Simulation and Optimization of the Fluid Solidification Process in Brazed Plate Heat Exchangers

When a brazed plate heat exchanger is used as an evaporator,the working mass in the channel may undergo soli-dification,thereby hindering the refrigeration cycle.In this study the liquid solidification process and its optimi-zation in a brazed plate heat exchanger are investigated numerically for different inlet velocities;moreover,different levels of corrugation are considered.The results indicate that solidificationfirst occurs around the con-tacts,followed by the area behind the contacts.It is also shown that deadflow zones exist in the sharp areas and such areas are prone to liquid solidification.After optimization,the solidification area attains its smallest value when a corrugation spacingλ=4.2 mm is considered.

NUMERICAL STUDY ON FLOW DISTRIBUTION IN PLATE-FIN HEAT EXCHANGERS

Objective To investigate the flow distribution in plate fin heat exchangers and optimize the design of header configuration for plate fin heat exchangers. Methods A mathematical model of header was proposed. The effects of the header configuration on the flow distribution in plate fin heat exchangers were investigated by CFD. The second header configuration with a two stage distributing structure was brought forward to improve the performance of flow distribution. Results It is found that the flow maldistribution is very serious in the direction of header length for the conventional header used in industry. The numerical predictions indicate that the improved header configurations can effectively improve the performance of flow distribution in plate fin heat exchangers. Conclusion The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution. The method has a wide variety of applications in the design of plate fin heat exchangers.

Conductive and corrosion behaviors of silver-doped carbon-coated stainless steel as PEMFC bipolar plates

Ni–Cr enrichment on stainless steel SS316 L resulting from chemical activation enabled the deposition of carbon by spraying a stable suspension of carbon nanoparticles; trace Ag was deposited in situ to prepare a thin continuous Ag-doped carbon film on a porous carbon-coated SS316 L substrate. The corrosion resistance of this film in 0.5 mol·L^（-1） H_2SO_4 solution containing 5 ppm F- at 80°C was investigated using polarization tests. The results showed that the surface treatment of the SS316 L strongly affected the adhesion of the carbon coating to the stainless steel. Compared to the bare SS316 L, the Ag-doped carbon-coated SS316 L bipolar plate was remarkably more stable in both the anode and cathode environments of proton exchange membrane fuel cell（PEMFC） and the interface contact resistance between the specimen and Toray 060 carbon paper was reduced from 333.0 m？·cm^2 to 21.6 m？·cm^2 at a compaction pressure of 1.2 MPa.

Optimizing Grids Demand Reduction through Enhanced Heat Transfer in Low-Temperature Waste Heat Driven ORC Systems

With increasing awareness of energy conservation and environmental protection, the Organic Rankine Cycle (ORC) system has gained significant attention. This technology enables the recovery of industrial waste heat, waste incineration heat, and renewable energy sources such as geothermal heat, biomass energy, and solar energy at lower temperatures. However, the low-grade heat source utilized in ORC systems faces a challenge to achieving high power generation efficiency and output power. Therefore, enhancing the power generation capacity of ORC systems is a key research focus in this field. An entranced heat exchanger ORC system with the screw expander driven by the low-temperature heat source is established to investigate the relevant performance. Hot water temperature from 77°C to 132°C is adopted for performance analysis, while the environmental temperature is approximately 25°C. Refrigerant R245fa is selected as the working fluid, and the screw expander is employed for power generation. It is worth noting that the entranced heat exchanger ORC system has significant potential for low-temperature heat recovery. Experimental results indicate that the maximum power output is 12.83 kW, which is obtained at around 105°C hot water inlet temperature. Correspondingly, the average power output remains 11.75 kW, revealing the system’s high stability for power generation. The implementation of a plate heat exchanger for enhanced heat transfer has enabled a 50% reduction in system size compared to traditional shell-tube type ORC systems. Besides, economic calculations demonstrate substantial benefits associated with the ORC system. The calculations indicate an internal benefit of 560,000 RMB/year, accompanied by notable external benefits such as an energy saving and emission reduction potential of up to 784 t CO2 per year. Moreover, the payback period is 2.23 years. It shows a remarkable improvement in terms of performance and excellent economic benefits. As a result, the novel ORC presents a promising alternative for low-grade heat utilization as compared to conventional small-scale ORC systems.

SIMULATION AND ANALYSIS OF FLOW PATTERN IN CROSS-CORRUGATED PLATE HEAT EXCHANGERS

Using numerical methodology, the flow fields between two corrugated plates with different values of the corrugation inclination angle β were simulated. The simulation results directly indicate that β affects the flow pattern between corrugated plates, and the results are in good agreement with the experimental results reported by interrelated literature. The results show that the flow pattern between the two plates changes from ＂double cross-flow＂ to ＂zigzag flow＂ with the increase in β. The reason for the effect on the flow pattern between the two corrugated plates was discussed from the view of the variation of momentum in the direction of corrugation with the variation in β.

FLOW RESISTANCE AND HEAT TRANSFER CHARACTERISTICS OF A NEW-TYPE PLATE HEAT EXCHANGER

A new-type corrugation Plate Heat Exchanger （PHE） was designed. Results from both numerical simulations and experiments showed that the flow resistance of the working fluid in this new corrugation PHE, compared with the traditional chevron-type one, was decreased by more than 50%, and corresponding heat transfer performance was decreased by about 25%. The flow field of the working fluid in the corrugation PHE was transformed and hence performance difference in both flow resistance and heat transfer was generated. Such a novel plate, consisting of longitudinal and transverse corrugations, can effectively avoid the problem of flow path blockage, which will help to extend the application of PHEs to the situation with unclean working fluids.

Numerical simulation and experimental research on heat transfer and flow resistance characteristics of asymmetric plate heat exchangers

The asymmetric plate heat exchanger(APHE)has the possibility of achieving balanced pressure drops on both hot and cold sides for situations with unbalanced flow,which may in turn enhance the heat transfer.In this paper,the single-phase water flow and heat transfer of an APHE consisted of two types of plates are numerically(400≤Re≤12000)and experimentally(400≤Re≤3400)investigated.The numerical model is verified by the experimental results.Simulations are conducted to study the effects of,an asymmetric index proposed to describe the geometry of APHEs.The correlations of the Nusselt number and friction factor in the APHEs are determined by taking and working fluids into account.It is found that an optimal exists where the pressure drops are balanced and the heat transfer area reaches the minimum.The comparison between heat transfer and flow characteristics of the APHEs and the conventional plate heat exchanger(CPHE)is made under various flow rate ratios of the hot side and the cold side and different allowable pressure drops.The situations under which APHE may perform better are identified based on a comprehensive index.

Comprehensive comparison of small-scale natural gas liquefaction processes using brazed plate heat exchangers

The brazed plate heat exchanger(BPHE)has some advantages over the plate-fin heat exchanger(PFHE)when used in natural gas liquefaction processes,such as the convenient installation and transportation,as well as the high tolerance of carbon dioxide(CO2)impurities.However,the BPHEs with only two channels cannot be applied directly in the conventional liquefaction processes which are designed for multi-stream heat exchangers.Therefore,the liquefaction processes using BPHEs are different from the conventional PFHE processes.In this paper,four different liquefaction processes using BPHEs are optimized and comprehensively compared under respective optimal conditions.The processes are compared with respect to energy consumption,economic performance,and robustness.The genetic algorithm(GA)is applied as the optimization method and the total revenue requirement(TRR)method is adopted in the economic analysis.The results show that the modified single mixed refrigerant(MSMR)process with part of the refrigerant flowing back to the compressor at low temperatures has the lowest specific energy consumption but the worst robustness of the four processes.The MSMR with fully utilization of cold capacity of the refrigerant shows a satisfying robustness and the best economic performance.The research in this paper is helpful for the application of BPHEs in natural gas liquefaction processes.

Technological Perspectives for Propulsion on Nuclear Attack Submarines

This work aimed at proposing a new combination of technologies to improve military performances and reduce costs of nuclear attack submarines, without overlooking safety constraints. The last generation of nuclear attack submarines increased size to meet safety and operational requirements, imposing huge burden on costs side, reducing fleet size. The limitations of current Technologies employed were qualitatively discussed, explaining their limitations. There are new technologies (plate and shell heat exchangers) and architectural choices, like passive safety, and segregation of safety and normal systems, which may lead to reduction of costs and size of submarines. A qualitative analysis was provided on this combination of technologies, stressing their commercial nature and maturity, which reduced risks. The qualitative analysis showed the strong and weak points of this proposal, which adopted the concept of strength in numbers. Concluding, new Technologies enabled the existence of 3800 t nuclear attack submarines with powerful propulsion systems and good acoustic discretion.

Utilisation of waste heat from exhaust gases of drying process

Nowadays a lot of low-grade heat is wasted from the industry through the off- and flue-gasses with different compositions. These gases provide the sensitive heat with utilisation potential and latent heat with the components for condensation. In this paper, process integration methodology has been applied to the partly condensed streams. A hot composite curve that represents the gas mixture cooling according to equation of state for real gases was drawn to account the gas-liquid equilibrium. According to the pinch analysis methodology, the pinch point was specified and optimal minimal temperature difference was determined. The location of the point where gas and liquid phases can be split for better recuperation of heat energy within heat exchangers is estimated using the developed methodology. The industrial case study of tobacco drying process off-gasses is analysed for heat recovery. The mathematical model was developed by using MathCad software to minimise the total annualised cost using compact plate heat exchangers for waste heat utilisation. The obtained payback period for the required investments is less than six months. The presented method was validated by comparison with industrial test data.

Augmentation plating in hypertrophic non-union after nail fixation of femoral shaft fracture compared with exchange plating Augmentation plating in hypertrophic non-union after nail fixation of femoral shaft fracture compared with exchange plating Augmentation plating in hypertrophic non-union after nail fixation of femoral shaft fracture compared with exchange plating

Background Augmentation plating has been used successfully to treat hypertrophic non-union after nail fixation.This study compared the efficacy of augmentation plating and exchange plating for treating hypertrophic non-union of femoral shaft fracture after intramedullary nail fixation.Methods A total of 12 patients received augmentation plating and 15 patients received exchange plating as treatment for femoral shaft hypertrophic non-union.The procedures were conducted at our medical centre between January 2005and January 2012.Clinical follow-up was conducted at 2 weeks,1 month and then monthly until union was achieved to compare union time,operation time,bleeding and complications between the two groups.Results All patients underwent follow-up examinations until fracture union was achieved.The average length of followup time after the second treatment was （18.37±3.28） months.The time needed for union was （4.17±0.94） months in the augmentation plating group and （5.33±1.72） months in the exchange plating group.The operation time was （90.00±17.58） minutes in the augmentation plating group and （160.00±25.35） minutes in the exchange plating group.The amount of blood loss during the operation was （270.00±43.32） ml in the augmentation plating group and （530.00±103.65） ml in the exchange plating group.Both groups showed significant difference （P 〈0.05） in their results.No complications were reported after the second operation.Conclusions Augmentation plating after nail fixation could remove local rotation instability,facilitate simple operation,create minimal damage and enable exercise for early functional recovery.Therefore,augmentation plating is excellent for treating hypertrophic non-union after nail fixation in femoral shaft fracture.