Thermal Design of Tube Banks in Cross Flow Based on Minimum Thermodynamic Losses

Abstract eng:
Tube banks are widely used in crossflow heat exchangers. Usually, the methods for its design are the NTU or LMTD methods, while in this research the Entropy Generation Method is used. By assuming constant tube wall temperature, a general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying the conservation equations for mass and energy with the entropy balance. A comparison of the design is accomplished for a tube banks of different stream velocity, lengths and diameters. The heat transferred rate, ambient and tube wall temperatures are 20kW, 300K, and 365K, respectively. From the comparison of the design with the entropy generation rates, the optimal design is obtained. A single objective function is used which is the dimensionless entropy generation rate Ns subjected to the constraints of diameters and pitch ratio. This method of optimization can be applied for any constraints on the system which is the Lagrange optimization method. The effects of tube diameter, tube length, dimensionless pitch ratios, front cross-sectional area of the tube bank, and heat load are examined with respect to its role in influencing optimum design conditions and the overall performance of the tube banks. It is demonstrated that the performance is better for higher air velocities and larger dimensionless pitch ratios. Compact tube banks perform better performance for smaller tube diameters.

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