Investigation of the impact of pipe geometric form on earth-to-air heat exchanger performance using Complex Finite Fourier Transform analysis. Part I: Operation in cooling mode

Abstract

This paper deals with the impact of the geometric form of pipe section upon the performance of earth-to-air heat exchangers operating in cooling mode. For this purpose, three different configurations, namely circular, rectangular and elliptical were selected. Based on the heat balance analysis, a transient two-dimensional model was developed for the disturbed soil around the buried pipe. The discretization of the partial differential equation by applying the Finite Control-Volume Method yields a new generalized heat equation valid for all kinds of cylinders. Then, the resulting set of ordinary differential time-equations was linearized by using the technique of Complex Finite Fourier Transform. After successfully validating the model against available data in the literature, a thorough sensitivity analysis was carried out for assessing the impact of geometric and operating parameters including pipe length and cross-section and air speed. Further, the effect of pipe flatness relating to the rectangular and elliptical configurations was also considered through the geometric aspect ratio.The results show that the rectangular and elliptical buried pipes are more effective because they possess large exchange surfaces compared to the circular (conventional) one. For instance, at a pipe cross-section of 0.07 m2 and a geometric aspect of 9, the exchange surface between the flowing air and pipe wall in the rectangular and elliptical exchangers are larger of 88.3% and 93.6% than that of the conventional system, respectively. Moreover, at a geometric aspect of 4 and a pipe length of 60 m, the outlet temperatures supplied by the rectangular and elliptical systems are of 1.5 and 1.3 °C less than that obtained with the conventional one. Regarding the influence of pipe dimensions, it is shown that the effect of pipe length is dominant compared to that of pipe section and the performance of the system is proportional to the pipe length. Likewise, the EAHE performance increases with the increasing of the pipe geometric aspect, but a value of 4 is found to be a good compromise. Though, the performance of EAHE is found to drop dramatically at high speeds of air.