UWE Bristol Engineering showcase 2015

Convective heat transfer coefficeint
(W/m2K)
Thermal conductivity (W/mK)
Wasim Ahmed
BEng – Mechanical Engineering
Automotive Thermoelectric Generator and Heat Exchanger Design
Analysis, Using Computational Fluid Dynamics, For Exhaust Gas Waste
Heat Recovery.
Introduction
The "Energy Crisis" has become a major challenge in front of engineers across the globe due to rapidly increasing demands and consumption of energy. For
almost two hundred years, the main energy resource has been fossil fuel and will continue to supply much of the energy for the next two and half decades.
Worldwide oil consumption is expected to rise from 80 million barrels per day in 2003 to 98 million barrels per day in 2015 and
then to 118 million barrels per day in 2030.The investigation carried out in this report focuses on different methods of waste heat recovery systems, to give a
brief overview of the project background and relation heat recovery systems have to transportation. An internal combustion engine can be taken as an
example, the engine has an efficiency of around 30%, where 30% is wasted in cooling water; 10% loss is unaccountable and 30% is wasted as exhaust gas.
Wasted heat in the form of exhaust gas heat is what we will concentrate on in particular within this project. The aim is to capture the wasted heat via some
form of heat transfer process, temporarily store this heat and convert this heat into electrical energy in order to increase overall efficiency of an engine and
therefore reduce fuel consumption.
Automotive Thermoelectric Generators
A Thermoelectric generator is a heat recovery system incorporated into the
exhaust of a car that is used in conjunction with the internal combustion
engine. The exhaust pipe contains a block consisting of thermoelectric
materials with a hot side heat exchanger and cold side heat exchanger. A
Thermoelectric Generator is used to convert thermal energy from different
temperature gradients existing between hot and cold ends of a
semiconductor into electric energy that can be used to power various
electrical purposes within the car, This phenomenon was discovered by
Thomas Johann Seebeck in 1821 and is called the ‘Seebeck effect’.
Heat Exchanger Numerical Modelling
A rectangular plate fin heat exchanger of a specific geometry
and dimension was numerically modelled using Excel. Changes
in fin geometry dimension were made for the heat exchanger
component and effects that this had on the efficiency and heat
transfer convective coefficient were recorded from results,
represented graphically and discussed. An optimum fin design
and geometry was found from numerical modelling and put
forward for CFD analysis to make comparisons to the initial
design.
CFD Analysis Hot Side Heat Exchanger
From Numerical modelling we found that increases in fin thickness led to
greater convective coefficient values and temperature drop across the hot
side heat exchanger, which allowed greater heat transfer possible, which
would in turn increase the power output from the thermoelectric generator
device. Both designs were modelled, analysed and compared using CFD
analysis results. Boundary conditions were input in CFD from experiments
carried out in a research study ‘Automobile Exhaust Thermo-Electric
Generator Design & Performance Analysis’. Comparisons were also made to
the results found from the experimental study against CFD.
Thermoelectric Module Numerical modelling
Thermoelectric material consists of p-type and n-type semiconductors. When
the hot exhaust from the engine passes through the exhaust, charge carriers
of semi conductors within the generator diffuse from the hot side of the heat
exchanger to the cold side. When this happens, a temperature difference is
created between the two surfaces of the block resulting in a net charge; such
a temperature difference is capable of generating 500-750W of electricity.
Numerical Modelling was carried out on the HI-Z HZ-20 thermoelectric
module and various temperature dependant properties were recorded
graphically against varied temperature differences. Behaviour of these
properties were then analysed and discussed.
Fin Thickness vs Convective heat transfer
coefficient
98
96
94
92
90
88
86
1 2 3 4 5
Fin thickness (mm)
Fin thickness vs
convective heat transfer
coefficient
2.2
2.15
2.1
2.05
2
1.95
1.9
1.85
1.8
1.75
Thermal conductivity vs Temperature
difference (cold surface temp 30 degrees)
70 100 130 160 190 220 250 280 310 340
Temperature difference (degrees)
Thermal conductivity vs
Temperature difference
(cold surface temp 30
degrees)
Project Supervisor
Dr Abdessalem Bouferrouk
Project summary
1. Background research into Automotive Thermoelectric
Generators and their components.
2. Numerical modelling of hot side heat exchanger
component and performance output for varied fin
geometries.
3. Numerical modelling of thermoelectric module
component at varied temperature differences.
4. Proposed redesign model to be carried forward for
CFD analysis, comparisons made from CFD of original
design, redesign and experimental results gathered
from research study.
5. Any relevant validations made from CFD to numerical
modelling.
Project Objectives
The objective of the project is to gather an understanding,
by using numerical and computational techniques, into
how changing the design of an automotive thermoelectric
generator component can enhance the performance
output of the device. By the end of this project we would
have gathered an understanding of the components used
in automotive thermoelectric generators and how they
work at different temperature and flow boundary
conditions
Project Conclusion
From research into the study carried out on
automotive thermoelectric generators performance
and analysis, the first stage of CFD analysis on the hot
side heat exchanger component was able to prove
the accuracy and correlation of the results gathered
from the experiment carried out on the initial design.
Furthermore, carrying out the redesign process for
CFD analysis, a performance output increase of
24.27% was found in heat transfer from redesign of
the hot side plate fin heat exchanger. Which in turn
from research carried out would lead to a greater
power output from the thermoelectric generator
device.