In turbomachines, the transfer of energy between the rotor and the fluid does not—in theory—result in lateral forces on the rotor. In positive displacement machines, on the other hand, the transfer of energy between the moving components and the working fluid usually results in unbalanced pressure fields and forces. Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135 (6), p. 062502) developed a model to predict the dynamic forces in twin-screw pumps, showing that the helical screw shape generates hydraulic forces that oscillate at multiples of running speed. The work presented here attempts to validate the model of Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135 (6), p. 062502) using a clear-casing twin-screw pump. The pump runs in both single and multiphase conditions with exit pressure up to 300 kPa and a flow rate 0.6 l/s. The pump was instrumented with dynamic pressure probes across the axial length of the screw in two perpendicular directions to validate the dynamic model. Two proximity probes measured the dynamic rotor displacement at the outlet to validate the rotordynamics model and the hydrodynamic cyclic forces predicted by Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135 (6), p. 062502). The predictions were found to be in good agreement with the measurements. The amplitude of the dynamic pressure measurements in two perpendicular plans supported the main assumptions of the model (constant pressure inside the chambers and linear pressure drop across the screw lands). The predicted rotor orbits at the pump outlet in the middle of the rotor matched the experimental orbits closely. The spectrum of the response showed harmonics of the running speed as predicted by the model. The pump rotor's calculated critical speed was at 24.8 krpm, roughly 14 times the rotor's running speed of 1750 rpm. The measured and observed excitation frequencies extended out to nine times running speed, still well below the first critical speed. However, for longer twin-screw pumps running at higher speed, the coincidence of a higher-harmonic excitation frequency with the lightly damped first critical speed should be considered.

The screw expander discussed in this work was part of a 50 kW organic Rankine cycle (ORC) system. The ORC was tested under different conditions in heat source and heat sink. In conjunction with collecting data for the ORC system, experimental data were also collected for the individual components of the ORC, viz. evaporator, preheater, screw expander, working fluid pump, and condenser. Experimental data for the screw expander were used to develop the two empirical models discussed in this paper for estimating screw expander performance. As the physical parameters of the screw expander discussed in this article are not known, a “black-box” approach was followed to estimate screw expander power output, based on expander inlet and outlet pressure and temperature data. Refrigerant R245fa was used as the working fluid in the ORC. The experimental data showed that the screw expander had ranges of pressure ratio (2.70 to 6.54), volume ratio (2.54 to 6.20), and power output (10 to 51.5 kW). Of the two empirical models, the first model is based on the polytropic expansion process, in which an expression for the polytropic exponent is found by applying regression curve-fitting analysis as a function of the expander pressure ratio and volume ratio. In the second model, an expression for screw expander work output is found by applying regression curve-fitting analysis as a function of the expander isentropic work output. The predicted screw expander power output using the polytropic exponent model was within ±10% of experimental values; the predicted screw expander power output using the isentropic work output model was within ±7.5% of experimental values.

Twin screw pumps are positive displacement machines. Two meshing screws connected by timing gears push the fluid trapped in the screw cavities axially from suction to discharge. Available steady state hydraulic models predict pump performance and axial pressure distribution in the chambers in single- and two-phase flow conditions. However, no model is available for their rotordynamics behavior. Due to the helix angle of the screw, the pressure distribution around the rotor is not balanced, giving rise to both static and dynamic lateral forces. The work presented here introduces a starting point for rotordynamic analysis of twin screw pumps. First, we show that the screw rotor's geometry can be represented by axisymmetric beam elements. Second, we extend the steady state hydraulic model to predict both the static and dynamic lateral forces resulting from the unbalanced pressure field. Finally, hydraulic forces are applied to the rotor to predict static, synchronous, and nonsynchronous responses. Predictions of the dynamic pressure were compared to measurements from the literature and were found to be in good agreement.

A microscale electrically heated rotary kiln for slow pyrolysis of biomass and waste was designed and built at the University of Perugia. The reactor is connected to a wet scrubbing section, for tar removal, and to a monitored combustion chamber to evaluate the lower heating value of the syngas. The system allows the evaluation of gas, tar, and char yields for different pyrolysis temperatures and residence times. The feeding screw conveyor and the kiln are rigidly connected; therefore, a modification of the flow rate implies a modification of the inside solid motion and of residence time. The paper provides the theoretical and experimental calculation of the relationships between residence time and flow rate used to determine the working envelope of the reactor as a function of the feedstock bulk density and moisture content, given the actual heat rate of the electric heaters. The methodology is extendable to any rotary kiln reactor with a rigidly connected feeding screw conveyor, given its geometric and energetic specifications. Part II of the paper will extend the energy balance, also introducing the yields of pyrolysis products.

Fuel gas compressors installed in cogeneration systems must be highly reliable and efficient machines. The screw compressor can usually be designed to meet most of the gas flow rates and pressure conditions generally required for such installations. To an ever-increasing degree, alternative sources are being found for the fuel gas supply, such as coke-oven gas, blast-furnace gas, flare gas, landfill gas, and synthesis gas from coal gasification or from pyrolysis. A feature of the oil-free screw compressor when such gases are being considered is the isolation of the gas compression space from the bearing and gear lubrication system by using positive shaft seals. This ensures that the process gas cannot be contaminated by the lubricating oil, and that there is no risk of loss of lubricant viscosity by gas solution in the oil. This feature enables the compressed gas to contain relatively high levels of particulate contamination without danger of “sludge” formation, and also permits the injection of water or liquid solvents into the compression space, to reduce the temperature rise due to the heat of compression, or to “wash” any particulate matter through the compressor.

Fuel gas compressors installed in cogeneration systems must be highly reliable and efficient machines, like the other main components, such as gas turbines, gas engines, etc. In the range of gas flow rate and pressure conditions generally required for such systems, the oil-injected screw compressor is often the most suitable compressor type for these requirements. Advantages of oil injected screw compressors are: improved compression efficiency; low discharge gas temperature; high reliability; simple mechanical construction; which all result from injection of lubricant into the compressor. Injected lubricant is discharged together with compressed gas on the high-pressure side but the oil is separated by a fine oil separation system down to a level that causes no problems for the downstream combustion equipment. The oil-injected screw compressor is equipped with an integral stepless capacity control by means of a slide valve, which makes part-load operation possible with reduced power consumption and improves overall system efficiency. As cogeneration systems, which are energy efficient and environmentally sound, are now increasing in number, so oil-injected screw compressors are expected to be used more widely.

An analytical procedure for calculating the performance of a two-phase flow screw-type expander is presented. Predicted results are compared with experimental measurements made with a recently developed prototype expander. This expander was designed for investigating the applicability of this type of machine as the expansion device in refrigeration or heat pump cycles with the objective of taking advantage of the power-producing capability of the expanding fluid. It has two rotors each with a diameter of 81.6 mm and a length of 135 mm, and was operated with Freon-12 entering at a pressure of 1.588 MPa (230.4 psia) and subcooling of between 2 and 12 K. Maximum power generated was 10 kW at a rotor speed of 3000 rpm. The internal (machine) efficiency is predicted to increase from about 30 to 70 percent as the rotor speed increases from 500 to 3000 rpm. Experimental results over this same speed range increase from 30 to 60 percent. The lower experimental values at the higher rotor speeds are attributed primarily to frictional losses not included in the analysis. Estimates of the performance of machines with larger diameter rotors yielded internal efficiencies reaching 80 percent and increases in power output proportional to the square of the rotor diameter.

An analysis is first performed to determine the exposed frontal area and effective volume as a function of rotor angle for circular lobed helical screw turbines. It is shown that once the female to male lobe ratio is set, the entire design is determined with respect to the male rotor pitch radius. These volume and area functions are then used in an overall dynamic model of the turbine. The model includes inlet and exhaust dynamics, leakage throughout the turbine, and the thermodynamics within the lobes. “Start up” of the system can be tested which is particularly important in determining critical speeds. The principal intended use of the model is for comparative parametric studies.

The dynamic acceleration performance of a single screw destroyer escort driven by two FT4A-2 gas turbine engines through a reversing reduction gear was analyzed. The analysis was carried out on a digital computer using a new method of a second modified advance coefficient to represent propeller thrust and torque coefficients. Quantitative results for all the major ship and propulsion plant parameters are given for the ship in a calm sea with no turning motions during fuel scheduled acceleration in the base and base-plus-boost operating modes. Control of fuel flow rates using fuel ramps with varying time bases was found to be effective in limiting engine overtorque conditions during acceleration. Other conclusions on transient thrust, acceleration time, and head reach are also presented.