Heat Transfer Enhancement

Advances in technology miniaturization with increasing power density call for new technologies for enhancing heat transfer. Enhancement of heat transfer with the use of nanofluids has been a hectic topic of research and development since the term “nanofluid” was first used in 1995, mainly because the thermophysical properties of nanofluids in most reports in the literature showed supremacy or improvement over their base fluids, which may not allow fulfillment of the present cutting-edge technology needs. Significant progress in this field has been made in the past two decades. This review summarizes a variety of the experimentally measured thermal properties of common nanofluids, the enhancement mechanisms discovered or hypothesised, the models used for properties and heat transfer characteristics, and the applications of nanofluids for enhancing heat transfer. The model of an artificial neutral network is particularly emphasized. Applications to cooling technology, renewable energy and energy systems, and building technology are detailed. Challenges and areas for future research are identified.

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A review on heat transfer enhancement with nanofluids

Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems’ thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).

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A Review of Recent Passive Heat Transfer Enhancement Methods

Enhancing heat transfer is crucial for improving the performance of heat transfer applications. Therefore, a novel method is proposed in this study for improving the heat transfer using a new rib geometry within a circular channel. The performance of the novel method was evaluated by comparing the results of the channel with ribs with that of an ordinary channel. In this study, numerical investigations were conducted on the temperature distribution and fluid flow properties of a 500 mm tube with an external diameter of 50 mm and an internal surface of 47 mm. To evaluate the effects of the ribs on the heat transfer via the tube wall, the tube was maintained at a steady temperature of 673 K, and the Reynolds number (Re) was set in the range of 1000–20000. Different cases were simulated using CFD-FLUENT (v. 16.1). For the three cases, three types of ribs (7 in total) with triangular transverse sections of 5 mm were set 62.5 mm apart in the tube. Subsequently, the turbulent parameters (k, ε) affecting the turbulent flow were evaluated. Moreover, the heat transfer coefficient and friction coefficient of the tube with ribs were calculated using the enhancement technique and compared with those of the plain tube. It was observed that the ribs inside the tube increased the temperature of the cooling air in the first, second, and third cases by 6.25%, 12.5%, and 17.5%, respectively. Furthermore, the difference in the velocity and temperature distribution across the centreline of the tube was observed for all three cases and a smooth tube. It was observed that the heat flow rate increased, particularly around Re = 14,000, in the ribbed tube. The tube with ribs exhibited a 90% increase in the flow rate compared to the flow rate in the tube with no ribs. The tube with ribs also exhibited better performance in terms of turbulent flow. 

Control of heat transfer in circular channels using oblique triangular ribs

In this study, the effects of copper foam and ultrasonic vibration on the melting process of lowmelting gallium in a rectangular vessel were experimentally investigated. The effective thermal conductivity, heating wall temperature, most melting duration, and total melting duration during the gallium’s melting process were examined for various heating powers. We found that a portion of the gallium remained solid in the corners of the vessel in the late melting stage, and melting the remaining solid portion accounted for approximately 28% of the total melting duration of the pure gallium. In our test, the corresponding heating wall temperature rose significantly during this stage, which reduced the usable volume of the vessel. However, by the addition of copper foam and ultrasonic waves the remaining solid portion was greatly reduced, and the heating wall temperature was controlled. The total melting time for the gallium with added copper foam and ultrasonic waves was reduced by 10% and 17%, respectively, below that for the pure gallium melting process. When heat was added using ultrasonic waves for a long period of the melting process, the temperature of the heating surface was consistently lower than a pure gallium heat regenerator without ultrasonic waves. This would enhance the value of workable condition power for devices.

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Enhancement of gallium phase-change heat transfer by copper foam and ultrasonic vibration

Organic phase-change material for enhanced energy storage was prepared by adding SiO2 particles in a certain proportion into molten palmitic acid (PA) with stirring and intensive sonication at a constant temperature. The thermal properties of the SiO2/PA composite were experimentally characterized. As compared with the pure PA, the latent heat capacity of the composite with addition of 3 wt% SiO2 particles is increased by 51.7 kJ/kg, reaching 214.7 kJ/kg, which is the highest value among the composites tested with varying SiO2 ratio from 1 to 5 wt%; the thermal conductivity of the 3 wt% SiO2/PA composite is also increased by 12% at 30 oC (solid state) and 7% at 70 oC (liquid state), respectively. A water heater was built and tested with embedded capsules containing the 3 wt% SiO2/PA composite and pure PA, respectively. The lab-built phase-change water heater is much smaller in size than an ordinary household water heater in comparison and uses reduced electrical power. However, it responds faster and has a larger volume ratio of hot water output to water heater size. During the 2 turning-off test, the water heater with 3 wt% SiO2/PA can provide 2.23 times hot water of heater size; while the ordinary water heater can only provide 1.41 times hot water volume of tank size.

Le Cai

Papers

Using organic phase-change materials for enhanced energy storage in water heaters: an experimental study