Effects of thermal conductivity and wettability of porous materials on the boiling heat transfer
Introduction
With the miniaturization and power-density enhancement of power devices, the timely heat dissipation is urgent to be enhanced. Recently, the gas-liquid phase transition has been widely applied for heat dissipation because of the high latent heat. Pool boiling as an application of gas-liquid phase-change method, can take away heat from the solid surface by generating and separating bubbles from the surface. The performances of boiling heat transfer are mainly characterized by heat transfer coefficient (HTC) and critical heat flux (CHF). Among them, CHF is the upper limit of the nucleate boiling. Once the heat flux exceeds the peak value, the boiling mode will change from nucleate boiling to modal boiling, and the degree of superheat will increase sharply, which may cause the equipment to burn down.
In 1934, Nukiyama [1] published the first work on enhancing boiling heat transfer. Since then, more and more attentions were focused on the mechanism of the boiling heat transfer and also on enhancing boiling performance. Until now, the method to enhance pool boiling performance can be categories into two types: the active and passive technologies. The active technology could directly change the physical properties and morphology of the working fluid to enhance heat transfer, like by using electric field to increase the attraction of water film [[2], [3], [4], [5]], and by spraying cooling methods [6,7]. The passive technology is usually easier to be realized compared with the active one, which includes by applying surface coating [8], layered porous structure [9], and other complex fabricated surface [[10], [11], [12]].
Many efforts have been exerted on developing new surface structures to enhance the HTC of passive technology, while the active technology could be still complex to be applied in real application. To increase the active evaporation centers in the passive technology, the surface coating is commonly applied. For example, carbon nanotubes [[13], [14], [15]], graphene [16], and graphene-oxide [17] have been coated on the surface to enhance roughness, due to their high thermal conductivity, excellent mechanical and chemical properties. Compared with the surface coating methods, the porous structure show more expectations, because of its natural structure which could supply more nucleation sites and large specific surface areas [18,19]. Although many new surface structures have been applied to improve the pool boiling performance, the thermophysical properties of surface should also have important impacts which have been rarely considered [20].
In this work, the effects of surface thermophysical properties like thermal conductivity and wettability on boiling heat transfer were focused, by preparing porous structures with copper and nickel powders through the cold pressing method. The pool boiling curves and bubble dynamics were experimentally obtained in deionized water. Results show that a high thermal conductivity could provide more heat for bubble formula ion and separation, thus reduce the temperature difference between the top and bottom surfaces of the sample. Moreover, hydrophilic modification on the surface of samples with high thermal conductivity could improve pool boiling performance.
Section snippets
Experimental apparatus
Fig. 1 shows the experimental setup. The container is filled with deionized water as working substance. Above the container, a tube heat exchanger is used to condense steam, which is connected with a cold-water tank and a water pump to form a condensation system. An acrylic (PMMA) chamber with a size of 120 × 120 × 120 mm was used to hold water which have a height of about 90 mm in the chamber. The bottom of the chamber is connected with a column copper rod which holds 8 pieces of heating rods
Results and discussions
In this section, the reliability of the experimental system was tested firstly. Then, the pool boiling curves of different samples were given and analyzed. To reveal the underlying mechanism of enhancement effects of porous copper on pool boiling, bubble dynamics analysis was further carried out (section 3.3).
Conclusions
In this work, copper and nickel porous structure with different thermal conductivity and wettability were prepared by cold pressing method. After validation of the experiment setup, saturated pool boiling experiments were carried out on smooth copper surface and the prepared samples by using deionized water as working fluid. Finally, the effects of thermal conductivity and wettability on the pool boiling performance were discussed. The following conclusions can be drawn from this study.
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The
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
This work has been supported by the National Science Fund for Distinguished Young Scholars of China (No. 51525602), and National Natural Science Foundation of China (No. 52076211, 51936004).
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