Design and development of an experimental teaching system for measuring hemispherical total emissivity
[Objective]Hemispherical total emissivity is a fundamental thermophysical parameter that describes the surface thermal radiation capability of materials and has important applications in various fields,such as energy power,aerospace,and new materials.In thermal engineering series courses,the introduction to the measurement of hemispherical total emissivity parameters is insufficient,and arrangements for teaching experiments on hemispherical total emissivity measurement are lacking.Devising approaches to help students better understand the concept of hemispherical total emissivity and its application in radiation heat transfer has always been difficult in the implementation of course knowledge.[Methods]To solve the problems of low measurement accuracy and limited temperature range in traditional teaching experimental devices for measuring hemispherical total emissivity,this study designs an experimental teaching system for measuring the hemispherical total emissivity at high temperatures based on the steady-state calorimetry method.The proposed system includes a vacuum chamber,vacuum pump,vacuum gauge,power supply,sample holder,voltmeter,ammeter,and temperature sensor.A hemispherical total emissivity calculation method based on steady-state calorimetry was established,and a calibration analysis for heat loss in the sample testing area was conducted.Based on theoretical formulas,the relation between hemispherical total emissivity and spectral emissivity,as well as the correlation between hemispherical total emissivity and material thermal radiation capability,were discussed.To verify the measurement performance of the hemispherical total emissivity teaching experimental device and calculation method,oxidized and unoxidized Ni-Cr high-temperature alloy samples were selected for testing,and the hemispherical total emissivity of samples at different temperatures was measured.[Results]The experimental results showed that the experimental measurement device has good measurement accuracy under high-temperature conditions,and uncertainty in the hemispherical total emissivity measurement decreases with increasing temperature and hemispherical total emissivity values.When the temperature exceeds 600℃,the measurement uncertainty is＜1.5％,which is better than the accuracy exhibited by existing experimental teaching devices and can meet the teaching requirements.The maximum proportion of thermal conductivity and wire thermal losses caused by the temperature gradient in the testing area to heating power is approximately 3％,which decreases overall with increasing temperature.The sample with higher emissivity has a relatively small proportion of wire heat loss because of its higher radiation heat dissipation power.The heating power of the sample increases with increasing temperature and hemispherical total emissivity and is positively correlated with the fourth power of temperature and first power of hemispherical total emissivity,which is consistent with the theoretical formula.The heating power of the oxidized sample was significantly higher than that of the unoxidized sample,indicating that the sample with high hemispherical total emissivity exhibits high thermal radiation capability.The experimental results show that the corresponding relation between hemispherical total emissivity and radiation heat transfer capability can be intuitively understood.[Conclusions]The system has the advantages of simple principles and high measurement accuracy,enabling students to understand the correlation between hemispherical total emissivity and material thermal radiation capability,as well as the correlation between spectral emissivity and hemispherical total emissivity at different temperatures.The experimental results are reliable and meet the needs of research-oriented experimental teaching in thermal engineering series courses.
hemispherical total emissivitysteady-state calorimetryexperimental teaching
万方数据
维普
