Optimal Pressure Drop Research Articles

Procedure for the Simultaneous Synthesis of Heat Exchanger Networks at Process and Total Site Level

Although Heat Exchanger Network (HEN) synthesis methods have been in existence for forty years, several issues still need to be resolved in order to obtain more realistic networks. Moreover, the complexity of the problem further increases when Heat Integration is performed at the Total Site level rather than at the process level. The usual approach is first to perform Heat Integration within the processes, and later to consider the non-integrated parts for Total Site Heat Integration. This sequential approach can omit some promising solutions and thus generally leads to worse results compared to the approach where the entire Total Site is synthesized simultaneously (Nemet et al., 2014). In order to obtain more realistic results, several practical constraints should be accounted for, such as transport between processes, optimal intermediate utility temperature level(s), pipeline design, heat losses and pressure drop (Nemet et al., 2015).In this work a synthesis of the Total Site is performed, which simultaneously considers integration within and between plants (at the plant and at the Total Site levels). For this purpose, the superstructure optimization approach is used. The superstructure contains all the possible matches for heat exchange within and between processes. Heat exchange between processes can be performed in the following ways: i) directly, by exchanging heat between the hot stream of one process and the cold stream of another process, or ii) indirectly, by utilizing an intermediate utility at optimal temperature levels (these are optimization variables). In both cases the transport of the heat carrier, either the process stream or the intermediate utility stream, is considered. Because there are severe nonlinearities and numerous options for heat recovery, the model is difficult to implement even for small-scale problems. However, when evaluating potential matches, it usually happens that most matches are either infeasible given heat transfer limitations, or unviable for economic reasons. A two-step approach is therefore proposed, where in the first step match alternatives are pre- screened with respect to infeasibility and unviability, and in the second step, a more detailed design is synthesized, taking into consideration the reduced superstructure obtained in the first step. The proposed two- step procedure yields results simultaneously at both the process and the Total Site levels, while also accounting for important properties such as heat losses, pipeline design and cost, temperature/pressure drop during transport between processes, and different types of heat exchangers.

A study on scaled up proton exchange membrane fuel cell with various flow channels for optimizing power output by effective water management using numerical technique

Water management in a Proton Exchange Membrane Fuel Cell (PEMFC) is of great concern for effective cell performance. Existing studies for scaled up model of PEMFC are limited only to experimental work where effective water management with respect to flow channels are very few, time consuming and less economical. In this connection, an extensive numerical study is carried out for optimizing the flow channels with respect to optimal pressure drop and water management for the scaled up model (225 cm2) of PEMFC which is a novel approach in the field of PEMFC. This numerical study involves four different configurations of flow channels. In the first configuration, predominantly existing serpentine parallel flow channel for 25 cm2 is extended to 225 cm2 cross-sectional area. Serpentine zig-zag, straight parallel and straight zig-zag are the other three flow channels considered respectively in this study. The three dimensional flow through the PEMFC is simulated by solving the governing principles namely mass, momentum, energy, species and electro-chemical equations. It is found that the power density developed by straight flow channel with zig-zag flow path is 0.3758 W/cm2 and is the maximum of the configurations considered and this is due to effective water management with minimal pressure drop.

Selecting the optimum pressure drop per unit length of district heating piping network based on operating strategies

A mathematical model used to obtain the optimum pressure drop per unit length (PDPUL) and the minimum annual cost (MAC) of a district heating (DH) piping network based on operating strategies was established. A case study based on a DH system in Hebei Province, China, was carried out. Four operating strategies were used respectively. Sensitivity analysis method was used to investigate the impact of pump overall efficiency, electricity price, heat price and interest rate on the optimum PDPUL and the MAC of the DH piping network. Results showed that the optimum PDPUL and the MAC of the DH piping network could be obtained by using the mathematical model. The MAC of the DH piping network for variable mass flow rate (VMFR) operating strategies was lower than that for constant mass flow rate (CMFR) operating strategies. Also, the decrease of the design supply temperature in the primary side caused the reduction of the optimum PDPUL. But the MAC increased with the decrease of the design supply temperature in the primary side. Among all the parameters, the pump overall efficiency had the greatest impact on the optimum PDPUL and the MAC of the DH piping network.

Molecular Dynamics Simulation of Transport and Separation of Carbon Dioxide–Alkane Mixtures in a Nanoporous Membrane Under Sub- and Supercritical Conditions

Extensive nonequilibrium molecular dynamics simulations were carried out to study the transport and separation of binary mixtures of $$\hbox {CO}_2$$ and n-alkane chains in a molecular pore network model of nanoporous carbon molecular-sieve membrane. Separation of both sub- and supercritical mixtures was studied. The membrane was generated atomistically by the Voronoi tessellation of the space, using tens of thousands of atoms. The simulations indicate that there is an optimal pressure drop for separation of mixtures of $$\hbox {CO}_2$$ and n-alkanes, when the mixtures enter the membrane under supercritical conditions. In addition, separation of $$\hbox {CO}_2$$ from n-alkanes becomes increasingly more efficient as the size of the alkane increases.

Optimal Frictional Pressure Drop and Vapor Quality Relationship of Ammonia and R22 in Two-Phase Flow

Research in two-phase flow in heat exchanging devices plays an important part in today’s applications in miniaturization of engineering systems. The phase change process factors in the flow conditions and heat transfer in evaporators and condensers. Numerous studies in the past have looked at the predicted and measured frictional pressure drop of coolants as the vapor quality increases. This paper reports a preliminary attempt at modeling of the relationship between the frictional pressure drop and vapor quality in an ammonia-cooled and R22-cooled mini-channel of 1.5 mm diameter under optimized conditions using multi-objective genetic algorithm. R22 is a being phased-out due to its ozone-depleting characteristic and ammonia is being considered as its potential replacement. The properties of ammonia and R22 used have been obtained experimentally at the saturation temperature of 5°C and 10°C respectively. Modeling of the minimized pressure drop per unit tube length together with the Lockhart-Martinelli parameter was completed under optimized flow rate and vapor quality.The outcomes obtained are similar to those that have been reported experimentally with other coolants, increasing pressure drop with increasing vapor quality.

Evaluation of the optimal turbine pressure drop ratio for a solar chimney power plant

The optimal turbine pressure drop ratio fopt for a solar chimney power plant (SCPP) is investigated using an analytical approach and 3D numerical simulations. Results indicate that the solar radiation and ambient temperature have obvious influences to the optimal turbine pressure drop ratio fopt and an improved performance of the SCPP system leads to a high fopt. The performance comparison between the cases with different collector shapes, a circular collector and a square collector with the same area, is conducted for the first time. It is found that the m-th power law assumption can also be applied to the SCPP system with a square collector; the values of fopt for the cases with a square collector are close to those for the cases with a circular collector. A fitting equation with variables of solar radiation and ambient temperature is obtained using the simulation results of the Spanish prototype; the results reveal that the fopt of the Spanish prototype ranges from 0.90 to 0.94 under normal climate conditions. This paper provides an approach to the preliminary estimation of plant performance, and reference data for an optimal pressure drop through solar chimney turbines.

Development and validation of a desiccant wheel model calibrated under transient operating conditions

The current study presents model based predictions of a desiccant wheel performance using the transient measurements obtained from a real system. The model is based on a set of equations to simulate the optimal and measured transient performance as a function of measurable input variables related to the desiccant wheel material and structure. The model is adapted to analyze the influence of different working conditions on the desiccant wheel performance: rotation speeds, air velocity, inlet temperature, and inlet air humidity for both process and regeneration air. The model is capable of estimating the optimal rotation speed and pressure drop of the desiccant wheel. Moreover, the developed model can be applied in both, dehumidification and enthalpy modes. The model is validated in comparison to the published data and measurements from the real building desiccant wheel installation. The specific enthalpy at the outlet of process air is considered performance parameter. The obtained results are in agreement with the published data, while the resulting maximum and minimum validation root mean square error (mean percentage error) between the simulated and measured transient performance is 3.6 kJ/kg (4.6%) and 1.9 kJ/kg (0.2%), respectively.

Thermodynamic optimization of a triple-shaft open intercooled, recuperated gas turbine cycle. Part 2: power and efficiency optimization

The power and the efficiency of a triple-shaft open intercooled, recuperated gas turbine cycle are analyzed and optimized based on the model established using thermodynamic optimization theory in Part 1 of this paper by adjusting the low-pressure compressor inlet relative pressure drop, the mass flow rate and the distribution of pressure losses along the flow path. First, the power output is optimized by adjusting the intercooling pressure ratio, the air mass flow rate or the distribution of pressure losses along the flow path. Second, the thermodynamic first-law efficiency is optimized subject to a fixed fuel flow rate and a fixed overall size by seeking the optimal intercooling pressure ratio, the compressor inlet pressure drop and optimal flow area allocation ratio between the low-pressure compressor inlet and the power turbine outlet. The numerical examples show that increase in effectiveness of intercooler increases power output and its corresponding efficiency and increase in effectiveness of recuperator decreases power output appreciably but increases its corresponding efficiency; there exist an optimal low-pressure compressor inlet relative pressure drop and an optimal intercooling pressure ratio, which lead to a maximum power. For a fixed fuel mass rate and a fixed overall area of low-pressure compressor inlet and power turbine outlet, maximum thermodynamic first-law efficiency is obtained by optimizing low-pressure compressor inlet relative pressure drop and intercooling pressure ratio. The double-maximum thermodynamic first- law efficiency is obtained by searching optimal flow area allocation between low-pressure compressor inlet and power turbine outlet.

Numerical analysis of the optimal turbine pressure drop ratio in a solar chimney power plant

In a solar chimney power plant, only a fraction of the available total pressure difference can be used to run the turbine to generate electrical power. The optimal ratio of the turbine pressure drop to the available total pressure difference in a solar chimney system is investigated using theoretical analysis and 3D numerical simulations. The values found in the literature for the optimal ratio vary between 2/3 and 0.97. In this study, however, the optimal ratio was found to vary with the intensity of solar radiation, and to be around 0.9 for the Spanish prototype. In addition, the optimal ratios obtained from the analytical approach are close to those from the numerical simulation and their differences are mainly caused by the neglect of aerodynamic losses associated with skin friction, flow separation, and secondary flow in the theoretical analysis. This study may be useful for the preliminary estimation of power plant performance and the power-regulating strategy option for solar chimney turbines.

Evaluation of the immersion vacuum cooling of cooked beef joints—mathematical simulation of variations in beef size and porosity and pressure reduction rates

A detailed evaluation of the influence of variations in product (beef size and porosity) and process parameters (vacuum chamber pressure reduction rate) on cooling time, total mass loss, pressure and temperature distributions during the immersion vacuum cooling of large cooked beef joint samples was performed using a previously developed and validated mathematical model. Beef size variations were done by changing radius values (between 5.5 and 8 cm) and porosity levels (between 2 and 5%). Pressure reductions rates varied between circa 5 to 25 mbar/min. The simulation indicated that using the maximum pressure drop rate tested (circa 25 mbars/min) beef samples (3% porosity) of up to 6.5 and 7 cm radius can be cooled within Irish and American official safety guidelines, respectively. An increase in porosity could further reduce cooling time but incur higher mass losses, which can be detrimental to product quality. The identified effects and relevant trends due to variations on the different parameters studied on the cooling process performance lead to recommendations in relation to optimal pressure drop rate, in relation to size and porosity ranges. Necessary modifications to the equipment design, such as the introduction of agitation in the cooling medium, are also emphasised. Meat is typically cooked before consumption, and if it is not for immediate consumption, it should be cooled as soon as possible thereafter. Commercial products such as large cured and precooked meat pieces undergo heat treatment, and hence need to be cooled prior to packing, storage and distribution. Product temperature should be reduced as quickly as possible to its final target, to minimise quality loss and prevent or minimise microbial growth or spore germination. After relatively mild heat treatments such as pasteurisation (heat processes in the range of 60–80 °C) the microorganisms of concern are often heat resistant micro flora such as mesophilic spores of Bacillus and Clostridium species that under adequate conditions might germinate and grow, posing a safety risk. The difficulties of cooling large commercial meat pieces or joints have been reported, and the responsibility is on manufacturers to ensure that procedures are adequate and safety risks are minimised. Recent investigations into vacuum cooling have shown advances for the meat industry. ► An inverse relationship between sample radius and calculated mass losses was found. ► Effect of vacuum rate on mass loss evolution increases with cooling time. ► Higher vacuum rates lead to faster cooling, but effect reduces with rate increase. ► Pressure control during 1st cooling phase can help minimise mass losses. ► Pressure control during 2nd cooling phase can help minimise total cooling time.

Multi-Objective Optimization of Double-Tube Once-Through Steam Generator

This paper presents a double-tube once-through steam generator (DOTSG) consisting of the outer straight tube and the inner helical tube. The tube length and pressure drop of are important parameters in optimal design of DOTSG. For optimal design of such a system, it was modeled to estimate its tube length and pressure drop. Pitch of inner helical tube, flow distribution ratio of the primary fluid, and tube assemblage are considered as design parameters. Then fast and elitist nondominated sorting genetic algorithm-II (NSGA-II) method was applied to find the optimum values of design parameters. In the presented optimal design approach, the tube length and the total pressure drop are two objective functions. The results of optimal designs were a set of multiple optimum solutions, called “Pareto optimal solutions.” The sensitivity analysis of change in optimum tube length and pressure drop with change in design parameters of the DOTSG is also performed and the results are reported.

Performance characteristics of ejector expander transcritical CO2 refrigeration cycle

The effects on the performance of the basic CO2 refrigeration cycle when using an ejector as the expander to recover expansion work were investigated. A two-phase constant area ejector flow model was used in the ejector analysis. The coefficient of cooling performance and gas cooler pressure that yielded the maximum exergy efficiency, suction nozzle pressure drop, and optimum values for the ejector area ratio were determined for various evaporator and gas cooler outlet temperatures. Parametric studies were performed using engineering equation solver. The suction nozzle pressure drop had a significant effect on the ejector area ratio, coefficient of cooling performance, and exergy efficiency. It is necessary to design an ejector with the optimum area ratio to achieve the optimum pressure drop and maximum performance. Cycle that use the ejector as an expander always has higher coefficient of performance and exergy efficiency than conventional cycle under any operating condition. The irreversibility decreases compared to the classic refrigeration cycle when the ejector or turbine are used as an expander. The analysis results showed that – under a gas cooler pressure of 9.5 MPa, gas cooler outlet temperature of 40 °C, evaporator temperature of 5 °C, and cooling capacity of 3.5 kW – the total irreversibility of the ejector system was lower than those of the basic and turbine expander systems by 39.1% and 5.46%, respectively.

Prediction of flow crossover in the GDL of PEFC using serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for Polymer electrolyte fuel cells (PEFCs) since it ensures the removal of water produced in the cell with an acceptable parasitic load. The operating parameters such as temperature, pres- sure and flow distribution in the flow channel and gas diffusion layer (GDL) has a great influence on the performance of PEFCs. It is desired to have an optimum pressure drop because a certain pressure drop helps to remove excess liquid water from the fuel cell, too much of pressure drop would increase parasitic power needed for the pumping air through the fuel cell. In order to accurately estimate the pressure drop precise calculation of mass conservation is necessary. Flow crossover in the serpentine channel and GDL of PEFC has been investigated by using a transient, non-isothermal and three-dimensional numerical model. Considerable amount of cross flow through GDL is found and its influence on the pressure variation in the channel is identified. The results obtained by numerical simula- tion are also compared with the experimental as well as theoretical solution.

Exploration of a simple, low cost, micro gas turbine recuperator solution for a domestic combined heat and power unit

Abstract For micro gas turbines (MGT) of around 1 kW or less, a commercially suitable recuperator must be used to produce a thermal efficiency suitable for use in UK Domestic Combined Heat and Power (DCHP). This paper uses computational fluid dynamics (CFD) to investigate a recuperator design based on a helically coiled pipe-in-pipe heat exchanger which utilises industry standard stock materials and manufacturing techniques. A suitable mesh strategy was established by geometrically modelling separate boundary layer volumes to satisfy y+ near wall conditions. A higher mesh density was then used to resolve the core flow. A coiled pipe-in-pipe recuperator solution for a 1 kW MGT DCHP unit was established within the volume envelope suitable for a domestic wall-hung boiler. Using a low MGT pressure ratio (necessitated by using a turbocharger oil cooled journal bearing platform) meant unit size was larger than anticipated. Raising MGT pressure ratio from 2.15 to 2.5 could significantly reduce recuperator volume. Dimensional reasoning confirmed the existence of optimum pipe diameter combinations for minimum pressure drop. Maximum heat exchanger effectiveness was achieved using an optimum or minimum pressure drop pipe combination with large pipe length as opposed to a large pressure drop pipe combination with shorter pipe length.

Optimal mixed convection for maximal energy recovery with vertical porous channel (solar wall)

A vertical open-ended channel filled with a porous medium, with an imposed heat flux and a heat loss coefficient on one of its walls, is studied numerically. A fan can enhance the self-driven flow, and therefore a mixed convection regime is considered. The objective is to maximize the overall energy recovery (heat transfer to the fluid minus fan power). Correlations are developed for optimal pressure drop to be imposed by the fan and maximal energy recovery, as a function of the Rayleigh number, the channel aspect ratio, and the heat loss coefficient. The optimal allocation of the total energy losses (i.e., sum of the heat loss and fan power) is shown. Potential applications include solar wall and solar chimney used for ventilation and preheating of makeup air in buildings.

Cardiac Axial Blood PUmp Analysis and Performance Prediction

Computational fluid dynamics (CFD) has been used for developing and evaluating the performance of a novel design of the cardiac axial blood pump (CABP). This device could be used as an implantable pump for boosting blood circulation in patients whose hearts are not providing sufficient output. Based on the Berlin Heart configuration the blood pump has been designed for a flow rate of 5 L/mine and 100 mmHg of head pressure. Finite element analysis method has been performed to predict the shear stress, pressure, velocity, pressure drop on the fluid through the pump and the shear stress on the pump impeller. Also, flow streamlines has been discussed in detail in this study to predict the flow streamlines behavior and the stagnation points. The goal of this work is to design an efficient blood pump to support the blood circulatory system and reduce the shear stress and blood hemolysis during transport through the pump. Our design simulated at several rotational speeds (5000 to 7000) rpm to investigate the relationship between the rotational speeds and shear stress. Results indicate that the rotational speed has a direct correlation with shear stress and pressure drop and at 6500 rpm the pump gives its optimal pressure drop.

Effect of Bed Pressure Drop on Performance of a CFB Boiler

The effect of bed pressure drop and bed inventory on the performances of a circulating fluidized bed (CFB) boiler was studied. By using the state specification design theory, the fluidization state of the gas−solids flow in the furnace of conventional CFB boilers was reconstructed to operate at a much lower bed pressure drop by reducing bed inventory and control bed quality. Through theoretical analysis, it was suggested that there would exist a theoretical optimal value of bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan energy consumption. The analysis was validated by field tests carried out in a 75 t/h CFB boiler. At full boiler load, when bed pressure drop was reduced from 7.3 to 3.2 kPa, the height of the dense zone in the lower furnace decreased, but the solid suspension density profile in the upper furnace and solid flow rate were barely influenced. Consequently, the average heat transfer coefficient in the furnace was kept nearly the same and the furnace temperature increment was less than 17 °C. It was also found that the carbon content in the fly ash decreased first with decreasing bed pressure drop and then increased with further increasing bed pressure drop. The turning point with minimal carbon content was referred to as the point with optimal bed pressure drop. For this boiler, at the optimum point the bed pressure was around 5.7 kPa with the overall excess air ratio of 1.06. When the boiler was operated around this optimal point, not only the combustion efficiency was improved, but also fan energy consumption and wear of heating surface were reduced.

Multi-objective optimization of rotary regenerator using genetic algorithm

The rotary regenerator (heat wheel) is an important heat recovery equipment, which rotates between two cold and hot streams. The pressure drop and effectiveness of rotary regenerator are important parameters in optimal design of this equipment for industrial applications. For optimal design of such a system, it was thermally modeled using ɛ-NTU method to estimate its pressure drop and effectiveness. Frontal area, ratio of hot to cold frontal heat transfer area, matrix thickness, matrix rotational speed, matrix rod diameter and porosity were considered as design parameters. Then fast and elitist non-dominated sorting genetic algorithm (NSGA-II) method was applied to find the optimum values of design parameters. In the presented optimal design approach, the effectiveness and the total pressure drop are two objective functions. The results of optimal designs were a set of multiple optimum solutions, called ‘Pareto optimal solutions’. The sensitivity analysis of change in optimum effectiveness and pressure drop with change in design parameters of the regenerator was also performed and the results are reported.

Numerical investigation of heat transfer and pressure drop characteristics of tube–fin heat exchangers in ice slurry HVAC system

This paper analyzes the heat transfer and pressure drop characteristics of a tube–fin heat exchanger in ice slurry HVAC system. Ice slurry is a suspension of crystallized water based - ice solution with a freezing point depressant like ethylene glycol. The ice- slurry is pumpable, hence it is also called pumpable ice. The composition of ice slurry considered for analysis is 14% ice fraction, 16% ethylene glycol, and 70% water by volume. It is deduced that the ice slurry HVAC system results in 7.4% increase in temperature drop over the conventional chilled water system The latent heat absorbed by ice slurry on melting makes it an attractive choice for achieving high degree of cooling. The numerical analysis was conducted by simulating the ice slurry tube flow region and air flow region of tube–fin heat exchanger in the air-handling unit of HVAC system. For the simulation six different louver patterns with 10 to 55 louver angle were considered. The design of the tube–fin heat exchanger for optimal heat transfer and pressure drop characteristics was also determined with the optimization parameter like louver angle, fin pitch, and ice slurry flow velocity.

Optimal pressure drop ratio for micro recuperators in small sized gas turbines

Heat recuperation is often used to improve the overall cycle efficiency of standard gas turbines. Hereby a high heat exchanger effectiveness and low pressure drops are favourable to achieve maximal cycle efficiency. Finding a compromise between these conflicting requirements is the main challenge in recuperator design. In this paper it is shown that the optimal cold and hot side pressure drops of a micro recuperator are uniquely correlated when integrated in a gas turbine. For an optimized microchannel recuperator, the ratio of the cold side pressure drop over the hot side one is only a function of gas turbine properties and primarily depends on the compressor pressure ratio. Based on these findings, a simplified optimization procedure is applied to design a micro recuperator in a 1.5 kW gas turbine. It is shown that the results of this optimization are almost identical to the full multi-dimensional optimization with an accuracy of approximately 1% on geometrical parameters and an excellent agreement for cycle efficiency.