Influence of geometrical parameters on the flow characteristics of multi-pipe earth-to-air heat exchangers -experimental and CFD investigations

Efficiency of the earth-to-air heat exchangers depends not only on their thermal performance but also on the total pressure losses that are the cost of harvesting a geothermal heat. In this paper the sensitivity analysis of the flow characteristics to the change of multi-pipe exchanger geometry is presented. Experimental investigation and CFD simulation results present total pressure losses in the considered exchangers and airflows in each branch pipes. Considered geometrical structures varies in the number of parallel pipes, pipes length, main pipes diameters and supply type. The experimental investigations were conducted on the exchangers models in a scale 1:4. To investigate the real size exchangers, validated CFD flow performance model was used. A costless modification of heat exchanger supply-type from Z-type to U-type structure (change in air inlet location) is verified as a simple method of decreasing total pressure losses by 6-36% and improving airflow division uniformity by 11-80%. It is shown that main pipes diameter that are 1.4 times bigger than parallel pipes diameter can result in diminished total pressure losses by 56-73% and improved airflow division uniformity by 6-59%. The least significant effect on the flow characteristics has the branch-pipe length. Total pressure losses of long branch pipes exchangers can be 15-32% higher than for short ones and the airflow division uniformity can be 8-35% higher. Results can be used for choosing the proper geometry of multi-pipe earth-to-air heat exchangers from the flow performance point of view. Presented flow characteristics can be used in detailed analysis and energy assessment of exchangers cooperating with the mechanical ventilation system in building.