## 1 Introduction  
Continuous miniaturization and performance enhancement of electronic devices have resulted in a rapid increase in the amount of heat generated in electronic devices [1; 2; 3; 4]. Without proper thermal management, high rates of heat generation cause excessively high junction temperatures, which degrade the performance and reliability of devices. For example, in the case of light-emitting diodes (LEDs), the light output decreases as the temperature rises, because the quantum efficiency decreases at higher temperatures due to an increase in non-radiative recombinations through defect states [5; 6; 7; 8]. The long-term stability and lifetime of LEDs also decrease with the increasing junction temperature [9; 10]. Therefore, thermal management is essential for the maintenance of the performance and the durability of electronic devices [11; 12]. Among several cooling techniques suggested for thermal management, natural convective heat sinks are the most widely used because of their high reliability, low cost, zero acoustic noise, and zero power consumption [5; 13]. There has been a great deal of research work on the heat sinks under free convection [14; 15]. In particular, finned horizontal tubes under free convection have been investigated by many previous researchers. This is mainly because finned horizontal tubes are very widely used in various heat exchangers [16]. In the case of the gas-to-liquid heat exchanger, thermal resistance of the gas side is much greater than that of the liquid side, if there are no fins on the gas side [16]. As a result, finned horizontal tubes are commonly used to reduce thermal resistance of the gas side to improve overall thermal performance of the heat exchanger. Heat exchangers with finned horizontal tubes are widely used in many engineering applications, including refrigerators, air conditioners, and heat pumps. Square vertical fins mounted on horizontal tubes under free convection were studied by [PERSON] and [PERSON], and [PERSON] and [PERSON] [17; 18]. Annular fins of horizontal finned tube were investigated experimentally, and Nusselt number correlations were developed by [PERSON] and [PERSON], and [PERSON] and [PERSON] [19; 20]. Rectangular fins of radial heat sinks under free convection were investigated experimentally and numerically by [PERSON] et al. [21; 22]. Tubes with radial rectangular fins, as shown in Figure 1, are some of the most widely used heat sinks for the cooling of LED lighting. These finned tubes were investigated experimentally and Nusselt number correlations were developed by the present authors [23; 24]. Considerable efforts have been aimed at investigating heat transfer through tilted fins. For example, the thermal performances of tilted pin fins in rectangular ducts were investigated by [PERSON] et al. [25]. They concluded that tilted pin fins dissipate heat more effectively compared to normal pin fins because of the greater surface area of the tilted pin fins. Experimental and numerical investigation on tilted rectangular fins mounted on vertical plates under free convection was conducted by [PERSON] and [PERSON] [26]. They showed that tilted rectangular fins have higher thermal performances compared to normal rectangular fins when the tilt angle is \(60^{\circ}\). Recently, the present authors conducted an investigation to compare the thermal performances of tilted rectangular fins mounted on vertical tubes under free convection to those of conventional radial rectangular fins [27]. It was shown that the cooling performance of a vertical tube with rectangular fins can be enhanced by as much as \(30\%\) by tilting the rectangular fins. Therefore, it is expected that the cooling performance of radial rectangular fins mounted on horizontal tubes, shown in Figure 1, under free convection can be enhanced by tilting fins, as shown in Figure 2. However, to the best of our knowledge, tilted rectangular fins mounted on horizontal tubes under free convection have not been investigated experimentally yet.