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Department of Mechanical Engineering
Name of Student: Class: SE/ T.E. /BE Div.
Subject: Heat Transfer (302043) Academic Year: 2019-
Date of Completion : __________________ Date of Submission: __________________
Sr. No.
Criteria Possible Marks
Marks Obtained 1 Team Work (15)^ 6, 9, 12, 15 2 Understanding experiment and its components (25)
10, 15, 20, 25
3 Diagram(s)/Calculations (15) 6, 9, 12, 15 4 Interpretation of^ Result and Conclusions (20)
8,12,16,
5 Q/A (10) 4,6,8, 6 Timely Submission (15) 6, 9, 12, 15 Total
Date: Name & Signature of Teacher
Experiment 5
Title. Heat Transfer in Forced Convection
5.1 Aim. To determine the heat transfer coefficient in forced convection.
5.2 Prior Knowledge: Phenomenon of heat convection, Newton’s law of cooling
and concept of boundary layer, and average heat transfer coefficient..
5.3 Practical Outcomes. After experiment performance students must be able to
PrO 1. Calculate local and average value of heat transfer coefficient over the
entire height of heating rod experimentally.
PrO 2. Calculation of average value of heat transfer coefficient using standard
empirical relations.
PrO 3. Nature of variation of heat transfer coefficient along length of heating
rod.
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5.4 Introduction
The heat transfer in forced convection occurs mechanically and depends on the motion of the fluid. When a fluid motion is caused by some external means, such as a pump or blower, then the convection is called forced convection. In forced convection, the velocity gradients are more effective than the density gradient. Forced convection heat transfer occurs in heat exchangers such as automobile radiators, condensers, and coolers.
5.5EXPERIMENTAL ANALYSIS 5.5.1 Experimental Setup
The experimental setup consists of a blower unit fitted with a test pipe as shown in Figure 5.1. Four band-type nichrome heaters surround the test section of the pipe wall. A portion of the heat supplied is conducted through the pipe wall of the test section and then to the flowing air by forced convection.
Fig.5.2 Experimental setup for forced convection
Three thermocouples (T 2 to T 4 ) are attached to the test section. Thermocouples T 1 and T 5 record the temperature of the incoming and outgoing air. Thermocouple T6 measures the temperature of the atmospheric air. The test pipe is connected with an orifice to measure the flow rate of air through the pipe. A level valve is fitted in the delivery pipe in order to regulate
Department of Mechanical Engineering
Where m =mass flow rate of oil = V^ a
Cp =specific heat of air = 1005 kJ/kg.K, Δ T = temperature difference of air = (T s – T ∞ ) h = heat transfer coefficient As = surface area of the test pipe The mean fluid temperature is T =^1 2
T T
Average temperature of heating pipe surface is given by
Ts =^2 33
T T T
The density of atmospheric air at the mean temperature of a =
p R T (^) The discharge (volume flow) rate V of air through the orifice can be calculated as
V = 4 o^2^ d^2 w^ w a
(^) d C g H^ where ρ w = density of water = 1000 kg/m^3 , Cd = 0.64, coefficient of discharge do = diameter of the orifice Hw = level of difference of water in manometer arms Then the mass flow rate of air is obtained by m^ ρ^ aV The surface are of heating pipe is
A s = πd L m^2 Now the heat transfer coefficient is obtained practically by
h = ( ) s s
Q A T T The heat transfer coefficient can also be obtained by using of empirical relations as
Nu = 0 .8^ 0. a ir
h d 0 .0 2 3 R e (^) d P r k
Where Re = ν
u^ md with u m = 2 4
V (^) d And Pr = a i r
μ Cp k
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The properties of air can be taken at mean film temperature Tf fromTableA.4.
Tf = 2
T (^) s T
5.6 Specifications
- Pipe outer diameter (^) d= 38 mm
- Pipe inner diameter d = 33 mm
- Length of testsection L= 500 mm
- Orifice diameter do =22mm
- Size of the duct = 200 mm × 200 mm × 700 mm
- Multichannel Digital Temperature Indicator 0-300C using Chromel/Alomel thermocouples.
- Blower to force the air through pipe.
- Water manometer to measure air pressure difference.
- Dimmer-stat 2 Amp. 240 Volts
- Controls -a) Voltmeter- 0 to 200 volts. b) Ammeter- 0 to 2 Ampere
5.7 Procedure
- Switch on the heater and adjust the voltage input to the heater through the dimmer- stat.
- Adjust the air flow rate with the help of the valve fitted in the delivery valve.
- After reaching the steady state, take the readings of all thermocouples, voltmeter, and ammeter.
- Measure the manometer deflection in the U tube with the help of the attached scale.
- Repeat the procedure two to three times with a different power input to the heater and air flow rate.
5.8 Observation Table
Sr. No.
Heater input
Manomete r reading, H w
Thermocouple readings, °C
V I 1 2 3 4 5 6
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- Calculate the approximate Reynolds numbers and state if the flow is laminar or turbulent for
the following :
(i) A 10 m long yatch sailing at 13 km/h in sea water, = 1000 kg/m^3 and = 1.3 × 10–^3 kg/ms. (ii) A compressor disc of radius 0.3 m rotating at 15000 r.p.m. in air at 5 bar and 400°C and dynamic viscosity is given by
(iii) 0.05 kg/s of CO 2 gas at 400 K flowing in a 20 mm dia. pipe and
ANSWERS: Attached separate sheet.
Multiple Choice Questions
1. In force convection, the Nusselt number (Nu) depends on a. Pr and Re b. Gr and Re c. Gr and Pr d. none of the above 2. What is the correct formula for the rate of heat transfer by convection form a surface of area A at temperature T to the surrounding fluid at temperature T 0? a. qc = h A (T – T 0 )^4 b. qc = h A (T^4 – T 04 ) c. qc = h A (T – T 0 ) d. none of the above 3.What is the mode of heat transfer from the hot surface to the adjacent layer of fluid which surrounds the surface? a. conduction mode of heat transfer b. radiation mode of heat transfer c. convection mode of heat transfer d. none of the above
- Mark the system where heat transfer is given by forced convection a) Chilling effect of cold wind on warm body b) Fluid passing through the tubes of a condenser and other heat exchange equipment c) Heat flow from a hot pavement to surrounding atmosphere d) Heat exchange on the outside of cold and warm pipes
- Nusselt number is given by a) h L/k b) 2 h L/k c) 3 h L/k d) 4 h L/k
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- The value of film coefficient is dependent upon (i) Boundary layer configuration (ii) Geometry and orientation of the surface (iii) Surface conditions a) 1 and 2 b) 2 and 3 c) 1 and 2 d) 1, 2 and 3
- The convection coefficients for boiling and condensation lie in the range a) 5000-12500 W/m^2 K b) 2500-100000 W/m^2 K c) 2500-5000 W/m^2 K d) 2500-12500 W/m^2. K
- Forced convection in a liquid bath is caused by a) Intense stirring by an external agency b) Molecular energy interactions c) Density difference brought about by temperature gradients d) Flow of electrons in a random fashion
- A body cooling from 80 degree Celsius to 70 degree Celsius takes 10 minutes when left exposed to environmental conditions. If the body is to cool further from 70 degree Celsius to 60 degree Celsius under the same external conditions, it will take a) Same time of 10 minutes b) More than 10 minutes c) Less than 10 minutes d) Time will depend upon the environmental conditions
- On a summer day, a scooter rider feels more comfortable while moving faster than at a stop or slower moving because a) An object in motion captures less radiation b) Air has a low specific heat and hence it is cooler c) More heat is loss by convection and radiation while in motion d) Air is transparent to radiation and hence it is cooler than the body
- For a given value of Nusselt number, the convective surface coefficient h is directly proportional to a) Length b) Mass c) Thermal conductivity d) Density
