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The low thermal conductivity of most phase change materials (PCMs) significantly limits the heat transfer rate, which is challenging for further application in thermal energy storage, energy saving of buildings, electronics cooling, etc. Here, we use the effective protection time of PCM heat sinks to represent the effect of copper foam on the melting behaviors of paraffin. Moreover, the effects of heat flux and copper foam filling ratio (β) were discussed on the effective protection time, the quantified contribution of natural convection, and the melting fraction of paraffin. The results prove that a good match between the physical parameters of the PCM heat sink and the working parameters of electronic components is essential to make full use of the latent heat. By contrast, a mismatch of 10 °C in allowable temperature or 0.21 W cm−2 in heat flux can be eradicated by a full filling of copper foam in paraffin. Specifically, the copper foam extends the effective protection time by enhancing heat conduction at β ≤ 60%, then it is further prolonged by improving the natural convection at β >60%. The results are expected to guide the synergistic enhancement of heat conduction and natural convection inside PCMs filled with copper foams. • Quantified contribution of natural convection inside PCM filled copper foams is obtained. • Copper foam suppresses the contribution of natural convection during paraffin melting. • A full filling of copper foam in paraffin is essential for the slight mismatch conditions. [ABSTRACT FROM AUTHOR]

A thermal buffer filled with phase change materials (PCM) can effectively prevent the intermittently working electronic devices from overheating and prolong the working time of the disposable electronic devices. This paper reported a novel composite PCM consisting of low-melting-temperature alloy bars embedded in pure paraffin that can concurrently improve thermal conductivity and natural convection during melting. Moreover, the effects of heat flux, filling height, and alloy mass fraction on the transient temperature of electronic devices and the effective protection time of thermal buffer were discussed experimentally. The results demonstrated that the heat conduction in stage one was enhanced by embedding alloy bars in pure paraffin, meanwhile the dripping of the alloy droplets in stage two not only enhanced the intensity of natural convection but also significantly delayed the increase of surface temperature. Moreover, correlations with a maximum error of 46∼126 s were obtained to predict the matching status between the thermal buffer and electronic devices, the effective protection time, etc., which was expected to guide the design of thermal buffers. In the future, some efforts should be made to alleviate the mismatching conditions, such as disturbing the boundary layers of natural convection by the elastic potential energy of spring or clockwork, etc. [ABSTRACT FROM AUTHOR]

• Effect of natural convection on cooling performance of PCM heat sink is explored. • Cooling performance of PCM heat sink can be improved by natural convection for about 59%. • Some synergistic enhancement technologies should be developed in future works. Phase change materials (PCMs) present great potential in extending the safe working time of electronic components that work intermittently or in an adiabatic cavity. Therefore, a clear understanding of the quantified contribution of natural convection and heat conduction in the cooling performance of PCM heat sink is essential to prolong the safe working time. In this paper, the quantified contribution of natural convection (η) was experimentally investigated by changing the heating position (bottom heat and top heating). Moreover, the effects of heat flux (q) and physical parameters were discussed on the average temperature during the overall fluctuation stage (T af), the η , safe working time, etc. The results proved that the dominant factor for the overall melting process of paraffin gradually changed from heat conduction to natural convection when the temperature fluctuation stage appeared. Specifically, the cooling performance of the PCM heat sink at q =1.24∼3.11 W·cm−2 could be improved by natural convection for 50%∼58.7%, which depended on the T af. Moreover, correlations consisted of the maximum allowable temperature, the liquidus temperature, and the heat flux were obtained to predict the η of paraffin in the range of q =1.24∼3.73 W·cm−2. It also presented an acceptable error (17%) in predicting the η of other organic PCMs. It should be noted that some synergistic enhancement technologies for heat conduction and natural convection should be developed in future works to prolong the safe working time of electronic components with high heat flux and low allowable temperature. [ABSTRACT FROM AUTHOR]