LOCAL HEAT-TRANSFER AND FLOW LOSSES IN FIN-AND-TUBE HEAT-EXCHANGERS WITH VORTEX GENERATORS - A COMPARISON OF ROUND AND FLAT TUBES

Local heat transfer on the plate fin of a fin-and-tube heat exchanger element with flat tubes in staggered arrangement was measured by liqui d crystal thermography in the Reynolds number range of 600-3000. The f low loss was estimated from the measured pressure loss. The influence of longitudinal vortex generators on fin heat transfer and flow losses was investigated. The results were compared with similar experimental results for round tubes. For the staggered fin-and-tube arrangement, the longitudinal vortices increase heat transfer only marginally (10%) for round tubes but dramatically (100%) for flat tubes. The heat exch anger element with flat tubes and vortex generators gives nearly twice as much heat transfer and only half as much pressure loss as the corr esponding heat exchanger element with round tubes.