A Visualized Study Of Melting Heat Transfer Of Phase Change Material Saturated In Metal Foam

An effective approach to enhancing heat transfer within phase change materials(PCM)is to adopt highly-conductive porous matrixes.Metal foams are preferred for heat transfer enhancement of PCM due to their open-celled pore structure,high porosity,large surface area,and high thermal conductivity of the metal ligaments.This research focused on the application of infrared thermal imaging to investigate melting of a phase change material filled in an open-cell metal foam at the pore-scale and exploring the pore size effects on melting heat transfer.The visualized study at the pore-scale was enabled using an infrared video camera equipped with a macro lens.Firstly,the total emissivity of the paraffin was determined experimentally by a surface emissivity apparatus that is based upon the principle of comparing the steady-state radiative energy.In order to validate the accuracy of the IR thermal imaging method,the thermocouples(TCs)and IR camera were employed simultaneously to measure the transient temperature variations during melting of the copper foam/paraffin composite.In the characterization of the composite sample,the porosity of 15ppi,30ppi and 50ppi copper foams and the melting point of the paraffin were measured measured by a differential scanning calorimetry.The IR camera with the kit macro lens was used to observe top surface of the sample within a circular region(diameter 5mm)for thermal visualization of transient temperature fields.To explore the pore size and boundary condition effects on melting heat transfer of the copper foam/paraffin composite sample,three types of composite samples and two boundary conditions(superheat 20? and 30?)were used.The relative mean deviation between IR and TC is less than 2%and 4%,so the thermal imaging method in this experiment is relatively accurate.The porosity of three types of copper foams is about 95%.In addition,the melting temperature of the paraffin is 46.4??50.9?.After establishing a confidence on both resolution and accuracy of the IR thermal imaging method,the results were obtained that visualize the temperature fields and phase interface motions during melting as well as quantify the local thermal non-equilibrium effect between the metal ligaments and PCM at pore-scale.The local thermal non-equilibrium between the copper ligaments and their surrounding paraffin was shown to be time-dependent that varies from near-zero to the order of 10 ? as melting proceeds.Based on the results measured by IR camera and TCs,it was observed that at the wall superheat of 20?,the 30 ppi and 50 ppi copper foams lead to almost identical overall melting rates,which are both much faster than that for the 15 ppi copper foam.However,when the wall superheat was increased to 30?,the 30 ppi copper foam appears to be the best filler that causes much faster melting than that of the 50 ppi one.In sum,the use of copper foams with relatively small pores was deemed to be desirable,which may lead to both faster melting and lower local thermal nonequilibrium effect.