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This paper analyzes the thermal performance of a co-current flow heat exchanger with transient gas outflow. The temperature distributions of the working fluid, heating fluid, and the wall over the length of the heat exchanger are predicted by an integral formulation. The heat transfer rates are determined at various stages of the heat exchanger operation. An integral formulation of the nondimensionalized governing equations is solved numerically, using a time-marching algorithm. The temperature distributions of the working fluid and the wall have an exponential increase from the inlet to the outlet of the heat exchanger. The heating fluid shows an initial decrease and subsequent increase of temperature. A base model for the step change in the mass flow of the working fluid is developed and compared against past data for purposes of validation. In addition, results are presented and discussed for the time-varying performance, during pressure regulated gas outflow from the heat exchanger.

Transient operation of a co-current heat exchanger is analyzed in this paper, using a new predictive formulation for laminar and turbulent flows with mass discharge from the heat exchanger. The model includes time-varying mass discharge due to pressure regulated gas outflow. The temperature variations of the working fluid, heating fluid and the wall are predicted and validated against past data. It is found that the temperature of the working fluid rises sharply to a peak and then gradually decreases over time, due to mass discharge effects. The wall temperature decreases exponentially, and the temperature of the heating fluid falls sharply, and then gradually decreases. A benchmark case of a step change in the mass flow rate of the incoming fluid is analyzed and compared against past data for validation, after which results are presented and discussed for transient step changes of the incoming mass flow rate. List of symbolsA Heat transfer area (m 2 ) C p Specific heat (kJ/kg K) f, g Functions h Heat transfer coefficient (W/m 2 K) L Length of the heat exchanger (m) m Mass (kg) m Á Mass flow rate (kg/s) N Dimensionless parameter NTU Number of transfer units Nu Nusselt number P Pressure (kPa) Pe Perimeter (m) Pr Prandtl number Re Reynolds number T Temperature (°C or K) t Time (s) t cycle Total time of the heat exchanger operation (s) x Length of a heat exchanger section (m)Greek symbols c Ratio of final and initial mass flow rates q Density (kg/m 3 ) D Difference k 1,2 Roots of characteristic equations eHeat exchanger effectiveness Superscripts * Dimensionless form 0Initial condition at t = t 0 ? Condition at t = t ?