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dezoksy
3 months ago
15

The temperature distribution across a wall 0.3 m thick at a certain instant of time is T(x) a bx cx2 , where T is in degrees Cel

sius and x is in meters, a 200 C, b 200 C/m, and c 30 C/m2 . The wall has a thermal conductivity of 1 W/mK. (a) On a unit surface area basis, determine the rate of heat transfer into and out of the wall and the rate of change of energy stored by the wall. (b) If the cold surface is exposed to a fluid at 100 C, what is the convection coefficient
Engineering
1 answer:
iogann1982 [368]3 months ago
6 0

Answer:

The heat transfer rate into the wall is \mathbf{q__{in}} \mathbf{ = 200 W/m^2}

The heat output rate is \mathbf{q_{out} =182 \ W/m^2}

The change in stored energy in the wall is \mathbf{ \Delta E_{stored} = 18 \ W/m^2 }

The convection coefficient is h = 4.26 W/m².K

Explanation:

Considering the problem:

The temperature profile across the wall is expressed as:

T(x) = ax+bx+cx^2

where:

T = temperature in °C

and a, b, & c are constants.

Substituting a = 200° C, b = -200° C/m, and c = 30° C/m² results in:

T(x) = 200x-200x+30x^2

This follows the application of Fourier's Law of heat conduction.

q_x = -k \dfrac{dT}{dx}

where the heat input rate q_{in} = q_k; Then x= 0

<pThus:

q_{in}= -k (\dfrac{d( 200x-200x+30x^2)}{dx})_{x=0}

q_{in}= -1 (-200+60x)_{x=0}

\mathbf{q__{in}} \mathbf{ = 200 W/m^2}

Consequently, the heat transfer rate into the wall measures \mathbf{q__{in}} \mathbf{ = 200 W/m^2}

The heat output rate is:

q_{out} = q_{x=L}; where x = 0.3

q_{out} = -k (\dfrac{dT}{dx})_{x=0.3}

Replacing T with 200x-200x+30x^2 and k with 1 W/m.K

q_{out} = -1 (\dfrac{d(200x-200x+30x^2)}{dx})_{x=0.3}

q_{out} = -1 (-200+60x)_{x=0.3}

q_{out} = 200-60*0.3

\mathbf{q_{out} =182 \ W/m^2}

Thus, the heat output rate is \mathbf{q_{out} =182 \ W/m^2}

Applying the energy balance to find the change in energy (internal energy) stored in the wall.

\Delta E_{stored} = E_{in}-E_{out} \\ \\ \Delta E_{stored} = q_{in}- q_{out} \\ \\ \Delta E_{stored} = (200 - 182 ) W/m^2 \\ \\

\mathbf{ \Delta E_{stored} = 18 \ W/m^2 }

Thus, the energetic change rate stored in the wall is \mathbf{ \Delta E_{stored} = 18 \ W/m^2 }

We know that in a steady state, the heat reaching the end of the plate must be convected to the surrounding fluid.

Thus:

q_{x=L} = q_{convected}

q_{x=L} = h(T(L) - T _ \infty)

where;

h represents the convective heat transfer coefficient.

Therefore:

Replacing \ 182 W/m^2 \ for \ q_{x=L}, (200-200x +30x \ for \ T(x) \, 0.3 m \ for \ x \ and \ 100^0 C for \ T We find:

182 = h(200-200×0.3 + 30 ×0.3² - 100 )

182 = h (42.7)

h = 4.26 W/m².K

Thus, the convection coefficient equals h = 4.26 W/m².K

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<ptherefore the="" mass="" of="" jar="" is:="">

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5. To calculate the weight and volume of the displaced water, we first need to ascertain how deep the jar sinks (H), as the volume of displaced water equals the submerged volume of the jar. The jar will descend until the gravitational force downwards equals the buoyancy force upwards. The displaced water volume is πa^2 • H, and the buoyancy is calculated as ρw • g • Vd (where ρw is the density of water, defined as 62.5 lb/ft^3 / 1728 = 0.036 lb/in^3, and Vd is the displaced water volume).

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ρw • g • πa^2 • H = m • g

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ρw • πa^2 • H = m

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