Why is transport phenomena so hard

OK, then how are they all related? They all are similar in their behavior. They all move stuff Momentum, Heat, or Mass from a place where there is a lot of the stuff to a place where there is less stuff.

Here are some examples:. In Fluid Mechanics momentum is transferred from a place where we have a lot of momentum to a place where we have less. A good analogy is the flow of traffic on a busy freeway.

The far right lane on the freeway typically move slower than the far left lane, with the lanes in the middle going faster the further left you move. This can be compared to flow over a flat plate, where the slower flow the right lane on the freeway is right next to the plate, and the faster flow the left lane on the freeway on the surface of the fluid. The transfer of momentum is like the cars changing lanes, as slower cars pull into faster lanes the lanes slow down to allow the car to accelerate and not cause a pile-up on the freeway , and the faster cars pulling into slower lanes and speeding up the lane a little bit.

In Heat Transfer, energy moves from a place where there is a lot to a place where there is less. For example, if you heat up a brick, then drop it into cold water, the brick gets colder and the water gets warmer.

Once the brick and the water are at the same temperature, no more energy can be transferred. Mass Transfer. Back to the red dye example: the red dye when it is first dropped into the water is at a high concentration and the water is at zero concentration. But, as the dye spreads out, the concentration of the dye slowly increases, until, it is all at the same low concentration everywhere. Once the concentration of the dye in the water is the same everywhere, no more mass transfer can take place.

The one of the most important similarities between all of these examples is: There is a driving force momentum, temperature, or concentration difference or 'gradient' , which becomes smaller as time progresses in each of the examples, and eventually becomes zero when no more transfer of stuff takes place. The math for all of these "transport phenomena" all are based on 2 ideas:.

The first idea is summed up by three similar laws for each of the three "transport phenomena" here in one dimension and rectangular coordinates :. Conservation of Mass and Energy. The idea behind the conservation of mass and energy can be thought of a little like a shoe box: if you put stuff into it, you can either take it out or leave it in the box, you can't make the stuff disappear.

The small droplets would be useful from a mass transfer perspective because there would be more contact area between the phases. The two fluid densities would determine how rapidly they would settle. The viscosities would determine the power required to move the impeller and the impeller speed would dictate the mixing rate and droplet size. The conductivity of the fluids would determine what the cooling requirements might be if this is an exothermic reaction. You decide to encapsulate the urea-destroying enzyme urease in a solid pellet and build a "reactor" artificial organ to do the task.

A smaller pellet would allow better contact between the bulk fluid blood and the pellet reducing mass transfer limitations; however, small particles will also make it more difficult to "push" the fluid past the particles ever try to pack flour into a straw and then suck water through it?!

As in the previous example, we need to worry about heat transfer either to or from the pellet center depending on whether the reaction requires or releases heat. Other than that, it would be useful if the pellet was porous so that we could put more catalyst on the "surface", but we need to worry about how fast mass transfer will occur through those pores. You plan on designing a solar collector to go on the roof of your house and harness the energy of the sun.

The most obvious thing is that cloudy days will absorb much of the radiant energy from reaching the collectors. Also, the efficiency of the collectors may depend on temperature so that you may need to devise a way of keeping them warm.

The traditional way is to heat up a fluid like air or water and simply flow that through the house.