Advances in Boiling Heat Transfer Enhancement using Micro / Nano Structured Surfaces

In this article we present an inclusive review of research carried out in the field of phase change heat transfer enhancement. First, we discuss about different kinds of conventional heat transfer enhancement techniques performed in convection heat transfer related heat exchangers. Next, we present the advantages of implementing phase change heat transfer and report a brief introduction to the physics behind the phase change (boiling) heat transfer phenomenon. We present a well explained data about different kinds of enhancement techniques using micro and nano scale structures on heat transfer surface/device to increase the limit of boiling heat transfer. The entire review article is broadly divided into two categories: first the investigation related to fluid flow or transport mechanism over the micro/nano structured surface which is of crucial importance, second is the actual computational and experimental methods to achieve higher heat transfer capability in terms of critical heat flux (CHF) for a given surface/device. From the ongoing work, we are able to conclude and put forward three major stages of doing research in CHF enhancement using micro/nano structures/devices viz.: (i) selection and construction of micro/nano structures, (ii) perceiving the fluid transport through capillary over the micro/nano structured surface and (iii) actual experiment/computation to compare CHF of modified device with the base device.


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Abstract-In this article we present an inclusive review of research carried out in the field of phase change heat transfer enhancement.First, we discuss about different kinds of conventional heat transfer enhancement techniques performed in convection heat transfer related heat exchangers.Next, we present the advantages of implementing phase change heat transfer and report a brief introduction to the physics behind the phase change (boiling) heat transfer phenomenon.We present a well explained data about different kinds of enhancement techniques using micro and nano scale structures on heat transfer surface/device to increase the limit of boiling heat transfer.The entire review article is broadly divided into two categories: first the investigation related to fluid flow or transport mechanism over the micro/nano structured surface which is of crucial importance, second is the actual computational and experimental methods to achieve higher heat transfer capability in terms of critical heat flux (CHF) for a given surface/device.From the ongoing work, we are able to conclude and put forward three major stages of doing research in CHF enhancement using micro/nano structures/devices viz.:

I. INTRODUCTION
Heat transfer enhancement has become an immense topic of discussion in the last few decades.In the field of applications ranging from electronics to power plants, automobiles, aeronautics and even space, efficient heat exchange is always desirable for effective functioning of devices/machines as well as proper thermal and energy management.Enhancement techniques are adopted in order to increase the limit of heat transfer capability as well as to eliminate the undesirable loss of energy/heat to the surrounding.Conventionally, it has been a practice of selecting materials with good thermal properties to be used in heat transfer devices which helps a lot for better heat conduction in a wide range of applications.And, if we discuss about convection mode of heat transfer, it is well known that, thermal boundary layer is a major hindrance for efficient heat exchange.Thus, numerous experimental and theoretical works have been carried out to destroy or postpone the thermal boundary layer formation on such heat transfer surfaces with an intention of acquiring an enhanced Published on November 29, 2019.S. Agarwal, postgraduate student, Visvesvaraya Technological University, India (e-mail: ashreya96@ yahoo.com).R. Kumar, student of Mechanical Engineering, Bangalore Institute of Technology, Bangalore, India heat transfer.People have used various kinds of insertions into the heat exchange tubes to destroy the thermal boundary layers which in turn enhances the heat transfer coefficients (Nusselt number Nu) [1][2][3][4][5][6].In supplement to that, various other techniques like use of magnetohydrodynamic MHD convection flow is also exploited a lot to enhance the heat transfer competency [7][8][9][10][11][12][13][14].Such kinds of heat transfer enhancement techniques work pretty fine in a nominal working range and condition; nevertheless, the amount of heat transfer enhancement is not very high since the mode of heat transfer is merely conduction/convection with sensible heat only.
On the other hand, phase change of material offers significantly very large amount of heat transfer as compared to sensible heat; this has motivated the use of phase change heat exchanger (evaporator or boiler) in modern day's heat exchange and thermal management applications.Thus a number of ways are being studied in depth to increase heat transfer potential of any device by using phase change.In this paper, we first discuss about the basic idea of phase change heat transfer, then we discuss about the modification of heat transfer surface by using various micro and nano scale structures to be utilized in such devices.We perform the inclusive review of different research activities related to the devices with enhanced heat transfer capability and put forward the major findings of each one of them which will be of great aid to carry out future research in this area.

II. PHASE CHANGE HEAT TRANSFER
Liquid (e.g.water) absorbs 2,260 kJ/kg of heat energy at 100 o C while transiting from liquid phase to vapor phase without change in temperature which is way larger as compared to sensible heating where only 4.2 kJ/kg of heat is absorbed with 1 o C rise in temperature.This is the major motivation of using phase change heat transfer where very large amount of heat is to be extracted including the applications like nuclear power plants, boiler, etc.While dealing with the heat transfer associated with phase change, one should understand the physics behind the actual phenomenon.When boiling (phase change) occurs on a heated surface, the initial stage in boiling is nucleation (formation of vapor bubbles).Later, the bubble density increases and more bubbles on the surface results into higher heat transfer through phase change.However, the major drawback of this kind of heat transfer comes forth when the surface is saturated with the bubbles formation and growth; ultimately the surface is sealed with low conducting vapor layer only.At this stage, heat flux at the boiling surface doesn't rise anymore.This limit of heat flux due to vapor seal on the surface is called the critical heat flux CHF during Advances in Boiling Heat Transfer Enhancement using Micro/Nano Structured Surfaces Shreya Agarwal, and Ranjan Kumar boiling.Many of the researches carried out in heat transfer enhancement in last few years are associated with delaying of this limit or to increase the CHF for a particular surface or device.Copyright (2010) American Chemical Society.

A. Role of micro/nano structures in CHF enhancement
It is an understood fact that the heat transfer enhancement using phase change on a particular surface is associated with wettability of the surface [34].Wettability or wickability of the heat transfer surface increases the potential of the surface to adsorb liquid (water) for a longer duration of time and the formation of thin film of vapor seal is delayed, ultimately shifting CHF to higher limit.This delay is possible only if liquid can travel from the reservoir to the spot of vapor formation so that the vapor seal formation can be halted.Thus, we can say that the mechanism of liquid flow over the micro/nano structured surface during that stage is of crucial importance which ultimately dictates the enhanced CHF.Since the scale of fluid flow in such structure is too small, with Bond number Bo << 1, major forces acting are capillary as well as viscous which determine the extent of spread of liquid on such surfaces or the wettability.Fig. 1 and 2 demonstrate different kinds of micro/nano structures constructed on heat transfer surfaces/devices used by different researchers [19][20].Accordingly, numerous studies are carried out to understand the fundamentals of capillary motion on such surfaces as well as to delay the CHF during boiling.We thus divide the present study into two broad categories: capillary flow over micro/nano structured surface and CHF enhancement using micro/nano structured devices.B. Capillary flow over micro/nano structured surface Consider the most common example of capillary flow: capillary rise in a vertical narrow tube, where the rise of liquid inside the tube takes place due to capillary flow which is against gravity.The scale (diameter) of the narrow tube being too small, capillary force due to surface tension is not negligible which actually rises certain weight of liquid till the weight itself balances the gravity.This typical example can be considered as a base line for studying capillary flow in micro/nano structured surface in association with phase change heat transfer enhancement.Similar concept for studying the magnitude of liquid spread/rise has been applied and proved experimentally and computationally by various researchers [15][16][17][18][19][20][21][22][23] which is elaborated in Table I.

C. CHF enhancement using micro/nano structured device
Use of micro/nano structured surfaces for heat transfer enhancement is rising now-a-days especially by employing phase change heat transfer.As we know that, for a particular surface there exists a limit of heat flux beyond which heat transfer surface is sealed with vapor layer and further heat transfer is blocked, the major goal of research is to shift this limit, usually by surface modification.Surface modification by using micro and nano scale structures is performed and the obtained CHF value is compared with the base CHF value (without surface modification).For an instance, we can observe the results obtained by Raghupathi & Kandlikar [32] in Fig. 3 which demonstrates enhanced CHF due to micro/nano structures.This way, people have performed many experimental and computational work to study the effect of micro/nano structures on CHF as in Table II.

III. CONCLUSION
After reviewing different research articles in the field of heat transfer enhancement, especially one with phase change, we come to a major conclusion that CHF enhancement is ultimately due to surface modification of heat transfer device.Additionally, the process of enhancing the heat transfer capability by raising CHF to higher value is to be studied in three steps: (i) selection of proper micro or nano scale structures to be built over the heat transfer device (ii) study of fluid transport (propagation) over that modified surface with micro/nano structures and (iii) actual experiment or computational analysis for CHF enhancement on the new device.In the present work we are able to illustrate an understanding of the use of different kinds of micro/nano structures for CHF enhancement as well as the systematic study of fluid flow over such structures.In future, in order to design and develop a micro/nano device for heat transfer enhancement, we have to look into the material aspect of such device in terms of fabrication and deposition onto the surface as well as the potential mechanism of fluid propagation through capillary i.e. rewetting of hot spot during boiling.This paper gives the overview of related researches and offers a wider viewpoint for thoughtfulness in the overall phenomenon of heat transfer enhancement using micro/nano structured devices in a concise manner.
(i) selection and construction of micro/nano structures, (ii) perceiving the fluid transport through capillary over the microto compare CHF of modified device with the base device.Index Terms-Heat Transfer Enhancement, Micro and Nano Structures, CHF, Boiling.

Fig. 3 .
Fig. 3. Plot of CHF versus contact line length showing enhanced CHF for higher contact length acquired by using micro/nano structures.Adapted with permission from Ref. [32].Copyright (2017) American Institute of Physics.

TABLE I :
DIFFERENT RESEARCHES IN THE FIELD OF CAPILLARY FLOW OVER

TABLE II :
DIFFERENT RESEARCHES IN THE FIELD OF CHF ENHANCEMENT