5.4. Material Selection
5.4.1. Jet Body
In order to choose a suitable material for the jet body, it was necessary to first determine the maximum pressures which could be expected within the engine during operation.
These pressures were obtained from the preliminary thermodynamic analysis in section 5.3. Also considered were pressure cycle plots obtained from existing pulsejet engines.
The analysis in section 5.3 provided a reasonable estimate for the stagnation pressure in the combustion chamber as
P03 = 4.1 Bar
Adding in a factor of safety of approximately 2, the jet body should be capable of withstanding maximum internal pressures of up to 8 Bar.
Comparing to existing pulsejet analysis as shown in figure 3.1 and fig 5.8, 8 Bar pressure appears to provide a huge factor of safety. Figure 3.1 shows a peak pressure of 27 psi or 1.86 Bar and figure 5.8 shows a peak pressure of 28 psi or 1.93 Bar.
Considering that both jets are at different ends of the thrust scale with the project jet lying between them, it seemed reasonable to assume that the actual pressures experienced in the jet would be very similar. Therefore, by choosing the material to withstand 8 Bar pressure, it could be ensured that the safety concerns of the university could be comfortably met.
Since the not a majo steel or sta Mild steel material o application A stress a thick-wall 78.1mm a
Figure
project jet w or concern.
ainless steel l has more a ower cost an etter machin etter weld-ab uperior heat
ade from st seconds” ( his problem he combina eam structur under opera of choice a
ns.
analysis of led cylinder and 5.49mm
5.8 Pressure Cy
was to be a Amateur-bu l.
appeal as a p nd higher av ne-ability
bility t radiation p
ainless stee (Beck, 2005 is reported ation of pre
ral steel tub ating condit as this was
the 3” Sch r theory. Th m respectivel
= 39.0 5.49
Cycle plot in AS
a purely stat uilt pulsejet
pulsejet mat vailability
properties.
el should no 5) as the bo
to be elimin essure and bing was un
tions. A106 s designed
h40 pipe ch he pipe has ly.
05
9 = 7.1
S-014 pulsejet e
tic test engi ts are most
terial over s
According ot be operat ody can ove nated if mil high temp ndesirable a 6 seamless s for use in
hosen in se s a nomina
10 = ℎ
ngine [Bressma
ne, the total commonly
stainless ste
to one pul ted staticall erheat and b ld steel is us perature in as the welde teel pipe wa high-temp
ction 5.1.1 l diameter
ℎ
an, 1946]
al weight of constructed
eel for sever
lsejet websi ly for “mor burn holes i
sed.
the engine ed seam wa was decided perature, hig
was condu and wall th
31 f the jet was d from mild
ral reasons:
ite, engines re than 30 -in the steel.
e, common as at risk of upon as the gh-pressure
32 According to thick-walled cylinder theory:
= − 1 +
5.22 Since the maximum stress is needed and maximum stress occurs at the outer wall, then r
= ro and equation 5.22 can be simplified to:
= 2
− 5.23
Using the dimensions of the pipe and the expected pressures then the hoop stresses were calculated as follows:
Table 5.2 Max Hoop Stress in Jet Tube
Pressure (Bar) 8 1.93
Max Hoop Stress, σh max (MPa) 5.36 1.29
While no information was found for the yield strength of A106 steel at high temperature, the stress values in table 5.2 were compared against information found for several other weaker grades of carbon steel. The comparison showed that the stresses in the jet tube were significantly lower than the yield strengths of mild steel grades which were not designed to operate under high temperature. An example of the temperature dependant properties of common grade 43A steel is shown in figure 5.9. From the plot, the yield stress of 43A steel at 800°C can be approximated to ≈15Mpa.
33
Figure 5.9 Mechanical Properties of Grade 43A Steel at Elevated Temperatures [Bailey, 2009]
5.4.2. Valve Plate
The valve plate must be able to withstand the repeated impact of the valve tips up to 150-200 times per second. As well as being able to withstand this impacting, the valve plate should also provide a certain amount of damping force to the valve tips. This damping force should absorb some of the impact energy from the valves and thereby reduce the amount of stress the valve tip experiences during impact. This would help to increase the operating life of the valves. The concept of energy being absorbed by a material during an impact was related back to strain energy theory.
“The energy stored within a material when work has been done on it is termed the strain energy” (Hearn, 1997).
Since the work being done on the valve plate is the impact from the valve, the material which stores the most amount of strain energy will be the best material choice for the valve plate. Some of the kinetic energy of the valve tip will be converted to strain energy in the valve plate material.
34 Strain energy can be expressed as:
= 2 Or:
= 2 5.24
V will be constant for a given valve plate design. σ will also be constant for a given valve impact. If the only variable is the material of the valve plate, then:
∝ 1 2
A material with smaller E will result in a smaller value of U
To maximise U, aluminium with E = 70GPa was chosen for the valve plate material over steel with E = 200GPa. The downside to using aluminium is that aluminium has a very low surface hardness. This would most likely result in the repeated impact of the valves damaging the surface of the valve plate and affecting the seal between the valve plate and the valves. To avoid this, it was decided to hard-anodise the machined valve plate. The thin layer of aluminium oxide would increase surface hardness significantly without affecting the underlying material properties. A table showing Vickers hardness values for different materials is included below in table 5.3 as a comparison.
Table 5.3 Material Hardness Comparison Table [Hard Anodising Ltd, 2005]
Material Vickers Hardness Number
Untreated Al 6082 100 – 120
Hard Anodised Al 6082 400 – 460
Mild Steel 200 – 220
Stainless Steel 300 – 350
35 Additionally, it was determined that an aluminium valve plate would conduct heat from the valves quicker than steel and help keep them from overheating.
5754 aluminium alloy was chosen as the final valve plate material due to its excellent anodising properties and local availability.
36 6. Construction
This section details the manufacturing and construction of the project engine, the problems encountered and how they were overcome.
The majority of manufacturing of the components was carried out in the university’s engineering workshop. Detailed engineering drawings of all components can be found in appendix A of this report.