Purdue University
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International Compressor Engineering Conference School of Mechanical Engineering
2016
Analysis of Indicator Diagrams of a Water Injected
Twin-sha# Screw-type Expander
Alexander Nikolov
Chair of Fluidics, TU Dortmund University, alexander.nikolov@tu-dortmund.de
Andreas Brümmer
Chair of Fluidics, TU Dortmund University, andreas.bruemmer@tu-dortmund.de
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Nikolov, Alexander and Brümmer, Andreas, "Analysis of Indicator Diagrams of a Water Injected Twin-sha> Screw-type Expander"
(2016). International Compressor Engineering Conference. Paper 2492.
h?ps://docs.lib.purdue.edu/icec/2492
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rd
International Compressor Engineering Conference at Purdue, July 11-14, 2016
Analysis of Indicator Diagrams of a Water Injected Twin-shaft Screw-type
Expander
Alexander NIKOLOV*, Andreas BRÜMMER
Chair of Fluidics, TU Dortmund University,
Dortmund, Germany
alexander.nikolov@tu-dortmund.de
andreas.bruemmer@tu-dortmund.de
* Corresponding Author
ABSTRACT
Twin-shaft screw-type expanders offer a high potential for energy conversion in the lower and medium power range,
for instance as expansion engines in Rankine cycles for exhaust heat recovery. With regard to minimizing internal
leakages and lubricating moving machine parts, an auxiliary liquid or liquid working fluid, for example in organic
Rankine cycles (ORC)
1
, can be fed to the screw expander.
In this paper, indicator diagrams of a twin-shaft screw-type expander prototype SE 51.2 designed at the Chair of
Fluidics at TU Dortmund University are presented and analyzed in detail. The experimental investigations are
carried out on a hot-air test rig with an expander inlet manifold water injection. The time-dependent working
chamber pressure is recorded by means of high-resolution absolute pressure transmitters. Hereby, specific aspects
of working chamber pressure measurements are mentioned. Based on the indicator diagrams, relevant influence
mechanisms on the expander's operational behavior resulting from water injection are determined. Additionally, the
impact of the injected water on the expander’s delivery rate and mechanical efficiency is presented.
1. INTRODUCTION
Due to the growing shortage of non-renewable fossil fuel reserves and the resulting increase in primary energy costs,
little developed energy potentials increasingly move into the focus of economic interest. Available heat sources in
the field of decentralized energy systems of small and medium power ranges, such as industrial exhaust gases or
waste heat in vehicles’ engines, geothermal or solar thermal energy can be converted into useable mechanical power
by means of expanders or turbines within a Rankine cycle.
In this context, twin-shaft screw-type expanders in Rankine cycles possess clear advantages compared with turbo
machines or even other expander concepts. In general, twin-shaft screw-type machines are characterized by
relatively high energy density and efficiency, good part load behavior, and rather a simple design. Furthermore,
screw expanders in Rankine cycles are suitable for wet-vapor operation, so that an overheating system is not
essential. On the one hand, the number of ORC components and the overall system costs can be reduced. On the
other hand, liquid in screw expanders is even beneficial with regard to minimizing internal leakages and reducing
noises or vibrations.
Whereas injecting oil is a common method in screw-type compressors without timing gears, water as an auxiliary
fluid offers sufficient potential for investigation. There are commercial water injection concepts for screw-type
compressors with timing gears which aim to reduce gas temperature and thermal stress in the working camber.
Investigations of water and oil injection into screw-type expanders with timing gears were done by
Zellermann (1996). Rinder et al. (2004) presented benefits and disadvantages of water compared to oil injection
with regard to a screw-type compressor without timing gears. Furthermore, Rinder and Moser (1990) have shown
oil distribution in a screw compressor. Kliem (2005) investigated a screw expander application in a trilateral flash
1
(Organic) Rankine cycles are thermodynamic cycles that operate as a closed loop. In Rankine cycles an operating
fluid, water or organic fluid, is continuously pressurized, evaporated, expanded and eventually condensed.
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cycle theoretically and experimentally. Here, overheated water is injected into the working chamber where it
expands and evaporates. An approach for the theoretical investigation of screw expanders in trilateral flash cycles is
presented by Vasuthevan and Brümmer (2016). An application of screw-type expanders in trilateral flash cycles is
predicted to be a promising approach in the medium and low power operating range in ORC by Ohman and
Lundqvist (2015).
In order to determine hydraulic friction losses in screw expander clearances using an auxiliary liquid, an analytical
approach was presented by Gräßer and Brümmer (2014) with regard to water and by Gräßer and Brümmer (2015) to
oil. Nikolov and Brümmer (2014) presented a water injected screw-type expander, focusing on an integral
investigation of the test expander without differentiation between the impact mechanisms resulting from the
auxiliary fluid. The most important expected advantage of water injection into screw-type expanders is sealing of
clearances as well as lubrication of the hardened and hard-coated screw rotors during operation. At the same time
depending on the liquid injection temperature, the auxiliary fluid lifts the temperature level within the working
chamber due to its higher heat capacity. Compared to a dry-running expander, injected water reduces temperature
drops during expansion which result in a slight difference between the inlet and outlet temperature of the working
fluid, as well as lower thermal stress alongside the expander.
Kovacevic and Rane (2013) presented a 3D analysis of a twin screw expander. Indicator diagrams, mass flow rates
and expander power at different operating conditions were estimated and compared with experimental results.
Cao et al. (2011) presented the results of an experimental investigation of pressure distribution inside the working
chamber of a twin screw compressor for multiphase duties. Additionally, a mathematical model for describing the
pressure distribution inside the working chamber is proposed.
Within the framework of the following paper, indicator diagrams of a water injected screw expander are presented.
Based on the indicator diagrams, relevant influence mechanisms on the expander's operational behavior resulting
from water injection are determined and explained in detail.
2. EXPERIMENTAL SETUP
The experimental rig, measurement devices which were used, and the test screw-type expander are introduced
below. In addition, an overview of the test parameters is given.
Diverse measurement devices record information about the fluid state in the rig and the test screw-type expander.
Insulated thermocouples and static relative pressure transducers are installed at different points of the test rig. Mean
static pressure and temperature of the gaseous working fluid is metered in the test rig before water injection. Inlet
and outlet static pressure as well as overall fluid temperature is recorded both in the high and low pressure domains
of the test expander. The effective power of the test screw-type expander is calculated by means of rotational speed
and driving torque metered using a torque transducer.
Within the framework of this paper, the results of experimental investigations on a water injected screw-type
expander SE 51.2 without timing gears (Figure 1) are presented. The expander’s geometrical parameters are listed
in Table 1. SE 51.2 is a further development of a twin screw charger GL 51 designed at Chair of Fluidics by
Temming (2007). The screw rotors are hardened and have a tough wear-protection coating, so that seizure can be
avoided at dry running or water injection. Both fixed and loose bearing sets are grease lubricated, so that no oil
supply is necessary. To avoid water flow through the fixed bearing sets on the high pressure expander side and
protect them from damage, a pressure cushion at constant level near the expander’s inlet conditions is set by a
pressurized air in the coupling casing.
In the high pressure domain of SE 51.2, a variable plate, which is shaped like the inlet control edge, is used. Thus,
the inner volume ratio and the utilizable pressure ratio can be adjusted. Moreover within the framework of
experimental investigations, inlet throttling effects can be investigated in detail. However, the inner volume ratio is
set constant for the experimental results presented below. Due to its modular structure, the geometry parameters of
the screw-type expander SE 51.2 can easily be modified. Apart from front clearance heights, housing and intermesh
clearance heights can be varied by changing the corresponding expander module.
In order to have a more detailed look into the physical effects of liquid phase within the working chamber, indicator
diagrams for dry- and wet-running operation of SE 51.2 are recorded by means of six high-resolution absolute
piezo-resistive pressure transmitters, Figure 1. Hereby, two different sensor types and sizes are used, which are
installed flush with the rotor bore surface and at the high-pressure front side. The pressure transmitters of the series
M5 manufactured by “KELLER AG für Druckmesstechnik” at positon 0, 1 and 6 record the static pressure at the
inlet port (position 0), within a rotational angle in the range between 85° and 130° (position 1) and during the fluid
discharge on the low pressure side of the expander (position 6). Each of these three sensors delivers the absolute
pressure at the metering point. With regard to the manufacturer’s data sheet of these transducers, a resonance
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frequency of more than 50 kHz can be expected. Nevertheless, a small cavity at sensor tip is constructively
available. Therefore especially at water injection, acoustical Helmholtz resonances may occur as a result of the rotor
tip gliding at the sensor. In this case, dynamic effects induce an impulse which can excite strong resonance
oscillations at frequencies far lower than given natural frequency in the sensor data sheet. This aspect has to be
considered within the evaluation of the pressure signal in order to avoid inaccurate interpretation. However, this
sensor type is essential for the experiments to record the working chamber pressure for very small chamber volume
at sensor position 1 in the high pressure front section, where an extremely narrow mounting space in the expander’s
casing is available.
Figure 1: Positions of pressure indication transmitters (0…6), inlet and outlet area, as well as volume curve of
the test screw expander SE 51.2
At positon 2 to position 5 XTM-190 pressure transducers manufactured by “Kulite Semiconductor Products, Inc”
with natural frequency higher than 425 kHz are mounted flush with the rotor bore surface and have no dead space
between their tip and the considered working chamber. Hence, no acoustic issues at the metering point with regard
to acoustic Eigen frequencies are expected. Within the calibration of these sensors, the pressure sensitivity proved
to be nearly constant. However, a relatively great temperature depending offset could be observed. Hence,
insulated thermocouples are installed at each pressure measurement position in order to compensate the impact of
temperature changes on the sensor sensitivity.
Table 1: Parameters of the test screw expander SE 51.2
designation unit designation unit male rotor female rotor
axis-center distance a [mm] 51.2 number of lobes z [-] 3 5
internal volume ratio v
i
[-] 2.5 diameter d [mm] 71.8 67.5
displaced volume per male rotor rotation V [cm
3
] 286 wrap angle φ [°] 200 -120
front clearance height h
fc
,
h
p
(high pressure) [mm] 0.1 rotor lead s [mm] 181.8 -303
front clearance height h
fc
,
l
p
(low pressure) [mm] 0.17 rotor length l [mm] 101
housing clearance height h
hc
[mm] 0.08 rotor profile [-] modified asymmetric SRM
Due to relatively high temperature sensitivity and offset of this sensor type, only the relative change in pressure to a
reference pressure level is considered. At this point, the reference pressure is determined at the overlapping scope of
two adjacent transducers (Figure 1). To begin with, the static pressure in the outlet port of the screw expander is
considered as the starting reference pressure.
Within the scope of this indication pressure measurement, a rotational angle range between 130° and 140° cannot be
covered due to lack of space in the casing, so the pressure has to be estimated. Preliminary analysis of the recorded
pressure delivered a linear pressure distribution in the missing range to be qualitatively and quantitatively
sufficiently accurate. For the area which was not surveyed at the beginning of the working cycle from 45° (start of
male rotor rotational angle [°]
chamber volume [cm
3
]
inlet and outlet area [cm
2
]
volume curve inlet area curve outlet area curve
100
90
80
70
60
50
40
30
20
10
0
100 200 300 400 500 600 700 800
0
10
9
8
7
6
5
4
3
2
1
0
0 1 2 3 4 6 5
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chamber filling) to 85° as shown in Figure 1, the dynamically recorded inlet port pressure is considered identical to
the chamber pressure. In this way, the assumed chamber pressure is mapped with regard to an ideal chamber filling
without pressure drop over the inlet. In theory, this results in an upper limit for the converted internal working of the
screw expander within this working cycle range. Another approach to map the chamber pressure profile in this
section is to assume a linear pressure increase starting from the outlet and ending at the pressure level of the
transmitter at position 1. This method is more appropriate with respect to increasing inlet throttling losses either at
higher expander’s rotational speed or increasing amount of injected water. Therefore, the pressure level and the
induced inner work within the missing range of the indicator diagram are expected to be lower than the real one
during the chamber growth. This approach represents a lower limit for the converted inner work within the first 40°
of chamber formation between 45° and 85°.
The relation between the time dependent high-resolution pressure signals and rotational angle and chamber volume
is provided by an optical trigger mounted on the male rotor shaft. Thereby, a TTL impulse is generated once per
male rotor revolution, which corresponds to a defined position of both rotors in relation to each other and the
working chamber volume. Between two trigger signals, an equidistant correlation between time and rotational angle
axis is considered. In terms of sufficient high resolution of the time depending signals, a sampling rate of 100 kHz
with regard to pressure transducers and trigger signal is provided.
Within the framework of this experimental investigation, both the system and the screw expander’s operating
parameters were varied. Water was injected at temperature levels of ϑ
i,w
≈ 30 °C, ϑ
i,w
≈ 60 °C, and ϑ
i,w
≈ 90 °C for a
constant air inlet pressure of p
i
= 4·10
5
Pa and back pressure of p
o
= 1·10
5
Pa and at hot air temperature of
ϑ
i,a
= 90 °C. The resulting mixture temperature at the expander’s inlet is ϑ
i
< 40 °C, ϑ
i
≈ 50 °C, and ϑ
i
> 60 °C
respectively depending on water volume flow and expander rotational speed.
With regard to steam or vapour mass fraction, the amount of injected water into the expander’s inlet port is
represented by the relative air mass fraction related to the overall air and water mass flow:
x =
m
a
m
a
+
m
w
. (1)
Here, measured air mass flow m
a
(Coriolis mass flow meter) corresponds to almost dry air, since a refrigerant type
dryer is used after air compression. Air mass fraction of x = 1 corresponds to a dry-running operation and 0 < x <1
to water injection operation and a two-phase flow respectively. The air mass fraction x is varied in the range from
1.0 to 0.4 depending on water temperature and expander’s rotational speed. For water injection, two nozzle sizes are
used at two adjacent water flow ranges with respect to a satisfactory water injection quality at the expander’s inlet.
The maximum expander rotational speed is n
MR
= 18,000 min
-1
, which equals a male rotor circumferential speed of
u
MR
= 67.7 m·s
-1
.
3. CHARACTERISTIC NUMBERS
The operational behaviour of screw-type expanders is evaluated by means of characteristic numbers. Effects such as
the sealing of expander clearances, inlet pressure drops, hydraulic losses, etc., can be evaluated integrally by taking
the system and expander operating parameters into account. The characteristic numbers used here are delivery rate
and mechanical efficiency.
3.1 Delivery Rate
Delivery rate is a characteristic number which describes different loss mechanisms such as internal or external
leakages, inlet pressure drops during the chamber filling, or thermal effects within a displacement machine. For a
screw-type expander, delivery rate is defined as:
λ
L
=
m
a
m
th
.
(2)
Here, m
th
represents theoretical air mass flow with:
m
th
= V
th,ex
·ρ
da,i
·n
MR
·z
MR
.
(3)