Air-side heat transfer coefficients, Visualization method, Coating, Tracer gas, Color change
Low air-side heat transfer coefficients (HTC) are often the bottleneck of heat exchanger performance. However, the measurement of local air-side heat transfer coefficients for the entire heat transfer surface is not a trivial task. Current experimental methods often only work on scaled-up samples which typically cannot deliver continuous HTC distributions across the surface of interest. Moreover, some methods require very precise and costly equipment such as lasers. Therefore, a novel visualization method is designed to obtain local air-side HTC distributions of an entire heat exchanger with a relatively simple experimental facility. The method relies on measuring mass transfer and applying the analogy between heat and mass transfer to determine heat transfer. The process involves a thin acidic coating on the heat transfer surface which is exposed in a wind tunnel to a suitable trace gas, in this case, a low concentration ammonia-air mixture. The coating will absorb the ammonia and gradually change color from yellow to green to blue, depending on the exposure time and local ammonia concentration. By observing the color difference, the mass transfer could be acquired and local HTC is calculated subsequently. In order to develop this method, different metal samples such as aluminum, copper and stainless steel were coated and tested. The thickness and evenness of coatings are measured by a stylus profiler. The results show that the coating characteristic will not have a significant impact on airflow and the boundary layer conditions. So, the conditions of applying the analogy between mass and heat transfer are fulfilled. Preliminary mass transfer experiments show continuous color change on a flat plate which is proportional to mass transfer of ammonia from the free stream flow to the coated surface. Thus, this new method is very promising to acquire local HTC with a visualization approach on different geometries.