# Heat Transfer Enhancement of Nanofluids with Non-Spherical Nanoparticles: A Review
## 1 Introduction  
Studies on heat transfer enhancement of nanofluids have largely matured over the past decade. Since [PERSON] and [PERSON] [1] introduced the concept of \"nanofluids,\" which are fluids with suspended nanoparticles, scholars have reported significant heat transfer enhancement of nanofluids with low particle loadings. In addition, the suspension is stable and induces a small amount of pumping power compared to the base fluid [2]. Therefore, nanofluids are considered the next generation of a heat transfer medium that will be extensively applied in advanced heating or cooling technology. Due to the complexity of the nanofluid flow transport, the mechanism underlying the excellent heat transfer performance of nanofluids is poorly understood. The bulk medium shows rheological behavior after the nanoparticles are dispersed in the base fluid [3, 4]. [PERSON] and [PERSON] reviewed the viscosity of nanofluids with different types of nanoparticles or base fluids, particle concentration, shear rate, etc. [5]. However, they discovered scattered and inconsistent literature data from different researchers. Moreover, the conventional and proposed empirical models could not effectively predict the viscosityof nanofluids containing non-spherical nanoparticles. Similar scenarios have been reported in heat transfer characteristics of nanofluids. First, there is a substantial variance in the thermal conductivity of nanofluids measured in the literature [6; 7]. On the other hand, the convective heat transfer coefficient or Nusselt number provided in literature for nanofluid flow is different and inconsistent [8; 9]. The contributing factors for nanofluids' heat transfer characteristics multiply, resulting in nonlinear correlations. Therefore, reviewing the results from previous studies is essential to identify research gaps. Heat transfer enhancement of nanofluids is influenced by several factors, including particle material, size, shape, concentration, type of base fluid, the temperature of bulk fluid, etc. For convective heat transfer (CHT), the flow parameters and thermal boundary conditions promote nanofluids flow. Most of the previous studies focused on the importance of fluid type and particle concentration. Based on the effective medium theory, the enhancement of nanofluids heat transfer increases with the concentration and the thermal conductivity of the particle material. Nonetheless, the effect of particle shape and size distribution was partially reported and considered a constant in experiments or theoretical studies. The nanoparticles prepared for experiments in laboratories or applications in industries are polydisperse. The shape and size of particles vary around the nominal value given by nanomaterial producers. Moreover, most studies on nanofluids' heat transfer enhancement are based on bulk fluid with spherical nanoparticles. With nanotechnology, more nanoparticles with different shapes can be industrially developed. In recent years, several experimental, theoretical, and numerical studies reported that particle shape modulates the CHT characteristic of nanofluids [10; 11]. Nanoparticles with shapes such as cylinders, bricks, blades, and platelets have been discussed (Figure 1). Furthermore, a theoretical and numerical investigation of heat transfer enhancement of nanofluids containing non-spherical nanoparticles (NSN nanofluids) has recently gained a premium [12; 13]. Therefore, it is essential to review the existing research on the importance of the particle shape effect on the nanofluid application in heat transfer enhancement. This article reviews research findings and the development of the heat transfer enhancement of NSN nanofluids. We collect and analyze thermal conductivity enhancement (TCE) measured in experiments based on NSN nanofluids. This work also summarizes the classical and recently proposed thermal conductivity models for NSN nanofluids and the related mechanisms. Further, we evaluate the experimental data in studies on CHT of NSN nanofluids.