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Journal of Computing and Electronic Information Management
ISSN: 2413-1660
56
Design and analysis of Air-powered Rotary Engine
Di Zou a, Gongxiang Zhong b
MOE Key Laboratory of Oil & Gas Equipment, Southwest Petroleum University, Sichuan
Chengdu 610500, China
[email protected], [email protected]
Abstract: In view of the phenomenon that the pressure energy of natural gas is
wasted in the process of using the Downhole Throttling technology in the high pressure
well, the idea of using the air-powered rotary engine to transform natural gas pressure
energy into mechanical energy to drive the generator is put forward. Based on the
structural feature of traditional Wankel-type rotary engines the overall structural
design calculations of a new air-powered rotary engine are carried out and a new
decompression scheme is set up, and the flow field simulation is performed using
advanced CFD numerical simulation methods. According to simulation data the
correctness of the design calculation of the first stage rotary engine is verified and the
structural defect improvement basis of the second rotary engine is provided. The
problem of hydrate generation in the cylinder is solved when the second stage air inlet
temperature is increased from 343K to 353K. A new set of precise air-powered rotary
engine FLUENT simulation scheme was proposed and a new set of ideas of the
research on air-powered rotary engine is provided.
Keywords: CFD, FLUENT, air-powered rotary engine, dynamic mesh, numerical
simulation.
1. Introduction
The outlet pressure of natural gas well is between 20 and 100MPa and meanwhile the
pressure of pipeline is between 6 and 10Mpa[1]. Thus the throttle device is set to
decreases the gas pressure from well till the pipeline networks allowed.
However the throttle process wastes the energy from the high pressure from natural
gas in the field of oil and gas development. A simple calculation can demonstrates that
the high pressure natural gas contains plenty of energy which can be quite valuable in
economical and practical area by proper using[2].
The main use of natural gas pressure energy is electric generation and refrigeration.
The foreign scholars research the electric generation by pressure energy of natural gas
Volume 5 Issue 7 2018
57
and get some results[4-9]. The air-powered engine was refitted by the rotary engine
invented by Felix Wankel. However, because of the blockade on technique, few of
relative information can be gained.
A system of air-powered engine which follows the Rankine cycle performs better in low
power output. A scheme of refitting the rotary engine to air-powered engine is put
forward. The scheme remains the superiority of rotary engine about low-vibration,
small volume and continues working which drives much scholars research on that. But
an important disadvantage of rotary engine that compares with the combustion
efficiency of reciprocating internal combustion engine, the air-powered rotary engine
performs poor because of the low compression ratio which leads the high consumption
and heavy pollution. Those disadvantages are not been avoided till now.
2.The Physical Design of Air-powered Rotary Engine
Air-powered rotary engine consists of rotor, end cap, cylinder block and bent axel etc.
In the paper a air-powered rotary engine applies to natural gas well in high pressure
difference, high rotation speed and mass flow working condition based on the existing
result of rotary engine[14,15].
2.1 The Theory Introduction of Air-powered Rotary Engine
The object line of cylinder block of air-powered rotary engine is double arc short-range
cycloid, the object line of rotor is the inner envelope of the object line of cylinder block.
An offset e exists between the center of cylinder block and rotor.
The inner gear of rotor and outer gear of end cap keeps engage which the gear ratio is
3:2. The cylinder block is treat as a stator as a fixed end, the rotor moves eccentrically
drives by eccentric bent axel. The 3 peaks of rotor slide on the inner wall of cylinder
block which separates the working chamber into 3 parts as air intake, inflation and air
outtake(Fig.1)[16]. Fig.2 shows the volume changing of working chamber by the
rotation of bent axel.
Fig.1 The physical design of air-powered rotary engine
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Fig.2 The relationship between the volume of 1st working chamber and the angel of
bent axel
2.2 The Scheme Design of Depressurization
A 2-class depressurization scheme is selected, and the designed parameters is showed
as:
(1)40MPa to 18MPa in the first class depressurization, 18MPa to 8MPa in the second
class depressurization.
(2)The initial temperature of 40MPa natural gas is 80℃;
(3)The flow of natural gas is 80× 104m3/d(standard condition);
(4)The rotation speed of bent axel is 3000r/min;
(5)The pure methane is selected as working fluid which the density is
0.717kg/m3(standard condition).
Because of the clearance volume while engine working, the exhaust coefficient is
introduced as dλ :
lTd (1)
λ is pressure coefficient; Tλ is temperature coefficient; lλ is leakage coefficient.
Table 1 The exhaust coefficient of each class cylinder block
calss Ⅰ Ⅱ
λ 0.98 0.99
Tλ 0.95 0.96
lλ 0.97 0.98
lTd 0.91 0.93
The exhaust gas temperature after depressurization is given by:
kk
sd TT1
1
(2)
dT is exhaust gas temperature; sT is intake air temperature; 1 is pressure drop ratio.
The 1st exhaust gas temperature is 293.6K after calculation. The aquo-complex will not
generate based on Fig.3[17].
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59
Fig.3 The pressure-temperature curves of methane aquo-complex generation
The temperature is decreased to 20.6 after 1st depressurization. To make sure that the
aquo-complex won’t generate after 2nd depressurization, the gas is heated to 70; The
1st exhaust gas temperature is 285K. The aquo-complex will not generate based on
Fig.3.
2.3 The calculation of structural design
The inner object line (epicycloid) can be written as:
3
3
sinsin
coscos
Rey
ReX (3)
In (3), R is generating radius; e is offset.
The object line of Wankel rotor is the inner envelope of epicycloid of cylinder block
which can be written as:
21
2
2
21
2
2
3sin1sin5sin
4sin8sin2sin
3sin1cos5cos
4cos8cos2cos
29
2
3
29
2
3
vvve
vvvRX
vvve
vvvRX
Re
Re
Re
Re
(4)
In (4), 12v ,
6
21
6
19
6
13
6
11
6
5
2,,,,, ππππππv 。
The main dimension of the first and second class air-powered engine is given in table
2.
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60
Table 2 The dimension parameters of air-powered rotary engine
object First class Second class
Generating radius(mm) 78 90
offset(mm) 13 15
Width of rotor and cylinder block(mm) 37 45
Thickness of cylinder block(mm) 20 20
Diameter of bend axel eccentric disc(mm) 65 75
Spindle diameter of bent axel(mm) 35 41
3 The simulation of air-powered rotary engine based on FLUENT
3.1 The geometric model construction of air-powered rotary engine
A simplified 3D model of air-powered rotary engine based on SolidWorks is given:
Fig.4 The 3D model of 1st and 2nd class rotor
3.2 The construction of channel model and meshing
Lead the 3D model of air-powered rotary engine into the DM module of Workbench and
the channel model is constructed and processed, and the 2D section model of 3D
channel is generated; Because of the contact relation between the rotor peak and inner
wall of cylinder block, the rotor size is narrowed 0.99 times to gain a 0.65mm,0.85mm
gap between the 1st and 2nd rotor and cylinder block (showed as Fig. 5a), the following
simulation setting uses a “cotton plug” method to solve the leak problem of the gaps._
4 definitions is added in cyl fluid in UDF to define the 3 peaks of rotor in cyl fluid, and
the flow resistance is added to its both sides to avoid the leak from the gaps; The
pressure cloud diagram is showed in Fig.5b about after setting the “cotton plug” which
demonstrates no air leaking in the situation of high pressure difference between each
cylinder blocks and gaps existence.
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61
(a) Reserved gap (b)The pressure cloud diagram after setting “cotton plug”
Fig.5 The setting of “cotton plug”
The type of mesh is non-structural triangular mesh which its size is 1mm showed as
Fig.6, and the mesh number is 25010,34024. The update of dynamic meshing is
achieved by Smoothing Layering and Remshing method.
Fig.6 The meshing in GAMBIT
While setting the boundary condition, the “valve” is set to air inlet and outlet with a 5
time control which is set by Define Event.
The transient solving method is chosen and the k-ε model is chosen for analyzing the
turbulence situation[18]. The control equation is given as:
0
dAnpvdxdydxAvt
(5)
the first item of left side of (5) is the control of mass increment, the second item is the
net flow from control surface to control volume. A is control surface, V is control
bolume
The moment conservation equation is given as:
z
p
y
p
x
pzdt
du
z
p
y
p
x
pydt
dv
z
p
y
p
x
pxdt
du
zxyxxx
zxyxxx
zxyxxx
F
F
F
(6)
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62
In (6), xF , yF , yF is the mass force component of unit mass fluid of x,y,z direction, P is
the internal stress tensor of fluid.
The energy reservation equation is given as:
2
21
11
1
up
heffijijjjxeffx
ixt
hE
SuJhk
pEuE
T
(7)
In (7), is the effective index of heat conduction, is the diffusion flux of component j, is
the volume heat source.
The PISO algorithm is chosen for calculation and the type of pressure discretization is
PRWESTO! method.
3.3 The result and analysis of simulation of air-powered rotary engine
The simulation time is 15ms because of the exhaust time is 15ms; The simulation
verified whether the pressure and temperature variation of air-powered engine in the
process of air suction and inflation can fit the design calculation. The pressure cloud
diagram and pressure-time variation curve is given as:
(a)Gas intake completion time (b)Gas exhaust time
The pressure cloud diagram of 1st class air-powered engine
(a)Gas intake completion time (b)Gas exhaust time
The pressure cloud diagram of 2nd class air-powered engine
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The pressure-time variation curve of 1st/2nd class air-powered engine
Fig.7 The simulation results of 2 class air-powered engine pressure
The gas pressure of inlet is 40MPa, the average pressure of inlet cavity is 39.92MPa and
its volume is 150.1cm3 after the gas intake completion and the bent axel rotates
170°(t=9.4ms) from initial position; then the intake cavity pressure drop to 19.4MPa
while the cavity pressure is 208.3cm3 after the bent axel rotates 270 (t=15ms) which
the inflation and pressure decrease completes; the deviation between design pressure
drop and simulation is 7.7% which within the tolerance interval, the first class pressure
drop is proved.
The gas pressure of inlet is 18MPa, the average pressure of inlet cavity is 17.92MPa and
its volume is 253.3cm3 after the gas intake completion and the bent axel rotates
143°(t=7.94ms) from initial position; then the intake cavity pressure drop to 7.52MPa
while the cavity pressure is 439.1cm3 after the bent axel rotates 270 (t=15ms) which
the inflation and pressure decrease completes; the deviation between design pressure
drop and simulation is 6.6% which within the tolerance interval, the second class
pressure drop is proved.
The temperature and temperature –time variation curve is showed in Fig.8. The first
class gas intake temperature of air-powered rotary engine is 353K, the cavity average
temperature after gas intake completion is 346.5K, and the gas exhaust temperature
drops to 298K after inflation completion while no aquo-complex generating and the
deviation value is acceptable, so the first class temperature drop is proved.
Gas intake completion time Gas exhaust time
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64
The temperature cloud diagram of 1st class air-powered engine
Gas intake completion time Gas exhaust time
The temperature cloud diagram of 2nd class air-powered engine
The temerature-time variation curve of 1st/2nd class air-powered engine
Fig.8 The simulation results of 2 class air-powered engine pressure
The second class inlet temperature of air-powered rotary engine is 343K, the cavity
average temperature after gas intake completion is 338.5 based on simulation results,
the exhaust gas temperature is drops to 273.3K after inflation completion which is
lower than design temperature 285K, so the second class design is inacceptable which
needs improvement.
3.4 The structural improvement based on flow field simulation of FLUWENT
Considering the problem of the temperature drop leads the aquo-complex generation,
the temperature is raised to 353K from 343K of gas inlet of the second class
air-powered rotary engine. The optimized parameters of second class air-powered
rotary engine is showed in table 3.
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The contrast between the original and improved parameters
Object Original design Optimized
design
Generation radius(mm) 90 90
Offset(mm) 15 15
Width of cylinder block and rotor(mm)(optimized)
45 47
Thickness of cylinder block(mm) 20 20
Diameter of bend axel eccentric disc(mm) 75 75
Spindle diameter of bent axel(mm) 41 41
Gas pressure of intake/exhaust(MPa) 18MPa,8MPa 18MPa,8MPa
Gas temperature of intake/exhaust((K)(optimized)
343K,285K 353K,292.6K
Bent axel angle of gas intake/exhaust (optimized)
intake:143°
exhaust:270°
intake:138.2°
exhasut:260°
Time switch of intake and exhaust “valve” (ms)(optimized)
Intake "valve” closes:7.94ms
Exhaust “valve” opens:15ms
Intake "valve” closes::
7.67ms Exhaust “valve” opens:
14.45ms
The optimized gas pressure of inlet is 18MPa, the average pressure of inlet cavity is
17.93MPa and its volume is 190.5cm3 after the gas intake completion and the bent axel
rotates 138.2°(t=7.67ms) from initial position; then the intake cavity pressure drop to
7.71MPa while the cavity pressure is 342.8cm3 after the bent axel rotates 260
(t=14.45ms) which the inflation and pressure decrease completes; the deviation
between design pressure drop and simulation is 3.62% which within the tolerance
interval, the second class pressure drop is proved. The optimized second class
presswure-time variation curve is showed in Fig.9a:
(a)Pressure-time (b)Temperature-time
Fig.9 The optimized 2nd class pressure/temperature-time variation curves
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66
The optimized temperature of gas inlet of air-powered rotary engine is 353K which the
temperature –time variation curve is showed in Fig.9b. The average cavity temperature
after gas intake completion is 347.5K, the temperature drops to 282.3K after inflation.
Though the temperature is still lower than the design value, aquo-complex will not
generate based on Fig.3 and the deviation is acceptable. The optimization and
improvement solves the problem of aquo-complex generation.
4. Conclusion
(1)A depressurization scheme and structure design is given of 40MPa pressure of
natural gas wellhead of 2 classes air-powered rotary engine, 1st class design is verified
by FLUENT dynamic mesh flux field simulation; the problem of aquo-complex
generation is solved by elevating the temperature of 2nd class inlet from 343K to 353K.
(2)The “cotton plug”, ”valve”, ”UDF” method is used to solved the problem of
simulating the special flux field air-powered rotary engine, a novel and precise
simulation method of air-powered rotary engine is put forward.
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