Radiation & Cooling Effect Video Lecture | Heat Transfer - Mechanical Engineering

welcome to a lecture online then here's
another very practical example of
radiation and the cooling effect of
radiation so let's assume that you're
out in the woods camping out and you're
in your sleeping bag at night and let's
say that you just on the tarp open sky
above you how much heat are you losing
through radiation well imagine that the
outside of the sleeping bag is at the
temperature of five degrees centigrade
and a good sleeping bag is is one where
there's so much insulation that the
outside of the sleeping bag is nearly
the same temperature as the air around
you because if the outside of the
sleeping bag is warm that means that a
lot of heat is transferring from the
inside the outside the sleeping bag and
that would not be a good insulated bag
and secondly the warmer the outside of
the temperature of the sleeping bag the
more that it will radiate out into the
into space because the dq/dt
the amount of here radiated per unit
time is equal to e Sigma the area and
then of course the temperature to the
fourth power so the warmer the outside
of the bag is the more heat it's
radiating out secondly you want to be
somewhere where you're covered over by
some sort of shelter because then you're
not exposed to space then actually the
shelter even though the shelter may be
at a very cold temperature let's say
minus five degrees centigrade same as
the outside temperature it will still
radiate heat back to you and you'll be a
lot warmer when you're underneath the
shelter like that so let's calculate the
difference so first we're going to do
this case right here the area of the bag
let's say that it's about two meters
about two square meters in area then we
get this is equal to 0.9 which would
probably a good approximation for the
emissivity of the bag times Sigma which
is five point six seven times ten to the
minus eight watts per square meter per
Kelvin to the fourth power times the
area let's assume that the bag has an
air of about two square meters and the
temperature would be five degrees
centigrade
of course we have to convert that to
Kelvin and five plus 273 is 278 Kelvin
so that would be the amount of heat
being expelled to to space and if
there's nothing above you and a twin
still day then and
there's no clouds in the sky then a lot
of heat will be generated and the bag
outside back will get very very cold now
keep in mind that if it's a cloudy day
then the clouds kind of act like a
shelter like that and on a cloudy day
you will not nearly get as cold as on a
very car I should say on a cloudy night
you will not get as cold as you went on
a clear night so this let's see what the
numbers are here so we have point nine
times five point six seven eight to the
eight minus x two and x to 78 and of
course we have to take that and raise it
to the fourth power we can't forget that
okay and so then we have six hundred and
ten watts so that would be why the heat
loss so you definitely do not want to be
exposed to the atmosphere if it's -5
degrees centigrade outside and your
radiant on to space you could lose a lot
of heat and you might be fairly
uncomfortable in that bag of yours but
if you put yourself in need a shelter
then what it becomes is now we have the
Q DT is equal to e Sigma a times the
temperature of the back to the fourth
power minus the temperature of the
surroundings to the fourth power and if
the roof of the shelter is at minus five
degrees centigrade let's say it's open
to the atmosphere and it's the same
temperature the atmosphere still you'll
get quite a bit of radiation back from
that so let's calculate what that would
be so this is equal to zero point nine
times five point six seven times ten to
the minus eight watts per meter squared
Kelvin to the fourth power
that's four right there the area still
two meters squared and the temperature
that would be two hundred whether we say
was five degree centigrade that's 278
Kelvin to the fourth power minus the
temperature of the roof of the shelter
which would be about 268 Kelvin five
below in centigrade and we have to raise
up to the fourth power as well so what
would be the difference here so I have
two seventy eight to the fourth power
minus two 68 to the fourth power
multiply the times to multiply times
five point six seven e to the eighth
minus multiply times two point nine
equals and I get eighty-three watts to
notice what a tremendous difference they
can make on radiation alone you'd only
be losing eighty three Watts versus six
hundred ten watts by being an Anita
shelter first is being in the open so if
you ever caught any in the wilderness
and you want to protect yourself at
night definitely try to find shelter
try to keep that radiation out into
space of course if it's really windy and
it's wind blowing all over the place and
it's snowing that kind of thing that's
still another story but at least from
the perspective radiation and the
shelter is a lot better than being wide
out in the open