A general transient-based technique for the evaluation of overall heat transfer coefficient in a heat exchanger.

摘要

In order to evaluate the performance of a heat exchanger, it is essential to determine the overall heat transfer coefficient. This can be determined using a steady state method which is time consuming and involves long waiting time for the steady state to be reached. In this thesis, a general technique is developed using the transient-based method to evaluate the overall heat transfer coefficient in a concentric tube heat exchanger. The transient-based method described here provides a speedy measurement of the overall heat transfer coefficient. This technique can be used for a parallel flow or counter flow heat exchanger with equal or unequal velocities of fluids involving liquid-to-liquid or liquid-to-gas heat transfer. Euler's implicit method and a finite element method combined with the Galerkin approach is used to solve the energy equations of a concentric tube heat exchanger for the determination of the overall heat transfer coefficient. In this work, the proposed method has also been applied to study the performance of an augmentation device for heat transfer enhancement. Several heat exchanger tubes enhanced with wire coil inserts having different wire diameters (0.813 1.016 mm, 1.575 mm) and coil pitches (8.47 mm, 5.08 mm, 3.63 mm, 2.82 mm) are tested. The experimental results indicate that the wire coil inserts improve the convective heat transfer coefficient by a factor of 1.56 to 2.34 compared with a smooth tube at the expense of an increase in pumping power by about 8 to 25 times.