E-PolyLearning

16. Dielectric heating apparatus consists of
a. 4 electrodes
b. Elemental strip
c. No Insulating material
d. 4 plates
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Answer: (b).Elemental strip

17. A slab of insulating material of thickness 6 cm and thermal conductivity 1.4kJ/m hr deg is placed between and is in contact with two parallel electrodes, and is then subjected to high frequency dielectric heating at a uniform rate of 140,000kJ/m³ hr. At steady state coefficients of combined radiation and convection are 42 and 48 kJ/m² hr deg. If atmospheric temperature is 25 degree Celsius, find surface temperatures?
a. 144.10 degree Celsius and 134.47 degree Celsius
b. 123.50 degree Celsius and 154.34 degree Celsius
c. 121.60 degree Celsius and 115.45 degree Celsius
d. 165.40 degree Celsius and 165.45 degree Celsius
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Answer: (c).121.60 degree Celsius and 115.45 degree Celsius

18. A 25 mm diameter egg roll (k = 1 W/m degree) is roasted with the help of microwave heating. For good quality roasting, it is desired that temperature at the center of roll is maintained at 100 degree Celsius when the surrounding temperature is 25 degree Celsius. What should be the heating capacity in W/m³ of the microwave if the heat transfer coefficient on the surface of egg roll is 20 W/m² degree?
a. 113.31 k W/m³
b. 213.31 k W/m³
c. 313.31 k W/m³
d. 413.31 k W/m³
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Answer: (b).213.31 k W/m³

19. The constants of integration are to be determined from the relevant boundary conditions which are
(i) t = t w at r = R
(ii) q g (π L R²) = -k (2 π R L) (d t/d r)
(iii) Increasing temperature gradient
Choose the correct option?
a. Only i
b. Only ii
c. i and iii
d. i and ii
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Answer: (d).i and ii

20. A concrete column used in bridge construction is cylindrical in shape with a diameter of 1 meter. The column is completely poured in a short interval of time and the hydration of concrete results in the equivalent of a uniform source strength of 0.7 W/kg. Determine the temperature at the center of the cylinder at a time when the outside surface temperature is 75 degree Celsius. The column is sufficiently long so that temperature variation along its length may be neglected. For concrete
Average thermal conductivity = 0.95 W/m K
Average density = 2300 kg/m³
a. 190.92 degree Celsius
b. 180.92 degree Celsius
c. 170.92 degree Celsius
d. 160.92 degree Celsius
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Answer: (b).180.92 degree Celsius

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