Experimental and numerical analysis of the effect of local non-uniformity on convective heat and mass transfer in porous media.

摘要

The role of non-uniformity on convective transport processes namely fluid flow, heat transfer and mass transfer in porous media using experimental and numerical analysis was studied. Experimental techniques reported here provided instantaneous results on fluid flow and heat transfer and mass transfer characteristics as well as local temperature and velocity variations specifically aimed at better understanding the role of non-uniformity. The numerical model presented here takes into account the various transport processes involved as well as local heterogeneities, which are inherent in many porous media. Experimental results on fluid flow through dry sheet structures with and without artificially induced non-uniformity enabled us to verify the accuracy of the model predictions. There was a reasonable comparison between the model predictions and experimental data on fluid flow vs. pressure drop under variety of non-uniformity conditions. Numerical simulations were then conducted to further study the role of non-uniformity under varying conditions of non-uniformity including increasing the degree of non-uniformity, random vs. non-random distribution, non-uniformity by means of inducing initial moisture variation etc. Structural or formation non-uniformity caused by artificially introduced holes was found to cause higher degree of non-uniform drying than the initial moisture content variation. The results indicate that spatially concentrated distribution of initial variations, i.e. moisture, formation, structure was found to introduce more drying non-uniformity than randomly distributed variations. While exhaust air temperature variation may be more easily measurable, this may not fully represent the complexities of non-uniformity and hence may not be a true characteristic to represent non-uniformity. Based on the results obtained in this study and the mechanisms alluded to earlier, exhaust air local velocity variation, a direct representation of the local moisture content and formation variation, is a better characteristics than temperature variation to directly represent the instantaneous physical mechanism of heat and mass transfer and non-uniformity during convective through flow drying. While moisture variation in the sheet is a direct result of drying non-uniformity, considering the difficulties in measuring the instantaneous local moisture content variation, the velocity variation might be a better characteristic which can accurately measure and truly represent the non-uniformity during drying.