Assignment 1 (Kalam) PDF

Preview

Summary

Download Assignment 1 (Kalam) PDF

Description

ASSIGNMENT 1

BMM3513_2021/I

UMP

COLLEGE OF ENGINEERING BMM3513 HEAT TRANSFER Assignment 1 (CO1) NAME

/100

MATRIC NO.

QUESTION 1 [10 Marks] a) Two surfaces of a 2-cm-thick flat plate are maintained at 0oC and 80oC respectively. If it is determined that heat is transfer through the plate at a rate of 500 W/m 2, determine its thermal conductivity. (2 Marks) b) In a power plant, pipes are used for transporting superheated vapour. Superheated vapour is flowing at the rate of 0.3 kg/s inside a pipe with 5 cm diameter and 10 m in length. The pipe is located in the power plant which is at 20 oC and the pipe surface has a uniformly temperature of 100oC. If the temperature drop between the inlet and exit of the pipe is 30oC, and the specific heat of the vapour is 2190 J/kgK, determine the heat transfer coefficient as a result of convection between the pipe surface and the surrounding as given in Figure Q1b. (5 Marks)

Figure Q1b

ASSIGNMENT 1

BMM3513_2021/I

c) Consider a sealed 20-cm-high electronic box which has a base dimensions 50 cm x 50 cm placed in a vacuum chamber. The emissivity of the outer surface of the box is 0.95. If the electronic components in the box dissipating a total of 120 W of power and the outer surface temperature of the box is not to exceed 55C as in Figure Q1c, determine the temperature at which the surrounding surfaces must be kept if this box is to be cooled by radiation alone. Assume the heat transfer from the bottom surface of the box to the stand to be negligible. (3 Marks)

Figure Q1c

ASSIGNMENT 1

BMM3513_2021/I

QUESTION 2 [10 Marks] A pipe in a manufacturing plant is transporting superheated vapour at a mass flow rate of 0.5 kg/s. The pipe is 10 m long, has an inner diameter of 5 cm and the pipe wall thickness of 6 mm. The pipe has a thermal conductivity of 17 W/m∙K, and the inner pipe surface is at a uniform temperature of 120 oC as shown in Figure Q2. The temperature drop between the inlet and the exit of the pipe is 7 oC, and the constant pressure specific heat of the vapour is 2190 J/kg∙C. If the air in the manufacturing plant is 25oC, determine the heat transfer coefficient as a result of convection between the outer pipe surface and the surrounding air. (10 Marks)

Figure Q2

ASSIGNMENT 1

BMM3513_2021/I

QUESTION 3 [10 Marks] a)

Briefly discuss the following terms in heat transfer: i. Blackbody ii. Absorptivity iii. Convection iv.

Specific heat

v. Thermal conductivity (5 Marks) b) An industrial galvanized pipe (Figure Q3) that carries SiO2 nanofluid as cooling liquid passes through a factory in which the air is at 27 oC. The outside diameter of the pipe is 75 mm, and its surface temperature and emissivity are 200 oC and 0.8, respectively. If the coefficient associated with free convection heat transfer from the surface to the air is 16 W/m2.K, what is the rate of heat loss from the pipe surface per unit length of pipe? (  5.67 10  8 ) (5 Marks)

Figure Q3

ASSIGNMENT 1

BMM3513_2021/I

QUESTION 4 [10 Marks] Consider a solid cylindrical rod of length 0.15 m and diameter 0.05 m as shown in Figure Q4. The top and bottom surfaces of the rod are maintained at constant temperatures of 20°C and 95°C, respectively, while the side surface is perfectly insulated. Determine the rate of heat transfer through the rod if it is made of the following materials: i.

copper, k =380 W/m·°C,

(2 Mark)

ii.

Aluminum, k =273 W/m·°C,

(2 Mark)

iii.

Iron, k =80.2 W/m·°C,

(2 Mark)

iv.

steel, k =18 W/m·°C, and

(2 Mark)

v.

granite, k =1.2 W/m·°C

(2 Mark)

Figure Q4

ASSIGNMENT 1

BMM3513_2021/I

QUESTION 5 [10 Marks]

5 kg of liquid water initially at 18°C is to be heated to 96°C in a teapot equipped with a 1200-W electric heating element inside. The teapot is 0.8 kg and has an average specific heat of 0.6 kJ/kg·°C. Taking the specific heat of water to be 4.18 kJ/kg · °C and disregarding any heat loss from the teapot, determine how long it will take for the water to be heated.

QUESTION 6 [10 Marks]

A 4-m-long section of an air heating system of a house passes through an unheated space in the attic. The inner diameter of the circular duct of the heating system is 20 cm. Hot air enters the duct at 50 kPa and 65°C at an average velocity of 3 m/s. The temperature of the air in the duct drops to 60°C as a result of heat loss to the cool space in the attic. Determine the rate of heat loss from the air in the duct to the attic under steady conditions. Also, determine the cost of this heat loss per hour if the house is heated by a natural gas furnace having an efficiency of 82 percent, and the cost of the natural gas in that area is $0.58/therm (1 therm = 105,500 kJ).

QUESTION 7 [10 Marks]

Water flows through a shower head steadily at a rate of 20 L/min. An electric resistance heater placed in the water pipe heats the water from 16°C to 43°C. Taking the density of water to be 1 kg/L, determine the electric power input to the heater, in kW. In an effort to conserve energy, it is proposed to pass the drained warm water at a temperature of 39°C through a heat exchanger to preheat the incoming cold water. If the heat ex-changer has an effectiveness of 0.50 (that is, it recovers only half of the energy that can possibly be transferred from the drained water to incoming cold water), determine the electric power input required in this case. If the price of the electric en-ergy is 8.5 ¢/kWh, determine how much money is saved during a 10-minute shower as a result of installing this heat exchanger.

ASSIGNMENT 1

BMM3513_2021/I

QUESTION 8 [10 Marks]

It is proposed to have a water heater that consists of an insulated pipe of 5 cm diameter and an electrical resistor in-side. Cold water at 15°C enters the heating section steadily at a rate of 20 L/min. If water is to be heated to 50°C, determine (a) the power rating of the resistance heater and (b) the average velocity of the water in the pipe.

QUESTION 9 [10 Marks]

A passive solar house that is losing heat to the out-doors at an average rate of 50,000 kJ/h is maintained at 22°C at all times during a winter night for 15 hours. The house is to be heated by 50 glass containers each containing 50 L of water heated to 80°C during the day by absorbing solar energy. A thermostatcontrolled 15-kW back-up electric resistance heater turns on whenever necessary to keep the house at 22°C. (a)

How long did the electric heating system run that night?

(b)

How long would the electric heater have run that night if the house incorporated no solar

heating?

QUESTION 10 [10 Marks] A 0.5-L glass of water at 20°C is to be cooled with ice to 5°C. Determine how much ice needs to be added to the water, in grams, if the ice is at 0°C. Also, determine how much water would be needed if the cooling is to be done with cold water at 0°C. The melting temperature and the heat of fusion of ice at atmospheric pressure are 0°C and 333.7 kJ/kg, respectively, and the density of water is 1 kg/L....