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The Effectiveness-NTU 方法 is used in heat exchanger analysis when fluid inlet temperatures are known, but outlet temperatures are not. Effectiveness ([latex]\epsilon[/latex]) is the ratio of actual heat transfer to the maximum possible heat transfer. The Number of Transfer Units (NTU) is a dimensionless measure of the heat exchanger’s size, defined as [latex]NTU = \frac{UA}{C_{min}}[/latex].
The Effectiveness-NTU method provides a powerful alternative to the LMTD method, especially in situations where iterating to find outlet temperatures would be cumbersome. The core of the method lies in three dimensionless parameters: effectiveness ([latex]\epsilon[/latex]), the number of transfer units (NTU), and the heat capacity rate ratio ([latex]C_r = C_{min}/C_{max}[/latex]). The maximum possible heat transfer rate, [latex]Q_{max}[/latex], occurs in a hypothetical infinitely long counter-flow heat exchanger, where the fluid with the smaller heat capacity rate ([latex]C_{min}[/latex]) undergoes the maximum possible temperature change, [latex]\Delta T_{max} = T_{h,in} – T_{c,in}[/latex]. Thus, [latex]Q_{max} = C_{min}(T_{h,in} – T_{c,in})[/latex]. The actual heat transfer is then simply [latex]Q = \epsilon Q_{max}[/latex]. The key is that effectiveness ([latex]\epsilon[/latex]) can be expressed as a function of only NTU and [latex]C_r[/latex] for a given flow arrangement. For example, for a parallel-flow exchanger, the relationship is [latex]\epsilon = \frac{1 – \exp[-NTU(1+C_r)]}{1+C_r}[/latex]. These relationships have been derived and charted for numerous common heat exchanger configurations, allowing engineers to quickly determine the performance of a given exchanger or to size a new one without knowing the outlet temperatures beforehand. This method is particularly useful in design and optimization studies where the impact of changing the exchanger’s size (and thus NTU) on its performance (effectiveness) is being investigated.
有效性-NTU 方法
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