The transient behavior of crossflow heat exchangers with one fluid mixed is discussed in this paper. The response to a step change in the inlet temperature of the mixed fluid is evaluated for cases in which the wall capacitance may be considered large. An approximate integral technique was used to obtain this solution to the governing differential equations. These results are summarized by a design chart for calculating the time required to attain 90 percent response. Design curves for calculating the entire transient as a function of time for the special case of an evaporator or condenser are additionally determined to extend the work of previous investigators. A new set of parameters for use in transient analysis is suggested which allows greater insight into the transient problem than does the more conventional set that includes the steady-state parameters. The range of validity of the large wall capacitance approximation is evaluated.

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