and the distribution thermocapillary-driven motion is studied. The thermocapillary flow patterns in SRFs are shown to be strikingly different when compared to the NFs: For otherwise the same conditions, the SRFs result in eight periodic counterrotating thermocapillary convection rolls, while the NFs exhibit only four such vortices. Moreover, the direction of the circulating fluid motion in such vortical structures for the SRFs is found to be towards the hotter zones on the interfaces, which is opposite to that in NFs. These features are found to be sustained even as the interfaces deforms in simulations. By tuning the sensitivity coefficients of the surface tension on temperature, it is shown that not only the magnitude of the thermocapillary velocity can be significantly manipulated, but also the overall flow patterns as well. It is also demonstrated that the thermocapillary convection can be enhanced if the SRF layer adjacent to the nonuniformly heated wall is made relatively thinner or has higher thermal conductivity ratio or has smaller viscosity when compared to that of the other fluid layer. The peak Marangoni velocity is found to be increased by a factor of 2 by doubling the dimensionless quadratic surface tension sensitivity coefficient and by about an order of magnitude as the fluid thickness ratio is changed from 1/3 to 3.
An experimental study is reported about a copper closed-loop flat plate pulsating heat pipe tested in both horizontal and vertical -bottom heated mode- orientations and at two cold source ...temperatures (20 °C and 40 °C); several working fluids were tested after having modified of their surface tension compared to pure water, verified through dedicated sessile droplets test bench. After which, tests were performed using pure water, pure ethanol, binary aqueous mixtures (water/ethanol, water/butan-1-ol and water/butan-2-ol, the last two being self-rewetting fluids) and water/surfactant mixtures (Tween® 20 and Tween® 40) as working fluids. The thermal performances of the device tested in horizontal orientation were significantly better for the aqueous mixtures such as water/ethanol and water/butan-2-ol than for the others (with values of thermal resistances down to 63%, and 52%, respectively, lower than those of pure water at 20 °C cooling temperature). Concerning pure fluids, the device presents severe temperature and pressure instabilities. Water with surfactants also presents enormous instabilities, calling into question the interstitial influence of the tested surfactants in the hydraulic slug flow pattern occurring in horizontal inclination. The results in vertical orientation, although often studied in the literature, are less conclusive and more difficult to analyze given that surface tension effects are less influential compared to the gravity effects in the annular or bubbly flow patterns occurring in this operating mode. Finally, zeotropic mixtures with butan-2-ol and ethanol performing very similarly, and considering their identical surface tension, the expected rewetting effect of SRWF is not proven, and may not even explain improved FPPHP operation, which is rather assumed to be largely caused by enhanced wettability or increased (∂P/∂T)sat.
Display omitted
•A copper Flat Plate Pulsating Heat Pipe was tested with pure fluids, alcohol aqueous mixtures and water/surfactants mixtures.•Thermal performances of the pulsating heat pipe are improved with zeotropic binary aqueous mixtures compared to pure water in horizontal orientation.•Surfactants with water do not improve thermal performances of flat plate pulsating heat pipe tested in horizontal orientation.•The effect of wettability is hardly notable on thermal performances of flat plate pulsating heat pipe tested in vertical orientation.•The operation improvement is mainly due to wettability rather than self-rewetting effect induced by adding butan-1-ol or butan-2-ol in water.
In this study, the stability, surface tension, and thermal conductivity of Al2O3/water nanofluids with different types of alcohol fluids were investigated. Different types of alcohols (butanol, ...pentanol, and hexanol) were added to the Al2O3/water nanofluid. From the UV–vis spectroscopy results, Al2O3/water nanofluids did not show a significant absorbance difference for concentrations more than 0.5 wt%. From 0.1 to 0.5 wt% the absorbance enhancement ratio was 101.3%; however, it was 9.0% from 0.5 to 0.9 wt%. Therefore, 0.5 wt% Al2O3 nanofluids are effective in terms of fluid dispersibility. During the zeta potential measurement of the nanofluid stability, increasing the Al2O3 in the nanofluids caused more instability. However, the type and concentration of the aqueous alcohol solutions without Al2O3 did not significantly affect the stability characteristics. The contact angle of the alcohol-based nanofluids at saturated concentrations showed a decrease of at least 18.7%. Adding Al2O3 nanoparticles to the alcohol-based nanofluids increased the surface tension by an average of 3.4%. The thermal conductivity of the alcohol-based nanofluids was lower than that of distilled water. However, the addition of Al2O3 can enhance their thermal conductivity. The thermal conductivity lowered by the alcohol-based fluids enhanced by an average of 73.4% after adding Al2O3.
The aim of this study is to oversee the impact of various techniques on thermal performance of heat pipes and to comprehensively cover the progress made so far in improving thermal performance. ...Thermal performance of heat pipes has been considerably improved by applying very novel techniques proposed by different investigators. Some major techniques have been reviewed and discussed: use of nanofluids, manufacturing different types of grooves and fins, use of different types of wicks, by inner surface treatment, use of self-rewetting fluids, use of embedded heat pipes that is passive cooling mechanism, using various inclination angles in heat pipes, etc. The presented study concludes that there are diverse methods for thermal performance enhancement of heat pipes each having its own impact level and constraints such as optimized parameters, manufacturing constraints, economic feasibility and commercial barriers. Some techniques show reversion in results when exceeded a certain level, e.g., crossing optimum concentration of nanofluid would reduce thermal performance of heat pipes. This review yields enough knowledge to optimize alteration parameters to get maximum augmentation in results and provides strong insight to decide about, which specific technique should be used for a case.
Graphic abstract
Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, ...functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.
•Gold nanofluid and butanol self-rewetting fluid represent a new and innovative class of heat transfer fluids for advanced cooling.•An experimental analysis is performed on the capillary evaporator ...section of capillary heat pipe.•Thermal performance is investigated by measuring the thermal resistance of the evaporator.•Gold nanofluid, with 1% Cv, showed a decreasing in the thermal resistance of the system by 13%.•A mixture of self-rewetting gold nanofluid showed the highest thermal performance by decreasing the thermal resistance by 22%.•The phase change is visualized by infrared camera.•This improvement has been explained due to the thermo-physical properties of nanofluid and self-rewetting fluid.
This paper presents an experimental analysis of the phenomenon of phase change inside a porous medium using different types of working fluids. It represents the impact of these fluids on improving the characteristics of heat and mass transfer in a Capillary Heat Pipe (CHP). In this study, gold nanoparticles (5 nm in diameter with 1% Cv), a self-rewetting binary solution (butanol with 3% Cv), and a mixture of self-rewetting butanol and gold nanofluid are considered to be the operating fluids within the CHP. The experiments are carried out after designing and developing the capillary heat pipe section. It consists of a water tank with a pump, an evaporator attached to a copper porous medium on which thermocouples and power supplies are placed. The experimental results showed the positive influence of gold nanoparticles on the thermal system's performance by reducing the thermal resistance by 13% compared to pure water as the base working fluid. In addition, a self-rewetting butanol solution showed improvement in the performance of the capillary evaporator by decreasing its casing temperature. While a mixture of self-rewetting butanol solution (3 % Cv) and gold nanofluid (1% Cv) exhibited the best performance of heat and mass transfer performance by reducing the thermal resistance of the system by approximately 22 %. To explain the mechanism for improving heat transfer, the phase change phenomenon was visualized by an infrared camera for the three working fluids. It is shown that as the applied power increases, the shape of the vapor pocket developed in the wick also increases, for pure water, until it reaches a stable form. Whereas, with respect to nanofluid and self-rewetting fluid, the shape of the vapor pockets was smaller than that of pure water allowing more efficient mass and heat transfer. The thermophysical properties of these fluids such as thermal conductivity, stability, surface tension, Marangoni, wettability, and capillary forces were presented to ensure and validate the decrease in the vapor pocket as well as the enhancement of the CHP thermal system.
•Self-rewetting fluids can enhance capillary evaporator performance.•The phase change is visualized.•The enhancement of capillary evaporator with self-rewetting fluids is explained.
This paper ...presents an experimental study of the phase change phenomenon in a porous medium. The tested working fluids are pure water and butanol aqueous solutions with different concentrations. In contrast to ordinary fluids, the surface tension of self-rewetting fluids exhibits a positive gradient beyond a certain temperature value. The experimental results indicate that the use of self-rewetting fluids (water/butanol) as working fluid significantly improves the performance of the capillary evaporator by decreasing the casing temperature. To explain the heat transfer enhancement mechanism, the phase change phenomenon is visualized for the two working fluids. It is shown that as the applied power increases, the shape of the vapor pocket that developed within the porous wick also increases for pure water until it reaches a stable shape. With respect to self-rewetting fluids, the shape of the vapor pocket decreases with increasing applied power allowing more efficient mass and heat transfers. Wettability, capillary pressure and Marangoni forces are the factors related to surface tension and contact angle that seem to be responsible for this heat transfer improvement for self-rewetting fluids.
Boiling experiments of pure water, aqueous n-butanol solutions and pure butanol were conducted in arrays of parallel microchannels with a cross-section of 25×25μm and 50×50μm. The introduction of 2% ...and 6% n-butanol solutions into microchannels with the mass fluxes ranging from 83kg/m2s to 208kg/m2s demonstrated an enhanced heat transfer during boiling compared to pure water and pure butanol. Both concentrations of butanol lowered the maximum temperature measured during boiling in the microchannel test section for approximately 10K and 30K compared to pure water and pure butanol, respectively. High-speed visualization, measurements of the contact angles and analysis of the surface roughness indicated that enhanced heat transfer originates from the improved wettability of the butanol solutions during boiling in microchannels, which is directly related to the positive surface tension gradient and the Marangoni effect. The self-rewetting property of the butanol solutions stimulated the formation of a well pronounced annular flow, enhanced the heat transfer and substantially lowered the temperatures measured in the microchannels during boiling.
•Flow visualization in a groove heat pipe model.•Effects of Marangoni convection on two-phase systems.•Reverse Marangoni effect on dry-out limit of two-phase heat transfer devices.
Experiments are ...performed to study the fundamental physics and the two-phase flow heat transfer in diluted water/butanol solutions with unusual dependence of the surface tension with temperature (self-rewetting fluids). The heat transfer and the dry-out behaviour are investigated in a grooved heat pipe model, increasing the power supplied at the evaporator section with an electric cartridge heater and cooling the condenser section with a water loop. The experimental configuration includes a top transparent window and a lighting system with diffused light, enabling visualization of the liquid in the groove with a CCD camera. Self-rewetting fluid behaviour is compared with ordinary heat transfer liquids, including an environmentally sustainable engineered liquid, Novec™ 7100 (Methoxy-nonafluorobutane), and bi-distilled water. Experiments with ethanol, butanol and a water/ethanol mixture have been also carried out to compare self-rewetting fluids with ordinary liquids and binary solution having no minimum in surface tension. It is found that the heat pipe filled with Novec™ exhibits good heat transfer performances at relatively low input power and temperatures, before the evaporator begins to dry. Similar behaviour is observed when ethanol is considered. For the heat pipe filled with water the evaporator becomes completely dry at larger power (40W). At the same input power, in the case of the self-rewetting fluid, the liquid is strongly flowing from the cold to the hot region due to the inverse Marangoni effect and this beneficially, rewets the evaporator. This phenomenon is never observed when dry out is established with other working fluids. Numerical simulations are carried out to explain some experimental results in case of pure liquid. A possible explanation of rewetting in the case of water/butanol mixture is given.
PDF
