Effect of inclination angle on the charging process of flat heat pipe-assisted latent heat storage unit

This study aims to conduct an experiment to understand the influence of inclination angle on coupled heat transfer process of heat pipe and the phase change material (PCM) under constant heat flux conditions. An experimental platform for an angle-adjustable latent heat storage unit using flat heat pipe and lauric acid was designed and fabricated. Melting time of PCM and time-varying temperature pattern of flat heat pipe and PCM under different inclination angles were recorded. Effective thermal resistance and conductivity of flat heat pipe, thermal resistance, and heat transfer coefficient between flat heat pipe and PCM were also calculated and analyzed. Results showed that (1) the overall melting time of the PCM is extended although heat transfer on the PCM side enhances with the decrease of inclination angle and the average thermal resistance between PCM and flat heat pipe reduces from 0.084 K.W-1 to 0.071 K.W-1 (heat transfer enhancement of 15.5%) while the melting time prolongs from 78.6 min to 80.1 min (1.9% increment) when the input heat is 20 W and the inclination angle decreases from 90 degrees to 15 degrees (83.3% reduction) and (2) the effective thermal conductivity of the flat heat pipe first decreases and then increases with the increment of the inclination angle under an inclination angle of 30 degrees-90 degrees. Maximum and minimum thermal resistance values are 0.058 (3.7% increase compared with those at 90 degrees) and 0.053 (6.1% reduction compared with those at 90 degrees) K.W-1 at 30 degrees when the heating power is 20 W and the inclination angle is 75 degrees. However, the heat transfer performance of the flat heat pipe deteriorates when the input heat is high (>30 W) under the condition of small inclination angles (<30 degrees). For example, the heat transfer resistance of the flat heat pipe is with the range of 0.056-0.058 K.W-1 at an inclination angle of 30 degrees-90 degrees when the heating power is 50 W and the thermal resistance suddenly increases from 0.056 K.W-1 to 0.074 K.W-1 (an increase of 31.5%) when the inclination angle decreases from 30 degrees to 15 degrees.