Abstract
Heat transfer enhancement by surface modification has been extensively studied in the last twenty years. However, there remains a large discrepancy among researchers on the performance of enhanced surfaces even for the same fluid and surface preparation technique. The reasons of this discrepancy are not understood and are not discussed in past papers, including paper reviews. Part II of this two-part paper aims to present a detailed assessment of pool boiling heat transfer enhancement, relating this to Part I [1], which presented a critical assessment of fundamental concepts of heterogeneous nucleation. Current challenges in evaluating the performance of enhanced surfaces is first discussed. The performance of smooth and roughened surfaces is then discussed and the effect of fluid type is explained. Pool boiling data of two fluids, namely water and FC-72, on two enhanced substrate materials, i.e. copper and silicon were digitized and assessed in order to elucidate the reason for the discrepancy in published works and present future recommendations for heat transfer enhancement. The heat transfer enhancement mechanisms adopted by researchers were presented and critically discussed and compared. The paper contributes to the understanding of the effect of fluid-surface combinations and suggest guidelines for researchers to consider when evaluating the performance of enhanced surfaces. This will help the research community and industry to conclude on the best surface structure and surface manufacturing technique matching particular fluid of interest.
Highlights
Boiling heat transfer enhancement has attracted the focus of re searchers in the last twenty years, motivated by the need to improve the heat transfer rates in heat exchangers used in, for example, the power generation, chemical and refrigeration industries where such improve ments will result in higher plant efficiencies, reduced size equipment and overall plant size and fluid inventory
The review in this paper presented studies for two fluids – water and FC-72 – and two materials – silicon and copper - in order to help elucidate the enhancement mech anism(s) and quantify the changes in the heat transfer rates and critical heat flux
This study revealed that the different enhancement mechanisms, assumed by researchers, can be classified as follows, see Fig. 11: 1. The increase of the number of active nucleation sites induced by the modified surface texture (Fig. 11b)
Summary
Boiling heat transfer enhancement has attracted the focus of re searchers in the last twenty years, motivated by the need to improve the heat transfer rates in heat exchangers used in, for example, the power generation, chemical and refrigeration industries where such improve ments will result in higher plant efficiencies, reduced size equipment and overall plant size and fluid inventory. Active techniques include surface or fluid vibration, surface rotation or the application of high intensity electric fields The latter require the use of an external force and are less common than passive heat transfer enhancement techniques. A review of the past reports reveals that there is still a large discrepancy among researchers on the performance of enhanced sur faces for the same fluid and enhancement technique, despite the large number of researches conducted in this area The reasons of this discrepancy are not understood and were not discussed in the past re view papers. In the past review papers there was no segregation to the fluidsurface combination and instead various fluids and various surface modification techniques were included in the discussion This approach makes it difficult to understand the performance of a certain fluid on a wide range of surface microstructures. It is worth mentioning that the performance of other fluids such as cryo genic liquids, refrigerants and hydrocarbons are not included in this paper so as to make the paper focused and conclusive
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