Free Convection Boiling MCQ

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• A 1.0 mm diameter and 300 mm long nickel wire is submerged horizontal in water at atmospheric pressure. At burnout, the wire has a current of 195 A. Calculate the voltage at burnout. The relevant thermos-physical properties are p f (fluid) = 959.52 kg/m3 p g (vapor) = 0.597 kg/m3 h f g = 2257000 J/kg σ = 0.0533 N/m
  ক

  6.15 V

  খ

  7.15 V
• Estimate the peak heat flux for water boiling at normal atmospheric pressure. The relevant thermo-physical properties are p f (liquid) = 958.45 kg/m3 p g (vapor) = 0.61 kg/m3 h f g = 2.25 * 10 6 J/kg σ = 0.0585 N/m
  ক

  1.53 * 10 8 W/m2

  খ

  1.53 * 10 7 W/m2
• Identify the wrong statement with respect to boiling heat transfer?
  ক

  The steam boilers employing natural convection have steam raised through pool boiling

  খ

  Boiling occurs when a heated surface is exposed to a liquid and maintained at a temperature lower than the saturation temperature of the liquid

  গ

  Leiden-frost effect refers to the phenomenon of stable film boiling

  ঘ

  The nucleation boiling is characterized by the formation of bubbles at the nucleation sites and the resulting liquid agitation
• In nucleate pool boiling, the heat flux depends on
  ক

  Liquid properties, material and condition of the surface

  খ

  Material of the surface only

  গ

  Material and roughness of the surface

  ঘ

  Liquid properties and material of the surface
• The heat flux in nucleate boiling varies in accordance with
  ক

  h f g

  খ

  (h f g) 0.5

  গ

  1/(h f g) 2

  ঘ

  (h f g) 3
• Natural convection heat transfer coefficients over surface of a vertical pipe and a vertical flat plate for same height. What is/are the possible reasons for this? (i) Same height (ii) Both vertical (iii) Same fluid (iv) Same fluid flow pattern Select the correct answer
  ক

  iv

  খ

  i and ii

  গ

  i

  ঘ

  iii and iv
• Consider the above problem, find the excess temperature of the wire
  ক

  18.01 degree Celsius

  খ

  19.01 degree Celsius

  গ

  20.01 degree Celsius

  ঘ

  21.01 degree Celsius
• A 0.10 cm diameter and 15 cm long wire has been laid horizontally and submerged in water at atmospheric pressure. The wire has a steady state voltage drop of 14.5 V and a current of 42.5 A. Determine the heat flux of the wire. The following equation applies for water boiling on a horizontal submerged surface H = 1.54 (Q/A) 0.75 = 5.58 (d t) 3 W/m2 K where Q/A is the heat flux rate in W/m2 and d t is the temperature difference between surface and saturation
  ক

  1.308 * 10 8 W/m2

  খ

  1.308 * 10 7 W/m2

  গ

  1.308 * 10 6 W/m2

  ঘ

  1.308 * 10 5 W/m2
• Fritz criterion is given by
  ক

  h = 1.973 (Q/A) 0.45

  খ

  h = 1.973 (Q/A) 0.55

  গ

  h = 1.973 (Q/A) 0.65

  ঘ

  h = 1.973 (Q/A) 0.75
• When evaporation takes place at the liquid-vapor interface, the heat transfer is solely due to free convection and the film coefficient follows the relation
  ক

  Nu = f 1 (G r) f 2 (P r)

  খ

  Nu = 2 f 1 (G r) f 2 (P r)

  গ

  Nu = 3 f 1 (G r) f 2 (P r)

  ঘ

  Nu = 4 f 1 (G r) f 2 (P r)