## 1 Introduction  
Heat pipes are widely used in the application of solar energy [1], which is a cost-effective and green energy resource for space heating [2], distillation [3], and absorption chillers [4]. In traditional solar thermal systems, round copper heat pipes are applied to the solar heat collectors [5; 6; 7]; however, this type of heat pipe is not a desired solution due to its high weight, small surface-volume ratio, and large size [8]. A novel micro flat aluminium heat pipe array (MF-AHPA) [9] provides a promising method for significantly improving solar heat collection due to its high in-plane thermal conductivity, high reliability, strength, and relatively low cost. The concept of micro heat pipes was proposed by [PERSON] et al. in 1984 [10]. A micro wick with a high surface-volume ratio shows potential heat transfer coefficient enhancement [11]. In past decades, extensive studies have been conducted because it outperforms a regular heat pipe. However, the heat transfer capacity of a single micro heat pipe is restricted by its relatively smaller cross-sectional areas. A micro heat pipe array can drastically enhance the heat transfer capacity due to its high surface-volume ratio. A micro heat pipe array was firstly investigated on silicon substrates [12; 13]. Aluminium extrusion provides a feasible and cost-effective technique to fabricate MF-AHPAs. Most recently, the solar thermal systems using MF-AHPAs demonstrated substantially enhanced performances. For example, [PERSON] et al. developed a flat plate solar collector using a MF-AHPA to achieve a maximum efficiency of 80%, accounting for 25% enhancement over the average efficiency on other alternatives [8]. [PERSON] et al. developed a solar air heater using flat micro heat pipe arrays. The average thermal efficiency of the novel solar air heater was reportedly up to 70% [14] and 73% [15] during steady-state operations, which is significantly better than a traditional flat plate heater. The micro heat pipe array also presented good performance in solar concentration thermoelectric generators [16], solar photovoltaic-thermal systems [17; 18], thermal energy storage systems [19], solar air collectors [20; 21], and heat exchangers [22]. Although the performance of a thermal system integrated with micro heat pipe arrays has been extensively studied, the micro heat pipe array has not been well characterised. The length-diameter ratio of the MF-AHPA is as high as 808.3, far surpassing the conventional heat pipe values of 51.5 [23], 50 [24], and 33.3 [25]. Knowledge of the heat transfer characteristic of heat pipes with such a high length-diameter ratio is insufficient in literature. In particular, the effect of orientation on MF-AHPA performance should be quantified, considering their critical roles in determining the efficiency of thermal systems [26]. In addition, working fluids have been known to play a critical role in governing heat pipe performance, no matter whether they are in an oscillating heat pipe [27] or a loop heat pipe [28]. However, study on working fluids on aluminium heat pipes is still insufficient. It is needed to systematically characterise the orientation and effect of working fluids on such aluminium micro heat pipe arrays. It is significant to investigate the effect of the inclination angle and working fluid on the MF-AHPA, since both can improve the efficiency of a solar collector. The optimal orientation of a solar collector well balances the heat pipe thermal resistance and light absorption efficiency. The inclination angle results of the MF-AHPA provide guidance on designing the orientation of a solar thermal collector. Furthermore, the mechanism of working fluid property and liquid film distribution will be analyzed to better understand the principle of selecting working fluid and inclination angle. Finally, the best thermal performance of the MF-AHPA will be achieved. The optimal inclination angle of heat pipes is significant, and has attracted extensive interest.