•Boiling of self-rewetting fluid in ppi-gradient foam structure was tested.•Bubble departure phenomenon was substantially attenuated.•One-layer structure can enhance boiling obviously for water, but ...slightly for the solution.•Foam of 110 ppi and its gradient structure enhance boiling obviously for the solution.•The enhancement combines the surface properties and the liquid interfacial properties.
Light-weight and high-surface-area metal foams used in phase change heat transfer may suffer flow resistance from the porous matrix and cause boiling deterioration. To alleviate the flow resistance, metal foams with pore-density gradient was proposed and significant enhancement of pool boiling heat transfer was achieved for fluids such as water and refrigerants. In this work, a self-rewetting fluid (aqueous n-butanol solution) was used for boiling on copper foams with pore-density gradient structures formed by using several layers of foam covers. The experimental results show that, comparing with the one-layer foam, the bubble departure phenomenon was substantially attenuated due to the largely increase of pore density and hence the bubble moving resistance when using a two- or three-layer foam structure. However, the increase of pore density can enhance the pool boiling of water when the foam thicknesses are the same due to more active cavity sites being formed in a denser metal foam. While the enhancement for the solution is not obvious especially for that in the foam structure with higher pore density and heat transfer deterioration may emerge at high heat fluxes, the boiling heat transfer of the solution can generally be enhanced by using the 110 ppi foam and its gradient structures as compared to the polished surface. This provides new insight into enhancing the boiling heat transfer utilizing both the surface properties formed in the pore-density gradient structure and the unique interfacial properties of the self-rewetting fluids.
•The thermal and solutal Marangoni number have been quantitatively analyzed.•The positive thermal Marangoni number of SRWF indicates positive Marangoni flow.•The solutal Marangoni numbers of the SRWF ...exceed magnitude of 106.•The flow patterns of Marangoni flow with difference fluids have been discussed.•The CHF of the SRWF increased up to 1.91 times the CHF of water.
To clarify the Marangoni effect on nucleate pool boiling with different working fluids, a serial boiling experiments with the monocomponent fluid (water), the tradition binary fluid (5 wt% ethanol aqueous solution) and self-rewetting fluid (5 wt% butanol aqueous solution) have been comparatively carried out. During the pool boiling experiments, a boiling system using a horizontal heated wire was employed. The experimental results show that, due to Marangoni convection induced by the surface tension gradient, the critical heat flux (CHF) of the binary fluid (5 wt% ethanol aqueous solution) increased up to 1.52 times that of water, and the CHF of the self-rewetting fluid (5 wt% butanol aqueous solution) increased up to 1.91 times that of water. Utilizing dimensionless parameters with thermal Marangoni number (MaT) and solutal Marangoni number (MaS), the Marangoni effect have been quantitatively analyzed. It can be concluded that the Marangoni convection should be the key factors causing the pool boiling heat transfer enhancement.
Nonlinear dynamic analysis of a self-rewetting fluid (SRWF) film flowing down the surface of a vertical cylinder is performed in this paper. The effect of the Biot number, Marangoni number and the ...substrate curvature on the interfacial evolution patterns and flow stabilities are discussed by linear stability analysis and numerical simulations of the evolution equation of the film thickness. Starting from the characteristic temperature T0 relating to the minimum surface tension and the interfacial temperature Ti, the nonlinear dynamics of the liquid film is investigated numerically in the cases of either Ti>T0 or Ti<T0. Good agreement of linear stability analysis with numerical simulations proves that the LSA could predict the development of thin liquid film flows in the early-time evolution. Through the analysis, we demonstrate that the Marangoni number Ma and the Biot number Bi play contrary roles for the two cases. For Ti<T0, the fingering instability is enhanced by the Marangoni effect while the inverse Marangoni effect stabilizes the interface for Ti>T0. The growth rate changes linearly with the increase of Marangoni number while it changes in forms of arched shapes versus lg(Bi). The growth rate reaches a maximum/minimum value at Bi=1, corresponding to the most/least unstable state. The radius of the cylinder R plays a significant role in the long wavelength modes, showing a stabilizing effect. For perturbations of short waves, increment of R expands the instability region. Nonlinear oscillatory waves (Rayleigh–Plateau instability) appears when the radius is smaller than 1 and fingering pattern tends to occur if the cylinder has a large radius (R≥1).
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•Instability regimes of the thin film flows is directly affected by Marangoni effect or inverse Marangoni effect, due to the nonlinear surface tension property of self-rewetting fluid.•The Biot number Bi=1 corresponds to the most unstable or least stable state, while the stability or the instability is strengthened with the increase of the Marangoni number.•The radius of the cylinder R plays a significant role in the long wavelength modes independent of the thermal parameter Φ, showing a stabilizing effect.
•The thermal and solutal Marangoni number have been quantitatively analyzed.•The strong Marangoni convection generated along the interface induces MEB.•The solutal Marangoni effect should be the main ...factor to induce MEB.•3 × 106 should be the critical magnitude induces the MEB for 0.1 wt% heptanol solution.•The thermal performance of the SRWF is improved with the occurrence of MEB.
In order to clarify the Marangoni effect on microbubbles emission boiling (MEB) generation during the pool boiling of self-rewetting fluid (SRWF), a plenty of boiling experiments were carried out, which used heptanol aqueous solution as SRWF. A horizontal platinum wire was used as heated surface during the pool boiling experiments. Additionally, a high speed video camera was used to record the nucleation boiling process. The experimental results show that, the MEB phenomenon only appeared in the heptanol aqueous solution case. Due to the occurrence of MEB phenomenon, the 0.1 wt% heptanol aqueous solution shows much better critical heat flux (CHF) and heat transfer coefficient. Furthermore, the Marangoni effect was quantitatively analyzed combining dimensionless parameters of the thermal Marangoni number (MaT) and solutal Marangoni number (MaS). The results show that, the Marangoni convection along the vapor/liquid interface is the key factor to induce the MEB. Since the Marangoni number reaches about 3 × 106 when the MEB appeared, the Marangoni number of 3 × 106 should be the critical magnitude that induces the MEB phenomenon for 0.1 wt% heptanol aqueous solution in this case.
The thermo-physical properties of working fluids play an important role in the heat transport performance of an oscillating heat pipe (OHP). In the present study, the heat transport performance of ...the OHP is investigated using two types of binary fluids and a type of ternary fluid as the working fluids. The working fluids include a nanofluid, self-rewetting fluid, and a mixed solution of self-rewetting fluid and a nanofluid called self-rewetting nanofluid. The tilt angle of the OHP is 90° with a charge ratio of 50%. In contrast to de-ionized water used as a base working fluid in the OHP, all working fluids can enhance the heat transport performance of the OHP. However, through an analysis of the enhancement ratios, it is found that nanofluids can only enhance the performance of an OHP within a heat load range of 30–70W. The maximal enhancement ratio is about 11% as the heat load is 60W. If the heat load exceeds 70W, the heat transport performance of the OHP degrades. For a self-rewetting fluid, the maximum of enhancement ratio is only 6% as the heat load is less than 30W, and then, as the heat load increases, the enhancement ratio decreases gradually. The OHP that uses self-rewetting nanofluids shows excellent heat transport performance over the entire heat load range. The maximal enhancement ratio is approximately 15%. Therefore, self-rewetting nanofluids are considered to be appropriate working fluids for use in the OHPs.
•An OHP with external expansion structure was designed and fabricated.•The variation law of the overall and local thermal resistance was revealed.•The OHP with water shows well adaptability to the ...heat load and filling ratio.•The OHP with SRWF has smaller thermal resistance at specific range of heat load.
Aiming to combining the strength of flexibility and adaptation of oscillating heat pipe (OHP) with specific application in the field of waste heat recovery and storage, an OHP with external expansion structure was designed and fabricated. The start-up characteristic, working status, overall thermal resistance, local thermal resistance and dispersion ratio for the local thermal resistances under different filling ratios (FR), working fluids and heat loads were revealed and compared experimentally. The results showed that for the OHP with water, it exhibits well start-up performance under wide range of FRs and is able to work effectively under wide range of FR at proper heat load. Meanwhile, it also shows well adaptability to heat load under proper FR. The OHP has not obvious difference for each branch in the heat transfer performance under wide working conditions. When filled with self-rewetting fluid (SRWF), it has smaller overall thermal resistance than the water case at specific range of heat load. Comparing to the case with water, the OHP with SRWF can show better uniformity in the heat transfer capacity for each branch under appropriate condition. However, the uniformity of branch does not always contribute toincreasing the heat transfer performance.
•Self-rewetting and ordinary mixture liquids evaporating in capillaries are experimentally studied.•The walls of the capillaries are heated with a transparent heating and various powers are ...applied.•The evaporation rates of the liquids inside the capillaries are measured.•Dependence of evaporation on applied powers is highlighted.•Thermocapillary effects are analysed.•Wettability of the liquids inside the capillaries is measured and reported.•Convection within the evaporating liquid is reported.
Evaporation heat and mass transfer in open microchannels was investigated using pure liquids (ethanol, deionized water and butanol), an ordinary (non-self-rewetting) mixture (5% v/v ethanol/water mixture) and a self-rewetting fluid (5% v/v butanol/water mixture). The applied heating power ranged from 0 mW to 282.76 mW (giving heat fluxes from 0 W/m2 to 4501 W/m2) with wall temperatures ranging from 24 °C to 95 °C. When the microchannel had no external heating, diffusion was found to be the main mechanism of mass transfer. With external heating, the diffusion model underestimated the experiment results, and buoyancy-driven convection may account for this. Evaporation rates remain high for the self-rewetting fluid even when the meniscus recedes significantly (by 6 times the inner diameter) in the microchannel under heating conditions. Additionally, the self-rewetting fluid showed the highest conduction rate in the axial direction in high heating power conditions. Meniscus-wall contact angles and flow visualizations were obtained for various heating rates. The “contact angle shift” phenomenon of the self-rewetting fluid was observed. Marangoni-induced convection in the microchannel accounts for the mechanism of better heat and mass transfer as well as the “contact angle shift” phenomenon for the self-rewetting fluid.
The attractive and repulsive behaviours of a pair of initially spherical gas bubbles rising side-by-side in a channel with non-uniformly heated walls containing a self-rewetting liquid are ...investigated numerically. The surface tension of a self-rewetting fluid exhibits a parabolic temperature dependence with a well-defined minimum, as opposed to linear (common) fluids whose surface tension decreases almost linearly with the increasing temperature. It is found that, for low Reynolds numbers, while in an isothermal medium, two gas bubbles display a repulsive behaviour, they attract in non-isothermal systems. The bubbles in the self-rewetting fluid undergo a plastic collision and show a ‘squeezing and relaxing’ behaviour, whereas they attract and then bounce in the linear fluid. A regime map demarcating the repulsive and attractive behaviours for a self-rewetting fluid is plotted in the Weber number (We) and the dimensionless linear component of the surface tension gradient (M1) space. It is found that the bubbles in the self-rewetting fluid remain spherical even for high Weber numbers while they deform considerably in the case of the linear fluid indicating that the attractive behaviour of the bubbles in the self-rewetting fluid is due to the lift force generated by the thermocapillary stresses and not due to the deformation. The mechanism underlying the observed phenomenon is elucidated by studying the drag and lift forces acting on the bubbles, their orientations, and the flow field around them.
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•Attractive and repulsive behaviours of a pair of spherical gas bubbles is studied numerically.•The channel contains a self-rewetting liquid which exhibits a parabolic temperature dependence.•In non-isothermal systems, the bubbles display an attraction for low Reynolds numbers.•In self-rewetting fluid, the collision is plastic.•The mechanism is elucidated through the drag and lift forces.
To clarify the fundamental heat transfer characteristic and heat transfer enhancement of subcooled pool boiling with self-rewetting fluid (SRWF), using dilute heptanol aqueous solution as SRWF, a ...serial boiling experiments have been carried out. During the experiments, a boiling system using a horizontal heated wire was employed. The boiling performance of SRWF with different subcooling (1 degreesC, 5 degreesC, 10 degreesC, 15 degreesC) and different concentrations (0, 0.01 wt%, 0.05 wt%, 0.1 wt% heptanol aqueous solution) have been demonstrated in this paper. The experimental results show that, the higher subcooled temperature leads to the better heat transfer performance, and it can effectively prevent the dry out phenomenon. Due to better surface tension and concentration gradient, the 0.1 wt% heptanol aqueous solution shows the best critical heat flux, which is beneficial for increasing the dry out limit. With a high speed video camera, the nucleation boiling process on the heated wire has been recorded. Furthermore, it is found from the boiling phenomenon that, the boiling bubbles of SRWF are visualized to be much smaller and more, which is conducive to the application in small thermal devices.
Recently, many countries have become increasingly interested in unused but possibly useful energy resources. Among these unused resources, the thermal energy produced around us can be used as a ...potential energy source for heating, cooling and power generation. This thermal energy is relatively stable on the supply side as waste heat in the industrial field. Heat transport devices are one of the important technology for the effective use of unused heat energy. This paper conducts basic research on devices that effectively transport heat below 200ºC. A pulsating heat pipe (PHP) is an excellent heat transport device based on the phase change of a working fluid. Experiments are performed to investigate the thermal performance of a PHP using different working fluids. The PHP consists of 20 parallel channels made of a copper capillary tube with an internal diameter of 1.8 mm. The PHP is filled with deionized water and an aqueous solution of 1-butanol as working fluids, with different filling ratios (FRs) in the range 50-60 vol.%. The 1-butanol aqueous solution is known as a self-rewetting fluid. The experimental results indicate that, in the case of self-rewetting fluid, stable oscillating motion in the PHP arises at the heat load regime lower than that with water. In addition, the effective thermal conductivity of the PHP with the highest concentration of self-rewetting fluid is higher than that with other fluids in the high heat load regime.
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