No abstract

Since diluted suspensions of nanoparticles were first called nanofluids and presented as viable solutions for heat transfer applications, this subject has received much attention and related investigations have expanded to many paths. In order to comprehend how nanoscale-related effects could influence the macroscopic transport behavior of nanofluids under single or phase-change conditions, researchers have studied, for example, the stability of these solutions, variation of thermal and rheological properties, and the convective heat transfer behavior of a great variety of nanofillers in common fluids, mainly water. The deposition of nanofillers over heated surfaces has also been investigated due to the role of surface nanostructuring in modifying wettability, thermal resistance, and delaying the occurrence of critical heat flux. Despite the considerable number of publications regarding nanofluids, scattered results for transport properties or convective behavior of nanofluids under similar experimental conditions are often found, which hinders their applications due to a lack of comprehension on the mechanisms related to the behavior of these fluids and, consequently, to the difficulty in predicting it. In this context, this work concerns a review about the heat transfer behavior of nanofluids under single-phase flow, pool boiling, and flow boiling conditions. In general, there is a consensus that the heat transfer coefficient of single-phase flow is enhanced by the addition of nanoparticles to base fluids, although overall benefits of their application cannot be assured due to increases in viscosity. In contrast, either increase or decrease in heat transfer coefficient could be observed for pool and flow boiling conditions. Such behavior can be attributed to surface modifications due to interactions between the bare surface texture and the deposited nanoparticles; however, information on the surface texture is commonly missing in most works. Finally, the main mechanisms reported in the literature pointed out as responsible for the heat transfer coefficient behaviors are summarized, where it can be seen that modifications of transport properties and particles movements impact single-phase flow, while phase-change heat transfer is also influenced by variations of surface characteristics.

No abstract

Heat transfer coefficient is a basic parameter used in the calculation of convective heat transfer problems. Due to the importance of the experimental measurements for the development of convective heat transfer, this review identifies, classifies and describes the experimental methods used for the measurement of heat transfer coefficient. The methods were classified into five major groups: (1) direct method, (2) transient method, (3) Wilson method, (4) heat/momentum/mass transfer analogy method and (5) boundary layer thickness method. Their applications, limitations and the reported accuracy were evaluated in the context of new developments in temperature and heat flux measurement techniques. Finally, this review provides criteria for the selection of the most suitable technique for measurements of heat transfer coefficient according to the aspects of spatial resolution, geometric scale, intrusiveness, fluid type, response time and accuracy. Keywords Heat transfer coefficient • Temperature • Heat flux • Convection • Experimental measurements List of symbols 1D One-dimensional 2D Two-dimensional A Surface area (m 2) A e Outer tube surface area (m 2) A i Inner tube surface area (m 2) A s Surface area (m 2) C e Thermal resistances outside the tube and the tube wall (K/W) (constant) C i Constant C f Fanning friction factor (-), C f = s V 2 ∕2 c Constant (-) c p Specific heat capacity (J/kg K) c p,i Specific heat capacity at constant pressure (J/ kg K) d Diameter (m) d e Outer tube diameter (m) d i Inner tube diameter (m) D m Mass diffusivity (m 2 /s) f Darcy friction factor (-), f = 4 ⋅ C f * Cristiano Bigonha Tibiriçá