# Forced Convection Heat Transfer from 3D Solids Experiment

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```					                      ME 252 Lab – Heat Exchanger Experiment

Objective: to measure the performance of a liquid-gas, shell-and-tube heat exchanger.
Specifically,
   determine the heat transfer rate, effectiveness, number of transfer units (NTU), and
overall heat transfer coefficient at four different operating conditions.

Background: The shell-and-tube heat exchanger used here is an ITT Standard TCF
two tube-pass model with a thermally efficient plate fin design. There are a total of
thirty, 3/8” OD, 0.022” wall, copper tubes that are 24” long. It was originally designed for
cooling hot lubrication oil or hydraulic fluid. In this experiment, it is used to heat water
on the tube-side by passing hot air through the shell. A centrifugal blower provides the
airflow, which passes through an adjustable electric heater before entering the heat
exchanger shell. The water flow is supplied to the heat exchanger tubes by the
laboratory cold water main. Electronic flow meters and thermocouples are used with
computer data acquisition to measure the required fluid flow rates and temperatures.

Theory: Read section 11.4 in Incropera & DeWitt (2002) on the effectiveness-NTU
method of analyzing heat exchangers. The heat transfer rate (q) of any heat exchanger
that conveys liquids or ideal gases can be obtained from the hot-side or cold-side
energy balance, given by:

q h  m h c ph Thi  Tho  ,
                                             (1)
qc  m c c pc Tco  Tci  .
                                      (2)

Heat exchanger effectiveness ( ) is the actual heat transfer rate divided by the
maximum possible heat transfer rate:
q              q
      
qmax mc p min Thi  Tci 
.                (3)


The number of transfer units (NTU) provided by a heat exchanger is a nondimensional
parameter that is primarily a measure of heat exchanger size – the larger the value of
NTU, the closer the heat exchanger approaches its maximum heat transfer rate. The
definition of NTU is
UA
NTU 
mc p min
,                    (4)


where U is the overall heat transfer coefficient and A is the corresponding surface area.
The relationship between  and NTU for a two tube-pass shell-and-tube heat
exchanger is given by the following equations:

 E 1
NTU  1  C r2 
1 / 2
ln      ,            (5)
 E 1

041102                                                                                     1
2 /   1  Cr
E                       ,                      (6)
1  C 
2 1/ 2
r

mc 

Cr 
p min

mc 
.                             (7)
  p max

Apparatus and Instrumentation:

   ITT Standard TCF heat exchanger
   Variac-controlled centrifugal blower
   Variac-controlled electric heater with three ranges
   Glass tube rotameter for air
   Totalizing turbine water meter
   Precision needle valve for water flow control
   Omega FTB936 pulse-output turbine air flow meter
   Omega FTB4607 pulse-output turbine water flow meter
   Two Omega DPF700 ratemeters with analog output
   Eight K-type thermocouples
   Omega DP119 miniature panel thermometer
   Fluke Hydra Data Logger with PC and software
   Safety glasses

Testing Conditions:

Air Flow Rate (lpm)   Water Flow Rate (lpm)                Heater Setting   Thermostat Set Pt.
500                    5                             Range 3, 50%         200F
500                    10                            Range 3, 50%         200F
1000                   5                             Range 3, 50%         250F
1000                   10                            Range 3, 50%         250F

Procedure:

1. Connect the garden hose to the water inlet and make sure that the exit hose drains
into the floor trough. Make sure the heater rheostat is in the “off” position. Plug in
power cord.
2. Set up computer data acquisition system and make sure all thermocouples are
reading correct temperature values. Set up the Fluke Hydra data logger with a
sampling interval of 10 sec and enable data recording to a named file with an
Elapsed Time tag and Append mode. Enable the Quick Plot feature to monitor all
temperatures and flow rates.
3. Turn the water gate valve at the supply spigot to the fully open position. Adjust the
needle valve to yield the correct water flow rate for the given testing condition. Turn
041102                                                                                           2
on the blower and adjust the speed rheostat to yield the correct airflow rate for the
given testing condition. Check to make sure that the data logger is reading the
correct flow rates.
4.   Always make sure that there is water and air flowing through the heat exchanger
before turning on the electric heater. Adjust the heater rheostat for the given testing
condition.
5.   Record data until steady-state temperatures are reached; this will require about 15
minutes from a cold start. Run heat exchanger for about 2 minutes at “steady-state”.
6.   Stop the data logger and adjust the flow rates and heater for the next testing
condition. Resume data logging.
7.   Repeat steps 5-6 for all four testing conditions.
8.   Turn off the heater at the end of the experiment, but keep the blower and water flow
on for about 10 minutes to cool down the electric heater core and heat exchanger.
9.   Have a 3.5” diskette available for downloading your data files. Be sure to identify
each channel number with the measured quantity.

Results:

Average the steady-state data (approx. 10-12 readings) for each testing condition.
Show the steady-state data and averages in a separate worksheet. Perform the
required calculations and complete a summary table as shown below. Be sure to use
the blower inlet temperature to compute the air density for the air mass flow rate
calculation. Clearly present all data reduction with sample hand calculations. Be sure to
show all material property evaluations (, cp).

Summary of Results

Air Mass      Water Mass        Thi      Tci      qh       qc               NTU          Uc
Flow Rate     Flow Rate                          (W)      (W)                          (W/m2-K)
(C)     (C)                       (%)

m h (kg/s)    
m c (kg/s)

Note: In theory, qh and qc should be identical if no heat loss from the heat exchanger
to the surroundings occurs. In practice, however, this is not the case due to some heat
loss and measurement errors in flow rate and temperatures. When calculating  , use qh
for the actual heat transfer rate since it will produce less uncertainty for two reasons: i)
the air experiences a greater temperature difference between inlet and exit than the
water, and ii) any errors in the measured air flow rate will cancel in equation (3).

041102                                                                                     3
Questions:

1. Which flow rate (water or air) has the greatest effect on the heat transfer rate?
Explain why.

2. How would you rate the effectiveness () of this heat exchanger? What trends, if
any, do you observe in the effectiveness as a function of the water or airflow rate?

3. Suppose the hot-side fluid was engine oil instead of air. If the oil had the same mass
flow rate and inlet temperature as the air, what differences would you see, if any, in
the calculated heat transfer rates, effectiveness, NTU, and cold-side overall heat
transfer coefficient (Uc) ?

041102                                                                                    4

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