Convection Flashcards
What is convection?
Convection is the transfer of heat from one place to
another by movement of a fluid itself (liquid or gas).
It does not occur in solids
What is free convection?
This kind of convection is called natural or free convection
because it does not need an outside power source other
than the heat source
Which cases do free convection only work in?
If the fluid expands on heating.
* In the presence of gravity. Thus, there can be no free
convection in space.
What is forced convection?
The fluid is forced to move
over a surface by a pump or fan, and
transfers heat to or from that surface by
conduction
What does free convection involve?
It involves the transfer of heat by the motion and
mixing of portions of a fluid
Describe forced convection in a motor engine?
Inside the engine, a pump circulates water to remove heat from
the combustion chambers (by conduction) and deliver it to the
“radiator”.
* The motion of the vehicle forces air to flow through the radiator
and over the engine block.
* When the vehicle stops, free convection may be inadequate, and
an electric fan is started automatically to maintain the cooling.
What are the factors that influence convection?
The fluid concerned and its physical properties: specific
heat, density, conductivity, viscosity, etc.
The velocity: the rate of heat transfer increases as the
fluid velocity increases
Temperature difference or surface area
Describe Newtons Law of cooling?
Applies when a body is cooling under conditions of forced convection.
States that the rate of loss of heat of a body is proportional to the difference in temperature between the body and its surroundings as well as the bodys surface area
What is the formula for Newtons law of cooling for convection and conductiion? For forced convection
Q=hAx change in temperaute
h= a constant
A= Available area
Change in temperature= Change in temperature between the surface and fluid
For conduction
Q=k(T1-T2)XA/L
Newtons cooling laws in free convection
Q=KxAx(Change in temp) ^3/4
K is a constant that must be found experimentally
Difference between heat coefficient h and thermal conductivity k?
h relates to the amount of heat crossing a surface rather than what goes on inside it
A thin steel plate 10 cm square and of mass 80 g is heated in boiling
water and then suspended in still air of 21
C. It is noted later, the
plate drops in temperature from 50°C to 49°C in 10 seconds. Estimate
the heat transfer coefficient h. (The specific heat for steel is c=460 Jkg-
1K-1)
h=Q/(A x (Change in temperature))
3.68/2 x 0.1^2 x 28.5=6.5Wm^-2 K^-1
What is the U-value?
The overall heat transfers coefficient represents the overall thermal conductance to outside to inside through all modes of heat transfer
What does the u-value measure
It measures the rate of heat transfer through a building element over
a given area, under standardized conditions:
20ºC indoors and 0ºC outdoors, with a 4.4 m/s wind
Units for U-Value?
W/m² K).
What does U = 1 W/m²K mean?
For every degree of temperature difference between the inside surface and the
outside surface, 1 Watt of heat energy would flow through each meter squared of
its surface
Overall heat flow rate?
For a building containing a number of materials, the
overall heat flow rate is the sum of the heat flow rates of all
its subcomponents
Overall heat flow rate equation
Q=U x A x Change in temperature
How to calculate heat loss?
Q=mc
Q=Energy
m= Mass
c= specific heat capacity
What does a lower value of U-Value indicate and what does a higher value of U-value indicate?
Lower values mean the material is a better insulator and the higher the u value means the material is a better conductor
What is the formula of heat loss including U-Value?
Q=UxAx(T2-T1)
Q=Heat loss (W)
U= U-Value (W/m(^2)K)
A=Area (m^2)
(T2-T1)= Change in temperature
What is the typical temperature difference under standard conditions between inside and outside?
20 C
In a hot country, an office in a multi-story building has two external walls. The
room is 5.0 m wide and 6.0 m long and has a ceiling height of 3.0 m. In the
middle of one of the external walls, there is a glass window which measures 2.0
m wide by 1.5 m tall. When the average outside temperature is 33 C, how much
power is required from an air-conditioner to keep the room temperature at 22
C? (Ignore the heat transfer between the room and the inside of the building.
Average U-value for the walls and the glass are 0.60 Wm-2K-1 and 5.5 Wm-2K-1
respectively)
Q=Q(wall) + Q (window) =198+181.5=379.5W
Thermal Resistance Formula
R=L/kA
R=Thermal resistance (K/W)
L=Thickness of material in (m)
k=Thermal conductivity of the matieral (W/mxk)
A=( Cross sectional area)
Trend of material thickness in relation to thermal resistance
Higher thermal resistance as heat has to travel a longer distance
What does a high k or low k mean?
High K is a good conductor
Low K is a Poor conductor
Trend of Area in relations to thermal resistance
The larger the area the lower the thermal resistance
Three building materials, plasterboard (k=0.3W/mK), brick (k=0.6 W/mK),
and wood (k=0.10 W/mK) are sandwiched together as shown. The
temperature at the inside and outside surfaces are 27 C and 0 C
respectively. Each material has the same cross-section area of 3 m2 and the
thickness of plasterboard, brick and wood are 1mm, 1.5mm and 3mm
respectively. Find the heat flow rate through the building materials
R(p)=0.001/0.3x3= 0.0011K/W
R(b) =0.0015/0.6x3=0.00083
R(w)-0.003/0.10x3=0.001 K/W
Total R=0.01193 K/W
Q=(T2-T1)/R
27-0/0.01193=2263W
Time-Dependant Heat Transfer formula
Rate of heat transfer=Heat transfer coefficient x Surface Area x (T2-T1)
dQ/dt=h x A x (T2-T1)
A metal plate of area
0.5m^2 is at 80∘𝐶 In air with an ambient temperature of 25∘C
The heat transfer coefficient is 15 W/m^2xK
Calculate the rate of heat transfer at𝑡=0
dt/dQ=15⋅0.5⋅(80−25)=15⋅0.5⋅55=412.5W
Average Rate of Heat Loss formula
Q average=
Q(2)xT(2)-Q(1)x(t1)/t2-t1
Heat energy drops from
5000J to 3000 over 20 seconds. Find the average heat loss rate.
3000-5000/20=-100W
Average Heat Loss using Temperature
Q average= hA (T2+T1)/2 - T0)
A plate with a surface area of
1m^2 cools from 60 to 50 C
The ambient temperature is 25 and h=15 w/m^2xk
T avg = 60+50/2=55
Q average=15⋅1⋅(55−25)=15⋅30=450W
A person loses heat at a rate of 10 W when exposed to the wind at 12ºC. At
what rate would they lose heat, with the same clothing and at the same wind
speed, at 0ºC? Take the normal body temperature as 37ºC
Q1/10=37/25
Q1=14.8
