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3 years ago

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In an adiabatic mixing device (two inlets and one exit) operating at steady state, a 10 kg/s flow of fluid-A (with a specific en

tropy of 5 kJ/kg.K) is mixed with a 5 kg/s flow of fluid-B (with a specific entropy of 10 kJ/kg.K). At the exit (the flow rate is 15 kg/s), the specific entropy is measured as 7 kJ/kg.K. Determine the rate of entropy generation in the device.

1 answer:

MAXImum [283]3 years ago

8 0

Explanation:

Formula for entropy balance for the given situation is as follows.

$\frac{Q}{T} + m_{A}S_{A} + m_{B}S_{B} - m_{AB}S_{AB} + S_{gen}$ = 0

As the given system is adiabatic, hence Q = 0.

So, $S_{gen} = m_{AB}S_{AB} - (m_{A}S_{A} + m_{B}S_{B})$

As the given data is as follows.

$m_{A}$ = 10 kg/s, $S_{A}$ = 5 kJ/kg K

$m_{B}$ = 5 kg/sec, $S_{B}$ = 10 kJ/kg K

$S_{AB}$ = 7 kJ/kg K,

$m_{AB} = m_{A} + m_{B}$

= 10 + 5

= 15 kg/sec

$S_{gen} = 15 \times 7 - (10 \times 5 + 5 \times 10)$

= 5 kW/K

Thus, we can conclude that the rate of entropy generation in the device is 5 kW/K.

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Answer: (a) 1.065 N (b) 2.13 N

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<h2>(a) average force exerted by the rain on the roof</h2><h2 />

According Newton's 2nd Law of Motion the force $F$ is defined as <u>the variation of linear momentum</u> $p$ <u>in time:</u>

$F=\frac{dp}{dt}$ (1)

Where the linear momentum is:

$p=mV$ (2) Being $m$ the mass and $V$ the velocity.

In the case of the rain drops, which initial velocity is $V_{i}=15m/s$ and final velocity is $V_{f}=0$ (we are told the drops come to rest after striking the roof). The momentum of the drops $p_{drops}$ is:

$p_{drops}=mV_{i}+mV_{f}$ (3)

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$p_{drops}=mV_{i}$ (4)

Now the force $F_{drops}$ exerted by the drops is:

$F_{drops}=\frac{dp_{drops}}{dt}=\frac{d}{dt}mV_{i}$ (5)

$F_{drops}=\frac{dm}{dt}V_{i}+m\frac{dV_{i}}{dt}$ (6)

At this point we know the mass of rain per second (mass rate) $\frac{dm}{dt}=0.071 kg/s$ and we also know the initial velocity does not change with time, because that is the velocity at that exact moment (instantaneous velocity). Therefore is a constant, and the derivation of a constant is zero.

This means (6) must be rewritten as:

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$F_{drops}=(0.071 kg/s)(15m/s)$ (8)

$F_{drops}=1.065kg.m/s^{2}=1.065N$ (9) This is the force exerted by the rain drops on the roof of the car.

<h2>(b) average force exerted by hailstones on the roof </h2><h2 />

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3 years ago

Does resistance in a circuit remain constant if the temperature changes?

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Answer:

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