What is the difference between the convective heat transfer coefficient a and the heat transfer coefficient K?
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What is the difference between the convective heat transfer coefficient a and the heat transfer coefficient K? What are the main differences? In practical applications, how is it mainly used?
K is the overall heat transfer coefficient, and a is the heat transfer coefficient of various substances. 1/k is equal to the sum of all 1/a values.
The overall heat transfer coefficient a of the K-type heat exchanger is given by 1/K = d_out/(a_in*d_in) + b*d_out/((λ*d_in) + 1/a_out) + R_in + R_out. The subscripts may not be perfect; just take a look at it. You can find this information in textbooks on chemical engineering principles – it’s not difficult
a is the convective heat transfer coefficient, which refers to the coefficient of heat transfer in the process between the fluid and the \"solid surface\"; it takes into account the effects of thermal convection within the fluid itself as well as heat conduction in the boundary layer. K is the overall heat transfer coefficient of the heat exchanger, which includes many factors. For heat to transfer from a hot fluid to a cold fluid, these three processes must occur: from the hot fluid to the hot-side surface of the wall, from the hot-side surface to the cold-side surface (i.e., through conduction through the wall), and from the cold-side surface to the cold fluid. Therefore, the calculation of the K value takes into account the convective heat transfer coefficients on both sides of the wall, as well as the thermal conductivity of the wall itself. In addition, the fouling that occurs over time (on both sides) must also be considered; thus, the K value consists of 5 components. In practical problems, some components with low thermal resistance are ignored; for example, when there is phase change and the convective heat transfer coefficient is high, the convective thermal resistance on that side can usually be disregarded. If the heat exchanger is in use for a long time, its fouling thermal resistance may account for a large proportion.
In addition to convection, there are also conduction and radiation; all three involve K. At low temperatures, only convective and conductive heat are generally considered. If conduction is ignored, the value of a at this point equals K, which corresponds to considering only convective heat transfer.
Thank you, I’ve learned it; that’s great.
GB151-2014, page 118. K is the total, and a is the fluid.