Evaluation of enhanced nucleate boiling performance through wall-temperature distributions on PDMS-silica coated and non-coated laser textured stainless steel surfaces

https://doi.org/10.1016/j.ijheatmasstransfer.2017.03.128Get rights and content

Highlights

  • Boiling on functionalized steel foils was investigated by high-speed IR imaging.

  • Wall-temperature distributions were calculated at various heat fluxes.

  • Distributions’ parameters revealed important differences in boiling performance.

  • Surface with microcavities provided the lowest standard deviation of wall superheat.

  • Compared to coatings, laser texturing didn’t increase heaters’ thermal resistance.

Abstract

Nucleate boiling was examined on Joule heated stainless steel foils, functionalized by PDMS-silica coating and/or nanosecond-laser texturing. The nucleating bubbles and transient temperature fields were visualized through high-speed IR and video recordings. The differences in boiling performance were evaluated through wall-temperature distributions. Results confirmed that smooth surfaces require high activation temperatures and produce larger bubbles, while wall-temperature distributions display higher standard deviations, higher local superheats, and bimodal shapes. Similarly, relatively high activation temperatures were observed on the superhydrophilic surface, where the enhanced liquid replenishment on the active nucleation sites reduces the bubble departure diameters and prevents formation of local hotspots. Consequently, the analyzed temperature distributions have negative skewness and decreased standard deviation. The highest heat transfer coefficient was achieved on a laser textured surface with non-uniform wettability and multi-scale microcavities. Here, nucleation site density was as high as 200 cm−2 at 300 kW/m2, while wall-temperature distributions demonstrated by far the lowest standard deviation. The temperature distributions also proved that annealed PDMS-silica coating significantly increased the thermal resistance of the entire heater. On the contrary, laser textured surfaces provided an even better boiling performance compared to coated surfaces and did not increase heater’s thermal resistance. This additionally endorses the coating-free, direct laser texturing method as a cutting-edge technology in the development of surfaces capable of significantly enhanced boiling heat transfer.

Introduction

Phase change heat transfer allows for the dissipation of high heat fluxes and provides very high heat transfer coefficients and is consequently indispensable in many heat transfer applications. Especially nucleate boiling has an important role in cooling of microelectronics, supercomputers, and nuclear power plant fuel rods [1], [2], [3], [4]. Due to constant advancements in the aforementioned fields, perpetual enhancements of boiling heat transfer are also required.

In pool boiling, a seemingly infinite and stationary amount of boiling liquid is considered. One of the pool boiling regimes is nucleate boiling, during which the liquid is vaporized and leaves the heating surface in the form of vapor bubbles of various sizes. This boiling regime is characterized by a relatively low surface superheat, i.e. the difference between the temperature of the surface and the saturation temperature of the surrounding liquid. Additionally, high heat fluxes can be removed from the surface and high heat transfer coefficients are obtainable. Therefore, it is the most prudent regime for the majority of boiling heat transfer applications. As both the boiling process and the accompanying heat transfer can be significantly influenced through the modification of the heating surface and its interaction with the liquid [5], [6], [7], [8], the desire to enhance both antecedent crucial pool boiling factors lead to the development of various surface modification techniques [9], [10], [11], [12].

In recent years, laser texturing is gaining popularity as a surface modification approach for the fabrication of surfaces capable of enhanced boiling heat transfer due to its many advantages compared to most other methods. Laser texturing is relatively fast and flexible; furthermore, laser textured surfaces are quite durable and the production generally does not involve any coatings or chemicals thus reducing or eliminating the thermal resistance of the modified surfaces and - at the same time - making the method more environmentally acceptable. Both the wettability and the micro/nanostructure of surfaces can be modified using laser texturing [13], [14], [15], [16]. Both parameters greatly influence boiling heat transfer and are frequently closely intertwined; in many cases changing the micro/nanostructure causes the wettability of the surface to change and vice versa. Through careful manipulation and combination of both parameters, especially by means of laser surface texturing, considerable enhancements of boiling heat transfer can be achieved [17], [18].

Several different approaches are possible when it comes to analyzing the boiling heat transfer performance of various surfaces. The most common mean of analysis is the so-called boiling curve, which defines the relation between the surface superheat and the heat flux [19]. A significant advantage of this approach is the fact that the experimental data needed to plot a boiling curve is easily obtained. However, a major drawback of the boiling curve is temporal and spatial averaging; in other words, local features of the boiling phenomena [20], [21], [22] are completely disregarded. Therefore, the boiling process is not characterized in its entirety. As we have previously shown [23], the evaluation of the boiling process based on wall-temperature distributions grants a significantly better insight into the phenomena during boiling on a particular surface. However, the prerequisite for the calculation of the distributions is an abundant amount of data regarding the transient temperature fields on the boiling surface, which requires a rather specific approach to the temperature measurement. The distributions show not only the minimum, maximum, and mean temperatures, but also the standard deviation and the overall shape of wall-temperature distribution. This allows for a better understanding of the boiling performance of each individual surface as high local superheats and large standard deviations of the surface temperature are most undesired due to the possibility of the onset of a boiling crisis [24] as well as the additional thermal stress and fatigue as a result of locally concentrated thermal loads [25].

In this study, we present an in-depth analysis of enhanced pool boiling performance on several recently developed coated and non-coated surfaces with an emphasis on the comparison of spatiotemporal temperature fields during boiling. Stainless steel foil with a thickness of 25 μm was used as the base material and high-speed IR thermography was employed to record unsteady temperature fields on the foil. Manufacturing procedures and nucleate boiling curves for these surfaces were already shown in our previous studies [12], [18]. Here, we aim to exploit the concept of wall-temperature distributions [23] and show the important differences in the boiling performance of various surfaces, which cannot be observed solely on the basis of the boiling curves.

Section snippets

Pool boiling experiments

The schematic representation of the pool boiling experimental setup is shown in Fig. 1. As the experimental setup has been described in details in our previous publications [12], [18], we are hereby providing only a brief overview of its vital components. The setup consisted of a double glazed glass boiling chamber with external dimensions of 170 × 100 × 100 mm3 mounted between two steel plates. The 25 μm thick stainless steel (AISI 316) foil with an effective heat transfer area of 17 × 27 mm2 was

High-speed visualization

Representative images showing nucleate boiling at 25 kW/m2 and 100 kW/m2 were extracted from the high-speed camera recordings and are shown in Fig. 3(a). The recordings obtained at 100 kW/m2 were further analyzed and the average bubble departure diameters were measured with the results provided in Fig. 3(b). At 25 kW/m2, the nucleate boiling regime has not yet been fully established on surfaces SS, F1, F2, and F3. Therefore, laser textured samples F1–F3 provided no enhancement regarding the onset of

Conclusions

Nucleate boiling performance was evaluated on 25-μm stainless steel foils with the use of double-distilled water as the working fluid. Several surfaces were functionalized either by the application of a PDMS-silica coating, which was potentially followed by a heat treatment; or solely by nanosecond–laser texturing. The recently proposed concept of wall-temperature distributions was utilized in order to emphasize important differences in the boiling process on all of the tested samples. Our

Acknowledgements

The authors acknowledge the financial support from the state budget by the Slovenian Research Agency (Program Nos. P2-0223 and P2-0392).

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