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A hair dryer is basically a duct of constant diameter in which a few layers of electric resistors are placed. A small fan pulls the air in and forces it through the resistors where it is heated. If the density of air is 1.20 kg/m3 at the inlet and 1.035 kg/m3 at the exit, determine the percent increase in the velocity

Answer :

Given:

density of air at inlet, [tex]\rho_{a} = 1.20 kg/m_{3}[/tex]

density of air at inlet, [tex]\rho_{b} = 1.05 kg/m_{3}[/tex]

Solution:

Now,

[tex]\dot{m} = \dot{m_{a}} = \dot{m_{b}}[/tex]

[tex]\rho_{a} A v_{a} = \rho _{b} Av_{b}[/tex]                        (1)

where

A = Area of cross section

[tex]v_{a}[/tex] = velocity of air at inlet

[tex]v_{b}[/tex] = velocity of air at outlet

Now, using eqn (1), we get:

[tex]\frac{v_{b}}{v_{a}} = \frac{\rho_{a}}{\rho_{b}}[/tex]

[tex]\frac{v_{b}}{v_{a}} = \frac{1.20}{1.05}[/tex] = 1.14

% increase in velocity = [tex]1.14\times 100[/tex] =114%

which is 14% more

Therefore % increase in velocity is 14%

Answer:

The percent increase in the velocity is 15.9%.

Explanation:

Given that,

Density of air [tex]\rho_{in}= 1.20\ kg/m^3[/tex]

Density of air [tex]\rho_{ex}=1.035\ kg/m^3[/tex]

We need to calculate the percent increase in the velocity

Using continuity equation

[tex]\rho_{in}Av=\rho_{ex}Av'[/tex]

[tex]\dfrac{\rho_{in}}{\rho_{ex}}=\dfrac{v'}{v}[/tex]

Put the value into the formula

[tex]\dfrac{v'}{v}=\dfrac{1.20}{1.035}[/tex]

[tex]\dfrac{v'}{v}=1.159[/tex]

[tex]v'=1.159v[/tex]

For percentage,

[tex]v'=\dfrac{1.159v-v}{v}\times100[/tex]

[tex]v'=15.9\%[/tex]

Hence, The percent increase in the velocity is 15.9%.

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