PRACTICAL 5: Convection
Concept questions:
C1. What is forced convection? How does it differ from natural convection? Is convection caused by winds forced or natural convection?
C2. In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural or forced convection? Why?
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Solution protocol for convection problems:
· Draw a schematic diagram
· Identify the type of convection: forced/natural/boiling/condensing
· Identify the geometry of the situation: Cylinder/duct/sphere?
· If forced
· Identify nature of flow:internal/external flow?
· Identify flow regime: Calculate Re – is flow laminar or turbulent?
· If natural
· Identify flow regime: Calculate Ra
· Choose appropriate dimensionless expression based on the nature of the convection, the geometry and flow regime
· Solve for heat transfer coefficient
1. A double-pipe heat exchanger consists of one pipe within another pipe, with fluids at different temperature flowing through the inner and outer pipes. Heat is transferred through the inner pipe wall from the hot fluid to the cold fluid. In this case, air at 1 atm and 200 oC is heated as it flows at 10 ms-1 through the inside tube of a double-pipe heat exchanger. The inner tube has inside diameter ID of 5 cm. Calculate:
a. the average heat transfer coefficient h in the inner tube;
b. the rate of heat transferred to the air, if a constant heat flux condition is maintained along the tube, and the temperature difference between the bulk fluid and the wall is 20 oC.
2. Milk at 60 oC enters the tubes of a shell and tube heat exchanger (3.0 m long, ID 2.54 cm) at a velocity of 2 cms-1. The wall temperature is a constant 80 oC. Calculate:
a. the average heat transfer coefficient h in the tubes;
b. the exit temperature of the milk.
THIS QUESTION LEADS INTO HEAT EXCHANGERS – ENSURE YOU COMPLETE THIS QUESTION BEFORE THE NEXT LECTURE
3. A spherical light of diameter 20 mm maintains a constant surface temperature of 60 oC when suspended in air at 20 oC, if the velocity of the air is 5 ms-1. Calculate:
a. the heat transfer coefficient h for convection from the sphere to the air
b. what must be the rate of energy consumption of the light to maintain the constant temperature under these conditions?
a. If the fan near the light breaks down, and the surrounding air becomes still, determine the heat transfer coefficient h for convection from the sphere.
b. assuming the same energy consumption by the light when the fan is not workiing, what will be the new temperature of the surface of the spherical light?
c. e. comment on your results, in terms of the light operation – can it survive if the fan breaks down?
4. Saturated steam is transported between two buildings, separated by 500m, at a temperature of 800K, and a rate of 3 litres per second. The steel pipe has an inside diameter of 0.15 m, with walls 5mm thick. It is located on a process plant in a cold place, where windspeeds of 50kmh-1 are a common occurrence, and the average winter temperature is 8oC. In order to minimize energy losses, it has been decided to insulate the pipe so that heat loss from the steam as it is transported between the buildings is equivalent to a temperature drop of 10oC, with no phase change. The insulation material chosen has a thermal conductivity of 0.04 Wm-1K-1.
a. What is the heat transfer coefficient for convection from the steam to the wall of the pipe?
b. What is the heat transfer coefficient for convection from the pipe to the surrounds?
c. What is the maximum acceptable heat loss from the pipe?
d. What thickness of insulation is required?
e. If the pipe were not insulated, what effect would the following have on heat loss:
· Changing the pipe diameter
· Locating the pipe in a sheltered area (ie reducing the wind speed)
· Using exhaust gases (properties similar to air) rather than steam
5. You are on a camping trip, and unfortunately a person with limited heat-transfer expertise has packed your esky – by the end of the first day, all the ice is gone, and the contents of the esky are at 35 oC. Luckily, there is a nearby stream with water at 10oC – you can cool your cans of beverage down! Your companions have placed the cans in a still part of the creek, where the water is not moving. They want to know how to orient the cans for fastest cooling, so you calculate:
a. the heat transfer coefficient h for cans placed vertically
b. the heat transfer coefficient h for cans placed horizontally
c. which way will you orient them?
You suggest that placing the cans in the flowing part of the creek, where the water is moving at 5ms-1, would greatly enhance heat transfer. Your companions are skeptical, and cite concerns about the safety of the cans. You argue that placing the cans in the flow will substantially increase the rate of cooling, and so reduce the time that the cans spend in the creek, and hence reduce overall the risk of cans escaping downstream. To support your case, you get out your calculator and determine:
d. the heat transfer coefficient h for cans placed vertically in the flow;
e. the heat transfer coefficient h for cans placed horizontally, in cross-flow arrangement, i.e. the axis of the cylinder perpendicular to the direction of stream flow;
f. the heat transfer coefficient h for cans placed horizontally, with the axis of the cylinder parallel to the direction of stream flow.
g. what do you recommend to your companions, and how do you justify this?
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