Write a lab report with: 1)Abstract2)introduction 3)Experiment /Procedures4)Discussion and Results5)Conclusionmust have pictures of eqiupment used and equations in the sections 3 and 4Lab handout is attached along with the powerpoint and data..the lab conducted is in the powerpoint in a YouTube link.THERMAL/FLUID SCIENCE LAB
MEEG-423
Evaluation of Heat Transfer Coefficient of a Row of Tubes in Forced Convection
Equipment: HT10XC Heat Transfer Service Unit, HT19 Free and Force Convection Heat
Transfer Module, Computer.
Introduction
A heated surface dissipates heat primarily through a process called convection. Air in
contact with a hot surface is heated by the surface and rises due to reduction in density.
The heated air is replaced by cooler air which in turn is heated by the surface and rises.
This process is called free convection.
In free convection, the heat transfer rate from the surface is limited by the small
movements of air generated by the heat. More heat is transferred if the air velocity is
increased over the heated surface. This process of assisting the movement of air over the
heated surface is called forced convection. Therefore, a heated surface experiencing forced
convection will have a lower surface temperature than that of the same surface in free
convection for the same power input.
Theory
In the study of thermodynamics, the average heat transfer coefficient, h, is used in
calculating the convection heat transfer between a moving fluid and a solid. Knowledge of
h is necessary for heat transfer design and the heat transfer coefficient is critical for
designing and developing better flow process control resulting in reduced energy
consumption and enhanced energy conservation. Heat transfer through a bank of tubes has
several applications in industry and can be used in many applications from the design of
heat exchangers to the study of forced convection over pipes [1].
When tube banks are used in heat exchangers, two arrangements of the tubes, aligned and
staggered, can be considered as shown in Figure 1 and there are many correlations that can
be used to predict the heat transfer coefficient from cross flow over a bank of tubes in both
Figure 1: Arrangements of the tubes (a) in-line arrangement, (b) staggered arrangement
forced and natural convection situations. The correlation proposed by Zukaukas [2] will be
used in this analysis. In this correlation, dimensionless parameters, such as Nusselt number
(Nu), Reynold’s number (Re) and Prandtl number (Pr), for different geometries, can be
used to calculate values of h.
The arrangement of the tubes in the HT-19 pinned heater is staggered in the direction of
flow as shown in Figure 1b. The arrangement of the tubes is characterized by a transverse
pitch ST, a longitudinal pitch SL and a diagonal pitch SD, where SD can be determined from


= + ( )

For the HT-19, these values are ST = 28mm, SL = 17mm and SD = 22mm respectively.
In Zukaukas correlation, the Nusselt number for cross flow over a bank of tubes is
expressed as
=

= ( / ) .

where the values of the constants C, m and n depends on the Reynolds number, h is the
average heat transfer coefficient, D the diameter of one tube and k the thermal
conductivity of the flow. Values of the constants C, m, and n for the staggered arrangement
is given in the Table 1 below.
Table 1 represents Nusselt number correlations for crossflow over tube banks with more
than 16 rows (Nr). Since the HT-19 has less than 16 rows, a correction factor needs to be
applied to the Nusselt number of Equation 2. Thus
Nu = f Nu (Nr < 16) 3 For the HT-19 this correction factor is f=0.93. Table 1: Average Nusselt Number Correlation [2] Range of Re Correlation . 0 – 500 . . ( / ) . 500 – 1000 . . . ( / ) . 1000 – 2x105 . ( / ) . . . ( / ) . 2x105 – 2x106 . ( / ) . . . ( / ) . The Reynolds number can be determined from = where and are the density and dynamic viscosity of the fluid (air) and VMAX is the maximum flow velocity respectively. The maximum average velocity, VMAX, between tubes is used in calculating Re (Equation 4). For the staggered arrangement, if >
+

Then
=

If not, then
=

( − )

In Equations 2 and 4 above, all properties are to be evaluated at the arithmetic mean of the
inlet and exit temperatures of the fluid (T1 + T2)/2, except PrS which is to be evaluated at
the surface temperature TS (the average temperature of T3 – T6). Table 2 below can be used
to obtain these properties.
Table 2: Properties of Air at 1atm Pressure
Heat Transfer Rate
Once the heat transfer coefficient is determined from the above analysis, the heat transfer
rate can be numerically obtained from application of Newtons Law of cooling as
̇ = ∆

where AS is the heat transfer surface area and ∆ is called the mean logarithmic
temperature difference. These values can be determined from
AS = DLPN
9
where LP is the length of the tubes (82mm), D the tube diameter (12mm) and N the number
of tubes (17) in the heater and
( − ) − ( − )
∆ =

[( − )/( − )]
Experimental Procedure
Place the pinned heater into the duct of the HT-19 and connect the HT-19 to the HT-10XC
service unit as shown in Figure 2. The heater is fitted with thermocouples that can be read
from the HT10XC console. The HT10XC provides the necessary measurement facilities
and power control for the module.
Set the heater power control to 60 watts and allow sufficient time to achieve steady state
conditions before noting and recording the heated plate temperature (T3). Also note and
record the upstream air temperature T1 and the temperatures T4, T5 and T6 of the pinned
heater. The distance of the three access holes on the pinner heater are given in Table 3
below.
Set the fan speed control to give air speed Ua of 1m/s using the air velocity sensor. When
the temperatures are stable (monitor the temperatures on the PC display screen), record
and save the displayed data. Repeat the procedure at 1.5 and 2.0 m/s.
Table 3: Thermocouple Identification
T1
Input air temperature in duct
T2
Output air temperature in duct
T3
Heater temperature
T4
Surface temperature at root of pin
T5
Surface temperature at mid height of pin
T6
Surface temperature at tip of pin
Distance of nearest hole in pinned heater 10mm
Distance of middle hole in pinned heater 36mm
Distance of farthest hole in pinned heater 62mm
Figure 2; Armfield HT-19 Convective Heat Transfer Module
Results
Plot graphs of surface temperature against distance from the back plate of the heater at the
various flow velocities and comment on your results.
Determine the average heat transfer coefficient h and the heat transfer rate Q using the
properties in the attached table and compare your result to the actual power input applied
to the pinned heater. Address any differences in results and comment on your findings.
References
1. Manohar, K. and Ramroop, K.,“A Comparison of Correlations for Heat Transfer
from Inclined Pipes,” International Journal of Engineering (IJE), Volume: 4, Issue:
4, 268.
2. Zukauskas, A. “Heat Transfer from Tubes in Crossflow,” Advances in Heat
Transfer, 8:87-159, 1987.
CONVECTIVE HEAT TRANSFER
FREE & FORCED CONVECTION OVER A BANK OF TUBES
Convection
Convective heat transfer, often referred to simply as convection,
is the transfer of heat from one place to another by the
movement of fluids. Occurs by the mixing of one portion of the
fluid with another portion due to gross movements of the mass
of fluid.
Can be subdivided into free convection and forced convection.
Natural or Free Convection:
Caused by buoyancy forces due to density differences caused
by temperature variations in the fluid. At heating the
density change in the boundary layer will cause the fluid to
rise and be replaced by cooler fluid that also will heat and
rise.
Forced or Assisted Convection:
Forced convection occurs when a fluid flow is induced by an
external force, such as a pump or fan.
Free and Forced Convection
Air in contact with the hot surface is heated by the surface
and rises due to a reduction in density. The heated air is
replaced by cooler air which is in turn heated by the surface
and rises.
More heat is transferred if the air velocity is increased over
the heated surface. This process of assisting the movement of
air over the heated surface is called forced convection.
Therefore, a heated surface experiencing forced convection
will have a lower surface temperature than that of the same
surface in free convection for the same power input.
Heat transfer to or from
a bank (or bundle) of
tubes in cross flow
• Flow around the tubes in the first
row of a tube bank is like that for a
single (isolated) cylinder in cross
flow.
• Correspondingly, the heat transfer
coefficient for a tube in the first row
is approximately equal to that for a
single tube in cross flow.
• However, we wish to know the
average heat transfer coefficient for
the entire tube bank.
For tube banks, two arrangements of the tubes can
be considered
in-line arrangement
staggered arrangement
Zukaukas Correlation
Zukaukas proposed the following equation for the cross flow
of air over a bank or bundle of tubes.
=

= Τ

Range of Re
0.25
Correlation
0 – 500
. . . /
.
500 – 1000
. . . /
.
1000 – 2×105
. /
2×105 – 2×106
. /
.
. . /
.
. . /
.
.
All properties above except Prs (Prandtl Number at the surface
temperature) are to be evaluated at the arithmetic mean of the
fluid inlet (Ti=T1) and outlet (To = T2) temperatures.
The need to evaluate fluid properties at the arithmetic mean
of the inlet and outlet temperatures is dictated by the fact
that the fluid temperature will decrease or increase,
respectively, due to heat transfer to or from the tubes.
If the change of the mean fluid temperature, │Ti -To│, is
large, significant error could result from the evaluation of
the properties at the inlet temperature.

=

For the staggered configuration, the maximum velocity may
occur at either the transverse plane AT or the diagonal plane
AD.
It will occur at AD if the rows are spaced such that
2( − ) < ( − ) Hence Vmax occurs at AD if = + < + in which case it is given by = − If Vmax occurs at AT then it may be computed from = − If: >
:
+
2
=


: =

2 −
Heat Transfer Rate
Newton’s law of cooling
ሶ = ∆
AS = DLPN
− 1 − − 2
∆ =
− 2 Τ − 1
Since the fluid may experience a large change in temperature
as it moves through the tube bank, the heat transfer rate could
be significantly overpredicted by using ΔT= Ts – T∞ as the
temperature difference in Newton’s law of cooling.
As the fluid moves through the bank, its temperature approaches
Ts and │ΔT│ decreases. Thus, the appropriate form of ΔT is
shown to be a log-mean temperature difference.
Experimental Apparatus
In free convection the heat transfer rate from the surface is
limited by the small movements of air generated by this
heat.
More heat can be transferred if the air velocity is increased
over the heated surface.
This process of assisting the movement of air over the heated
surface is called Forced Convection.
Therefore a heated surface experiencing forced convection,
for the same power input, will have a lower surface
temperature than that of the same surface in free convection.
Experiment

LAB #4 DATA
Air
Upstream Output Heater
Pin Root
Mid Pin
Pin Tip
Heater Heater Heater
Velocity
Temp
Temp
Temp Surface Temp Surface Temp Surface Temp Voltage Current Power
Ua
T1
T2
T3
T4
T5
T6
V
I
P
[m/s]
[°C]
[°C]
[°C]
[°C]
[°C]
[°C]
[V]
[A]
[W]
0.0
1.0
1.5
2.0
27.5
27.5
28.5
29.1
38.5
40.4
37.8
35.8
92.2
76.6
65.0
57.8
88.5
67.6
57.6
51.3
88.2
66.8
56.4
49.8
84.7
61.5
52.3
46.4
13.1
13.1
13.1
13.2
4.57
4.58
4.58
4.58
60.09
60.15
60.21
60.33

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