Friday, April 29, 2011

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...

Wednesday, April 27, 2011

But...What is wall friction anyways? Part 2

Now, Jenike proposed a measuring device to determine wall friction, that device is now known as the Jenike tester, other testers have sprung up which could be used to determine wall friction, but the Jenike cell has become the industry standard. The basic method consists of taking a sample of the powder and a sample piece of the hopper or silo wall, the powder is then poured into a ring (usually metallic) and then a normal load is applied (by placing dead weights on the cell), then the ring is pushed by an actuator and the shear force is measured. The ring displacement can be measured too. This is done for different loads so as to cover a wide range of normal loads.

If one plots shear force (or shear stress by dividing over the area of the cell) against normal force (or normal stress), a line is usually formed, but this is not always the case. For solids this line is straight, most of the times, but not for powders (at least not that often). I have seen more than one publication where the shear stress is plotted against the normal stress and a line is created for each point (the second point being the origin), and an envelop of lines is presented. Other times, the researchers have reported their wall friction by fitting a line to the data, which could produce a low linear correlation value.

A better way to understand the data is by plotting the wall friction against the applied normal stress. The wall friction angle is calculated by taking the arc tangent of the ratio of shear stress to normal stress. Usually, a decaying curve is obtained so that at low normal stresses, a high friction angle is obtained. This plot is then used to determine the wall friction at a certain normal stress.

To be continued...

Tuesday, April 26, 2011

But...What is wall friction anyways?

Yes, that's my thesis topic. The problem with this concept (experimentally determined) is that no predicting method or model exists that could help design equipment given known parameters. It is a very old problem, many people have tackled it, but so far, no explicit, extensively used solution exists.

Now, why is this so important? To answer that question, we have to go back to the 1960's when Andrew Jenike published his now extensively cited report. But even before, people like Janssen had understood that powders did not behave like liquids when they are stored in containers. Janssen found that unlike liquids where the pressure is proportional to the height of the container (so that as the container becomes higher, the pressure at the bottom increases), the pressure increases and at some point it becomes constant so that no matter how high the silo is, the pressure will be constant at the bottom. Janssen found that the reason this is so, it's because of friction, the powder and the wall develop friction at contact, which helps in supporting the whole mass of the powder.

Then Jenike came along and used this previous findings to develop a comprehensive method for designing silos, bunkers, and any vessel that may contain powder. But wall friction is one of the main parameters needed for the selection of the correct hopper for the silo. Since the powder has to flow by gravity, the hopper has to be used in aiding the powder flow out of the silo, this hopper can have many converging geometries. The correct angle of this hopper depends strongly on the wall friction created by the contact of the powder and the wall.

To continue...

Friday, April 22, 2011

Why I entered Particle Science & Technology (continues)

Dr. Brian Scarlett passed away on 2004 (send me an email if you want a special paper on him). I remember the day he announced he was going to go under treatment for cancer (2003), it was a surprise to me, but even more of a surprise that he passed away so quickly. I had just come on board the previous year and started working on photoelastic methods to determine stresses at the bottom of a photoelastic plate using a Jenike tester.
I was able to find an undergrad student to help me set up a polariscope, we were able to take some pictures, but the stresses were so small that the material barely registered something. I was able to find a manufacturer of photelastic material that had a very low Young's modulus, but from the preliminary tests it was obvious that determining the stresses from the images was going to be a tough task.
It is interesting to find that Dr. Scarlett had a great faith on photoelasticity, he conducted doctoral thesis on its application to powder flow, but for what we wanted to do, it was not going to work as expected.
I love optical methods for stress analysis, I would have stayed in the field if it weren't because of the large amount of people already in it, it so competitive it is hard to make any noticeable contribution. I especially like Moire and Speckle interferometry, using light to measure stresses is really exciting. At that time I knew I had to find a field where I could make a contribution and some impact. I had to put on hold my dreams of working on optical methods for stress analysis.
I am proud of having worked for him, I talked to him a couple of times and he was a man who could carry any conversation in any topic. He loved his students and he truly cared for them, I miss him, things would have been a little different for me if he was living today.

Why I entered Particle Science & Technology

This has been simply an amazing adventure. I am a mechanical engineer by training, but my Ph.D. will be in chemical engineering. I remember my Dad suggesting me entering chemical engineering when I was deciding my major in college. Like many sons, I decided to go against his suggestion and entered ME. Things have come full circle, and now I am doing exactly what he wanted me to do at the beginning. But I also want to do it to honor his memory, my Dad passed away last September 2010.
I finished my MS in MEMS (micro-electro-mechanical systems) with the IMG (Interdisciplinary Microsystems Group) at the Department of Mechanical and Aerospace Engineering (f.k.a. AEMeS) at the University of Florida. Just as another recession (2001-2003) was sweeping the country, and many people were struggling finding work, all this made my employment prospects pretty gloomy, so I decided to start a Ph.D. I would have love to stay at IMG, but there was no room for me. I started looking for another advisor at MAE. I talked to three professors and only Dr. Nicolaie D. Cristescu promised to think about it. At that time, I decided to join a group of friends in a trip to Peru, the experience was amazing. I still feel the peace of walking through the Andes mountains, hearing the water dripping from the glaciers and walking the steps the Incas walked centuries ago, I will never forget that visit.
A few days before coming back, I received an email from Dr. Cristescu asking me to come to his office because he might have an opportunity at PERC (Particle Engineering Research Center). After I arrived, the next Monday I went to his office and he walked me to the PERC building and introduced me to Dr. Brian Scarlett and Dr. Kerry Johanson. I remembered going into Dr. Scarlett's office for an interview with both of them, both made lots of questions about my background in optics, especially photoelasticity.
Next week Dr. Cristescu offered me the position and there I went into Particle Technology and Science.