Low-pressure Cycle Research Articles

EVALUATION OF THE GRAPHENE DISPERSIONS FOR KEVLAR FABRICS FUNCTIONALIZATION OBTAINED BY THE LIQUID-EXFOLIATION OF GRAPHITE

The process where pristine graphite is subjected to a solvent treatment seems simple and scalable and has attracted the attention of researchers over many years. However, for successful exfoliation, overcoming the van der Waals attractions between the adjacent layers of graphite is necessary. Over time, several methods have been created to overcome the attraction between layers. Graphene’s liquid phase exfoliation (LPE) occurs due to the strong interactions between the solvent molecules and the basal planes of graphite, overcoming the energetic resistance to exfoliation and subsequent dispersion. Ultra sonication used for exfoliation can produce single or few layered graphene flakes. During sonication the sound waves propagate through liquid medium in alternating high- and low-pressure cycles, strong mechanical and thermal energy released by acoustic cavitation results in splitting up large particles into fine ones and dispersing them. Simultaneous insertion of solvent and/or intercalation molecules in between the graphite layers takes place supporting graphite separation into graphene layers. ; The dispersion capacity of graphene flakes depends on how appropriate the solvent properties are to the corresponding properties of graphene, such as surface tension, Hildebrand, and Hansen solubility parameters. Only certain solvents can disperse graphene well and form a dispersion appropriate for specific future applications. In addition, after exfoliation by ultra sonication graphene flakes aggregation due to the van der Waals attractions must be overcome to prepare in long-term stable dispersions of nanometres-size graphene sheets. ; The research has focused on the preparation of stable dispersion ready for transfer without intermediate processes to the Kevlar fabrics. An experimental comparison of the potentials of the 4 solvents for the application resulted in three corresponding to the intended use, two of them examined in detail, supplementing the composition of the LPE liquid medium with triethanolamine to obtain sufficient performance graphene coverage on Kevlar textile fibres.

High-efficiency CO2 conversion via mechano-driven dynamic strain engineering of ZnO nanostructures

Strain engineering involves intentionally inducing lattice distortion in materials to manipulate their electronic and geometric properties, along with the accompanying bond strength between reactants and catalysts. This approach presents an appealing pathway to optimize catalytic performance. However, it confronts challenges in achieving precise control, scalability and controllable modulation of intermediate species’ adsorption and desorption. Herein, we report a dynamic strain engineering method achieved through ultrasonic cavitation-induced high and low-pressure cycles, enabling periodically adjustable adsorption/desorption properties while bypassing complex synthesis procedures. Illustrated using ZnO and CO2 piezo-reduction reaction as a demonstration, theoretical studies initially predict that adsorption of intermediates *COOH can be regulated within a specific range of strains. Under ultrasonic stimulation, ZnO catalyst with dynamic strain engineering exhibits a CO yield of ∼ 98.8 μmol·g−1·h−1, approximately 16.5 times higher than that achieved under ultraviolet light irradiation without dynamic strain engineering, despite the latter having a considerably stronger input power. Furthermore, we delve into the factors linked to dynamic strain amplitude and frequency. This dynamic strain engineering approach streamlines catalyst preparation and presents innovative possibilities for controlled manipulation of intermediate specie’s adsorption/desorption, with potential applications across a wide range of catalytic systems.

Sliding Pressure Inventory Control of a Supercritical CO2 Cycle for Concentrated Solar Power—Analysis and Implications

Abstract This paper presents the use of sliding pressure inventory control (SPIC) of a 10 MW supercritical carbon dioxide Brayton cycle for concentrated solar power, incorporating printed circuit heat exchangers. Load regulation using SPIC for three representative ambient conditions 45 °C, 30 °C, and 15 °C are considered. While a wide operating range from 10 MW to less than 1 MW part load is obtained, a notable cycle efficiency decline at part load is also seen. Irreversibility analysis reveals that deterioration in recuperator and turbomachinery performance are primarily responsible for cycle performance degradation at part load. Nevertheless, useful inferences are obtained from the 10 MW SPIC irreversibility study. With a slightly increased value of heat exchanger length, a non-condensing 1 MW subcritical CO2 cycle operating between 35 bar/53 bar is found to be as efficient as a 1 MW supercritical CO2 cycle operating between 88 bar/210 bar. The major benefit of choosing the subcritical CO2 cycle for 1 MW scale applications is the significantly reduced turbomachinery speed (∼26,000 rpm) in comparison with supercritical CO2 turbomachinery (∼67,000 rpm) for the same power scale. These advantages are found to be true for air-based ideal gas cycles operating between 35 bar/53 bar too, with the latter requiring nominally smaller heat exchangers than the subcritical CO2 cycle. The final choice of working fluid, however, for these low-pressure cycles would depend on practical considerations, such as material compatibilities at high temperatures, corrosion considerations, and cost.

EXPERIMENTAL COMPARISON OF ECO-FRIENDLY REFRIGERANT COUPLES R407C/R32/R600a (90/10 BY WT.%) AND R407C/R32 TO REVEAL THE GWP AND THERMODYNAMIC PROPERTIES

In this experimental study, based on cooling couples a new mixture (90%R32 + 10%R600a) has been proven to be better than R32 for the global warming potential (GWP) in a low-pressure cycle (LPC), within a high-pressure cycle (HPC) R407C by utilizing a small laboratory cooling system. This system consists of two cooling cycles connected by a heat exchanger and has also been termed as a mini cascade refrigeration system (MCRS). The main aim of this study is to study a suitable gas or mixture for MCRS at the lowest GWP value and a low-temperature cooling room to reach less than -30°C. In the same way, the following conditions have been determined in this research: the inlet and outlet temperatures of the evaporator, condenser, heat exchanger, compressors, with evaporator pressure as well as superheat. Consequently, MCRS can be used with thermostats at the lowest degree of -32°C or less, the mixture has results close to R32 and the coefficient of performance for MCRS COPMCRS is equal to 1.63 and 1.78 for the new mixture and R32, respectively, under the same operating conditions. Therefore, the ozone depletion potentials (ODP) for R32 and the new mixture are equal to zero, and the GWP for the new mixture is less by amount of 63 than that for R32. The MCRS can be used in a small place, for instance, transport blood samples, blood tests, and vaccines by small cars. Both R32 and R600a are flammable gases, so this study will be carried on in the future for safer replacements.

Study on the migration performance of Cs(I) in the treatment of simulated radioactive wastewater by electrodialysis.

As a competitive radioactive wastewater treatment technology, electrodialysis (ED) has the advantages of low operating pressure and high cycles of concentration. In order to analyze the migration performance of radionuclides during the treatment of radioactive wastewater by ED, a radionuclide migration model was constructed based on the mass conservation law and Faraday's law with the typical radionuclide cesium as the research object. Experiments were carried out for the treatment of simulated radioactive wastewater in a small-scale ED system, and the average migration rate of radionuclides under different operating conditions was predicted by the model. The results showed that the experimental values of concentration and average migration rate of Cs(I) were significantly correlated with the calculated values of the model, in which the relative error of the average migration rate was 4.54%. The variation characteristics of Cs(I) concentration in diluted solution under different current and volume ratio conditions can be predicted by the model. The average variation rate of Cs(I) concentration decreases significantly with the increase of current and volume ratio.

Thermodynamic Analysis of Air-Cycle Refrigeration Systems with Expansion Work Recovery for Compartment Air Conditioning

As a requirement for sustainable development, air-cycle refrigeration has received wide attention as a candidate for environmentally friendly air conditioning technology. In this study, the thermodynamic performance of air refrigeration cycles is investigated in compartment air conditioning. The effects of compressor efficiency, expander efficiency, ambient humidity, all-fresh-air supply and ambient pressure on the cycle performance are presented. The effects of compressor arrangement in the high-pressure cycle and the low-pressure cycle are compared. An open-loop high-pressure cycle has a larger COP than that of an open-loop low-pressure cycle but requires larger heat exchange. The performance of air refrigeration cycles with full fresh air is studied, and the influence of fresh air is discussed. Schemes for condensed water recirculation with wet compression are proposed, which can improve the COPs of open-loop low-pressure cycles by 44.7%, 48.8% and 48.4%. In the air conditioning of plateau trains, open-loop high-pressure cycles have slightly lower COPs, but they can supply air with elevated pressure and oxygen concentration.

Deep feature learning of in-cylinder flow fields to analyze cycle-to-cycle variations in an SI engine

Machine learning (ML) models based on a large data set of in-cylinder flow fields of an IC engine obtained by high-speed particle image velocimetry allow the identification of relevant flow structures underlying cycle-to-cycle variations of engine performance. To this end, deep feature learning is employed to train ML models that predict cycles of high and low in-cylinder maximum pressure. Deep convolutional autoencoders are self-supervised-trained to encode flow field features in low dimensional latent space. Without the limitations ascribable to manual feature engineering, ML models based on these learned features are able to classify high energy cycles already from the flow field during late intake and the compression stroke as early as 290 crank angle degrees before top dead center ([Formula: see text]) with a mean accuracy above chance level. The prediction accuracy from [Formula: see text] to [Formula: see text] is comparable to baseline ML approaches utilizing an extensive set of engineered features. Relevant flow structures in the compression stroke are revealed by feature analysis of ML models and are interpreted using conditional averaged flow quantities. This analysis unveils the importance of the horizontal velocity component of in-cylinder flows in predicting engine performance. Combining deep learning and conventional flow analysis techniques promises to be a powerful tool for ultimately revealing high-level flow features relevant to the prediction of cycle-to-cycle variations and further engine optimization.

Aliphatic amine decorating metal–organic framework for durable SO2 capture from flue gas

The efficient capture of SO2 using porous adsorbents is of great significance in flue gas desulfurization (FGD) process as the reversible physical adsorption can avoid solvents/water consumption and minimize waste generation. Metal organic frameworks (MOFs) are interesting and promising candidates to sequestrate SO2 in flue gas as their high porosities, regular pore sizes and easy functionalization. However, the coordination bond nature of MOFs makes stably and persistently capture acidic SO2 gas remains a challenge in MOF. Herein, an aliphatic amine decorating strategy was introduced in a MOF (MIL-101(Cr)) with open metal sites for reversible and durable SO2 capture. The successful synthesis of N,N'-dimethylethylenediamine (mmen) decorated MIL-101(Cr) named mmen-MIL-101(Cr), was confirmed by powder X-ray diffraction (PXRD), Fourier transform-infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive spectra (EDS) and nitrogen adsorption–desorption isotherm. Compared with chemisorption of SO2 on open Cr sites in MIL-101(Cr), the SO2 adsorption sites are transferred from Cr to N atom in mmen-MIL-101(Cr), leading to reversible SO2 adsorption and desorption. The dipole–dipole interaction between N atom and S(SO2) atom, combining with hydrogen bonding interaction between O(SO2) and H in mmen can increase the SO2 absorption capacity of mmen-MIL-101(Cr) under low SO2 partial pressure. The SO2 adsorption capacity and cycle stability properties were studied in detail by adsorption isotherm and dynamic breakthrough measurement. An unprecedented high SO2 adsorb capacity (upto 4.27 mmol·g−1) at SO2 partial pressure (0.5 mbar) is achieved in mmen-MIL-101(Cr), which is a great improvement compared to 2.92 mmol·g−1 SO2 adsorption capacity for primordial MIL-101(Cr) under the same condition. Moreover, mmen-MIL-101(Cr) manifests reversible SO2 adsorption–desorption performance while MIL-101(Cr) gradually loses its adsorption capacity during dynamic breakthrough cycles. The high SO2 sorption capacity in low partial pressure and impressive cycle stability of mmen-MIL-101(Cr) demonstrate aliphatic amine decorating strategy is an efficient way to improve MOF’s SO2 capture capability, which also can extend to other porous adsorbents for low concentration FGD process.

Thermodynamic analysis and parametric optimization of a novel S–CO2 power cycle for the waste heat recovery of internal combustion engines

With the aim to recover the exhaust heat of internal combustion engine efficiently, a novel supercritical CO2 (S–CO2) power cycle is proposed on the basis of recompression cycle configuration. The corresponding thermodynamic models are established to analyze the energetic and exergetic performances of the system. The effects of turbine inlet temperatures and system pressures are investigated. Thereafter, in order to output the maximum net work, genetic algorithm (GA) is employed to optimize these key parameters. Under design conditions, the calculated results indicate that the thermal efficiency and the exergy efficiency of the system can reach up to 35.86% and 67.90% respectively. Furthermore, as the inlet temperature of high pressure turbine increases, cycle efficiency increases while output work decreases. However, for the effect of inlet temperature of low pressure turbine, cycle efficiency and output work increase with the increase of temperature. As for the intermediate pressure of this system, there exist pressures to get the maximum net work and cycle efficiency. Based on GA optimization, an optimal combination of system parameters is obtained. The corresponding maximum cycle power is 39.49 kW, and the recovery efficiency of waste heat can reach up to 74.83%.

Numerical study on the approach for super-high thermal efficiency in a gasoline homogeneous charge compression ignition lean-burn engine

The approach for achieving super-high thermal efficiency in a gasoline homogeneous charge compression ignition lean-burn engine was studied using numerical simulation. A model engine was designed based on the split cycles, including the low-pressure cycle consisted of the turbocharger system and the high-pressure cycle controlled by the variable effective compression ratio ( ε) and the exhaust gas recirculation (EGR). Based on the model engine, load (L) – Φoxy (F) – EGR (E) – ε (E) cooperative control strategy was proposed to optimize the thermal efficiency and its interactive mechanism was clarified. The results revealed that the core of the load (L) – Φoxy (F) – EGR (E) – ε (E) strategy was the simultaneous optimization of the combustion process and the specific heat ratio ( γ) contributing to the piston work maximization. The optimum combustion phase was found in the range of 4°–9° crank angle after top dead center, and highest combustion rate under the rough combustion restriction was also required. Under this precondition, reducing ε to retard the combustion phase appropriately could mitigate the EGR usage to improve the γ. Based on the load (L) – Φoxy (F) – EGR (E) – ε (E) strategy, increasing the load was found to improve the thermal efficiency effectively by reducing the heat transfer loss. The highest brake thermal efficiency of 50% was reached when the gross indicated mean effective pressure was increased to 15 bar under the conventional engine condition. Further increasing the gross indicated mean effective pressure to 35 bar with elevated peak cylinder pressure of 400 bar could improve the brake thermal efficiency to 54% under the enhanced mechanical strength condition. To pursue super-high thermal efficiency, the approach of thermal insulation for the engine was proved to be more effective. It showed the potential to achieve the super-high brake thermal efficiency over 60% and maintain clean combustion by adopting the load (L) – Φoxy (F) – EGR (E) – ε (E) strategy in the model engine with thermal insulation and high mechanical strength.

Simulation of Space Charge Impact on Partial Discharge Inception Voltage in Power Busbars Dedicated to Future Hybrid Aircrafts

Reducing greenhouse gases, saving energy resources and mass optimization require technological changes towards increasingly electric vehicles. At the same time, performance improvement of semiconductor and dielectric materials further promotes electronic components confinement, resulting in a significant increase of embedded power densities. In the particular case of future hybrid propulsion aircrafts, electrical power that intended to supply reactors would be converted through power electronics components mounted on power busbars and insulated by solid dielectrics materials. These dielectrics materials have to respond to various electrical constraints of use (HVDC), in spite of environment change of aircraft parameters such as low pressure, temperature and thermal cycles, humidity... Unfortunately, partial discharges phenomenon is the most problem within electrical insulation system (EIS). Based on a topological model of power busbars designed for power converters dedicated to hybrid aircraft, partial discharge studies were conducted by simulation in various charging conditions of a PTFE insulator. Simulation results, which focus on electric field thresholds criteria of partial discharge inception voltage in air, reveal a net sensitivity of a space charge accumulation and distribution on dielectrics behaviour even for low space charge density, depending on their location in dielectrics. Compared to the behaviour observed with implanted homocharges, when by increasing homocharges density from 0.5 C/m3 to 2 C/m3 we observe a decrease of electric field by 450%, simulation results show a highest risk of partial discharge inception when heterocharges are accumulated inside dielectrics. Their accumulation increases the electric field in triple points beyond electric field thresholds of partial discharge inception in air. The simulated electric field reaching 22 kV/mm with only 2 C/m3 of heterocharges density accumulated in dielectric/busbars interfaces.

Isolation of Mitochondrial DNA from Single, Short Hairs without Roots Using Pressure Cycling Technology.

Hairs are commonly submitted as evidence to forensic laboratories, but standard nuclear DNA analysis is not always possible. Mitochondria (mt) provide another source of genetic material; however, manual isolation is laborious. In a proof-of-concept study, we assessed pressure cycling technology (PCT; an automated approach that subjects samples to varying cycles of high and low pressure) for extracting mtDNA from single, short hairs without roots. Using three microscopically similar donors, we determined the ideal PCT conditions and compared those yields to those obtained using the traditional manual micro-tissue grinder method. Higher yields were recovered from grinder extracts, but yields from PCT extracts exceeded the requirements for forensic analysis, with the DNA quality confirmed through sequencing. Automated extraction of mtDNA from hairs without roots using PCT could be useful for forensic laboratories processing numerous samples.

Experimental investigation on the influences of exhaust gas recirculation coupling with intake tumble on gasoline engine economy and emission performance

To improve the economy and emission performance of gasoline engine under part load, the approach of exhaust gas recirculation coupling with intake tumble was investigated by bench testing. Based on a naturally aspirated gasoline engine, the sweeping test of exhaust gas recirculation rate was conducted in two intake modes (with/without intake tumble), and the parameters related to engine heat-work conversion process and emission performance were measured. Through comparing and analyzing the measured data, the effects of exhaust gas recirculation coupling with intake tumble on gasoline engine economy and emission performance were revealed. The results show that pumping loss decreases gradually while in-cylinder residual gas fraction increases linearly with the exhaust gas recirculation rate increasing; the high-pressure cycle efficiency ascends with exhaust gas recirculation rate increasing due to the decrease of heat transfer loss and exhaust gas energy loss. Thus, the improvement of indicated thermal efficiency is the superposition of double benefits of low-pressure cycle and high-pressure cycle. At 1600r/min and 2.94bar, the indicated thermal efficiency can be increased by 4.29%. With the increase of exhaust gas recirculation rate, nitrogen oxide emissions almost fall linearly, but hydrocarbon and carbonic oxide emissions have no obvious change in the effective range of exhaust gas recirculation rate. The biggest advantage of intake tumble is that it can extend the effective range of exhaust gas recirculation rate. As a result, the potential of energy conservation and emission reduction of exhaust gas recirculation is largely improved.

Application of Ultrasonic Wavesfor Degassing of Drilling Fluids and Crude OilsApplication of Ultrasonic Waves for Degassing of Drilling Fluids and Crude Oils

In the oil and gas industry, both the produced oil and the drilling fluids used while drilling will contain gases that are entrapped within their liquid systems. These gases are removed, or degassed, using several methods such as separator tanks (for crude oil) and vacuum degassers (for drilling mud). This project, however, proposes a novel, environment friendly and cheap method of degassing that uses ultrasonic waves to remove gas bubbles from liquid systems. This method could be incorporated with already existing degassing technologies to increase their efficiency. The objective of this work, therefore, is to investigate the feasibility of using ultrasonic waves as a method of degassing drilling fluids and crude oil samples. The basic idea of this new method is based on the effect that ultrasonic waves generate when in contact with a liquid medium as they create repeated compressions (high-pressure cycles) and rarefactions (low-pressure cycles) in which small vacuum bubbles (voids) are formed in the liquid. Dissolved gases will migrate into these small voids which will coalesce and then rapidly grow into large size bubbles that are easily removed out of the liquid. Hence, this method insures more effective removal of dissolved gases that are entrapped within the liquid. Also, as this whole process happens rapidly, gas bubbles will have shorter time in contact with the liquid particles which reduces the possibility of gas redissolving; especially in the case of highly viscous liquids such as oil. This further adds to the advantages of using this new method. For the purpose of this project, several testing methods such as sonication, density, pH, Particle Size Distribution (PSD), Fourier Transform Infra-Red (FTIR), and corrosion testing were conducted. These tests aimed to evaluate the physical and chemical impacts that ultrasonic waves have on the tested systems. The results of these tests prove, to a great extent, the effectiveness of ultrasonic waves in removing gases from water based mud and crude oil samples. The impact of ultrasonic waves on the physical and chemical properties of the tested fluid systems, however, requires further investigation.

A novel low-grade heat-driven absorption refrigeration system with LiCl–H2O and LiBr–H2O working pairs

A novel low grade heat-driven absorption refrigeration system is proposed, where LiCl–H2O with higher vapor pressure is used in the high-pressure cycle and LiBr–H2O with lower vapor pressure is employed in the low-pressure cycle. Effects of key parameters on the system performance are analyzed, and different heat source utilization modes are considered: parallel modes (PM-1 and PM-2) and serial mode (SM). What's more, comparisons among the PM-1, PM-2, SM and the traditional double-stage LiBr–H2O absorption system (TDS) are made. Results show that the PM-1 has much higher COP than the TDS under small ranges of working conditions, with the maximum COP improvement 26.7%, while the PM-2 shows prominent advantages under wider ranges, with the maximum COP improvement 35%. In addition, The PM-1 shows much higher COP than the SM under higher condensing temperature and evaporation temperature, while the SM has much lower heat source outlet temperature which is at most 12 °C lower than that of the PM-1. The intermediate pressure is important for the system performance and the optimum value is 2.27 kPa in the PM-1 and 2.5 kPa in the SM. For the PM-2, the intermediate pressure should be chosen to achieve low circulation ratio in the low pressure cycle.

Double Compression Expansion Engine Concepts: A Path to High Efficiency

Internal combustion engine (ICE) fuel efficiency is a balance between good indicated efficiency and mechanical efficiency. High indicated efficiency is reached with a very diluted air/fuel-mixture and high load resulting in high peak cylinder pressure (PCP). On the other hand, high mechanical efficiency is obtained with very low peak cylinder pressure as the piston rings and bearings can be made with less friction. This paper presents studies of a combustion engine which consists of a two stage compression and expansion cycle. By splitting the engine into two different cycles, high-pressure (HP) and low-pressure (LP) cycles respectively, it is possible to reach high levels of both indicated and mechanical efficiency simultaneously. The HP cycle is designed similar to today's turbo-charged diesel engine but with an even higher boost pressure, resulting in high PCP. To cope with high PCP, the engine needs to be rigid. The usage of higher piston ring tension and larger bearings are examples of measures to cope with higher PCP. These measures will cost in terms of friction. Hence, mechanical efficiency is not as good as other engine concepts with lower PCP. The low-pressure cycle on the other hand, uses a design more similar to current naturally aspirated (NA) spark ignited (SI) engines, but designed for even lower PCP. Because of this, the engine does not need to be as rigidly designed and the overall friction levels will be much lower. By combining these two engine philosophies, a total engine concept with both high indicated and mechanical efficiencies can be achieved. Simulations show net indicated efficiency above 60% and a brake efficiency of 56%. (Less)

Comparative performance evaluation of cascaded air-source hydronic heat pumps

The results are reported of an investigation of the effects of cascading air-source heat pumps on performance for hydronic residential systems. Three heat pump systems are modeled as single-stage, single-refrigerant cascaded, and two-refrigerant cascaded. Energy and exergy analyses are performed, and a comparative performance analysis is carried out, considering energy efficiency, refrigerant mass flow rates, evaporator pressure, exergy efficiency, and several other criteria. Three sets of source and supply temperatures, representing different climates and different water sink systems (low, medium and high temperature), are used to provide more comprehensive behavior assessments of the systems. Additionally, the optimum intermediate pressure of the cascaded systems for all working temperature pairs is found for the highest energetic COP and exergetic COP. Compared to a single stage heat pump, cascading improves the overall energy efficiency of the system for low-ambient temperatures, but not for high-ambient temperatures. Although this improvement is minor, the exergetic COP is increased by 67% for the single refrigerant cascaded system and 70% for a two-refrigerant cascaded system, at low ambient temperatures. Using refrigerant R404A in the low-pressure cycle marginally improves the energetic COP of the cascaded heat pump, but increases the evaporator working pressure, making it possible to use smaller compressors. However, the overall refrigerant mass flow rates increase with cascading. The two cascaded systems have higher exergy destructions (by almost four times) compared to the single stage system, mainly due to having more components, including an intermediate heat exchanger. Also, cascading shifts the major exergy destruction centers from the compressors and expansion valves to the evaporators. A comparison of cascaded and single-stage heat pumps shows that the exergy analysis results exhibit a different trend than energy analysis results with source and supply temperatures, highlighting the advantages of exergy methods in determining if cascading is appropriate for a given application.

Novel contact ultrasound system for the accelerated freeze-drying of vegetables

Abstract A contact ultrasound system for the integration into freeze-drying processes for vegetables was developed and process parameters were estimated. Red bell pepper cubes were freeze-dried on a stainless steel screen used as the sound transmitting surface. Ultrasound induced heating effects due to attenuation and absorption in the product and relevant process parameters were investigated. Continuous ultrasound application at an excitation amplitude of 6.7 μm resulted in immediate sample heating at reduced ambient pressure and a loss of freeze-drying conditions. A reduction of the excitation amplitude to 4.9 μm and of the net sonication time to 10% by applying an intermittent treatment with an interval of 10 s ultrasound and 90 s recovery phase allowed freeze-drying at increased sublimation rates without causing sample heating for 7 h of freeze-drying. As drying proceeds less sublimation energy is required in the process and sound energy was partially transformed into heat, which could be used for accelerated moisture removal during secondary drying in combination with further ultrasound effects improving heat and mass transfer (moisture migration, effect on boundary layers, improved evaporation at low pressure cycles). The ultrasound treatment reduced the drying time required to reach a final moisture content of 10% d.b. by 11.5%. The ultrasound treatment did not affect product quality in terms of bulk density, color, ascorbic acid content, and rehydration characteristics. Industrial relevance : Freeze-drying is a unique drying process for the production of high quality food products. However, high energy requirements for sublimation and vacuum generation turn it into a very cost intensive treatment. Current technologies used for the improvement of drying rates (heated plates, infrared radiation, microwave application) are limited to heating effects and can easily impair product quality. In this study a contact ultrasound treatment has been developed close to industrial freeze-drying, where the product is dried on trays. Ultrasound combines the positive effects of heating due to attenuation and adsorption with mechanical effects of pressure waves to improve drying rates and can thus be used to shorten drying times and has the potential to reduce related processing costs.

A numerical model for the design of a mixed flow cryogenic turbine

Present day cryogenic gas turbines are in more popular as they meet the growing need for low pressure cycles. This calls for improved methods of turbine wheel design. The present study is aimed at the design of the turbine wheel of mixed flow impellers with radial entry and axial discharge. In this paper, a computer code in detail has been developed for designing such turbine wheel. To determine the principal dimensions of the turbine wheel, optimum operating speed has been taken from design charts based on Similarity principles. The algorithm developed, allows any arbitrary combination of fluid properties, inlet conditions and expansion ratio, since the fluid properties are properly taken care of in the relevant equations. The computational process is illustrated with an example. The main dimensions, thermodynamic properties at different states, velocity and angles at entry and exit of the turbine wheel were worked out. The work may help the researchers for further design and development of cryogenic turboexpander depending on their operating parameters. Keywords: Cryogenic, turboexpander, mixed flow turbine, flow angle, flow velocity

A computational approach to the design of a cryogenic turbine blade profile

The present day cryogenic gas turbines are in high use as they meet the growing needs for low pressure cycles. This calls for improved methods of blade profile design. The present study is aimed at the design of the blade profile of mixed flow impellers with radial entry and axial discharge. In this paper, details of a computer code have been developed for designing such blade profile. The computational procedure developed describes the three-dimensional contours of the blades for the turbine wheel. The flow of fluid in a turbine blade passage depends on the length of the flow path and the curvature of such path. This has been given due weightage. Effect of some of the turbine operating and design parameters on the flow path and its curvature have been analyzed and presented. Optimum solution for some of the free parameters and angles between velocity components was found. On that basis, co-ordinates of this blade profile were computed. Although manufacturers design the blade profiles for their production, till date no academic literature is available for this purpose. The work may help researchers for further developmental work on this topic as well as the manufacturers of cryogenic expanders. Keywords: Radial turbine, Blade profile parameter, Meridional streamlength, Axial coordinate, Radial coordinate, Angular coordinate