Influence of the Flow From an Axial Fan on the Performance of a Heat Exchanger

Abstract

Limited water supplies in arid regions that have abundant solar resources eliminates the use of water as a feasible means of cooling condensers in a Concentrated Solar Power (CSP) plant condenser. This has triggered the need to optimise existing air-cooled condenser technology, which is currently extremely inefficient. This paper aims to investigate the influence of various fan parameters on the performance of a cross-flow heat exchanger. The study first focuses on the effect of varying the distance between the fan and the heat exchanger in order to establish if uniform airflow distributions can be achieved with acceptable axial spacing between the fan and the heat exchanger. This was achieved by mapping the velocity field at the outlet from the heat exchanger by means of a Particle Image Velocimetry (PIV) analysis. The analysis was carried out for two air flow scenarios; the fan mounted at the inlet to the heat exchanger (forced draught) and the fan mounted at the outlet of the heat exchanger (induced draught). An investigation into the effect of fan speed on velocity distribution was also carried out. The measurements which are presented show that uniform velocity distributions can be achieved with relatively small fan to heat exchanger spacing for the case of the induced draught, whilst for the forced draft, although increasing the fan to heat exchanger spacing resulted in increased flow uniformity, the flow was still highly non uniform at fan to heat exchanger spacing of up to 1.4 times the fan tip radius. The measurements also showed little effect of fan speed on normalised velocity distribution. Combining the fore mentioned measurements with an analytical calculation technique, the heat flux per unit area across the heat exchanger was calculated. The results highlight the limitations on heat transfer in various regions of the heat exchanger in both flow scenarios. These measurements and calculations will facilitate designers of air cooled heat exchangers in achieving the minimum fan to heat exchanger spacing which gives no further increase in total heat transfer.