Bff3203 Heat Transfer

Matric No Program Table No

Universiti Malaysia PAHANG

* Erg re'.rirç Technccjy Crc*Uvty

FACULTY OF MANUFACTURING ENGINEERING

FINAL EXAMINATION

COURSE : HEAT TRANSFER

COURSE CODE : BFF32031BFF3242

LECTURER : ASSOC. PROF DR ABDUL AZIZ JAAFAR

DATE : 17J TJNE 2015

DURATION : 3 HOURS

SESSION/SEMESTER : SESSION 2014/2015 SEMESTER II

PROGRAMME CODE : BFF/BFM

INSTRUCTIONS TO CANDIDATE:

1. This Question Book has SIX (6) Questions. 2. Answer ONLY FIVE (5) Questions. 3. All answers MUST be written in the Question Book.

EXAMINATION REQUIREMENTS:

NIL

DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO

This examination paper consists of FIFTEEN (15) printed pages including front page

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ANSWER ONLY FIVE (5) QUESTIONS

QUESTION 1 [20 MARKS]

Using your own word, express your understanding on the following concepts and their significance on engineering design.

a) Blot Number and Fourier Number [5 marks]

b) Consider two plane-walls of different conductivity. Both walls have identical geometrical dimension and are subjected to an identical temperature boundary conditions. Provide an argument on the temperature distribution and the rate of conduction heat transfer in both walls. [15 marks]

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QUESTION 2 [20 MARKS]

A piece of aluminum with irregular shape weighing 6 kg and at 300°C is suddenly immersed in a fluid bath at 20°C. You are required to model the physical setup and express the governing equation as well as the boundary and initial conditions, if any, to estimate the followings:

a. the time period to reduce the temperature of the aluminum below 90°C

b. the amount of heat loss during the time period, and

c. the temperature history at the center of mass of the aluminum

Take the convection heat transfer coefficient to be 58 W/m2K

a) Sketch the engineering model: [5 marks]

b) Engineering Assumptions [5 marks]

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c) Solution Procedure: [10 marks]

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QUESTION 3 [20 MARKS]

A long 10-cm-thick brass sheet at 20°C are heat treated by drawing the metal sheet at a velocity of 2 mlmin through a 6-rn long oven maintained at 700°C.

You are required to propose the appropriate speed for drawing the metal sheet through the oven such that the center plane temperature of the sheet is greater than 600°C.

Given the heat transfer coefficient in the oven is 220 W/m2K.

a) Sketch the engineering model: [5 marks]

b) Engineering Assumptions [5 marks]

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c) Solution Procedure: [10 marks]

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QUESTION 4[20 MARKS]

A 25-cm diameter steel ball is removed from the oven at a uniform temperature of 300T to be cooled by convection of air flowing at 3 m/s in a room conditions.

You are required to evaluate the uniformity of internal temperature of the steel ball by providing the average convection heat transfer coefficient and duration of the cooling process if the surface temperature of the ball drops to 200°C

Given the dynamic viscosity of air at room and at 200°C are given as 1.846 x 10 Pa.s and 2.849 x 10 Pa.s respectively

a) Sketch the engineering model: 15 marks]

b) Engineering Assumptions [5 marks]

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c) Solution Procedure: [10 marks]

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QUESTION 5 [20 MARKS]

A long roll of 2-rn-wide and 0.5-cm thick Manganese steel plate coming off a furnace at 820°C and entering a 9-rn long oil bath at a steady velocity of 20 mlrnin for quenching process at 45°C.

You are required to evaluate the quenching process and the sizing of a cooling equipment for maintaining the oil bath temperature at 45°C if it is assumed that the heat transfer coefficients on both sides of the plate are 860 W/m2K,

a) Sketch the engineering model: [5 marks]

b) Engineering Assumption [5 marks]

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c) Solution Procedure: [10 marks]

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QUESTION 6 [20 MARKS]

A manufacturing section of a plastic plant deliver a continuous 1.2-m wide and 0.1-cm thick sheet of a plastic at a velocity of 9 rn/mm, The temperature of the plastic sheet is 95°C when it is the manufacturing section.

During the delivery, a 0.6 —m-long section of the plastic sheet is exposed to a surrounding air at 25°C. The air flows at 3 m/s on both sides of the sheet surfaces in the crosswise direction to the plastic motion.

You are required to model a heat transfer system for evaluating the rate of heat transfer from the plastic sheet to the flowing air and the temperature of the plastic sheet at the end of 0.6-rn long exposure section cooling section assuming that the emissivity of the plastic is 0.9. Take the dynamic viscosity of air at 300°C is 2.76x10 5 Pa.s

a) Sketch the engineering model: -- 110 marks]

b) Engineering Assumptions [5 marks]

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c) Solution Procedure [10 marks]

END OF EXAMINATION PAPER

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APPENDIX

Fluid Pronerties substance Thermal Density Specific heat Dynamic

conductivity (kg/rn3) at constant viscosity (W/mK) pressure (Pa.$)

(J/kgK) Air atlatmand 0.02514 1.204 1007 1.825x105

20 OC

Oil 0.1404 863.9 2048 0.07399

Water 0.598 998 4180 1.002

Solid Properties substance Thermal

conductivity (W/mK)

Density (kg/m)

Specific heat at constant

pressure (J/kgK)

aluminum 237 2702 903

brass 110 8530 380

manganese 15.1 +8055 480

stainless steel 63.9 7832 434

plastic 0.2 1200 1700

Emnirical Correlation Physical setup Average Nusselt number

0.664ReL°'5Pr°33 Laminar Flow over a isothermal flat plate

for Pr> 0.6 and ReL <5x105

0.037Re8Pr033 Turbulent flow over an isothermal flat plat

for 0.6 Pr < 60 and 5x10 5 ReL < 10

Turbulent Flow over a spheref \O.25

2 + [0.4Re° 5 + 0,06Re0667]Pr04 /j (-\ jig

Where Re and Pr are Reynolds number and Prandtl number respectively. Subscript 00 and s refers to values at surrounding and surface respectively

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Biot number and Integration Constants for one-term approximation of transient one dimensional conduction heat transfer through plane wall

BJ X, 001 0.0998 1.0017 0,02 : 0.1410 1 0033 OOt 01 7 10066 0.06 0,2425 .L0098 008 0.271 1.0130 0_1 03111 10161 O.2 04328 1.0311 :03 O5218 1.04.50 04 05932 10560 0.5 0.6533 1.0701. 0.6 07051 1,0814 07 0.756 10918 0$ O.790 1.1016 0.: 0.274 1.1107 IC 08603 1,1101 2.0 1-0769 1,1785 3.0 11925 1,2103 40 12646 12287 5.0 1-31 38 1.2403 6.0 1,346 1,2479 7.0 143766 1.2532 6.0 1..378 1.2570

0 1.41.40 1.2598 10.0 1.4289 12620 200 1.4961 1.269 300 1.5202 L2717 40.0 1.53;25 1.2,723

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Graphical and Analytical Solution for One-term Approximation of Thermal Characteristics for Heat Transfer Syustem Plane Wall

o T-7 T IyTro

hO

0. 9

0.8

0.7

0.6

03

0.4

0.3

02

0.1 U 0.01

I - k Iii hL

Temperature Distribution in a plane wall

4sin A ,, -A2

2A -- sn2A) e cos (Ax IL)

A, tan A - Bi

0.1 .1.0 10 100

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