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QUESTION
Q-1
Q-2
EASTERN MEDITERRANEAN UNIVERSITY
FACULTY OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
MENG 345 HEAT TRANSFER
MID-TERM EXAM
(Spring 2014-2015)
Q-3
Q-4
TOTAL:
(Out of 25)
STUDENT’S
NAME, SURNAME:………………………………….
NUMBER
:………………………………….
GROUP NO
:……….
Type of examination: B.S. Examination – Mid Term
Time allowed: 90 mins
Date: April 24, 2015
Instructions to candidates
1. This is a closed book exam.
2. Do not ask any questions to the invigilators.
3. Answers are expected to all questions.
4. Answers are to be written in the spaces provided under each question.
5. Only non-programmable calculators are allowed.
6. Be clear and specific and give units to answers.
7. Give explanatory notes where necessary.
MARKS
Question 1:
Consider the base plate of a 1200-W household iron that has a thickness of L = 0.5
cm, base area of A =300 cm2, and thermal conductivity of k = 15 W/m·°C. The inner
surface of the base plate is subjected to uniform heat flux generated by the
resistance heaters inside, and the outer surface loses heat to the surroundings at T =
20°C by convection, as shown in Figure. Taking the convection heat transfer
coefficient to be h = 80 W/m2·°C and disregarding heat loss by radiation, obtain an
expression for the variation of temperature in the base plate, and evaluate the
temperatures at the inner and the outer surfaces. (5 marks)
Question 2:
Clothing made of several thin layers of fabric with trapped air in between, often called
ski clothing, is commonly used in cold climates because it is light, fashionable, and a
very effective thermal insulator. So it is no surprise that such clothing has largely
replaced thick and heavy old-fashioned coats. Consider a jacket made of five layers
of 0.1-mm-thick synthetic fabric (k = 0.13 W/m · °C) with 1.5-mm-thick air space (k =
0.026 W/m · °C) between the layers. Assuming the inner surface temperature of the
jacket to be 28°C and the surface area to be 1.1 m2, determine the rate of heat loss
through the jacket when the temperature of the outdoors is -5°C and the heat transfer
coefficient at the outer surface is 25 W/m2 · °C. (5 marks)
Question 3:
In Betty Crocker’s Cookbook, it is stated that it takes 2 h 45 min to roast a 3.2-kg rib
initially at 4.5°C “rare” in an oven maintained at 163°C. It is recommended that a
meat thermometer be used to monitor the cooking, and the rib is considered rare
done when the thermometer inserted into the center of the thickest part of the meat
registers 60°C. The rib can be treated as a homogeneous spherical object with the
properties ρ=1200 kg/m3, Cp = 4.1 kJ/kg · °C, k = 0.45 W/m · °C, and α= 0.91 x 10-7
m2/s. Determine (a) the heat transfer coefficient at the surface of the rib, (b) the
temperature of the outer surface of the rib when it is done, and (c) the amount of heat
transferred to the rib. (d) Using the values obtained, predict how long it will take to
roast this rib to “medium” level, which occurs when the innermost temperature of the
rib reaches 71°C. Compare your result to the listed value of 3 h 20 min.
If the roast rib is to be set on the counter for about 15 min before it is sliced, it is
recommended that the rib be taken out of the oven when the thermometer registers
about 4°C below the indicated value because the rib will continue cooking even after
it is taken out of the oven. Do you agree with this recommendation? (10 marks).
Question 4:
The temperature of a gas stream is to be measured by a thermocouple whose
junction can be approximated as a 1-mm-diameter sphere, as shown in Figure. The
properties of the junction are k = 35 W/m · °C, ρ = 8500 kg/m3, and Cp = 320 J/kg ·
°C, and the convection heat transfer coefficient between the junction and the gas is h
= 210 W/m2 · °C. Determine how long it will take for the thermocouple to read 99
percent of the initial temperature difference. (5 marks).
Formula:
𝑞̇ =
𝑄̇
𝐴
∆𝑇
𝑄̇ =
𝑅
𝑅𝑐𝑜𝑛𝑑 =
𝐿
𝑘𝐴
𝐴𝑐𝑦𝑙,𝑠 = π𝐷𝐿
𝐴𝑐𝑦𝑙,𝑐 = π
𝑝𝑐𝑦𝑙 = π𝐷
D2
4