ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
It is a widely adopted practice??even adopted in ASHRAE/ANSI/ARI engineering standards??to use the arithmetic mean of two heat transfer measurements for the evaluation of heat-exchanger performance. However@ this approach does not generally lead to a minimized experimental uncertainty because uncertainties of redundant measurements can vary considerably depending on the experimental techniques and the test conditions. Moreover@ based on this approach it is preferred to discard information when its uncertainty exceeds some limit. It is proposed in this paper that Qave should be calculated based on a form of weighted-linear average@ with weighting factors depending on the individual uncertainties in Qh and Qc. Heat-transfer rate which has larger uncertainty will be weighed less in the average@ and the other one with smaller uncertainty will be weighed more accordingly. Implementing this new methodology will minimize the uncertainty in heat-transfer coefficient and Colburn j factors@ which will consequently provide more accurate data for use in the development of correlations or for performance comparison purposes. Through analysis of experimental data with different uncertainties@ the benefit of weighted average method was demonstrated. The results showed that the weighted averaging method recuded the average relative uncertainty in j factors from 11% to 10.3% for dry condition data@ and from 21.8% to 13.1% for wet condition data. The benefit was more pronounced as the air-side Reynolds number increased. Because the air-side uncertainty is usually much higher under wet operating conditions@ the weighted average method is highly recommended for data reduction with dehumidifying conditions.