1B-10 | Dimensionless Groups and Equations | 5 pts |
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The following dimensionless equation correlates the values of variables in a system in which solid particles are suspended in a flowing gas. | |||||||||
Eqn 1 | |||||||||
Both of the terms in parentheses are dimensionless groups. kg is a coefficient that expresses the rate at which a particular chemical is transferred from the gas to the surface of the solid particles. The value of kg is needed to design a catalytic reactor. Since this coefficient is difficult to determine directly, values of the other variables are measured and kg is calculated from the given equation. | |||||||||
The following values were measured : | |||||||||
dp | 5.00 |
mm | μ | 1.00E-05 |
N-s/m2 | ||||
y | 0.100 |
(dimensionless) | ρ | 1.00E-03 |
g/cm3 | ||||
D | 0.100 |
cm2/s | v | 10.0 |
m/s | ||||
What is the estimated value of kg ? What are the units of kg ? Show your work. | |||||||||
Read : | The key here is that the equation and the groups in parentheses are dimensionless. | ||||||||
This tells us that the constants 2.00 and 0.600 are also dimensionless. | |||||||||
So, we can plug numbers and units into the right-hand side of the equation to determine the value of the dimensionless group on the left-hand side. Then, we use the values and units of dp, y and D to determine the value and units of kg. | |||||||||
It is probably wise to begin the problem by converting all of the given values to one consistent system of units. I chose the MKS system. | |||||||||
Given : | dp | 5.00 |
mm | μ | 1.00E-05 |
N-s/m2 | |||
y | 0.100 |
(dimensionless) | ρ | 1.00E-03 |
g/cm3 | ||||
D | 0.100 |
cm2/s | v | 10.0 |
m/s | ||||
Solution : | Once all the given values are in a consistent set of units, we can evaluate each of the dimensionless groups in Eqn 1. | ||||||||
5000 |
1.00 |
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Let's double check the units on these groups using the MKS units. | |||||||||
Eqn 2 |
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Eqn 3 |
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All the units cancel, so we can conclude that both of these groups are indeed dimensionless. | |||||||||
44.426 |
(dimensionless) | ||||||||
First, let's determine the units for kg. | |||||||||
Eqn 4 |
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0.889 | |||||||||
Answer : | kg | 0.889 |
m/s | ||||||
88.9 |
cm/s |