Volumetric expanders I
Chair: Prof. Vincent Lemort
EXPERIMENTAL STUDY ON THE PERFORMANCE OF SINGLE SCREW EXPANDERS WITH 195 MM DIAMETER SCREW
Guo-Dong Xia, Ye-Qiang Zhang, Yu-Ting Wu, Chong-Fang Ma, Yan-Hai Peng, Wei-Ning Ji
An Organic Rankine Cycle (ORC) is a promising process for conversion of low and medium temperature heat to electricity. The ORC process works like a Rankine steam power plant but uses an organic matter as working fluid instead of water. Today, Organic Rankine Cycles are commercially available in the MW power range, but very fewsolutions are actually suitable for the 1kW~100s kW. However, a lot of small and middle scale ORC technology applications exist such as biomass combined heat and power, solar power plant, geothermal energy, waste heat recoveryand so on [1,2]. Performance of the ORC system strongly correlates with those of the expander.Turbines are mainly designed for larger-scale applications andshow a higher degree of technical maturity. While, displacement expanders are more appropriate to the small-scale ORC system,because they are characterized by lower flow rates and higher expansion ratios than turbines.
Single-screw expander is a new type of displacement expander with one rotor and two gaterotors. In an action cycle, three working processes,which are gas admission, gas expansion, and gas discharge, are shown by Figure 1.
Figure 1.Working principle graph of single-screw expander
Single-screw expander can be used as prime mover in small-sized ORC system. It has many advantages, such as long working life, balanced loading of the main screw, high volumetric efficiency, low noise, low leakage, low vibration and simple configuration, etc. Single screw expander can realize 1-200 kW range of power output, and it is suitable for saturatedsteam,superheated steam, gas-liquid two phases or heat liquid.
A single screw expander prototype with 195 mm diameter screw is developed by us. The main task of this paper is to carry out the performance test for the single-screw expander, and obtain the influence law of rotational speed and inlet pressure of working fluid on the prototype performance.
The experiment used compressed air as working fluid. Air compressor provided high pressure air to gas tank, whichwas the stable gas source for the test. Compressed airwith different pressure from gas tank entered into expander by adjusting the inlet valve. Exhaust gasled to outdoors. Through aneddy current dynamometerfor the load, theshaft power of expander was changed to heat carried away by cooling water. The parameters of volume flow, inlet and outlet pressure,inlet and outlet temperature, rotational speed and torque, thepower and temperature drop performance of the prototypewere obtained. Experimental platform is shown in Figure 2.
Figure 2. Experimental platform
EXPERIMENTAL PERFORMANCES ANALYSIS
Performance tests are carried out by varying the inlet pressure (pin) from ranged 7bar to 15bar and by varying the speed of 1200rpm to 3200rpm.
The power output ( ) is an important parameter for evaluating the performance of the single-screw expander. The effects of inlet pressure and rotational speed to power output are shown in Figure 3.
Figure 3:Power output versus inlet pressure and rotating speed
Adiabatic efficiency is an important parameter for evaluating the effect of internal irreversibilities during adiabatic processes, which is defined as
Where is the actual enthalpy drop of work fluid between inlet and outlet of expander, is the ideal enthalpy drop of work fluid during adiabatic processes.
The variation of adiabatic efficiency with inlet pressure and rotational speed are shown in Figure 4.
Figure 4: Adiabatic efficiency versus inlet pressure and rotating speed
The total efficiency is the most important parameter for evaluating the performance of the single-screw expander, which is defined by
Where is the ideal power output during adiabatic processes.
The variation of total efficiency versus inlet pressure and rotational speed are shown in Figure 5.
Figure 5:Total efficiency versus inlet pressure and rotating speed
Through the experiment for the prototype of single-screw expander, following conclusions can be given:
(1) The power output increases with the increase of intake pressure and rotational speed. The total efficiency and air-consumption ratio are not regular, but can obtain best values at 2800rpm.
(2) Higher power output and higher total efficiency are along with higher intake pressure.
(3) For the self-developed single-screw expander, the highest adiabatic efficiency is 66.43% and total efficiency are 66.43% and 62.23%, respectively. The biggest power output is 51.08kW.
The authors are also grateful to the financial support by the National Basic Research Program (also called 973 Program) of China under grant number 2011CB710704 and 2011CB707202.
 Canada, S., G. Cohen, R. Cable, D. Brosseau, H. Price. Parabolic trough organic rankine cyclesolar power plant, NREL/CP-550-37077.In: The 2004 DOE Solar Energy Technologies, Denver, USA.
 Drescher, U., D. Bruggemann. Fluid selection for the Organic Rankine Cycle (ORC) in biomasspower and heat plants. Applied Thermal Engineering, 2007(27): 223-228.
 Wei Wang, Yu-ting Wu, Chong-fang Ma, et al.Preliminary experimental study of single screw expander prototype. Applied Thermal Engineering, 2011(31):3684-3688.
NUMERICAL SIMULATION OF AN OIL INJECTED TWIN SCREW EXPANDER FOR SMALL SCALE ORC SYSTEMS
Iva Papes, Joris Degroote, Jan Vierendeels
Abstract: A number of studies have shown a big potential of Organic Rankine Cycles (ORC) for waste heat recovery. Although ORC systems are now well developed, efforts have been increasingly directed towards higher efficiencies and power outputs. The key element for the power generation in ORC systems is the expander. One of the advantages when using screw and scroll expanders (among other positive displacement machines) is the possibility to use existing compressors with opposite sense of rotation. In this paper, a complete 3D CFD calculation for an oil injected twin screw expander is performed. With an in house code, a block structured grid is generated using the solution of a Laplace problem. In order to lubricate the rotor motion and seal the gaps, oil is injected into the expansion chambers. The working fluid inside the screw expander is treated compressible and the oil phase is modeled with a multiphase model. The results of these 3D CFD calculations will lead to improvements for the maximization of the power output in ORC systems.
GEOMETRIC, THERMODYNAMIC AND CFD ANALYSES OF A REAL SCROLL EXPANDER FOR MICRO ORC APPLICATIONS
Mirko Morini, Claudio Pavan, Michele Pinelli, Eva Romito, Alessio Suman
Abstract: At the beginning, scroll architecture was applied for the design of refrigeration compressors due to its small dimension, quiet operation and highly compact architecture. Recently, the concept was reinvented to build scroll air motors (expanders), which are adopted to drive generators for electric power generation because of the scroll inherent advantages. For example, Sanden scroll compressors have been recently adapted to be used as an expander for microCHP applications. In [1,2], a complete study of the Sanden TRSA05-3373 has been performed. The study included the definition of the mathematical relation for the spiral profile generation, the definition of the volumes isolated and transferred by the scroll, and the implementation of a thermodynamic control volume model.
In this paper, an integrated Reverse Engineering (RE)-Computational Fluid Dynamics (CFD) methodology is applied in order to study the adaptation of a commercial scroll compressor to be used as an expander in a micro ORC system. The analysis consists of: (i) the acquisition of the scroll compressor Sanden TRSA09-3658 geometry through an RE procedure, (ii) the transient simulation with a Dynamic Mesh (DM) strategy of the scroll in compression and expansion operations and (iii) the analysis of the performance in terms of pressure and mass flow rate profiles and volumetric efficiency. The CFD model allows the evaluation of the influence of leakage flows, e.g. due to radial (flank) gaps, which play a key role in the determination of the performances of the machine. Moreover, it allows the tuning of analytical and thermodynamic models with fewer resources in the design phase. Finally, CFD results (e.g. pressure and temperature closer to reality than those resulting from simplified thermodynamic models) can be used as boundary conditions in mechanical and structural analyses of the spiral profiles.