Proceedings of the Institution of Mechanical Engineers,Part E: Journal of Process Mechanical Engineering

The paper deals with the optimisation of the geometry of screw machines, focussing on the wrap angle of the rotors. The Pi-theorem is used to identify independent thermodynamic and geometric factors for the optimisation of dry running twin screw compressors with uniform and dual lead rotors. Dual lead rotors possess two segments with different rotor leads, which can be selected to optimise the progression of chamber volume, clearance size and outlet area. Energy efficiency of the compressors is determined using a multi-chamber model simulation in which fluids are treated as ideal gases. The wrap angles of the rotors are optimised for a given set of characteristic numbers using the Nelder–Mead algorithm. Optimisation results reveal that the optimal geometry mainly depends on the circumferential Mach number and isentropic exponent of the fluid since these influence discharge throttling and the pressure progression due to volumetric compression. The results point out that dual lead compressors are most beneficial for fluids with low speed of sound and low isentropic exponent.

In this paper, reducing the friction losses in a radial inflow turbine rotor surface by adding engineered features (riblets) is explored. Initially, computational fluid dynamics analysis was used to study the operating mechanism of riblets and to test their ability to reduce drag within the rotor passage when running the turbine at the design point. Thereafter, riblets with different heights and spacing have been implemented at the rotor hub to study the effect of riblets geometry and arrangement on the drag reduction, which leads to determine the riblet geometry where the maximum benefit on turbine performance can be achieved. The effect of riblets on boundary layer development and on the secondary flow generation within the rotor passage has been examined. It was found that the introduction of riblets could reduce the wall shear stress at the hub surface, and on the other hand, they contribute to increasing the stream-wise vorticity within the rotor passage. The maximum wall shear reduction was achieved with riblet with relative height h_rel = 2.5% equivalent to 19.3 wall units, while the maximum performance happens when using riblets with h_rel = 1.5% equivalent to 11.8 wall units as the later contributes less in secondary flow generation within the passage. For riblets with height more than 19.3 wall units, the overall effect is negative, as they cause an increase in drag and give rise to secondary flow leading to lower turbine performance.

Reed valves are widely used in hermetic reciprocating compressors and are responsible for a major share of thermodynamic losses. The suction valve in particular, which is opened for almost the entire suction phase, has a very significant improvement potential. Suction valves in hermetic reciprocating compressors are usually opened only by the pressure difference created by the moving piston and should be closed before the compression phase starts, in order to avoid a reversed mass-flow through the valve. The valves are thus preloaded, which, on the other hand, results in a higher flow resistance. In this work, a force-assisted suction valve is investigated. An electromagnetically actuated spring generates an additional force acting on the conventional suction reed valve. This force leads to a completely different valve movement and a significant reduction of the losses caused by the suction valve. A conventional suction valve flutters during the suction phase. Thus, it hits the valve seat several times which has, beside an increased flow resistance, negative influences on the reliability of the reed valve and noise emissions. With the help of the additional force, it is able to overcome the opening delay due to the preload and the oil ‘stiction’ effect. Furthermore, the suction valve can be held open during the whole suction phase, and it closes smoothly in time near the bottom dead centre of the piston. The investigation of the force-assisted suction valve is done by numerical simulations as well as by experiments. Results show an improvement of the coefficient of performance by approximately 0.7%–1.6% depending on the operating conditions.

Condensation and its effects on turbomachinery operation are well understood and have been widely investigated. However, only little scientific work on condensation in positive displacement machines has been published. Although, depending on machine type, high expansion rates and, as a consequence, significant supersaturation can be achieved for working fluids with a negative saturation vapour curve. In this paper, the effects of spontaneous steam condensation in screw expanders are discussed. Classical nucleation theory is used for the thermodynamic simulation of operational behaviour. The study shows at which point during the expansion phase spontaneous condensation can be expected and typical nucleation rates are determined. The impact of released latent heat during expansion on chamber states is depicted. Furthermore, a comparison of purely metastable expansion with equilibrium expansion is provided in order to show the full-range discrepancies. Additionally, the influence of internal leakage and throttling effects during the inlet phase on the course of spontaneous condensation and droplet growth is analysed. Typical operating parameters are widely varied in simulation so as to identify the impact of steam parameters and expander parameters on the condensation process.

The transient flow model of a scroll refrigeration compressor which included axial clearance was established in the paper using PUMPLINX based on the computational fluid dynamics and the dynamic grid methodology. According to the actual size in the physical compressor, the flank and the axial clearances were set equal to 0.030 mm and 0.015 mm, respectively. As the fluid domain of the axial clearance was too small to generate enough layers of grids in PUMPLINX, it was first magnified 100 times to generate grids of high quality, and then the fluid domain and grids were recompressed to the original size. The simulation results were verified experimentally. The influence of the radial leakage on the downstream flow field was investigated. Moreover, the characteristics of radial and tangential leakage flows were analyzed. It was concluded that the predictions of the compressor performance, except the discharge temperature, are improved after the axial clearance was included in the model. The radial leakage is dominant while a portion of involute leakage flow exists near the flank clearance. The influence of radial leakage on the downstream temperature is lower than that of the tangential leakage in flank clearance. The tangential leakage flow is turbulent basically while the radial leakage flow is laminar. The tangential leakage is severer at the beginning of the discharge process, and the radial leakage is more remarkable at the end of the compression process. When the pressure ratio across the leakage path is higher than the critical pressure ratio, the tangential leakage flow becomes supersonic for the converging–diverging clearance structure, while the radial leakage flow remains subsonic due to the larger flow losses. The results obtained through this research could be applied to modify the current leakage models and optimize the clearances of scroll compressors.

The requirements for compressors in refrigeration industry become more demanding each year. Requirements regarding efficiency, durability, and also the possibility / demand of dry running machines are the main driving factors. So far, the majority of compressors are mainly manufactured out of cast iron, aluminum, steel, and other metals (not including sealing elements), which lead to high costs as well as weights. The scope of this work includes a demonstration of new manufacturing methods and material choices for small-scale compressors as well as a possible transfer to even larger machines. A detailed look on the potential of different material classes and compounds will be presented. The paper also takes into consideration if the new materials have to be used for lubricated compressors or if there is even a possibility for an operation without any oil. Thermodynamic and strength limits will be pointed out as well as the potential to substitute the nowadays used metals-based components by lightweight materials.

Water refrigerant heat pump system with a water vapour turbo compressor is developed. Water (R718) is an ideal refrigerant that is considered perfectly environment friendly. Although water refrigerant heat pump is studied extensively, the development of turbo compressors of high pressure ratio is still a technical challenge, in terms of both aerodynamics of the impellers and high-speed rotordynamics. In this study, the high-speed rotor is supported by journal bearings lubricated with water refrigerant. Additionally, other system components such as sprayed direct intercooler heat exchanger and anti-cavitation flow control valve have to be designed and developed with the new requirements as well. An experimental test rig of closed-loop heat pump is constructed which achieves the cooling capacity of 100 kW on the rated condition with COP = 5. The experimental results and the experienced challenges and their undertaken solutions are discussed in terms of the efficiency and the vibrations of the turbo compressor and the heat pump system.

Recently, linear compressor technology has gained increased attention in vapor compression systems due to its compactness and lower frictional losses compared with the conventional reciprocating compressors. Since the absence of the crank-mechanism eliminates the rotation and reduces the side loads on the piston centered inside the cylinder, it is possible to operate the linear compressors without oil lubrication. A number of advantages can be obtained in terms of the refrigerant compatibility, operation conditions and the system cost. In order to enable oil-free operation, while minimizing frictional losses and leakage flow between the piston and the cylinder wall, a gas bearing approach is applied. Little work was found in the open literature related to the numerical analysis of gas bearings coupled with a comprehensive dynamic linear compressor model. This paper presents a 1-D gas bearing model based on a Finite Volume Method (FVM) applied to a linear compressor. The dynamic characteristics of the gas bearings on the gas film pressure field are analyzed using the model. When integrated together with a comprehensive dynamic linear compressor model, it is possible to predict leakage and frictional losses within the compressor as well as the overall performance.

This paper reports an investigation on the phenomenon of refrigerant outgassing in the screw pump of a hermetic reciprocating compressor oil supply system. The interfacial mass transfer between the refrigerant and the lubricating oil is modelled by means of a cavitation model based on the Rayleigh-Plesset equation, for cases of fixed and variable mass fractions of refrigerant dissolved in the lubricant. The influence of compressor speed and compressor crankcase pressure is evaluated by comparing the lubricant volumetric flow rate and the lubricant volume fraction field for each condition. The results reveal that significant outgassing of refrigerant may take place in the oil pump, reducing the lubricant flow rate supplied by the pump.

Conventional twin-screw compressors are positive displacement machines that form working chambers between two helical, parallel rotors and an outer casing. This study investigates an alternative configuration of rotors for compressor applications, in which the working chambers are formed by continuous contact between meshing inner an outer rotors. This is a similar configuration to gerotor machines, which are commonly used in fuel and oil pumping applications and as hydraulic motors. For compressor applications, the use of helical rotors with appropriate porting in a gerotor-type configuration has been identified as having potentially lower rotor contact forces and larger port areas than occur with non-helical rotors. This study investigates how key performance parameters of a gerotor-type screw compressor (volume, port flow areas and tip leakage areas) are influenced by the profile, dimensions and wrap angle of the rotors. The results of this geometric analysis are compared with a conventional twin-screw compressor, and suggest that the gerotor-type screw compressor can achieve higher axial port areas and lower tip leakage area over most of the compression cycle.

The present study establishes the optimum process condition for additive mixed electrical discharge machining of Al7075–5%B₄Cp metal matrix composite by performing experimental investigation. The suspension of chromium particles in a dielectric fluid is used as an additive. The input process parameters selected for experimentation are specifically pulse on-time, gap voltage, pulse off-time and peak current, for analysing their influence on wear of the tool along with surface roughness of the composite. Comparative study of the machined surface is done by analysing microstructures, cracks and recast layers formed at different settings of input parameters using a scanning electron microscope. Rise in amount of current and pulse on-time led to increased height of the recast layer generated on the surface of the machined workpiece. Furthermore, a confirmatory experiment was performed at the optimal setting. The result revealed an error of 5.5% and 7.5% between experimental and predicted value of tool wear rate and surface roughness.

The two-stage ejector has been suggested to replace the single-stage ejector geometrical configuration better to utilize the discharge flow’s redundant momentum to induce secondary flow. In this study, the one-dimensional gas dynamic constant rate of momentum change theory has been utilized to model a two-stage ejector along with a single-stage ejector. The proposed theory has been utilized in the computation of geometry and flow parameters of both the ejectors. The commercial computational fluid dynamics tool ANSYS-Fluent 14.0 has been utilized to predict performance and visualize the flow. The performance in terms of entrainment ratio has been compared under on- design and off-design conditions. The result shows that the two-stage ejector configuration has improved (≈57%) entrainment capacity than the single-stage ejector under the on-design condition.

In the present work, nanoboron carbide is integrated in the aluminum matrix using friction stir processing: by varying process parameters, that is, tool pin profile, tool rotational speed and tool traverse speed, based on Taguchi L₁₆ design of experiment. A self-assembled monolayer is successfully developed on the substrate to homogeneously and uniformly distribute the reinforcement particles. Response surface methodology and artificial neural network models are developed using ultimate tensile strength and total elongation as responses. Percentage absolute error between the experimental and predicted values of ultimate tensile strength and total elongation for the response surface methodology model is 3.537 and 2.865, respectively, and for artificial neural network is 2.788 and 2.578, respectively. For both the developed models experimental and forecasted values are in close approximation. The artificial neural network model showed slightly better predictive capacity compared to the response surface methodology model. From the scanning electron microscopy micrograph, it is evident that throughout the matrix B₄C reinforcement particles are well distributed also; with increasing tool rotational speed grain size decreases up to 1200 r/min; on further increasing the tool rotational speed particles starts clustering.

The present investigation deals with the finite-element analysis of the high strain rate deformation behavior of the quenched and tempered armor-grade rolled and homogeneous armor steel. The rolled and homogeneous armor steel is extensively used in civil and military structures such as battle tanks, armament combat vehicles, combat helicopter, etc. The dynamic deformation behavior of rolled and homogeneous armor steel, that is, resistance against ballistic circumstances relates to its mechanical behavior under high strain rate conditions. In the present research work, a finite-element analysis investigation (using Abaqus finite-element analysis code) has been carried out to evaluate the influence of specimen l/d ratios and high loading strain rates on the deformation behavior and stress–strain responses of the rolled and homogeneous armor steel. Further, an attempt has also been made to check the high strain rate and specimen l/d ratio influence on the strain amplitudes of incident, reflected, and transmitted pulses. The numerical investigation has been carried out with the rolled and homogeneous armor steel specimen with l/d ratios of 1, 0.8, and 0.6. In addition, three high impact strain rates of 2130, 2907, and 3105 s⁻¹ are considered to evaluate the stress–strain responses. The results revealed that the l/d ratio and strain rate have a significant influence on the specimen stress–strain response and the strain amplitudes of incident, reflected, and transmitted pulses. The peak stress value is increased with the increase in the l/d ratio and strain rate. The developed finite-element analysis model has predicted the stress–strain responses with <3% percentage error. The obtained finite-element analysis results have been validated with the experimental investigation with an l/d ratio of 0.6 and a strain rate of 3105 s⁻¹ for rolled and homogeneous armor steel.

The present investigation deals with the development of AA 5052-based metal matrix composites (MMCs) by utilizing industrial wastes, spent alumina catalyst, chrome-containing leather waste, and grinding sludge as a reinforcement material. The chrome-containing leather waste has been utilized to extract the collagen powder, which is a form of chromium oxide. The presence of Al₂O₃, Fe₂O₃, and SiO₂ phases in the spent alumina catalyst and grinding sludge ball-milled powder encourages its utilization as reinforcement material (in the form of Cr) for the development of MMCs. The stir-casting technique has been used to develop the aluminum-based MMC with waste spent alumina catalyst, chrome-containing leather waste, and grinding sludge. Further, results revealed that the matrix material mechanical properties compressive strength, tensile strength, and hardness were significantly increased by 12.93%, 5.34%, and 31.81% after adding spent alumina catalyst, Cr, and grinding sludge with the weight percentage (wt.%) of 4.5%, 1.5%, and 4.5%, respectively, but the toughness was reduced. The microstructural investigation indicated the uniform distribution of reinforcing elements spent alumina catalyst (4.5 wt.%), GS (4.5%), and Cr (1.5%) in the aluminum matrix material. Further, the influence of given reinforcement elements on the thermal expansion and corrosion weight loss properties of aluminum alloy matrix material has also been investigated.

Steels alloyed with high phosphorus should be developed for those applications where both strength and corrosion resistance are required. Structural steels such as rebars are manufactured using high-temperature deformation. Hence, thermo-mechanical behaviour of Fe-0.13P-0.05C -0.015N steel is considered using high temperature compression experiments after austenitization at 1050 °C for 10 s. The temperatures selected were between 750 and 1050 °C and at intervals of 50 °C. The rates of strain varied from 0.001 to 10 s⁻¹. Microstructural evolution was studied with the help of optical and scanning electron microscopy. Modified dynamic materials model is used to draw a processing map. The processing map helps in determining the domains which are safe for hot working. This alloy steel can be safely processed in the area confined by the rates of strain between 1 and 10 s⁻¹ as well as the temperature interval of 900 to 1000 °C.

In this work, metal matrix composites were fabricated using the electromagnetic stir casting process by adding 5 and 10 wt% silicon carbide in Al6063 alloy. Hardness, ultimate tensile strength, and yield strength of the developed Al6063/SiC/5p metal matrix composites have been improved by 17%, 18%, and 37%, respectively, in comparison with Al6063 alloy. Further, an improvement of 25%, 37%, and 71% in hardness, ultimate tensile strength, and yield strength, respectively, have been noted for Al6063/SiC/10p metal matrix composite in comparison with the Al6063 alloy. Results revealed that the hardness and strength of metal matrix composites were increased with silicon carbide addition in Al6063 alloy. The presence of different elements in metal matrix composites was identified by energy-dispersive X-ray spectroscopy and X-ray diffraction techniques. Energy-dispersive X-ray spectroscopy was used for elemental mapping observation of the metal matrix composites. Uniform distribution of reinforcement particles in the matrix with improved mechanical properties of metal matrix composites proved the adequacy of the electromagnetic stir casting process. The presence of facets and dimples in fractographs indicated the mixed mode of fracture. The average percentage porosity presented in Al6063/silicon carbide/5p and Al6063/SiC/10p metal matrix composites was found to be 4.68% and 5.22%, respectively.

Fabrication of complex shape micro-channels is a major challenge for manufacturing industries. Currently in commercial applications, lithography and non-conventional machining processes like lasers, electro-discharge machining (EDM) and chemical etching are commonly used for fabrication of these channels. In the present work, a novel Abrasive water jet (AWJ) milling based tool fabrication strategy has been proposed and implemented to make micro-tools (die/ electrode). The path strategy for jet movement is considered in a manner to selectively remove metal from a piece of material such that the resulting three-dimensional features become the required die shape that can be used as tool for the texturing process. Micro-tool of complex shape as fabricated on hard material (EN 31) sheet of 12 mm thickness and its geometry were analysed by controlling the step over (SO) distance. Hydraulic control based hard press contact texturing setup was developed to analyse the performance of such fabricated tools. Experiments were conducted on soft materials like, PMMA, Copper, Brass, aluminium and Nylon. Taper along the depth of the channels was observed because of the taper of the tool during fabrication. During fabrication, width of the tool less than 200 μm was found wavi in nature as there was not enough backing material present to bear the side wise jet pressure of impinging jet. Buckling on the tools was observed with tool height greater than 1 mm.