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[en] Experimental investigations were carried out to study heat transfer characteristics of titanium (commercially pure titanium, TA2)/water two-phase closed thermosyphon (Ti/H2O TPCT). Experiments of copper/water (Cu/H2O) TPCT with same dimension and manufacturing process had also been performed for contrast. Experimental results show that there are no remarkable differences of heat transfer coefficients in evaporator (he) between the two kinds of TPCTs, whereas surprisingly the experimental results of heat transfer coefficient in condenser (hc) of Ti/H2O TPCTs are about 2-3 times more than that of Cu/H2O TPCTs, moreover the Nusselt's theoretical correlation based on laminar filmwise condensation is not suitable for simulating the hc of Ti/H2O TPCTs. Experimental results and theoretical analysis of surface free energy difference between condensate and solid surface indicate that the mixed condensation mode with dropwise and filmwise condensation coexisting on titanium surface result in the higher hc for Ti/H2O TPCTs. Experiments on condensation mechanism of titanium surface are ongoing to further validate the point.
[en] Highlights: • A variable conductance loop thermosyphon (VCLT) is designed. • VCLT behaviors under static and dynamic adjustment modes are examined with R134a. • The maximum ratio of heat transfer adjustment is approximately 85%. • The response time for each dynamic adjustment is generally within 30 s. - Abstract: Given that loop thermosyphons are widely used as simple and high-efficiency heat transfer devices, their passive heat transfer performance has attracted considerable research interest. The capability of loop thermosyphons for active heat transfer adjustment can potentially improve the precision and efficiency of heat and temperature management in many fields, but has not received much attention. A variable conductance loop thermosyphon (VCLT) is developed in this study. Heat transfer is precisely adjusted by actively regulating the internal flow resistance of the working fluid. The performance of VCLT under variable flow resistances is tested. The boundary conditions are set to those of a novel cool-storage refrigerator, wherein temperature control directly affects the freshness of stored food. VCLT behaviors under static and dynamic adjustment modes are also examined with the working fluid R134a. The maximum ratio of heat transfer adjustment is approximately 85%, when decreases the mass flow rate from 1.67 g/s to 0.21 g/s with increasing the equivalent thermal resistance from 0.0074 K/W to 0.54 K/W. Finally, the performance is verified with R404a, R407a and R410a. Results demonstrate that VCLT is an efficient and low-cost temperature adjustment device for refrigerators and other applications that require precise temperature control.
[en] Highlights: • Thermosyphon with phase change heat exchanger computational model. • Construction and experimentation of a prototype. • ±9% of maximum deviation from experimental values of the main outputs. • Influence of the auxiliary equipment on the net power generation. - Abstract: An important issue in thermoelectric generators is the thermal design of the heat exchangers since it can improve their performance by increasing the heat absorbed or dissipated by the thermoelectric modules. Due to its several advantages, compared to conventional dissipation systems, a thermosyphon heat exchanger with phase change is proposed to be placed on the cold side of thermoelectric generators. Some of these advantages are: high heat-transfer rates; absence of moving parts and lack of auxiliary consumption (because fans or pumps are not required); and the fact that these systems are wickless. A computational model is developed to design and predict the behaviour of this heat exchangers. Furthermore, a prototype has been built and tested in order to demonstrate its performance and validate the computational model. The model predicts the thermal resistance of the heat exchanger with a relative error in the interval [−8.09; 7.83] in the 95% of the cases. Finally, the use of thermosyphons with phase change in thermoelectric generators has been studied in a waste-heat recovery application, stating that including them on the cold side of the generators improves the net thermoelectric production by 36% compared to that obtained with finned dissipators under forced convection.
[en] Highlights: • The hybrid GCHP system with multi-functions is proposed. • The system maintains the soil temperature and heating reliability steady. • The multi-functional operation of HCUT can save more energy of the system. - Abstract: Underground thermal imbalance is a significant problem for ground-coupled heat pump (GCHP) systems that serve predominately heated buildings in cold regions, which extract more heat from the ground and inject less heat, especially in buildings requiring domestic hot water (DHW). To solve this problem, a previously developed heat compensation unit with thermosyphon (HCUT) is integrated with a GCHP unit to build a hybrid GCHP system. To improve the energy savings of this hybrid GCHP system, the HCUT unit is set to have multiple functions (heat compensation, direct DHW and direct space heating) in this paper. To analyze the improved system performance, a hotel requiring air-conditioning and DHW is selected and simulated in three typical cold cities using the dynamic software DeST and TRNSYS. The results indicate that the hybrid GCHP system can maintain the underground thermal balance while keeping the indoor air temperature within the design range. Furthermore, the HCUT unit efficiently reduces the energy consumption via its multi-functional operations. Compared to the previous system that only used HCUT for heat compensation, adding the direct DHW function further saves 7.5–11.0% energy in heat compensation (HC) and DHW (i.e., 3.6–4.8% of the whole system). Simultaneously adding the direct DHW and space heating functions to the HCUT can save 9.8–12.9% energy in HC and DHW (i.e., 5.1–6.0% of the whole system). The hybrid GCHP system with a multi-functional HCUT provides more energy savings while maintaining the underground thermal balance in cold regions that demand both air-conditioning and DHW
[en] Highlights: • Numerical study of nanofluid heat transfer in thermosyphon heat pipes is performed. • Effect of nanoparticle concentration and operating temperature are studied. • Fe2O3–water nanofluid with 5.3% volume concentration shows the best performance. • Results show the improvement the thermal performances of thermosyphon heat pipe with nanofluids. - Abstract: In this work, a three-dimensional analysis is used to investigate the heat transfer of thermosyphon heat pipe using water and nanofluids as the working fluid. The study focused mainly on the effects of volume concentrations of nanoparticles and the operating temperature on the heat transfer performance of the thermosyphon heat pipe using the nanofluids. The analysis was performed for water and γ-Fe2O3 nanoparticles, three volume concentrations of nanoparticles (0 vol.%, 2 vol.% and 5.3 vol.%) and four operating temperatures (60, 70, 80 and 90 °C). The numerical results show that the volume concentration of nanoparticles had a significant effect in reducing the temperature difference between the evaporator and condenser. Experimental and numerical results show qualitatively that the thermosyphon heat pipe using the nanofluid has better heat transfer characteristics than the thermosyphon heat pipe using water
[en] Highlights: • A micro-scale thermosyphon heat pipe technology for 3D chip cooling is proposed. • CHF for micro-scale thermosyphon can be roughly predicted by the conventional correlation. • Adding suitable SDS into R113 can significantly improve heat transfer in thermosyphon. • Adding suitable nanoparticles into R113 can further improve heat transfer in thermosyphon. - Abstract: A micro-scale thermosyphon heat pipe technology which passively utilizes 3D chip’s specific micro-channels structure as the evaporating section of the thermosyphon to form a thermosyphon boiling in micro-channels for 3D chip cooling is proposed. The maximum heat flux and heat transfer coefficient of thermosyphon boiling in micro-channels which simulated actual 3D chip structure were experimentally studied. Experiments were carried out using four kind of working liquids: two pure fluids (deionized water and R113), super-moist fluids (R113 + surfactant) and nanofluids (R113 + surfactant + CuO nanoparticles). The height and gap of channels used were in the range of 30 mm to 100 mm and 30 μm to 1 mm, respectively. Experimental results show that nanofluids can significantly enhance both the maximum heat flux and heat transfer coefficient of thermosyphon boiling in micro-channels due to super-wettability of the liquid and the formation of a porous sedimentary layer consisted of nanoparticles on the wall. The results show that the micro thermosyphon heat pipe structure is a promising technology for 3D chip cooling.
[en] Highlights: • Highly dispersed GNP-based water nanofluids are prepared with the microwave-assisted method. • Rheological and thermo-physical properties of all treated samples are shown good enhancements. • Different heat transfer parameters are investigated in a thermosyphon. • An industrially scalable and cost-effective route is introduced. - Abstract: Graphene Nanoplatelets (GNP) were stably dispersed in aqueous media by covalent and non-covalent functionalization. Covalent functionalization was performed by a rapid microwave-assisted approach. Surface functionality groups and morphology of acid-treated GNP were analyzed by Fourier transform infrared spectroscopy and transmission electron microscopy. The GNP-based water nanofluids were then prepared with different concentrations of GNP to evaluate the thermo-physical and rheological properties. It was found that the rheological and thermo-physical properties of all treated samples were significantly enhanced compared to the pure water. The amount of enhancement also increased as the weight concentration increased. Thermo-physical results also confirmed that the thermal conductivity varied significantly depending on the functionalization approaches. At a constant concentration, the measurement showed that the thermal conductivity of covalent nanofluid (GNP-COOH/water) is larger than the non-covalent nanofluid (GNP-SDBS/water), which is larger than distilled water. The GNP-COOH/water nanofluids were found to be especially more effective in the thermosyphon in terms of overall thermal properties such as net heat transfer, entropy, and thermal efficiency, and rheological property such as effective viscosity, as well as, total pressure drop in comparison to GNP-SDBS/water nanofluids and certainly distilled water. The relative degradation of thermal conductivity and heat transfer efficiency of non-covalent nanofluids (GNP-SDBS/water) is due to the reduction of effective heat transfer surface of GNP nanoparticles in suspension, implying lower formation of surface nanolayers. Since the covalent functionalization with microwave radiation is a fast and cost-effective, it would provide an economical approach for industrial applications, an environmentally friendly alternative to the surfactant methods
[en] Highlights: • Atmospheric plate thermosyphon (APT) cooling solar cells was first proposed. • APT can reduce the temperature of PV panels without parasitic energy consumption. • The maximum temperature difference was less than 6 °C. • The heat transfer resistance at the evaporator is between 0.00486 and 0.02368 K/W. - Abstract: Since the heat pipe has no parasitic energy consumption, it is an important method for cooling the photovoltaic. In this paper, a novel type of atmospheric plate thermosyphon (APT) cooling system has been designed, which can be used for the heat dissipation of the single or low concentrated solar cells. In the experiments, the non-condensable gas (NCG) was collected by a gas reservoir. The coolant, ethanol, formed a liquid film on the porous medium and directly cooled the photovoltaic panel. The effect of various parameters such as heat flux density, tilt angle and inlet temperature have been studied. The results demonstrated that APT cooling system could effectively reduce the temperature of PV cells, and the higher heat flux density was, the shorter start-up time. The temperature of evaporator was uniform, and the larger inclined angle was, the greater surface temperature difference which maximumly was 5.6 °C. The thermal resistance at the evaporator was between 0.00486 and 0.02368 K/W.