RESUMO
The thermal properties of bipolar plates, being key elements of polymer electrolyte membrane fuel cells, significantly affect their heat conduction and management. This study employed an innovative approach known as a heat flow loop integral method to experimentally assess the in-plane thermal conductivity of graphite bipolar plates, addressing the constraints of traditional methods that have strict demands for thermal stimulation, boundary or initial conditions, and sample size. This method employs infrared thermal imaging to gather information from the surface temperature field of the sample, which is induced by laser stimulation. An enclosed test loop on the infrared image of the sample's surface, situated between the heat source and the sample's boundary, is utilized to calculate the in-plane heat flow density by integrating the temperature at the sampling locations on the loop and the in-plane thermal conductivity can be determined based on Fourier's law of heat conduction. The numerical simulation analysis of the graphite models and the experimental tests with aluminum have confirmed the precision and practicality of this method. The results of 1060 aluminum and 6061 aluminum samples, each 1 and 2 mm in thickness, show a deviation between the reference and actual measurements of the in-plane thermal conductivity within 4.3% and repeatability within 2.7%. Using the loop integral method, the in-plane thermal conductivities of three graphite bipolar plates with thicknesses of 0.5 mm, 1 mm, and 1.5 mm were tested, resulting in 311.98 W(m·K)-1, 314.41 W(m·K)-1, and 323.48 W(m·K)-1, with repeatabilities of 0.9%, 3.0%, and 2.0%, respectively. A comparison with the reference value from the simulation model for graphite bipolar plates with the same thickness showed a deviation of 4.7%. The test results for three different thicknesses of graphite bipolar plates show a repeatability of 2.6%, indicating the high consistency and reliability of this measurement method. Consequently, as a supplement to existing technology, this method can achieve a rapid and nondestructive measurement of materials such as graphite bipolar plates' in-plane thermal conductivity.
RESUMO
In a low-temperature environment, the actuation performance of the piezoelectric stack results from the synergic action of the thermo-electro-mechanical field; the actuation performance is influenced by the change in temperature, compressive preload, and excitation voltage. A special and novel instrumentation system is proposed and developed in this study to measure the relationship between the actuation performance of the piezoelectric stack and the change in temperature, preload, and voltage. The bending strain of the cantilever beam driven by the piezoelectric stack reflects its actuation performance, and the corresponding theoretical model is established to optimize the experimental conditions and maximize the strain and signal-to-noise ratio. Based on the experimental results, it can be seen that the actuation performance of the piezoelectric stack increases linearly with the excitation voltage under different temperatures and preload conditions. The static actuation performance increased by 79%-90% when the prestress increased from 0 to 6 MPa, corresponding to a decrease of 15%-30% when the temperature decreased from 20 to -70 °C, and the dynamic actuation performance decreased with an increase in the frequency of the excitation voltage. Consequently, the design methods and ideas are informative to develop an instrumentation system that can measure the influence of thermo-electro-mechanical synergistic effects on the actuation performance of piezoelectric stacks under different temperatures, preloads, and voltages.