Heat Transfer

281. Nucleate boiling is promoted

  1. On polished surfaces
  2. On rough surfaces
  3. In the absence of agitation
  4. None of these
Correct answer: (B)
On rough surfaces

282. Nusselt number (for forced convection heat transfer) is a function of the _________ number.

  1. Prandtl
  2. Reynolds
  3. Both (A) & (B)
  4. Neither (A) nor (B)
Correct answer: (C)
Both (A) & (B)

283. Nusselt number is a function of Prandtl number and _________ number of fluid in natural convection heat transfer.

  1. Grashoff
  2. Biot
  3. Stanton
  4. Reynolds
Correct answer: (A)
Grashoff

284. Nusselt number is the ratio of the

  1. Temperature gradient of the wall to that across the entire pipe
  2. Temperature difference to the temperature gradient at the wall
  3. Heat flux at the wall to that across the entire pipe
  4. None of these
Correct answer: (C)
Heat flux at the wall to that across the entire pipe

285. Nusselt number is the ratio of the temperature gradient at the wall to

  1. Temperature difference
  2. Heat flux
  3. That across the entire pipe
  4. None of these
Correct answer: (C)
That across the entire pipe

286. One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to

  1. Heat the water from 0°C to 100°C
  2. Evaporate the water
  3. To superheat the steam
  4. Data insufficient, can't be predicted
Correct answer: (B)
Evaporate the water

287. Open pan evaporators are preferred to be used, when the solution to be concentrated is

  1. Scaling
  2. Highly viscous
  3. Corrosive
  4. Salty
Correct answer: (B)
Highly viscous

288. Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be

  1. 0.35
  2. 0.45
  3. 0.55
  4. 0.85
Correct answer: (B)
0.45

289. Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (Δt1 - Δt2)/ln(Δt1/Δt2)], which one is subject to the largest deviation in practice ?

  1. Constant overall heat transfer co-efficient.
  2. Constant rate of fluid flow
  3. Constant specific heat
  4. No partial phase change in the system
Correct answer: (B)
Constant rate of fluid flow

290. Overall heat transfer co-efficient for cooling of hydrocarbons by water is about

  1. 50 -100 Kcal/hr.m2.°C
  2. 50 -100 W/m2.°K
  3. 50 -100 BTU/hr. ft.2°F
  4. 1000 - 1500 BTU/hr. ft.2°F
Correct answer: (C)

50 -100 BTU/hr. ft.2°F

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