UWE Bristol Engineering showcase 2015

Lawrence Dubey
MEng Mechanical Engineering
Project Supervisor
Rachel Szadziewska
Increasing Heat Transfer by Pipe Roughening
Experimental Study
The primary aim of the proposed project was to
investigate whether roughened tube had a
significant impact on heat transfer, and whether it
significantly affected the pressure drop and thus
the pump power demand. Therefore the
temperate and pressure values needed to
measured over a length of smooth pipe and a
length of roughened pipe.
The experimental rig was set up as shown in the
schematic diagram below. It consists of four main
parts: water heating, temperature measurement,
pressure measurement and flow rate
measurement. The test pipes were made of
copper, to represent common domestic water
pipes. Both pipes were of the same dimensions,
1m long and ID of 20mm, except that one of the
pipes had been internally roughened by manual
abrasion.
Theoretical Analysis
It was evident from the results, that the
experiment produced unexpected and
contradictory results. The rough pipe both felt and
looked rougher. However the empirically
calculated equivalent sand grain roughness‘
conflicted with the reality of the pipes internal
surface. This meant the heat transfer model could
calculated, instead the focus changed to fluid flow.
An analysis was conducted in Excel to determine
the reason for this discrepancy. It was discovered
that the pipe roughness ε was very small
compared to the pipe diameter and the viscous
sub layer completely submerged the effect of ε.
This meant that the pipes acted as hydraulically
smooth, so therefore smooth regime laws were
applied. This showed again that the smooth pipe,
had a higher friction factor as it had a slightly
higher flow rate, due to error at the water valve.
Filonenko
correlation
Blasius
correlation
Darcy-
Weisbach
equation
Friction Factor Formula Comparison
0.023179566
0.022852982
0.023624008
0.023277335
0.026230914
0.02 0.022 0.024 0.026 0.028 0.03
Friction Factor
Friction
Factor
Smooth
Pipe
Friction
Factor
Rough
Pipe
0.029581001
Pressure (Pa)
Computational Fluid Dynamics (CFD)
The aim was to use the ANSYS software to run a
number of CFD simulations for the flow through a
pipe. The parameters of the experiment were
utilised as the inputs for the first series of CFD
runs. This then meant that the empirical and CFD
calculated pressure drops could be used for
comparison purposes. Another set of runs were
carried out with parameters that would produce
the results expected of this investigation, the ideal
results, which would help validate the theory
The CFD results confirmed the hypothesis that the
experimental pipes behaved hydraulically smooth.
It also verified that increased roughness does lead
to an increase in pressure loss.
1600
1400
1200
1000
800
600
400
200
0
Pressure (Pa)
Comparing Rough Pipe vs Smooth Pipe
1 1.2 1.4 1.6 1.8 2
Distance (m)
3000
2500
2000
1500
1000
500
0
Pressure loss difference between a smooth and roughened pipe (idealised
situation)
1 1.2 1.4 1.6 1.8 2
Distance (m)
Smooth CFD
Rough CFD
Smooth
Theoretical
(Smooth Regime)
Rough Theoretical
(Smooth Regime)
Smooth
Experimental
Rough
Experimental
Smooth Ideal
Rough Ideal
Project summary
A wide variety of industrial processes involve the
transfer of heat energy. These processes provide a
source for energy efficiency increases. Enhanced heat
transfer surfaces can be designed through a
combination of factors that include: increasing fluid
turbulence, generating secondary fluid flow patterns,
reducing the thermal boundary layer thickness and
increasing the heat transfer surface area.
Project Objectives
The project was an investigation into roughened pipe
with the main objectives being the improvement of
the heat transfer efficiency whilst minimising the
pressure loss induced by turbulence. The paper
discusses the theory behind the heat transfer and
fluid mechanics and how this relates to heat transfer
enhancement. A practical experiment,
complemented by theoretical analysis and
computational fluid dynamics, was conducted to the
see how the theoretical results compared to the
empirical data.
Project Conclusion
It was found that the experimental pipe had a relative
roughness that when compared to the Reynolds
number, resulted in the pipes acting hydraulically
smooth, despite the fact that one pipe was in reality
rougher than the other. This was because the
boundary layer was thicker than the roughness
height. This report details how these problems would
be overcome in a revised experiment, so that the
heat transfer could be analysed effectively. The CFD
model was also employed to simulate an ideal
experiment, in which the roughened pipe exhibited a
rough regime, this verified that this does lead to an
increased pressure loss.