What is wall friction? Part 3

Now, after measuring your wall friction angles, and selecting your hopper, and having it fabricated and installed, you go ahead and test your new hopper. Theory says the new hopper will work, and in many instances it will, but if you find yourself in the situation that It would not work, that you get hang-ups, or maybe ratholes, it is time to go back and retrace the whole process to find the culprit.

Although there could be other causes for hopper problems, wall friction is one of them, and I will concentrate on this problem for the time being. What many designers fail to recognize is that wall friction is not a static value. Many things can change the wall friction angle. The fabrication process might have affected the wall characteristics, there is also the possibility that the hopper is being used with a different material, since it is not unusual to have a silo handling other kinds  of bulk materials. The hopper and silo walls will sustain wearing, this changes the wall friction. The bulk solid is affected by humidity or temperature, even if the wall friction was measured taking those into account, you might still have problems during discharge, this is so because the influence of humidity on wall friction is not well understood at this point.

Of course, the best solution, after the fact, is to install a flow-promoting device. This adds cost to the original silo project, a little design sense can go a long way. You could use some criteria to make sure variations of the wall friction can be accounted for. Unfortunately, other than empirical methods, there is not a formal procedure than would help on this.

Previous researchers have identified the many variables that can affect wall friction, this makes the wall friction angle a mutivariable problem, which complicates matters a lot. But in some cases, some parameters might be important and others not so much. Of course this gives hope to investigators like me. In summary, many people have attacked the problem but a few have come up with a model to estimate wall friction.

Most of the studies out there are essentially presentations of collected data from which they make qualitative analysis to find trends when a variable is changed. I have only encountered three papers (as recent as 1980, before that I am not considering, since so much has changed since the 1970s) that suggest a model, and all of them obtained the model from macro parameters from the bulk testing...