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ELEN [110]
3 years ago
8

An alloy is evaluated for potential creep deformation in a short-term laboratory experiment. The creep rate (ϵ˙) is found to be

1% per hour at 800 ∘C and 5.5×10−2% per hour at 700 ∘C. Calculate the activation energy for creep in this temperature range.
Engineering
1 answer:
cupoosta [38]3 years ago
3 0

Answer:

Activation energy for creep in this temperature range is Q = 252.2 kJ/mol

Explanation:

To calculate the creep rate at a particular temperature

creep rate, \zeta_{\theta} = C \exp(\frac{-Q}{R \theta} )

Creep rate at 800⁰C, \zeta_{800} = C \exp(\frac{-Q}{R (800+273)} )

\zeta_{800} = C \exp(\frac{-Q}{1073R} )\\\zeta_{800} = 1 \% per hour =0.01\\

0.01 = C \exp(\frac{-Q}{1073R} ).........................(1)

Creep rate at 700⁰C

\zeta_{700} = C \exp(\frac{-Q}{R (700+273)} )

\zeta_{800} = C \exp(\frac{-Q}{973R} )\\\zeta_{800} = 5.5 * 10^{-2}  \% per hour =5.5 * 10^{-4}

5.5 * 10^{-4}  = C \exp(\frac{-Q}{1073R} ).................(2)

Divide equation (1) by equation (2)

\frac{0.01}{5.5 * 10^{-4} } = \exp[\frac{-Q}{1073R} -\frac{-Q}{973R} ]\\18.182= \exp[\frac{-Q}{1073R} +\frac{Q}{973R} ]\\R = 8.314\\18.182= \exp[\frac{-Q}{1073*8.314} +\frac{Q}{973*8.314} ]\\18.182= \exp[0.0000115 Q]\\

Take the natural log of both sides

ln 18.182= 0.0000115Q\\2.9004 = 0.0000115Q\\Q = 2.9004/0.0000115\\Q = 252211.49 J/mol\\Q = 252.2 kJ/mol

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Do not mix Antifreeze with used oil
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The following statements are about the laminar boundary layer over a flat plate. For each statement, answer whether the statemen
Nikolay [14]

Answer:

1. B. False

2.  B. False

3. A. True

4. B. False

5. A. True

6. A. True

7. A. True

Explanation:

1. B. False

The relation of Reynolds' number, Reₓ to boundary layer thickness δ at a point x is given by the relation

\delta = \dfrac{x \times C}{\sqrt{Re_x} }

That is the boundary layer thickness is inversely proportional to the square root of the Reynolds' number so that if the Reynolds' number were to increase, the boundary layer thickness would decrease

Therefore, the correct option is B. False

2.  B. False

From the relation

Re_x = \dfrac{U_o \times x}{v}

As the outer flow velocity increases, the boundary layer thickness diminishes

3. A. True

As the viscous force is increased the boundary layer thickness increases

4. B. False

Boundary layer thickness is inversely proportional to velocity

5. A. True

The boundary layer model developed by Ludwig Prandtl is a special case of the Navier-Stokes equation

6. A. True

Given a definite boundary layer thickness, the curve representing the boundary layer thickness is a streamline

7. A. True

The boundary layer approximation by Prandtl Euler bridges the gap between the Euler (slip boundary conditions) and Navier-Stokes (no slip boundary conditions) equations.

6 0
3 years ago
Propane (C3H8) burns completely with 150% of theoretical air entering at 74°F, 1 atm, 50% relative humidity. The dry air compone
yarga [219]

Answer:

y=0.12 lbmol(water)/lbmol(products)

Explanation:

First we find the humidity of air. Using humidity tables and the temperature we find that is 0.01 g water/L air.

Now we set the equation assuming dry air:

C_{3}H_{8}+7.5(O_{2}+3.76N_{2}) \longrightarrow 3CO_{2}+4H_{2}O+2.5O_{2}+28.2N_{2}

With this equation we have almost all moles that exit the reactor, we are just missing the initial moles of water due to the humidity. So we proceed to calculate it with the ideal gas law:

PV=nRT

Vair=867.7L

With the volume and the fraction of water, we can calculate the mass of water:

0.01 * 867.7=8.677 g of water

Now we calculate the moles of water:

8.677 g / 18 g/mol = 0.48 moles of water

Now we can calculate the total moles of water in the exit of the reactor:

0.48 + 4 = 4.48 moles of water

And finally we just need to sum all moles at the exit of the reactor and divide:

3 moles of CO2 + 2.5 moles of O2+ 4.48 moles of H2O + 28.2 moles of N2

And we have 38.18 moles in total, then:

4.48/38.18=y=0.12 moles of water/moles of products

As the relation moles/moles is equal to lb-moles/lb-moles, we have our fina result:

y=0.12 lbmol(water)/lbmol(products)

6 0
3 years ago
What is a yield criterion? What are the assumptions that we normally make when applying a yield criterion?
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Answer:

Yield criteria is a hypothetical criteria which defines the elastic limit in a material and the starting of permanent or plastic deformation under different stress combinations.

or

We can say that relationship among different combinations of stress provides an idea of the yield of any material can be termed as yield criteria.

Assumptions while applying yield criteria:

  • The material to be used is incompressible
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4 0
3 years ago
An electrical heater is a form of sensible heating process, and heats 0.1 m/s of air from 15°C and 80% RH to 50°C? The barometri
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Answer:

heater wattage = 4.32 kW

Explanation:

given data

volume flow rate of air = 0.1 m³/s

temperature t1 =  15°C

temperature t2 =  50°C

relative humidity RH = 80%

pressure = 101.325 kPa

to find out

heater wattage

solution

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that is

h1 = 37 kJ/kg

and

h2 = 73 kJ/kg

and v1 = 0.829 m³/kg

so here heater wattage formula is

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m = V/v1 = 0.1 / 0.819 = 0.12 kg/s

so from equation 1

heater wattage = 0.12 ( 75 -39 )

heater wattage = 4.32 kW

5 0
3 years ago
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