1. The assignment sheet contains 6-pages (excluding this cover page)
2. Print off all pages except this cover page and put all your answers in this printed document for
3. Question 1: Put only your answers in this section. No calculations are needed. Full marks for correct
4. Question 2: In experimental work you will be faced with large data sets as an engineer, which you
have to be able to filter and analyse. You are given the raw data in a text file. Process the raw data,
convert the temperature from K to Celcius, and plot using spreadsheet software e.g. MS Excel, Google
Docs, or Matlab, according the instructions given in each question. After you have plotted, sketch
what you see, onto the graphs provided in this paper.
5. The same data can also be obtained through Week-9 Computer Lab files (we will provide question and
answer time during this session to help you).
6. Show all your calculations for Question 2g.
7. Question 3: Show all calculations.
8. Write all your answers on the assignment sheet and submit this sheet. Do not submit this cover sheet.
Page 1 of 6
|MIET1081 Assignment C
Sem 1, 2014
Due Date: 26/5/14
Question 1 [20 marks] – External flow over a cylinder
Consider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing their body to
|airflow.||Heat transfer by convection and radation is tak|
|ing place simultaneously.||The air temperature is 32oC, and|
|the fan is blowing air at a velocity of 5m/s.||The sur|
|rounding surfaces are at 40oC, and the emissivity of the per|
|son can be taken to be 0.9.||If the person is doing light|
|work and generating sensible heat at a rate of 90W, deter
mine the average temperature of the outer surface (neglect sur
|face, skin or clothing) of the person.||The average human|
|body can be treated as a 30-cm-diameter cylinder with an ex|
|posed surface area of 1.7m2.||The properties of air at 32oC|
Pr = 0:72; k = 0:02625W=m:K; ν = 1:655 × 10-5kg=ms
Use the Nu number correlations that are in the form of Nu = CRemP rn where C,m, and n are constants.
1. (a) [2marks] What is the heat transfer coefficient, h? h =
(b) [6marks] Rewrite the energy balance equation, Q_ gen + Q_ conv + Q_ rad = Q_ net by substituting in
its parameters (e.g. h; A; T; ; σ).
(c) [8marks] Solve for the person’s surface temperature Ts either by using a guess, and iterative method
or solving tools such as MS Excel’s Goal Seek, or mathematics such as finding roots/zero’s of equations.
(d) [4marks] Why is the radiation heat transfer acting in the opposite direction to the convection flow
(i.e. on the opposite sides of the equation)?
Due Date:26/5/14 MIET1081 Assignment C Page 2 of 6
Question 2 [40 marks] – Fully Developed Pipe Flows
Internal forced convection heat transfer can be found in various industrial applications. Ansys-Fluent is
used to simulate flow in a pipe with air and water. The geometry is given below (note: not to scale).
The material properties are as follow:
air: ρ = 1:225kg=m3; Cp = 1006J=kg:K; k = 0:0242W=m:K; µ = 1:79 × 10-5kg=ms
water: ρ = 998kg=m3; Cp = 4182J=kg:K; k = 0:6W=m:K; µ = 1:0 × 10-3kg=ms
The fluid velocity is adjusted for each case to achieve a Reynolds number of Re=100.
You are given raw data points for velocity and temperature in a pipe at different x-locations along the
pipe (data files inside the zipped file). The first column of data is the radius r, as the distance from the
bottom wall beginning at r=0m to the symmetry line at r=0.1m. The second column of data is the flow
variable, either velocity, u in [m/s] or temperature, T in [K].
2. (a) [4marks] Sketch the velocity profiles at x=0.4m, x=1.0m, x=3.0m for air, and for water. Re-sketch
the profiles on the axes below, include the range of values for the velocity axis:
(b) [2marks] Plot the velocity profile for air, and water at x=3.0m. Re-sketch the profiles on the axes
below. Include the range of values for the velocity axis.
(c) [4marks] Re-plot the normalised velocity profiles (u=umax) for air, and water at x=3.0m. Re-sketch
the profiles on the axes below. Include the range of values for the velocity axis.
Due Date:26/5/14 MIET1081 Assignment C Page 3 of 6
(d) [6marks] Plot the temperature profile for air, and water at x=3.0m. Re-sketch the profiles on the
axes below. Include the range of values for the velocity axis.
(e) [2marks] What are the hydrodynamic entrance lengths for air: water:
(f) [2marks] What are the thermal entrance lengths for air: water:
(g) [20marks] What is the heat transfer rate for air? (Show all your calculations)
Due Date:26/5/14 MIET1081 Assignment C Page 4 of 6
(Show all your calculations)
Due Date:26/5/14 MIET1081 Assignment C Page 5 of 6
Question 3 [40 marks] – Natural Convection
A double-pane window has a vertical height of 0.9-m
and a width of 1.4-m that consists of two layers of
glass separated by a 10-cm air gap at atmospheric
pressure. The room temperature is 26oC while the inner
glass temperature is 18oC. Radiation heat transfer, is
neglected. Steady operating conditions exist.
Properties For natural convection between the
inner surface of the window and the room air,
the properties of air at 1 atm and the film temperature of (Ts+T1)/2 = (18+26)/2 = 22oC are:
Pr = 0:7304; k = 0:02529W=m:K; ν = 1:534 × 10-5kg=ms
Properties For natural convection between the two glass sheets separated by an air gap, we can set
an initial guess of T2 = 0C The properties of air at 1 atm and the anticipated average temperature of
(T1+T2)/2 = (18+0)/2 = 9oC are: Pr = 0:7339; k = 0:02431W=m:K; ν = 1:417 × 10-5kg=ms
3. [40marks] (a) the temperature (T2) of the outer glass layer and (b) rate of heat loss (Q_ ) through the
window by natural convection.(Show all calculations)
Due Date:26/5/14 MIET1081 Assignment C Page 6 of 6
(Show all your calculations)
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