Heat Transfer 2 Flashcards
In natural convection heat transfer, the correlating parameter is the
Grashoff number
Boiling of milk in an open vessel is an example of __________ boiling.
film
Pick out the wrong statement.
The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1.
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
natural convection
Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that
material is anisotropic.
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80�C and the right face is exposed to air at 30�C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in �C is
48.7
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
This is valid for the value of NRe equal to
> 10000
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2�C.
5000-15000
Correction is applied to LMTD for __________ flow.
cross
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
always between h1 and h2
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
evaporate the water.
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
The radiation heat flux from a heating element at a temperature of 800�C, in a furnace maintained at 300�C is 8 kW/m2. The flux, when the element temperature is increased to 1000�C for the same furnace temperature is
16.5 kW/m2
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
D/5 and D
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
logarithmic
In a gas-liquid shell and tube heat exchanger, the
all ‘a’, ‘b’ & ‘c’
Which is the most suitable for the concentration of foamy & frothy liquors?
Long tube vertical evaporator
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
heat transfer area
Which of the following has the lowest overall heat transfer co-efficient?
Air
Which of the following has the minimum absorptivity?
Aluminum foil
A black body when hot, emits heat radiation of __________ wavelengths.
all
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA < 0.5kB
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
all (a), (b) and (c)
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
Sensible heat of hot industrial flue gases can not be recovered by a/an
none of these
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
In pipe flow, heat is transferred from hot wall to the liquid by
forced convection and conduction
The value of Stefan-Boltazman constant in SI unit is
5.6697 x 10-8W/m2.�K4
The unit of heat transfer co-efficient is
BTU/hr. ft2�F
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
The inner wall of a furnace is at a temperature of 700�C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.�C-1 respectively. The ambient air is at 30�C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.�C-1. The rate of heat loss from the outer surface in W.m-2is
165.4
Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
2
Low thermal conductivity of heat insulating materials is due to its
high proportion of air space
The most conducive surface for dropwise condensation to occur is the __________ surface.
oily
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
concentrations
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
Pick out the correct equation.
jH = (St)(Pr)^2/3 = f/2
Multiple effect evaporators ar used to
increase the steam economy & the capacity.
Prandtl number is given by
(Cp � ?)/k
Small scale evaporation is done in a
steam jacketed kettle
Heat exchanger tubes are never made of
lead
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
Reason for operating an evaporator in multiple effect is to secure
both (a) and (b).
In a cooling tower, water becomes cool by
loosing sensible heat.
In Fourier’s law, the proportionality constant is called the
thermal conductivity
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
lower than
Pick out the wrong statement.
Thermal conductivity of a dry material is more than that of the damp material.
Duhring’s plot’ is of use in
evaporation
Which has the lowest Prandtl number?
Liquid metal
Pick out the wrong statement.
Duhring’s plot is used for calculating the concentration of solution.
Prandtl number for most of dry gases is about
0.72
Prandtl number is the reciprocal of
Thermal diffusivity/Momentum diffusivity
In a heat exchanger with steam outside the tubes, a liquid gets heated to 45�C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be
more than 45�C
For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is
Nu = 2/pi x Gz
Multiple effect evaporation accounts for
steam economy
In case of a supercooled solution, which is on the verge of crystallisation, the free energy of the solution as compared to that of the solid is
more
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
volume to surface area
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
85% magnesia cement and glass wool.
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of
Reynolds number for the flow involved is < 2100.
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
Triangular pitch
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
and the wall are proportional to individual resistances.
The purpose of providing expansion bellows in the shell of tubular exchanger is to
account for the uneven expansion of shell and tube bundles.
A graph between __________ is called Wilson plot.
1/U Vs. 1/V^0.8
Grashhoff number is given by
gD^3.?.?t?2/?^2
The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m2.K. The overall heat transfer co-efficient at this time is
333W/m2.K
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is
5000
Pick out the wrong statement.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
What is the geometric mean of two heat transfer areas A1 and A2?
sqrt*(A1 . A2)
Evaporation by thermocompression results in the
both (a) and (b).
Forced circulation evaporators are normally used for concentrating liquids having
both(a) & (b)
Steam economy in case of a triple effect evaporator will be
> 1
