The realization of the importance of high-performance heat transfer units, in this case a two-phase closed thermosyphon (TPCT), has gained a foothold in recent years. To this end, the ...super-hydrophobic coating has been applied in the TPCT condenser surface and 1-Butanol is added as a self-rewetting aqueous solution in the working fluid. The super-hydrophobic coating has been created by using surface engineering and synthesis super-hydrophobic SiO2 nanoparticles. In this work, in a bid to measure some well-established parameters (heat input, filling ratio, and coolant mass flow rate) which are capable of getting thermal efficiency and resistance surging toward desirable numbers, experimentally considered TPCT implement. In order to illustrate boiling and condensation phenomena, the numerically simulation have been conducted assists by volume of fluid (VOF) multiphase model and user-define function (UDF) house code. Results indicate that in the heat input of 250 watts, the condenser convective heat transfer coefficient of TPCT with super-hydrophobic coating is 13.34% higher than the TPCT with non-modified condenser. Also, increasing heat input, filling ratio and 1-Butanol mass concentration decreases the TPCT thermal resistance opposite of the coolant mass flow rate. The numerical simulation illustrates that the boiling in a TPCT with 1-Butanol-water working fluid has occurred faster than TPCT with water working fluid. Also, the condensation has been appeared droplet due to high contact angle in the condenser with super-hydrophobic coating.
•TPCT heat transfer characteristics havre been investigated experimentally.•1-Butanol is used as self-rewetting fluid in TPCT working fluid.•Super-hydrophobic coating is applied in condenser section of TPCT.•The numerical simulation has been conducted to illustrate phase change phenomena.
•A thin-film model is developed to study the modulation of chemical reactions.•Linear stability analysis shows that chemical reactions play diverse roles.•The interfacial temperature determines the ...direction of Marangoni flows.•Traveling wave solutions and transient simulations are performed for validation.
This paper examines the Marangoni instability of a self-rewetting film coating on a vertical heated fibre and its modulation by chemical reactions. A single evolution equation based on the classic thin-film model is derived to study the flow dynamics. Linear stability analysis shows that without chemical reactions, wall heating plays a stabilizing role when Ti>T0 (Ti is the interfacial temperature and T0 is the temperature corresponding to the minimum of surface tension in self-rewetting fluids), and it plays a destabilizing role when Ti<T0. However, chemical reactions can strongly modulate how the wall heating affects the flow dynamics. For both situations (Ti>T0 or Ti<T0), the endothermic reaction further enhances respective Marangoni effects, and the exothermic reaction suppresses respective Marangoni effects. In particular, a strong exothermic reaction can totally neutralize and even reverse Marangoni effects. These findings are further confirmed by performing nonlinear traveling wave solutions and transient numerical simulations.
•Applying self-rewetting fluid on the PCS can enhance HTC, but reduce CHF.•The decrease in CHF of self-rewetting fluid boiling is mainly caused by gas–liquid conflict.•Coupling the self-rewetting ...fluid and BPSs can significantly enhance boiling heat transfer.•Small pores promote liquid supply and large pores promote steam discharge inside the BPS.
Enhancing boiling heat transfer by changing the working fluid or surface modification is an effective and simple method. In order to investigate the effect of the coupling characteristics of porous sintered surfaces with self-rewetting fluid, pool boiling experiments were carried out on the polished copper surface (PCS) and the bi-porous sintered surface (BPS) with n-heptanol aqueous solution as the working fluid. The experimental results showed that, for the PCS cases, the self-rewetting fluid enhanced the heat transfer coefficient (HTC), while reduced the critical heat flux (CHF). This is because while self-rewetting fluid promoted the generation and detachment of bubbles, it also caused gas–liquid conflict, resulting in the surface was covered by a layer of vapor film. As for the boiling experiments on the BPS, the self-rewetting fluid not only enhanced the HTC but also enhanced the CHF. The Marangoni effect, generated by the self-rewetting fluid, replenished the BPS and coupled with its stronger capillary force. That led to surface wetting and reduced conflict between gas and liquid flow. Consequently, under this coupling effect, the heat transfer coefficient (HTC) of 0.1 wt% n-heptanol aqueous solution was 178.7 % higher than that of deionized water with PCS.
•Effects of DC and pulse duration are explored in high-temperature applications.•Intermittent spray of self-rewetting fluid has different dominant mechanisms.•Faster cooling with better thermal ...uniformity is realized with less fluid.•Spray pulse of 1.5 s and DC of 15% are preferred.
Comparing to continuous spray, intermittent spray is able to offer effective cooling by consuming less liquid. Targeting at relatively high-temperature applications, this study explores the influences of pulse duration and duty cycle (DC) in the cooling performance of the intermittent spray of a self-rewetting fluid, the 1.5% wt.% aqueous solution of 1-pentanol. Unlike water, using the self-rewetting fluid in intermittent spray is able to induce the inverse Marangoni convection which helps to replenish the hot spot with cool liquid even though the surface temperature exceeds 145°C. This leads to a further reduction in fluid consumption because comparable cooling rate can be delivered by spraying the self-rewetting fluid at a much lower frequency. Although dryout can still occur in the low DC regime, the inverse Marangoni convection is proven to sustain effective cooling during spray-off period in the high DC regime. As a result, cooling rate is nearly independent of DC in the high DC regime. For the self-rewetting fluid, cooling performance is not benefited from increasing the pulse duration as excess fluid simply drains out without contributing much to heat removal. Hence, a pulse duration of 1.5 s with a DC of 15% is recommended for the intermittent spray cooling of 1-pentanol/water mixtures at high wall superheats. Since the spray cooling efficiency of the self-rewetting fluid is always higher than that of water in this study, it opens up new possibility in spray quenching with more rapid cooling, better thermal uniformity, and further reduction in liquid consumption.
•Overview of the study on boiling heat transfer using SRWF.•The thermophysical properties of SRWF have been reported.•The pool and flow boiling experiments of SRWF have been summarized.•The heat pipe ...applications of SRWF have been summarized.
The self-rewetting fluid is the liquid which have an anomalous surface tension increasing with temperature when the temperature exceeds a certain value. This particular property can cause the subcooled liquid to be drawn towards the heated surface if a dry patch appears, thus making it possible for the self-rewetting fluids to become promising working fluids. This paper presents an overview of the recent developments of the research on boiling heat transfer using self-rewetting fluids. Thermophysical properties, pool and flow boiling experiments and heat pipe applications of self-rewetting fluids have been reported in this paper. It can be found that the use of self-rewetting fluids in a wide range of applications appears promising. However, further detailed and valuable theoretical and numerical investigations are necessary for us to better understand the boiling phenomenon of self-rewetting fluids.
A new annular structure of pulsating heat pipe (APHP) was designed for evacuated solar collector. To reduce the contact thermal resistance and alleviate the overheating problem, each tube of the ...pulsating heat pipe can be installed to fit snugly against the inner wall of evacuated tube. Experiments on filling ratio, heat transfer distance, inclination angle, and working fluid were carried out. The flow circumstances and heat transfer performance were analyzed based on the difference in frequency and amplitude of temperature oscillation curve and thermal resistance data. The results showed that high-frequency and low-amplitude temperature curve appeared when the APHPs were in the situation of low flow resistance, normally accompanied with better heat transfer effect. The optimum filling ratio and inclination angle are 60% and 60 °, respectively. The heat transfer performance of APHP decreased with the increase of heat transfer distance. The APHPs charged with self-rewetting fluid demonstrated a significant effect on heat transfer enhancement. Over different heat transfer distance, the application of self-rewetting fluid reduced the thermal resistance on average by 32.46%, 34.15% and 37.40%, respectively.
•An annular pulsating heat pipe was designed for solar evacuated collector.•The temperature oscillation characteristic was used to analyze the internal heat and mass transfer.•The excellent thermal performance of SRWF in three-dimensional pulsating heat pipe was obtained.
•A large-scale OHP was manufactured and injected with self-rewetting fluid.•The OHP with self-rewetting fluid has greater heat-transport limit and can work normally under the larger heat load.•The ...heat-transport capacity of the OHP can be increased by 13.19 times comparing to copper.•The OHP has lower thermal resistance and larger heat density compared to those of other research groups.
The maximization of oscillating heat pipe (OHP) has important significance on OHP based large-scale thermal energy storage (TES) system. In order to understand the thermal performance of OHP with long heat-transport distance for thermal energy storage (TES) application, a large-OHP testing platform was constructed. The influence of working medium type, heating load, cooling condition on the thermal efficiency, thermal resistance and effective thermal conductivity of OHP were investigated and analyzed. The results mainly showed that the OHP with self-rewetting fluid (SRWF) has greater heat transfer limit and can work normally under larger heating load compared to that with water or ethanol as working medium. The OHP with filling ratio (FR) of 40–80% can endure higher heating load compared to the case of FR=30%. It can be concluded that the best FR is about 40% under large heat load (the effective thermal conductivity reaches 5676Wm−1°C−1 at 700W). The SRWF based OHP with long heat-transport distance in this paper has lower thermal resistance and larger heat density compared to those of some other research groups.
This paper discusses the heat transfer enhancement of micro oscillating heat pipes (MOHPs) using self-rewetting fluid (SRWF). To clarify the heat transfer enhancement mechanism, the thermo-physical ...properties (including surface tensions, contact angles and thermal conductivities) of SRWFs and deionized water have been comparatively analyzed. Furthermore, to find out the strengthening effect, experimental studies were performed on MOHPs. During the experiments, MOHPs with heat transfer length (L) of 100, 150 and 200mm, consisting of 4 meandering turns and inner diameter (Di) of 0.4, 0.8, 1.3mm were adopted. SRWF and deionized water were employed as the working fluids. The results showed that, due to the unique property that the surface tension increases with increasing temperature, the SRWF can spontaneously wet the hotter region. The capillary resistance of the SRWF was much smaller than that of the deionized water, which is conductive to improving the circulation efficiency of the working fluid. Compared with the water, as the working fluid of the MOHPs, the SRWF exhibited much better thermal performance, which can decrease the thermal resistance and extend the effective operation range of MOHPs.
This study investigated the application of self-rewetting fluid to loop heat pipe (LHP) and its effect on the heat transfer performance of LHP, proposing a most suitable solute and concentration for ...LHP. At a certain temperature, a self-rewetting fluid, contrary to common newtonian fluids, has the ability to reverse its surface tension's trend, inducing colder fluid to flow to the heated surface and delaying the occurrence of dry-out in LHP during operation. The most important variables for a self-rewetting fluid are its concentration and solute. Thus, this study focused on a few self-rewetting fluids (including butanol, pentanol, and hexanol aqueous solutions); the surface tensions of these fluids at different concentrations were measured to find the best self-rewetting fluid, and then it was applied to LHP as working fluid to investigate its effect on the heat transfer performance.
Results of surface tension measurement showed that, concerning the concentration of a self-rewetting fluid, the optimal concentration for a self-rewetting fluid was its saturation concentration; concerning the solute of a self-rewetting fluid, after comparing all the tested working fluids at the optimal concentration, 6% butanol aqueous solution was the best type of self-rewetting fluid. Results from applying 6% butanol to LHP as working fluid for performance testing showed that the critical heat load was 650 W and the total thermal resistance was 0.25 ° C/W. Compared with LHP with regular working fluid, the critical heat load was 2.5 times higher and total thermal resistance decreased by about 60%, indicating high potential for self-rewetting fluids to enhance the heat transfer performance of LHPs.
•First applied self-rewetting fluid to LHP as working fluid.•Best concentrations of the self-rewetting fluids of the same solute were found.•Best type of self-rewetting fluid was found.•Self-rewetting fluid can enhance LHP's performance by 160%.
Heat transfer performances of the ultrapure water, n-butanol self-rewetting fluid (SRF), multi-walled carbon nanotubes (MWCNTs) nanofluid (NF), and n-butanol self-rewetting MWCNTs nanofluid (SRNF) ...were investigated in a pulsating heat pipe (PHP). The highlight of this study was to analyze their thermal performances under different heating powers, inclination angles, and ambient temperatures. Experimental results indicated that all functional working fluids generally gave advantages in thermal transport performances relative to the ultrapure water in most cases. Furthermore, advantage of the SRF gradually emerged with increasing thermal input when compared with the ultrapure water and its enhancement ratio reached a maximum of 22%. In contrary, advantage of NF was demonstrated only when the thermal input exceeded 20W, and its enhancement percentage reached 16% at 35W. The SRNF exhibited an outstanding increase of the thermal transfer performance at a low heating power range. The augmented percentage of the SRNF was around 7–17% with the maximum value obtained at 35 W. The inclination angle also had a non-negligible impact on thermal performances of different working fluids, particularly, their thermal transfer limits in the PHP. In addition, a lower ambient temperature could generally raise the heat transmission performance of the PHP with operating fluids.
