Question

**What does Nusselt number equals? Show that it is the
dimensionless temperature gradient at the wall, in the process of
external convection heat transfer.**

Answer #1

A flat wall is exposed to an environmental temperature of 38
degrees C. The wall is covered with a layer of insulation 2.5 cm
thick whose thermal conductivity is 1.8 W/(m.K), and the
temperature of the wall on the inside of the insulation is 320
degrees C. The wall loses heat to the environment by convection.
Compute the value of the convection heat transfer coefficient that
must be maintained on the outer surface of the insulation to ensure
that the...

A hot black painted pipe(2.54-cmO.D.and50-mlong)passesthrougha
20°C room to heat it. The temperature of the pipe surface is 150°C.
If the heat transfer coefficient by convection to the air is 10
W/m2-K :
a) What is the total heat rate loss?
b) What is the radiation heat transfer coefficient?
c) At what temperature of the pipe, the heat rate by convection
equals the heat rate by radiation.

Air at freestream velocity U∞=16
ms, and free steam temperature
T∞=50℃ is
flowing over a plate surface that is at temperature
Ts=100℃. The velocity and thermal
boundary layers developing on the surface have been shown in the
figure. Also shown are the tangents to the velocity and temperature
profiles at the surface y=0. If the density of air is
ρ∞=1.1
kgm3, viscosity
μ=1.963×10-5kgm∙s, and
thermal conductivity k=0.0274Wm∙K then,
calculate
(a) the wall shear
stress, τw
(b) coefficient of skin
friction,...

Why is a temperature gradient used in gas chromatography? What
effect does increasing the temperature have on eluting
compounds?

Air is flowing at a Mach number of M = 0.8 and a temperature of
35 Celsius. What temperature would the air be if it were brought to
rest with no external work or heat transfer?

Water at 15°C enters a tube of 2 cm of diameter with flow rate
3953 kg/h. Assume the ratio L/D>10, and the wall temperature is
constant at 80-C. The outlet temperature is 50°C The properties of
water at the film temperature are density rho = p = 985 kg/m3,
specific heat Cp = 4180 J/kgk, conductivity k = 0.651 W/mK, dynamic
viscosity mu= u = 4.71 × 10- kg/ms, At the wall temperature of 80°C
we have dynamic viscosity muw=...

Heat and mass transfer question:
Why blackbody does not exist? What if there is a "blackbody" in
outer space without any contact to any object, so no conduction no
convection, can I emit its 100% energy? Why not?

A thin metallic wall may be constructed using copper (properties
are given in the table). The piping is required
to have a radius r = 0.008 m and carries steam at 385 K. The wall
is inside a room surrounded by air at a temperature
of 298 K. The wall is insulated with a material (properties are
given in the table).
Properties
Copper
Insulation material
Thermal Conductivity (W/m per ˚C)
385
0.071
Density (kg/m3)
8940
453
i. If the external...

1.What is the relationship between the change in length and
change in temperature of a body?
2. When is a system considered to be in thermal equilibrium?
When does heat transfer from one body to another stop?
3. Where does the amount of heat absorbed by ice go if its
temperature remains the same?

An aluminum transmission line with a diameter of 20 mm has an
electrical resistance of R'elec= 2.636 × 10-4 Ω/m and
carries a current of 700 A. The line is subjected to frequent and
severe cross winds, increasing the probability of contact between
adjacent lines, thereby causing sparks and creating a potential
fire hazard for nearby vegetation. The remedy is to insulate the
line, but with the adverse effect of increasing the conductor
operating temperature.
(a) Calculate the conductor temperature...

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