Thermal conductivity of metal walls
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There is a multiple-choice question: When the flow velocity of smoke increases, what happens to the thermal conductivity of the metal wall? A) It increases, B) It remains unchanged, C) It decreases. Which one is the answer? Please explain your reasoning; it will earn extra points
In heat transfer, as the flow velocity increases, the convective heat transfer coefficient rises; in other words, the thermal conductivity increases, and thus the heat exchange capacity is enhanced.
A. The thermal conductivity remains constant. The thermal conductivity is determined by the material
The thermal conductivity of a metal wall is determined by the properties of the material
The thermal conductivity of the material itself should remain unchanged; as the flow rates of flue gas and air increase, the overall heat transfer coefficient increases
The thermal conductivity of metals is related to the temperature at which it is measured; generally, the higher the temperature of a material, the lower its thermal conductivity.
From the flue gas to the medium, heat transfer occurs through convection + conduction + convection. The temperature order is: flue gas main stream > metal outer wall > metal inner wall > medium main stream. As the flue gas velocity increases, the convective heat transfer coefficient between the flue gas and the metal outer wall rises, resulting in an increase in the temperature of the metal outer wall； The average temperatures of the inner and outer walls of the metal increase; the temperature difference between these walls grows, resulting in an increased heat transfer rate. The heat conduction flux is equal to the convective heat flux from the flue gas and the outer wall of the metal, and the temperature of the inner wall of the metal also rises ； As the temperature difference between the metal inner wall and the main flow of the medium increases, the heat transfer amount also increases, while the heat flux remains the same as in the previous two cases. The thermal conductivity of metals is an inherent property that decreases as temperature rises. Although the thermal conductivity of the metal decreases, the convective heat transfer coefficients on both sides increase, and convective heat transfer coefficients are usually an order of magnitude higher than the thermal conductivity; overall, the total heat transfer coefficient increases.